Concurrent Deformation in the Longmen Shan and the Sichuan Basin: A Critical Wedge Captured by Limit Analysis

Abstract

The southern Longmen Shan region has seismically active faults in its western part, the Beichuan fault (BCF) and the Pengguan fault (PGF), both rooting on a deep décollement, and has flat topography in its eastern part that marks the current deformation front due to thrusting from a shallow décollement. The concurrent faulting at the rear and at the front is separated by the flat Sichuan Basin. The objective of this contribution is to propose a mechanical approach based on the limit analysis theory to estimate the frictional properties that could explain how this heterogeneous wedge could be critical in the sense that faulting occurs at various places concurrently. A simple prototype with bilinear topography is first proposed to construct analytical solutions. The activations of the BCF, the PGF, and of the upper décollement up to the deformation front are examples of collapse mechanisms captured analytically by the limit analysis. Deformation maps are constructed in the space of the friction angles of the upper and lower décollements, identifying the domain of activation of the various collapse mechanisms. The critical conditions in terms of friction angles for the concomitant activation of these various collapse mechanisms are established. A more realistic prototype is then analyzed with the numerical code OptumG2, which is also based on the limit analysis theory. The deformation maps established for the first prototype are then applied to evaluate the critical conditions of the complex prototype with a limited number of numerical simulations.

Structural Significance of the Mid-level Décollement Within the Western Sichuan Fold-And-Thrust Belt (WSFTB), Insights From Sandbox Modeling

“…There is no doubt that the presence of décollements controlled the first-order style of the WSFTB. Their role in shaping fold-andthrust belt ahead of the Longmen Shan has been tested by both physical and numerical modeling (Zhang et al, 2019;Cui et al, 2020;Fan et al, 2020), as well as by analytical solution (Hubbard et al, 2010;Zhang et al, 2018). One remarkable phenomenon associated with the décollements is the nucleation of Longquan anticline that extends far into the basin (Figure 1).…”

Influence of Surface Processes on Strain Localization and Seismic Activity in the Longmen Shan Fold‐and‐Thrust Belt: Insights From Discrete‐Element Modeling

“…Previous studies investigated the relationship between the detachment fault strength and topography relief in the Longmen Shan range fronts and adjacent foreland basin using critical wedge taper theory, sandbox physical modeling, and numerical simulation (Cui et al., 2020; Hubbard et al., 2010; Sun et al., 2016; Z. Zhang et al., 2018). These studies reveal that the mechanical strength differences of basal and shallow detachments control the crustal shortening and topography of the Longmen Shan range front and WSFB.…”

“…Previous studies investigated the relationship between the detachment fault strength and topography relief in the Longmen Shan range fronts and adjacent foreland basin using critical wedge taper theory, sandbox physical modeling, and numerical simulation (Cui et al, 2020;Hubbard et al, 2010;Sun et al, 2016;Z. Zhang et al, 2018).…”