The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand
Ultrasonic image logs acquired in the DFDP‐2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation...
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Veröffentlicht in: | Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2018-08, Vol.19 (8), p.2492-2515 |
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Zusammenfassung: | Ultrasonic image logs acquired in the DFDP‐2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation and foliation‐parallel veins and fractures, has a constant orientation. The average dip direction of 145° is subparallel to the dip direction of the Alpine Fault, and the average dip magnitude of 60° is similar to nearby outcrop observations of foliation in the Alpine mylonites that occur immediately above the Alpine Fault. We suggest that this foliation orientation is similar to the Alpine Fault plane at ∼1 km depth in the Whataroa valley. The other two auxiliary feature sets are interpreted as joints based on their morphology and orientation. Subvertical joints with NW‐SE (137°) strike occurring dominantly above ∼500 m are interpreted as being formed during the exhumation and unloading of the Alpine Fault's hangingwall. Gently dipping joints, predominantly observed below ∼500 m, are interpreted as inherited hydrofractures exhumed from their depth of formation. These three fracture sets, combined with subsidiary brecciated fault zones, define the fluid pathways and anisotropic permeability directions. In addition, high topographic relief, which perturbs the stress tensor, likely enhances the slip potential and thus permeability of subvertical fractures below the ridges, and of gently dipping fractures below the valleys. Thus, DFDP‐2B borehole observations support the inference of a large zone of enhanced permeability in the hangingwall of the Alpine Fault.
Plain Language Summary
Fluids circulating near faults affect the occurrence of earthquakes. Near the Alpine Fault, South Island of New Zealand, fluids circulate through fractures. For the first time, measurements made in the DFDP‐2B borehole reveal the nature and orientation of >2200 fractures and other features along a transect approaching the Alpine Fault to a depth of 818m below surface. Based on these measurements, we propose an orientation of the Alpine Fault at ~1km below surface in the Whataroa Valley, which to date has not been precisely imaged from other surveys. We observe three fracture sets: one present all along the borehole and parallel to the inferred Alpine Fault orientation; and two others whose proportions switch with depth, reflecting deep fracturing under high fluid pressures and unloadin |
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ISSN: | 1525-2027 1525-2027 |
DOI: | 10.1029/2017GC007368 |