We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a west-dipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo–Santa Clara–Pajarito fault system, a collection of northeast- and north-striking faults in the center of the Española Basin, defines the western boundary of the half graben and was active throughout rifting. Throw rates near the middle of the fault system (i.e., the Santa Clara and north Pajarito faults) and associated hanging-wall tilt rates progressively increased during the middle Miocene. East of Española, hanging-wall tilt rates decreased after 10–12 Ma, coinciding with increased throw rates on the Cañada del Almagre fault. This fault may have temporarily shunted slip from the north Pajarito fault during ca. 8–11 Ma, resulting in lower strain rates on the Santa Clara fault. East of the Embudo–Santa Clara–Pajarito fault system, deformation of the southern Barrancos monocline and the Cañada Ancha graben peaked during the early–middle Miocene and effectively ceased by the late Pliocene. The north-striking Gabeldon faulted monocline lies at the base of the Sangre de Cristo Mountains, where stratal dip relations indicate late Oligocene and Miocene tilting. Shifting of strain toward the Embudo–Santa Clara–Pajarito fault system culminated during the late Pliocene–Quaternary. Collectively, our data suggest that extensional tectonism in the eastern Española Basin increased in the early Miocene and probably peaked between 14–15 Ma and 9–10 Ma, preceding and partly accompanying major volcanism, and decreased in the Plio-Pleistocene.

The geometry of major basin-bounding faults in the central Rio Grande rift indicates that extension is characterized by overlapping and nested half grabens linked together by accommodation zones and bounded by tilted block uplifts. The Albuquerque-Belen Basin is composed of two separate, opposite-facing, structurally complex basins linked by an accommodation zone. South of the Albuquerque-Belen Basin, the rift broadens southwestward around the Colorado Plateau. A zone of distributed sinistral shear at the margin of the plateau accommodated differential extension between the little-extended plateau and the half grabens (Milligan Gulch, La Jencia, and Socorro) of the rift. Extension in the latter accommodation zone is expressed in the sedimentary cover rocks by north-striking, high-angle normal faults and a segmented, northeast-striking, sinistral fault. The Ladron horst and the Lucero uplift border the rift on the western side of the Albuquerque-Belen Basin and were isostatically uplifted as a result of tectonic denudation along the bounding faults of adjacent half grabens. Footwall structures include folded Paleozoic and Mesozoic rocks, intrabed thrust faults, and high-angle shear zones. We present new models for the Neogene evolution of the central Rio Grande rift and integrate contractional and extensional structures previously attributed to different stress regimes. Many of the structures in the central Rio Grande rift previously attributed to Laramide compression can be adequately explained by late Tertiary extension.

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation. From the model, a critical value of the ratio of basalt recharge-to-assimilation, (r/a) c , is deduced, which increases with decreasing crustal temperature. The r/a value must exceed (r/a) c to allow the volume of differentiated magma to increase, a prerequisite for developing large volumes of silicic magma. Strongly peraluminous (or S-type) magmas (NCI = 0·8–1), form under conditions of high crustal temperature and low basalt supply. Metaluminous or I-type granites form over a wide range of conditions (NCI = 0·1–1), generally where basalt supply is substantial. In individual long-lived volcanic centres, the large-volume high-silica ignimbrites are associated with the highest r/a and lowest NCI values, indicating that these magmas are typically differentiates of mantle-derived basaltic parents.