LARAMIDE SHORTENING AND EXHUMATION OF THE ZUNI MOUNTAINS, WEST-CENTRAL NEW MEXICO

Structural and thermochronologic analyses highlight the kinematic deformation history and regional implications of the Zuni Mountains, a Colorado Plateau Laramide arch in west-central New Mexico. The Zuni Mountains trend NW-SE and display a markedly deformed southwest forelimb (Nutria monocline), a sinuous basement-involved range crest, and a gentle northeast-dipping backlimb (Chaco Slope). Structural analysis of minor faults in tilted strata across the range suggest a mean horizontal shortening azimuth of 061° that was established prior to significant faulting and folding. Apatite (U-Th)/He (AHe) and fission-track analyses on samples collected in the forelimb, range crest, and backlimb of the arch display varied patterns according to their structural and stratigraphic position. Relatively long track lengths and flat AHe date-eU trends are consistent with moderate to rapid cooling on the forelimb and range crest. Relatively short track lengths with broader distributions and positive AHe date-eU trends from backlimb samples show a protracted cooling history. Coupled with HeFTy inverse modeling, results are interpreted to represent southwest-directed thrust-driven exhumation of the forelimb and range crest while exhumed detritus buried the backlimb as it tilted northeastward. Exhumation was between ca. 80–45 Ma, and started slightly earlier than commonly considered for Colorado Plateau Laramide structures. Taken together, the structural and thermochronologic analysis of the Zuni Mountains (1) corroborates previous studies that show regional ENE-directed shortening across a wide expanse of the Laramide foreland region, and (2) fit into an easterly-directed sweep of Laramide exhumation that was driven by Farallon flat-slab processes.