KINEMATIC EVOLUTION OF LOWER AND MIDDLE CRUSTAL SHEAR ZONES DURING OROGENIC COLLAPSE

Extensional metamorphic core complexes have been identified as products of orogenic collapse in mountain belts around the globe. Limited exposure in many ancient examples, however, makes it difficult to directly observe how deformation operating at different crustal depths is related. The pre-Cenozoic configuration of western New Zealand allowed us to examine the geometry and kinematic evolution, including kinematic vorticity (Wk), of shear zones that evolved simultaneously at different depths within an orogenic belt.

By 108-105 Ma, a series of subhorizontal, 0.5-1 km thick shear zones formed at lower and mid-crustal depths while brittle normal faults formed in the upper crust. These events took place as the tectonic setting of the western province of New Zealand changed from contraction to extension, preceding the breakup of Gondwana at ~84 Ma. Shear zones in the lower crust (36-48 km paleodepths) preserve high pressure (12-14 kbars) garnet-granulite facies fabrics. During exhumation these high pressure fabrics were hydrated and retrogressed to upper amphibolite facies conditions. Mid-upper crustal shear zones preserve upper greenshist facies fabrics cut by brittle normal faults. We compared the kinematic evolution of these shear zones at multiple length scales and obtained the following scale independent results: All shear zones contain anastomosing zones of high strain surrounding low strain pods, and record oblate strains. Kinematic vorticity analysis using asymmetric plagioclase porphyroclasts indicates that all shear zones formed at this time were vertically thinning. The degree of non-coaxiality was variable in the lower crust (0.57<Wk<0.72), and consistently dominated by simple shear in the mid-upper crust (0.80<Wk<0.82). Sense of shear indicators record NE-SW sub-horizontal stretching at all levels. The lack of asymmetries on planes perpendicular to lineation suggests monoclinic symmetry.

Our results indicate strong vertical kinematic coupling between the lower, middle, and upper crust during the collapse of the Fiordland-Westland orogen in the mid-Cretaceous. This scenario reflects the simultaneous extensional failure of all crustal levels driven by changes in plate motions. Vertical coupling was aided by a strong, viscous lower crust at the time of extension.