scholarly journals Controls on fault zone structure and brittle fracturing in the foliated hanging-wall of the Alpine Fault

2017 ◽  
Author(s):  
Jack N. Williams ◽  
Virginia G. Toy ◽  
Cécile Massiot ◽  
David D. McNamara ◽  
Steven A. F. Smith ◽  
...  
Keyword(s):  
Fault Zone ◽  
Hanging Wall ◽  
Plate Boundary ◽  
Field Measurements ◽  
Mechanical Anisotropy ◽  
Zone Structure ◽  
Fault Zone Structure ◽  
Alpine Fault ◽  
Fracture Damage ◽  
Borehole Televiewer

Abstract. The orientations and densities of fractures in the foliated hanging-wall of the Alpine Fault provide insights into the role of a mechanical anisotropy in upper crustal deformation, and the extent to which existing models of fault zone structure can be applied to active plate-boundary faults. Three datasets were used to quantify fracture damage at different distances from the Alpine Fault principal slip zones (PSZs): (1) X-ray computed tomography (CT) images of drill-core collected within 25 m of the PSZs during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections at

scholarly journals Supplementary material to "Controls on fault zone structure and brittle fracturing in the foliated hanging-wall of the Alpine Fault"

2017 ◽  
Author(s):  
Jack N. Williams ◽  
Virginia G. Toy ◽  
Cécile Massiot ◽  
David D. McNamara ◽  
Steven A. F. Smith ◽  
...  
Keyword(s):  
Fault Zone ◽  
Hanging Wall ◽  
Zone Structure ◽  
Fault Zone Structure ◽  
Alpine Fault ◽  
Supplementary Material

scholarly journals Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the Alpine Fault

Solid Earth ◽  
2018 ◽  
Vol 9 (2) ◽  
pp. 469-489 ◽  
Author(s):  
Jack N. Williams ◽  
Virginia G. Toy ◽  
Cécile Massiot ◽  
David D. McNamara ◽  
Steven A. F. Smith ◽  
...  
Keyword(s):  
Fault Zone ◽  
Hanging Wall ◽  
Damage Zone ◽  
Ct Images ◽  
Mechanical Anisotropy ◽  
Drill Core ◽  
Fracture Density ◽  
Alpine Fault ◽  
Wide Range ◽  
Confining Pressures

Abstract. Three datasets are used to quantify fracture density, orientation, and fill in the foliated hanging wall of the Alpine Fault: (1) X-ray computed tomography (CT) images of drill core collected within 25 m of its principal slip zones (PSZs) during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections up to 500 m from the PSZs, and (3) CT images of oriented drill core collected during the Amethyst Hydro Project at distances of  ∼  0.7–2 km from the PSZs. Results show that within 160 m of the PSZs in foliated cataclasites and ultramylonites, gouge-filled fractures exhibit a wide range of orientations. At these distances, fractures are interpreted to have formed at relatively high confining pressures and/or in rocks that had a weak mechanical anisotropy. Conversely, at distances greater than 160 m from the PSZs, fractures are typically open and subparallel to the mylonitic or schistose foliation, implying that fracturing occurred at low confining pressures and/or in rocks that were mechanically anisotropic. Fracture density is similar across the  ∼  500 m width of the field transects. By combining our datasets with measurements of permeability and seismic velocity around the Alpine Fault, we further develop the hierarchical model for hanging-wall damage structure that was proposed by Townend et al. (2017). The wider zone of foliation-parallel fractures represents an outer damage zone that forms at shallow depths. The distinct < 160 m wide interval of widely oriented gouge-filled fractures constitutes an inner damage zone. This zone is interpreted to extend towards the base of the seismogenic crust given that its width is comparable to (1) the Alpine Fault low-velocity zone detected by fault zone guided waves and (2) damage zones reported from other exhumed large-displacement faults. In summary, a narrow zone of fracturing at the base of the Alpine Fault's hanging-wall seismogenic crust is anticipated to widen at shallow depths, which is consistent with fault zone flower structure models.

Weak at what scale? Insights from a late interseismic interplate fault

2021 ◽  
Author(s):  
Carolyn Boulton ◽  
Catriona Menzies ◽  
Virginia Toy ◽  
Ludmila Adam ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Hanging Wall ◽  
Damage Zone ◽  
Plate Boundary ◽  
Seismogenic Zone ◽  
Low Permeability ◽  
Temperature Sensitive ◽  
Fault Rock ◽  
Alpine Fault ◽  
Brittle Ductile Transition

&lt;p&gt;The central section of the Alpine Fault accommodates a majority (~75%) of the total relative Pacific-Australian plate boundary motion on a single structure. For strain localization to occur to such an extent, the Alpine Fault must accommodate deformation at spatially and temporally averaged work rates that are lower than those required by hanging wall and footwall structures. Exhumation of a complete fault rock sequence (mylonites-cataclasites-gouges) from ~35 km depth in &lt;5 million years provides us with an unparalleled opportunity to identify the weakening mechanisms underpinning the fault&amp;#8217;s remarkable efficiency. We summarize the results of experimental, geochemical, geophysical, seismological, and geological research facilitated by the Deep Fault Drilling Project (DFDP).&lt;/p&gt;&lt;p&gt;Three main factors promote crustal-scale weakness on Alpine Fault: (1) high heat flow associated with rapid exhumation results in a shallow frictional-viscous transition at 8-10 km depth. In turn, temperature-sensitive creep (initially crystal-plasticity with an increasing contribution from grain size sensitive mechanisms during exhumation) can accommodate deformation at strain rates on the order of, and episodically higher than, 10&lt;sup&gt;&amp;#8211;12&lt;/sup&gt;s&lt;sup&gt;&amp;#8211;1&lt;/sup&gt;across a broad portion of the fault zone (from ~8 to 35 km depth). (2) Above the frictional-viscous transition, cataclastic processes associated with quasiperiodic large-magnitude earthquakes have permanently reduced the elastic moduli of damage zone rocks; and (3) cataclastic processes, combined with fluid-rock interactions, have formed low-permeability principal slip zone gouges and cataclasites. The near-ubiquitous presence of juxtaposed, low-permeability fault core gouges and cataclasites promotes dynamic (coseismic) weakening mechanisms such as thermal pressurization.&lt;/p&gt;&lt;p&gt;Clay mineral alteration reactions are commonly thought to result in fault zone weakening through a reduction in the static coefficient of friction, but fluid-rock interactions on the central Alpine Fault largely result in the precipitation of frictionally strong minerals such as calcite and, locally, K-feldspar. Although relatively narrow in down-dip extent, the brittle seismogenic zone of the central Alpine Fault is not misoriented with respect to the maximum principal stress when a full 3D stress analysis is performed. Moreover, the fault comprises frictionally strong gouges and cataclasites that can sustain high differential stresses. Combined, these factors have important implications for estimating dynamic stress drops and the extent to which future earthquake ruptures may propagate beneath the brittle-ductile transition, thereby increasing moment magnitude.&lt;/p&gt;

scholarly journals Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

2021 ◽  
Author(s):  
JD Eccles ◽  
AK Gulley ◽  
PE Malin ◽  
CM Boese ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Plate Boundary ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Catchment Scale ◽  
Near Surface ◽  
Low Velocity Zone ◽  
Alpine Fault ◽  
Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

Sign in / Sign up

Export Citation Format

Share Document