GD5.2

The subduction channel is located directly above a downgoing oceanic plate and forms by dehydration of this plate. The ascending water-rich fluids react with the mantle to hydrous minerals such as chlorite and amphibole. This process rheologically weakens the mantle and reduces its density so that an upwards-directed mass flow is continuously generated as long as the oceanic plate is subducted. However at great depth, the fluids ascending from the subducting plate do not produce hydrous minerals anymore due to too high pressure-temperature (P-T) conditions. Thus, the question arises how high can these conditions become in order to still generate such hydrous minerals in the mantle. To answer this question, thermodynamic modelling was undertaken with PERPLE_X using different data sets of Holland and Powell (1998, 2011), corresponding solid-solution models for relevant minerals, and the bulk-rock composition of a common lherzolite + 2.5 wt% H₂O. In addition, results of experiments at high pressure on the P-T stability of hydrous minerals such as chlorite were considered.

Under the assumption of a relatively steeply and fast dipping oceanic plate, the geothermal gradient at the interface between this plate and the overlying mantle wedge should be below 7.5 °C/km (100 km = 3.2 GPa). At such low gradients, that are common in modern subduction zones, chlorite is the only (nominally) hydrous mineral in the lherzolite considered because amphibole shows an upper pressure limit, for example 2.3 GPa using model cAmph(G), in the calculation results. Calculations with the data set of Holland and Powell (1998) lead to results at pressures >3 GPa, which are, due to the used equation-of-state for minerals, incompatible with experimental results, whereas the results produced with the more recent data set (Holland and Powell, 2011) are compatible. Along gradients of 7.5, 5, and 3.5 °C/km, chlorite decomposes to form garnet in lherzolite at about 740 (3.15 GPa), 660 (4.3 GPa), and 570 °C (5.3 GPa), respectively. These temperatures are 60-80 °C lower than calculated for the reaction of chlorite + enstatite = forsterite + pyrope + H₂O in the system MgO-Al₂O₃-SiO₂-H₂O.

The aforementioned P-T conditions limit the subduction channel towards great depths, which should be less than 160 km (5.2 GPa) even at very low thermal gradients, and are compatible with peak P-T conditions of many eclogites exhumed in the subduction channel from the surface of the downgoing oceanic plate. A few exceptions were reported which suggest exhumation of eclogite from depths > 200 km (e.g., Ye et al., 2000). The reason for these greater depths could be another exhumation mechanism. However, a misinterpretation of so-called exsolution lamellae in eclogitic minerals, taken as evidence for unusual mineral compositions and, thus, depths > 200 km, is more likely (see Liu and Massonne, 2022).

Holland, T.J.B., Powell, R., 1998. J. Metamorph. Geol. 16, 309-343.

Holland, T.J.B., Powell, R., 2011. J. Metamorph. Geol. 29, 333–383.

Liu, P., Massonne, H.-J., 2022. J. Metamorph. Geol., doi: 10.1111/jmg.12649

Ye, K., et al., 2000. Nature 407, 734–736.

How to cite: Massonne, H.-J.: The maximum depth of the subduction channel in modern subduction zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1293, https://doi.org/10.5194/egusphere-egu22-1293, 2022.

The ‘channel flow’ concept is generally associated with the collisional mountain belts (such as the Himalaya) to explain the exhumation of deeper crustal materials. According to the concept, the top part of the subducting plate gets ‘molten’ and tries to return to the surface following the ‘pipe flow’ mechanism via a combination of Poiseuille- and Couette Flows. In this study, we employed these concepts to address a long-standing debate related to the existence and cryptic nature (normal/ reverse) of an orogen parallel discontinuity, named the Jhala Normal Fault (JNF) present in the Bhagirathi River section of the Garhwal Higher Himalaya. More importantly, while a group of researchers consider the JNF to be the northern boundary of the Higher Himalayan channel (i.e., the South Tibetan Detachment System), another group put the JNF well inside the channel. In this scenario, understanding the mechanism of deformation at the JNF will not only solve this local issue but will also provide us with new insights into the geodynamic evolution of an orogeny. Based on fresh field observations and SHRIMP geochronological data (zircon and monazite), a model is being proposed in the current study to explain the origin and evolution of the JNF. The presence of amphibolite-grade rocks across the JNF, along with the lack of well-developed extensional markers, confirm that the fault is located within the Higher Himalayan channel, and not at the channel boundary. The U-Pb zircon rim ages of 33.8 ± 0.8 Ma and 30.7 ± 0.5 Ma obtained from the JNF hanging wall (northern block) and footwall (southern block), respectively, are considered as the ages of peak metamorphism. The hanging wall, which was present at the slow-moving marginal part of the channel during Eocene, eventually lagged behind the relatively faster and warmer central part. As a result, the footwall (southern) block experienced a faster exhumation, resulting in normal-sense movement along the JNF, as documented by sparse extension markers. At 21.4 ± 2.3 Ma (monazite U-Pb age), tourmaline-bearing leucogranite intruded in the JNF hanging wall, rupturing the host. This indicates the passive uplift of the JNF hanging wall (in a brittle domain) as a part of the Higher Himalaya. Hence the JNF originated as an intra-channel discontinuity, and our proposed model predicts the origin of a ‘normal fault’ during crustal channel flow.

How to cite: Bose, N., Imayama, T., Kawabata, R., Gupta, S., and Yi, K.: Channel-flow induced ‘normal faulting’ in the Himalaya: a case study from the Jhala Normal Fault, Garhwal Higher Himalaya, NW India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13213, https://doi.org/10.5194/egusphere-egu22-13213, 2022.

The NE striking Longmen Shan (LMS) mountains are located at the eastern margin of the Tibetan plateau, and towers nearly 5000m above the Sichuan basin, which is considered to be the greatest relief than anywhere else around the plateau. From west to east, three major sub-parallel faults straddle the Longmen Shan: Wenchuan-Maoxian fault (WMF), Yingxiu-Beichuan fault and Guanxian-Anxian fault. Several models have been proposed to explain the Cenozoic uplift of the Longmen Shan. The major two models are lower crustal channel flow and upper crustal shortening, which imply different movement sense on the Wenchuan-Maoxian fault. The former suggests that the LMS were uplifted above a lower crustal flow expulsed from below the Tibetan plateau and would require a normal sense movement on the MWF. The latter implies that a series of upper crustal thrusts controlled the uplift of the LMS, and the WMF should have a reverse sense. Here we present field observations, fault gouge structural analysis and authigenic illite K-Ar geochronology data of fault gouge in the Wenchuan-Maoxian fault, showing that the Maoxian-Wenchuan fault was dextral with a reverse component at ~7Ma. Reconstruction of offsets of river valleys along the Wenchuan-Maoxian fault suggests that the corresponding total horizontal dextral offset is ~25km. Analysis of the thermochronology data acquired on both side of the fault suggest that dextral-reverse faulting started at ~13 Ma and possibly lasted until today. Our conclusions support the upper crustal shortening model and suggest the channel model maybe not applicable to Longmen Shan uplifting in the Miocene.

How to cite: Ge, C., Leloup, P. H., Zheng, Y., and Li, H.: No channel flow in the Longmen Shan: evidence from the Maoxian-Wenchuan fault Cenozoic kinematics (SE Tibet), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13458, https://doi.org/10.5194/egusphere-egu22-13458, 2022.

The role of subducted fluids on the generation of deep earthquakes (300 – 700 km) has been a topic of much research and debate for decades. While fluids are commonly believed to play a role in the genesis of intermediate depth earthquakes (70 – 300 km), it is often argued that fluids (i.e., water- or carbonate-bearing) cannot be transported to sufficient depth to play a role in the triggering or propagation of deep earthquakes. However, recent investigations show evidence of up to ~1.5 wt% water in a ringwoodite inclusion in a diamond from the mantle transition zone [1]. Additionally, heavy iron (δ⁵⁶Fe = 0.79–0.90‰) and unradiogenic osmium (187Os/188Os = 0.111) isotopic compositions of metallic inclusions in sublithospheric diamonds trace the pathway of serpentinized slabs from the trench to the top of the lower mantle [2]. Given this evidence for slab derived fluids at transition zone depths, we investigate the ability of fluids to reach these depths in subducted slabs by compiling a) new subduction zone thermal models, b) slab earthquake locations within these modeled subduction zones, and c) phase relations of hydrated or carbonated mantle peridotite and basaltic crust. Our results show a distinctive pattern that is consistent with the necessity of fluids in the generation of deep seismicity [3]. Specifically, those slabs capable of transporting water to the bottom of the transition zone (via dense hydrous magnesium silicates (DHMS)) produce earthquakes at transition zone depths. Conversely, virtually all slabs that do not transport water to these depths do not generate deep earthquakes. We also note that the depths of deep earthquakes coincide with the P/T conditions at which oceanic crust is predicted to intersect the carbonate-bearing basalt solidus to produce carbonatitic melts. We suggest that hydrous and/or carbonated fluids released from subducted slabs at these depths lead to fluid-triggered seismicity, fluid migration, diamond precipitation, and inclusion crystallization. Deep focus earthquake hypocenters would then track the general region of deep fluid release and migration in the mantle transition zone [3].

[1] Pearson, D. G., Brenker, F. E., Nestola, F., Mcneill, J., Nasdala, L., Hutchison, M. T., et al. (2014). Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 507, 221–224. https://doi.org/10.1038/nature13080 [2] Smith EM, Ni P, Shirey SB, Richardson SH, Wang W, and Shahar, A (2021) Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor. Science Advances 7: eabe9773 https://doi.org/10.1126/sciadv.abe9773 [3] Shirey SB,  Wagner LS, Walter MJ, Pearson DG, and van Keken PE (2021) Slab Transport of Fluids to Deep Focus Earthquake Depths – Thermal Modeling Constraints and Evidence From Diamonds. AGU Advances: 2, e2020AV000304.    https://doi.org/10.1029/2020AV000304

How to cite: Wagner, L., Shirey, S., Walter, M., Pearson, D. G., and van Keken, P.: The role of subducted fluids on the genesis of deep earthquakes: evidence from deep diamonds and subduction zone thermal modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8968, https://doi.org/10.5194/egusphere-egu22-8968, 2022.

Volatiles play a pivotal role in subduction zones dynamics, associated geological hazards and mineralization, yet their pathways remain partially understood. The Lesser Antilles subduction zone can yield insights to volatile recycling as a global end-member, where old oceanic lithosphere formed by slow spreading slowly subducts. Here we use seismograms from local earthquakes recorded by a temporary deployment of ocean-bottom seismometers in the fore- and back-arc during the VoiLA (Volatile Recycling in the Lesser Antilles) experiment to characterize the 3-D properties of the slab, back-arc and mantle wedge in the north-central Lesser Antilles subduction zone. Along the top of the slab, defined by the underlying Wadati-Benioff seismicity, we find low P-wave velocity extending to 130–150 km depth, deeper than expected for magmatic oceanic crust. The deep low velocities together with high Vp/Vs at 60–80 km and 120–150 km depth are consistent with a significantly tectonised and serpentinised slab top, as expected for lithosphere formed by slow spreading. The most prominent high Vp/Vs anomalies in the slab correlates with two projected fracture zones and the obliquely subducting boundary between Proto-Caribbean and Equatorial Atlantic lithosphere, indicating these structures enhance hydration of the oceanic lithosphere and subsequent dehydration when subducted. Deep dehydration of slab mantle serpentinite is evidenced by high Vp/Vs anomalies in the back-arc offshore Guadeloupe and Dominica. Right above the slab, the asthenospheric mantle wedge is imaged beneath the back-arc as high Vp/Vs and moderate Vp feature, indicative for fluids rising from the slab through the overlaying cold boundary layer. The fluids might be dragged down with the subducting slab before rising upwards to induce melting further to the west. The variation in seismic properties along the subducting slab and in the back-arc mantle wedge shows that the changes in hydration of the incoming plate govern the dehydration processes at depth. The highest Vp/Vs anomaly in the back-arc west of Dominica at depth greater than 120 km, together with the anomaly at 60–80 km depth on the slab east of the island, appear to track the source and path of excess volatiles that may explain the relatively high magmatic output observed on the north-central islands of the Lesser Antilles arc.

How to cite: Bie, L., Hicks, S., Rietbrock, A., Goes, S., Collier, J., Rychert, C., Harmon, N., and Maunder, B. and the VoiLA Consortium: Fluid migration, deep dehydration, and melt generation in the Lesser Antilles subduction zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4399, https://doi.org/10.5194/egusphere-egu22-4399, 2022.

Intermediate-depth earthquakes in many subduction zones occur in two distinct layers, forming an upper and a lower seismic zone separated vertically by an aseismic or weakly seismic region. These Double Seismic Zones (DSZs) have been related to dehydration reactions in the downgoing crust and mantle lithosphere. Notably, intermediate-depth seismicity in Northern Chile shows a pattern of intraslab seismicity which is quite different from a conventional DSZ. Here, two parallel seismicity planes are present in the updip part of the slab, but at a depth of ∼80–90 km, there is a sharp transition to a highly seismogenic volume of 25–30 km thickness, which corresponds to a closing of the gap between the two seismicity planes.

While such an observation is unique to Northern Chile, understanding the processes behind the formation of this feature should provide important constraints on the mineral processes that govern seismicity in DSZs as well as the role and involvement of fluids. As seismic velocities contain important information about mineralogy and fluid content, we aim at a high-resolution characterization of the seismic wavespeeds of the Northern Chile subduction zone, mainly focusing on the downgoing Nazca slab. We use the seismicity catalog of Sippl et al. (2018) that contains >100,000 earthquakes and 1,200,404 P- and 688,904 S-phase picks for the years 2007 to 2014 to perform local earthquake tomography using the FMTOMO algorithm (Rawlinson et. al., 2006). Data from the seismic stations of the permanent IPOC (Integrated Plate boundary Observatory Chile) deployment in the Northern Chile forearc form the backbone of the dataset, but are complemented by several temporary deployments that span shorter time sequences.

We will present first 3D models of P- and S-wavespeeds from the Northern Chile forearc between about 19°S and 23°S, using a subset of the earthquake catalog mentioned above, as well as images of ray coverage, relocated seismicity and synthetic resolution tests.

The presented seismic velocity distribution will eventually be compared with theoretical wavespeeds that are forward calculated assuming different mineralogical compositions in order to narrow the range of possible reactions that may be occurring at depth.

How to cite: Hassan, N. and Sippl, C.: Towards imaging dehydration reactions in the downgoing Nazca plate with local earthquake tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4461, https://doi.org/10.5194/egusphere-egu22-4461, 2022.

Subduction zones provide a key link between the surficial biogenic, atmospheric and hydrospheric geochemical cycles with the Earth’s internal reservoirs. Sediment compaction and dehydration of variably altered oceanic lithosphere during subduction release volatile species (containing e.g., S, H, C, N) to the overlying mantle wedge. In particular, sulfur plays a key role in the formation of porphyry ore deposits and has a major control on redox processes in subduction zones, given it occurs in variable oxidation states from oxidized sulfate (S⁶⁺) to reduced sulfide (S^2-). Here we studied samples from a contact between serpentinite and partly metasomatized eclogitic metagabbros in the Voltri Massif (Italy). We determined the bulk rock and in situ sulfur isotope composition of pyrite grains and combined this with detailed mineralogic and petrologic investigations. Along the serpentinite-metagabbro contact, the metagabbros are metasomatized to actinolite-chlorite schists and metagabbros rich in epidote and Na- and Na-Ca amphiboles. The serpentinites as well as the actinolite-chlorite schists along the serpentinite-metagabbro contact have very low sulfide contents and provide evidence for the oxidation of sulfides, including formation of Fe-oxides. Sulfur input from the serpentinite-metagabbro contact towards the less metasomatized eclogitic metagabbros is observed. This sulfur input is reflected by bulk rock δ³⁴S values that increase from initially around +1.5‰ in samples distant from the contact to +7.3 to +12.5‰ in samples near the contact. This trend correlates with a general increase in the in situ δ³⁴S values from core to rim of individual pyrite grains. Distinct Co and Ni growth zones in pyrite and variations in the in situ δ³⁴S values indicate multiple phases of pyrite growth during subduction and exhumation of these rocks, with the last stage of pyrite growth clearly related to Mg-metasomatism along the serpentinite-metagabbro contact. Thus, this study provides new insight into processes of sulfur migration during metasomatism of gabbroic rocks within the subducting slab and at the slab–mantle interface.

How to cite: Schwarzenbach, E., Streicher, L., Dragovic, B., Scicchitano, M. R., Wiechert, U., Codillo, E., Klein, F., Marschall, H., and Scambelluri, M.: Sulfur transfer along a metasomatized serpentinite-metagabbro contact in the Voltri Massif, Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5283, https://doi.org/10.5194/egusphere-egu22-5283, 2022.

Although many different mechanisms for subduction initiation have been proposed, few of them are viable in terms of agreement with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra-oceanic subduction triggered by an upwelling mantle plume could contribute greatly to the onset and functioning of plate tectonics in the early Earth and, to a lesser extent, in the modern Earth. In contrast, the onset of intracontinental subduction is still underestimated. Here we review 1) observations demonstrating the upwelling of hot mantle material flanked by sinking proto-slabs of the continental mantle lithosphere, and 2) previously published and new numerical models of plume-induced subduction initiation. Numerical modelling shows that under the condition of a sufficiently thick (> 100 km) continental plate, incipient down thrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies with moderate temperatures and without significant strain and/or melt-induced weakening of the overlying rocks. This finding is in contrast to the requirements for plume-induced subduction initiation in oceanic or thin continental lithosphere. Consequently, plume-lithosphere interactions in the continental interior of Paleozoic-Proterozoic (Archean) platforms are the least demanding (and therefore potentially very common) mechanism for triggering subduction-like foundering in Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in a variety of intracontinental settings. A better understanding of the role of intracontinental mantle downthrusting and foundering in global plate tectonics and, in particular, in triggering "classic" oceanic-continental subduction will benefit from further detailed follow-up studies.

How to cite: Cloetingh, S., Koptev, A., Kovacs, I., Gerya, T., Beniest, A., Willingshofer, E., Ehlers, T., Andric-Tomasevic, N., Botsyun, S., Eizenhofer, P., Francois, T., and Beekman, F.: Plume-induced sinking of the intracontinental lithosphereas a fundamentally new mechanism of subduction initiation., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4976, https://doi.org/10.5194/egusphere-egu22-4976, 2022.

Conversion of a passive margin, which is the transition between oceanic and continental lithosphere formed by sedimentation above an ancient rift, into an active converging plate boundary is still ambiguous. According to the Wilson Cycle (Wilson, 1966), which describes the repeated opening and closing of the oceans, the collapse of a passive margin is a key factor in the closing phase of the Wilson Cycle. However, the lack of any Cenozoic examples of conversion of passive margins into subduction zones and the existence of old oceanic plates along Atlantic passive margins indicate the difficulty of subduction initiation at passive margins. Due to lack of observational evidence, modeling studies play a key role in understanding the kinematics and dynamics of transforming a passive into active margin. During the last decades, they proposed several facilitating mechanisms to collapse a passive margin such as sediment loading (Cloetingh et al., 1982), water weakening (Regenauer-Lieb et al., 2001), STEP faults (Subduction-Transform-Edge-Propagator; Govers and Wortel, 2005) near passive margins (Baes et al., 2011), mantle suction forces derived from detached slabs and/or neighboring subduction zones (Baes and Sobolev, 2017), convergence forces induced from neighboring plates (Zhong and Li, 2019) and propagation of subduction along passive margins (Baes and Sobolev, 2017; Zhou et al., 2020).

 In this study, we extend the work of  Baes and Sobolev (2017) by using 3D models. As breaking a 3D lithosphere is more difficult than a 2D plate, 3D numerical models may lead to different conclusions than those of 2d ones. To study the effect of mantle suction flow on the destabilisation of passive margins, we set up 3D models, using the ASPECT finite element code (Kronbichler et al., 2012). We investigate the effect of different parameters such as the magnitude, spatial size and location of suction flow, the age of oceanic lithosphere and the existence of a STEP (Subduction-Transform-Edge-Propagator; Govers and Wortel, 2005) fault near margin. Our preliminary results show over-thrusting of continental crust from the earliest stage of deformation. This continued over-thrusting along with suction force, which imposes shear stresses below the lithosphere, causes breaking of the oceanic plate and its sinking into the mantle and eventually subduction initiation at the passive margin. The time of subduction initiation, which depends on several factors such as magnitude and location of the suction force, is more than 30 Myr indicating difficulty in the converting passive margins into converging plate boundaries. We believe that subduction initiation at some Atlantic passive margins such as those in the north of the South Sandwich subduction zone, southwest of the Iberia and north of the Caribbean region, where considerable suction forces induced by sinking slabs or neighboring subduction zones are available, will occur in a few tens of million years.

References:

Baes et al., 2011. Geophys. J. Int.

Baes, and Sobolev, 2017. Geochem. Geophys. Geosyst.

Cloetingh et al., 1982. Nature.

Govers and Wortel, 2005, Earth Planet. Sci. Lett.

Kronbichler et al., 2012, Geophys. J. Int.

Regenauer-Lieb et al., 2001. Sci.

Wilson, 1966, Nature

Zhou et al., 2020. Sci. Adv.

Zhong and Li, 2019. Geophys. Res. Lett.

How to cite: Baes, M., Sobolev, S., Hampel, A., and Glerum, A.: 3D numerical modeling of suction-induced subduction initiation at passive margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4261, https://doi.org/10.5194/egusphere-egu22-4261, 2022.

Subduction invasion has been referred to as the process by which subduction zones from a subducting ocean invade or trigger subduction initiation in a contiguous ocean. This can, in principle, happen in different ways that can vary from a direct migration by rollback along an oceanic corridor connecting the two oceans (e.g., the Gibraltar Arc into the Atlantic) or by polarity reversal across a narrow continental land bridge, potentially involving the collision of an ocean plateau with the pre-existent trench (the Scotia and the Caribbean arcs). This process is important because new subduction zones are difficult to start in the present plate tectonics context and most known examples of initiation seem to be forced by pre-existent subduction zones. The problem is that in internal Atlantic-type oceans there are no pre-existent subduction zones, and therefore, they must be introduced from the outside. Luckily, the Atlantic seems to be just passing through a phase of invasion, as evidenced by the three referred examples. But while the Caribbean and the Scotia arcs are already two fully formed Atlantic-subduction systems, the Gibraltar Arc is currently in the process of migrating between oceanic basins. In the future, the Arc can evolve according to two different scenarios. In the first, the Gibraltar Arc is stuck between Africa and Iberia and the subduction is waning. In the other scenario, after a period of quiescence, the arc manages to go through and invade the Atlantic. In order to understand which is more feasible, we have developed 3D numerical models using the code LaMEM to gain some insights into how this system may evolve. We have simulated the development of the Mediterranean arc-back-arc system, with rollback and the retreat of the subduction zones in a fully dynamic framework (no active kinematic boundaries). Our model shows that under the studied parameters, the Gibraltar subduction zone manages to invade the Atlantic, even in the cases of a very narrow oceanic corridor. However, this led to a very significant decrease in the subduction velocity, suggesting that in the natural prototype, a period of quiescence is expected before the Mediterranean subduction zone manages to go through and invade the Atlantic.

J.C. Duarte and F.M. Rosas acknowledge financial support by FCT through the project UIDB/50019/2020 – Instituto Dom Luiz (IDL)

How to cite: Duarte, J. C., Riel, N., Kaus, B. J. P., and Rosas, F. M.: Subduction invasion of the Atlantic by Mediterranean subduction zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4774, https://doi.org/10.5194/egusphere-egu22-4774, 2022.

The structure of oceanic back-arc basins reflects the dynamics of the subduction zone they are associated with. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. These features are a direct consequence of the transient nature of subduction zones. Here, we show results from different types of numerical models that aim at understanding how back-arc basins are shaped by subduction dynamics.

We present 3D numerical models of back-arc spreading centre jumps evolving naturally in a homogeneous subduction system surrounded by continents without a trigger event (Schliffke et al., 2022). We find that jumps to a new spreading centre occur when the resistance on the boundary transform faults enabling relative motion of back-arc and neighbouring plates is larger than the resistance to break the overriding plate closer to trench. Time and distance of spreading centres jumps are, thus, controlled by the ratio between the transform fault and overriding plate strengths. We also present results from 2D numerical models of lithospheric extension with asymmetric and time-dependent boundary conditions that simulate multiple phases of extension due to episodic trench retreat (Magni et al., 2021). We show that multiphase extension can result in asymmetric margins, mantle exhumation and continental fragment formations. We find that the duration of the first extensional phase controls the final architecture of the basin. Finally, we show that our models can explain many features observed in present-day and extinct back-arc basins.

Magni, V., Naliboff, J., Prada, M., & Gaina, C. (2021). Ridge Jumps and Mantle Exhumation in Back-Arc Basins. Geosciences, 11(11), 475.

Schliffke, N., van Hunen, J., Gueydan, F., Magni, V., & Allen, M (2022). Episodic back-arc spreading centre jumps controlled by transform fault to overriding plate strength ratio. Accepted for publication in Nature Communications.

How to cite: Magni, V., Schliffke, N., van Hunen, J., Gueydan, F., Allen, M., Naliboff, J., Prada, M., and Gaina, C.: Modelling ridge jumps in back-arc basins at different scales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4810, https://doi.org/10.5194/egusphere-egu22-4810, 2022.

Slab detachment causes a reorganization of the forces acting on orogenic systems and can have a distinctive signature in the geological record that may be identified through the structural,  metamorphic and topographic evolution of the orogen. However, this signature is hidden within other signals relating to the general complexity of the mountain building processes. In addition, slab detachment (or slab tearing in 3D) is a complex process that occurs on different timescales as a function of the inherent rheological properties of the lithosphere and the weakening mechanism occurring within the slab (viscous, plastic or thermal weakening).

How these properties affect the slab detachment process and to which extent these controls are reflected in the topograhic evolution of the orogenetic system is not yet fully understood. As slab detachment may occur at different depths and rates, it has different effects on the overall pull force acting on the orogen and on its post-detachment response.

Here, we employ 2D numerical experiments to systematically explore first order controls on slab detachment (slab rheology, geometry and weakening mechanisms) and the corresponding topographic evolution. Apart from the effect of lithosphere rheology and weakening mechanisms, we put particular focus on the effects of plate coupling and breakoff depth.

How to cite: Piccolo, A. and Thielmann, M.: Controls on slab detachment and subsequent topography evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7483, https://doi.org/10.5194/egusphere-egu22-7483, 2022.

Subduction is the main driver of tectonic activity on Earth. Termination of subduction is followed by diverse and unexpected tectonic activity, such as anomalous magmatism, exhumation, subsidence and subsequent rapid uplift. What fundamentally drives these processes remain enigmatic. A prime example of subduction termination can be found in northern Borneo (Malaysia), where subduction ceased in the late Miocene and was followed by puzzling tectonic activity, as reconstructed from geological and petrological evidence. Our current understanding of the subduction cycle cannot be reconciled with evidence of post-subduction tectonics in both the near-surface geology and mantle of northern Borneo.

We use new passive-seismic data to image at unprecedent detail a sub-vertical lithospheric drip that developed as a Rayleigh-Taylor gravitational instability from the root of a volcanic arc, which formed above a subducting plate. We use thermo-mechanical simulations to reconcile these images with time-dependent dynamical processes within the crust and underlying mantle, following subduction termination. Our model predictions illustrate how significant extension from a downwelling lithospheric drip can thin the crust in an adjacent orogenic belt, causing lower crustal melting and possible exhumation of subcontinental material, which can explain core-complex formations seen in other areas of recent subduction termination.

How to cite: Pilia, S., Davies, R., Hall, R., Bacon, C., Gilligan, A., Greenfield, T., Tongkul, F., and Rawlinson, N.: Effects of subduction termination processes on continental lithosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5344, https://doi.org/10.5194/egusphere-egu22-5344, 2022.