Abstract
Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.
Highlights
Metamorphic core complexes are exhumed sections of the ductile middle crust brought to the surface during horizontal crustal extension and vertical thinning
(1) How do the mylonites and other deformed rocks in a core complex footwall relate to the detachment that exhumes them? (2) What do these relationships tell us about the rheological structure of the middle and lower crust? In an attempt to answer these questions, we present a detailed investigation of crustal rheology in the footwalls of the Whipple Mountains, the northern Snake Range, and the Ruby Mountains– East Humboldt Range in northeast Nevada (Fig. 2)
We relate our observations of two spatially and temporally distinct zones of ductile deformation in all three core complexes to exhumation of the footwalls through two rheological transitions: a localized–distributed transition (LDT) and a brittle–ductile transition (BDT)
Summary
Metamorphic core complexes are exhumed sections of the ductile middle crust brought to the surface during horizontal crustal extension and vertical thinning They were first recognized and described in the North American Cordillera (Coney, 1980), where they form a discontinuous NW–SEtrending belt extending from Canada to Mexico in the hinterland of the Sevier fold and thrust belt (Fig. 1). The bulk of the high-strain rocks preserved in the footwall are, a pre-existing feature of the ductile middle crust that was subsequently captured and exhumed by a brittle detachment (the northern Snake Range décollement, NSRD) In this interpretation, the brittle hanging wall to the NSRD represents a series of upper crustal normal faults, whereas the ductile footwall represents exhumed middle crustal ductile deformation, much of which is not directly related to the detachment. Of, and a mechanical explanation for, the geometry of the detachments in these three core complexes and many like them; and (iv) show that the detachments in all three core complexes formed during the Miocene and post-date early phases of extension and exhumation in the exhumed midcrustal metamorphic rocks
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
