HR: 0800h
AN: V21A-0590    [Abstracts]
TI: Can Seismology Detect Ductile Deformation in the Deep Continental Crust?
AU: * Lloyd, G E
EM: g.lloyd@earth.leeds.ac.uk
AF: School of Earth and Environment, The University, Leeds, LS2 9JT United Kingdom
AU: Halliday, J M
V21A-0590 AF: School of Earth and Environment, The University, Leeds, LS2 9JT United Kingdom
AU: Butler, R W
EM: r.butler@earth.leeds.ac.uk
AF: School of Earth and Environment, The University, Leeds, LS2 9JT United Kingdom
AU: Casey, M
EM: m.casey@earth.leeds.ac.uk
AF: School of Earth and Environment, The University, Leeds, LS2 9JT United Kingdom
AU: Kendall, J
V21A-0590 AF: School of Earth and Environment, The University, Leeds, LS2 9JT United Kingdom
AB: Various models exist to explain deformation in deep continental crust, broadly divisible into localized simple shear and distributed pure shear. Although seismic imaging is perhaps the most effective way to determine deformation at depth, the veracity of any interpretation depends critically upon knowledge of the seismic properties of the rocks. As a region of active crustal thickening and exhumation, the Nanga Parbat Massif (NPM), Pakistan Himalaya, offers insight into how localized thrust faults at the Earth's surface couple with distributed strain at depth. Gneissic, mylonitic and cataclastic rocks at the surface were sampled as proxies for lithologies and fabrics currently accommodating deformation at depth. The LPO of individual minerals were measured using SEM/EBSD, from which elastic constants, 3D seismic velocities and anisotropies were predicted. Micas contribute most to the bulk anisotropy; only when mica content is low and/or LPO is weak do other minerals contribute significantly. The anisotropy varies with deformation style: background gneiss samples have highest AVs anisotropy (9.5 percent) compared with highly strained gneiss samples (5.1-6.3 percent) and cataclasite (0.4 percent). This suggests that some shear zones are characterized by low anisotropy. Sample elastic constants were used to construct models, based on current ideas of NPM tectonics, through which P-, S- and converted waves were ray traced. Foliation orientation and intensity have dramatic impacts on seismic waves: rocks with 9.5 percent AVs and vertically aligned foliation persistent through 40km of crust induce ~1.5s AVs compared to horizontal alignment that induces only 0.2s AVs. Mode conversions and transverse component energy are also diagnostic of foliation intensity and orientation. The models indicate that it is possible to discriminate between different deformation processes active at depth using seismic measurements and we conclude that the dominant process in the NPM is distributed strain accommodated by horizontal shortening and vertical stretching.
DE: 3625 Petrography, microstructures, and textures
DE: 3909 Elasticity and anelasticity
DE: 7205 Continental crust (1219)
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8108 Continental tectonics: compressional
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005