Comment on egusphere-2022-62

Abstract

Geological settings characterized by the simultaneous action of multiple tectonic regimes provide a unique opportunity to understand complex interactions among different geodynamic processes. From an experimental point of view, these contexts remain comparatively less studied than areas with more simple patterns of deformation resulting from primary plate-boundary interactions. Here, we carried out analog experiments involving simultaneous shortening and orthogonal extension under different rheological conditions, and including the effect of crustal inheritance. We performed brittle experiments and brittle-ductile experiments to simulate cases of “strong” and “weak” crusts, respectively. We present two types of experiments: i) one stage experiments with either shortening-only or synchronous orthogonal shortening and stretching, and ii) two stages experiments with a first phase of stretching and a second phase with either shortening-only or synchronous orthogonal shortening and stretching. In our models, deformation occurs by a combination of normal, thrust, and strike-slip faults with structures location depending on boundary conditions and crustal inheritance. For brittle models, we show that the three types of structures can develop at the same time for intermediate ratios of extrusion over indentation rates (1.4 < Ve/Vs < 2). For brittle-ductile models, we observe either shortening-orthogonal thrust faults associated with conjugate strike-slip faults (mod- els with low Ve/Vs and no initial extensional phase) or stretching-orthogonal normal faults associated with conjugate strike-slip faults (models with high Ve/Vs and initial extensional phase). Whatever the crustal strength, the past deformation history, and the extrusion/indentation ratio, both normal and thrust faults remain with similar orientations, i.e. stretching-orthogonal and shortening-orthogonal, respectively. Instead, strike-slip faults exhibit variable orientations with respect to the indentation direction, which may be indicative of the strength of the crust and/or of the extrusion/indentation ratio. We also show that extensional structures formed during a first stage of deformation are never inverted under orthogonal shortening but can be reactivated as normal or strike-slip faults depending on Ve/Vs. The models replicate some deformation patterns documented in nature. Independently of the crustal rheology or the presence of crustal weaknesses, conjugate strike-slips faults develop along with variable normal faulting during compression/indentation, reminiscent of tectonic escape processes along the Himalayas-Alpine chain.