CENOZOIC EXHUMATION OF THE RUBY MOUNTAIN-EAST HUMBOLDT-WOOD HILLS-PEQUOP MOUNTAINS METAMORPHIC CORE COMPLEX: NEW CONSTRAINTS FROM REMNANTS OF PALEOGENE TO NEOGENE EXTENSIONAL BASINS

The Ruby-East Humboldt-Wood Hills-Pequop Mountains metamorphic core complex region contains a hallmark normal-sense mylonite zone that, as defined by earlier workers, facilitated significant decompression and partial exhumation of mid-crustal metamorphic rocks between 29 and 17-15 Ma, and possibly earlier. There are no known extensional basins or surface-breaking normal faults linked to this decompression event, and hence this conundrum has led to the notion that the middle and upper crust were structurally decoupled, with decompression reflecting diapiric rise of metamorphic rock into the middle crust. This event was followed by widespread exhumation and surface exposure of the metamorphic rocks between ca. 16 and 8-3 Ma along the north-trending, Knoll-Ruby and Thousand Springs fault systems, which had well-developed sediment-filled basins.

New geologic mapping and detrital zircon data from the Wood Hills part of the core complex reveals two successive, surface-breaking normal faults with Cenozoic basin-fill preserved in their hanging walls. The younger normal fault is part of the Knoll-Ruby fault system. Geologic map relations, coupled with detrital zircon data from the older fault’s (Moor fault) basin-fill, suggests that this fault formed after 40 Ma with slip and basin filling beginning by the late Oligocene and continuing to at least 17-16 Ma. These age constraints are consistent with published thermochronologic data that indicate protracted Paleogene-early Neogene cooling of footwall metamorphic rocks in the Wood Hills.

Regionally, we correlate the Moor fault’s basin-fill in the Wood Hills with two other similar-aged remnants of normal-fault-bounded basin-fill that are preserved at the northern ends of the Pequop and East Humboldt ranges. Collectively, these three basin remnants were probably once part of an extensional basin system that defined the northern termination of the core complex between 40 and 16 Ma. On the basis of compatible age constraints, we infer that this fault and basin system was linked to mylonitization and decompression of core complex metamorphic rocks at depth. Consequently, decoupling of the middle and upper crust may not be required to explain early decompression, cooling, and mylonitization of this core complex.