METAMOPRHIC CORE COMPLEXES IN FOUR DIMENSIONS:THE ROLE OF POLYPHASE DEFORMATION, NON-CYLINDRICAL STRUCTURES, AND TECTONIC HEREDITY IN THE EVOLUTION OF THE SNAKE RANGE-KERN MOUNTAINS-DEEP CREEK COMPLEX, EASTERN NEVADA

The juxtaposition of mylonitic mid-crustal rocks and faulted supracrustal rocks in metamorphic core complexes (MMCs) is generally portrayed in 2 dimensions and attributed to a single extensional “event” involving large-scale slip ± isostatic doming along a low-angle “detachment fault"/ shear zone. This paradigm does not explain dramatic along strike (3-D) variations in architecture, metamorphic grade, and exhumation histories of many MMCs nor the polyphase (4-D) history of deformation recorded by both hanging wall and footwall rocks. The question arises: Are MMCs a consequence of a specifc style of extension, or are they composite features reflecting superimposed tectonic events and structures? The Snake Range-Kern Mts-Deep Creek Mts (SKDC) complex in eastern Nevada offers unparalleled insights into this question.

The SKDC records a rich polyphase history of Jurassic to Miocene magmatiism, metamorphjism, and deformation. Large-scale non-cylindrical recumbent folds locally thickened the crust during the late Cretaceous, resulting in deep burial (650°C, 6-8 Kbar) of portions of the footwall, but metamorphic grade decreases dramatically to the N and S. Subsequent Paleocene-Eocene extension (±diapirism) as recorded by supracrustal normal faulting and ESE-directed mylonitic shearing of the footwall brought the highest grade rocks back to depths of~10-12 km and is concentrated in areas of greatest thickening. After a ≥10 Ma quiescence, rapid E-directed slip initiated along the mainly brittle Snake Range detachment at ~20 Ma - reactivating the Eocene shear zone. At this time, the W part of the footwall had cooled to temperatures of ≤150°C, whereas the E part of the footwall was still ~300-350°C, implying that the Miocene slip surface originated as a moderate to steeply E-dipping normal fault.

These relations suggest that the SKDC MMC is the composite product of localized crustal thickening followed by Paleogene extensional exhumation and mylonitization, followed by Miocene brittle high-angle (rotational) normal faulting. The importance of tectonic inheritance is demonstrated by the coincidence of high modern surface elevations with the areas of greatest Miocene slip which in turn coincide with areas of greatest Paleogene exhumation and greatest Cretaceous crustal thickening.