Himalayan hinterland-verging superstructure folds related to foreland-directed infrastructure ductile flow: Insights from centrifuge analogue modelling

The orogenic superstructure (SS) and infrastructure (IS) constitute two levels of a mountain belt with contrasting structural styles. In the Nepal Himalaya, N-verging back folds, which oppose the orogenic vergence, dominate the SS. Competing explanations for these folds are tested using centrifuge analogue models. Modelling suggests that SS folding occurs during bulk shortening accompanied by IS thickening before IS flow. Focused erosion then instigates IS lateral flow and stretching, decoupling of the SS, and transposition of the lower SS into a detachment zone. Decoupling at the IS–SS interface separates an SS dominated by older folds and an IS characterised by younger horizontal transposition and stretching of early folds. Extrusive ductile flow of the IS locally modifies fold vergence in the SS. The fold asymmetry is thus controlled by the efficiency of coupling between IS and SS; a low viscosity at the IS–SS interface favours complete decoupling and hinders modification of fold vergence, whereas a higher viscosity IS-SS interface favours fold vergence modification. Modelling supports a tectonic scenario in which Himalayan hinterland-verging folds are the product of early shortening of the SS followed by local modification of fold geometry when the IS subsequently stretches and flows during focused erosion and melt-enhanced IS weakening.