Displays

Critical ingredients and conditions necessary to initiate a new subduction zone are debated. General agreement is that subduction initiation likely takes advantage of previously weakened lithosphere and may prefer to nucleate along pre-existing plate boundaries. To evaluate how past tectonic regimes and lithospheric structures might facilitate underthrusting and lead to self-sustaining subduction, we present an analysis of the Puysegur Margin, a young subduction zone with a rapidly evolving tectonic history.

The Puysegur Margin, south of New Zealand, currently accommodates convergence between the Australian and Pacific plates, exhibits an active seismic Benioff zone, a deep ocean trench, and young adakitic volcanism on the overriding plate. Tectonic plate reconstructions show that the margin experienced a complicated transformation from rifting to seafloor spreading, to strike-slip motion, and most recently to incipient subduction, all in the last ~45 million years. Details of this tectonic record remained incomplete due to the lack of high-quality seismic data throughout much of the margin.

Here we present seismic images from the South Island Subduction Initiation Experiment (SISIE) which surveyed the Puysegur region February-March, 2018. SISIE acquired 1252 km of deep-penetrating multichannel seismic (MCS) data on 7 transects, including 2 regional dip lines coincident with Ocean Bottom Seismometers (OBS) deployments which extend (west to east) from the incoming Australian plate, across the Puysegur Trench and Puysegur Ridge, over the Solander Basin and onto the continental Campbell Plateau margin.

We integrate pre-stack depth migrated MCS profiles with OBS tomography models to constrain the tectonic development of the Puysegur Margin. Based on our results we propose the following Cenozoic evolution: (1) The entire Solander Basin contains thinned continental crust which formed from orthogonal stretching between the Campbell and Challenger plateaus during the Eocene-Oligocene. This phase of rifting was more pronounced to the south, producing thinner crust with abundant syn-rift volcanism across a wider rift-basin, in contrast to the relatively thicker crust, moderate syn-rift volcanism and narrower rift basin in the north. (2) Strike-slip deformation subsequently developed along Puysegur Ridge, west of the locus of rifting and within relatively unstretched continental lithosphere. This young strike-slip plate boundary translated unstretched crust northward causing an oblique continent-collision zone, which led to a transpressional pattern of distributed left-stepping, right-lateral faults. (3) Subduction initiation was aided by large density contrasts as oceanic lithosphere translated from the south was forcibly underthrust beneath the continent-collision zone. Early development of oblique subduction generated modest and widespread reactivation of faults in the upper plate. (4) Present-day, the Puysegur Trench shows a spatiotemporal transition from nearly mature subduction in the north to a recently initiated stage along the southernmost margin, requiring a southward propagation of subduction through time.

Our new seismic images suggest subduction initiation at the Puysegur Margin was assisted by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike-slip along the developing plate boundary. The Puysegur Margin demonstrates that forced nucleation along a strike-slip boundary is a viable subduction initiation model and should be considered throughout Earth’s history.

How to cite: Shuck, B., Van Avendonk, H., Gulick, S., Gurnis, M., Sutherland, R., Stock, J., Patel, J., Hightower, E., Saustrup, S., and Hess, T.: Strike-Slip Enables Subduction Initiation Beneath a Failed Rift: New Seismic Constraints from Puysegur Margin, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10317, https://doi.org/10.5194/egusphere-egu2020-10317, 2020.

The crystalline crust that underlies the Western Canada Sedimentary Basin in northern Alberta is composed of tectonic domains that accreted to the margin of the Archean Rae province of western Laurentia, ca. 2.1-1.9 Ga. Geophysical data indicate that the basement crust in this region hosts a vast, mid-crustal reflection sequence (Winagami Reflection Sequence) interpreted as assemblage of mafic sills and an unusually wide domain of Paleoproterozoic magmatic arcs (Taltson Magmatic Zone). The latter are interpreted to have formed during Paleoproterozoic tectonic assembly through near-synchronous closure of small oceanic basins along subduction systems of opposing polarity. Here, we introduce a new tectonic model, which postulates that the Taltson Magmatic Zone represents collated fragments that formed within a single subduction system. Comparison with modern analogs suggest that observed temporal relationships and present-day configuration of Paleoproterozoic arcs can be explained by plate-margin processes of slab rollback and back-arc rifting. Our model is consistent with re-interpreted basement-drillcore petrology, provides a genetic link for the association between magmatic arcs and the Winagami sill complex, explains an extraordinary fit between aeromagnetically defined “conjugate margins” and provides a tectonic framework for the origin of the enigmatic low^-δ18O magmatic zone (Kimiwan anomaly) along the southern Chinchaga domain.

How to cite: Ekpo, E., Eaton, D., and Nair, R.: Evidence for Slab Rollback, Back-Arc Rifting and Arc Dismemberment During Assembly of Western Laurentia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3661, https://doi.org/10.5194/egusphere-egu2020-3661, 2020.

Serpentinized peridotites are weaker than other mantle rocks, with an internal friction coefficient μ_i~0.3 vs. ~0.6. Therefore they often promote strain localization. Serpentinite is also considerably lower in density (r=2.5-2.6 g/cm³) than most rocks. In the presence of denser material, its buoyancy can mobilize upwelling masses and aid exhumation. Serpentinized peridotites can therefore influence the evolution of tectonic plate boundaries: their presence enhances shear processes, and serpentinite-hosted faults can evolve into zones of permanent lithospheric weakness that can be reactivated during different tectonic phases. Fault reactivation also provides paths for fluid infiltration and upward remobilization of serpentinized peridotites that can also interact diapirically with overlying rocks.

We have compiled observations that document the near-surface journey of serpentinized peridotites that are exhumed during rifting and continental break-up, reactivated as buoyant material during subduction, and ultimately emplaced as ‘ophiolite-like’ fragments within orogenic belts. This lifecycle is particularly well documented in former Tethys margins that now subduct beneath the Calabrian Arc. Here recent studies describe serpentinized peridotites that diapirically rose from a subducting lithospheric slab to be emplaced into the accretionary prism in front of the continental arc. We show that this newly recognized mode of subduction-linked serpentine diapirism from the downgoing lithospheric slab is consistent with the origin of some exhumed mantle rocks in the Apennines, with these assemblages having been ultimately emplaced into their present locations during Alpine Orogenesis. Transfer of serpentinized peridotites from the mantle lithosphere of the subducting slab to the overriding plate motivates the concept of a potentially “leaky” subduction channel.  In addition to passing vertically through a shallow subduction channel, weak serpentine bodies may also rise into and preferentially migrate within the intraplate shear zone, leading to strong lateral heterogeneities in its composition, mechanical strength and seismic characteristics.

How to cite: Vannucchi, P., Morgan, J., Polonia, A., and Molli, G.: How serpentine peridotites can leak through subduction channels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10250, https://doi.org/10.5194/egusphere-egu2020-10250, 2020.

Subduction-zone dynamics, kinematics, and seismicity are strongly affected by the rheology of hydrous phyllosilicates. Although there is growing evidence for hydrous minerals in the subducting plate, mantle wedge, and the interface between the plates, we are continuing to learn more about the rheological behavior of phyllosilicates at the relevant pressures. Talc is stable to depths of ≈100 km and has been found in fault rocks and subduction-zones mélanges as the product of metasomatism and/or mineral breakdown (e.g., breakdown of antigorite). The frictional strength of talc under low to intermediate pressures (up to ~400 MPa) was studied and demonstrated some of the mineral’s unique rheology; however, there is a lack of data for pressures of P > 0.5 GPa. Here we present the first rheological and microstructural analysis of experimentally deformed talc under pressure and temperature conditions relevant for the rheology of a subducted slab or mantle wedge.

We analyzed the mechanical and microstructural evolution of 15 samples of natural talc cylinders deformed using a high P-T deformation ‘Griggs’ type apparatus. We used natural samples comprise of >98 % talc and analyzed the post-mortem microstructure and chemistry of the samples using optical microscopy, scanning electron microscopy, and electron microprobe. The experiments were performed at confining pressures from 0.5 to 2 GPa and temperatures of 25 to 700°C; all within the talc stability field. Results show that the strength of talc at 25°C or 400°C is pressure-dependent up to the highest pressure tested (2 GPa). This behavior is attributed to brittle/semi-brittle mechanisms. At higher temperatures (500-700° C) and above a pressure threshold the strength becomes independent of pressure (e.g., when P > 1 GPa at T = 600 ° C), indicating that dilatant cracking is suppressed at these pressures. However, microstructural analysis indicates that fracturing is evident in all samples at all conditions examined. Interestingly, samples deformed at higher temperatures (>600°C) show more localized deformation. A synthesis of results from this study and previously published studies demonstrate that the strength of talc only becomes temperature-dependent at higher pressures. It is suggested that an increasing P-T geotherm of a subducted slab is likely to induce weakening and localization of talc-rich layers with possible implications for the mechanism to induce/hinder regional seismicity and affect the plate-coupling between the subducted and riding plates.   

How to cite: Boneh, Y., Pec, M., and Hirth, G.: The rheology of talc at high P-T conditions with implications for subduction-zone dynamics , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12696, https://doi.org/10.5194/egusphere-egu2020-12696, 2020.

Eclogite formation in the subducting crust was the first metamorphic transformation to be acknowledged as important in the dynamics of convergent plate boundaries. It is indeed expected to affect the mass distribution via density change, but it also influence the fluid content of crustal and possibly lithospheric wedges; both density and fluids being first order in values measured by passive geophysical imaging such as tomography of receiver functions. Recent high accuracy focal mechanism solutions showing singular signatures in deep orogens actually imply that eclogitization could also have a signature in the seismological source signals, and hence have an impact at much shorter time-scales. This presentation aims at bridging what we know from the field and the lab at smaller time and space scales, to what we observe at larger scales in collision zones. Field-based studies show the ways a pristine rock can evolve from metastable to fully eclogitized from the thin section to the kilometre scale. More than the contrast between eclogitized and non-eclogitized domains, the eclogitization front itself is expected to be detected in the geophysics, especially when driven by strain. Indeed strain-assisted eclogitization develops a characteristic shear zone network pattern with a significant anisotropy. This network itself evolves with the eclogitization progress. The observed progressive widening and increasing connectivity of eclogite-facies shear zones with increasing fluid availability could actually be controlled by the transient properties of the newly formed assemblages, inducing fluid pressure gradients for instance. In this context it appears that the competition between reaction kinetics and strain-rate is a key factor. This is also the case at shorter time scales. Experimental studies show that strain of metastable assemblages in the eclogite facies is more likely to lead to mechanical instabilities for intermediate reaction kinetics, implying again that not the eclogite but the eclogitization rate is the smoking gun. Eclogitization of plagioclase-bearing rocks is the finite result of a large set of reactions involving different chemical subsystem (Na or Ca end-members, with or without fluid available), not reacting at the same pace. Further work is therefore needed on the kinetics of the different reactions and their interactions to distinguish the one(s) that controls the eclogitization front signature, and hence improve the seismological imaging acuity.

How to cite: Labrousse, L., Sarah, I., Sascha, Z., Marie, B., Lisa, K., Alexandre, S., Timm, J., Torgeir B., A., Frederik, T., Julien, G., Evangelos, M., Stefan, S., Johannes C., V., and Joerg, R.: Up-scaling eclogitization : from experimental and natural aggregates behaviours to seismological signatures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11073, https://doi.org/10.5194/egusphere-egu2020-11073, 2020.

The origin of earthquakes in the lower crust at depth of 20-40 km, where dominantly ductile deformation is expected, is highly debated. Exhumed networks of lower crustal coeval pseudotachylytes (quenched frictional melt produced during seismic slip) and mylonites (produced during the post- and interseismic viscous creep) provide a snapshot of the earthquake cycle at anomalously deep conditions in the crust. Such natural laboratories offer the opportunity to investigate the origin and the tectonic setting of lower crustal earthquakes.

The Nusfjord East shear zone network (Lofoten, northern Norway) represents an exhumed lower crustal earthquake source, where mutually overprinting mylonites and pseudotachylytes record the interplay between coseismic slip and viscous creep (Menegon et al., 2017; Campbell and Menegon, 2019). The network is well exposed over an area of 4 km² and consists of three main intersecting sets of ductile shear zones ranging in width from 1 cm to 1 m, which commonly nucleate on former pseudotachylyte veins. Mutual crosscutting relationships indicate that the three sets were active at the same time. Amphibole-plagioclase geothermobarometry yields consistent P-T estimates in all three sets (700-750 °C, 0.7-0.8 GPa). The shear zones separate relatively undeformed blocks of anorthosite that contain pristine pseudotachylyte fault veins. These pseudotachylytes link adjacent or intersecting shear zones, and are interpreted as fossil seismogenic faults representing earthquake nucleation as a transient consequence of ongoing, localised aseismic creep along the shear zones (Campbell et al., under review).

The coeval activity of the three shear zone sets is consistent with a local extensional setting, with a bulk vertical shortening and a horizontal NNW-SSE extension. This extension direction is subparallel to the convergence direction between Baltica and Laurentia during the Caledonian Orogeny, and with the dominant direction of nappe thrusting in the Scandinavian Caledonides. ⁴⁰Ar‐³⁹Ar dating of localized upper amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord East network yielded an age range of 433–413 Ma (Fournier et al., 2014; Steltenpohl et al., 2003), which indicates an origin during the collisional (Scandian) stage of the Caledonian Orogeny.

We propose that the Nusfjord East brittle-viscous extensional shear zone network represents the rheological response of the lower crust to the bending of the lower plate during continental collision. (Micro)seismicity in the lower crust in collisional orogens is commonly localized in the lower plate and has extensional focal mechanisms. This has been tentatively correlated with slab rollback and bending of the lower plate (Singer et al., 2014). We interpret the Nusfjord East shear zone network as the geological record of this type of lower crustal seismicity.

How to cite: Menegon, L., Campbell, L., Fagereng, Å., and Pennacchioni, G.: The Nusfjord exhumed earthquake source (Lofoten, Norway): deep crustal seismicity driven by bending of the lower plate during continental collision, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7129, https://doi.org/10.5194/egusphere-egu2020-7129, 2020.

Shortening of Archaean and Paleoproterozoic continental lithospheres: large strains, but no orogeny

Denis Gapais¹, Jonathan Poh¹, Philippe Yamato¹, Thibault Duretz^1, Florence Cagnard²

• (1) Géosciences Rennes, UMR CNRS 6118, Université de Rennes 1, 35042 Rennes cedex, France

• (2) Bureau de Recherche Géologique et minière, 3 avenue Claude-Guillemin, BP 36009 45060 Orléans Cedex 2, France

Denis.gapais@univ-rennes1.fr, jonathanpoh87@gmail.com, philippe.yamato@gmail.com, thibault.duretz@univ-rennes1.fr, f.cagnard@brgm.fr

In many ancient deformation belts of Archaean and Paleoproterozoic age (e.g. Terre Adélie in East Antarctica, Finnish Svecofennides in Southern Finland, Murchison Belt in South Africa, Thompson Nickel Belt in Manitoba, Dharwar Craton in western India, Abitibi sub-Province in Québec, Trans-Hudson belt of Canada, Trans-Amazonian belt of Suriname), latest recorded deformations are compressive or transpressive. In these belts that involved hot and weak continental crusts, deformations are distributed with basically vertical tectonics and important crustal thickening. On the other hand, there is no evidence of syn-orogenic extension or late-orogenic collapse, as classically observed in modern orogens where extensional detachments are widespread.

Analogue and numerical models emphasize that shortening of weak and hot lithospheres basically favour downward motions, which result in limited topographies. Field evidence further point to metamorphic isogrades rather parallel to the Earth surface at belt scale. Hence, metamorphic conditions are rather monotonous at the scale of individual belts, with limited metamorphic jumps and typical P-T paths with no significant adiabatic retrograde segments. Consistently, localized deep detrital sedimentary basins like foreland or intra-mountain basins, are not documented. Sedimentary records rather suggest distributed sedimentation processes. In addition, several lines of evidence tend to point out that cooling of ancient hot deformation belts was rather slow, which is consistent with distributed topographies and long-lasting erosion-driven exhumation processes.

 On these bases, we propose that gravity-driven collapse had no reason to occur in ancient hot deformation belts because important topographic gradients and orogeny could not develop as observed in modern mountain chains.

How to cite: Gapais, D.: Shortening of Archaean and Paleoproterozoic continental lithospheres: large strains, but no orogeny, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1496, https://doi.org/10.5194/egusphere-egu2020-1496, 2020.

Knowledge on the thermal state of orogens and subduction zones is crucial in trying to understand the processes that take place in these zones, since temperature controls, e.g., rock strength, metamorphic reactions and fluid flow. These are all critical parameters for the dynamics of orogens and subduction zones and conversely, these parameters feed back on the thermal state in various ways. We investigated an example of a former subduction zone, exposed in the central Tauern Window (Eastern Alps), with the aim of reconstructing its three-dimensional temperature variations.

Structural and petrological observations in the central Tauern Window reveal a tens-of-kilometre-scale sheath fold that formed under high-pressure (HP) conditions (ca. 2 GPa). The fold is a composite structure that isoclinally folded the thrust of an oceanic nappe derived from Alpine Tethys onto a unit of the distal European continental margin, also affected by HP conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes are associated with temperature-dominated Barrow-type metamorphism that overprints the HP-metamorphism partly preserved in the sheath fold.

Using Raman spectroscopy on carbonaceous material (RSCM) on 100 samples from this area, we were able to distinguish domains with the original, subduction-related peak temperature conditions from domains that were overprinted during later temperature-dominated (Barrovian) metamorphism. The distribution of RSCM-temperatures in the Barrovian domains indicates a decrease of peak temperature with increasing distance from the centres of the thermal domes, both in map view and cross section. This represents a geotherm where paleo-temperature increases downward, in line with previous studies using, e.g., oxygen isotope fractionation and calcite-dolomite equilibria. However, we observe the opposite temperature trend in the lower limb of the sheath fold, viz., tendentially an upward increase in paleo-temperature. We interpret this inverted temperature domain as the relic of a subduction-related temperature field. Towards the central part of the sheath fold’s upper limb, measured temperatures increase to a maximum of ca. 520°C. Further upsection in the hanging wall of the sheath fold, temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Isograds (i.e. contours of equal peak-temperature) are oriented roughly parallel to the nappe contacts and lithological layering, which results in an eye-shaped concentric isograd pattern in cross-section. This reflects a sheath-like three-dimensional geometry of the isograds. We propose the following hypothesis to explain the subduction-related peak-temperature pattern: The pattern reflects sheath folding of a subduction-related temperature field. Possibly, sheath folding occurred during exhumation, after the equilibration at peak pressure and temperature conditions. The preservation of the pattern implies fast exhumation and cooling of the rocks.

How to cite: Groß, P., Pleuger, J., Handy, M. R., and John, T.: Three-dimensional temperature variations in a fossil subduction zone resolved by RSCM thermometry (Tauern Window, Eastern Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8054, https://doi.org/10.5194/egusphere-egu2020-8054, 2020.

The Adula Nappe in the Central Alps and the Pohorje Nappe in the Eastern Alps are among the highest-pressure metamorphic complexes in the Alps. In both cases, Variscan continental crust containing post-Variscan intrusions was subducted, during the Cenomanian-Turonian in the case of Pohorje and during the Eocene in the case of Adula.

The Pohorje Nappe is exceptional in that ultrahigh pressures of 3.0 to 4.0 GPa are recorded by different rocks contrasting in rheology: competent lenses of kyanite eclogite and garnet peridotite as well as the surrounding incompetent matrix of diamond-bearing paragneiss. If pressure had been strongly non-lithostatic, rheologically different rock types would be expected to record different pressures. This is not the case, which rather suggests near-lithostatic pressure and, consequently, subduction to >100 km depth. Lu-Hf ages for UHP metamorphism in eclogite and garnet peridotite are similar (c. 96–92 Ma). Paragneiss yielded Permian to Triassic zircon cores and Cretaceous (c. 92 Ma) rims grown during UHP metamorphism. Hence, the rocks were subducted and exhumed together as a coherent, although strongly deformed unit.

The Adula Nappe originated from the southern passive continental margin of Europe. It was buried in and exhumed from a south-dipping subduction zone after Europe-Adria continent collision. Previous interpretations as a tectonic mélange were based on the mixture of gneiss with eclogite and garnet peridotite lenses. However, the eclogites also record an older (Variscan) metamorphism and thus do not represent Mesozoic oceanic crust but pre-Alpine continental basement, just like the gneisses. The Alpine subduction culminated around 37 Ma. Alpine metamorphic pressures show a strong gradient from c. 1.2 GPa at the front of the nappe in the North to >3 GPa in garnet peridotite and eclogite in the southernmost part (e.g. Alpe Arami), over a north-south distance of only c. 40 km. In contrast to Pohorje, indications of UHP metamorphism have not yet been found in the gneissic matrix surrounding eclogite and peridotite. During exhumation, the nappe was intensely sheared and folded but stayed coherent and did not mix with the surrounding units.  The exhumation of the Adula from deep in the subduction zone is recorded by mylonitic shearing in the gneissic matrix. Structures, strain, and textures indicate strongly three-dimensional, non-plane-strain flow. Differential loading, not buoyancy, is proposed to have caused the exhumation.

The main results from these two case studies are: (1) Subduction of continental crust to mantle depth is real and not a misinterpretation of non-lithostatic pressure; (2) not all subducted units are mélanges but some stay coherent during subduction and exhumation.

How to cite: Froitzheim, N.: Deep subduction and exhumation of continental crust in the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8378, https://doi.org/10.5194/egusphere-egu2020-8378, 2020.

The Attic-Cycladic Massif (ACM) preserves the entire evolution of a NE dipping subduction zone.  This includes the intra-oceanic subduction initiation associated with ophiolite obduction and formation of a metamorphic sole, to subduction-termination associated with burial and exhumation of the Cycladic continental margin to eclogite-blueschist facies conditions. The Tsiknias Ophiolite represents a piece of ca. 162 Ma Tethyan oceanic lower crust and mantle that was thrust towards the SW onto the ACM during a subduction initiation/ophiolite obduction event during the initial stages of oceanic closure. Beneath the Tsiknias Ophiolite lies a ~250 m thick sequence of amphibolites which represent the lower plate. These record an inverted metamorphic gradient at ca. 8.5 kbar reaching > 750°C at the top (associated with small-scale partial melting) and 600°C at the base and formed during high-grade metamorphism of ca. 190 Ma oceanic crust along the subduction zone interface beneath a major thrust fault (Tsiknias Thrust) under geothermal gradients of 30°C/km. U-Pb zircon dating of leucodioritic melt veins constrains the timing of metamorphism to ca. 74 Ma, which may correlate with the switch in the motion of the Nubian plate from transcurrent to convergent with respect to Eurasia. Highly deformed greenschist facies pelagic metasediments underlie the amphibolites suggesting an inverted lithological sequence. This can be explained by the zone of active thrusting propagating down structural level with ongoing subduction, such that underplated material became accreted to the base of the ophiolite. A Miocene aged greenschist-facies shear zone truncates the metamorphic sole rocks and metasediments, placing them directly against the Cycladic Blueschist Unit (CBU) associated with burial of the Cycladic continental margin down the same NE-dipping subduction zone some 25 Myr later. Lawsonite bearing eclogite and blueschist-facies rocks crop-out < 1 km structurally beneath the metamorphic sole and record P-T conditions of 23 kbar and 550°C at ca. 53-46 Ma. These rocks experienced variable retrogression through blueschist and then greenschist facies conditions. This retrogression was largely due to differential growth of lawsonite depending on bulk rock composition during prograde and peak metamorphic conditions causing some rocks to hold large quantities of water at peak conditions. Subsequent exhumation caused lawsonite to break down, hydrating the adjacent rocks and facilitating growth of secondary amphibole and epidote. These P-T-t conditions imply the CBU experienced geotherms of 6-7°C/km during peak metamorphism, which suggests the subduction zone cooled at an average rate of ca. 1.5°C/km/Myr between ca. 74 and ca. 53-46 Ma. This decrease in cooling rate raises two questions:  (1) is this cooling rate a result of thermal conduction due to the burial of cold old oceanic lithosphere following subduction initiation?, or (2) are the hot apparent geothermal gradients recorded in the metamorphic sole due to processes other than conduction from the overriding lithospheric mantle?. Our thermometry data from the Tsiknias metamorphic sole suggest that: (1) the maximum temperatures increase structurally upwards towards the Tsiknias Thrust, (2) peak metamorphic temperatures are superimposed on the structure, and (3) the length scale of heating is inconsistent with thermal conduction alone.

How to cite: Lamont, T., Searle, M., White, R., Roberts, N., Gopon, P., Wade, J., and Waters, D.: The Cycladic subduction zone from birth to death: Insights into the subduction cooling rate conundrum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21985, https://doi.org/10.5194/egusphere-egu2020-21985, 2020.

Rocks of the Easton Metamorphic Suite and San Juan Islands preserve an inverted metamorphic sequence with ultramafic rocks underlain by amphibolite and high-temperature blueschist juxtaposed above low-temperature blueschists. The sequence is interpreted as a metamorphic sole and younger accreted rocks that formed during and after the initiation of Farallon plate subduction beneath North America in Jurassic time. Thermobarometry, Ar/Ar dating, and structural observations constrain a relatively continuous deformation history and the rheology of rocks during subduction.  The data suggest HT metamorphism and accretion of oceanic crust at the initiation of subduction was followed by rapid cooling, underplating, exhumation, and later underplating and HP/LT metamorphism that persisted for >30 m.y. at a thermal steady state.

The earliest deformation event in the metamorphic sole at ~10 kbar, 760 °C formed S₁^A in amphibolite followed by cooling through hornblende closure temperature by 167 Ma. Strain was variable, with high strain in amphibolite interlayered with quartzite and quartz-mica schist and weaker S₁^A fabric in homogeneous blocks of amphibolite. Metasomatism due to contact with hot hangingwall rocks may have occurred before, during, and after S₁^A, as locally preserved blackwall assemblages occur at the contact of relatively undeformed amphibolite and ultramafic rocks, but metasomatic assemblages also overprint hornblende-dominated fabrics. Recrystallization during isoclinal folding of amphibolite formed a second fabric (S₂^A) at 590°C, >165 Ma.  S₂^A is mylonitic where amphibolite blocks are in contact with quartzite, quartz-mica schist, and tremolite schist; foliation in the schists is discordant to and wraps blocks.  The S₂^A event overlaps with the earliest metamorphism and strong deformation (S₁^N) of high-grade Na-amphibole schist at ~530°C, 10 kbar, which cooled below 400°C by 165 Ma. We interpret the Na-amphibole schist to have been underplated as a lower metamorphic sole during this event. Retrograde metamorphism, cooling, and partial uplift to ~350°C, 7 kbar by 157 Ma is evidenced by a crenulation cleavage in the Na-amphibole schist (S₂^N) during brittle deformation in the amphibolite and metasomatic schist evidenced by glaucophane-filled fractures in hornblende. 

Younger accretion and exhumation events occurred as HP/LT conditions persisted, including underplating of regional phyllite at ~7 kbar, ~320°C from ~154-142 Ma and metavolcanic greenschist-blueschist at ~7 kbar, 360°C at ~140 Ma.  Exhumation to ~5 kbar, ≤300˚C occurred between ~140-125 Ma during later deformation of greenschist-blueschist and underplating of structurally lower metagraywacke and greenstone.  Low-T fabrics are characterized by early pressure solution cleavage followed by tight to isoclinal folding and local shearing with weak to strong recrystallization in the second cleavage.  Strain partitioning at this stage was high, with non-coaxial strain focused in phyllite and flattening fabric dominant in metagraywacke.  No deformation is evident in the high grade rocks at this time, showing the locus of strain had stepped to lower structural levels.  Meso-scale and microstructures throughout the deformation history are consistent with initial high-T deformation and limited rheological differences between lithologies, rapidly followed by weakening of metasomatized rocks and lower-T ductile and ductile-brittle deformation where strong strength contrasts favored strain partitioning into weaker units.

How to cite: Schermer, E., Cordova, J., and Mulcahy, S.: Thermal and structural history of underplated rocks from subduction initiation to thermal steady state: Easton Metamorphic Suite and related units, WA, USA , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10164, https://doi.org/10.5194/egusphere-egu2020-10164, 2020.

The metamorphic history of exhumed high-grade rocks provides invaluable insight into the thermomechanical processes of subduction zones. While subduction in most orogens has been terminated by continent collision entailing variably strong overprint of related units, the Franciscan Complex of California allows studying a >150 Myr long subduction history that started at ~175 Ma and ended by transformation into a transform plate boundary (San Andreas fault) without significant metamorphic overprint. The highest grade metamorphic rocks of the Franciscan Complex of California are found as blocks in serpentinite and shale matrix mélanges. They include amphibolites, eclogites, blueschists, and blueschist facies metasediments. These Franciscan mélanges inspired the subduction channel return-flow model, but other processes e.g., buoyancy-driven serpentinite diapirism have been argued to be concordant with our current understanding of their metamorphic history, too.

We investigate a suite of metabasite blocks from serpentinite and shale matrix mélanges of the Califonia Coast Ranges. Our new dataset consists of U-Pb dates of metamorphic zircon and ⁴⁰Ar/³⁹Ar dates of calcic amphibole and white mica. Combined with published geochronology, particularly prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. We find: (i) Exhumation from the eclogite-amphibolite facies occurred only in a short episode at 165­–160 Ma with an apparent southward younging trend. (ii) Exhumation of the blocks was uniform and fast in the eclogite-amphibolite facies with rates of 2–8 km/Myr. In the blueschist facies exhumation of the blocks was less uniform and slowed by an order of magnitude. (iii) The age of amphibole in a metasomatic reaction zone indicates that at least one amphibolite was enclosed in a serpentinite matrix by ~155 Ma. Considering the entire subduction zone system, the high-grade exhumation temporally correlates with a significant pulse of magmatism in the respective magmatic arc (Sierra Nevada) and termination of forearc spreading (Coast Range Ophiolite).

Our findings do not support a steady-state process that is continuously exhuming high-grade rocks. Instead the subduction zone system changed with an eventlike character resulting in exhumation of high-grade rocks enclosed in serpentinite.

How to cite: Rutte, D., Garber, J., Kylander-Clark, A., and Renne, P.: Eventlike exhumation of high-grade blocks in the young Franciscan subduction zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4218, https://doi.org/10.5194/egusphere-egu2020-4218, 2020.

The residence time of rocks within subduction channels provides a narrative on the physical processes that reflect the interplay between subduction rate and angle, coupling between the lower and upper plate and hydration of the mantle wedge.  In oceanic subduction systems, it is now recognised that rocks can reside within subduction channels for 10’s of millions of years.   These apparently long-lived durations of entrainment in the subduction channel probably require circulatory motions that recover material from terminal subduction and simple one-cycle exhumation.  In turn, these residence times can plausibly be used to deduce geodynamic variables that control the subduction system.

Establishing the duration a rock has been stored within a subduction environment typically requires application of multi-mineral geochronology coupled with considerations of closure systematics.  However because subduction environments are commonly fluid-rich, a mineral with great potential to reveal durations rocks can reside within subduction channels is zircon.  In subduction environments, several studies have documented apparently long-lived records of zircon growth, but seemingly have not recognised the potential for zircon to extract information on the duration a rock experienced subduction channel metamorphism.

Lawsonite-bearing eclogite in eastern Australia has a remarkable microstructural record of zircon growth.  Thin section-scale 1-3 micron resolution synchrotron mapping by X-ray Fluorescence (XFM) reveals the presence of 1000’s of micron-sized zircons which occasionally range up to 15 microns in size.  Zircon: (1) defines inclusion trails in garnets, (2) is a foliation defining matrix mineral and (3) occurs in retrograde chlorite-bearing veins that formed during post-eclogite blueschist paragenesis.   In-situ U-Pb geochronology shows that zircon growth occurred over the interval c. 520-400 Ma.  The zircons have hydrothermal characteristics with elevated LREE and simple tetragonal morphologies.  The apparently long duration of zircon growth is generally consistent with other geochronology from the eclogite: garnet Sm-Nd and Lu-Hf ages between 530-490 Ma, matrix foliation titanite U-Pb c. 450 Ma, and matrix foliation phengite Ar-Ar and Rb-Sr ages of 460-450 Ma.   

The small size of the zircons means they cannot be readily extracted using bulk rock methods.  Instead, fast, high-resolution imaging methods such as synchrotron XFM mapping coupled with spatially precise U-Pb-trace element analysis reveal a long history of HFSE element mobility resulting in microstructurally organised zircon growth that allows rock residence time in a subduction channel to be determined.

If lawsonite eclogite from eastern Australia records more than 100 Ma of zircon growth at eclogite-blueschist facies conditions, the single eclogite sample reflects around 5000-7000 km of consumption of the palaeo-pacific plate under the east Gondwana margin while remaining trapped in the subduction channel.

How to cite: Hand, M., Tamblyn, R., Zivak, D., and Raimondo, T.: Long-lived zircon growth in trapped eclogite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12636, https://doi.org/10.5194/egusphere-egu2020-12636, 2020.

Stratigraphic and structural context of the early evolution of the Cascadia convergent margin, following major subduction reconfiguration associated with accretion of the igneous Siletzia terrane at 50−45 Ma, remains insufficiently understood. Here, we have applied a novel approach that uses combined Nd, Sr and stable isotope analyses of ancient methane-seep carbonates to constrain the early hydrogeological regime of the Cascadia margin. Analyses included the oldest-known seep deposits of Cascadia, formed during mid-Eocene time (42.5−40.5 Ma). A combination of exceptionally high ε_Nd and low ⁸⁷Sr/⁸⁶Sr signatures observed in these carbonates consistently point to former interactions between the seeping fluids and mafic, igneous constituents of the forearc basement. Moderately negative δ¹³C_carbonate values imply thermogenic origin of hydrocarbons at three out of four studied seeps, with likely contribution of biogenic methane at a single, landward-most site. When combined with structural constraints, the recorded signals point to discharges of fluids originating from deep portions of the young subduction wedge, and their channeled ascent through the Siletzia terrane. The results document the presence of a fluid expulsion system indicative of active convergence prior to maturation of typical arc magmatism in the Cascades at 40 Ma. The exceptionally pronounced role of exotic, ¹⁴³Nd-enriched, ⁸⁷Sr- and ¹⁸O-depleted fluids recorded for early Cascadia reflects its distinctive structural architecture, including the relatively thin sedimentary cover of the young forearc, its extensional tectonics, and the near-trench position of the volcanic terrane that the descending plate-derived fluids must have migrated through prior to reaching the seafloor.

How to cite: Jakubowicz, M., Kiel, S., Goedert, J., Dopieralska, J., and Belka, Z.: Nd, Sr and stable isotope signatures of ancient methane-seep carbonates (Eocene, Washington, USA) as a record of incipient subduction at the Cascadia convergent margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2412, https://doi.org/10.5194/egusphere-egu2020-2412, 2020.

The Khondalite Belt is an east-west-trending Paleoproterozoic continental-continental collisional belt, separating the Western Block of the North China Craton into the Yinshan Block and the Ordos Block from north to south. In the past years, extensive metamorphic and geochronological investigations for pelitic granulites have been carried out in the Khondalite Belt. However, felsic granulites attract just a little attention although they are widely exposed in the field and potentially preserve key high-pressure information, thus hindering better understanding of the tectonic processes and settings of this critical area. In this study, a link between ‘inter-layered’ felsic and pelitic granulites from the Qianlishan Complex of the Khondalite Belt was established based on comprehensive metamorphic analysis. Three distinct metamorphic stages including peak pressure (M₁), post-peak decompression (M₂) and late retrograde cooling (M₃) stages have been identified in the felsic and pelitic granulites. Felsic granulites experienced high-pressure metamorphism up to ~12 kbar, while estimated peak pressure for pelitic granulites is 11-15 kbar. The decompression stage (M₂) is represented by cordierite + sillimanite symplectite and/or cordierite coronae with conditions of 5.7-6.5 kbar/800-835 °C in pelitic granulites, and by garnet-sillimanite assemblages formed at conditions of >6.5 kbar/810-865 °C in felsic granulites. The later cooling stage (M₃) is indicated by sub-solidus biotite-quartz-plagioclase symplectite and later melt crystallization. Clockwise P-T paths involving near-isothermal decompression and near-isobaric cooling were defined by these mineral assemblages and approximate P-T conditions, which suggest a continent-continent collisional event. SIMS zircon U-Pb dating yields a consistent metamorphic age of ~1.95 Ga from felsic granulites, interpreted as the timing of peak metamorphism. The results, combined with previously reported data, suggest that the Khondalite Belt formed by collision between the Yinshan and Ordos blocks at ~1.95 Ga.

How to cite: Wu, S., Yin, C., Davis, D. W., Zhang, J., Qian, J., Qiao, H., Xia, Y., and Liu, J.: Metamorphic P-T-t paths of high-pressure felsic and pelitic granulites from the Qianlishan Complex and tectonic implications for the Khondalite Belt in the North China Craton , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22312, https://doi.org/10.5194/egusphere-egu2020-22312, 2020.

NE China recorded the key tectonic evolution history of the Eurasian Plate from the Paleozoic-Mesozoic
collisional formation of the Central Asian Orogenic Belt to the Mesozoic subduction of the Paleo-Pacific Ocean.
To better understand this tectonic transition, it is crucial to constrain the time and pattern of the initial subduc-
tion of the Paleo-Pacific Ocean. Recently, someresearchers proposed that theMudanjiang Ocean existed between
the Songnen and Jiamusi blockswas part of the Paleo-Pacific Ocean. Here, through geochemical and geochrono-
logical studies on the widespread granitoids in the Lesser Xing'an-Zhangguangcai Range in the eastern Songnen
Block, we verify that these magmatic rocks show volcanic arc affinity with increased mantle contribution from
east to thewest of the range, likely related to a flattening subduction of theMudanjiang Ocean. In addition, a uni-
versal westward younging trend for over 70 Myr can be observed for the granitoids throughout the Lesser
Xing'an-ZhangguangcaiRange, indicating a long-lastingsubductionof theMudanjiangOcean.More interestingly,
the oldest ages of the granitoids in the east display a northward younging trend from275Ma to 218Ma, suggest-
ing that the subduction of the Mudanjiang Ocean had been initiated at latest by 275 Ma in the south and then
progressively expanded to the north. Based on these observations, we proposed a new tectonic evolution
model for theMudanjiang Ocean, i.e., a Triassic-Jurassicwestward scissor-like subduction and closure, to contrib-
ute to the understanding of the early subduction of the Paleo-PacificOcean

How to cite: Ge, M., Zhang, J., Li, L., and Liu, K.: A Triassic-Jurassic westward scissor-like subduction history of the Mudanjiang Ocean and amalgamation of the Jiamusi Block in NE China: Constraints from whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes of the Zhangguangcai Range granitoids, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-606, https://doi.org/10.5194/egusphere-egu2020-606, 2020.

  Paleoproterozoic is a pivotal time for understanding the geochronological framework of the Tarim Craton. Located on the southeastern margin of the Tarim Craton, the northern Altyn Tagh is the main exposed region for Paleoproterozoic magmatic-metamorphic rocks. These rocks are diverse, diachronous and modified by multiple magmatic and/or metamorphic events. In this study, we performed systematic analyses on the amphibolite, felsic gneisses, and metasedimentary rocks in the Aketashitage area, southeastern Tarim Craton, including petrography, mineral chemistry, and whole-rock geochemistry, as well as in-situ zircon U-Pb ages and Hf isotopes, to examine the Paleoproterozoic magmatic-metamorphic events in the northern Altyn Tagh. Geochemically, the amphibolite and felsic gneisses in the Aketashitage area seemingly represent the typical bimodal associations of mafic and acidic volcanic rocks. In addition, the felsic gneisses are characterized by high Sr and low Y contents, with high Sr/Y and La_N/Yb_N ratios, and indistinctive Eu anomalies, closely resembling high-SiO₂ adakites derived from subducted basaltic slab-melt. The palimpsest textures and geochemical features of the Aketashitage metasedimentary rocks suggest that their protoliths are argillaceous rocks. The amphibolite has a metamorphic age of 1.96 Ga, and the felsic gneisses yield crystallization ages of 2.54-2.52 Ga. For the metasedimentary rocks, the major age peaks of 2.72 Ga, 2.05 Ga and 1.97 Ga are consistent with the magmatic and/or metamorphic events in the study area. The minimum age peak suggests that the depositional age is no earlier than 1.97 Ga. The geochemical and geochronological evidences documented by the exposed rock associations in the Aketashitage area suggest a subduction-related tectonic setting in the Paleoproterozoic. Our new data combined with the previous studies indicate that the Paleoproterozoic magmatism and metamorphism in the northern Altyn Tagh area are nearly synchronous, and both are likely related to oceanic subduction.

How to cite: Wang, Z., Han, C., Xiao, W., and Sakyi, P. A.: Polyphase tectonothermal events recorded in metamorphic basement rocks from the northern Altyn Tagh, southeastern Tarim Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3195, https://doi.org/10.5194/egusphere-egu2020-3195, 2020.

Despite significant progress in our understanding of the thermal history of ultra-high pressure (UHP) metamorphosed oceanic eclogite, the mechanisms of detachment and exhumation of these rocks in the subduction channel are still debatable. Opinions vary from their exhumation as detached blocks due to circulation in a weak and loose serpentinite mélange to coherent bodies in large-scale imbricated slices. In this study, we integrate published metamorphic P-T path and peak P-T data with new metamorphic reconstruction of oceanic eclogites from two locations in the Nagaland Ophiolite Complex (NOC), NE India to establish its UHP signature and complicated  multistage exhumation history. Previous studies reveal the NOC to be the largest exposed remnant of an array of HP/LT metamorphic rocks within the eastern Neo-Tethys with the subduction burial-exhumation cycle of eclogites being bracketed between ca. 205 and 172 Ma. In both the locations near Thewati and Mokie villages, the eclogites occur as ~5 to ~50 m long and ~2-5 m wide tectonic lenses within a lawsonite blueschist facies metamorphosed package of oceanic basalt-limestone-radiolarian chert (peak P-T at ~11.5 kbar, ~340^oC).  The Thewati eclogite records a clockwise (CW) P-T path of evolution with an epidote blueschist facies prograde burial at ~18.8 kbar, 555°C, peak epidote eclogite  facies metamorphism at ~25–28 kbar, ~650°C and a two stage exhumation: an early one along a steep dP/dT gradient in amphibole-eclogite facies at ~18.3 kbar, 630°C and a later one along a gentler dP/dT gradient through epidote blueschist facies to the transitional lawsonite blueschist and greenschist facies metamorphic conditions at ~6 kbar, 300°C. In the Mokie locality, thin discontinuous stringers of highly magnesian (Mg# = 73) and eclogite facies altered basaltic crust (peak P-T at ~23.8 kbar and ~555°C) separate the eclogitic core (Mg# = 44) from the blueschist host. The Mokie eclogite core records an epidote blueschist facies prograde burial at ~12.5 kbar, ~510°C, peak UHP epidote eclogite facies metamorphism at ~32.0 kbar, ~700°C, an initial, eclogite facies exhumation at ~17.3 kbar, 560^oC that retraces the prograde burial path, but at a higher temperature, a subsequent phase of eclogite facies prograde heating and the final exhumation and cooling at metamorphic conditions transitional between lawsonite blueschist and prehnite-pumpellyite facies. We interpret the P-T history of the Nagaland blueschists and eclogites in terms of a Jurassic-aged ultra-cool (thermobaric ratio at metamorphic peak between ~220^oC/GPa and ~300^oC/GPa) intra-oceanic subduction system within the Neo-Tethys, subduction burial of the Mokie eclogite core to ~100 kms of depth, putting it in the select category of rare global UHP oceanic eclogite facies metamorphism during the cold mature stage of subduction and a change in its exhumation style from an initial buoyancy-driven material transport in a rheologically weak and fluidised subduction channel, often involving prograde heating of partially exhumed rocks to later thrust stacking and tectonic mixing of the eclogites from different crustal levels with the cooler, prograde blueschists at shallower crustal levels (P~5-6 kbar). This stage two exhumation led to the assembly of the Nagaland Accretionary Complex.

How to cite: Bhowmik, S. K. and Rajkakati, M.: Multistage Exhumation History of Ultra-cool Oceanic (U)HP eclogites: New evidence from the Nagaland Ophiolite Complex (NOC), NE India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9583, https://doi.org/10.5194/egusphere-egu2020-9583, 2020.

The Taiwan orogen formed as a consequence of the oblique subduction of the Eurasian continental margin below the Luzon volcanic arc of the Philippine Sea Plate since the late Miocene. The Yuli Belt of the eastern Taiwan Central Range, exposed in the retro-wedge of the fold-and-thrust belt, hosts slivers of a heterogeneous unit of blueschist-facies rocks that are among the youngest blueschist units worldwide. However, the palaeogeographic provenance of this unit is still debated. This is due to the fact that numerous structural aspects, including the kinematics of the Yuli Belt’s tectonic contacts with adjacent units, are improperly understood.

Our studies form part of an ongoing reinvestigation of the tectonic evolution of the Yuli Belt. A revised geological map of the Yuli Belt was generated, incorporating own structural data from several river transects. Fieldwork and microstructural analyses suggest that the Yuli Belt was polyphasely deformed. Based on newly constructed cross sections we suspect that the blueschist-facies units were tectonically emplaced along thrusts on top of a mostly greenschist-facies metasedimentary unit that locally exhibits characteristics of a mélange. Later, both blueschist-facies and metasedimentary units were tightly folded, likely during the emplacement of the Yuli Belt onto the westerly adjacent Eurasia-derived Tailuko Belt along the so-called Shoufeng Fault. Lithological and fabric transitions across this fault are gradual, suggesting that the juxtaposition of Yuli and Tailuko Belts occurred during an early W-directed transport direction before becoming refolded during later E-vergent backfolding. Peak metamorphic temperatures in the greenschist-facies metasediments, estimated by Raman spectroscopic analyses of carbonaceous material (RSCM), reveal systematic spatial variations across the Yuli belt, supporting the idea of an allochthonous nature of the blueschist units on top of the lower grade metasedimentary unit.

The incorporation of published geochronological and whole-rock geochemical data and their combination with own paleogeographic reconstructions led us to fundamentally reinterpret the structural position of the Yuli Belt. We suggest that the blueschist-facies unit most likely represents a mid-Miocene fragment of oceanic crust and mantle issued in the South China Sea before having been subducted, exhumed and ‘sandwiched’ between the (Eurasia-derived) Tailuko Belt and the easterly adjacent Coastal Ranges derived from the Philippine Sea plate. The Yuli Belt should hence be considered to contain the suture between the Eurasian and the Philippine Sea plates.

How to cite: Zhang, Y., Tsai, C.-H., Froitzheim, N., and Ustaszewski, K.: Yuli Belt in the eastern Taiwan orogen: a part of suture zone separating Eurasian and Philippine Sea plates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-540, https://doi.org/10.5194/egusphere-egu2020-540, 2020.

Based on field investigations, microscopic observations, and available geophysical, geochemical and geochronological data, this study intends to better understand the structural characteristics of the Eurasian continental margin (e.g., the eastern Central Range in Taiwan) during subduction and exhumation while the Philippine Sea plate has been approaching in the vicinity of Taiwan since the Miocene time.  The eastern Central Range is composed of two major geological units: 1) the Tailuko belt, the Mesozoic metamorphic subduction complex, retro-metamorphosed in green schist facies and exhumed since late Miocene, and 2) the Yuli belt, continental margin rocks that contain high-pressure minerals (omphacite, glaucophane, garnet) with Miocene-Pliocene ages suggesting rapid exhumation from mantle depths of 40-50 km.

We conducted detailed field surveys around the Shoufeng fault which represents the boundary between the Tailuko belt and the Yuli belt. We found a mylonite zone of several kilometers wide in the boundary of these two belts. Based on the meso- and microscopic scale observations we define the boundary as ultra-mylonite, mylonite, and proto-mylonite zones. Within the ultra-mylonite and mylonite zones, rocks from two belts are intercalated each other in varied widths. The main dominant schistosity/cleavage in the mylonite zones (Sm/S3) remains the same orientation of striking in NE/NNE and dipping to the west. Also, the main composition layers, which we tentatively called S2 for the sake of field investigations, were more intensively deformed (i.e., crenulated, folded, etc.) from outside toward the core of the mylonite zones. As a result, the Sm/S3 becomes less persistent outside of the mylonite zones in the Yuli belt.

The mylonite zones exhibit left-lateral Sm/S3-related shearing without significant down-dip component. We also observed a general S2/S3-related top-to-west sense of shear across the two belts. As a consequence, we tend to interpret that the Yuli belt and the Tailuko belt have been mylonitically sheared (Sm/S3) in a left-lateral movement at the depth and that they exhumed coevally up to the surface level. The schistosity of the main composition layers S2 probably occurred before the mylonization during the transition from subduction to exhumation. The shallow dipping and less dominant S3 outside the mylonite zone might imply an upward unroofing process during the rapid exhumation of the eastern Central Range of Taiwan.

How to cite: Lee, J.-C., Ho, G.-R., Lee, Y.-H., Yeh, E.-C., Byrne, T., and Chu, H.-T.: Left-lateral shearing before coeval exhumation between the high pressure Yuli belt and the underlying Mesozoic Tailuko belt? Insights from the Shoufeng fault in eastern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3989, https://doi.org/10.5194/egusphere-egu2020-3989, 2020.

The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is subducting near the Taitao peninsula. Around CTJ, presence of high heat flow on the continental slope and near-trench young granitic rocks on the Taitao peninsula suggests the thermal and petrological impact of subducting ridge on the continental side. The tectonic history of the southeast Pacific since early Cenozoic to the present suggests that ridge subduction continuously occurred along the Chile trench, which migrated northward.

In January 2019, the MR18-06 cruise Leg 2 was conducted at CTJ, as a part of 'EPIC' expedition by using R.V Mirai of JAMSTEC. During the leg, we completed 4 SCS lines, 6 piston coring with heat flow measurements, 2 dredges, and underway geophysics observations, as well as deployment of 13 OBSs. Coring/heatflow sites were located across the ridge axis, HP5 on the seaward plateau of axial graben, HP1/HP2/HP6 on the axis, and HP3/HP7 on the forearc slope near the trench axis. The primary object of heat flow measurement at CTJ is to better constrain the thermal regime around CTJ by adding new data right above CTJ. The key question is whether CTJ is thermally dominated by ridge activity (magmatic, tectonic, and/or hydrothermal) or by subduction initiation (tectonic thickening, accretion, and/or erosion). The ultimate goal is to model the temperature at the plate interface from the heat flow and other data, and to infer how the thermal regime at CTJ contributes the seismogenic behavior at the M~9 megathrust zone.

Onboard and post-cruise measurements include; bulk density, porosity, Vp, resistivity, CT imags, iTracks element scan, age dating, etc. Core saples seaward of ridge axis (HP5) has few turbidites with higher density (~2 g/cc) and low sedimentation rate (SR; 0.2 m/ky), whereas cores on the axis the density are turbidite dominant with lower (1.6~1.8 g/cc) and very high SR (1~3 m/ky). The accretionary prism (landward of trench) cores have the density of 1.6~1.7 g/cc and SR=0.5~1 m/ky. They suggest that the turbidite covers only the axial graben.

Heat flow in the axial graben range 140-210 mW/m^2, which is lower than on the seaward plateau (370 mW/m^2). This apparent controversy may be due to lower magmatic activity and/or high sedimentation rate on the axis. The lower spreading rate (2.6 cm/yr one side) and the rapid convergent rate at the trench (7.2 cm/yr) may suppress sufficient magma supply or hydrothermal circulation. Heat flow on the accretionary prism (230 mW/m^2) is higher than borehole or BSR-derived heat flow (~<100 mW/m^2). It is suggestive of fluid upwelling along the decollement as proposed in the previous study. Some numerical thermal models will be presented to show the effect of ridge subduction.

How to cite: Kinoshita, M., Anma, R., Yokoyama, Y., Ohta, K., Yokoyama, Y., Nishikawa, T., Abe, N., Iwamori, H., and Villar, L.: Thermal regime around the Chile Triple Junction based on JAMSTEC MR18-06 cruise 'EPIC', EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6414, https://doi.org/10.5194/egusphere-egu2020-6414, 2020.

The Banda Arc system is sited in a junction of convergence between the Eurasian, Indo-Australian, Philippine and Pacific plates. It has a remarkable 180° curve in the Benioff zone. Two fundamental ideas have been invoked to explain this significant subduction-arc orientation change: (1) bent subduction zone around the Banda Sea (Hamilton, 1979; Spakman and Hall, 2010; Hall, 2012), or (2) oppositely dipping subduction zones (Cardwell and Isacks, 1978; McCaffrey, 1989), but no general agreement exists as to the cause of this curvature. However, a WNW-trending strike-slip fault, i.e. Seram-Kumawa fault, is observed at the north-eastern end of the Arc, cutting through the Seram accretionary wedge, prism and trench and seems to continue on the subducting plate (Hall et al., 2017). This fault is either inactive or locked temporarily at the present day, because there are very few strike-slip events along its trend while there are many thrust earthquakes on its north and northwest side. A few essential questions remain unanswered about this fault in relation to the evolution of the Banda Arc. For instance, what is the origin of this fault, what role does it play in the tectonic processes and large earthquakes along the Banda Arc. Could this fault eventually break-up the Banda Arc? What is its tectonic implication on the evolution of other highly curved subduction-arc systems? To address these questions, we will carry out a comprehensive investigation into active tectonics and seismicity occurrence along the northeast Banda Arc using high-resolution bathymetry, 2D marine seismic profiles and earthquake data.

Reference:

Cardwell, R.K. and Isacks, B.L., 1978. Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions. Journal of Geophysical Research: Solid Earth, 83(B6): 2825-2838.

Hall, R., 2012. Late Jurassic–Cenozoic reconstructions of the Indonesian region and the Indian Ocean. Tectonophysics, 570: 1-41.

Hall, R., Patria, A., Adhitama, R., Pownall, J.M. and White, L.T., 2017. Seram, the Seram Trough, the Aru Trough, the Tanimbar Trough and the Weber Deep: A new look at major structures in the eastern Banda Arc.

Hamilton, W.B., 1979. Tectonics of the Indonesian region. US Govt. Print. Off.

McCaffrey, R., 1989. Seismological constraints and speculations on Banda Arc tectonics. Netherlands Journal of Sea Research, 24(2-3): 141-152.

Spakman, W. and Hall, R., 2010. Surface deformation and slab–mantle interaction during Banda arc subduction rollback. Nature Geoscience, 3(8): 562.

How to cite: Yang, X., Singh, S. C., and Deighton, I.: The role of the remarkable Seram-Kumawa strike-slip fault in the tectonic process of the northern Banda Arc system , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7300, https://doi.org/10.5194/egusphere-egu2020-7300, 2020.

The nature and processes occurring at the subduction plate interface remain poorly constrained. In particular, the behavior of fluids and its impact on the rheology and the chemistry of the plate interface are mostly unknown. Based on detailed fieldwork, petrographic, geochemical analyses and thermodynamic modelling, the present study documents an example of a reacted “mélange” and metasomatism along the subduction interface: the Lia mélange zone on Syros island. Syros island is located in the Cycladic Archipelago in the centre of the Aegean domain which corresponds to the deepest exhumed parts of the Hellenides–Taurides belt. We show that this particular mélange zone is a disrupted yet still relatively coherent fragment of transitional lithosphere (i.e., OCT type from the Pindos Ocean), which has undergone dominant exhumation-related deformation with top to the east shearing. A large part of the “mélange” structure is inherited from the initial lithostratigraphic setting. Through detailed mapping and a statistical study of the nature of blocks and matrix we show that, as a first approximation, metasomatism occurs in contact between metavolcanite layers and serpentinite, with diffusion of Ca from the metavolcanites to the matrix and diffusion of Mg from matrix to metavolcanite. Most of the metavolcanite layers and blocks (mafic and carbonate) are mostly only partly digested but the ultramafic matrix has been largely metasomatised forming a tremolite-chlorite-talc schist, a “hybrid” rock, with an intermediate chemical composition. Geochemical data suggest that exhumation-related metasomatism is probably triggered and/or enhanced by the arrival of fluids from the dehydrating slab underneath. The Lia mélange zone shows that hybrid rocks can be formed by metasomatism along the subduction interface. Due to the absence of major tectonic mixing and of evidence of prograde reactions, this metasomatism may not be representative of deeper hybridization (as a potential source of arc volcanism). However, by changing the mineralogy of the matrix, the metasomatism changes the rheological properties of the mélange and thus could impact that of the subduction interface and the exhumation processes. This study highlights the significance of rock hybridization through metasomatism, largely in the context of a syn-convergent exhumation, along the slab interface and emphasizes its potential chemical and rheological impacts.

How to cite: Gyomlai, T., Agard, P., and Jolivet, L.: Syros, blocks and matrix structure: evolution of a mélange along the subduction interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7338, https://doi.org/10.5194/egusphere-egu2020-7338, 2020.

Important amounts of fluids are released in subduction zones by successive dehydration reactions occurring both in the previously hydrated oceanic crust (and mantle) and overlying sedimentary cover. The release and circulation of such fluids in rocks have major consequences on both their mechanical and chemical behavior. Indeed, the presence of a free fluid phase strongly modifies the rock rheology, fracturing properties, and could be implicated in both intermediate-depth earthquake and slow slip events nucleation. Moreover, the scale of mass transfer, associated chemical changes in infiltrated rocks and element recycling in subduction zones are controlled by both the rock permeability and the amount and composition of such fluids. Thus, there is a crucial need to identify the major fluid sources, amounts and pathways to better constrain their impact on subduction dynamics.

Metamorphic veins, as well as mineralized fractures and shear zones in exhumed fossil subduction zones are the best witnesses of fluid-rock interactions and fluid circulation pathways. However, their interpretation in terms of fluid sources, residence time, scale of circulation requires a good knowledge of the composition of potential fluid sources. In order to determine the composition of the fluid released by both oceanic crust and sediments at various depth along their subduction, we analyzed the composition of fluid inclusions contained in vein minerals formed at peak P-T conditions, in rock units buried at various depths in the Alpine subduction zone.

The Schistes Lustrés complex is a slice-stack representing the deep, underplated part of the former Alpine accretionary wedge. These Alpine Tethys rocks are mainly composed of oceanic calcschists with fewer mafic and ultramafic rocks, buried to various depths before exhumation. From West to East, the juxtaposed Schistes Lustrés units show increasing peak P-T conditions from blueschist (300-350°C - 1.2-1.3 GPa) to eclogite facies (580°C - 2.8 GPa). This study focuses on the Schistes Lustrés - Monviso transect, which shows an almost continuous increase in metamorphic grade.

In the Schistes Lustrés blueschist-facies sediments, fluid inclusions were analyzed in quartz from high-pressure veins, i.e. quartz that co-crystallized with prograde to peak metamorphic minerals such as lawsonite and Fe-Mg carpholite. In the metamorphosed mafic rocks, we analyzed fluid inclusions from the peak metamorphic assemblages, i.e. glaucophane +/- omphacite in blueschist facies rocks, omphacite in eclogite-facies slices. Raman spectroscopy data on these fluid inclusions suggest that fluids released during dehydration of calcschists in blueschist-facies conditions are aqueous fluids with low-salinity and small amounts of CO₂ and CH₄. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N₂ content. These results, which will be associated with LA-ICP-MS analysis of fluid inclusions in metasedimentary quartz veins, will contribute to better constrain the evolution of composition of the fluids liberated by dehydration reactions with depth and protolith composition along the subduction interface.

How to cite: Herviou, C., Verlaguet, A., Agard, P., Raimbourg, H., Locatelli, M., and Plunder, A.: Composition of fluids released along a subduction interface with increasing depth: insights from fluid inclusions analysis on the Schistes Lustrés - Monviso transect (Western Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16975, https://doi.org/10.5194/egusphere-egu2020-16975, 2020.

Extending across Anglesey and Llŷn Peninsula in North Wales, UK, the Mona Complex is a collection of Neoproterozoic-Cambrian units formed through the collision of the Iapetus oceanic plate with the Avalonian microcontinent [1].  One of these units, the Gwna Complex, represents accreted ocean floor material that is largely characterised as a regional-scale tectonic mélange.  Detrital zircon ages in terrigenous sediments suggest that subduction occurred around 600-540 Ma [2].  Accreted sequences of volcanics, pelagic sea floor sediments and turbidites can be used to reconstruct the history, stratigraphy and origin of the ancient ocean floor, whilst the presence of these different lithologies also have major influences on structural controls of accretion.

In Newborough, Anglesey, sub-greenschist (T < 300°C) Gwna Complex material has been accreted in the form of imbricated semi-coherent lenticular slices 5 – 200 m thick with a subvertical orientation.  Large volumes of terrigenous sediment (turbidite-derived muds and fine sands) are present elsewhere in the Gwna Complex, acting as the mélange matrix, incorporating blocks of stronger, more brittle surrounding units.  In Newborough, however, the Gwna Complex has experienced comparatively little terrigenous input, localising mélange formation to metre-scale layers towards the upper unit interfaces.  This leads to the semi-coherent preservation of ocean floor stratigraphy.  Highly foliated hyaloclastite layers within thick volcanic sequences were exploited as weak horizons during accretion, allowing relatively thick, coherent volcanic sequences to be preserved.  Hyaloclastites typically make up to basal unit of lenticular slices.

Lenticular units record a stratigraphy consisting of relatively undeformed pillow basalts with intermittent hyaloclastite horizons, grading upwards into peperites and then carbonates as sea floor sedimentation becomes more prominent. Overlying layers of pelagic cherts and terrigenous turbiditic sediment are typically more dismembered and mélange formation is localised within turbiditic sediment, and rarely within clast-poor hyaloclastites.  The geochemistry of pillow basalts and associated volcanics from throughout the Gwna Complex is similar, albeit not identical, to typical modern MORB.  This suggests that the volcanics originated from a mid-ocean ridge source, with overlying sediments accumulating on the sea floor representing different stages in the life cycle of the oceanic crust leading up to subduction and accretion.  A small series of accreted sills and related amygdalar hyaloclastites that occur in Newborough show a distinct OIB signature and are likely related to a later episode of minor intraplate magmatism.

References:

[1] Horák J et al. (1996) J Geol Soc London 153: 91-99

[2] Asanuma H et al. (2017) Tectonophysics 706-707: 164-195

How to cite: Groome, N., Buchs, D., Fagereng, Å., Wood, M., Campbell, S., and Horák, J.: Accretionary processes and stratigraphic reconstruction of Neoproterozoic oceanic crust in North Wales, UK, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21122, https://doi.org/10.5194/egusphere-egu2020-21122, 2020.

The Coastal Terrane of the Kaoko Belt in Namibia was originally defined as a Neoproterozoic arc terrane that originated outboard of the attenuated Congo Craton margin. Early (~650–630 Ma) igneous activity and high-grade metamorphism were interpreted as connected with subduction of the Adamastor Ocean and related arc magmatism. Protoliths of metasedimentary lithologies were interpreted as juvenile clastic sediments originating from the arc erosion. Later deformation (~580 Ma) was associated with lower amphibolite-facies conditions during thrusting over the Congo Craton margin.

Our research, however, suggests different evolutionary scenario.  The structurally lowermost part of the metasedimentary complex contains amphibolites and orthogneisses with U–Pb zircon ages between ~820–785 Ma, interpreted as metamorphosed syn-sedimentary bimodal volcanics. Detrital zircon ages from associated metamorphosed clastic sediments show identical patterns as observed in the metasedimentary cover of the underlying Congo Craton. Towards the structural hanging wall, the metasediments are devoid of metavolcanic rocks, and their detrital zircon age spectra are comparable with those from flysch sediments in the eastern, less metamorphosed parts of the Kaoko Belt.

The structurally lowermost part of the Coastal Terrane shows signs of partial melting broadly coeval with intrusion of ~650 Ma (U–Pb zircon) granitic–dioritic/gabbroic rocks. The temperature and depth of this migmatization event remains unconstrained, because the original mineral assemblages were overprinted during thrusting over the Congo Craton margin.

The thrusting period is characterized by solid-state reworking and partial retrogression of the migmatites in the lower part, and by pervasive metamorphism in the upper part, of the metasedimentary complex. Lu–Hf age (583 ± 2 Ma) of garnet from reworked migmatite shows that the garnet-bearing mineral assemblage represents conditions of thrusting, which were determined at ~660–670°C and 5.5–6 kbar. The ~580 Ma (and beyond) period of deformation started with development of flat-lying metamorphic fabric, later overprinted by folds with step axial planes, steep cleavage and isolated shear zones with general N–S to NNW–SSE trend. The associated intrusions show steep magmatic fabric, which transits into solid-state deformation in bodies close to the base of the Coastal Terrane.

Rather than an arc, the Coastal Terrane probably represents the inner part of an early Neoproterozoic rift. This interpretation is supported by the zircon provenance data and the presence and age of the bimodal volcanic rocks. The early, ~650–630 Ma magmatic activity and migmatitization coincides with the early period of rift inversion that took place along the western edge of the rift system in the Dom Feliciano Belt (Brazil and Uruguay). At this period, the former rift centre was established as the high-grade hinterland system of the developing Kaoko–Dom Feliciano–Gariep orogen. Inversion of the eastern rift edge started at ~580 Ma, as recorded in the Coastal Terrane, and continued up to ~550 Ma, which is the timing of the metamorphic peak in the Kaoko Belt foreland.

Financial support of the Czech Science Foundation (GACR 18-24281S) is appreciated.

How to cite: Konopásek, J., Jeřábek, P., Anczkiewicz, R., and Sláma, J.: Evolution of the south-eastern hinterland in the South Atlantic Neoproterozoic Orogenic System – the Coastal Terrane of the Kaoko Belt (NW Namibia) revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13351, https://doi.org/10.5194/egusphere-egu2020-13351, 2020.

Nappes of the Scandinavian Caledonides are the repository of information on both Caledonian orogenic evolution and pre-Caledonian geologic evolution of the Baltica and Laurentia margins and the Iapetus ocean. We report geological mapping, zircon U–Pb geochronological data on 33 samples, and mica 40Ar/39Ar data on 4 samples, along five profiles in the southernmost Caledonides in the Stavanger-Ryfylke region (Stavanger, Suldal, Nedstrand, Randøy, Røldal). 
In Stavanger, the lowermost phyllite nappe –Buadalen nappe– is overlain by the Madla and Sola nappes (former Jæren Nappe). The Madla nappe comprises c. 1510–1495 Ma orthogneiss with Sveconorwegian metamorphism (c. 1025 Ma). The overlying Sola nappe comprises a sequence of mica schist, metasandstone, marble, amphibolite and felsic metavolcanic rocks. The metavolcanic rocks – Snøda metadacite-rhyolite – are fine-grained, frequently porphyritic, mica gneisses, with calc-alkaline, peraluminous, composition and negative Nb-Ta anomaly. Their extrusion ages of c. 941 and 934 Ma date deposition of the whole sequence. Detrital zircons in a metasandstone sample (n=138) yield main age modes at c. 1050 and 1150 Ma, significant Proterozoic and Archaean modes, and a maximum deposition age of c.990 Ma. The Sola nappe was affected by Taconian metamorphism peaking in eclogite-facies conditions at c.470 Ma (Smit et al., 2010), followed by regional cooling between c.446 Ma (white-mica) and 438 Ma (biotite). Trondhjemite dykes intruded at c.429 Ma, cutting the pre-Scandian fabric. 
At regional scale, the lower nappes correlate over long distances. The lowest phyllite nappes –Buadalen, Holmasjø, Lower Finse and Synnfjell– represent the Cambro-Ordovician sediment cover of the Baltic margin, containing thin tectonic slivers of the underlying c. 1521 to 1225 Ma orthogneiss. The overlying nappes –Madla, Storheia, Dyrskard, Hallingskarvet, Espedalen– consist of felsic metavolcanic or metaplutonic rocks with a consistent age between c. 1525 and 1493 Ma with c. 1040 Ma intrusive, corresponding to the Telemarkian crystalline basement in S Norway. The Kvitenut nappe hosts metaplutonic rocks ranging from c. 1625 to 1039 Ma and metasedimentary rocks. It requires additional characterization. The overlying far-travelled nappes do not correlate well. The metasedimentary Revseggi nappe in Røldal is affected by a Taconian metamorphism (470–450 Ma) and hosts c. 434–428 Ma felsic intrusives (Roffeis & Corfu, 2014). Detrital zircons (n=33) in a kyanite-mica-gneiss sample constrain deposition of the sequence after c. 890 Ma. The Revseggi nappe may correlate with the Sola nappe. In Nedstrand, a c. 932 Ma augen gneiss is overlain by amphibolite and mica schist, tentatively attributed to the Boknafjord nappe. Detrital zircon data (n=11) imply an Ordovician (<459 Ma) deposition, therefore refuting a correlation of this transect with the Sola nappe.
The Sola nappe exposes a far-travelled Tonian marine volcanic-sedimentary sequence. The Taconian metamorphism suggests an evolution in the Iapetus ocenic realm. The Sola sequence may represent the microcontinent onto which the Karmøy ophiolite complex (c. 493–470 Ma) was obducted. By analogy to several other Tonian sequences preserved in far-travelled allochthons in the Scandinavian and Greenland Caledonides, the Sola sequence may originate from the active Neoproterozoic Renlandian margin of Laurentia and Rodinia before opening of Iapetus.

How to cite: Bingen, B., Torgersen, E., Ganerød, M., and Roberts, N. M. W.: Southernmost nappes in the Scandinavian Caledonides: correlations, evidence for a Tonian marine volcanic-sedimentary terrane and paleogeographic implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18891, https://doi.org/10.5194/egusphere-egu2020-18891, 2020.

Slabs in subduction zones with geotherms of 7 K km^-1 or higher are expected to dehydrate effectively in the forearc. Nevertheless, large amounts of water are released from these slabs at and beyond subarc depth, indicating that H₂O remains slab-bound to much greater depth than expected. It is possible that this reflects a transient sealing effect exerted by the subducting lower crust—a section of the lithosphere that typically undergoes delayed recation and is effectively impermeable until then. To test this concept, we investigated gabbros that were partially transformed to hydrous eclogite along shear zones during subduction. The rocks were subjected to a textural, petrological and Li-chronometric analysis. The observations characterize the progressive stages of transformation, and provide detailed insight into the governing feedbacks among fluid flow, deformation, and reaction. Lithium chronometry indicates that it took only a few weeks for the shear zone network to develop and for the externally derived fluids to traverse this network and drive eclogitization; the switch in these rocks—going from strong to weak and from impermeable to sustaining long-range fluid flow—thus was essentially instanteneous on subduction time scales. The re-equilibration of the rocks occurred well beyond equilibrium at c. 90 km depth, which is where large fluid-filled channel system typically emanate from warm slabs. Our data suggest that the fluids that are produced in the slab mantle throughout the forearc accumulate beneath the Moho until the lower crust is breached by dynamic fluid vents and commences its delayed transformation. The subducting lower crust may thus be a exert a strong control on H₂O and element budgets, and the rheology of slabs in warm subduction zones.

How to cite: Smit, M. and Pogge von Strandmann, P.: Deep fluid release beneath arcs from delayed breaching of the slab lower crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6323, https://doi.org/10.5194/egusphere-egu2020-6323, 2020.

Subduction initiation is commonly identified as a major enigma in plate tectonics. Attention to subduction initiation is growing in the community, as is our understanding of the sequences of geologic events that precede and postdate this critical stage of the Wilson cycle. Nevertheless, the direct triggers of subduction initiation and their regional to global consequences remain uncertain. The New Caledonia ophiolite has formed in a supra-subduction zone setting in the vicinity of an active spreading centre. The metamorphic sole, which represents the ancient subduction interface, is locally preserved beneath the ophiolite. Unravelling its tectono-metamorphic record is essential in order to determine the timing of subduction initiation and the tectonic processes operating at the plate interface during the early stages of subduction.  We have sampled and studied amphibole-bearing rocks of the metamorphic sole that crop out in three newly found and three previously known localities that are scattered across the island (160 km * 50 km in size). The amphibolites form laterally discontinuous meter-size lenses that crop out within or beneath the serpentinite sole at the base of the ophiolite nappe.  Preliminary U-Pb zircon ID-TIMS geochronology yields a crystallization age of 56±1 Ma in agreement, but with a narrower timespan compared to previously published data (Cluzel et al., 2012).  We use whole-rock geochemistry, mineral chemistry and thermodynamic modelling to constrain the Pressure-Temperature-time history of the amphibolites.  Microstructural data such as dominant deformation mechanisms, crystallographic preferred orientations, grain size distributions determined by EBSD allow to constrain the deformation processes and rheological behavior of the amphibolites during subduction infancy.

Cluzel, D., Jourdan, F., Meffre, S., Maurizot, P., and Lesimple, S., 2012. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge. Tectonics, VOL. 31, TC3016, doi:10.1029/2011TC003085.

How to cite: Cenki-Tok, B., Gürer, D., Chatzaras, V., Collot, J., Corfu, F., and Maurizot, P.: Intra-oceanic subduction initiation recorded by the metamorphic sole of the New Caledonia ophiolite: petrological, structural and age constraints, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11835, https://doi.org/10.5194/egusphere-egu2020-11835, 2020.

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large- scale ophiolites (usually ~20 km thick). They typically show a steep inverted metamorphic structure where the pressure and temperature (T) conditions of crystallization increase upward, from the base of the sole (500 ± 100°C at 0.5 ± 0.2 GPa) to the contact with the overlying peridotite (800 ± 100°C at 1.0 ± 0.2 GPa). The inverted  T gradient was historically interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab synchronously with the overlying ophiolite formation (within only 1-2 Myrs). Their mineralogical assemblage and deformation pattern provide major constraints on the nature and the timing of the processes controlling the dynamics of the plate interface during early subduction.

Soret et al. (2017, 2019) recently reappraised the tectonic–petrological model for the formation of metamorphic soles below ophiolites, showing that the present-day structure of the sole results from the successive stacking of several homogeneous oceanic crustal slivers (without internal T gradient). This stacking marks the evolution of rheological properties of slab material and peridotites of the upper plate as the plate interface progressively cools (Agard et al., 2016). These findings outline the thermal and mechanical complexity of early subduction dynamics, and highlight the need for refined numerical modelling studies.

Lu-Hf geochronology on garnet from the Oman metamorphic sole has recently shown that the earliest accreted subunit, found directly against the upper plate mantle, was initially buried ≥ 8 Ma earlier than previously estimated (Guilmette et al., 2017). These results imply initiation ≥ 8 Ma before the formation of the ophiolite, which underscores the common belief that ophiolite-sole couples record spontaneous subduction initiation and rather indicates far-field forcing long before upper plate extension and mantle upwelling.

We herein present new U-Pb titanite and monazite petrochronology across the different sub-units of the Oman metamorphic sole. Our results confirm the time lag of several million years between subduction initiation and the ophiolite formation, therefore supporting the recently proposed model of far-field forced subduction initiation. They also reveal a significant time lag between the underplating and exhumation of each sub-unit of the sole.

How to cite: Soret, M., Bonnet, G., Agard, P., Larson, K., Cottle, J., Dubacq, B., and Button, M.: Slow subduction initiation forces fast ophiolite formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20130, https://doi.org/10.5194/egusphere-egu2020-20130, 2020.

The offshore north Oman margin, located north of the Hajar Mountains in the Gulf of Oman,
remains a key area for understanding the evolution of the obduction Emails Ophiolite. With the
help of a grid of 2D-multichannel seismic lines linked to well data, we present a new view of
the obduction and post-obduction history of the Oman margin. Offshore deposits, overlying on
what we interpret as being the offshore extension of the ophiolites, can be divided into two
mega-sequences. The older one is comprised of late Cretaceous to Paleogene deposits mainly
located in the Sohar basin and offshore of the Abat trough. In the Sohar basin, the latest stages
of obduction are recorded by the deposition of the erosional products of the Autochthonous
Arabian sediments and the ophiolite, in a flexural basin induced by a volcanic high. Offshore
of the Abat trough, a Maastrichtian-Paleocene basin develops above a detachment fault
system linked to the extension phase associated to the exhumation/expulsion of the subducted
continental margin. Both sectors are divided by a structured high located offshore of the Semail
Gap transfer fault. We propose that this transfer fault, likely a major Pan-African structure,
impacted both the architecture of the passive margin following the rifting of the Neotethys and
later ophiolite emplacement, during (continental) subduction and obduction.

How to cite: Ninkabou, D., Agard, P., Nielsen, C., Smit, J., Haq, B., Rodriguez, M., and Gorini, C.: The syn- and post-obduction history of the offshore north Oman margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22476, https://doi.org/10.5194/egusphere-egu2020-22476, 2020.

Terrane accretion and tectonothermal activity associated with the Penokean and Yavapai Orogenies are recorded in various geologic elements of the Lake Superior region, USA, including: (1) mafic–ultramafic terranes comprising tholeiitic basalts and gabbros, boninites and calc-alkaline volcanics and intrusives (e.g., the Pembine–Wausau Terrane), and (2) multiple and distinct, short-length-scale (5–15 km) chlorite–biotite–garnet–staurolite–(kyanite–)sillimanite regional metamorphic isograd sequences. These geologic associations reflect development of a suprasubduction zone system (subduction initiation?) within a Paleoproterozoic ocean in the Orosirian Period, followed by episodes of short-duration (limited-length-scale) tectonometamorphism during accretionary orogenesis in the Statherian Period.

The geologic processes recorded in the Paleoproterozoic terranes of the Lake Superior region are very common in the Phanerozoic. We suggest that Paleoproterozoic tectonism in the Lake Superior region may reflect a West Pacific-type setting, involving distinct, short-lived tectonothermal events marking periods of subduction rollback and lithospheric extension, punctuated by episodes of arc/microcontinent collision, terrane accretion and lithospheric shortening.

The apparent operation of modern-like plate tectonics—accretionary tectonics involving rapid switching between lithospheric extension and shortening—in the Paleoproterozoic requires that a scenario of temporally-varying buoyancy forces at the subduction zone (spatially-varying density of the subducting slab?) be reconciled with the thicker (slower-densifying) oceanic lithosphere expected for a hotter Earth. Such a scenario may be explained by: (1) an anomalously cool mantle (producing anomalously thin oceanic crust) beneath the ocean basin whose closure led to the accretionary orogenesis recorded in the Lake Superior region, or (2) an incredibly long-lived (>> 100 Myr) ocean basin that allowed widespread development of critically-overdense lithosphere prior to subduction initiation and onset of accretionary orogenesis associated with the Penokean and Yavapai Orogenies.

We are currently investigating geologic associations in the Lake Superior region and their potential tectonic origins, using whole-rock geochemistry to test for the tectonic origins of the Pembine–Wausau Terrane, and ⁴⁰Ar/³⁹Ar geochronology/geospeedometry to constrain time scales for the tectonometamorphism that produced the metamorphic isograd sequence in the region of Republic, Michigan. Results will provide new insights into accretionary tectonics during the Paleoproterozoic, and processes controlling the emergence and evolution of plate tectonics on Earth.

How to cite: Viete, D. R. and Holder, R. M.: Accretionary orogenesis in the Lake Superior region, USA: modern-like tectonics during the Paleoproterozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21104, https://doi.org/10.5194/egusphere-egu2020-21104, 2020.

During the amalgamation, tenure and break up of Pangea several oceans played a major tectonic role. Remnants of them now occur mostly along the margins of the Atlantic, Mediterranean, Black and Caspian seas, as well as in the Alpine-Himalayan and adjacent orogens. Of those oceans, three (Iapetus, Tornsquist and Rheic) were closed during the amalgamation of Pangea and another (Neo-Tethys) is the main witness of its break-up.

The Paleotethys is the enigmatic ocean that shared an internal position during most of Pangea’s tenure. There is no consensus about its origin, some suggest that opened during the latest stages of Pangea’s amalgamation (Devonian-Carboniferous) whereas others considert it a remnant of the mostly subducted Rheic ocean after Gondwana-Laurussia collision. The Shanderman eclogites, in NW Iran are a potential candidate to represent the Paleotethys ocean. They are metamorphosed oceanic rocks (protolith oceanic tholeiitic basalt with MORB composition). Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation.

In this contribution I will show the new petrological, geochemical and geochronological results from this eclogites to shed light on the Paleotethyan problem. The piece of oceanic crust preserved at Shanderman area (Iran) crystallized some time in the mid-Carboniferous (~330 Ma) showing the paleotethys kept expanding during the Gondwana-Laurussia collisions that amalgamated Pangea. Metamorphic ages, suggest that subdution initiated in this segment of the Paleotethys between 310 and 290Ma. We integrate this results into a tectonic reconstruction that shows a major plate reorganization within Pangea during the late Carboniferous and early Permian (320-270 Ma) that questions its role as a supercontinent.

How to cite: Pastor-Galán, D., Tsujimori, T., López-Carmona, A., and Yi, K.: Shanderman eclogite (Iran): A supercontinent killer subduction., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13417, https://doi.org/10.5194/egusphere-egu2020-13417, 2020.

High-pressure complexes along the Earth's surface provide evidence of the processes involved in both the crystallization of rocks in the subduction channel and its exhumation. Such processes are key to understand the dynamics and evolution of subduction zones and to try to reconstruct P-T trajectories for these complexes.

Previous studies on the Raspas complex (southern Ecuador) agree to state that it is composed of metamorphic rocks, mainly blueschists and eclogites, containing the mineral assemblage: glaucophane + garnet + epidote + omphacite + white mica + rutile ± quartz ± apatite ± pyrite ± calcite; which stabilized in metamorphic conditions of high pressure and low temperature. Additionally, the Raspas Complex has been genetically related to accretion and subduction processes of seamounts, which occurred in South America during the Late Jurassic - Early Cretaceous interval; and the exhumation of the complex was related to subduction channels. However, the evidence presented in the existing literature makes little emphasis on the reconstruction of thermobarometric models for the rocks of this complex.

By combining petrographic observations, whole-rock chemistry, and mineral chemistry in this work; it was possible to determine that pressure values of 10 ± 3 Kbar and temperature values of 630 ± 30 ° C, (obtained by simulations with THERMOCALC®) correspond to an event of retrograde metamorphism, suffered by the complex during its exhumation. This theory is complemented by the specific textures (that suggest this retrograde process) observed during petrographic analysis, such as amphibole replacing pyroxene, garnet chloritization, plagioclase crystallization and rutile replacement by titanite.

The results obtained, together with the thermobarometry data published for the Arquía complex in Colombia, allow us to establish a P-T trajectory, that may suggest a genetic relationship between these two complexes as a result of the tectonic processes associated with an active subduction margin that affected the NW margin of the South American plate at the end of the Jurassic.

How to cite: Arrieta-Prieto, M., Zuluaga-Castrillón, C., Castellanos-Alarcón, O., and Ríos-Reyes, C.: Metamorphic evolution of Raspas complex (Ecuador) and its relation with a J-K belt of melanges in NW of the South American plate., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12395, https://doi.org/10.5194/egusphere-egu2020-12395, 2020.

The Andes Mountains are formed at a destructive plate margin, where dense oceanic crust descends beneath relatively buoyant continental crust. In this geological setting, we typically would not expect to see such a high and wide mountain belt forming. Numerical modelling shows that if slabs roll back, continents are stretched, causing tension and potentially back-arc extension. The formation of the Andes has been hypothesized to be due to anchoring of the slab in the lower mantle, subduction of buoyant features in the Nazca plate, or compression driven by large-scale convection cells underneath South America.

Previous research suggests a clear correlation between slab age and overriding plate crustal thickness, globally, but in particular for South America. In this project, we hypothesize that this age variation plays a significant role in the formation of the Andes. As subducting slabs descend into the mantle, their properties differ in conjunction with their age affecting their buoyancy and strength, thereby generating different dynamics, surface tectonics, and slab morphologies. Using numerical modelling code ASPECT, we examined the role of slab properties and related dynamics on the state of stress in the overriding plate.

We quantify how much compression occurs in the overriding plate to use as a proxy for topographic growth. Typically, older slabs anchor more readily, causing more rollback and therefore extension. Our models however, predict that a stronger pull force acting on the overriding plate from older slabs causes stronger coupling than their younger counterparts, due to these buoyancy controls and increased density, resulting in greater compression. But, in doing so mantle convection contributes to corner flow in the static mantle wedge, increasing compression further. An increase in overriding plate thickness from 50 to 100km increases the amount of compression in the overriding plate by 10 Mpa , while an increase in slab age from 40 to 80 Myrs generates a similar increase in compression. Finally, slab morphology effects the geometry and vigour of convection cells beneath the overriding plate, which also affects the compressional state of the plate. This is in qualitative agreement with previous work.

How to cite: Withers, C., van Hunen, J., and Allen, M.: Modelling slab age and crustal thickness: numerical approaches to drivers of compression in the overriding plate in Andean style subduction zone systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18333, https://doi.org/10.5194/egusphere-egu2020-18333, 2020.

The Northandean plate margin underwent a fundamental change in its structural configuration during a Cretaceous subduction cycle, as evidenced by the formation and accretion of a province of basic igneous arc rocks that gave rise to the basement of an Northandean Western Cordillera. Further north, this igneous terrane links to the Caribbean Large Igneous Province and has been associated, with respect to its origin, to an actively spreading ridge of the Farallon plate, implying a far-travelled origin with respect to Southamerica and calling for the existence of giant strike-slip faults. We challenge this allochthonous scenario by an alternative option of a forearc origin, invoking the possibility of a forearc opening by the forcing of a toroidal mantle flow at the northern end of the Andean trench, which would have introduced mantle material from the Pacific into the Andean realm through a Central American gap. Support for such an opening mode of a forearc basin comes from extensional tectonics, that accompanied the emplacement of the basic arc units and a concomitant subduction of the extrusive basic units at the inner border of this postulated forearc basin. This intraplate subduction comprises a distinct three-partite evolution: (I) Convergence first became manifest by the reactivation of a normal fault located within the supposed forearc basin and inboard of an inherited Triassic-Jurassic suture, but still failed at a crustal level. (II) A succeeding contractional stage involved the reactivation of the inherited Triassic-Jurassic suture and the tectonic erosion of a frontal compartment of the continental margin. After an incipient underplating, slivers of this continental compartment returned within a time span of about 20 Ma. (III) A final Late Cretaceous subduction stage evolved under the conditions of an oblique SW-NE oriented plate convergence and is characterized by extensional pulses, as may be concluded from the structural setting of the giant Antioquia batholith. In the Campanian subduction definitely locked, as evidenced by the regional buckling of the forearc realm and a rebound of the upper continental plate. Both onset and shutoff of this subduction cycle may be linked to deformation phases and are dated by syntectonic, fault-guided intrusions. This scenario of a forearc origin of the basic igneous province calls for the existence of two paired subduction zones: on its outer margin the subducting Farallon slab imposed a trench-parallel mantle flow and constrained an expansion of the forarc basin by slab rollback. On its inner margin, a secondary subduction compensated a surplus expansion of the actively forming forearc basin.

How to cite: Kammer, A. and Avila, M.: Structural framework and regional significance of the Northandean Cretaceous subduction cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18206, https://doi.org/10.5194/egusphere-egu2020-18206, 2020.