a b s t r a c t Structural studies of the Barmer Basin in Rajasthan, northwest India, demonstrate the important effect that pre-existing faults can have on the geometries of evolving fault systems at both the outcrop and basin-scale. Outcrop exposures on opposing rift margins reveal two distinct, non-coaxial extensional events. On the eastern rift margin northwest-southeast extension was accommodated on southwest and west-striking faults that form a complex, zig-zag fault network. On the western rift margin northeast-southwest extension was accommodated on northwest-striking faults that form classical extensional geometries.Combining these outcrop studies with subsurface interpretations demonstrates that northwest-southeast extension preceded northeast-southwest extension. Structures active during the early, previously unrecognised extensional event were variably incorporated into the evolving fault systems during the second. In the study area, an inherited rift-oblique fault transferred extension from the rift margin to a mid-rift fault, rather than linking rift margin fault systems directly. The resultant rift margin accommodation structure has important implications for early sediment routing and depocentre evolution, as well as wider reaching implications for the evolution of the rift basin and West Indian Rift System. The discovery of early rifting in the Barmer Basin supports that extension along the West Indian Rift System was long-lived, multi-event, and likely resulted from far-field plate reorganisations.

The Barmer Basin is a poorly understood rift basin in Rajasthan, northwest India. Exposures in the Sarnoo Hills, situated along the central eastern rift margin of the Barmer Basin, reveal a sedimentary succession that accumulated prior to the main Barmer Basin rift event, and a rift-oblique fault network that displays unusual geometries and characteristics. Here, we present a comprehensive study of Lower Cretaceous sedimentology on the basin margin, along with a detailed investigation of riftoblique faults that are exposed nowhere else in the region and provide critical insights into Barmer Basin evolution. Lower Cretaceous sediments were deposited within a rapidly subsiding alluvial plain fluvial system. Subsequent to deposition, the evolving Sarnoo Hills fault network was affected by structural inheritance during an early, previously unrecognised, rift-oblique extensional event attributed to transtension between India and Madagascar, and formed a juvenile fault network within the immediate rift-margin footwall. Ghaggar-Hakra Formation deposition may have been triggered by early rifting which tectonically destabilised the Marwar Craton prior to the main northeast-southwest Barmer Basin rift event. The identification of early rifting in the Barmer Basin demonstrates that regional extension and the associated rift systems were established throughout northwest India prior to the main phase of Deccan eruptions. Inheritance of early oblique fault systems within the evolving Barmer Basin provides a robust explanation for poorly understood structural complications interpreted in the subsurface throughout the rift. Critically, the presence of syn-rift sedimentary successions within older oblique rift systems obscured beneath the present-day Barmer Basin has significant implications for hydrocarbon exploration. INTRODUCTIONDespite being a significant structure within the West Indian Rift System (Fig. 1a), and an important hydrocarbon province (Dolson et al., 2015), the structural geology and evolution of the Barmer Basin rift remains relatively poorly understood. The current understanding of rift evolution is based predominantly upon subsurface data sets, where an abundance of complex structures and rift-oblique faults are variably imaged on seismic data throughout the rift (Bladon et al., 2015; Fig. 1b). However, the origin of these structures remains elusive, and limited exposure of the structure and stratigraphy of the rift at outcrop has restricted outcrop-based studies. This work characterises a rift-oblique fault network that is apparent nowhere else in the region, and is exposed on the eastern rift margin of the Barmer Basin, in the Sarnoo Hills. Detailed sedimentological and structural analyses are used to clarify the origin of similar rift-oblique faults imaged in the Barmer Basin subsurface, and subsequently to place the findings within the wider context of northwest India. Sediments were deposited in a maturing fluvial system within a rapidly subsiding continental alluvial plain environment. The expos...

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