2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 58-5
Presentation Time: 10:00 AM

QUANTIFIED VERTICAL STRAIN PROFILE OF MID-CRUSTAL DEFORMATION IN THE GREATER HIMALAYAN SEQUENCE, ANNAPURNA-DHAULAGIRI HIMALAYA, NEPAL


PARSONS, Andrew J., Institute of Geophysics & Tectonics, University of Leeds, Leeds, LS2 9JT, United Kingdom, PHILLIPS, Richard J., Institute of Geophysics and Tectonics, University of Leeds, Leeds, LS2 9JT, United Kingdom, LLOYD, Geoffrey E., Institute of Geophysics and Tectonics, University of Leeds, LS2 9JT, United Kingdom, SEARLE, Micheal P., Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR and LAW, Richard D., Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Understanding crustal processes that occur during continental collision requires knowledge of the distribution of strain across shear zones and exhumed terranes. Variations in the strength of crystallographic preferred orientation (CPO) fabrics in tectonically deformed rocks provide a proxy for the relative strain magnitudes under which the CPOs formed. We present a quantified relative strain magnitude profile for the Greater Himalayan Sequence (GHS), which forms the metamorphic core of the Himalayan orogeny, derived from CPO data.

With increased crystal plastic strain, the degree of mineral alignment, and thus CPO strength, increases. Samples were collected for CPO analysis from the GHS and bounding units in the Kali Gandaki valley of the Annapurna-Dhaulagiri region, central Nepal. CPO measurements were made via Electron Back Scattered Diffraction (EBSD). CPO strength of each sample is calculated with an eigenvalue-based intensity parameter and stratigraphically arranged into a single plot to produce a quantified relative strain magnitude profile for the GHS and bounding units.

CPO fabric strengths reveal high strain zones at the margins of, and possibly within the amphibolite facies rocks of the Upper GHS. Variations in CPO strength also reveal strain gradients above and below the Upper GHS that decrease vertically outwards over several kilometres. Associated deformation temperatures indicate that the migration of deformation mimics a decrease in metamorphic grade both up-section and down-section from the Upper GHS. As such, this strain profile reveals the extrusion and exhumation history of the GHS.

This technique reveals the kinematic evolution of the GHS and provides a method with which to investigate the detailed distribution of strain across other exhumed terranes and shear zones. As such, further application of this technique is encouraged to develop its usefulness and to extend our understanding of the distribution of strain in the crust.

Session No. 58

T26. Deformation Localization throughout the Crust (Posters)
Sunday, 19 October 2014: 9:00 AM-5:00 PM
Exhibition Hall C (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.158
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