Kinematics of the Central Alpine Fault Mylonite Zone, Tatare Stream, South Island, New Zealand

Abstract

The hanging wall of the Alpine Fault (AF) near Franz Josef Glacier has been exhumed during the past ~3 m. y. providing a sample of the ductilely deformed middle crust via obliquereverse slip on the AF. The former middle crust of the Pacific Plate occurs as an eastward-tilted slab that has been upramped from depths of ~25–35 km. A mylonitic high strain zone abuts the eastern edge of the AF in Tatare Stream. This ductile shear zone is locally ~2 km thick. The Tatare Stream locality is remarkable along the AF in the Central Southern Alps for the apparent lack of near surface segmentation of the fault there; instead its mylonitic shear zone appears uniformly inclined by ~63° to the SE. I infer this foliation is parallel to the shear zone boundary (SZB). In the distal part of the mylonite zone in extensional C' shear bands cross-cut the older non-mylonitic Alpine foliation (S3), and deflect that pre-existing fabric in a dextral-reverse sense. Based on the attitude of these shears the ductile shearing direction in the Alpine mylonite zone (AMZ) during extensional shear band activity is inferred to have trended 090 ± 6° (2σ), which is ~20° clockwise of sea floor spreading based estimates for the azimuth of the Pacific Plate motion. This indicates that slip on this central part of the AF is not fully “unpartitioned”. Measurements of the mean spacing, per-shear offset, C’ orientation, and per-shear thickness on >1000 extensional C’ shears provides perhaps the largest field-based data set of extensional shear band geometrical parameters so far compiled for a natural shear zone. The mean spacing between C’ shears decreases towards the AF from ~6 cm to ~0.2 cm. The per-shear offsets (8.2 ± 5 mm 1σ) and thickness (128 ± 20 1σ) of the extensional shears remains consistent despite a finite shear strain gradient. Using shear offset data I calculate a bulk finite shear strain accommodated by slip on C’ shears of 0.4 ± 0.3 (1σ), and a mean intra-shear band (C’ local) finite shear strain of 12.6 ± 5.4 (1σ). Consistency in the intra-shear band finite shear strain throughout the mylonite zone, together with increased C’ density implies that the quartzose rocks have behaved with a strain hardening rheology as the shears evolved. The dominant C’ (synthetic) extensional shears are disposed at a mean dihedral angle of 30° ± 2.2 (2σ), whereas the C’’ (antithetic) shears are 135 ± 3° (2σ) to the foliation (SZB). The C’ and C’’ shears appear to lie approximately parallel to planes of maximum instantaneous shear strain rate from which I obtain an estimate for Wk of 0.5 for the AMZ. I have measured the geometrical orientation of Mesozoic Alpine Schist garnet inclusion trails and tracked these pre-mylonitic age porphyroblastic garnets through the distal and main mylonite zones to determine their rotational response to late Cenozoic shearing. Electron microprobe analysis indicates that all the garnets examined in Tatare Stream are prograde from the regional (M2) Barrovian metamorphism. The mean inclusion trail orientations in the distal mylonite zone have been forward rotated by 35° relative to their equivalent orientation in the adjacent, less deformed non-mylonitic Alpine Schist. This rotation is synthetic to the dextralreverse shear of the AF zone. The rotation of approximately spherical shaped garnet porphyroblasts in the distal mylonite implies a finite shear strain of 1.2 in that zone. In the main part of the mylonite zone an additional forward rotation of 46° implies a finite shear strain there of 2.8. The inclusion trail rotational axis measured trends approximately perpendicular to the shear direction and parallel to the inferred late Cenozoic vorticity vector of ductile shearing. Using GhoshFlow, a program for simulating rotation of rigid passive objects in plane strain general shear a new kinematic vorticity number (Wn) estimate of 0.5 – 0.7 is established for the AMZ. The transition zone between the distal mylonite and the main mylonite zone, though little described in the literature, is well exposed in Tatare Stream. A distinct quartz rodding lineation, inherited from the non-mylonitic schist as an object into the mylonite zone, is distorted in the plane of the foliation across the transition from SW plunges to NE plunges. Because the foliation plane is here parallel to the SZB and by special reference to strongly curved lineation traces I have been able to isolate the pure shear component of deformation considering a simple 2D deformation on that slip plane; by modeling the distortional reorientation of inherited lineations in that plane. The direction of maximum finite elongation that I calculate in this plane trends 89 ± 3.8° (2σ). I believe this records the finite strain related to the co-axial component only. The parallelism of the previously calculated mylonitic ductile shearing direction to this stretching direction (also trending 090) indicates that the late Cenozoic ductile flow path in the central AMZ has been approximately monoclinic. I estimate a Wn of 0.8 ± 0.06 (2σ) based on the observed finite shearing in the mylonite zone (garnet rotation) and on the co-axial strain observed deforming the inherited lineations.