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

Abstract

The Alpine fault self‐exhumes its own ductile shear zone roots and has a known slip kinematics. Within ∼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 ∼28 ± 4° 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 σ1 was arranged at 60–80° 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 ∼0.7 to ∼0.85. Inversions of seismic focal mechanisms in the brittle crust of the Southern Alps indicate that σ1 is oriented ∼60° to the Alpine Fault; that shear bands form at ∼30° to this direction, and that σ2 and σ3 flip positions between the brittle and ductile parts of the crust.

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

“…This linkage and the Alpine Fault origin of the Alpine Schist mylonite zone have been proposed by many workers (e.g., Norris & Cooper, , ; Sibson et al, ) on the basis of high finite shear strains in the Alpine mylonite zone and quartz lattice‐preferred orientations in those rocks that record amphibolite‐facies conditions of ductile shearing. This shear deformation, localized to within ~0.5–1.5 km of the Alpine Fault, was uniformly dextral‐reverse in sense and featured a shearing vector that is indistinguishable from the modern slip direction on the Alpine Fault (Little et al, , ; Toy et al, ).…”

The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand

“…Geological observations along the central Alpine Fault suggest such a stress configuration prevails near the brittle-ductile transition (Little et al, 2016); at 3 km depth (Holm et al, 1989); and near the surface due to late-stage exhumation and deglaciation (Hanson et al, 1990;Norris & Copper, 1986). Switching between r 2 and r 3 has previously been interpreted to be possibly related to a postseismic transient stress rotations, or stress perturbations close to active faults (Little et al, 2016). In addition, recent numerical stress modeling of topographic and tectonic stresses shows that r 2 is currently rotated by topographic relief from near-vertical under the ridges to near-horizontal beneath valleys crossing the Alpine fault (such as the Whataroa Valley), resulting in a thrust stress regime rather than oblique strike-slip ( Figure 9; Upton et al, 2018).…”

Constraints on Alpine Fault (New Zealand) mylonitization temperatures and the geothermal gradient from Ti-in-quartz thermobarometry

“…The Alpine Fault is thus a rare location where the understanding of plate boundary phenomena such as major earthquakes (Sutherland et al, 2007) or slow slip events (Chamberlain et al, 2014) can be informed by observation of rocks that recently experienced these events. Because rock deformation was recent and continues today, aspects such as total strain and strain rate (Norris and Cooper, 2003), stress levels (Liu and Bird, 2002), exhumation rates (e.g., Little et al, 2005), and depth of seismicity (Leitner et al, 2001) are known with a relatively high degree of certainty.…”