How does structural and rheological evolution influence the seismicity of oceanic transform faults?
Abstract
Oceanic transform faults are deficient in crustal seismicity relative to faults of comparable length in other tectonic settings. They display enigmatic variations in along-strike and down-dip behaviour with ~85% displacement accommodated aseismically around seismically coupled patches that episodically generate earthquakes. From studies of active examples, oceanic transform faults are interpreted as segmented into two end-member seismic behaviours: one characterised by relatively intact rock, where moderately-sized earthquakes can nucleate, and another hosting microseismicity and aseismic creep, interpreted as highly damaged zones acting as rupture barriers. Although inferred from relatively high and low seismic velocities, respectively, there are no direct geological observations describing these variably deforming fault patches and their rheology.
Here, we constrain seismic style along an oceanic transform fault from direct geological observations. We use field data from the Southern Troodos Transform Fault Zone (STTFZ), a <5 km wide exhumed oceanic transform within the Cretaceous aged Troodos Ophiolite, Cyprus, preserving the 3D structure of an oceanic transform fault. The STTFZ allows direct observation of deformation within sheeted dolerite dykes, representative of oceanic crust deformation. Geological mapping from km to cm scale reveals variability in fault rocks at all scales. We interpret the range of fault rocks in the sheeted dykes to record a progressive finite strain gradient, increasing from discrete faults and breccias, predominately along dyke margins, to matrix-rich gouge zones containing angular to rounded dyke clasts. Grain size distributions of these fault rocks reveal fractal dimensions of 1.2 < D2D< 2.0, increasing with inferred shear strain. Our observations indicate that the structure and rheology of the STTFZ crust evolved, spatially and temporally, from localised to distributed slip as damage increased. This transition may relate to a change from velocity-weakening to velocity-strengthening behaviour as slip delocalises. This interpretation supports a correlation of increased fault zone damage with creeping fault segments, as inferred from changes in seismic velocity along active oceanic transforms.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43H0418C
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS