Deformation of brittle‐ductile thrust wedges in experiments and nature

Abstract

[1] Even though the rheology of thrust wedges is mostly frictional, a basal ductile decollement is often involved. By comparison with purely frictional wedges, such brittleductile wedges generally display anomalous structures such as backward vergence, widely spaced thrust units, and nonfrontward sequences of thrust development. Laboratory experiments are used here to study the deformation of brittle-ductile thrust wedges. Results are compared with natural systems in the Jura Mountains and the northern Pakistan Salt Range and Potwar Plateau. Two series of three models are used to illustrate the effects of varying the basal wedge angle (b) and shortening rate (V). These two parameters directly control variations in relative strength between brittle and ductile layers (BD coupling). Wedges with strong BD coupling (low b and high V) give almost regular frontward sequences with closely spaced thrust units and, as such, are not significantly different from purely frictional wedges. Weak BD coupling (high b and low V) gives dominantly backward thrusting sequences. Intermediate BD coupling produces frontwardbackward oscillating sequences. The spacing of thrust units increases as coupling decreases. Back thrusts develop in parts of a wedge where BD coupling is weak, regardless of the thrust sequence. Wedges with weak BD coupling need large amounts of bulk shortening (more than 30%) to attain a state of equilibrium, at which stable sliding along the base occurs. On this basis, we argue that a state of equilibrium has not yet been attained in at least some parts of the Jura Mountains and eastern Salt Range and Potwar Plateau thrust systems.

Feedback between erosion and active deformation: geomorphic constraints from the frontal Jura fold-and-thrust belt (eastern France)

“…Regional Plio-Pleistocene uplift and erosion may have transferred the critically tapered wedge of the thin-skinned Jura fold-and-thrust belt that had stalled around the Early Pliocene into a sub-critical stage, initiating renewed out-of-sequence deformation within the fold-and-thrust belt. Analogue models support such a hypothesis (Smit et al 2002). However, the seismicity of the region (Baer et al 2005(Baer et al , 2007 implies that at present deformation in the RBTZ area is thick-skinned and also involves the underlying basement (Figs.…”

Effects of mass waste events on thrust wedges: Analogue experiments and application to the Makran accretionary wedge

“…The basal décollement favors coeval activity along several thrusts. The amount of shortening needed to form a wedge with a stable surface slope depends on the efficacy of the basal décollement [Smit et al, 2003]. The efficacy itself is dependant on the shear-stress ratio between brittle cover (B) and ductile décollement (D) and increases with increasing B/D stress ratio.…”

“…terms of movement direction, spacing of thrust faults and inand out-of-sequence faulting [Costa and Vendeville, 2002;Smit et al, 2003]. The basal décollement favors coeval activity along several thrusts.…”

“…Because the maximum shear stress in a brittle layer and the shear stresses in a ductile layer depend in opposite ways on layer thickness, the stress ratio of brittle to ductile layer varies strongly from back to toe in wedge shaped systems [e.g., Smit et al, 2003]. Such systems cannot be described only by a brittle wedge whose basal resistance is zero.…”

“…[24] Previous experimental studies have analyzed the effects of brittle-ductile coupling in terms of relative strength between the brittle and ductile layers in both extensional [Nalpas and Brun, 1993;Brun, 1999Brun, , 2002 and compression settings [Bonini, 2001;Smit et al, 2003]. This approach allows taking into account lateral variations in rheology and geometry of separate layers.…”