Abstract

Lithospheric-scale thrusts from the west Newfoundland ophiolite belt (White Hills Peridotite shear zone) and the south-western Grenville Province (Parry Sound shear zone) involve rocks of lower crustal and (or) upper mantle origin that exhibit intense crystal-plastic deformation of plagioclase, K-feldspar, orthopyroxene, and clinopyroxene, minerals that are commonly viewed as representative of low-ductility phases. The occurrence of this extreme deformation in shear zones that exhibit similar lower crustal syntectonic PT conditions suggests a phenomenological link between the megathrust environment and both the generation and subsequent preservation of the observed deformation microstructures. An empirical homologous parameter is constructed in an attempt to characterize conditions for similar behaviour among different minerals and to explore the feasibility of refining a threshold recovery–preservation condition within the megathrusts studied. This parameter predicts, at the estimated syntectonic temperature of 800 °C, the similarity of microstructures in highly strained albite and orthopyroxene crystals observed in both megathrusts. This temperature is interpreted as a lower limit for the upper threshold of microstructure preservation in albite and orthopyroxene for the particular megathrust history. Comparison of tectonic constraints with strain rates calculated at the inferred threshold temperature for several minerals with tectonic constraints indicates that strain rates of at least 10−12 s−1 are both rheologically possible and geometrically plausible in shear zones of kilometre-scale widths. The associated lithosphere strength during megathrust displacement is on the order of 1–50 MPa. These data support formation of synkinematic records within shear zones that preserve evidence of lithospheric behaviour over crustal-thickness length scales.

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