Strain partitioning and localization due to heterogeneities in the fold - and - thrust belt 2 detachment: Analogue models of progressive arcs

Although most arcuate orogens are deformed as progressive arcs–curvature is acquired during12shortening-, they have been scarcely simulated by analogue modelling. Our team work designed13a backstop that deformed in map view building up fold-and-thrust belts (FTBs) that acquire14progressively its curved shape (Jiménez-Bonilla et al., 2020); silicone and sand layers15reproduced the brittle-ductile conditions expected to be common in external tectonic wedges.16However, natural cases usually includeheterogeneities in the detachment such as diapirs,17thickness variations of the viscous layer or pinch outs. Based on the same progressive arc model18setup, we present here seven new experiments including these three types of heterogeneities.19Our results show that strain was partitioned between shortening structures whose transport20directions draw a radial pattern and normal faults and oblique strike-slip faults that21accommodate arc-lengthening. Moreover, any heterogeneity conditions the wedge evolution and22the nucleation of structures. Both diapirs and the presence of a silicone pinch-out perpendicular23to the apex movement favour that frontal deformation slows down and the wedge thickens up to24reach the supercritical angle. Interestingly, the presence of diapirs or silicone thickness25variations favour the arc-parallel stretching localization close to these heterogeneities. In26addition, silicone bands parallel to the apex movement generate different structural styles along27the FTB. More frictional detachments favour thicker wedges and less frontal propagation.28Transfer zones accommodate the differential displacement between FTB segments. These29results may be useful to investigate geometric and kinematic changes along natural progressive30arcs such as the Gibraltar, Sulaiman and Zagros cases