1:24,000-Scale Geologic Map of the Western Half of the Franklin Lake NW 7.5-Minute Quadrangle, Ruby Mountains-East Humboldt Range Metamorphic Core Complex, Northeast Nevada

Session: Best Student Geologic Map Competition (Posters)

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Abstract:

Continental metamorphic core complexes (MCCs) commonly expose migmatitic infrastructure. These high-grade metamorphic terranes demonstrate elevated crustal mobility in the middle crust, as evidenced by kilometer-scale fold nappes, extreme ductile thinning, and mylonitic shear zone carapaces. Evaluating their tectono-magmatic history and structural architecture is essential for testing competing endmember models of MCC formation. The north-trending Ruby Mountains-East Humboldt Range MCC in northeastern Nevada consists of a migmatitic infrastructure to the north, and an unmetamorphosed suprastructure in the southern Ruby Mountains, where a nearly coherent Paleozoic stratigraphy is preserved. The critical junction between the infrastructure and suprastructure is located in the central Ruby Mountains, a key transition zone for recording distinct magmatic and structural features indicative of the along-strike variations in crustal mobility. Here, we present a 1:24,000-scale geologic map of the western half of Franklin Lake NW 7.5-minute quadrangle in the rugged, high-relief central Ruby Mountains. Based on field observations and map patterns, the infrastructure to the north is dominated by the kilometer-scale, NE- to E-verging King Peak nappe, which folded the Neoproterozoic–Ordovician metasedimentary rocks and is extensively intruded by a Late Cretaceous–Oligocene leucogranite injection complex. Intrusion density in the metasedimentary host rock is >75%. In the suprastructure south of the infrastructure, Mesozoic structures are well preserved and show no evidence of disruption by the Late Cretaceous–Oligocene melt injection complex. For example, a 15-km-wide undeformed Late Jurassic granitic pluton intrudes the Neoproterozoic–Cambrian sedimentary units and is dated by new zircon U-Pb ages of ~150 Ma. SE-verging, decameter-scale, open to tight folds are exposed in the roof pendant and are correlative with regional Middle-Late Jurassic contractional tectonism (i.e., the Elko orogeny). These field relationships suggest a strong spatial correlation between elevated crustal mobility in the infrastructure and the Late Cretaceous–Oligocene melt injection complex. Quaternary deposits and geomorphic indicators reveal a major history of Pleistocene glaciations, as exemplified by linear lateral moraines and curvilinear terminal moraines. Quaternary normal faulting is expressed by the NE-striking SE-dipping Ruby Valley fault zone that cuts the late Pleistocene alluvium with 15- to 35-m scarps. Integration of our detailed field mapping with ongoing analytical analyses will help quantify the role of protracted melt injections in modulating mid-crustal mobility and MCC development.