The Ancestral Rocky Mountains (ARM) represent an intraplate deformational event that resulted in a series of Precambrian-cored basement uplifts with adjacent basins that accumulated Pennsylvanian to early Permian strata. Tectonic models for the event are debated largely because of the lack of robust age control in the basin fill. In New Mexico, the ARM event resulted in a series of basins with some of the best biostratigraphic records across the orogenic province. This article utilizes published biostratigraphic and lithostratigraphic data to (1) reconcile the onset of subsidence by first accumulation of Pennsylvanian strata, (2) establish a period of peak subsidence estimated by maximum accumulation rate, (3) correlate estimated peak subsidence with the first appearance of arkose derived from the adjacent denuded Precambrian-cored block, and (4) demarcate synorogenic strata from Permian strata that are postorogenic. Results demonstrate that within New Mexico ARM basins, (1) onset was relatively synchronous, predominantly beginning in early Atokan time; (2) peak subsidence, while potentially younging southward, was relatively coeval in the Middle to Late Pennsylvanian; (3) first occurrence of arkose either predates the period of peak subsidence or is coeval with peak subsidence; and (4) early Permian strata across the study area onlap preexisting faults and folds, and/or form a buttress unconformity with Precambrian basement.

ABSTRACT The Mogollon-Datil volcanic field is a 40–24 Ma cluster of calderas that formed during ignimbrite flare-up eruptions in southern New Mexico associated with sub-duction, and possible delamination, of the Farallon plate beneath the North American plate. This study uses magmatic zircon sampled from four ignimbrites from a nested caldera system and an additional ignimbrite located outside of the nested system to compare the processes and timing of magma accumulation in southern New Mexico. These ignimbrites include: the Whitewater Tuff, the Cooney Canyon Tuff, the Davis Canyon Tuff, and the Shelley Peak Tuff from the Mogollon Mountains and the Bell Top 4 Tuff from the Uvas volcanic field. The ignimbrites range from crystal-poor, high-silica rhyolite to crystal-rich, low-silica rhyolite. We compare previous 40 Ar/ 39 Ar sanidine eruption ages to new U-Pb crystallization ages and trace-element compositions of zircon. Weighted mean zircon ages define two magmatic groups. Group one includes the Bell Top Tuff (34.5 ± 0.5 Ma), the Cooney Canyon Tuff (34.8 ± 0.8 Ma), and the Whitewater Creek Tuff (36.2 ± 0.4 Ma). The second group includes the Davis Canyon Tuff (28.7 ± 0.5 Ma) and the Shelley Peak Tuff (29.6 ± 0.5 Ma). Weighted mean zircon ages are within published 40 Ar/ 39 Ar ages, with the exception of the Shelley Peak Tuff, which is ~1 m.y. older. Hafnium contents and Th/U and Yb/Gd ratios suggest the dominant mechanism that produced eruptible melt was rejuvenation or remobilization of a crystal mush accompanied by minimal partial melting of the continental crust.

We studied the oxidation state of Fe in silicate glasses produced during the first atomic bomb blast at the Trinity test site (New Mexico) by X-ray absorption–near edge spectroscopy (XANES). The sample consists of green glass resulting from melting of the quartz-bearing sand present at the test site; some relict unmelted sand is still fused to the bottom of the sample. Comparison of the pre-edge peak data with model compounds of known Fe oxidation state and coordination number shows that in the Trinity glass sample, Fe is in the divalent state and, on average, in a mixture of 4- and 5-fold coordination. XANES spectra collected at various heights of the sample, from the bottom of the sample up to the exposed surface, show no variation of the pre-edge peak and, thus, of the Fe oxidation state with the distance from the sand-glass interface. However, XANES analysis of a portion of the sand at the bottom of the sample shows Fe to be a mixture of Fe 2+ and Fe 3+ , with a Fe 3+ /(Fe 2+ + Fe 3+ ) ratio close to 0.5. This demonstrates that during the nuclear explosion, the ground rock was instantaneously reduced, transforming all the iron from mostly trivalent state to almost exclusively divalent. Pre-edge peak features (intensity and energy) are consistent with those of tektites from the Ivory Coast studied here and with literature data of tektites from all the other known strewn fields (Australasian, Central European, and North American). The reduction of Fe to divalent state during Trinity glass formation, the homogeneity of the Fe oxidation state within the glass, and the Fe structural role suggest that this glass represents a good analog of tektite glass.