Rheology of selected continental lower crustal rocks

Abstract

To gain insight into the rheology of selected feldspar-bearing rocks from the continental lower crust (CLC), three compositionally-diverse assemblages from exposed PreCambrian lower crustal terrains and a microgabbro have been deformed to about 10% strain at confining pressures of 0.8-1.0 GPa, temperatures of 600°-900°C, and constant strain rates of 10^-4-1-^-7s^-1 The most extensive work was done on the mafic Pikwitonei transitional granulite and the felsic Adirondack granulite. The Pikwitonei (Mg-hornblende-plag (An[70])-2px) has only a limited T, ε range of apparent steady-state behavior, and strain-softens at T [greater than or equal to 800°C at ε = 10^-5-10^-6s^-1, apparently associated with developing shear zones and increased crystal plasticity in amphibole and plagioclase grains. This strain-softening is pressure-insensitive from P[c] = 0.8-1.0 GPa at T = 850°C, ε = 10^-6s^-1 even though the dominant strain accommodation mechanisms change from semibrittle/cataclastic to crystal plastic with increasing pressure. Plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs. Microprobe analyses suggest that amphibole breakdown is not involved in the experimental strain-softening. However, in CLC, strain-softening related to amphibole-out reactions may play a large role in the development of detachments in upper-amphibolite to granulite facies transition zones. Kapuskasing (plag-cpx-gt) granulite, deformed at 800°C and 850°C, 10^-6s^-1 has deformed in the steady-state at stress levels only slightly greater than the Pikwitonei. The Stillwater microgabbro and Adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady-state behavior over the entire range of conditions investigated. For the Adirondack (2 felds-2 px-gt-qtz) granulite, the creep rate at these low strains appears to be controlled primarily by ductile deformation of quartz (21 vol. %), and secondarily by flow in feldspars (50 vol. %); mechanical data fit a Weertman power-law with A = 8 x 10^-3 MPa[^-ns^-1], H = 243 kJ mol^-1 and n = 3.1. Extrapolation of this flow law and of preliminary power-law creep relations for the Pikwitonei and microgabbro to a range of pressure, temperature, and strain rate conditions assumed to encompass those at the continental Moho supports geological and geophysical inferences that mechanical discontinuity occurs at this transition zone, with the CLC generally being the weaker material...