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The Alpine Fault hanging-wall viewed from within: Structural analysis of the ultrasonic image logs in the DFDP-2B borehole, New Zealand

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posted on 10.02.2021, 22:14 by C Massiot, B Célérier, ML Doan, TA Little, John Townend, DD McNamara, J Williams, DR Schmitt, VG Toy, Rupert Sutherland, LJ Capova, P Upton, PA Pézard
©2018. American Geophysical Union. All Rights Reserved. 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.

History

Preferred citation

Massiot, C., Célérier, B., Doan, M. L., Little, T. A., Townend, J., McNamara, D. D., Williams, J., Schmitt, D. R., Toy, V. G., Sutherland, R., Capova, L. J., Upton, P. & Pézard, P. A. (2018). The Alpine Fault hanging-wall viewed from within: Structural analysis of the ultrasonic image logs in the DFDP-2B borehole, New Zealand. Geochemistry, Geophysics, Geosystems, 19(8), 2492-2515. https://doi.org/10.1029/2017GC007368

Journal title

Geochemistry, Geophysics, Geosystems

Volume

19

Issue

8

Publication date

01/01/2018

Pagination

2492-2515

Publisher

American Geophysical Union (AGU)

Publication status

Published

Contribution type

Article

Online publication date

01/01/2018

ISSN

1525-2027

eISSN

1525-2027

Language

en

Exports