Magnetic fabrics of soft-sediment folded strata within a neogene accretionary complex, the Miura group, central Japan

Anisotropy of magnetic susceptibility (AMS) on the middle Miocene–Pleistocene sedimentary sequence in the Boso and Miura Peninsulas of central Japan was used to study 18 sites in the northern tectonic setting and 37 sites in the southern setting. This sequence is associated with abundant synsedimentary deformation structures of folding and faulting generated in accretionary tectonics. AMS results in different deformation settings such as the forearc, the accretionary prism and the trench were analyzed. The shapes of the dissimilar magnetic fabrics are compared using the shape parameter (T) and the corrected anisotropy degree (P′) in the so-called T–P′ diagrams. Our results have implied that the oblate fabric of the trench sediments can be regarded as the result of depositional and compactional processes alone. The AMS shape parameters obtained from the northern sequence (forearc) closely resemble an indication of undeformed trench sediments. In contrast, a different pattern is observed in the highly prolate-shaped AMS results of the southern sequence. The difference apparently reflects the degree of deformation in the three tectonic provinces. In order to understand the deformation mechanism of the sedimentary fabric, a detailed AMS study was made on one anticline system. An AMS evolution from an oblate fabric to a prolate fabric in the anticline system was observed. We also found that an AMS tectonic fabric occurred in the center of the anticline. Thickness correlations of the strata under study indicate that strained sediments formed in the central portion of the fold. As a consequence, one can say that this mechanism can modify the magnetic fabric from the sedimentary form to the tectonic form in a compressional regime.