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Measurements of magnetic susceptibility anisotropy, orientated using stable remanences, are presented for Hole 808C, over the depth range 300 to 1050 meters below seafloor. Within the accretionary prism, magnetic fabric directions are remarkably consistent, the maximum susceptibility axes lying horizontal and at right angles to the direction of underthrusting, with minimum susceptibility axes generally close to vertical. Steeper inclinations of the minimum susceptibility axes are observed in the fold below the frontal thrust and in a disturbed zone just above the décollement. Anisotropy parameters are scattered, possibly reflecting local inhomogeneity of deformation, but the broad pattern is consistent with the model for magnetic fabric modification put forward by Graham (1966). Below the décollement the magnetic fabric is strongly oblate, with no preferred azimuthal orientation. There is evidence of deformation of remanence in the zone immediately below the frontal thrust. INTRODUCTION The tectonic environment of an accretionary prism provides an excellent opportunity to study the effects of deformation on both magnetic fabric and remanence. Hole 8O8C is perhaps unique in two respects: in the degree to which complementary information (e.g., on structural geology and physical properties) is available and in the control on orientation that was provided by remanence measurements. The theme of the present paper is, therefore, the modification of magnetic fabric and remanence by deformation, as evidenced by Hole 8O8C. The paper focuses on the depth interval from 300 to 1050 m below seafloor (mbsf), which extends from just above the frontal thrust to below the décollement (see review paper, this volume). Related studies have been carried out by Taira and Niitsuma (1985), in the Nankai Trough, and by Hounslow (1990), in the Barbados accretionary prism. The former concentrated on details of sedimentation of the turbidites that characterize the trench wedge. Hounslow employed an approach similar to that adopted here, but in a tectonic regime that proved to be of considerably greater complexity. For those unfamiliar with magnetic fabric studies, a useful starting reference is Collinson (1983). SAMPLING AND MEASUREMENT In view of the problems inherent in obtaining undisturbed samples when using sampling cubes, measurements were restricted to drilled, cylindrical samples. "Minicores" of nominally 2.54 cm (1 in.) diameter were taken from the working half of the core. To preserve parallelism of the minicore axis to the core reference direction, previously marked portions of the working half were removed, inverted onto a flat wooden surface, and drilled on a bench press from outside in. Samples were subsequently sawed to length (nominally 2.2 cm) onshore. Problems were experienced due to the friability of samples (exacerbated by samples drying out in the fan-driven cooling of the alternating field demagnetizer). These problems were particularly acute for samples taken from just above the décollement; the samples required some delicate reconstruction. 1 Hill, I. A., Taira, A., Firth, J.V., 1993. Proc. ODP, Sci. Results, 131: College Station, TX (Ocean Drilling Program). 2 School of Earth Sciences, University of Birmingham, Birmingham B15 2 TT, United Kingdom. Remanence measurements were made on a Molspin fluxgate spinner magnetometer. For alternating field demagnetization a Highmoor demagnetizer was used. This has a two-axis tumbler, which was modified to alternate the sense of tumbling to minimize the introduction of rotational remanences. Some of the weaker specimens were measured, at the University of Southampton, on a cryogenic magnetometer with an inbuilt static-sample demagnetizer. Anisotropy of initial (low field) magnetic susceptibility was measured on a Molspin Minisep instrument, in which anisotropy is measured by detecting the component of magnetization induced at right angles to the direction of an applied, weak (nominally 0.7 mT), alternating magnetic field. REMANENCE On Leg 131 remanence was measured for the purposes of magnetostratigraphy and to provide a means of sample orientation. The magnetostratigraphic applications have been reported previously (Shipboard Scientific Party, 1991) and are not discussed further, except to make two points that arise from that contribution: 1. The primary source of data for magnetostratigraphic purposes is the shipboard, 2G continuous-core cryogenic magnetometer. Discussion in Shipboard Scientific Party (1991) touched on the interpretation of the continuous-core records produced by the data-analysis program currently in use. Errors, in the form of oversteepened inclinations, can arise when, for example, uniformly magnetized core is broken into segments, which are short compared with the averaging distance of the sensors, and which are rotated relative to each other (Shipboard Scientific Party, 1991, pp. 45-^6). An obvious extension of this argument, although not stated explicitly previously, is that, in such cases, the lower limit of the scattered values, rather than the average, will provide the best estimate of the remanence inclination. This is well illustrated by the figure accompanying the discussion (Shipboard Scientific Party, 1991, Fig. 14). 2. Measurements on discrete, minicore samples are made as an adjunct to the continuous-core cryogenic measurements made on board, primarily because they allow alternating-field demagnetizations to be taken to higher fields than is permitted on the archive half of the core. The shipboard measurements made using a static-sample demagnetizer indicated that at high demagnetization fields spurious remanences were clearly being introduced. Subsequent shore-based measurements using a rotating-sample demagnetizer showed that some samples were prone to acquire a significant rotational rema-

Magnetic Fabric Studies of Samples from Hole 808C, Nankai Trough