Deformation and Orogenesis: the Geodynamic Significance of a Brittle-Ductile Fault Array in the Central Southern Alps, New Zealand

Abstract

An exhumed array of near-vertical brittle-ductile faults exposed in the hanging-wall of the Alpine Fault in the central Southern Alps have allowed a unique insight into the workings of an active orogen at lower crustal depths where brittle deformation might expand downward, temporarily, into otherwise ductilely deforming rocks. These brittle-ductile shears are only exposed in the central section of the Southern Alps around Franz Josef and Fox glaciers, located 5-7 km structurally above the Alpine Fault ramp in biotite-grade Alpine Schist, and strike sub-parallel to the Alpine Fault trace. This study has used a combination of field data, including extensive structural surveying, and laboratory analyses of rocks deformed by these brittle-ductile faults, including fluid inclusion analyses, stable isotopes, 40Ar/39Ar dating, and standard optical microscopy and electron backscattered diffraction analyses. These have been used to evaluate the deformation conditions under which the faults formed, their kinematics and geodynamic significance in the Southern Alps orogen, and the effect of water on quartz rheology under changing deformation conditions. The dextral-oblique brittle-ductile shears are inferred to have sequentially formed as a series of backshears in an escalator-like fashion due to the translation and tilting of the Pacific Plate onto the SE-dipping Alpine Fault ramp at depth during the late Cenozoic. The backshears are remarkably systematic in their spacing (45 ± 4 cm) and average offset (14.1 ± 1.2 cm, n=1230), with an up-to-the-northwest sense of shear. The backshears offset the quartzofeldspathic Alpine Schist host in a predominantly brittle manner, whereas pre-existing quartz veins in this schist were ductilely deformed to finite ductile shear-strains of 4.8 ± 0.8 across 1-3 cm-wide shear zones before being further offset brittlely as a consequence of fault-tip propagation. The backshears are infilled by syntectonic quartz-calcite veins that preserve a fault-surface lineation pitching 36 ± 5°SW, interpreted to represent the net-slip direction along the shears. The backshears are interpreted to have initiated brittlely in the schist at depths of~21 ± 5 km under near lithostatic fluid pressures by hybrid-extensional failure. Differential stresses at the time of failure are estimated to have been ≥100 MPa and deformation temperatures were 450-500 ± 50°C. Fluid pressure conditions during fault-infilling vein deposition were sub-lithostatic (λ=0.55-0.6), suggesting a large and transient drop in fluid pressure occurred post-failure that extended well below the base of the seismogenic zone. Metamorphic fluid flow driven by this near-hydrostatic pressure gradient may have been rapid (~14 m3/s), resulting in deposition of a 1 mm-thick infilling-vein in <100 days, sealing the backshear fracture. Backshear orientation is controlled by the strike and dip of the underlying fault ramp but also by the obliquity of the backshear to the plate motion vector. The rotation in backshear strike reflects a curvature and steepening of the Alpine Fault ramp in the central section of the Southern Alps. The backshears accommodated vertical tilting over a minimum ramp step at the base of the Alpine Fault of 22 ± 8°. Microscale shearing along the schistosity planes between the discrete backshears can account for the remaining vertical shear that is required to tilt the hanging-wall onto a 40-50° dipping Alpine Fault ramp. Maximum strain-rates focussed around the bend in the Alpine Fault ramp were as high as 1.0 x 10-8 s-1 for a single, sequentially activated backshear, and are likely to have been transient. Quartz veins deformed by the backshears often show components of both ductile and brittle deformation, reflecting changing deformation conditions with time. Initial ductile deformation of these veins is inferred to have been by dislocation creep, driven by high differential stresses surrounding the fault-tips. Contemporaneous brittle offset along the faults through the quartzofeldspathic schist was accommodated by a dissolution-precipitation creep process, the rate of which was limited by the strain-rate of ductile deformation in the quartz veins. Dynamic recrystallisation of these veins to <5 µm grain-size, in combination with a stress-drop down to 1-2 MPa due to translation of the rocks away from the ramp-step, allowed diffusion creep-accommodated grain boundary sliding to become the dominant deformation mechanism, which was followed by post-deformational annealing and grain-growth. Existing quartz flow laws for dislocation and diffusion creep are unable to explain the accumulation of the observed shear strain in these veins in the time estimated to exhume the backshears up the Alpine Fault ramp from 21 km depth. Enhanced rates of dislocation and diffusion creep due to the presence of intracrystalline water at high fugacity in the quartz veins are invoked to allow ductile deformation to have accumulated at strain-rates faster than those predicted from experimental flow laws. Rates of grain-growth are also inferred to have been enhanced due to this fluid phase, resulting in rapid grain growth from <5 µm to -100 µm in <1.5 Myrs. Quartz in naturally deformed rocks appears to be weaker than quartz tested in laboratories at geologically unrealistic strain-rates, attributable to the integral role that water played during quartz deformation in the backshear array and the non-steady-state conditions of deformation. Both of these aspects need to be taken under consideration when extrapolating experimental flow laws to natural conditions.