Extension and uplift of the northern Rio Grande Rift: Evidence from ⁴⁰Ar/³⁹Ar geochronology from the Sangre de Cristo Mountains, south-central Colorado and northern New Mexico

Abstract

New 40 Ar/ 39 Ar dating and geologic mapping of middle to late Cenozoic volcanic rocks and basin-µll sedimentary rocks preserved in the San Luis Valley and adjacent Sangre de Cristo Mountains give new insights into the timing of rift volcanism and opening of the northern Rio Grande Rift. The age, orientation, and elevation of these preserved sections delimit the timing of early rift volcanism, establishment of protorift or paleorift basins, and the uplift rate of the Sangre De Cristo crustal block along the eastern rift margin. We use offset geomorphic surfaces along with newly dated volcanic rocks to determine the exhumation or uplift rate of the Culebra Range relative to the adjacent San Luis Valley. On a µrst-order scale, we determined an exhumation rate of 58 m/m.y. based on the preserved section of 25 Ma volcanic rocks in the Culebra Range and ageequivalent volcanic rocks preserved in the San Luis Hills. A second uplift rate based on a 400 m offset section of Servilleta basalt dated as 4.7 Ma yields an exhumation rate of 87 m/m.y. Previous age data, exhumation and uplift rates, and µeld observations from the San Luis Valley region indicate that a major period of uplift and subsequent block faulting occurred during the middle Miocene (15 Ma), ~10 m.y. after the initiation of rifting.

Preliminary 40Ar/39Ar geochronology of igneous intrusions from Uvalde County, Texas: Defining a more precise eruption history for the southern Balcones Volcanic Province

“…However, confidence in the interpretation of complex age spectra from groundmass concentrates and mineral separates has been documented for igneous rocks associated with the northern Rio Grande Rift (Miggins, 2002 andMiggins et al, 2002). Moreover, there is a high degree of confidence in the ages for the K-feldspar samples from 01-CB-005 and 01-CB-006, as well as for the plagioclase and groundmass concentrate from sample 01-CB-008.…”

“…After irradiation, the 40 Ar radiogenic/ 39 Ar potassium ratios of the sample and standard were determined. Standard techniques were employed to produce 40 Ar/ 39 Ar spectra as described by Snee (1982), Snee et al, (1988), andSnee, (2002).…”

Depositional environments and cyclo- and chronostratigraphy of uppermost Carboniferous–Lower Triassic fluvial–lacustrine deposits, southern Bogda Mountains, NW China — A terrestrial paleoclimatic record of mid-latitude NE Pangea

“…Argon isotopic compositions were measured at the U.S. Geological Survey Argon Geochronology Laboratory in Denver, Colorado. Analytical techniques are given in Table C and in general followed procedures given in Miggins et al (2002Miggins et al ( , 2004. Standard methods were employed to produce 40 Ar/ 39 Ar age spectra and isochron diagrams (Snee, 2002).…”

“…The Sample was analyzed in a low blank resistance furnace. Mineral phases from the sample were prepared as described in Miggins et al (2002Miggins et al ( , 2004. The samples were irradiated for 50 MW h in the central thimble of the U.S Geological Survey TRIGA reactor (Dalrymple et al, 1981).…”

Sedimentary response to orogenic exhumation in the northern Rocky Mountain Basin and Range province, Flint Creek basin, west-central Montana

“…Argon isotopic compositions were measured at the U.S. Geological Survey Argon Geochronology Laboratory in Denver, Colorado. The complete step-heating procedure and analytical techniques generally follow Miggins et al (2002Miggins et al ( , 2004 and are available in Table S1 1 along with the complete incremental heating data. Samples were irradiated for 40 MW·h in the central thimble of the US Geological Survey TRIGA reactor in Denver, Colorado (Dalrymple et al 1981).…”