Extensional shearing deformation and kilometer‐scale “a”‐ type folds in a Cordilleran Metamorphic Core Complex (Raft River Mountains, northwestern Utah)

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Abstract
Cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations. In the Raft River Mountains a mylonitic zone and a late superimposed detachment fault separate Precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous Paleozoic rocks which suffered a brittle extensive deformation. We suggest that the mylonitic ductile deformation is the result of Tertiary extensional tectonics. Strain analysis shows that (1) strain results from large‐scale progressive shearing deformation in a non‐coaxial regime, (2) E–W regional stretching lineations represent the direction of shearing and transport, (3) shear criteria indicate an east directed movement, (4) strain magnitude is increasing eastward, (5) kilometer‐scale folds parallel to the lineations are interpreted as “a”‐type folds formed by eastward shearing of a previously northward dipping layering. Measurement of striated faults in the upper plate gives an E–W direction of extension parallel to the lineations. Thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics. A model is proposed to explain the observed geometry and kinematics of the structures. The Mesozoic to Paleocene Cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes. Important uplift, abnormal heating, and granite emplacement occurred later during the collapse of the former belt. The combined effects of thermal and gravitational instabilities during Tertiary extensional tectonics enabled the development of a ductile shear zone localized at the Precambrian basement‐Paleozoic cover interface. The brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust.