Geology and Geophysics of the Arabian Sea Region

The Arabian Sea region is an area of dramatic current and past tectonic and climatic activity. It may be considered the world type area for studying tectonic-climatic interactions related to orogenic uplift, this case the uplift of the Tibetan Plateau and the onset of the SW Monsoon. The Indus Fan drains the high topography of western Tibet, the Karakoram, and Himalaya, whose development is proposed to have initiated the SW Monsoon. [See related articles in Eos and Geoscientist]. The sedimentary record of the Arabian Sea should reflect both orogenic processes as well as changes in the ocean and atmospheric circulation.

Currently there are several IODP proposals to sample the sediments of the region. The goals of these projects are (1) to understand the millenial scale variations of the monsoon during Pleistocene-Recent times, (2) to reconstruct the erosion response of the western Himalaya to proposed monsoon strengthening at 8 Ma, and (3) to date the timing of Indus Fan initiation and subsequent erosion during the Paleogene through drilling of the Murray Ridge. In order to allow this work to occur geophysical surveying of the region is currently occurring and being planned. The Joint Oceanographic Institutions have supported mapping of the stratigraphy in the northern Arabian Sea to allow planning for a deep drill site in the Indus Fan. Data collected by the BGR Hannover, Germany is crucial to the current efforts. Click here to see a map of the current data coverage. Age resolution of the stratigraphy is limited by the lack of drilling data on the deep fan, but is sufficient to allow resolution of the Pleistocene, Pliocene, Upper Miocene, Middle Miocene, Lower Miocene, and Paleogene.

The goals of the drilling and other projects were the focus of a meeting held in April 2001 at the Geological Society of London. A subsequent special publication of that society (Number 195) has synthesized data from the diverse subdisciplines that are examining this region in order to better understand how the system as a whole operates. This meeting addressed not only marine geology and geophysics, but also the development of the onshore areas surrounding the sea. These regions have a profound effect on the development of the oceanic crust and continental margin, controlling both the onset of upwelling along the Oman Margin, the sedimentation on the Indus Fan, and the tectonic uplift of the Murray and Owen Ridges. The special publication will also be of interest to the hydrocarbon industry, which is currently exploring in the shelf areas of Pakistan and India and is seeking opportunities in the deep water slope areas of this exciting and relatively unexplored rifted volcanic margin. Click here to see the table of contents of the special publication.

The Joint Oceanographic Institutions are now supporting a planning meeting for 23-25th July 2003 to be held in Boulder, CO, convened by Peter Clift and Peter Molnar (U. Colorado) to formulate coherent drilling objectives and strategies for the Indian Ocean submarine fans. The objective of the meeting is to integrate the goals of the existing proposals on the Bengal and Indus Fans into a coherent program of deep drilling within the context of IODP, involving both riser and non-riser drilling. Interested parties should contact the convenors and visit the meeting web page.

Geophysical and geological investigations of the Arabian Sea continue using existing data and samples. In a recent development I have depth-converted the interpreted seismic profiles published by Clift et al. (2001) using the stacking velocities derived from the processing and from nearby LDEO sonobuoy data in order to generate a true depth section, which runs parallel to the Murray Ridge offshore. This section was then backstripped using the FLEX-DECOMP program of Alan Roberts and Nick Kusznir in order to generate a sediment budget for the fan that verifies the 1-D backstripping result of Clift & Gaedicke (2002). In this analysis the peak sedimentation in the Middle Miocene is confirmed.
In addition, by fully removing the sedimentary cover of the margin the original unloaded basement structure is revealed. This analysis shows that the shelf basement has a sharp deepening similar to that seen at other rifted volcanic margins, e.g., East Greenland. This allows the continent-ocean transition to be placed much closer to the coast than previously believed. This in turn implies that the Murray Ridge is underlain by oceanic crust. The subduction character of the volcanics analyzed by Burgath et al. (2002) must reflect contamination of the rift and earliest MOR volcanism during break-up rather than the presence of older continental basement in this area, as these authors suggest. The simple observation that the depth of the basement oceanward of the escarpment lies close to that of oceanic crust of that age argues strongly that this crust is oceanic, not continental.