Moore, J. C, Mascle, A., et al., 1990 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 110 21. FLUID FLOW WITHIN THE BARBADOS RIDGE COMPLEX, PART I: DEWATERING WITHIN THE TOE OF THE PRISM1 Elizabeth J. Screaton,2,3 Dennis R. Wuthrich,2,4 and Shirley J. Dreiss2 ABSTRACT Sediment compaction at convergent margins expels pore fluids, which in turn influence many aspects of subduction zone geology. Drilling in the Barbados Ridge complex during ODP Leg 110 and DSDP Leg 78A provided information about sediment types, porosities, and the geometry of the complex. In this paper, we use these observations to estimate the rates of sediment porosity loss and accompanying fluid expulsion from the prism, the decollement, and the under-thrust sediments. Rates of porosity loss depend on how rapidly the sediments move arcward through the complex. We compute rates of sediment movement in the prism and decollement as a function of distance from the deformation front. This calculation assumes that the rates of sediment movement decrease arcward as the result of porosity loss in the prism and decollement and thickening of the prism. According to computed rates of sediment movement for the prism, sediment deceleration is greatest within the first 3 to 5 km arcward from the deformation front. Similarly, most dewatering of the prism sediments takes place in this region. Beyond 5 km, rates of sediment dewatering become great-est in the underthrust sediments, assuming these sediments are carried downward at a uniform rate with the oceanic plate. INTRODUCTION At sediment-rich subduction zones, highly porous sediments are deformed by tectonic and gravitational stresses as they are either accreted or underthrust. Fluids expelled from the com-pacting sediments influence many aspects of subduction zone geology, including heat transport (Langseth and Hobart, 1984), diagenesis and metamorphism (Etheridge et al., 1983), and ben-thic biology (Kulm et al., 1986). In addition, the relative rates of sediment loading and fluid dissipation determine the magnitude and distribution of excess pore pressures. These excess pore pressures may affect the shape of the accretionary wedge (Davis et al., 1983) as well as thrust fault and fold geometries (Hubbert and Rubey, 1959; Seely, 1977). Despite the importance of fluids at convergent margins, little is known about the rates and directions of fluid flow and the pore-pressure distributions in these settings. The Barbados Ridge complex is primarily composed of fine-grained sediments (Marlow et al., 1984) and represents a low-permeability end member of accretionary complexes. In the Barbados complex, an extensive decollement zone separates the downgoing Atlantic ocean crust from the accretionary prism of the over-riding Caribbean Plate. Above the decollement, sediments are off-scraped, building up the accretionary prism and causing the deformation front to grow seaward. As these accreted sediments are deformed by tec-tonic compression and loading, they compact and lose porosity. Beneath the decollement, the sediments move with the downgo-ing plate. Tectonic stresses and the weight of the over-riding prism cause vertical compaction and porosity loss within these underthrust sediments. At Sites 671 and 672, ODP Leg 110 succeeded in drilling through the decollement zone. Sediment and pore-water sam-1 Moore, J. C, Mascle, A., 1990. Proc. ODP, Sci. Results, 110: College Sta-tion, TX (Ocean Drilling Program). 2 Earth Sciences Board, University of California at Santa Cruz, Santa Cruz, CA 95064. 3 Current address: Harding Lawson and Associates, P.O. Box 578, 7655 Red-wood Blvd., Novato, CA 94948. 4 Current address: Geomatrix Consultants, One Market Plaza, Spear Street Tower, San Francisco, CA 94105. pies collected from these sites contained chemical, thermal, and structural evidence of preferential fluid migration along the de-collement (Moore, Mascle, et al., 1987). In addition, the Leg 110 drilling, along with previous drilling and seismic reflection studies, provided information on the structure and physical properties of sediments within the accretionary prism, the de-collement, and the underthrust series. In this study, we use these observations to construct a numerical model of fluid flow in the toe of Barbados Ridge complex. Fluid migration through the toe of the complex results pri-marily from sediment compaction and porosity loss. Therefore, to describe the fluid flow, we must examine the fluxes of both solids and fluids. This paper is the first of a two-part series de-scribing the hydrogeologic modeling. Here, we quantify the rate and distribution of sediment porosity loss. To do this, we ex-trapolate a porosity distribution for the accretionary prism and the decollement from borehole porosity data. We use this distri-bution, the geometry of the complex, and equations for the conservation of solid mass to estimate the rates of sediment movement as a function of distance from the deformation front. We then compute rates of porosity loss and associated fluid expulsion throughout the complex. The second paper (Wuth-rich et al., this volume) describes the fluid flow model and simu-lated pore pressures and flow patterns, using the estimated rates of porosity loss to approximate the effects of sediment deforma-tion. PHYSICAL SETTING The Barbados Ridge complex (Fig. 1A) varies in width from 450 km in the south to 260 km in the vicinity of the ODP Leg 110 and DSDP Leg 78A transects (Moore and Biju-Duval, 1984). A number of studies have suggested that the Caribbean and North American Plates converge at a relatively slow rate of 2.0 to 2.2 cm/yr in an east-west direction (MacDonald and Holcombe, 1978; Minster and Jordan, 1978; Tovish and Schu-bert, 1978; Dorel, 1981). MacDonald and Holcombe (1978) esti-mate that this rate has been constant for the last 2.4 m.y. Alter-natively, Sykes et al. (1982) suggest a much higher convergence rate of 4.0 cm/yr in an east-northeast direction for the last 7 m.y. Stein et al. (1988) review these estimates and conclude that 2.0 cm/yr is most plausible. 321E. J. SCREATON, D. R. WUTHRICH, S. J. DREISS 19°N 15°40'N 15°30' -15°20' 59°00'W 58°50' 58640' 58°30' 58°20' Figure 1. (A) Location map showing sites drilled during ODP Leg 110 and DSDP Leg 78A, and the network of seismic reflection lines (from Mascle, Moore, et al., 1988). (B) Interpretation of seismic line CRV128 and projected sites of ODP Leg 110 and DSDP Leg 78A drill