SOME MUSINGS REGARDING STRUCTURAL EVOLUTION OF NE NEVADA-NW UTAH FROM JURASSIC TO RECENT

Based on field work, earthquake data, and geophysical surveys, the following thoughts/concepts are presented for consideration:

• Initial regional metamorphism of Paleozoic miogeoclinal rocks occurred during the Middle Jurassic Elko Orogeny from central Elko County to the Wasatch Front.
• Metamorphosed Cambrian-Mississippian rocks from the East Humboldt Rge to the Toana/Goshute Rge, that are not part of the layered migmatite succession in the Ruby-East Humboldt ranges, were never buried more than one normal stratigraphic depth based on field relations in numerous ranges and CAI in early Mesozoic strata.
• The Wells Fault, a major Jurassic and/or Cretaceous WNW structure, separates a terrain to the north with multiple sub-parallel tear faults that penetrate as deep as 15 km from terrain to the south that is lacking tear faults. These faults were active during the Elko and/or younger Sevier orogenies and are interpreted to be inherited, in large part, from the Precambrian basement structures.
• The Wells Fault and related tear faults in the southern Snake Mtns partitioned earthquake epicenters (as deep as 15 km) in 2007 to the present along the Wells earthquake fault at the eastern foot of the southern Snake Mtns.
• Field relations indicate that the juxtaposition of contrasting Paleozoic lithotectonic units across tear faults is pre-Eocene in age.
• Some tear faults were reactivated during late Miocene to Recent Basin and Range development, both as faults within ranges and as range-bounding faults.
• Eocene (42-37 Ma) andesitic flow breccias across the region predate and are synchronous with major ash-flow units and are locally derived, suggesting that the crust was being fractured/crackled across the region, but was not subjected to major extension.
• Concordant relationships between Miocene (as young as 14 Ma) and Eocene (42-37 Ma) volcanic rocks indicate that tilting is late Miocene and younger and thus does not support the concept of widespread pre-Miocene extension. This assertion does not preclude local extensional faulting.
• The continuum of tectonic events being superposed on earlier events makes it extremely tenuous/dangerous to assume that one can determine the age of a particular structure without crosscutting plutonic or volcanic units or stratigraphic data that clearly bracket the age of a structure.