The role of sediment subduction and buoyancy on subduction dynamics and geometry

Subducted sediments are thought to lubricate the subduction interface and promote faster plate speeds. However, global observations are not clear-cut on the relationship between the amount of sediments and plate motion. Sediments are also thought to influence slab dip, but variations in subduction geometry depend on multiple factors. Here we use 2D thermomechanical models to explore how sediments can influence subduction dynamics and geometry. We find that thick sediments can lead to slower subduction due to an increase of the megathrust shear stress as the accretionary wedge gets wider, and a decrease in slab pull as buoyant sediments are subducted. Our results also show that larger slab buoyancy and megathrust stress due to thick sediments increase the slab bending radius. This offers a new perspective on the role of sediments, suggesting that sediment buoyancy and wedge geometry also play an important role on large-scale subduction dynamics.

Load distribution in concrete solid slab bridges

Canadian Highway Bridge Design Code (CHBDC) specifies empirical equations for the moment and shear distribution factors for selected bridge configurations. These empirical equations were based on the orthotropic plate theory with equivalent slab bending and torsional rigidity. Also, they were based on analysis procedure and CHBDC truck loading condition slightly different from those specified in the current CHBDC code of 2006. In this study, a parametric study was conducted, using the finite-element modeling to determine the moment and shear distribution factors for solid slab bridges subjected to CHBDC truck loading. Shell elements were used to model the bridge deck slab supported over bearings on each side of the bridge at 1.2 m spacing. The results from the parametric study were correlated to those available in the CHBDC code. Results show considerable difference in FEA results and CHBDS equations, especially for shear distribution factors. This project provides research results that can be used further to develop more reliable expressions for moment and shear distribution factors for solid slab bridges.

Load distribution in concrete solid slab bridges

Canadian Highway Bridge Design Code (CHBDC) specifies empirical equations for the moment and shear distribution factors for selected bridge configurations. These empirical equations were based on the orthotropic plate theory with equivalent slab bending and torsional rigidity. Also, they were based on analysis procedure and CHBDC truck loading condition slightly different from those specified in the current CHBDC code of 2006. In this study, a parametric study was conducted, using the finite-element modeling to determine the moment and shear distribution factors for solid slab bridges subjected to CHBDC truck loading. Shell elements were used to model the bridge deck slab supported over bearings on each side of the bridge at 1.2 m spacing. The results from the parametric study were correlated to those available in the CHBDC code. Results show considerable difference in FEA results and CHBDS equations, especially for shear distribution factors. This project provides research results that can be used further to develop more reliable expressions for moment and shear distribution factors for solid slab bridges.

From sub-Rayleigh to intersonic crack propagation in snow slab avalanche release

<p>Snow slab avalanche release can be separated in four distinct phases : (i) failure initiation in a weak snow layer buried below a cohesive snow slab, (ii) the onset and, (iii) dynamic phase of crack propagation within the weak layer across the slope and (iv) the slab release. The highly porous character of the weak layer implies volumetric collapse during failure which leads to the closure of crack faces followed by the onset of frictional contact. To better understand the mechanisms of dynamic crack propagation, we performed numerical simulations, snow fracture experiments, and analyzed the release of full scale avalanches. Simulations of crack propagation are based on the Material Point Method (MPM) and finite strain elastoplasticity. Experiments consist of the so-called Propagation Saw Test (PST). Concerning full scale measurements, an algorithm is applied to detect changes in image pixel intensity induced by slab displacements. We report the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation following the Burridge-Andrews mechanism. In detail, after reaching the critical crack length, self-propagation starts in a sub-Rayleigh regime and is driven by slab bending induced by weak layer collapse. If the slope angle is larger than a critical value, and if a so-called super critical crack length is reached, supershear crack propagation occurs. The corresponding critical angle may be lower than the weak layer friction angle due to the loss of frictional resistance during volumetric collapse. The sub-Rayleigh regime is driven by mixed mode anticrack propagation while the supershear regime corresponds to a pure mode II propagation with intersonic crack speeds (v: crack speed, c<sub>s</sub>: shear wave speed, c<sub>p</sub>: longitudinal wave speed, E: slab Young's modulus and ρ: slab density). This intersonic regime of crack propagation thus leads to pure tensile slab fractures initiating from the bottom of the slab as opposed to top initiations induced by slab bending in the sub-Rayleigh regime. Key ingredients for the existence of this transition are discussed such as the role played by friction angle, collapse height and slab secondary fractures. </p>

Bending curvatures of subducting plates: old versus young slabs

SUMMARY By combining scaled laboratory experiments and numerical simulations, this study presents a quantitative analysis of the bending radius (RB) of subducting slabs within the upper mantle, taking into account the effects of age (A). Based on a half-space cooling model, we constrain the density (ρ), viscosity (η) and thickness (h) of slabs as a function of A, and develop representative models to estimate RB for different A. Laboratory subduction models produce visually contrasting bending curvatures for young (A = 10 Ma), intermediate (A = 70 Ma) and old (A = 120 Ma) slabs. Young slabs undergo rollback, resulting in a small bending radius (scaled up RB ∼ 150 km), whereas old slabs subduct along a uniformly dipping trajectory with large bending radius (RB ∼ 500 km). Equivalent real scale computational fluid dynamic simulations reproduce similar bending patterns of the subducting slabs, and yield RB versus A relations fairly in agreement with the laboratory results. We balance the buoyancy driven bending, flexural-resistive moments and viscous flow induced suction moment to theoretically evaluate the rate of slab bending. The analytical solution suggests an inverse relation of the bending rate with A, which supports our experimental findings. Finally, slab geometries of selected natural subduction zones, derived from high-resolution seismic tomographic images have been compiled to validate the experimental RB versus A regression. We also discuss the subduction settings in which this regression no longer remains valid.

A closer look at the relationship between slab (un)bending and double seismic zone seismicity

<p>The origin of double seismic zones (DSZs), parallel planes of intraslab seismicity observed in many subduction zones around the globe, is still highly debated. While most researchers assume that fluid release from prograde metamorphic reactions in the slab is an important control on DSZ occurrence, the role of slab unbending is currently unclear.<br>Slab bending at the outer rise is instrumental in hydrating the downgoing oceanic plate through bend faulting, and is evident from earthquake focal mechanisms (prevalence of shallow normal faulting events). Observations from NE Japan show that focal mechanisms of DSZ earthquakes are downdip compressive in the upper and downdip extensive in the lower plane of the DSZ, which strongly hints at slab unbending. This coincidence of slab unbending and DSZ seismicity in NE Japan has given rise to several models in which unbending forces are a prerequisite for DSZ occurrence.</p><p>To globally test a potential correlation of slab unbending with DSZ seismicity, we derived downdip slab surface curvatures on trench-perpendicular profiles every 50 km along all major oceanic slabs using the slab2 grids of slab surface depth. We here make a steady-state assumption, i.e. we assume that the slab geometry is relatively constant with time, so that the downdip gradient of slab curvature corresponds to slab (un)bending. We compiled the loci and depth extent of all DSZ observations avalable in literature, and compare these to the slab bending or unbending estimates.</p><p>Preliminary results indicate that while there is a clear correspondence between the depth of slab unbending to DSZ seismicity in the Japan-Kurile slab, most other slabs do not show this correlation. Moreover, some DSZs deviate from the above-mentioned focal mechanism pattern and exhibit downdip extension in both planes (e.g. Northern Chile, New Zealand). It appears that the global variability of slab geometries in the depth range 50-200 km is larger than anticipated, and DSZ seismicity is not limited to slabs where unbending is prevalent at these depths. The Northern Chile case is especially interesting because focal mechanisms there not only do not fit the pattern observed in NE Japan, but also can not be explained with the current slab geometry alone. This could indicate a direct influence of ongoing metamorphic reactions on focal mechanisms (e.g. via volume reduction and densification), or it may be a hint that our steady-state assumption is invalid for the Nazca slab here (i.e. that it is in the process of changing its geometry).</p>

Rupture characteristics of the 2019 North Peru intraslab earthquake (Mw8.0)

<p>According to GlobalCMT, the 2019/05/26 North Peru earthquake is the largest event since 1976 in the wide depth range between 70km and 550km. Its hypocentral location (at about 130km depth) inside the Nazca slab geometry, together with its normal focal mechanism, favor an origin related to slab bending. Owing to its magnitude and depth, this earthquake generated large coseismic displacements over a broad area, that were geodetically measured by InSAR and GNSS. By combining these observations with regional and teleseismic data, we invert for the rupture process of the event, and first focus on the actual focal plane. Inversion reveals that the steeper plane (dipping 55-60° to the East) is preferred. A clear northward propagation is also imaged, with rupture traveling ~200km in 60s, and with little extent in the dip direction. This narrow rupture aspect implies that the stress drop is significant, even if a simple duration-based measurement would not indicate so. These inversion results obtained at relatively low frequency (below 0.2Hz) are then thoroughly compared with back-propagation images obtained at higher frequency (at 0.5-4Hz), which also highlight the dominantly northward rupture propagation with an average rupture speed of about 3 km/s. Implication in terms of earthquake rupture dynamics and occurrence of such large intermediate depth earthquakes in slabs will finally be discussed.<br>    </p>

Late Orogenic Heating: Slab Breakoff or Slab Rollback?

<p>High- to ultrahigh pressure rocks ((U)HP) from some collisional orogens bear evidences of post collisional heating recorded by a β-shaped pressure–temperature–time (P–T–t) path. The post peak pressure heating segment of the P–T–t path, which can be well developed such as in the Bohemian Massif of the Variscan orogenic belt, occurs after the (U)HP rocks are exhumated from mantle depths to various crustal levels. This process is often explained by geologists as a result of mantle delamination or slab breakoff. Based on a two-dimensional coupled petrological–thermomechanical tectono-magmatic numerical model, we demonstrate that slab rollback during ongoing continental subduction can be considered as a possible mechanism responsible for the effective extraction of (ultra)high pressure metamorphic rocks and their later heating. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not directly related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust.</p>

Late Orogenic Heating of (Ultra)High Pressure Rocks: Slab Rollback vs. Slab Breakoff

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.