2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 304-3
Presentation Time: 9:00 AM-6:30 PM


ORLANDINI, Omero F.1, MAHAN, Kevin H.2, ALLAZ, Julien M.2, BRENNER, Kelly A.1, REGAN, Sean P.3 and WILLIAMS, Michael L.3, (1)Department of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309, (2)Department of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309-0399, (3)Department of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, Amherst, MA 01003, orlandin@colorado.edu

Pseudotachylytes (PST) are veins of amorphous material generated by highly localized strain, and one of the most reliable geological indicators of seismic slip. Frequently, the amorphous material forms from the quenching of a high temperature frictional flash-melt on sliding surfaces, yielding varyingly clast-rich veins of PST. It is commonly difficult to directly extract timing and ambient P-T conditions of PST formation, due to the disequilibrium of solidification resulting from an extreme cooling gradient (potentially > Δ1,000 °C cm-1), as well as the presence of scattered clastic host material that is difficult to distinguish from neoblastic phases. PST from at or below the traditional frictional-plastic transition of the crust (~12-18 km), however, is susceptible to overprint from both crystal-plastic deformation and associated metamorphic crystallization, which likely promotes re-equilibration from the metastable amorphous PST material.

The Cora Lake shear zone is a 4-6 km wide and >90 km long granulite to upper amphibolite-facies (1.06 – 0.8 GPa, 800-700 °C) sinistral strike-slip structure that was active in an intracontinental setting during the earliest stages of the Trans-Hudson Orogeny (~1.88 Ga). The shear zone hosts an extensive network of variably deformed and recrystallized PST. One sample contains four distinct generations of PST; the oldest three (1-3) are plastically deformed and contain chemically distinct garnet populations, whereas the youngest generation (4) shows no deformation and very little re/neo- crystallization. Generations 1-3 contain sinistral shear sense indicators, and the presence of neoblastic micron-sized orthopyroxene, garnet and plagioclase in the earliest generation argues strongly for the synkinematic formation of PST networks within the shear zone while in the deep crust. Preliminary thermobarometry of the earliest generation using the Al-in-Opx thermometer and the Grt-Opx-Pl-Qz barometer indicate minimum PST formation conditions of 0.74 GPa and 760 °C. Thermobarometric calculations and equilibrium phase diagrams for each of the three earliest PST veins can provide a rare window into the timing and conditions of alternating brittle and ductile processes in the lower crustal levels of an intracontinental strike-slip structure.