Tectonic evolution and thermal structure of the Gold Butte normal fault block, Nevada: New insights from multiple diffusion domain (MDD) 40Ar/39Ar K–feldspar analyses

The Gold Butte normal fault block in Nevada has been interpreted as one of the structurally thickest intact upper crustal sections exposed in the western United States. As such, this fault block has been extensively used as a natural laboratory both to investigate the tectonics of large magnitude extension, as well as to evaluate new and existing thermochronometers. However, recent work has led to debate about the tectonic history of the region and the degree to which the fault block is intact. We report new thermochronologic results derived from multiple diffusion domain (MDD) modeling of ⁴⁰Ar/³⁹Ar K-feldspar analyses that shed new light on the accuracy of this method as well as the tectonic evolution of the fault block. ⁴⁰Ar/³⁹Ar K-feldspar ages decrease systematically towards the west to a paleodepth of ∼14 km and MDD models accurately reproduce many key features of the known tectonothermal history of the fault block, including the inception, rate, and magnitude of rapid cooling produced by tectonic exhumation. These MDD models also yield Miocene temperatures that match the known pre-extensional thermal structure of the block from other thermochronologic studies. These results provide strong evidence that MDD modeling of K-feldspar analyses can provide geologically meaningful and accurate continuous thermal histories. At the deepest inferred paleodepths however, the ⁴⁰Ar/³⁹Ar K-feldspar data show a clear repetition of the age gradients and MDD models yield Miocene temperatures that are 80–100 °C cooler than models assuming an intact fault block predict. We propose that the deepest parts of the block lie in the hanging wall of a previously unrecognized normal fault that repeats the westernmost part of the block. An analysis of prior thermochronology strongly supports this hypothesis. These results demonstrate the utility of high precision ⁴⁰Ar/³⁹Ar K-feldspar analyses and MDD models for resolving structural and tectonic problems.