FIDELITY OF RIFT BASIN RECORDS: HIGH PRECISION CHRONOLOGY AND BASIN SUBSIDENCE IN THE RIO GRANDE RIFT, SOCORRO, NM

The process of creating new tectonic plates, plate boundaries, and ocean basins begins with continental rifting. However, rates and patterns of surface deformation, basin formation, and volcanism during continental rifting are uncertain. We investigate the timing and pattern of rift basin development in the La Jencia Basin of the central Rio Grande Rift by integrating stratigraphy and new U-Pb data into a Bayesian framework of subsidence analysis. Our geochronological work-flow included LA-ICPMS analysis of ash beds and siliciclastic rocks followed by CA-ID-TIMS analyses on the youngest grains to attain high-precision eruption and maximum depositional ages, respectively. LA-ICPMS analyses of zircon U-Pb were critical in identifying young versus old zircon, but TIMS analyses of young grains were necessary to provide precise and accurate age determinations of strata.

Our resulting age model shows that the onset of Rio Grande rifting in the La Jencia Basin is recorded by accumulation of a ~1.1 km thick section of volcanic rock, beginning with the eruption of the Hells Mesa Tuff at 33.442 ± 0.039 Ma. However, the La Jencia Basin preserves little sedimentary record until ca. 18 Ma, after which fluvial and intermittent lacustrine systems dominated, resulting in reworking of some ash layers during deposition. Geochronology data from four ash horizons and three sandstones and conglomerates as inputs into the Bayesian age model indicate punctuated and rapid Miocene subsidence (15.4 cm/Kyr) that accommodated approximately 1.5 km of sediment between 18.424 ± 0.035 to 13.445 ± 0.020 Ma. Furthermore, these Miocene strata contain relatively minor volcanic input and sit atop an unconformity that was incised into a 28.066 ± 0.021 Ma regional ignimbrite that marks the end of an earlier episode of extensive bimodal volcanism. These shifts in rates and dominant processes illustrate a punctuated and compartmentalized continental rift system that may serve in comparison to other locations along the Rio Grande Rift, as well as other continental rift systems in the geologic record.