Paper No. 9
Presentation Time: 11:00 AM


SICARD, Karri R., State of Alaska, Division of Geological and Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709 and SNOKE, Arthur W., Department of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 E. University Avenue, Laramie, WY 82071,

The southeastern East Humboldt Range, Nevada, is part of the Ruby-East Humboldt core complex that experienced large-scale Tertiary crustal extension subsequent to a complex Mesozoic contractional, magmatic, and metamorphic history. The Gordon Creek Quadrangle exposes a partial crustal cross-section from mid-crustal rocks (~5–6 kb) in the north to unmetamorphosed upper crustal rocks in the south.

The deepest exposed rocks are migmatitic, upper amphibolite-facies metasedimentary rocks correlated with the Neoproterozoic McCoy Creek Group. There is widespread evidence of partial melting. An isopleth was mapped that delineates an area containing greater than 65% granitic rocks (Cretaceous to Oligocene). Structurally above this migmatitic zone is a composite unit of Cambrian-Neoproterozoic metamorphosed Prospect Mountain Quartzite and McCoy Creek Group, including a distinctive, mappable Neoproterozoic calcite marble to calc-silicate rock unit. Overlying these is another composite Cambrian to Ordovician impure calcite marble and calc-silicate unit that is intruded by pegmatitic leucogranite and a variety of Tertiary igneous rocks. The Tertiary intrusive rocks occur as numerous sheets, some reaching tens of meters in thickness. Their compositions vary from gabbro/quartz diorite to leucogranite, and their ages range from middle Eocene to Oligocene.

Succeeding the Late Cretaceous regional metamorphism was the development of a km-scale, west-rooted, normal-sense mylonitic shear zone during Oligocene time. The southern end of the quadrangle displays a normal-sense brittle detachment fault that separates the mylonitic rocks from an upper structural level of non- to weakly metamorphosed, brittlely attenuated stratified rocks (Paleozoic through late Eocene). Between the upper crustal and mid-crustal rocks are lower amphibolite-facies rocks, consisting of Eureka Quartzite, a metadolomite unit, and graphic calcite marble. The protoliths of these metasedimentary rocks range from Ordovician to Upper Devonian in age and they are typically preserved as fault slivers in the detachment-fault zone. Subsequent range-bounding Quaternary normal faults tilted the partial crustal section and locally cut older alluvium fan deposits.