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Deformation and seismic style of oceanic transform faults: Evidence from the Southern Troodos Transform Fault Zone, Cyprus

Cox, Sophie 2021. Deformation and seismic style of oceanic transform faults: Evidence from the Southern Troodos Transform Fault Zone, Cyprus. PhD Thesis, Cardiff University.
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Oceanic transform faults are considered relatively weak structures and are dominantly aseismic, but occasionally host larger earthquakes (Mw >6.0) that occur quasiperiodically. The seismic behaviour varies along-strike, down-dip and between different oceanic transforms. Several mechanisms have been proposed to explain this mechanical weakness and observed variability in seismic behaviour. These include: (1) hydration and alteration of mafic and ultramafic minerals to frictionally weaker phases; (2) elevated fluid pressure; (3) fluid-driven weakening mechanisms and strain localisation and (4) spatial variation in fault zone damage and mechanical properties. There is, however, limited direct geological evidence on the mechanical properties of creeping and locked portions of oceanic transforms. This thesis investigates the geological controls on the seismic behaviour and strength of oceanic transforms using the exhumed Southern Troodos Transform Fault Zone (STTFZ) in Cyprus as an analogue for active oceanic transforms. Deformation in the STTFZ took place within the thermally-defined seismogenic zone (T <600◦C). Within serpentinite shear zones, ubiquitous fractures and macroscopically ductile serpentinite fabrics are mutually cross-cutting. These serpentinite fabrics developed progressively from lizardite-rich mesh-textured serpentinite to intensely-foliated, chrysotile-rich phyllonitic serpentinite with increased strain. Dissolution-precipitation is considered an important fabric-forming mechanism during this transition. Within the mafic crust, jointed dolerite dykes were progressively deformed to generate fault breccias and chlorite-rich fault zones, and brittle fracture and gouge formation were important deformation processes. Direct-shear friction experiments on natural samples show that chlorite-rich faults are weak (µ <0.3) and velocity-strengthening, while less deformed dolerite crust is strong (µ ∼0.7) and velocity-weakening. The geological data indicate that variable deformation and alteration can control the weakness and seismic style of oceanic transforms. In this model, foliated serpentinites and chlorite-rich fault gouges represent weak and dominantly creeping portions. In contrast, less deformed and altered sections remain strong and represent locked patches where earthquakes may nucleate.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 1 November 2021
Date of Acceptance: 29 October 2021
Last Modified: 27 Sep 2022 01:10

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