Extensional shear band development on the outer margin of the Alpine mylonite zone,Tatare Stream,Southern Alps,New Zealand |
| |
| Institution: | 1. Department of Geosciences and Center for Geomechanics, Geofluids, and Geohazards, Deike Building, The Pennsylvania State University, University Park, PA 16802, USA;2. Center for Tectonophysics and Department of Geology and Geophysics, MS 3115, Texas A&M University, College Station, TX 77843, USA;1. Faculty of Earth Sciences, Department of Geology, Shahid Beheshti University, Tehran, Iran;2. Museum of Natural History/Department of Earth Science, University of Bergen, Postboks 7803, N-5020 Bergen, Norway;1. Instituto de Geociências, Departamento de Mineralogia e Geotectônica, Universidade de São Paulo, Rua do Lago 562, CEP 05508-900 São Paulo, SP, Brazil;2. Universidade Federal do Paraná, Departamento de Geologia, Rua Francisco Heráclito dos Santos, 100, Bloco VI, CEP 35400000 Curitiba, PR, Brazil;3. School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia |
| |
| Abstract: | Schistose mylonitic rocks in the central part of the Alpine Fault (AF) at Tatare Stream, New Zealand are cut by pervasive extensional (C′) shear bands in a well-understood and young, natural ductile shear zone. The C′ shears cross-cut the pre-existing (Mesozoic—aged) foliation, displacing it ductilely synthetic to late Cenozoic motion on the AF. Using a transect approach, we evaluated changes in geometrical properties of the mm–cm-spaced C′ shear bands across a conspicuous finite strain gradient that intensifies towards the AF. Precise C′ attitudes, C′-foliation dihedral angles, and C′–S intersections were calculated from multiple sectional observations at both outcrop and thin-section scales. Based on these data the direction of ductile shearing in the Alpine mylonite zone during shear band activity is inferred to have trended >20° clockwise (down-dip) of the coeval Pacific-Australia plate motion, indicating some partitioning of oblique-slip motion to yield an excess of “dip-slip” relative to plate motion azimuth, or some up-dip ductile extrusion of the shear zone as a result of transpression, or both. Constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary (SZB). Across all examined parts of the shear zone, the mean dihedral angle between the C′ shears and the mylonitic foliation (S) remains a constant 30 ± 1° (1σ). The aggregated slip accommodated on the C′ shear bands contributed only a small bulk shear strain across the shear zone (γ = 0.6–0.8). Uniformity of per-shear slip on C′ shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain-hardening rheology, such that they locked up at a finite shear strain (inside C′ bands) of 12–15. Shear band boudins and foliation boudins both record extension parallel to the SZB, as do the occurrence of extensional shear band sets that have conjugate senses of slip. We infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with Wk < 0.8, and perhaps even as low as <0.5. The C′ shear bands near the AF formed in a thinning/stretching shear zone, which had monoclinic symmetry, where the direction of shear-zone stretching was parallel to the shearing direction. |
| |
| Keywords: | Shear zone Extensional shear bands Mylonite Shear zone kinematics Alpine fault New Zealand |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
