LOW TEMPERATURE COOLING HISTORY AND "THERMOSTRUCTURAL ANALYSIS" OF THE LARAMIDE ZUNI ARCH, WEST-CENTRAL NEW MEXICO

Apatite fission-track (AFT) and apatite (U-Th)/He (AHe) data are integrated with stratigraphic data to delineate the low-temperature (110–30 ºC) exhumational and structural history of the Zuni Mountains, a Laramide structural arch in west-central New Mexico. These AFT and AHe data are also used (together where available) to inversely produce continuous time-temperature path models. Stratigraphic results suggest 5 km of burial (decompacted) that was sufficient to reset AFT and AHe ages from prior exhumation (Ancestral Rocky Mountains). Thermochronologic data and modeling suggest that tectonically driven exhumation of the Zuni arch commenced ca. 84–77 Ma, a timeframe that is similar to (and perhaps earlier than) the accompanying San Juan basin and nearby Nacimiento arch. Thermochronologic ages show the absence of an AHe partial retention zone and the possible base of an AFT partial annealing zone, which suggests 4–5 km of exhumation during and since the Laramide orogeny.

We also present a “thermostructural analysis” based on samples collected in a transect across the southeastern Zuni arch, and reconstruct a stepwise deformation and tectonic history for the range. Thermochronologic data and modeling are markedly unique in each sample across the transect: the southwestern forelimb of the arch displays moderate to rapid cooling, while the range crest for the southeastern (and northwestern) Zuni arch displays rapid cooling; slow cooling characterizes the northeastern backlimb. These thermal characteristics are interpreted to represent initial southwest-directed thrusting all along the length of the arch via a northeast dipping master thrust, exhumationally driven erosion that buried the flanks of the arch during deformation (notably at the northeastern backlimb), later pulses of Laramide deformation, and post-orogenic cooling episodes that may reflect Colorado Plateau uplift and/or drainage reorganization. In all, our Zuni Mountains data exhibit (1) Laramide deformation at a timeframe similar to nearby Laramide features, (2) ≥4 km of structural and erosional exhumation during and since the Laramide orogeny, and (3) detailed clues into Zuni arch deformation and regional tectonics, which (4) highlights the importance of a structurally motivated sampling strategy.