Paper No. 3
Presentation Time: 8:30 AM


BEHR, Whitney M., Department of Geological Sciences, Jackson School of Geoscience, UT Austin, 2275 Speedway Stop C9000, Austin, TX 78757, COOPER, Frances J., School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, England and PLATT, John P., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740,

Mylonites in Cordilleran metamorphic core complexes represent ductile deformation in the middle crust, but it is unclear how these mylonites relate to the brittle detachment faults that overlie them. Field, microstructural, and geochronological data from the Whipple Mtns and northern Snake Range core complexes suggest the existence of two rheological transitions in the middle crust, which may help explain the observed structural relationships. 1) The brittle-ductile transition (BDT), which depends on temperature and strain rate, marks the switch from discrete brittle faulting to continuous, but still localized, ductile shear. 2) A deeper dominantly temperature-dependent transition marks the switch from localized ductile shear to distributed ductile flow; this is the localized-distributed transition or LDT. In this model, detachment faults are brittle in the upper crust, but persist as low angle ductile shear zones below the BDT, and become horizontal and sole into the LDT at greater depth.

In the Whipple Mtns, mylonitic foliation on the west side of the range dips away from the Whipple detachment and shows an abrupt strain gradient into undeformed rock above, forming a ‘mylonitic front’. Microstructural observations indicate that deformation temperatures along the front were > 450°C and differential stresses were < 25 MPa, suggesting that it represents an exhumed LDT. Above the front but below the Whipple detachment, we document several localized ductile shear zones that likely represent the transition from BDT to LDT. On the east side of the range, an ~100 m thick zone of mylonitic rock lies parallel to the Whipple detachment, and records temperatures from 300-400°C, and stresses from 70-140 MPa; this likely represents the down-dip, ductile extension of the Whipple fault below the BDT.

In the northern Snake Range, early Tertiary extension occurred along both E- and W-dipping normal-sense shear zones that penetrated through the BDT and soled into a distinct high strain zone along the LDT. Petrological evidence from rocks originally beneath the LDT suggests that it was subhorizontal at this time. Progressive exhumation caused the LDT to migrate downwards, stranding early mylonites between the LDT and BDT; these were captured and exhumed in the early Miocene by the Snake Range decollement.