Reconstructing the Transition from Laramide Contraction to Rio Grande Rift Extension
Exposure of tilted Eocene-Oligocene volcanic and sedimentary units unconformably overlying folded and thrusted Mesozoic rocks in the southern Indio Mountains provide an important geologic record of the transition from Laramide contraction to Rio Grande rift extension. Detailed geologic mapping and cross-section reconstruction combined with sedimentary analysis and geo/thermochronologic methods are used to test the hypothesis that Laramide contractional structures and post-Laramide topography focused ignimbrite and sedimentary deposition within two paleovalleys located in the northwest and northeast portions of the study area. Laramide deformation in the study area is confined to Mesozoic units and includes a series of low-angle thrust faults and outcrop- to map-scale folds that collectively indicate E-directed contraction. Sanidine 40Ar/39Ar geochronology methods applied to the oldest exposed ignimbrite place a minimum age of 38.166 ± 0.016 Ma on the cessation of Laramide folding and thrusting. Clast counts and paleoflow directions collected from AB-planes of Paleogene clasts in the paleovalley provide an opportunity to investigate the source and direction of local fluvial deposition. A sanidine 40Ar/39Ar date of 37.61 ± 0.07 Ma from a basalt flow interbedded within the Paleogene conglomerates constrains the age of changes in paleoflow from NE-SE-directed to South. Clast counts collected from basin fill sediments at nine locations in a Neogene half-graben allow for a quantitative and qualitative analysis of the sedimentological history of rift basin fill through progressive offset of the Indio fault, the largest normal fault in the southern Indio Mountains. A detrital sanidine 40Ar/39Ar age of 11.87 0.016 Ma collected from the basal portion of Neogene graben sediments constrains the age of half-graben formation associated with Rio Grande rift extension. Apatite (U-Th)/He (AHe) ages collected from the footwall of the Red Mountain normal fault methods to constrain the onset and duration of the initial phase of Rio Grande rift faulting between 29–19 Ma. Fault kinematic measurements collected from the Indio fault and other extensional structures throughout the mapping area reveal an overall SW-NE extension direction. Detailed geologic mapping and cross-section restoration supports the existence of a paleovalley in the northwest mapping area that, prior to offset by the Indio normal fault, was continuous with a paleovalley in the northeast mapping area. Abrupt thickening of ash flow and ash fall units in the northwest paleovalley indicate increased accommodation space for deposition compared to the northeast paleovalley. Additionally, abrupt pinching out of a lithic flow breccia (Garren Mt. Tuff Breccia) in the northeast may indicate connectivity between the two paleovalleys was intermittent, and that the extent of the Garren Mt. Tuff Breccia, a dense high-particle flow, corresponds to a topographic low that did not continue to the west across Laramide paleotopography. Clast counts and paleoflow directions of Paleogene clasts in the paleovalley indicate that all clasts are derived from the local underlying Mesozoic units or older Eocene volcanic deposits, and paleoflow was generally from west to east. Apatite (U-Th)/He (AHe) thermochronology from the footwall of the Red Mountain normal fault shows cooling between 29–19 Ma, in agreement with the onset and duration of the initial phase of Rio Grande rift extension in southern New Mexico. However, detrital sanidine 40Ar/39Ar analysis indicates that Rio Grande rift basin fill deposits adjacent to the Indio fault are younger than 11.87 ± 0.016 Ma, overlapping in time with the second phase of extension documented in the southern and central portions of the Rio Grande rift in New Mexico. Clast counts from this half-graben indicate a normal unroofing sequence in which Eocene-Oligocene volcanic clasts progressively decrease and Cretaceous marine and fluvial sediments increase up-section. Paleostrain analysis of fault kinematic data indicate SW-NE extension in the southern Indio Mountains and left-lateral offset along the E-W extensional structures in the northern portion of the mapping area suggest reactivation of older structures similar to those documented in the Border Corridor System in southern Trans-Pecos.
Conley, Aaron, "Reconstructing the Transition from Laramide Contraction to Rio Grande Rift Extension" (2021). ETD Collection for University of Texas, El Paso. AAI28870039.