Multiscale geometries and deformation processes of the pseudotachylyte "type locality" - Outer Hebrides, Scotland
Abstract
The Outer Hebrides fault zone, Scotland, is the 'type locality' for pseudotachylytes and is one of the model localities for crustal scale ductile to brittle fault zones (Sibson, 1975; Imber et al., 2001). Here we study what has been referred to as "quasi-conglomerate" by Sibson (1975) in the footwall to the main thrust. The quasi-conglomerate layers have a black continuous matrix containing mostly round rock fragments, hence resembling a conglomerate. These roughly foliation-parallel quasi-conglomerate layers have been associated with the Outer Hebrides fault zone, though they dip more steeply. Between the layers a pervasive network of pseudotachylytes with clear injection vein and generation surface geometries is present. We map and investigate these layers of quasi-conglomerate and the pseudotachylyte networks from the macro (km) to meso (m-cm) and micro (mm-μm) scale. The quasi-conglomerates occur in several-dm thick layers which cut the host rock gneissic foliation at low angle. They are very well exposed cutting in a headland, both along the coast and inland, and can be traced along-strike and down-dip over several tens to hundreds of meters. The majority of the fragments are round, but greater proportions of angular fragments are observed in places. From field observations, at least some of the rock fragments within the layers form by brittle fracturing of the host rock. Observations of curved fractures in the wall rock imply possible primarily formation of round fragments. Two of the potential mechanismcs for rounding of primarily angular fragments are abrasion and melting. On the micro-scale, fragments are pervasively fractured. We have not observed any mylonitic fabrics in outcrops of the quasi-conglomerate or pseudotachylyte networks. This study provides new and more detailed structural data from the macro- to the micro-scale which allows for a more in-depth evaluation of the slip history of this major fault zone. We evaluate the role of dynamic versus static fracturing, paleostress fields, and strengthening and weakening mechanisms active during deformation. References: Cowan, D. S. (1999): Do faults preserve a record of seismic slip? A field geologist's opinion. J Struct Geol, 21(8-9):995-1001. Imber, J., et al. (2001): A reappraisal of the Sibson-Scholz fault zone model: The nature of the frictional to viscous ('brittle-ductile') transition along a long-lived, crustal-scale fault, Outer Hebrides, Scotland. Tectonics, 20(5):601-624. Sibson, R. (1975): Generation of pseudotachylyte by ancient seismic faulting. Geophys J Int, 43(3):775-794.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T53C2599V
- Keywords:
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- 8004 STRUCTURAL GEOLOGY Dynamics and mechanics of faulting;
- 8118 TECTONOPHYSICS Dynamics and mechanics of faulting