Evolution of structures and fabrics in the Barbados Accretionary Prism ; Insights from Leg 110 of the Ocean Drilling Program.

Behrmann, Jan H. , Brown, Kevin, Moore, J.C., Mascle, A., Taylor, E., Alvarez, F., Andreieff, P., Barnes, R., Beck, C., Blanc, G., Clark, M., Dolan, J., Fisher, A., Gieskes, J., Hounslow, M., McLellan, P., Moran, K., Ogawa, Y., Sakai, T., Schoonmaker, J., Vroluk, P., Wilkens, R. and Williams, C. (1988) Evolution of structures and fabrics in the Barbados Accretionary Prism ; Insights from Leg 110 of the Ocean Drilling Program. Open Access Journal of Structural Geology, 10 (6). pp. 577-591. DOI 10.1016/0191-8141(88)90025-9.

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Abstract

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Document Type: Article
Keywords: Barbados Accretionary Prism, leg 110, Ocean Drilling Program,
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/0191-8141(88)90025-9
ISSN: 0191-8141
Date Deposited: 07 Feb 2013 11:50
Last Modified: 17 Jan 2018 09:51
URI: http://oceanrep.geomar.de/id/eprint/19479

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