Geological setting

The eastern Arabian Sea presents an intriguing case for the study of continental break-up, differing in important ways from both the classic nonvolcanic Iberia-Newfoundland conjugate (e.g., Boillot et al., 1995) and the volcanic Norway-Greenland sets (e.g., Skogseid et al., 2000). Similar to the North Atlantic, the northern Arabian Sea is characterized by the presence of a large continental block located between the western continental shelf of India and deep seafloor in the Arabian Basin. This block is known as Laxmi Ridge, which is separated from the western Indian margin proper by Laxmi Basin. The oldest seafloor spreading–related magnetic anomalies are Anomaly 27n (62.2–62.5 Ma) and 28n (63.5–64.7 Ma) located south of Laxmi Ridge in the Arabian Basin and north of the Seychelles, respectively (Chaubey et al., 2002a; Royer et al., 2002). Previous studies using magnetic anomalies describe the juxtaposition of India and the Seychelles immediately before the onset of extensional tectonics; however, most plate models for this region predict a wide, deep-water offshore region (Laxmi Basin and Offshore Indus Basin) of ~300 km width between the Seychelles and the Indian subcontinent before the onset of seafloor spreading between Laxmi Ridge and the Seychelles in the Paleocene. Numerous geophysical studies carried out to investigate the nature of crust in the deep Laxmi Basin remain inconclusive, with some authors favoring the presence of rifted continental crust (Naini, 1980; Naini and Talwani, 1982; Kolla and Coumes, 1990; Miles and Roest, 1993; Miles et al., 1998; Radha Krishna et al., 2002; Krishna et al., 2006; Minshull et al., 2008), whereas others favor oceanic crust (Biswas and Singh, 1988; Bhattacharya et al., 1994; Malod et al., 1997; Talwani and Reif, 1998; Singh, 1999; Bernard and Munschy, 2000). Testing of these competing models requires direct sampling of rocks from the basement of Laxmi Ridge and Laxmi Basin.

Significant sediment cover overlies the basement of Laxmi Basin, with the oldest parts representing a postrift passive margin sequence. Since the onset of India-Eurasia collision, the Indus and its associated tributaries have been the primary drainage system for sedimentation in the Arabian Sea (Clift, 2002) and this Indus-derived sediment accounts for most of the sedimentary section we target here. Far lesser amounts are discharged from small rivers on the steep western margin of India and from the Narmada River. Much of the present Indus discharge represents run-off during the summer monsoon rains, enhanced with the seasonal melting of Karakoram and Himalayan glaciers. The Indus Fan covers more than one million square kilometers, stretching 1500 km into the Arabian Sea from the present delta front. It is the second largest submarine fan in the world and is >10 km thick at the northernmost part (Clift et al., 2001). As the proto-Indus Fan prograded southward in the late Oligocene, characteristic sediment eroded from the Indus drainage began to accumulate on the distal parts, as observed at Deep Sea Drilling Project (DSDP) Site 221 (Kolla and Coumes, 1987). The Indus appears to have experienced major drainage capture during the Miocene (Clift and Blusztajn, 2005) but has otherwise been stable within the Indus Suture and western syntaxis of the Himalaya since the Eocene. Drilling can provide erosion records through analyses of the sediment cores, as well as by providing age control for regional seismic stratigraphy. It is only by quantifying the volume of sediment deposited in the fan that we will be able to mass balance the volume eroded from the mountains as constrained by thermochronology with the volume deposited in the offshore.