The use of asymmetric pressure shadows in mylonites to determine the sense of shear

Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. It is one of the most noticeable asymmetric microstructures, showing that the porphyroclasts have rotated during non-coaxial laminar flow in a ductile shear zone. The shadow domains are filled with recrystallized quartz and K-feldspar. Excepting APS, various asymmetric microstructures in the mylonites indicate a sinistral sense of displacement throughout the ductile shear zone along the MTL. Based on the shape analysis of APS in XZ section (parallel to the mylonitic lineation and normal to the mylonitic foliation), the following results were obtained: (1) the relative position of the APS with respect to a porphyroclast is not a reliable criterion for deducing the sense of shear; and (2) the drag angle (β) of the shadow boundaries with respect to the mylonitic foliation in each quartered domain is diagnostic of the sense of shear; when the shearing is sinistral, β in upper right- and lower left-hand side of a porphyroclast is larger than β in upper left- and lower right-hand side, and vice versa for dextral shearing. These results demonstrate that the drag patterns of APS around porphyroclasts in mylonites are highly reliable indicators for the determination of the sense of shear.