The Seismic Stratigraphy, Crustal Structure, Volcanic and Sedimentary History of the East Mariana and Pigafetta Basins of the Western Pacific

Abstract

In the following investigations regional multichannel seismic reflection and sonobuoy refraction data have been used to identify a site where Leg 129 of the Ocean Drilling Program (ODP) successfully drilled and recovered Jurassic material for the first time in the Pacific Ocean. Prior to drilling first-order stratigraphic relationships and crustal structure were determined. The sedimentary and volcanic history of the East Mariana Basin (EMB) and Pigafetta Basin (PB) were then inferred with emphasis on defining the extent (lateral as well as vertical) and type of deep sea volcanic provinces (sills/flows and or volcaniclastics) in this part of the western Pacific. The successful completion of ODP Leg 129, resulting in the first and only holes to penetrate igneous basement in the EMB and PB, allows the calibration of our regional multichannel seismic site surveys and the extrapolation of drilling results throughout these oldest Pacific basins. Our study indicates that mid Cretaceous flows/sills overlie Jurassic/ Early Cretaceous sediments and oceanic crust throughout an area - 500,000 km2 in the EMB and the southeast PB. Jurassic age oceanic crust and overlying upper Jurassic - lower Cretaceous pelagic sediments unquestionably exist at Site 801 and extend semicontinuously between Site 801 and Site 800. A "Rough-Smooth" Horizon B boundary marks the termination of continuous mid-Cretaceous volcanic overburden. Volcanogenic turbidite sequences of varying thicknesses and ages are ubiquitous features of both basins. The Ogasawara Fracture Zone, Magellan Seamounts and associated flexural moat separate the PB from the EMB and influence the source and distribution of this redeposited material. This study places constraints on the depth to basement and crustal thickness in these oldest ocean basins which, when coupled with recently acquired heatflow measurements (Lister et al., 1990; Stein and Abbot, 1991), favor a time-varying heat flow model as a possible explanation for the observed divergence of heatflow and depth versus age from that predicted by a simple cooling boundary layer. These observations are consitent with those compiled in Larson (1991a, b), where increased mantle temperatures due to vigorous global plume activity are rapidly initiated at approximately 124 ma and gradually decrease toward 80 ma.