GD4.3

The lithospheric sinking along subduction zones is part of the mantle convection. Therefore, computing the volume of lithosphere recycled within the mantle by subducting slabs quantifies the equivalent amount of mantle that should be displaced, for the mass conservation criterion. Starting from the analysis of the subduction hinge kinematics, that could either move towards (H-convergent) or away (H-divergent) with respect to the fixed upper plate, we compute the amount of lithosphere currently subducting below 31 subduction zones worldwide. Our results show that ~190 km³/yr and ~88 km³/yr of lithosphere are currently subducting below H-divergent and H-convergent subduction zones, respectively. This volume discrepancy is principally due to the difference in the two end-members subduction rate, that takes into account the hinge kinematics. We also propose supporting numerical models providing asymmetric volumes of subducted lithosphere, using the subduction rate, instead of plate convergence, as boundary condition. Subduction zones show a worldwide asymmetry from geological and geophysical observations, such as slab dip, structural elevation, gravity anomalies, heat flow, metamorphic evolution, subsidence and uplift rates or depth of the décollement planes. This asymmetry is expressed also in the behaviour of the subduction hinge, so that H-divergent subduction zones appears to be coincident with subduction zones having “westward”-directed slabs, whereas H-convergent are compatible with those that have “eastward-to-northeastward”-directed slabs. On the basis of this geographical polarity of subducting slabs, the obtained lithospheric volume estimation gives ~214 km³/yr and ~88 km³/yr of subducting lithosphere for subduction zones with W-directed and E-to-NE-directed slabs, respectively. This imply that W-directed subduction zones contribute more than twice in lithospheric sinking into the mantle with respect to E-to-NE-directed ones. In accordance with the conservation of mass principle, this volumetric asymmetry in the mantle suggests a displacement of ~120 km³/yr of mantle material from the west to the east, providing a constrain for a global asymmetric mantle convection.

How to cite: Ficini, E., Cuffaro, M., and Doglioni, C.: Asymmetric dynamics at subduction zones: from plate kinematic constraints to global mantle convection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2441, https://doi.org/10.5194/egusphere-egu21-2441, 2021.

Overriding plate deformation (OPD) and topography vary at different subduction zones, with some subduction zones showing mainly overriding plate extension and low topography (e.g. Mariana, Tonga, Izu-Bonin subduction zones), while some showing mainly shortening and elevated topography (e.g. Makran, southern Manila subduction zones). Here we investigate how different subduction modes, namely trench retreat and trench advance, affect OPD and generate corresponding topography with fully dynamic analogue models of time-evolving subduction in three-dimensional space. We conduct two sets of experiments, one of which is characterized by trench retreat and slab rollback, and the other characterized by trench advance and slab rollover. We compute the mantle flow, the overriding plate strain and topography during subduction using the particle image velocimetry technique (PIV). The overriding plate in the experiments showing continuous trench retreat experiences overall extension, while in the experiments with trench advance following trench retreat it experiences overall shortening. The overriding plate in both trench retreat and trench advance subduction modes present fore-arc shortening and intra-arc extension. Our experiments indicate that the overall OPD except in the fore-arc region is mainly driven by the horizontal mantle flow at the base of the OP inducing a viscous drag force (F_D), and is determined by the gradient of the horizontal mantle flow velocity (dv_x/dx). Furthermore, a large-scale trenchward overriding plate tilting and an overall subsidence of the overriding plate were observed in the experiments showing continuous trench retreat, while a landward tilting and an overall uplift of the overriding plate were observed during long-term trench advance. The two types of topography during the two different subduction modes can be ascribed to the large-scale trenchward and landward mantle flow, respectively, and thus represent forms of dynamic topography. Our models showing trench advance provide a possible mechanism for OPD in the Makran subduction zone, which has experienced overall trench-normal tectonic shortening in the overriding plate, but shows extension in a local region of the coastal Makran that is spatially comparable to that in our experiments.  In addition, these models might also provide an explanation for the regional topography at the Makran subduction zone, which shows a long-wavelength topographic high in the overriding plate near the trench that decreases northward.

How to cite: Xue, K., Schellart, W. P., and Strak, V.: Overriding plate deformation and topography during slab rollback and slab rollover: insights from subduction experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2922, https://doi.org/10.5194/egusphere-egu21-2922, 2021.

Trench retreat, or slab roll-back, has been proposed to account for various degrees of extensional deformation within the overriding plate in subduction zones, eg. Izu-Bonin-Mariana, Tonga etc. However, the relationship between trench retreat rate and the degree of extension has not been rigorously tested. Here we obtain a wide range of trench retreat rate by varying the initial age of subducting plate (SP, Age⁰_SP) and overriding plate (OP, Age⁰_OP) met at trench. Then we investigate how much trench retreat rate is needed to initiate rifting in the OP.

The results show that models would evolve from a non-steady state towards a steady state as the SP sinks to the transition zone at 660 km.

Before the SP starts to interact with the transition zone, the trench retreat rate accelerates with time reaching a maximum value (v_max), which can be very high but only lasts a short time (~0.5 Myr). For models with a given OP, v_max is Age⁰_SP-dependent. The trench retreat rate, on the other hand, determines the extensional extent within the OP. With increasing Age⁰_SP, a minimum trench retreat rate (v_rift) is needed to initiate rifting within the OP. For models with Age⁰_OP = 20 Myr and Age⁰_OP = 25 Myr, v_rift is ~19 cm/yr and ~27 cm/yr separately. This implies that an older OP is more resistant to extensional stress field driven by trench retreat. In all, three types of stretching states are observed within the OP in our models: i) minor extension, where v_max<v_rift and the OP lithosphere has little extension; ii) rift, where v_max≈v_rift and the OP would rift but not be torn apart; iii) break-up, where v_max>v_rift and the OP would rift when the trench retreat rate reaches v_rift, then breaks up into two parts after it exceeds v_rift. We note all three states involve different extents of mantle wedge erosion at ~100 km away from the trench underneath the OP, while rifting and break-up occur >700 km away from the trench. In the break-up cases, the two parts of the OP can be ~250 km apart.

After the SP reaches the transition zone, the trench retreat rate would drop to a constant magnitude around 2 cm/yr and lose the Age⁰_SP-dependency. This is because the viscosity jump at the transition zone prevents the SP from accelerating into the lower mantle. Meanwhile, the Age⁰_SP-dependent negative buoyancy loses its dominant role in driving the trench retreat.

We discuss two driving mechanisms to relate the initiation of extension with rapid trench retreat (trench suction): 1) focused upwelling from the transition zone; 2) horizontal basal drag. We conclude that the transient rapid trench retreat can lead to an extensional stress field through basal drag which is strong enough to initiate rifting or even break-up within a mobile overriding plate. A high negative SP buoyancy could play the driving force to generate this transient rapid trench retreat.

How to cite: Lei, Z. and Davies, H.: Investigating the role of transient rapid trench retreat in initiating rifting of a mobile overriding plate during subduction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1420, https://doi.org/10.5194/egusphere-egu21-1420, 2021.

Much of our understanding of the dynamics of slab break-off and its geological signatures rely on numerical models with a simplified set-up, in which slab break-off follows arrival of a continent in a mantle-stationary trench, the subsequent arrest of plate convergence, and after a delay time of 10 Ma or more, slab break off under the influence of slab pull. However, geological reconstructions show that plate tectonic reality deviates from this setup: post-collisional convergence is common, trenches are generally not stationary relative to mantle, neither before nor after collision, and there are many examples in which the mantle structure below collision zones is characterized by more, or fewer slabs than collisions.

A key example of the former is the India-Asia collision zone, where the mantle below India hosts two major, despite the common view of a single collision. Kinematic reconstructions reveal that post-collisional convergence amounted 1000s of kms, and was associated with ~1000 km of trench/collision zone advance. Collision between India-Asia collision zone may provide a good case study to determine the result of post-collisional convergence and absolute lower and upper plate motion on mantle structure, and to evaluate to what extent commonly assumed diagnostic geological phenomena of slab break-off apply.

In addition to the previously identified major India, Himalaya, and Burma slabs, we here map smaller slabs below Afghanistan and the Himalaya that reveal the latest phases of break-off. We show that west-dipping and east-dipping slabs west and east of India, respectively, are dragged northward parallel to the slab, slabs subducting north of India are overturned, and that the shallowest slab fragments are found in the location where the horizontally underthrust Indian lithosphere below Tibet is narrowest. Our results confirm that northward Indian absolute plate motion continued during two episodes of break-off of large (>1000 km wide) slabs, and decoupling of several smaller fragments. These slabs are currently found south of the present day trench locations. The slabs are located even farther south (>1000 km) of the leading edge of the Indian continental lithosphere, currently underthrust below Tibet, from which the slabs detached, signalling ongoing absolute Indian plate motion. We conclude that the multiple slab break-off events in this setting of ongoing plate convergence and trench advance is better explained by shearing off of slabs from the downgoing plate, possibly at a depth corresponding to the base of the Indian continental lithosphere, are not (necessarily) related to the timing of collision. A recently proposed, detailed diachronous record of deformation, uplift, and oroclinal bending in the Himalaya that was liked to slab break-off fits well with our kinematically reconstructed timing of the last slab shear-off, and may provide an important reference geological record for this process. We find that the commonly applied conceptual geological signatures of slab break-off do not apply to the India-Asia collision zone, or to similar settings and histories such as the Arabia-Eurasia collision zone. Our study provides more realistic boundary conditions for future numerical models that aim to assess the dynamics of subduction termination and its geological signatures.

How to cite: Qayyum, A., Lom, N., Advokaat, E. L., Spakman, W., and van Hinsbergen, D. J. J.: Geological signatures of slab shear-off during ongoing India-Asia convergence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13160, https://doi.org/10.5194/egusphere-egu21-13160, 2021.

The Ecuadorian subduction regularly hosts large earthquakes. Among them, the Mw 8.8 1906 earthquake is the 7th biggest known event. Following the recent 2016 Mw 7.8 Pedernales earthquake, a large deployment of onshore/offshore seismological stations, in addition to the permanent seismological/geodetical network, revealed a complex slip behavior including the presence  of  seismic and aseismic slip.

During the geophysical experiment HIPER, in march 2020, 47 Ocean Bottom Seismometers (OBS), were densely deployed along a 93-km-long trench-perpendicular profile, recording airgun shots (4990 cu.inch.) performed by R/V Atalante to obtain a high-resolution P-wave velocity image. The profile was located north of the 2016 Pedernales rupture zone passing through an area experiencing aseismic slip and a region of contrasted geodetic interseismic coupling.    

We used the traveltime tomography code « tomo2d » (Korenaga et al., 2000) to invert first arrivals and reflected phases recorded by our OBS.  A joint 2D-seismic-reflection profile was acquired (abstract by L. Schenini) and provides details on the oceanic basement topography and on Vp velocities in shallow sedimentary layers.

Regarding the structural complexity in the region, we decided to start the inversion  using an a priori 2D velocity model. Several geophysical experiments have already been conducted offshore-onshore Ecuador (SISTEUR, 2000 ; SALIERI, 2001 and ESMERALDAS, 2005). Compilation of velocity models from tomographic images were used to build two a priori 1D Vp velocity models for both the Nazca oceanic crust and the forearc seismic structure. A 2D a priori Vp velocity model was built by merging the results of the two localized inversions using a selection of OBS on each side of the trench.

We obtain the crustal structure of the upper and subducting plates down to 20 km depth. Beneath the trench, a ~30-km-wide low-Vp anomaly is observed at lithospheric scale. This velocity is 10% lower than the typical Vp values observed for hydrated Pacific-type oceanic crust near the trench (Grevemeyer et al., 2018). Recorded PmP phases will allow us to further constrain the crustal thickness. While we observe PmP phases in areas of low-Vp, the Moho reflectivity weakens and even disappears from the coincident MCS line. This intriguing observation could highlight processes, such as the presence of fluids or serpentinization, that need to be identified and better understood.

How to cite: Skrubej, A., Galve, A., Laigle, M., Rietbrock, A., Charvis, P., Vaca, S., Agurto-Detzel, H., Schenini, L., Bogelspacher, F., Oregioni, D., Vignon, D., Brotzer, A., Muñoz Muñoz, M., and Moreno Piña, M.: Travel-time tomography imaging the Ecuadorian subduction, north of the Mw 7.8 Pedernales earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10959, https://doi.org/10.5194/egusphere-egu21-10959, 2021.

The Grenada Basin separates the active Lesser Antilles Arc from the Aves Ridge, described as a Cretaceous-Paleocene remnant of the “Great Arc of the Caribbean.” Although various tectonic models have been proposed for the opening of the Grenada Basin, the data on which they rely are insufficient to reach definitive conclusions. We present a large set of deep-penetrating multichannel seismic reflection data and dredge samples acquired during the GARANTI cruise in 2017. By combining them with published data including seismic reflection data, wide-angle seismic data, well data and dredges, we refine the understanding of the basement structure, depositional history, tectonic deformation and vertical motions of the Grenada Basin and its margins as follows: (1) rifting occurred during the late Paleocene- early Eocene in a NW-SE direction and led to seafloor spreading during the middle Eocene; (2) this newly formed oceanic crust now extends across the eastern Grenada Basin between the latitude of Grenada and Martinique; (3) asymmetrical pre-Miocene depocenters support the hypothesis that the southern Grenada Basin originally extended beneath the present-day southern Lesser Antilles Arc and probably partly into the present-day forearc before the late Oligocene-Miocene rise of the Lesser Antilles Arc; and (4) the Aves Ridge has subsided along with the Grenada Basin since at least the middle Eocene, with a general subsidence slowdown or even an uplift during the late Oligocene, and a sharp acceleration on its southeastern flank during the late Miocene. Until this acceleration of subsidence, several bathymetric highs remained shallow enough to develop carbonate platforms.

How to cite: Garrocq, C., Lallemand, S., Marcaillou, B., Lebrun, J.-F., Padron, C., Klingelhoefer, F., Laigle, M., Münch, P., Gay, A., Schenini, L., Beslier, M.-O., Cornée, J.-J., Mercier de Lépinay, B., Quillévéré, F., and BouDagher-Fadel, M.: Genetic Relations Between the Aves Ridge and the Grenada Back-Arc Basin, East Caribbean Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10993, https://doi.org/10.5194/egusphere-egu21-10993, 2021.

Studying back-arc basins, where sedimentation is less deformed than in the forearc, provides complementary information about formation and tectonic evolution of subduction zones. At the Lesser Antilles subduction zone, the North and South American plates are subducting underneath the Caribbean plate at a velocity of 2 cm per year. The crescent-shaped Grenada back-arc basin is located between the Aves Ridge, which hosted the remnant Early Paleogene “Great Caribbean Arc”, and the Eocene to present Lesser Antilles Arc. In this study, based on wide-angle data, we provide constraints about lateral variations in basement thickness and velocity structure in the Lesser Antilles back-arc, and to a lesser extend in the arc and forearc domain, constraining for the first time the extent of oceanic crust in the Grenada Basin and shed light on the structure and compositions of the basin’s margins.

Three combined wide-angle and reflection seismic profiles, together with gravity and bathymetric data, were acquired in the Lesser Antilles back-arc basin. Direct modeling techniques were applied to the wide-angle seismic data in order to include shallow structures imaged by the coincident reflection seismic data. The resulting velocity models were additionally constrained by gravity modeling and synthetic seismogram calculation. The final models from direct modeling image variations in thickness and velocity structure of the sedimentary and crustal layers to a depth of up to 35 km. The sedimentary cover has a variable thickness from less than a kilometer on top of the ridges to nearly 10 km in the basin. North of Guadeloupe Island, the crust is ~20 km thick from back-arc to forearc, without significant change between the Aves Ridge, the Eocene and present Lesser Antilles volcanic arc. While based on the seismic velocities, the southern part of the basin is underlain by a 6.5-7 km thick crust of of mainly magmatic origin over a width of ~80 km, the northern part is underlain by thinned continental crust. At the western flank of the Lesser Antilles Arc, the crust is 17.5-km thick, about 5 km thinner than north of Martinique island. The velocity structure is typical of volcanic arcs or oceanic plateaus. Between Aves Ridge and the Grenada basin the crust thins in a 80-100 km wide transition zone. No anomalous high velocities indicating the presence of exhumed upper mantle material were detected at the transition zone. This narrow E-W arc-ocean transition zone suggests that opening might have proceeded in a direction highly oblique to the main convergence.

How to cite: Padron, C., Klingelhoefer, F., Marcaillou, B., Lebrun, J.-F., Lallemand, S., Garrocq, C., Laigle, M., Roest, W., Beslier, M.-O., Schenini, L., Graindorge, D., Gay, A., Audemard, F., and Münch, P.: Deep structure of the Grenada Basin from wide-angle seismic, bathymetric and gravity data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11712, https://doi.org/10.5194/egusphere-egu21-11712, 2021.