ABSTRACT Extensive fieldwork and supporting laboratory analyses by Murray and Busby in the Cerocahui-Guazapares region of the northern Sierra Madre Occidental silicic large igneous province have identified three Oligocene volcanic stratigraphic subdivisions that were erupted during distinct phases of the mid-Cenozoic ignimbrite flare-up in western North America. The ca. 27.5 Ma Parajes Group, an ~1-km-thick sequence of rhyodacitic welded ignimbrite sheets, represents medial outflow facies erupted outside of the study area from unidentified caldera sources during the Oligocene pulse of flare-up magmatism. The ca. 27.5–24.5 Ma Témoris Formation is composed of Southern Cordillera basaltic andesite (SCORBA) mafic-intermediate lavas and associated intrusions, alluvial deposits, and distal nonwelded silicic ignimbrites deposited in synvolcanic half-graben basins following the Oligocene ignimbrite pulse. The ca. 24.5–23 Ma Sierra Guazapares Group is a fissure-fed silicic ignimbrite and bimodal volcanic unit that was erupted during the initiation of the early Miocene pulse of the ignimbrite flare-up. Three new 40 Ar/ 39 Ar ages further refine the ages of deposition and extension in the Cerocahui-Guazapares region. The Chepe ignimbrite, the lowest stratigraphic unit of the Parajes Group, yields a late Eocene age of 34.89 ± 0.11 Ma. This age is older than the majority of the Parajes Group—a new date from the KM ignimbrite near the stratigraphic top of the Parajes Group yields an age of 27.62 ± 0.3 Ma, which corresponds well to the previous ca. 27.5 Ma zircon U-Pb geochronology ages. In the Cerocahui basin, upper Témoris Formation alluvial deposits are capped by a Sierra Guazapares Group basalt lava unit that yields an age of 23.99 ± 0.20 Ma. This basalt lava has only minor offset across the basin-bounding fault, and much of the Sierra Guazapares Group is relatively flat, suggesting that extension in the study region was active since at least ca. 27.5 Ma but was negligible after ca. 23 Ma. The timing of extension and volcanism in the Cerocahui-Guazapares region is older than in areas further west, supporting the wide-rift to narrow-rift evolution models proposed for the Sierra Madre Occidental and the Gulf of California divergent plate margin.

ABSTRACT The record of Permian–Triassic evolution in eastern North America indicates an important change in the tectonic regime from compression to extension as eastern Laurentia transitioned from the Alleghanian orogeny to continental rifting associated with the breakup of Pangea. The temporal pace (e.g., gradual vs. episodic, diachronous vs. synchronous), the accommodating structures, and the influential processes that characterized this transition provide critical insights into the late Paleozoic evolution of Laurentia and rifted continental margins in general. Connections between the formation of the South Georgia basin and regional cooling of the southernmost Appalachian crystalline rocks, along with the distribution of normal faults and discontinuities in metamorphic grade, indicate extensional collapse of the Alleghanian orogen along an extensive detachment system that was active from ca. 295 to 240 Ma. The 40 Ar/ 39 Ar cooling ages of biotites from low-angle normal shear zones cutting migmatitic gneisses of the southernmost Appalachians are interpreted to document extensional faulting ca. 280 Ma and to provide a snapshot of the prolonged orogenic collapse. The timing, orientation of structures, extent of reactivation, and character of late Alleghanian extension in the central and northern Appalachians provide an orogen-scale framework for this tectonic transition. This contribution focuses on correlations between the beginning of orogenic collapse and the initiation of continental rifting along with the tectonic processes that transformed eastern North America from a convergent to divergent plate boundary following the Alleghanian orogeny.

To better define the cooling history of the northern Oaxacan Complex, titanite and phlogopite from metasedimentary calc-silicate and biotite from a pegmatite were collected. All these rocks were involved in the granulite-facies Zapotecan orogeny between ca. 1004 and 978 ± 3 Ma, inferred to result from underthrusting the Oaxacan Complex beneath an arc or continent. Fragments of 2 × 5 cm 2 titanite crystals yielded a concordant U-Pb age of 968 ± 9 Ma, whereas 40 Ar/ 39 Ar analyses of phlogopite and biotite gave ages of 945 ± 10 Ma and 856 ± 10 Ma, respectively. These ages are inferred to date cooling through 660–700 °C, 450 °C, and 300–350 °C, respectively. When combined with published ages (Sm-Nd garnet, 40 Ar/ 39 Ar hornblende, Rb-Sr biotite and whole rock, and K-Ar biotite and K-feldspar) the data define a two-stage cooling curve: (1) 8 °C/m.y. between 978 and 945 Ma, cooling through 450 °C by which time the rocks had risen through a depth of 15 km; and (2) 2 °C/m.y., which, by extrapolation, brought the rocks to the surface between 710 and 760 Ma. The first stage of exhumation is interpreted in terms of tectonic switching between steep and flat slab subduction, a result of interactions of a ridge, a plume, or an oceanic plateau with the trench. The second stage may be related to thermal relaxation of the lithosphere, ending with the breakup of Rodinia, which brought the rocks to the surface.