Kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range

Abstract

Analysis of shear criteria enables the kinematics of two main ductile-shearing events (D1 and D2) to be established in the Raft River, Grouse Creek and Albion ‘metamorphic core complex’. The first event (D1) is a NNE-thrusting and corresponds to Mesozoic shortening. A well developed non-coaxial ductile deformation (D2), of Cenozoic age, is marked by the occurrence of opposing eastward (in Raft River) and westward shear criteria (in Albion-Grouse Creek). These characterize an arch structure where the shear strain increases outwards. In the axial zone of the complex, D2 seems coaxial. Cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust (involving uplift, erosion and abnormal heating). Brittle extension occurs in the upper part of the uplifted domain. It is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the Precambrian-Paleozoic interface. This extension of the middle and lower crust occurred in the vicinity of the root zones of the former Mesozoic thrusts, which may thus have been reactivated as ductile normal faults during Cenozoic extension.