Brittle Shear Tectonics in a Narrow Continental Rift: Asymmetric Nonvolcanic Barmer Basin (Rajasthan, India)

摘要

Our field studies emphasizing brittle shear P- and Y-planes along the margins of the Barmer basin (Rajasthan, India) support its two-phase (NW-SE, followed by NE-SW) extension during Early Cretaceous and Late Cretaceous-Paleocene periods. We also document nearly NE-trending megascale transfer zones along the northern margin of the Barmer basin. Preexisting brittle planes in the Malani basement rocks guided the relay structures here. Structures at the western basin shoulder margin indicate NE-SW extension, and the crosscut relation connotes the relative timing of the two extension phases. The crosscutting conjugate fault sets are non-Andersonian. The NW-trending faults produced by the second-phase extension and the inherited NNW-trending brittle features are dominantly dip-slip. Prior fractures of the Malani rocks at 45 degrees to the NE-SW principal extension direction extended the Barmer basin obliquely during the Late Cretaceous-Paleocene period. The asymmetric nature of the rift, too, connotes its oblique rifting. The extension direction of the first phase probably rotated clockwise. This is derived mainly from WSW-trending faults cutting NE-trending faults. Brittle planes of shear and fracture significantly promoted fluid flow, as understood from secondary hydrothermal mineral deposits (Barmer hill area) and pre-Deccan basalts (Sarnoo area). Reverse slip detected along subvertical faults on the western and eastern rift shoulders are probably due to isostatic flexure-related contraction or might be related to the far-field effect of ridge-push forces. The Mesozoic subsurface stratigraphy there and elsewhere within the Barmer basin requires more study to substantiate the potential for structural entrapment of hydrocarbon.