School of Earth Sciences - Theses

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Start date: 2021 × End date: 2022 × Type: PhD thesis × Subject: Thermochronology × Subject: Alpine Fault ×

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    Neotectonic Evolution of an Incipient Continental Plate Boundary Fault Intersection, Hope-Kelly Fault System, New Zealand
    Vermeer, Jessica LeeAnne ( 2022)
    The Hope-Kelly fault system forms the intersection between the plate boundary Hope and Alpine faults in the South Island of New Zealand. New fault mapping, paleoseismology, slip-rates, and low temperature thermochronology provide insights into the structure, kinematics and evolution of this fault intersection zone. Lidar, photogrammetry and field-based fault mapping reveals the transition from a dextral fault zone in the east to a splay-like zone of distributed oblique dextral-normal faults that abut the Alpine fault in the west. Structural interactions between the Hope-Kelly faults and the Alpine fault influence surface rupture geometries and kinematics, accommodate differential orogenic growth, and facilitate N-S extension that enables a slip rate change between the central and northern Alpine fault sections. Radiocarbon (14C), optically stimulated luminescence (OSL; quartz), and infrared stimulated luminescence (IRSL; feldspar) ages of fault-proximal sedimentary deposits are combined with geomorphic surface displacement measurements to derive fault slip-rates. Dextral slip-rates on the Hope Fault decrease westward from 5.6 (+2.0/-0.8) mm/yr to 1.7 (+1.0/-0.5) mm/yr. Dextral slip-rates on the Kelly Fault vary from 6.2 (+2.5/-1.2) mm/yr (east) to 2.0 (+2.5/-0.7) mm/yr (central) to 6.4 (+7.8/-1.4) mm/yr (west). Subsidiary faults have minimum slip-rates of 1.3 (+0.1/-0.4) mm/yr. Spatial variations in apparent slip rates are proposed to reflect complexities in slip localization and transfer across the complex deformation zone, slip on unrecognized, buried, and/or blind faults, and possible temporal transience in slip behaviours. Paleoseismic trenching and 14C dating of dead trees provides preliminary evidence for the most recent surface rupturing earthquake on the Taramakau section of the Hope Fault between ca. 1680 and 1840 AD, with a preferred age of ca. 1800-1840 AD. Coulomb fault stress transfer modelling of the 3D Hope-Kelly-Alpine fault intersection zone shows that slip on either the central Hope, Kelly, or central Alpine (source) faults increases Coulomb stress on the other (receiver) faults in the network, highlighting the potential for earthquake spatio-temporal clustering in this region. Zircon and apatite (U-Th)/He thermochronology is used to investigate the thermal-exhumational evolution of rocks in the Hope-Kelly-Alpine fault interaction zone. Late Miocene exhumation (3.4 - 0.8 km/Myr, assuming geothermal gradients of 33 - 40 degrees C/km) through crustal depths of approximately 5-6 km is interpreted to be controlled by proximity to the Alpine Fault, with rocks more proximal to the fault recording faster exhumation rates relative to more distal samples in the east. Establishment of the Hope-Kelly fault system in the Quaternary structurally juxtaposed rocks with discordant cooling histories. Rocks throughout the study region record increased cooling rates from circa 2 Ma. Possible causal mechanisms include increases in rock uplift and denudation rates associated with kinematic changes along the Australia-Pacific plate boundary, Quaternary glaciation, and/or increases in rock mass erodibility associated with Hope-Kelly fault system. This thesis provides new insights into a structurally complex plate boundary, with implications for analogous settings globally.