Abstract

The southeastern Great Basin of Nevada and Utah is a tectonically and magmatically active geologic region. This field trip will investigate not only its structural and igneous features but also their economic significance. The primary area of interest will be the Caliente caldera complex, an east-elongated (50 × 22 mi; 80 × 35 km) complex of inset calderas in Nevada and Utah. It spans an age of at least 10 million years (at least 23-13 Ma), unusually long lived for a caldera complex. It is bounded on its northern and southern sides by transverse zones, which are east-striking late Mesozoic to Cenozoic structures that cross the Great Basin and accommodate different amounts, types, and rates of crustal extension north and south of them. We will examine the Timpahute transverse zone, along the northern side of the caldera complex. The Caliente complex was highly extended along transverse zones and faults that are synchronous with caldera magmatism. Elsewhere in the southern Great Basin, the transverse zones control ground-water flow and, by inference, petroleum flow. The faults and magmas of the Caliente area belong to two episodes, first the middle Cenozoic pre-basin-range episode of calc-alkaline magmatism and northeast- and northwest-striking oblique-slip faults, then the basin-range episode of bimodal magmatism and north-striking normal faults. Gold deposits that surround the caldera complex are interpreted to represent leaching, transport, and deposition of metals by ground water moving through the transverse zones and heated to boiling by intracaldera magmas. We will visit the Delamar gold district on the southwestern side of the caldera complex, where these processes took place. We also will visit the Iron Springs iron district, Utah, where the structural setting is considerably different. Here magmatism was largely during the pre-basin-range episode and faulting was largely confined to the basin-range episode.