PRELIMINARY ANALYSIS OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND PALEOMAGNETIC FABRICS WITHIN EXHUMED PORTIONS OF THE NANKAI ACCRETIONARY COMPLEX, KII PENINSULA, JAPAN

Since the early Miocene, the Nankai margin has experienced periods of oblique subduction, resulting in changes in the pace of accretion and the development of trench-slope-basin depositional environments. We collected samples from the Kii Peninsula NNW of the NanTroSEIZE focus area in a feasibility study for the use of anisotropy of magnetic susceptibility (AMS) and/or paleomagnetism to provide kinematic constraints on the evolution of the Nankai margin. In particular, we aim to test the hypothesis that depositional environments evolve progressively with deformation. Preliminary analyses were conducted on seven hand samples of very fine-grained sandstone and siltstone collected from exhumed sections of the accretionary complex on the Kii Peninsula. Analysis of mean coercivities and dispersions suggests the magnetic mineralogy includes pedogenic magnetite, detrital magnetite, magnetofossils, hematite, and goethite. Six samples fall within the range of a pseudo-single domain magnetite (Mr/Ms 0.1-0.5, Hr/Hc 2-4), but one falls in the range of superparamagnetic domain magnetite (typical Mr/Ms values << 0.01, Hr/Hc > 10). Analysis of AMS cores prepared from the same samples yields highly consistent results. Most AMS ellipsoids are characterized by oblate shapes (i.e. T > 0). The corrected degree of anisotropy (Pj) is low and varies from 1.00-1.06. Within most samples, the variation in Pj was <0.01. Five of seven sites yield consistent orientations with respect to the orientation of the AMS ellipsoids. The long and short axes (K1, K3) roughly plot along steeply dipping great circles, while the intermediate axes (K2) tend to cluster shallow, NS orientation. Paleomagnetic results were far less consistent. All samples were subject to alternating field demagnetization up to 200 mT. Only three of seven sites analyzed yielded internally consistent results (a95 < 15˚). These sites are characterized by single or two component systems, but the high coercivity components were not orientated in the same direction. Our preliminary results indicate the targeted units are solid candidates for AMS analysis. The lack of consistency in the paleomagnetic analyses suggest (1) a large volume of samples from these are needed to produce statistically significant results and/or (2) alternate formations should be targeted.