Moore, J. C, Mascle, A., et al., 1990 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 110 17. GRAIN FABRIC MEASURED USING MAGNETIC SUSCEPTIBILITY ANISOTROPY IN DEFORMED SEDIMENTS OF THE BARBADOS ACCRETIONARY PRISM: LEG HO1 M. W. Hounslow2-3 ABSTRACT The anisotropy of magnetic susceptibility documents the generation of tectonically produced fabrics in sediments that macroscopically show no evidence of this disruption. The fabric observed in initial accretion is largely produced by overprinting of the original sedimentary susceptibility anisotropy by an E-W horizontal tectonic shortening and vertical extension. The response of the sediments to stress during initial accretion is variable, particularly near the sediment sur-face, and appears to reflect the inhomogeneous distribution of strain rate in the overthrust sequence. The susceptibility anisotropy of sediments possessing scaly fabric is consistent with the strong orientation of Phyllosilicates seen in thin section, producing a Kmin normal to the scalyness. The slope sediments deposited on the accreted sequence are also af-fected by tectonic shortening. The accreted sequences at Sites 673 and 674 show a complex history of fabric modifica-tion, with previous tectonic fabrics overprinted by later fabric modifications, pointing to continued tectonic shortening during the accretion process. The form of the susceptibility anisotropy axes at Sites 673 and 674 is consistent with NE-SW shortening, probably reflected in the NW-SE surface expression of the out-of-sequence thrusts. The susceptibility anisotropy appears to document a downhole change in the trend of shortening from E to W at the surface to more NE-SW at depth, probably as a result of the obliquely trending basement ridge, the Tiburon Rise. INTRODUCTION To a large extent the grain fabric of a rock is indicative of the stresses that were imposed upon it during its formation, and therefore the fabric can be used as a delimiter in defining these stresses. The deformed sediments in present day accretionary prisms provide the natural laboratory for studying the mecha-nisms of grain fabric modification, because the present and past conditions in them can be reasonably well defined. Such studies have important implications for understanding the grain fabric in ancient low-temperature metamorphic rocks. The sediments recovered on ODP Leg 110, through the toe of the northern Barbados Ridge accretionary prism, potentially reflect both the fabric generated in normal deep-sea sediments and the stresses imposed by incorporation in the prism. The fabric of the sediments at the toe of the prism, will be examined using anisotropy of magnetic susceptibility (AMS), a technique that has been widely used in examining the fabric of sediments and tectonic rocks under a wide variety of conditions (Hrouda, 1982). This study examines the development of grain fabric in the deformed overthrust sediments, with a view to determining the strain and stress history imposed during the structural modi-fications accompanying accretion. STRUCTURAL FEATURES The Barbados Ridge forms the leading edge of the Carib-bean Plate, under which the Atlantic Plate has been subducting at least since the Eocene, the process of which has lead to the formation of the Lesser Antilles Island Arc (Fig. 1). The forearc region consists of both forearc basins, such as the Tobago Trough, and material scraped off the incoming Atlantic Plate (Mascle et al., 1986; Brown and Westbrook, 1987). The only subaerially exposed part of the accretionary prism is that out-1 Moore, J. C, Mascle, A., et al., 1990. Proc. ODP, Sci. Results, 110: College Station, TX (Ocean Drilling Program). 2 Department of Geology, University of Sheffield, Sheffield S3 7HF, United Kingdom. 3 Current address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom. cropping on Barbados (Speed, 1983). The deformation front forms the eastern boundary of the prism, and is marked by the first occurrence of large-scale deformation. The sediments from the accretionary prism at the drill site latitude show a surface structural grain that is oriented approximately N to S (Fontas et al. 1984; Mauffret et al., 1984), presumably reflecting the local collision direction of the Tiburon Rise and the toe of the accre-tionary prism. The structural features at the toe of the prism (Sites 541, 542, 671, 675, 676, Fig. 1) document relatively simple folding and thrusting of the Pleistocene-to-Miocene sedimentary sequence above the decollement horizon, located in lower Miocene mud-stones (Behrmann et al., 1988; Mascle, Moore, et al., 1988). The decollement horizon is characterized by a penetrative sub-horizontal fracture termed scaly fabric, which is well documented from other forearc regions and is attributed to tectonic defor-mation (Moore et al., 1985). Sub-horizontal scaly fabric is also developed in several horizons above the level of the decollement. The underthrust sequence is for the most part structurally unaf-fected by the plate collision. Several kilometers arcward of the frontal thrust the sequence at Sites 673 and 674 document more extensively tectonized sedi-ments with overturned limbs, tight folding, stratal disruption, calcite veining, subvertical to horizontal dipping scaly fabrics, and stratigraphically complicated by further thrusting. A slope sequence of apparently normally deposited hemipelagic sedi-ments and slump deposits is also found above the accreted se-quence. This has resulted in a stratigraphic and structural pat-tern that is much more complicated than that found at Site 671. ANISOTROPY OF MAGNETIC SUSCEPTIBILITY In titanomagnetite-bearing sediments, the anisotropy of magnetic susceptibility (AMS) has been shown to represent a measure of the grain shape orientation of the detrital quartz grains (Rees, 1965), and has been widely used in examining deep-sea sediments (Ellwood and Ledbetter, 1977; Hailwood et al., 1987). In moderately deformed tectonic rocks it has been suggested that the AMS is quantitatively related to the magni-tude of the strain ellipsoid, with some authors suggesting a uni-versal relationship (Kligfield et al., 1977; Henry and Daly, 1983). 257M. W. HOUNSLOW 19°N 15°40'N 15°30' -15°20' i I 59°10'W 59°00' 58°50' 58°40' 58°30' 58°20' Figure 1. Location of ODP and DSDP sites relative to topographic features, seismic lines, and bathymetry. Contour interval, 100 m. The AMS is