Structure and ancient seismicity in the Moonlight Fault Zone, Matukituki Valley, Wanaka.
The Moonlight Fault Zone (MFZ) is a regionally significant structure in Otago that was reactivated as a high angle reverse fault in the Miocene. This movement exhumed fault rocks from the mid to upper crust which has provided the opportunity to study the structure of the fault zone from this depth, the dominant deformation processes that occurred during faulting and, significantly, the weakening mechanisms that may have facilitated high angle reverse movements. Field and microstructural examination of the Moonlight Fault Zone in the Matukituki Valley revealed that solidified frictional melts (pseudotachylytes) are present within the hanging wall greenschist at least 500 m from the main fault trace. The pseudotachylytes lie parallel or sub-parallel to the steeply-dipping host rock foliation. The presence of pseudotachylytes indicates that the hanging wall was exhumed from at least 5 km depth and that brittle failure within the MFZ occurred, at least in part, by localised seismic slip. Along the main trace of the Moonlight Fault there is a c. 15 m wide zone of deformation that contains a progressive transition from random fabric breccias to well foliated cataclasites ≤1 m from the fault trace. The foliated cataclasites contain microstructural evidence (e.g. dissolution seams enriched in titanite, overgrowths of chlorite in strain-shadows) of fluid-induced dissolution – precipitation reactions associated with diffusive mass transfer. Alteration of load bearing phases such as quartz and feldspar led to the widespread formation of chlorite and muscovite in the main fault. This produced well foliated, interconnected networks of weak phyllosilicate-rich fault rocks. The presence of interconnected phyllosilicates may have lowered the frictional strength of the Moonlight Fault and thus likely contributed towards reactivation of this poorly oriented, high angle reverse fault. The network of foliated phyllosilicates may also have acted as a fluid seal, allowing for build-up in fluid pressure in the footwall and leading to further weakening. The close association between pseudotachylytes and phyllosilicates (containing evidence for dissolution – precipitation) suggests that the MFZ preserves fault rock evidence for both seismic slip and slower aseismic creep.
x, 99 pages A4
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Location (WKT, WGS84):
POLYGON ((168.754416137838149 -44.500151899889865,168.750598124707892 -44.545741231960108,168.688426798921029 -44.541977734441211,168.69614379829369 -44.498324191733104,168.754416137838149 -44.500151899889865))
Alder, Simon, “Structure and ancient seismicity in the Moonlight Fault Zone, Matukituki Valley, Wanaka.,” Otago Geology Theses, accessed February 17, 2019, http://theses.otagogeology.org.nz/items/show/567.