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.

The Rosario segment of the Cretaceous Alisitos arc (Baja California, Mexico) is arguably the best-exposed structurally intact and unmetamorphosed oceanic arc crustal section on Earth. The gently tilted, 50-km-long section exposes the transition from upper-crustal volcanic rocks to mid-crustal plutonic rocks, formed in an extensional environment. This book presents a detailed geologic map, based on an exhaustive data set including geochemistry, geochronology, and annotated outcrop photos and photomicrographs. Subsegments within the Rosario segment include a subaerial edifice, a volcano-bounded basin, and a fault-bounded basin, each underpinned by separate plutons. The entire data set is integrated across these subsegments in a time slice reconstruction of arc evolution and the relationships between plutonism and volcanism. The data set provides constraints on the evolution of silicic calderas and tectonic triggers for caldera collapse, caldera resurgence by emplacement of sill complexes and by incremental growth of plutons, and comparison with velocity profiles in modern arcs.

ABSTRACT The Rosario segment of the Early Cretaceous Alisitos oceanic arc exposes the transition from upper-crustal volcanic and hypabyssal rocks to middle-crustal plutons, which formed in an extensional environment. The Rosario segment forms a structurally intact, unmetamorphosed, spectacularly well-exposed, gently tilted section that is 50 km long and 7 km deep. The top of the exposed section is unconformably overlain by flat-lying Late Cretaceous sedimentary rocks (Rosario Group, described elsewhere), and the base of the section passes downward into ductilely deformed metamorphic rocks (not mapped herein). We divided the Rosario segment into three subsegments: a central subaerial edifice, underpinned by the La Burra pluton; a southern volcano-bounded basin (dominantly shallow marine), underpinned by the San Fernando pluton; and a northern fault-bounded basin (dominantly deep marine), underpinned by the Los Martires pluton. Using a combination of published and new geochronologic data, we infer that the time span represented by the arc crustal section could be as little as 1.7 m.y., dated at ca. 111–110 Ma. Volcanic and plutonic samples show a continuum from basalt/basaltic andesite to rhyolite, are low to medium K, and are transitional tholeiite to calc-alkaline in character. Hf isotopic data from zircons indicate primitive magma, consistent with previously published whole-rock isotopic data. The volcanic stratigraphy can be correlated across all three subsegments using the tuff of Aguajito (Ki-A) , a distinctive rhyolite welded ignimbrite that fills the 15-km-wide, >3.6-km-deep La Burra caldera on the central subaerial edifice. Additionally, a second caldera is preserved below the tuff of Aguajito (Ki-A) in the northern fault-bounded basin, floored by a large rhyolite sill complex, up to 700 m thick with a lateral extent of >7 km. Up section from the tuff of Aguajito (Ki-A) , there is an abrupt shift to dominantly mafic volcanism that we correlated across all three subsegments of the Rosario segment, dividing the section into two distinct parts (phase 1 and phase 2). The pluton beneath the central subaerial edifice (La Burra) is associated with the caldera that produced the tuff of Aguajito (Ki-A) during phase 1. Plutons beneath the northern fault-bounded basin (Los Martires) and the southern volcano-bounded basin (San Fernando) were emplaced during phase 2. However, we infer that the La Burra pluton, which is associated with the phase 1 La Burra caldera, continued to grow incrementally during phase 2 because it intruded and tilted both phase 1 and phase 2 strata. The Rosario segment escaped postmagmatic deformation, other than gentle tilting (25°–35°) to the west as a single rigid block. The Rosario segment of the Cretaceous Alisitos arc represents an extensional oceanic arc with abundant silicic pyroclastic rocks, culminating in arc rifting with outpouring of mafic magmas. The excellent exposure and preservation provide us with the opportunity to herein describe the following: (1) caldera collapse features and the products of varying explosive eruptive styles; (2) caldera plumbing systems, including silicic sill complexes; (3) the transition from plutons through hypabyssal intrusions to eruptive products; (4) incremental pluton growth and its effects on the structure of the roof rocks; (5) the products of deep-water mafic to silicic eruptions; and (6) flow transformations that occur when hot pyroclastic flows enter marine basins on gentle slopes versus steep slopes. We also used this data set to address questions highly complementary to the work being done on understanding the growth of continental crust at subduction zones. Finally, this volume serves as a model for detailed geologic study of paleo-arcs.

Abstract This field guide examines Quaternary volcanism in the Canadian Cascade arc informally known as the Garibaldi Volcanic Belt. During the first day, the trip proceeds along the Sea-to-Sky Highway corridor from Vancouver to Pemberton and focuses on Quaternary glaciovolcanic deposits and lavas. Interactions between volcanoes and ice in the Garibaldi Volcanic Belt have been common during the past two million years, which has resulted in a diverse array of landforms, including subglacial domes, tuyas, impounded lava masses, and sinuous lavas that exploited within-ice drainage systems. On Days 2 and 3, the trip heads northwest of Pemberton, British Columbia, along logging roads to see deposits from the 2360 yr B.P. eruption of the Mount Meager volcanic complex. The eruption began Plinian-style, generating pyroclastic fall and density current deposits (Day 2) and ended with the production of welded and nonwelded block and ash flows by explosive (Vulcanian) collapse of a lava (Day 3). The guide will examine the deposits of the temporary lake impounded by a block and ash flow deposit dam, and the evidence of the catastrophic failure of the dam and the generation of a huge outburst flood and lahar. Many of the traits of the deposits seen on this three-day trip are a reflection of both the style of eruption and the nature of the surrounding landscape. In this regard, the trip provides a spectacular window into the nature and hazards of effusive and explosive volcanism occurring in mountainous terrains and the role of water and ice.

Abstract Recent structural analysis of the Grey’s Landing ignimbrite offers new insights into the emplacement of rheomorphic ignimbrites. We present several key localities, where volcanological and structural features reveal the emplacement history of a lavalike ignimbrite and the evolution of ductile deformation structures during and after deposition across complex topography. Excellent three-dimensional exposure allows us to interpret structural features of the Grey’s Landing ignimbrite in the context of diverse emplacement models for rheomorphic ignimbrites elsewhere and to consider field criteria to distinguish between lava-like ignimbrites and extensive silicic lavas .