T11G-05
Tectonic stress and fault rock fabrics in the vicinity of the Alpine Fault inferred from DFDP-2 borehole televiewer imagery

Monday, 14 December 2015: 09:00
306 (Moscone South)
Norio Shigematsu1, Cecile Massiot2, John Townend3, Mai-Linh Doan4, David D Mcnamara5, Virginia Toy6, Rupert Sutherland5 and DFDP-2 Science Team, (1)Geological Survey of Japan, Tsukuba Ibaraki, Japan, (2)Victoria University of Wellington, Wellington, New Zealand, (3)Victoria University of Wellington, School of Geography, Environment, and Earth Sciences,, Wellington, New Zealand, (4)ISTerre Institute of Earth Sciences, Saint Martin d'Hères, France, (5)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand, (6)University of Otago, Dunedin, New Zealand
Abstract:

The present-day upper crustal stress regime near the central Alpine Fault represents the superposition of topographic and tectonic stresses late in the fault’s typical earthquake cycle. Previous estimates based on focal mechanism analysis of the principal stress orientations at seismogenic depths reveal a regional axis of maximum horizontal compressive stress trending approximately 115°. However seismicity adjacent to the Alpine Fault is sparse and does not constrain the stress field close to the fault or with sufficient spatial resolution to distinguish variations. The analysis of fractures revealed in borehole televiewer (BHTV) logs collected during DFDP-2B drilling provides an opportunity to examine stresses in the hanging-wall of the Alpine Fault. Here we present the results of stress analysis utilizing a dataset consisting of 418 fractures. On average, many of the fractures dip at 50-60° towards the southeast, consistent with the regional attitude of the Alpine Fault. We compute stress parameters using the Hough transform method, which enables us to analyze incomplete fault slip measurements that do not contain slip directions. For this analysis, we assumed that all fractures represent reverse-faulting components in response to a single homogeneous stress tensor, and that fractures with similar geometries to the Alpine Fault involve similar top to the west shear. Our preliminary analysis of the dataset as a whole yields orientations (trends/plunge) for the maximum and minimum compressive stress axes σ1 and σ3 of 129/26 and 27/23 (±30°), respectively and a stress ratio of (σ2–σ3)/(σ1–σ3)=~0.3. These parameters yield an axis of maximum horizontal compressive stress of 136±30°, implying that the trend of σHmax is rotated clockwise from the regional azimuth into a direction more perpendicular to the strike of the Alpine Fault near the fault. The orientations are compatible with previously determined horizontal shortening from analysis of small scale fractures.

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