Preliminary Geochronological Results From the Fra Cristobal Range of South-Central New Mexico – Revision of Cretaceous-Age Units to Santa Fe Group

The Fra Cristobal range of south-central New Mexico is a north-south striking horst block bounded by the Engle basin to the west and the Jornada del Muerto basin to the east.Phanerozoic units in the range are generally southeast-dipping, exposing Proterozoic-age basement in the northern section of the range.Previous work in the northern section of the range revealed: i) a possible Laramide-age thick-skinned thrust fault, ii) a widespread geothermal system that altered basement rock and Phanerozoic cover, and iii) a previously known but unmapped volcanic tuff.We hypothesize that Precambrian basement rocks overthrust Paleozoic limestones during the Laramide orogeny (~75 Ma; Seager et al., 1997;Amato et al., 2017).After cessation of thrust faulting, the volcanic tuff was emplaced, followed by deposition of trough cross-bedded sandstones previously interpreted as syn-Laramide McRae Formation stratigraphically above the tuff (Nelson, 1986).The tuff's source and timing of emplacement is anomalous in the current understanding of the regional geologic history because the tuff's petrographic properties (for example, relatively large phenocryst size) implies a local caldera source.However, the closest known possible source is the Copper Flat complex, 10's of km away from the range.We therefore sampled the trough cross-bedded sandstone unit and the underlying tuff for 40 Ar/ 39 Ar dating to clarify the timing of tuff emplacement and subsequent sandstone deposition.Detrital sanidine samples from the sandstone yield a maximum preliminary age of ~24-35 Ma, rather than Cretaceous-age syn-Laramide sedimentation as previously hypothesized.Sedimentation is likely associated with early Rio Grande rift deposition, analogous to the Popotosa Fm. to the north of the range.This is an important addition to our understanding of the tectonic evolution of the Fra Cristobal range and will assist in ongoing structural evaluations of its geologic history.Ongoing research will integrate geochronological and thermochronological techniques with sequentially balanced and restored cross-section analysis to further constrain the range's structural evolution from the Laramide to Rio Grande rift tectonic regimes.