Elongated orthopyroxene porphyroclasts as a shear sense indicator and kinematic history of the Horoman Peridotite Complex, Hokkaido, Japan

Abstract

Elongated orthopyroxene porphyroclasts commonly form stretching lineations in deformed peridotites or metabasic rocks. Internal obliquity of (100) exsolution lamellae with respect to elongation direction is a candidate for a shear sense indicator in these rocks. However, it requires statistical analyses of shape and lattice preferred orientation of orthopyroxene porphyroclasts with reference to another reliable shear-sense indicator such as lattice preferred orientation (LPO) of olivine. We measured three geometrical parameters ; R : aspect ratio, θ : long axis orientation, φ : (100) exsolution lamellae orientation on the XZ (foliation normal, lineation parallel) plane, for 1100 porphyroclasts in 160 thin sections selected from 5 structural zones in the Horoman Peridotite Complex, Hokkaido, Japan.Bimodal distributions of internal obliquity are shown in R-φ and θ-φ diagrams of all structural zones. The equigranular zone and the internal shear zone (ISZ) show high contrast of aspect ratio between opposite internal obliquity. This asymmetry is explained by a higher R being attained by synthetic slip on the (100) slip plane, whereas the other peak consists of grains which were deformed by antithetic slip on (100) and rotated towards an unfavorable orientation for slip. The asymmetric features of geometry of elongated orthopyroxene porphyroclasts and LPO of olivine consistently indicate a top-to-the-south sense of shear. On the other hand, high R symmetric bimodal distribution of internal obliquity in the basal shear zone (BSZ), where LPO and shape preferred orientation (SPO) of olivine show apparent asymmetry (a top-to-the-north sense of shear), can not be explained by a single progressive shear deformation. But it implies inverted shear movement in the basal shear zone.Statistical analyses of internal obliquity and aspect ratio of elongated orthopyroxene porphyroclasts can be used as an independent shear-sense indicator for deformed peridotites. Furthermore, these analyses compared with the LPO of olivine can decipher a more complex kinematic history which can not be revealed only by olivine fabrics.