The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand
Abstract 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...
| Main Authors: | , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
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Wiley
2018-08-01
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| Series: | Geochemistry, Geophysics, Geosystems |
| Subjects: | |
| Online Access: | https://doi.org/10.1029/2017GC007368 |
| _version_ | 1797351772404056064 |
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| author | Cécile Massiot Bernard Célérier Mai‐Linh Doan Tim A. Little John Townend David D. McNamara Jack Williams Douglas R. Schmitt Virginia G. Toy Rupert Sutherland Lucie Janku‐Capova Phaedra Upton Philippe A. Pezard |
| author_facet | Cécile Massiot Bernard Célérier Mai‐Linh Doan Tim A. Little John Townend David D. McNamara Jack Williams Douglas R. Schmitt Virginia G. Toy Rupert Sutherland Lucie Janku‐Capova Phaedra Upton Philippe A. Pezard |
| author_sort | Cécile Massiot |
| collection | DOAJ |
| description | Abstract 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. |
| first_indexed | 2024-03-08T13:06:19Z |
| format | Article |
| id | doaj.art-91508e3f97cc4a35b8e9d5285a33f172 |
| institution | Directory Open Access Journal |
| issn | 1525-2027 |
| language | English |
| last_indexed | 2024-03-08T13:06:19Z |
| publishDate | 2018-08-01 |
| publisher | Wiley |
| record_format | Article |
| series | Geochemistry, Geophysics, Geosystems |
| spelling | doaj.art-91508e3f97cc4a35b8e9d5285a33f1722024-01-18T19:36:25ZengWileyGeochemistry, Geophysics, Geosystems1525-20272018-08-011982492251510.1029/2017GC007368The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New ZealandCécile Massiot0Bernard Célérier1Mai‐Linh Doan2Tim A. Little3John Townend4David D. McNamara5Jack Williams6Douglas R. Schmitt7Virginia G. Toy8Rupert Sutherland9Lucie Janku‐Capova10Phaedra Upton11Philippe A. Pezard12GNS Science, Avalon Lower Hutt New ZealandGéosciences Montpellier Université de Montpellier, CNRS Montpellier FranceUniv. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR ISTerre, Grenoble FranceSGEES, Victoria University of Wellington Wellington New ZealandSGEES, Victoria University of Wellington Wellington New ZealandSchool of Natural Sciences, University of Ireland Galway IrelandDepartment of Geology, University of Otago Dunedin New ZealandDepartment of Physics University of Alberta Edmonton AB CanadaDepartment of Geology, University of Otago Dunedin New ZealandSGEES, Victoria University of Wellington Wellington New ZealandSGEES, Victoria University of Wellington Wellington New ZealandGNS Science, Avalon Lower Hutt New ZealandGéosciences Montpellier Université de Montpellier, CNRS Montpellier FranceAbstract 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.https://doi.org/10.1029/2017GC007368Alpine Faultultrasonic image logfoliationfracturesanisotropy |
| spellingShingle | Cécile Massiot Bernard Célérier Mai‐Linh Doan Tim A. Little John Townend David D. McNamara Jack Williams Douglas R. Schmitt Virginia G. Toy Rupert Sutherland Lucie Janku‐Capova Phaedra Upton Philippe A. Pezard The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand Geochemistry, Geophysics, Geosystems Alpine Fault ultrasonic image log foliation fractures anisotropy |
| title | The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand |
| title_full | The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand |
| title_fullStr | The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand |
| title_full_unstemmed | The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand |
| title_short | The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand |
| title_sort | alpine fault hangingwall viewed from within structural analysis of ultrasonic image logs in the dfdp 2b borehole new zealand |
| topic | Alpine Fault ultrasonic image log foliation fractures anisotropy |
| url | https://doi.org/10.1029/2017GC007368 |
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