Thermochronometric constraints on the thermal anatomy and evolution of an extensional accommodation zone and implications on exploration for extensional-type geothermal systems

Abstract

The northern Basin and Range province is characterized by a significantly elevated heat flow and geothermal gradient as a result of shallow volcanism and lithospheric stretching and thinning occurring in tandem throughout the Cenozoic. Along the western margin of the northern Basin and Range, in west-central Nevada, early to middle Miocene volcanism immediately preceded large-magnitude extension, which is accommodated by high degrees of footwall rotation in normal faults. The Wassuk Range (WR) footwall is one such example, where Miocene and younger extension was preceded by the extrusion of the ~15 Ma Lincoln Flat andesite, and subsequent extension resulting in footwall rotation (≥ 60°), as recognized in 40Ar/39Ar dated, pre- and syn-extensional Neogene volcanic and sedimentary rocks. The structural architecture of the southern WR is that of a terminating extensional fault system in that the degree of tilting and extension decreases southward into a series of right-lateral faults of the Mina Deflection (MD) accommodation zone. Apatite (AHe) and zircon (ZHe) thermochronologic data from ~9 ¨C 4 km exhumed and exposed upper-crustal sections in the central and southern WR footwall preserve the thermal and tectonic evolution of the southern WR and MD accommodation zone. The entire WR and MD region underwent slow cooling (< 2 – 2.5° C/Ma) during the earliest Cenozoic related to the slow denudation of Mesozoic basement rocks. Middle Miocene heating overprinted much of the Paleogene cooling history towards the southernmost WR which was anomalously hot (≥ 65 ± 20 °C/km),, and was the result of magmatic advection focused along the MD. The onset of rapid footwall exhumation in the Wassuk Range began at ~15 Ma, due to Basin and Range extension and again at ~3 ¨C 4 Ma related to Walker Lane transtensional faulting. Pliocene to recent Walker Lane transtensional structures are associated with a number of extensional-type geothermal systems. AHe ages along the modern WR range front and from a ~1.4 km deep borehole in the hanging-wall basin show evidence of young (< 3 Ma) hydrothermal reheating (85° - 135° C) and correlate with the location of a known geothermal anomaly. Thermal modeling of hydrothermally reset AHe ages from the modern range front suggests that the geothermal likely has been active for ~0.1 ¨C 1 Ma, while borehole AHe ages in the hanging-wall basin suggests that its current plumbing system and manifestation as a geothermal anomaly is likely very young and that the borehole is not in conductive thermal equilibrium. This study demonstrates that thermochronometry can be an effective geothermal exploration tool to detect young and blind geothermal resources.