CENOZOIC NORMAL FAULTING IN NORTHERN GREAT BASIN: TIME-SPACE PATTERNS, INSIGHTS INTO THE PRE-EXTENSIONAL ARCHITECTURE, AND IMPLICATIONS FOR ORE DEPOSITS

The northern Great Basin produces ~75% of U.S. gold, largely due to Eocene Carlin-type gold deposits. The genesis of these deposits remains debated, as the original relations between geologic features that inform past processes have been obscured by partially superposed generations of normal faults that have variably dismembered and tilted pre-extensional stratified rocks that serve as strain markers. An increased understanding of the nature and distribution of normal faulting allows for examination of the northern Great Basin geology at the time of the Carlin-type mineralization.

Eocene normal faulting first is documented at: Gold Hill (>39 Ma, >200?% extension), White Horse Pass (>39 Ma, >100%), Copper Mountains (41-37 Ma, 400%), Spruce Mountain (≥38 Ma, 95%), Pequop Mountains (≥41-39 Ma, 15-25%), Piñon Range (38-34 Ma, 5-10?%), Emigrant Pass (40-38 Ma, 10%), Battle Mountain (42-38 Ma, 5-10%), and Cove-McCoy district (>38 Ma, 5-10%).

Likely Oligocene (though in some areas with poor age control, still potentially late Eocene) to early Miocene extension is documented at: White Horse Pass (30-20 Ma?, >100%), Piñon Range (31-24 Ma, 5-10?%), Emigrant Pass (25-15 Ma, 10%), Battle Mountain (38-25.5 Ma, 10%), and Tobin Range (<33 Ma, 50%).

Likely mid-Miocene and younger faulting is documented at: Leppy Hills (≤12 Ma, ≥70%), Toano Range (17-12 Ma, 50-100%), Spruce Mountain (<38-? Ma, 13%), southern Ruby Mountains (17-10 Ma, 200%), Piñon Range (16-? Ma, 5-10 %), northern Shoshone Range and Caetano caldera (16-10 Ma, 100%), Battle Mountain (≤25.5 Ma, but likely younger, 20%), and Sonoma Range (<15 Ma, 5-10%).

Previous workers have suggested pre-Miocene extension was localized to areas of active magmatism. Extension at these locales variably preceded, accompanied, or followed magmatism, with no clear correlation between magmatism and extension across the northern Great Basin. The locus of higher-magnitude extension was in the northeastern Great Basin in the Eocene and Oligocene, with locally higher-magnitude extension to the west (e.g., Tobin Range). Widespread high-magnitude extension was prevalent in the Miocene, decreasing in intensity post-10 Ma. Structural restorations reveal the Eocene geologic architecture of the Great Basin, with a Mesozoic ‘fold belt’ present from Battle Mountain to the Piñon Range.