LATE OLIGOCENE EXTENSION IN THE EAST HUMBOLDT-WOOD HILLS-PEQUOP METAMORPHIC CORE COMPLEX: NEW CONSTRAINTS FROM AGES OF DETRITAL ZIRCONS IN THE HUMBOLDT FORMATION ALONG THE NORTHERN MARGIN OF THE PEQUOP MOUNTAINS, NE NV

Exhumation of the East Humboldt-Wood-Hills-Pequop Mountains metamorphic core complex (EPCC) has been largely attributed to E-W extension between 16-8 Ma that was accommodated by north-trending fault systems with well-developed basins filled with the upper part of the Humboldt Formation. However, our recent work has recognized remnants of a > 45 km-long normal fault and basin system predating 16 Ma that was responsible for early NW-SE extension along the northern margin of the complex. These basin remnants, filled with the lower Humboldt Formation, were broadly bracketed between 39 and 16 Ma because lack of datable volcanic rock in the basin precluded further age refinement. To better constrain the age of the sediment and the timing of extension, we have been utilizing U-Pb dating of detrital zircons (DZ) to assess maximum depositional ages (MDA). Our earlier work in the Wood Hills yielded MDA’s of 27 to 18-16 Ma, which we interpret to likely be close to depositional ages.

Here, we report new MDA’s from the lower Humboldt Formation in the hanging wall of the west-trending Holborn fault along the northern margin of the Pequop Mtns, which forms the easternmost known exposure of this early basin and fault system. Mueller et al. (1994) constrained the age of Holborn fault and its basin-fill to be between 35-16 Ma on the basis of cross-cutting relationships. Two sandstone samples from the basin fill were collected for DZ analyses. Both samples yielded relatively continuous age distributions from 42 to 27 Ma with MDA’s of 29 and 28 Ma, which we infer are close to depositional age. The 42-39 Ma zircons are probably derived from locally exposed volcanic flows and tuffs of the same age. The majority of the younger 38-27 Ma zircons have no known local source, but most are consistent with being sourced from distal ash-fall deposits derived from Paleogene eruptions in the central and western Nevada, and Rocky Mountain, volcanic fields. Relative age-probability peaks from the Pequop Mountains samples are similar to those in the Wood Hills. Most notable are peaks at 29 Ma and 33-36 Ma that are consistent with eruptions in the central Nevada volcanic field. In summary, new DZ data from the Pequop Mtns and Wood Hills collectively indicate that the early basin and fault system was active by at least 29-28 Ma, and it provides additional evidence that the EPCC experienced significant extension along surface-breaking normal faults prior to 16 Ma.