GD5.2

EDI
Subduction dynamics, volatiles and melts: Investigations from surface to deep mantle

Subduction drives plate tectonics, generating the major proportion of subaerial volcanism, releasing >90% seismic moment magnitude, forming continents, and recycling lithosphere. Numerical and laboratory modeling studies have successfully built our understanding of many aspects of the geodynamics of subduction zones. Detailed geochemical studies, investigating compositional variation within and between volcanic arcs, provide further insights into systematic chemical processes at the slab surface and within the mantle wedge, providing constraints on thermal structures and material transport within subduction zones. However, with different technical and methodological approaches, model set-ups, inputs, and material properties, and in some cases conflicting conclusions between chemical and physical models, a consistent picture of the controlling parameters of subduction-zone processes has so far not emerged.

This session aims to follow subducting lithosphere on its journey from the surface down into the Earth's mantle and to understand the driving processes for deformation and magmatism in the over-riding plate. We aim to address topics such as: subduction initiation and dynamics; changes in mineral breakdown processes at the slab surface; the formation and migration of fluids and melts at the slab surface; primary melt generation in the wedge; subduction-related magmatism; controls on the position and width of the volcanic arc; subduction-induced seismicity; mantle wedge processes; the fate of subducted crust, sediments and volatiles; the importance of subducting seamounts, LIPs, and ridges; links between near-surface processes and slab dynamics and with regional tectonic evolution; slab delamination and break-off; the effect of subduction on mantle flow; and imaging subduction zone processes.

With this session, we aim to form an integrated picture of the subduction process, and invite contributions from a wide range of disciplines, such as geodynamics, modeling, geochemistry, petrology, volcanology, and seismology, to discuss subduction zone dynamics at all scales from the surface to the lower mantle, or in applications to natural laboratories.

Co-organized by GMPV2/SM4/TS7
Convener: Ágnes KirályECSECS | Co-conveners: Oğuz H Göğüş, Taras Gerya, Jeroen van Hunen
Presentations
| Mon, 23 May, 08:30–11:50 (CEST)
 
Room K1

Presentations: Mon, 23 May | Room K1

Chairpersons: Ágnes Király, Taras Gerya, Jeroen van Hunen
08:30–08:31
08:31–08:36
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EGU22-1293
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Presentation form not yet defined
Hans-Joachim Massonne

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.

08:36–08:38
08:38–08:43
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EGU22-13213
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ECS
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Virtual presentation
Narayan Bose, Takeshi Imayama, Ryoichi Kawabata, Saibal Gupta, and Keewook Yi

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.

08:43–08:45
08:45–08:50
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EGU22-13458
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ECS
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Virtual presentation
Chenglong Ge, Philippe Hervé Leloup, Yong Zheng, and Haibing Li

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.

08:50–08:52
08:52–09:02
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EGU22-5045
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ECS
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solicited
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On-site presentation
Artem Chanyshev, Takayuki Ishii, Dmitry Bondar, Shrikant Bhat, Eun Jeong Kim, Robert Farla, Keisuke Nishida, Zhaodong Liu, Lin Wang, Ayano Nakajima, Bingmin Yan, Hu Tang, Zhen Chen, Yuji Higo, Yoshinori Tange, and Tomoo Katsura

The 660-km seismic discontinuity (D660) is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as the dissociation of (Mg,Fe)2SiO4 ringwoodite to (Mg,Fe)SiO3 bridgmanite plus (Mg,Fe)O ferropericlase (post-spinel transition). Prominent features of D660 are significant depressions to 750 km and multiplicity beneath cold subduction zones. Previous high-pressure experiments provided negative but gentle Clapeyron slopes (−1.3 to −0.5 MPa/K) of the post-spinel transition. Thus, the post-spinel transition cannot interpret the D660 depression. Therefore, another phase transition with a steep negative slope is required, and the akimotoite−bridgmanite transition in (Mg,Fe)SiO3 is one candidate.

In the current study, we determined the boundaries of the post-spinel (RBP) and akimotoite−bridgmanite (AB) phase transitions in the MgO-SiO2 system over a temperature range of 1250–2085 K using advanced multi-anvil techniques with in situ X-ray diffraction. We judged a stable phase assemblage by observing relative increase/decrease in the ratio of coexisting high- and low-pressure assemblages at spontaneously and gradually decreasing pressure and a constant temperature from diffraction intensities. Since this strategy is strictly based on the principle of phase equilibrium, it excludes problems in determining phase stability caused by sluggish kinetics and surface energy.

We found that the RBP boundary has a slightly concave curve, whereas the AB boundary has a steep convex curve. The RBP boundary is located at pressures of 23.2–23.7 GPa in the temperature range of 1250–2040 K. Its slope varies from −0.1 MPa/K at temperatures less than 1700 K to −0.9 MPa/K at 2000 K with an averaged value of −0.5 MPa/K. The slope of the AB boundary gradually changes from −8.1 MPa/K at low temperatures up to 1300 K to −3.2 MPa/K above 1600 K. Based on these findings, we predict that, beneath cold subduction zones, ringwoodite should first dissociate into akimotoite plus periclase, and then akimotoite transforms to bridgmanite with increasing depth; these successive transitions cause the multiple D660. Moreover, the steep negative boundary of the AB transition should result in cold-slab stagnation due to significant upward buoyancy. Our predictions are supported by the seismological observations beneath cold (e.g., Tonga, Izu-Bonin) subduction zones.

How to cite: Chanyshev, A., Ishii, T., Bondar, D., Bhat, S., Kim, E. J., Farla, R., Nishida, K., Liu, Z., Wang, L., Nakajima, A., Yan, B., Tang, H., Chen, Z., Higo, Y., Tange, Y., and Katsura, T.: Depressed 660-km seismic discontinuity beneath cold subduction zones caused by akimotoite-bridgmanite phase transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5045, https://doi.org/10.5194/egusphere-egu22-5045, 2022.

09:02–09:04
09:04–09:09
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EGU22-8968
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Highlight
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On-site presentation
Lara Wagner, Steven Shirey, Michael Walter, D. Graham Pearson, and Peter van Keken

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.

09:09–09:11
09:11–09:16
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EGU22-4399
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ECS
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Virtual presentation
Lidong Bie, Stephen Hicks, Andreas Rietbrock, Saskia Goes, Jenny Collier, Catherine Rychert, Nicholas Harmon, and Benjamin Maunder and the VoiLA Consortium

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.

09:16–09:18
09:18–09:23
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EGU22-4461
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ECS
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On-site presentation
Nazia Hassan and Christian Sippl

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.

09:23–09:25
09:25–09:30
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EGU22-4233
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On-site presentation
Christian Sippl, Timm John, Stefan Schmalholz, and Armin Dielforder

Double seismic zones (DSZs), parallel planes of intermediate-depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may well be a ubiquitous feature of downgoing oceanic plates. Early focal mechanism observations from Japan and Alaska have shown downdip compressive events in the upper and downdip extensive events in the lower plane of the DSZ, which was interpreted as a signature of plate unbending at these depths. Such a pattern of compressive over extensive events has become a hallmark of DSZ seismicity, and some models of DSZ seismogenesis explicitely rely on an unbending-dominated intraslab stress field as a mechanism for deep slab hydration.

In this study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the “classical” pattern of compressive over extensive events, as in NE Japan, is only observed at about half of the DSZ locations around the globe. The occurrence of extensive mechanisms across both planes accounts for most other regions, whereas a “bending signature” of extensive over compressive events is not widely observed at all. To obtain an independent estimate of the (un)bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab unbending. Taking high-resolution focal mechanism information from the Northern Chile subduction zone as an example, we conclude that the intraslab stress field in subduction zones is primarily a superposition of (un)bending stresses and downdip extensive in-plane stresses. Depending on the sign (bending or unbending) and the relative contributions of these two principal stresses, an unbending signature as in NE Japan or a purely extensive pattern of focal mechanisms as in Northern Chile can emerge. We also consider possible additional contributing stresses that may further modify the intraslab stress field, such as friction along the plate interface and volume loss due to metamorphic phase changes.

How to cite: Sippl, C., John, T., Schmalholz, S., and Dielforder, A.: Global compilation of double seismic zones and their dependence on the intraslab stress field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4233, https://doi.org/10.5194/egusphere-egu22-4233, 2022.

09:30–09:32
09:32–09:37
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EGU22-5283
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On-site presentation
Esther Schwarzenbach, Linus Streicher, Besim Dragovic, Maria Rosa Scicchitano, Uwe Wiechert, Emmanuel Codillo, Frieder Klein, Horst Marschall, and Marco Scambelluri

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 (S2-). 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.

09:37–09:39
09:39–09:44
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EGU22-11903
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ECS
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On-site presentation
Qasid Ahmad, Martin Wille, Carolina Rosca, Jabrane Labidi, Timothy Schmid, Klaus Mezger, and Stephan König

Significant Mo mobility and isotope (δ98/95Mo) fractionation is induced during prograde metamorphism at present-day subduction zones. Depending on the redox conditions during early subduction and accompanied slab dehydration, isotopically heavy Mo is released towards the overlying mantle wedge, leaving behind a depleted, and isotopically light subducted slab. This isotopically light Mo signature has been detected in slab-melt influenced volcanic rocks and potentially will be traceable in ocean-island basalts, if their geochemical signatures are affected by previously subducted lithologies (i.e. slab and overlying sediments). Thus, the isotope composition of mantle plume-influenced volcanic rocks might reveal the nature of subducted and re-incorporated lithologies and possibly redox conditions during subduction.

In this study, we present new Mo isotope data for South-Mid Atlantic Ridge basalts that partly interacted with the enriched Discovery and Shona mantle plumes. Isotopically heavier Mo isotope ratios (δ98/95Mo > ambient depleted mantle) are observed in samples tapping a more enriched mantle source. Furthermore, δ98/95Mo correlates with radiogenic isotopes (Sr, Nd, Hf) indicating recycling of a Proterozoic sedimentary components with a Mo isotopic composition that was not modified during and before subduction by Mo mobility under oxidising conditions. Rather, the new Mo isotope data supports and expands on previous stable Se and S isotope evidence that suggests the incorporation of subducted anoxic Proterozoic deep-sea sediments into the mantle of the South-Mid Atlantic Ridge basalts.

How to cite: Ahmad, Q., Wille, M., Rosca, C., Labidi, J., Schmid, T., Mezger, K., and König, S.: Modern hotspot-influenced MORBs reveal anoxic conditions during deposition and subduction of recycled Proterozoic sediments in their source, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11903, https://doi.org/10.5194/egusphere-egu22-11903, 2022.

09:44–09:46
09:46–09:51
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EGU22-382
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ECS
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Virtual presentation
Olga Bergal-Kuvikas

The correlation of subducted plate parameters with generated volcanism was studied along the Kamchatka arc. Increased slab age controls dip angle (25-45o) and length of the seismic zone (200-700 km slab depth)  from the north (~530N) to the south (~490N) of the Kamchatka arc. All listed above parameters generate various aged volcanic belts with different parameters of volcanism. The natural boundary between various aged slabs is on ~530N, on the extension Avachinsky transform fault. It divides the Kamchatka arc on Southern Kamchatka with slab age ~ 103-105 Ma and Eastern volcanic belt, Central Kamchatkan Depression with slab age ~ 87-92 Ma. Complicated evolution and various ages of the slab control magmatism along the Kamchatka arc. Basic-intermediate magma compositions dominantly characterized Quaternary-Pliocene volcanoes in Central Kamchatkan Depression. In contrast, Neogene-Quaternary volcanism on Southern Kamchatka represents by strong explosions of acidic magmas (Gordeev, Bergal-Kuvikas, 2022).

Monogenetic volcanism marked a Malko-Petropavlovsk zone of transverse dislocations (MPZ), which is located on the extension Avachinsky transform fault. Monogenetic cinder cones in MPZ are randomly distributed along to these long-lived rupture zones. Here I present new geochemical and isotopic results of monogenetic volcanism in MPZ. Based on whole rock and trace element geochemistry, Pb-Sr-Nd isotopic ratios of monogenetic cinder cones magmas were shown to tap the enriched mantle source (low 143Nd/144Nd isotopic ratios (0.512959-0.512999), as variated 87Sr/86Sr (0.703356-0.703451) and 206Pb/204Pb (18.30-18.45), 208Pb/207Pb (38.00-38.12) isotopic ratios).  High Nb/Yb and La/Yb ratios, without significant inputs of the slab`s components (the lowest Ba, Th contents), indicate decompression melting predominately (Bergal-Kuvikas et al., 202X). Therefore, a combination of geophysical and geochemical methods enable us to conclude that monogenetic volcanism in MPZ   mark a natural boundary between various aged slab on Avachinsky transform fault. Various aged slabs under Southern Kamchatka and the Eastern volcanic belt generate volcanism with different magma compositions and ages of volcanoes.

This research was supported by Russian Science Foundation (grant number 21-17-00049,https://rscf.ru/project/21-17-00049/).

References

Bergal-Kuvikas O.V., Bindeman I.N., Chugaev A.V., Larionova Yu. O., Perepelov A.V., Khubaeva O.R. Pleistocene-Holocene monogenetic volcanism at Malko-Petropavlovsk zone of transverse dislocations on Kamchatka: geochemical features and genesis // Pure and Applied Geophysics. Special Issue: Geophysical Studies of Geodynamics and Natural Hazards in the Northwestern Pacific Region (in review)

Gordeev, E.I., Bergal-Kuvikas O.V. (2022). Structure of subduction zone and volcanism on Kamchatka. Doklady of the Earth Sciences. 2. 502. P. 26-30. 10.31857/S2686739722020086

 

 

 

How to cite: Bergal-Kuvikas, O.: Correlation slab heterogeneity and volcanism in Kamchatka arc, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-382, https://doi.org/10.5194/egusphere-egu22-382, 2022.

09:51–09:53
09:53–10:00
Coffee break
Chairpersons: Ágnes Király, Oğuz H Göğüş, Taras Gerya
Introduction 2
10:20–10:25
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EGU22-4976
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Presentation form not yet defined
Sierd Cloetingh, Alexander Koptev, Istvan Kovacs, Taras Gerya, Anouk Beniest, Ernst Willingshofer, Todd Ehlers, Nevena Andric-Tomasevic, Svetlana Botsyun, Paul Eizenhofer, Thomas Francois, and Fred Beekman

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.

10:25–10:27
10:27–10:32
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EGU22-4261
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Virtual presentation
Marzieh Baes, Stephan Sobolev, Andrea Hampel, and Anne Glerum

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.

10:32–10:34
10:34–10:39
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EGU22-12659
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ECS
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Virtual presentation
Francisco Bolrão, Jaime Almeida, João C. Duarte, and Filipe M. Rosas

The absence of a forearc is a recurrent simplification in numerical subduction models. This because, to our knowledge, there are no previous studies that have systematically investigate the role of this structure on subduction systems. Despite its short length (166 ± 60 km), the forearc has a significant impact in the maintenance of a stable subduction. It has already been proposed that the serpentinization of this region, by percolating fluids from the sinking slab, reduces the effective mechanical strength of the plate coupling zone interface, allowing the one-sided asymmetric subduction observable in nature. Moreover, the forearc could be the key stabilization mechanism in intra-oceanic subduction settings. In this scenarios, the oceanic overriding plate (OP) could be in a thermal state such that would also be negative buoyant. The ubiquitous presence of forearcs in all-active intra-oceanic subductions suggests that a weak interface alone could not be enough to prevent the OP to sink. Adding a positive buoyant forearc  to the tip of the OP could provide the counterforce required to prevent the OP to sink, and eventually, double-sided subduction setting. There are studies that already implement a forearc structure in their numerical models. However, since its dynamic influence has not been study yet, we can not predict its impact and/or ascribe a specific dynamic behaviour of the system to it. 

In this work we investigate the role of the forearc and its contribution to emergent features in subduction zones. We present a series of fully dynamic, buoyancy driven, thermo-mechanical numerical modelling experiments with a free surface carried out to gain insight on the dynamic role of the forearc.  We use the Underwolrd numerical code to perform a parametrization to geometric and rheologic parameters of this structure, namely the thickness (age of the OP), length and density. We consider a forearc that encompasses the arc (25 to 250 km wide) as well. We kept all physical properties of the subducting plate  constant throughout all models. Therefore, we are able to ascribe all dynamic changes solely to variations of the forearc properties. We test different forearc compositions based on its density, ranging between 2700 and 3300  kg.m−3, for 200  kg.m−3 intervals, mimicking a full granitic continental and an basaltic oceanic forearc, respectively. For all densities, we also test several possible lengths, for 130 km and for 200 to 470 km, for intervals of 90 km. Additionally, we test all possible density-length combinations for five different OPs, in terms of age, ranging between 20 and 100 Myr, for 20 Myr intervals. 

We expect a higher accommodation of strain in the tip of the OP in models where the forearc is implemented. The presence of this structure could favor slab roll-forward before this reaches the 660 km discontinuity, enhance subduction velocities and generate a more pronounced orogenic topography. This features would be enhanced with the decrease of density and thickness and  the increase of length of the forearc.

How to cite: Bolrão, F., Almeida, J., C. Duarte, J., and M. Rosas, F.: Numerical modeling of subduction zones: thermo-mechanical stabilization as a function of overriding plate rheology and thickness, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12659, https://doi.org/10.5194/egusphere-egu22-12659, 2022.

10:39–10:41
10:41–10:46
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EGU22-2400
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Presentation form not yet defined
Taras Gerya, David Bercovici, and Thorsten Becker

Subducting oceanic plates experience intense normal faulting during bending that accommodates the transition from horizontal to downward motion at the outer rise at subduction trenches. We investigated numerically the consequences of the plate bending on the mechanical properties of subducting slabs using 2D subduction models in which both brittle and ductile deformation, as well as grain size evolution, are tracked and coupled self-consistently. Numerical results suggest that pervasive brittle-ductile slab damage and segmentation can occur at the outer rise region and under the forearc that strongly affects subsequent evolution of subducting slabs in the mantle. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs, the development of large-offset normal faults near trenches and the occurrence of segmented seismic velocity anomalies and interfaces imaged within subducted slabs. Furthermore, brittle-viscously damaged slabs show a strong tendency for slab breakoff at elevated mantle temperatures that may have destabilized continued oceanic subduction and plate tectonics in the Precambrian (Gerya et al., 2021).

Gerya, T.V., Bercovici, D., Becker, T.W. (2021) Dynamic slab segmentation due to brittle-ductile damage in the outer rise. Nature, 599, 245-250.

How to cite: Gerya, T., Bercovici, D., and Becker, T.: Segmentation of subducting oceanic plates by brittle-ductile damage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2400, https://doi.org/10.5194/egusphere-egu22-2400, 2022.

10:46–10:48
10:48–10:53
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EGU22-4774
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Highlight
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On-site presentation
João C. Duarte, Nicolas Riel, Boris J.P. Kaus, and Filipe M. Rosas

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.

10:53–10:55
10:55–11:00
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EGU22-4810
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On-site presentation
Valentina Magni, Nicholas Schliffke, Jeroen van Hunen, Frédéric Gueydan, Mark Allen, John Naliboff, Manel Prada, and Carmen Gaina

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.

11:00–11:02
11:02–11:07
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EGU22-8243
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ECS
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Virtual presentation
Maaike Fonteijn, Elenora van Rijsingen, and Ylona van Dinther

The subduction of seamounts and its accompanying crustal deformation of the overriding plate is thought to have a large effect on the occurrence of megathrust earthquakes. Subducted seamounts can generally only be observed using seismic-reflection studies, which have shown that seamounts can subduct intact down to 30-40 km depth. On the other hand, there is evidence for accreted seamounts in e.g. the Costa Rica and Makran subduction zones. Because such observations only provide snapshots in space and time, little is still known about the exact evolution of seamount subduction and its effect on overriding-plate deformation and subduction zone seismicity through time. We investigate the different styles of seamount subduction and how these influence seismicity and overriding plate deformation. We use seismo-thermo-mechanical (STM) models with a visco-elasto-plastic rheology simulating seamount subduction over millions of years in a 2D realistic subduction setting. The momentum, mass and energy equations are solved and a strongly slip rate dependent friction allows for the spontaneous development of faults. The use of a realistic rheology allows us to evaluate faulting patterns and the state of stress in the overriding plate caused by seamount subduction. We find three scenarios for seamount subduction by varying the rock properties cohesion (C) and pore fluid pressure ratio (λ): (1) cutting off of the seamount at the trench leading to frontal accretion; (2) intact subduction through the trench, followed by flattening and stretching of the seamount; and (3) intact subduction of the seamount until seismogenic depths. Scenario’s 1 and 2 are most common, while scenario 3 only occurs under a limited range of material parameters. Particularly, a cohesion of the seamount and upper oceanic crust larger than 20 MPa is required for intact seamount subduction. Decreasing λ on locations with ample amounts of fluids increases the strength of the sediments, upper oceanic crust and seamount, but does not lead to intact seamount subduction. Subduction scenario’s 2 and 3 show more crustal deformation and seismicity within the fore-arc than subduction of a smooth interface (scenario 1 and models without a seamount). Seismicity patterns are also affected by λ and C. A low λ results in shorter and shallower megathrust ruptures and higher cohesions decrease the recurrence interval. Furthermore, the seamount itself introduces more frequent nucleation of smaller events at its edge.

How to cite: Fonteijn, M., van Rijsingen, E., and van Dinther, Y.: Styles of seamount subduction and overriding plate deformation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8243, https://doi.org/10.5194/egusphere-egu22-8243, 2022.

11:07–11:09
11:09–11:14
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EGU22-3822
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ECS
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On-site presentation
Nestor Cerpa, Karin Sigloch, Fanny Garel, Rhodri Davies, and Arnauld Heuret

Plate kinematics in the vicinity of subduction zones, as well as seismic tomography provide insights into the deep dynamics of subducting slabs. Velocities at which subducting plates are consumed at the trench (the subduction velocities) typically exceed 3–4 cm/yr at present-day. Absolute trench velocities (relative to a lower-mantle reference frame) are lower, between -2 and 2 cm/yr [Heuret and Lallemand, 2005]. This implies that the “accommodation space” created by the slab rollback associated with lateral trench migration is not nearly sufficient for accommodating the length of incoming slab in the horizontal dimension. In the vertical dimension, even the fastest estimates for slab sinking rates over long time scales amount to only a fraction of 3–4 cm/yr [Butterworth et al. 2014, van der Meer et al. 2010, Sigloch & Mihalynuk 2013]. Hence the rates at which the lithosphere typically subducts cannot be accommodated by fast vertical sinking either. Seismic tomography confirms the “traffic jam” conditions for slabs in the mantle that are implied by these numbers, with slab thickening imaged in and beneath the mantle transition zone (MTZ). These highly visible, thickened, slabs have been interpreted as the result of folding [Ribe et al., 2007], and their relative localization (massive,  near-vertical “slab walls”) supports the notion of near-stationary trenches over long time scales [Sigloch and Mihalynuk, 2013]. 

Buoyancy-driven analog and numerical models of subduction have commonly produced subduction and trench velocities that differ from the first-order observations above. Their subduction velocities typically drop below 1-2 cm/yr once the modelled slab enters the high-viscosity lower mantle, and their trench migration velocities remain almost equal to subduction velocities, thus accommodating the slab mainly in the horizontal direction. In addition, these models tend to produce trench retreat and slab “rollback” , unless the latter is very weak and/or the overriding plate is very strong [Goes et al., 2017]. These modelling results have led to the conclusion that near-vertical slab sinking and folding at the MTZ is an end-member regime restricted to very specific subduction set-ups. 

We have added a weak asthenospheric layer to typical 2-D thermo-mechanical models of subduction zones with a complex rheology [e. g., Garel et al., 2014], which partly reconciles the models and the observations. A weak asthenosphere appears as an intuitive candidate for increasing subduction velocity because a reduced mantle drag at the base of the subducting plate lowers the mantle’s resistance to the plate’s trench-ward motion. We further found that the models with a weak asthenospheric layer lessens the trench motion and thus tend to produce prominent vertical folding of slabs at the MTZ. Subduction velocities remain higher than trench velocities long after the slab reaches the MTZ, so that 300-to-400-km wide “slab walls” are continuously produced in the lower mantle over a relatively wide range of model parameters. The presence of a weak asthenosphere has often been speculated to explain seismic properties beneath oceanic plates, but seldom modelled. This study contributes to a quantification of its potential effects on subduction dynamics. 

How to cite: Cerpa, N., Sigloch, K., Garel, F., Davies, R., and Heuret, A.: Subduction dynamics through the mantle transition zone in the presence of a weak asthenospheric layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3822, https://doi.org/10.5194/egusphere-egu22-3822, 2022.

11:14–11:16
11:16–11:21
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EGU22-5284
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ECS
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On-site presentation
Michaël Pons, Stephan Sobolev, Sibiao Liu, and Derek Neuharth

The nature of the shortening of the Central Andes has been a matter of debate. The South American plate is advancing westwards forcing the subducting Nazca plate to roll back and the trench to retreat. But as the trench slowed its retreat the Andean mountain belt formed. This decrease of trench velocity has been attributed to the anchoring of the slab, but this process cannot explain the observed pulsatile behaviour of the shortening rate. Indeed, whereas the formation of the Central Andes started ~50 Ma ago, most of the shortening and elevation growth, including the formation of the Altiplano-Puna plateau, took place in two pulsatile steps at 15 Ma and 7 Ma as recognized from geological data. Thus we hypothesize that the deformation of the Central Andes is controlled by the subduction dynamics and a complex interaction between the overriding and subducting plates.

We used the FEM geodynamic code ASPECT to develop a self-consistent subduction E-W-oriented 2D high-resolution geodynamic model along the Altiplano-Puna plateau (21°S). This model incorporates the flat slab subduction episode at 35 Ma and follows the evolution of the lithospheric deformation. Our model results reproduced the observed spatial and temporal variations of tectonic shortening in Central Andes.

Three main conditions related to the plate interaction are of key importance to explain the observed shortening rate evolution in Central Andes. Firstly, the subduction dynamics affects the trench migration: each episode of slab steepening is followed by the blocking of the trench. The steepening occurs after the flat slab and at the end of two slab-buckling instabilities at 15 Ma and at 7 Ma. The second relevant process is the weakening of the overriding plate. This is ensured by the partial removal of a part of the lithospheric mantle after the re-steepening of the flat slab at 35 Ma and by weakening of the sediments in the Subandean Ranges after 10 Ma. Thirdly, a relatively high interplate friction coefficient (~0.05) is needed to ensure the stress transfer from the slab to the overriding plate, which is further enhanced by the delaminated mantle lithosphere eventually blocking the subduction corner flow.

The pulses of shortening rate occur at the end of each slab-buckling cycle when the trench is blocked. The deformation of the overriding plate is intensified by the eclogitization of the lower crust and the subsequent delamination of the sublithospheric mantle. Finally, at ~10 Ma, the deformation switches from pure-shear to simple-shear shortening, after the underthrusting of the Brazilian craton in presence of weak foreland sediments. 

How to cite: Pons, M., Sobolev, S., Liu, S., and Neuharth, D.: Variability of the shortening rate in Central Andes controlled by subduction dynamics and interaction between slab and overriding plate. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5284, https://doi.org/10.5194/egusphere-egu22-5284, 2022.

11:21–11:23
11:23–11:28
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EGU22-7483
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ECS
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Presentation form not yet defined
Andrea Piccolo and Marcel Thielmann

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.

11:28–11:30
11:30–11:35
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EGU22-5344
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On-site presentation
Simone Pilia, Rhodri Davies, Robert Hall, Conor Bacon, Amy Gilligan, Tim Greenfield, Felix Tongkul, and Nicholas Rawlinson

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.

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