THE ANGEL LAKE FOLD: UNRAVELING THE POLYPHASE DEFORMATIONAL HISTORY OF THE EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX, NORTHEASTERN NEVADA

The East Humboldt Range in central Elko County, Nevada forms the northern half of the Ruby Mountains (RM) - East Humboldt Range (EHR) metamorphic core complex. It hosts the oldest rocks in Nevada, the Neoarchean to Paleoproterozoic Angel Lake gneiss complex. These rocks are also the most deeply exhumed rocks in Nevada as previously documented by PT estimates ranging up to >700 °C, 10 kb for Late Cretaceous (78-85 Ma) peak metamorphism. The dominant structure is a southward-closing recumbent fold with a 7km lower limb, the Winchell Lake fold-nappe (WLN). The WLN is overprinted by a >500 m thick late Cenozoic high amphibolite facies, normal-sense mylonitic zone, the RM-EHR shear zone.

Complex structural relationships in a previously poorly understood area at the base of Angel Lake cirque near the northern end of the EHR are clarified by new, more detailed mapping. Petrographic comparison documents the interfolding and extreme plastic attenuation of the Precambrian gneiss complex with younger paragneiss units near the base of the cirque and requires the presence of a ~100m scale recumbent fold, here named the Angel Lake fold. Re-folding relationships suggest that the Angel Lake fold is not merely parasitic to the larger WLN, but rather represents a distinct fold phase. Understanding the age and tectonic significance of this structure depends on deciphering cross-cutting relationships relative to datable granitoid gneisses. In particular, hornblende biotite quartz dioritic orthogneiss appears to have intruded syntectonically into a normal-sense shear zone that cuts and therefore must postdate the Angel Lake fold. On the other hand, the involvement in folding of monzogranitic orthogneiss sheets, also inferred to be late Cenozoic in age, raises the possibility that the Angel Lake folding could have occurred during an earlier stage in Cenozoic tectonism. Geochronology currently being conducted at Texas Tech University is expected to resolve this problem and clarify the role of possible large-scale folding during an early, deeper-seated phase of Cenozoic extensional orogenesis.