HR: 17:15h
AN: V14A-06    [Abstracts]
TI: Transient Creep and Mechanical Instabilities in the Lower Crust: the Long and the Short of It All
AU: * White, J C
EM: clancy@unb.ca
AF: University of New Brunswick, Box 4400, Fredericton, NB E3B 5A3 Canada
AB: The recognition that non-steady, heterogeneous deformation is the natural response of the lower crust to imposed asthenosphere/lithosphere displacements enables the explanation of the ubiquitous occurrence of instabilities and strain localization that are central to the question of crustal strength. In essence, tectonic forcing over large characteristic length and time scales does not occur under homogeneous conditions leading to the need to understand how a volume of crust is deforming at any given time versus how it has deformed over an extended period. Strain rate as a scale-related normalization parameter is fundamentally related to the volume of rock in which imposed deformations are accommodated. Bulk shear strain rates for perfectly homogeneous strain can be calculated from velocity/width of accommodation zones. Lower limits can be estimated for natural strain rates by assuming that plate velocities are wholly and homogeneously accommodated though whole-mantle convection/shear, giving strain rates one the order of 10^-15s^-1. Similarly, homogeneous whole-crust deformation within continental crust would have to occur in accommodation zones 30-100 km thick, giving the "typical strain rate" of 10^-14s^-1. Partitioning of displacements into the heterogeneous geometries observed in lower crustal rocks requires that strain rates in these zones of narrower thickness be higher over a wide range of imposed particle velocities. Strain rate and strength will be directly linked. Microstructure records suggest two distinct strength regimes for the lower crust. Rocks are most commonly found in the low strength regime where stresses range from 1-50 MPa, with mylonites recording stresses order of 100 MPa. Less commonly observed are high strength records of 200 MPa or greater. Whereas seismic displacement rates and slow crustal deformation serve as end-member strain rates, microstructure studies indicate that fast strain rates approaching that of deformation experiments and high stresses occur, even at high homologous temperature. The transience of stress in the lower crust is demonstrated by the cyclic and mutual overprinting of high- and low-stress microstructures. These records correspond to development of mechanical instability during ductile deformation. The fast strain rates reflect build-up of high stress, and both are indicative of work hardening during plastic deformation. Although very localized, this behavior emphasizes the scale at which strain is actively accommodated as opposed to the finite volume of rock that is deformed. Mechanical instability in the lower crust and subsequent strain softening associated with localization are observed to have a set of common attributes. Evidence of work hardening is ubiquitous, in the form of high-stress microstructures, the limit being formation of brittle or plastic rupture. Rupture instabilities may be accompanied by cataclasis and are typically associated with the introduction of new materials such as veins, pegmatites, partial melts and pseudotachylyte. The introduction of new material as partial melts or veins suggests the presence of sufficient fluid to generate pressure transients associated with the rupture. Major textural changes and mechanism transformations that occur as a result of the instability contribute to work softening and stabilization of the rupture as a zone of localized displacement (high strain).
DE: 8110 Continental tectonics: general (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8160 Rheology: general (1236, 8032)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005