Structure and fault rock sequences of the Moonlight Fault Zone, West Otago: constraints on fault reactivation during basin inversion

Abstract

Large normal faults are often reactivated as high-angle reverse faults during compressional basin inversion. Although a common situation worldwide, basin inversion is poorly understood from a mechanical perspective, as high-angle reverse faults are severely misoriented for reactivation (frictional ‘lock-up’ should occur at dips of c. 60°). Prevailing models of failure on high-angle reverse faults rely on fluid overpressure, however such models are calculated on the assumption of Byerlee-type friction (friction coefficient of 0.6 – 0.85). The Moonlight Fault Zone in New Zealand, is a >200 km long Oligocene basin-bounding normal fault that reactivated in the Miocene as a high-angle reverse fault during basin inversion (present dip angle 65°-77°). Excellent exposures of the fault zone exhumed from c. 4-10 km depth are found in creek sections along the entire strike length. This thesis presents field and microstructural observations concerning the structure and fault rock assemblages found in five creek sections within the MFZ, and aims to provide some of the first observational constraints on the structure and possible mechanical properties of reactivated normal faults. In the MFZ wall rocks are mainly quartz-albite-muscovite-chlorite schists with a strong foliation that is everywhere sub-parallel to the Moonlight Fault (i.e. dip angle 65°-75°), with deformation varying in response to host rock composition. Where the hanging wall consists of well foliated, intact greenschist or quartzofeldspathic gneiss, pseudotachylyte is present lying largely sub-parallel to the foliation. Where the hanging wall consists of fissile greyschist it is host to foliation-parallel fault breccias. The footwall is mainly greyschist where fault movements resulted in the formation of meso to macro scale folds whose fold axial planes lie parallel to the orientation of the Moonlight Fault. Where folding has not accommodated all reverse-slip, Moonlight Fault-parallel breccias are present. Although the overall structure of the fault zone changes significantly along strike in response to wall rock composition, the fault core always contains interconnected layers of foliated cataclasite or gouge rich in authigenically-grown chlorite and muscovite which is regionally significant and, critically, interconnected on a regional scale. The fault core is regularly flanked by a zone of breccia which at times shows a strain transition into the <5 metre thick fault core. Microstructural evidence suggests deformation in the fault core was accommodated by a combination of cataclasis, frictional slip along phyllosilicate seams and dissolution-precipitation. Published frictional strength measurements for chlorite and muscovite (friction coefficient of 0.32-0.38) are used to explore mechanical models of frictional reactivation along high-angle reverse faults. Results show that low-friction fault cores increase the frictional lock-up angle to 71°, allowing for easier reactivation of faults that initially formed at 60°. These results indicate that low frictional strength may play an important role in slip on high-angle reverse faults during basin inversion.