ROLE OF REACTIVATED BASEMENT FAULTS IN LOCALIZING DEFORMATION IN THE UPPER CRUST: INSIGHTS FROM CENTRIFUGE ANALOGUE MODELLING

Recent studies document along-strike variations in deep crustal density, seismicity, and convergence rates along the Himalayan arc. Some of these variations have been linked to the pre-orogenic configuration of the colliding plates. Interpretation of enhanced gravity data highlights northeast-striking inherited faults in the crust of northern India underplating the Himalaya. These faults coincide with the location of subsurface horst of late Archean to early Proterozoic granitic and gneissic basement rocks continuing beneath the east-west Himalayan orogen. We use dynamically scaled centrifuge analogue models to investigate the reactivation of these faults in the subducting Indian crust during the collision of India and Asia in the Paleogene and early Neogene (ca. 50 Ma-15 Ma) to elucidate their role in localizing deformation in the upper crust.

Models based on the Himalayan prototype consist from bottom to top of a rigid basement material, a ductile mid-crust, a layered package representing the upper crust, and a surface brittle layer. Models are shortened at accelerations of 1,000 g and analysed using computed tomography scanning to provide a 3D view of the system. Models show localization of thrust and strike-slip faults in the upper brittle crust above the reactivated basement faults during early stage collision (ca. 40 Ma). Strain partitioning across the basement fault manifests in the upper crust as offset folds, tear faults, and wrench faults, and in the ductile mid-crust as post-depositional thickness variations. Initiation of east-west extension in the models, simulating the prototype during later stage collision (ca. 15 Ma), results in the development of relay faults and extensional structures striking perpendicular to the orogen. Results suggest inherited basement faults control, at least in part, fault patterns in the foreland basin developed during initial shortening, and structures in the Himalayan hinterland developed during later stage east-west extension. We propose that the orogen-perpendicular extensional structures represent upper crustal graben faults in southern Tibet and that they are spatially related to the deep-seated faults. The analogue models, though based on northern India and the Himalaya, can be used as examples for thick-skinned orogenic systems worldwide.