SOUTHERN RIO GRANDE RIFT BASIN SUBSIDENCE & CHRONOLOGY: COMPLEXITY OF DEPOSITIONAL RECORDS OF RIFTING

This study investigates Miocene rift basin development around the area of Socorro, NM during the predominant episode of extension in the southern Rio Grande rift (RGR) by using previously mapped stratigraphy and new age control. Although Miocene RGR tectonism was spatially widespread, regional variability of basin subsidence is only coarsely characterized. To further complicate the record of basin development, some “ash” beds, which can be used for age control within the Miocene section, bear evidence of pervasive reworking and mixing as demonstrated by their 1) fluvial-lacustrine facies, and 2) detrital zircon age spectra, which can be dominated by Oligocene grains. Therefore, to accurately approximate depositional age from radiometric minerals contained within these ash beds (e.g., zircon), it is necessary to assess their degree of sediment mixing via preliminary analyses (n =50–100), and, if multimodal, treat them more like a detrital sample rather than an ash bed (i.e., n >100). Through this approach, we highlight local depositional variability, driven by intra- and inter-basinal structural complexities. The two sections analyzed in this study are separated by ~6 km, but the timing of deposition and stratigraphic architecture suggest different burial histories. Both sections record rapid subsidence (>1.2–0.5 km/Myr), but at different times. The northern section, which lies to the west of the Silver Creek normal fault, suggests middle Miocene (~11–8 Ma) rapid extension, whereas the southern section, which lies across a relay ramp that links two normal faults (Silver Creek and Socorro Canyon) suggest late Miocene (~16–11 Ma) rapid extension. Adjacent to the southern section, previous thermochronologic work in the footwall of the breached relay ramp supports an independent model of rapid exhumation at the same time. Both stratigraphic sections record the regional Miocene–Pliocene Unconformity observed throughout much of the rift, and indicate notably lower rates of subsidence following development of this unconformity (< 0.05 km/Myr). This study provides a detailed look at rift basin subsidence variability, highlights caveats in developing age control in these basins, and compares independent records of lithospheric extension in a continental rift setting.