Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

Abstract

Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, have provided a superb natural laboratory for insight into the microstructural evolution of lower crustal shear zones during exhumation. Shears are exposed in the central section of the Southern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 km wide that is located 6–8 km structurally above the Alpine Fault. An array of systematically spaced shear zones that formed by embrittlement and faulting of quartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonate veins embedded within the schist. This study has used field-based structural mapping along with optical microscopy and universal stage measurements of crystallographic preferred orientations (CPO) to resolve the shear zone kinematics and rheology. On the basis of these data, the strain path can be reconstructed for the sheared veins during their progressive deformation. This began with their incidence as backshears at the base of the Alpine Fault ramp and ended with their subsequent recrystallisation, uplift, and exhumation. The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n = 780). They are inferred to have formed as backshears accommodating uplift of the Pacific Plate as it was translated onto the oblique footwall ramp of the Alpine Fault during late Cenozoic oblique convergence. Detailed fault offset transect surveys across the shears at Chancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of 25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideup shear sense. Fault offset geometry and a consistent arrangement of mineral fibre lineations that decorate fault surfaces, indicate that the mean displacement vector pitches 35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed marker veins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a mean finite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformation of the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain, an escalator kinematic model, and assumptions about the width of the deforming zone. Five deformation phases have affected the sheared veins during their transport up the fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding of mylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisation and grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformation and exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of ~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently high shear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPO of quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001) and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductile deformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have caused the rocks to undergo brittle failure, a situation that is confirmed by a late component of brittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calcite veins infill the shear fractures, and these themselves have been sheared. The deformation of the veins was not a simple shear process but one with triclinic flow symmetry. This is inferred from discordance between the shear direction and the near-vertical principle extension direction that is revealed by the pattern and symmetry of quartz and calcite CPO fabrics. After the shears move away from the ramp-step, grain boundary sliding (GBS) accommodated by solid-state diffusion creep is inferred to have affected quartz veins. This deformation mechanism takes place because of 1) the small 8 μm grain size inherited from Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPa that followed the initial up-ramping. Quartz CPO fabrics in the sheared veins are remarkably weak considering their large shear strains. GBS is inferred to have been a chief deformation mechanism that caused the weakening of quartz CPO fabrics in the highly sheared sections of deformed veins. Calcite has also affected the quartz fabric strength as those veins containing >5% calcite have very weak quartz CPO fabrics. In contrast to quartz, the CPO fabrics for the co-existing calcite remained strong and continued to develop by dislocation creep. The third phase of deformation, a process that may have contributed to subsequent weakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and an equigranular-polygonal grain shape fabric. This fabric was overprinted by late-stage dislocation creep microstructures in the fourth deformation phase in response increased differential stress encountered by the rocks at lower temperatures in the upper crust. The final phase of deformation to affect the sheared veins was semibrittle deformation at differential stresses of <189 MPa and temperatures of 200–280oC as the rocks passed through the steady-state brittle-ductile transition zone at depths of 8–10 km before being exhumed at the surface.