Transpressive ductile flow and oblique ramping of lower crust in a two‐sided orogen: Insight from quartz grain‐shape fabrics near the Alpine fault, New Zealand

Abstract

The three‐dimensional (3‐D) grain‐shape fabric (GSF) of deformed quartz veins was measured in 30 samples from the hanging wall of the oblique‐slip Alpine fault in the central Southern Alps, New Zealand. These record aspects of the neotectonic strain experienced by lower crustal rocks in the Pacific Plate as they were transported through a two‐sided orogen. Deformation was nonsteady state: transpression was followed by near‐vertical shearing as the rocks negotiated the oblique footwall ramp of the Alpine fault. At the brittle‐ductile transition, ramping up was accommodated by an escalator‐like back shearing process. This thinned the transpressed crust and boosted dip‐slip rate on the Alpine fault. Mechanically, this suggests a sharp step in the footwall ramp, adjacent to which differential stresses were transiently high. Margin‐parallel motion was partitioned onto these shears. Fabric development is simulated using a 3‐D kinematic model into which we input constraints on plate motion, fault slip rates, crustal structure, fabrics, and geodetic strain in the Southern Alps. This model can explain many aspects of the Southern Alps fabrics, including their nonvertical foliation and lineation. Similarities between the model predictions and the observed fabrics suggest that lower crustal flow in the Pacific Plate mimicked the geodetic velocity pattern at the surface today, with obliquely convergent deformation being distributed across a wide region to the east of the Alpine fault. The model shows that the timescale for an exhumational steady state with respect to deformational fabrics is slow (107 years), a condition which the ∼6 m.y. old Southern Alps have probably not yet achieved.