--> ABSTRACT: Nature of the Continent-Ocean Transition along the NW Palawan Continental Margin, South China Sea, by Franke, Dieter; Barckhausen, Udo; Baristeas, Niko; Engels, Martin; Ladage, Stefan; Lutz, Ruediger; Montano, Jennie; Pellejera, Nicole; Ramos, Emmanuel G.; Schnabel, Michael; #90135 (2011)

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Nature of the Continent-Ocean Transition along the NW Palawan Continental Margin, South China Sea

Franke, Dieter 1; Barckhausen, Udo 1; Baristeas, Niko 2; Engels, Martin 1; Ladage, Stefan 1; Lutz, Ruediger 1; Montano, Jennie 4; Pellejera, Nicole 4; Ramos, Emmanuel G.3; Schnabel, Michael 1
(1)B1.4, Federal Institute for Geosciences and Natural Resources, Hannover, Germany. (2) GFZ - Deutsches GeoForschungsZentrum, Potsdam, Germany. (3) DOE - Department of Energy, Continental Shelf Project, Fort Bonifacio, Taguig City, Philippines. (4) UP-NGIS - University of the Philippines, National Institute of Geological Sciences, College of Science, Diliman, Quezon City 1101, Philippines.

The South China Sea has become a classic area for examining rift and break-up tectonics. The oceanic basin formed by magma-poor rifting in the Paleogene. We investigate the rift to drift tectonics and crustal stretching at its southern margin, off NW Palawan between the continental block of Reed Bank and the islands of Palawan and Calamian. Much of the NW Palawan basin remains lightly explored despite the numerous oil and gas discoveries made in the shallow water areas of the basin including the single giant Malampaya gas field. Our study bases on several geophysical surveys, recorded by the BGR from 1979 to 2008, which established a comprehensive database of deep regional multichannel seismic lines, accompanied with magnetic and gravity profiles.

We distinguish two rift-basin systems across the continental slope: an inner and an outer rift basin, separated by a structural high of non volcanic origin.

The continent-ocean transition (COT) is interpreted at the seaward limit of the continental crust, when magnetic spreading anomalies terminate some 80-100 km further north. The area in between displays extensive volcanism, occurring after breakup, and documented with dykes, basaltic lava flows .and extrusions, occasionally reaching or cutting the seafloor.

The COT is highly variable along the NW Palawan slope: One type shows a distinct outer ridge at the COT with a steep modern seafloor relief. The other type is characterized by rotated fault blocks, bounded by listric normal faults ramping down to a common detachment surface. Half-grabens developed above a strongly eroded pre-rift basement. The seafloor relief is smooth across this other type of COT.

We propose that a close relation exists between the pre-rift structure and the modern continental margin as evidenced by the varying style in the COT. The thickness and firmness of the attenuated continental crust and the amount of magmatism during the transition to seafloor spreading is closely related to the structure of the COT.

Gravity modeling revealed an extremely thinned crust across the shelf. We suggest a depth-dependent extension model with crust being decoupled from mantle lithosphere, explaining the discrepancy of subsidence observed across the South China Sea region.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.