Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth

Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

Unraveling scaling properties of slow-slip events

A major debate in geophysics is whether earthquakes and slow‐slip events (SSEs) arise from similar failure mechanisms. Recent observations from different subduction zones suggest that SSEs follow the same moment‐duration scaling as earthquakes, unlike qualitatively different scaling proposed by earlier studies. Here, we examine the scaling properties using dynamic simulations of frictional sliding. The resulting sequences of SSEs match observations from the Cascadia subduction zone, including the earthquake‐like cubic moment‐duration scaling. In contrast to conventional and widely used assumptions of magnitude‐invariant rupture velocities and stress drops, both simulated and natural SSEs have rupture velocities and stress drops that increase with event magnitudes. These findings support the same frictional origin for both earthquakes and SSEs while suggesting a new explanation for the observed SSEs scaling

Slab Rollback Orogeny Model: A Test of Concept

Buoyancy forces associated with subducting lithosphere control the dynamics of convergent margins. In the postcollisional stage these forces are significantly reduced, yet mountain building and seismicity are ongoing, albeit at lower rates. We leverage advances of a newly developed seismo‐thermo‐mechanical modeling approach to simulate tectonic and seismicity processes in a self‐driven subduction and continental collision setting. We demonstrate that the rearrangement of forces due to slab breakoff, in the postcollisional stage, causes bending and rollback of the residual slab, suction forces, and mantle traction at the base of the upper plate, while stress coupling transfers to the shallow crust. Our results provide an explanation for the postcollisional evolution of the Central Alps, where the so‐called Slab Rollback Orogeny model explains the slow yet persistent upper plate advance, the height of the mountain range, and a seismicity pattern consistent with the different tectonic regimes throughout the orogen

Subduction earthquake sequences in a non-linear visco-elasto-plastic megathrust

ISSN:0956-540XISSN:1365-246

Bimodal seismicity in the Himalaya controlled by fault friction and geometry

There is increasing evidence that the Himalayan seismicity can be bimodal: blind earthquakes (up to Mw ~ 7.8) tend to cluster in the downdip part of the seismogenic zone, whereas infrequent great earthquakes (Mw 8+) propagate up to the Himalayan frontal thrust. To explore the causes of this bimodal seismicity, we developed a two-dimensional, seismic cycle model of the Nepal Himalaya. Our visco-elasto-plastic simulations reproduce important features of the earthquake cycle, including interseismic strain and a bimodal seismicity pattern. Bimodal seismicity emerges as a result of relatively higher friction and a non-planar geometry of the Main Himalayan Thrust fault. This introduces a region of large strength excess that can only be activated once enough stress is transferred upwards by blind earthquakes. This supports the view that most segments of the Himalaya might produce complete ruptures significantly larger than the 2015 Mw 7.8 Gorkha earthquake, which should be accounted for in future seismic hazard assessments

Studio preliminare del contesto strutturale dei boati del Fadalto

This study looks in detail at the Lapisina Valley and specifically the residential areas of Fadalto and Farra d’Alpago, which have been subject to rumbling. This area is interesting from a geo-tectonical point of view, as it presents various thrust, in particular the “Longhere-Fadalto-Cadola” line which runs through the area analyzed in my study. Due to these perceived rumbles in the early part of 2011 the OGS (Istituto nazionale di Oceanografia e Geofisica Sperimentale) decided to carry out an! investigation through the use of portable seismometers, which led them to discover relatively shallow microAearthquakes. Furthermore they formulated a hypothesis that these seismic events were responsible for the rumbles!and caused by a phenomenon known!as “Water Hammer”, where a liquid or even a gas is forced to change its natural course and crash into a subterranean rock wall. This thesis is!aimed to define whether these “Water Hammer” events have a structural control. The study is comprised of a detailed geological survey, a structural investigation and a remote-Asensing analysis (Ortho photos). Through accurate fieldwork I was able to draw up a detailed geological map (in:15,000!scale) and a geological profile of the interested area with the use of geographic information system software (ArcGIS); also during this fieldwork I set up three structural stations whose data allowed me to draw rose diagrams.The second part of my study comprised by the telemetric survey of the area allowed me to draw up another rose diagram to compare to the three diagrams previously obtained on the ground. According to this study, there is a possible structural conditioning of mask conduits where water hammer could have accused and on the niche of the Fadalto Landslid

Studio preliminare del contesto strutturale dei boati del Fadalto

This study looks in detail at the Lapisina Valley and specifically the residential areas of Fadalto and Farra d’Alpago, which have been subject to rumbling. This area is interesting from a geo-tectonical point of view, as it presents various thrust, in particular the “Longhere-Fadalto-Cadola” line which runs through the area analyzed in my study. Due to these perceived rumbles in the early part of 2011 the OGS (Istituto nazionale di Oceanografia e Geofisica Sperimentale) decided to carry out an! investigation through the use of portable seismometers, which led them to discover relatively shallow microAearthquakes. Furthermore they formulated a hypothesis that these seismic events were responsible for the rumbles!and caused by a phenomenon known!as “Water Hammer”, where a liquid or even a gas is forced to change its natural course and crash into a subterranean rock wall. This thesis is!aimed to define whether these “Water Hammer” events have a structural control. The study is comprised of a detailed geological survey, a structural investigation and a remote-Asensing analysis (Ortho photos). Through accurate fieldwork I was able to draw up a detailed geological map (in:15,000!scale) and a geological profile of the interested area with the use of geographic information system software (ArcGIS); also during this fieldwork I set up three structural stations whose data allowed me to draw rose diagrams.The second part of my study comprised by the telemetric survey of the area allowed me to draw up another rose diagram to compare to the three diagrams previously obtained on the ground. According to this study, there is a possible structural conditioning of mask conduits where water hammer could have accused and on the niche of the Fadalto Landslideope