--> Abstract: Nannofossil Evidence for Thrust Faulting and Sediment Mixing in an Accretionary Complex (New Hebrides Island Arc), by T. S. Staerker, S. W. Wise, N. S. Lundberg, P. Reid, J.-Y. Collot, and H. G. Greene; #90987 (1993).

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STAERKER, T. SCOTT, S. W. WISE, and N. S. LUNDBERG, Department of Geology, Florida State Univ., Tallahassee, FL; P. REID, RSMAS, Univ. of Miami, FL; J.-Y. COLLOT, Lab. Geodynamique, ORSTROM, Villefranche-sur-Mer, France; and H. G. Greene, U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Nannofossil Evidence for Thrust Faulting and Sediment Mixing in an Accretionary Complex (New Hebrides Island Arc)

As the Ocean Drilling Program explores increasingly more complex convergent margins, calcareous nannofossils have become an important key for documenting both tectonic and sedimentary processes. Nannofossil assemblages from upper Eocene to Pleistocene sequences drilled in the central New Hebrides Island Arc during ODP Leg 134 document imbricate thrust faults and intricate patterns of sedimentary mixing during deposition along the collisional boundary between the Australian-India and Pacific plates where the d'Entrecasteaux Ridge and Bougainville Guyot are being subducted.

Despite problems of reworked assemblages, poor preservation, overgrowths, and dilution by volcaniclastic sediments, nannofossil biostratigraphy delineates several repeated sections at Site 825 in the accretionary prism adjacent to Espiritu Santo Island. Paleogene pelagic sediments equivalent to those in a reference section at Site 828 appear to have been offscraped from the downgoing plate and accreted onto the forearc during the Pleistocene. Other sediments in the present forearc include Pleistocene olistostromal trench-fill deposits containing clasts of various ages and compositions. Some of the clasts and olistoliths have affinities resembling

rocks exposed on neighboring islands and environs whereas others are of uncertain origin. The matrix of the olistostromes is predominantly Pleistocene, although mixed ages are frequently encountered. Comparisons of the mixed nannofossil ages in the matrix of the olistostromes with sedimentological and structural data suggest that sediment mixing that results from fault movement is subordinate to that which occurs during deposition.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.