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Abstract In the southwest USA, the Colorado Plateau is encircled by Late Cenozoic volcanic fields, most of which have eruptive histories that are marginally constrained. Establishing the spatiotemporal evolution of these volcanic fields is key for quantifying volcanic hazards and understanding magma genesis. The Black Rock Desert (BRD) volcanic field covers ∼700 km²of west‐central Utah. We present 46 new⁴⁰Ar/³⁹Ar ages from the BRD ranging from 3.7 Ma to 8 ka, which includes⁴⁰Ar/³⁹Ar plateau ages from olivine separates. These new ages are combined with 13 recently published⁴⁰Ar/³⁹Ar ages from the Mineral Mountains to evaluate the spatiotemporal evolution of all five BRD subfields. The oldest lavas and domes are located to the southwest, whereas the youngest lavas, which are only a few hundred years old, are located ∼30 km to the NNE. However, BRD vent migration patterns over the last 2.5 Ma are non‐uniform. They are also not consistent with North American Plate motion over a partial melt zone nor have they migrated toward the center of the Colorado Plateau. BRD eruptions are almost always coincident with mapped Quaternary faults. A shear‐velocity (Vs) model beneath the BRD indicates that the lithosphere has been thinned and that asthenospheric melt has coalesced at the lithosphere‐asthenosphere boundary, which is supported by the trace element compositions of BRD lavas that signify that they have incorporated continental lithospheric mantle. Our data and observations suggest that the asthenosphere‐lithosphere‐volcanic system in the BRD is inherently complex. 

Abstract Improvements in analytical procedures in parallel with intercalibration of⁴⁰Ar/³⁹Ar and U–Pb methods and laboratories, spurred since 2003 by the EarthTime geochronology community initiative, have led to ±2σuncertainties of the order of 50–100 ka, or better, for Cretaceous ash beds. Assembled here are 57⁴⁰Ar/³⁹Ar ages and 17²³⁸U–²⁰⁶Pb ages of volcanic ash beds in strata of the Western Interior Basin of North America determined during the last 15 years since these improvements have been made. These age determinations span from 108 Ma in the middle Albian to 66 Ma in the latest Maastrichtian. Five of the⁴⁰Ar/³⁹Ar ages from Campanian and Maastrichtian strata are newly reported here, whereas the remainder are from the literature. Building on the pioneering work of John Obradovich and Bill Cobban, where possible these age determinations are tied to ammonite and inoceramid biostratigraphy. For most ash beds, the temporal uncertainties, unlike earlier timescales for the Western Interior Basin, are much shorter than the duration of fossil biozones. Proposed ages for stage boundaries based on this review of the radioisotopic ages include: Maastrichtian–Danian, 66.02 ± 0.08 Ma; Campanian–Maastrichtian, 72.20 ± 0.20 Ma; Santonian–Campanian, 84.19 ± 0.38 Ma; Coniacian–Santonian, 86.49 ± 0.44 Ma; Turonian–Coniacian, 89.75 ± 0.38 Ma; Cenomanian–Turonian, 93.95 ± 0.05 Ma; Albian–Cenomanian, 100.00 ± 0.40 Ma. Six bentonites that occur within theVascoceras diartianum, Neocardiocerus juddi, Prionocylus macombi, Scaphites preventricosus, Scaphites depressusandDesmoscaphites bassleriammonite zones, dated using both⁴⁰Ar/³⁹Ar and U–Pb methods, yield ages in agreement to within 150 ka and form the backbone of the Western Interior Basin timescale. In parallel, improvements in the taxonomy of ammonites, inoceramids and foraminifera, and recent field work, are better establishing the biostratigraphic framework for these age determinations. Each of these efforts contributes to the progressive refinement of the chronostratigraphic framework of the Western Interior Basin, and enhances its utility for global correlation.