Mechanics of Fold‐and‐Thrust Belts Based on Geomechanical Modeling

Abstract

We use a large strain geomechanical model and critical state soil mechanics to study the evolution of stress and deformation in an evolving fold-and-thrust belt and its underlying footwall sediments. Both mean effective stress and deviatoric stress contribute to porosity loss within the wedge with 35% of the porosity loss resulting from increased shear. As a result, porosity increases abruptly across the décollement because both mean-effective and shear stresses are much higher inside the wedge than in the footwall. As the basal friction coefficient (μ b ) increases, more shear stress is transmitted across the décollement, resulting in additional compaction of the footwall sediment and decrease in the porosity contrast across the décollement. As the internal friction coefficient (μ s ) increases, the wedge sediment is more compacted because it can withstand higher mean-effective and deviatoric stresses. Inside the wedge, the sediment experiences subhorizontal shortening strain and subvertical elongation strain. We predict a 10-30 km wide "transition zone" in which the shear-stress ratios and compaction curves change rapidly between compressional critical state failure and uniaxial strain (K 0 ) state. Our model results agree with the taper angles and the stress orientations predicted by critical taper theory. This large-strain drained modeling approach provides first-order insights into the mechanical processes of loading and compaction in fold-and-thrust belts and a foundation for understanding field observations of pressure, stress, and deformation in thrust belt systems.While critical taper theory provides insight into the relations among pore pressure, material strength, and wedge geometry, it does not address the mechanics of sediment deformation within and beneath the wedge. Sediments in the wedge experience great horizontal shortening and vertical thickening caused by lateral tectonic loading. As a result, systematic changes in porosity occur within and below wedges. For GAO ET AL. 4454

Coupled Evolution of Deformation, Pore Fluid Pressure, and Fluid Flow in Shallow Subduction Forearcs

“…Our current modeling approach suffices for evaluating key elements of the first‐order coupling between mechanical and hydrological processes at active convergent plate boundaries. Further improvements of the modeling method, to better account for additional mechanical and geological processes in driving deformation and sediment compaction, include (1) a quantitative description of shear‐induced fluid pressurization and shear‐enhanced sediment compaction (Flemings & Saffer, 2018; Gao et al, 2018; Nikolinakou et al, 2012), (2) a consideration of the contributions of metamorphic dehydration fluids both originating within the depth range of our model domain (Kastner et al, 1991; Moore & Vrolijk, 1992) and originating from greater depths (i.e., associated with thermally activated dehydration processes (van Keken et al, 2011) and fluid migration), and potentially (3) a more appropriate incorporation of elasticity for a model rheology that is more consistent with real‐world subduction forearcs. Consideration of the first two points would lead to an increase in the modeled fluid pressure and a resultant decrease in effective stress, although we expect that the systematic behaviors described in the current models would remain unchanged.…”

“…This formulation yields a rapid porosity reduction at shallow depths and low stresses, with the rate of reduction decreasing with depth and stress because of strain hardening (Figure 3a). In this approach, other sediment and soil mechanical and diagenetic processes, such as compaction enhancement caused by differential stress, cementation, or thermally driven consolidation (e.g., Flemings & Saffer, 2018; Gao et al, 2018), are not considered explicitly. For the more rigid subducting slab material, we use the same form as equation 2 but different choices of parameters ( σ ref and n 0 are set as 200 MPa and 0.17, respectively), yielding considerably lower values of loading efficiency (Figure 3b).…”

Predictability and controlling factors of overpressure in the North Alpine Foreland Basin, SE Germany: An interdisciplinary post-drill analysis of the Geretsried GEN-1 Deep Geothermal Well

“…resolved, yet. The study area is only a few kilometers away from the rst thrust front of the Eastern Alps and might therefore be in uenced by increased lateral stresses, which would in uence velocity-based and basin modelling-based pore pressure estimates (Gao et al 2018;Obradors-Prats et al 2017, 2016. Also, lateral pressure transfer (Lupa et al 2002;Yardley and Swarbrick 2000) could be a mechanism yielding either additional overpressure or even pressure regressions.…”

Modified Cam-Clay Model for Large Stress Ranges and its Predictions for Geological and Drilling Processes

“…The friction angle and the compression curve are two of the most critical rock parameters for many geological and hydrocarbon-production processes. Friction angle controls the geometry and activity of faults (Hubbert and Rubey, 1959;Suppe, 2007), the stability of earth slopes (Hubbert and Rubey, 1959;Sawyer et al, 2014;Stigall and Dugan, 2010), and the geometry of critical tapers such as in accretionary wedges and fold-and-thrust belts (Dahlen, 1990;Davis et al, 1983;Gao et al, 2018). Friction angle also impacts hydrocarbon production in different ways.…”