Are quartz LPOs predictably oriented with respect to the shear zone boundary?: A test from the Alpine Fault mylonites, New Zealand

摘要

The Alpine fault self-exhumes its own ductile shear zone roots and has a known slip kinematics. Within approximate to 1 km of the fault, the mylonitic foliation is subparallel to the boundary of the amphibolite-facies ductile shear zone in which it formed. Using EBSD, we analyzed quartz Lattice Preferred Orientations [LPOs) of mylonites along a central part of the Alpine Fault. All LPOs feature a strongest girdle of [c]-axes that is forward-inclined approximate to 284 degrees away from the pole to the fault. A maximum of < a > axes is inclined at the same angle relative the fault. The [c]-axis girdle is perpendicular to extensional (C') shear bands and the < a > maximum is parallel to their slip direction. [c]-axis girdles do not form perpendicular to the SZB. Schmid factor analysis suggests that sigma(1) was arranged at 60-80 degrees to the Alpine Fault. These observations indicate ductile transpression in the shear zone. The inclined arrangement of [c]-axis girdles, < a > axes, and C' planes relative to the fault can be explained by their alignment relative to planes of maximum shear-strain-rate in a general shear zone, a significant new insight regarding shear zones and how LPO fabrics may generally develop within them. For the Alpine mylonite zone, our data imply a kinematic vorticity number (Wk) of approximate to 0.7 to approximate to 0.85. Inversions of seismic focal mechanisms in the brittle crust of the Southern Alps indicate that sigma(1) is oriented approximate to 60 degrees to the Alpine Fau that shear bands form at approximate to 30 degrees to this direction, and that sigma(2) and sigma(3) flip positions between the brittle and ductile parts of the crust.