Synchronous and velocity‐partitioned thrusting and thrust polarity reversal in experimentally produced, doubly‐vergent thrust wedges: Implications for natural orogens

Abstract

Abstract. A sandbox analog modeling research program was used to study the detailed evolution of doubly-vergent thrust wedges. High-resolution multilayers of thin alternating sand and mica layers were used in contractional Coulomb wedge experiments to simulate deformation of anisotropic, brittle upper crustal strata in doubly-vergent orogens. Experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carded out. Our laboratory models evolved in two main stages: (1) initial high-velocity thrusting in the retrowedge and high-frequency together with low displacement folding and thrusting in the prowedge; and (2) low-frequency, high-displacement synchronous thrusting in the prowedge and low-velocity thrusting in the retrowedge. Transition from stage I to stage II occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion. Addition of syntectonic sediments increased the persistence of stage I and triggered out-of-sequence thrusting in the axial zone of the experimental orogens. Thrust motion was stick-slip. Retrovergent thrusting occurred along a long-lived ramp whose lower tip was located at the subduction slot. Provergent kink bands nucleated at the subduction slot in stage I. In contrast, during the second stage of wedge evolution, kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high-displacement faults that remained active up until the end of the experiments, at progressively decreasing rates of thrusting. The axial zones of the model wedges were characterized by fast uplift rates during stage I due to backward translation of the belt along the retrovergent, longlived ramp and due to the localization of deformation close to the subduction slot. Outward migration of the deformation front in the prowedge region during stage II caused the progressive decrease in the rate of the wedge uplift, until it eventually stopped. Analytical models to quantify the Coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening, triggered by the buildup of the topographic load during deformation. This thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant

The numerical sandbox: comparison of model results for a shortening and an extension experiment

“…Convergent thrust wedges have been studied with analytical techniques (Davis et al 1983;Dahlen 1984), analogue experiments (e.g., Mulugeta 1988;Gutscher et al 1998a;Storti et al 2000) and numerical models (e.g., Strayer et al 2001;Burbridge & Braun 2002). Similarly, brittleviscous extension has been investigated with analogue (e.g., Michon & Merle 2000;Bahroudi et al 2003) and numerical (e.g., Behn et al 2002;Wijns et al 2003) methods.…”

Analogue benchmarks of shortening and extension experiments

“…Numerous studies document experiments of thrust wedges under normal gravity conditions (e.g., Davis et al 1983;Malavieille 1984;Mulugeta & Koyi 1987, 1992Mulugeta 1988;Colletta et al 1991;Lallemand et al 1994;Koyi 1995;Storti & McClay 1995;Mugnier et al 1997;Gutscher et al 1996Gutscher et al , 1998Storti et al 2000;Schreurs et al 2001;Turrini et al 2001;Kukowski et al 2002;Costa et al 2004;Hoth et al 2005). Deformation in these experiments occurs either by displacing a back wall or by pulling a basal sheet below a fixed wall.…”

Late Paleozoic low-angle southward-dipping thrust in the Züünharaa area, Mongolia: tectonic implications for the geological structures in the Sayan-Baikal and Hangai-Daur belts

“…Simultaneously formed "doubly vergent (or bivergent; e.g., Willett et al 1993;Storti et al 2000;Naylor and Sinclair 2007;Mukherjee 2013b;Bose and Mukherjee 2015) asymmetric structures", similar to those present in the HD and SB belt, have been illustrated in the Alps, the Andes, and in other locations. For instance, the Taranaki Fault in the northern part of New Zealand is considered to be a back thrust antithetic to the Hikurangi margin subduction thrusts.…”