Role of reactivated basement faults in localizing deformation in the upper crust: Insights from centrifuge analogue modelling
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Dynamically scaled centrifuge analogue models are used to investigate recent interpretation of enhanced gravity data, which highlights northeast-striking inherited faults in the crust of northern India. The reactivation of these faults in the subducting Indian crust during the collision of India and Asia in the Paleogene (ca. 45–40 Ma) and early Neogene (ca. 20–15 Ma) is investigated to elucidate their potential role in localizing deformation in the upper crust. Models consist from bottom to top of a rigid basement material, a ductile middle, a layered package representing the upper crust, and a near-surface brittle layer. Viscoelastic materials used as analogues for the mid- and lower crust demonstrate complex dynamic behaviour; effective viscosity alone is not sufficient to determine if a material will be an effective analogue. Two series of rheological experiments are conducted using an oscillating parallel-plate rheometer to measure the storage and loss moduli and effective viscosities of potential materials. Inherently cohesive synthetic sands are suggested as suitable analogues for the brittle upper crust, historically not reproduced in centrifuge models. In models simulating early stage collision (ca. 10 Myr after onset), deformation is dominated by left-lateral strike-slip deformation along the inherited basement fault. This manifests in the upper brittle layer as three major sets of faults striking northwest–southeast (the same orientation as the basement fault), northeast–southwest, and east–west. Extension is accommodated in the northeast–southwest striking faults. In models simulating later stage collision (ca. 35 Myr after onset), deformation is dominated by shortening in the upper crust and two sets of folds develop. Strike-slip offset along the basement fault manifests in the ductile middle layer as lateral flow and the development of necking zones. These necking zones are interpreted to be discreet zones of strain transfer to the semi-ductile and brittle upper layers, localizing normal faults above. Three fault sets develop in the same orientation as the early stage collision models. We propose that the normal faults stepping en échelon along the basement fault are analogous to south Tibet graben in the prototype. These models, though based on the Himalaya, can be used as examples for thick-skinned orogenic systems worldwide.