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.

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 In this volume we present seven field trip guides that span the breadth of the geology of central California. The trips are associated with the 2013 Cordilleran Section meeting of the Geological Society of America, convened in Fresno, California. These trips provide guides to some of the most remarkable of geologic localities, which are not only iconic, but form type examples of key geologic phenomena and include Yosemite National Park, the San Andreas fault, the Franciscan complex, and the Sierra Nevada Foothills near Fresno, California. The topics covered by these field trips include the nature of continental transform faults, the initiation of subduction, the origin of the Sierra Nevada batholith, the initiation of the Sierra Nevada arc, Pleistocene vertebrate fossils of the Central Valley, and debris flows triggered from burned watersheds.

Abstract The San Andreas fault is arguably the best-studied strike-slip fault in the world, and our understanding of its evolution continues to improve as new data become available. This trip will take the participants to a selection of some classical as well as some new sites that will highlight a revised understanding of the San Andreas fault's recent history based on our geomorphological, paleoseismologic, and paleoclimate investigations.

Abstract Since the days of John Muir, the striking granitic topography of Yosemite Valley, California, has been understood to have been sculpted by glaciers and presently modified by rockfall. Glacial erosion has provided remarkably clean and extensive exposures of granitic rocks on the vertical walls that provide insights into intrusive relations and rockfall susceptibility. However, it is only with recent remote sensing methods that these exposures have been studied in detail. El Capitan presents an unparalleled exposure of the interior of a granitic plutonic system at the point of interaction between multiple intrusive suites and two sets of mafic dike swarms. The distribution and orientation of these units affected El Capitan's extensive rockfall history, including a huge postglacial rock avalanche at 3.6 ka. This two-day field trip will explore these ideas and apply them to some of the other classic cliffs of Yosemite Valley such as Glacier Point and Half Dome. We will present a new map of El Capitan and discuss the intrusive relationships exposed on the face while visiting several rockfall deposits and some of the classic vistas of Yosemite Valley, including El Capitan Meadow, Glacier Point, Taft Point, and Mirror Lake.

Abstract This two day field trip presents an overview of ~500 m.y. of diverse geologic history along the southwestern Sierra Nevada Foothills, with focus on several profound geologic features that distinguish this part of the western foothills. These features include (1) a highly deformed oceanic Moho section through the Kings River ophiolite, interpreted as a Carboniferous abyssal core complex; (2) Permo-Carboniferous ophiolitic mélange interpreted to have formed and been emplaced along the SW Cordilleran edge transform, along which the early Mesozoic convergent margin initiated; (3) Early Cretaceous mafic rocks of the western Sierra Nevada batholith, which also characterize much of the Great Valley basement; (4) evidence for 1000-m-scale paleo-relief of Late Cretaceous age; (5) anomalous subsidence of the Tulare subbasin of the Great Valley as an expression of ongoing mantle lithosphere delamination; and (6) the Kern arch and Kern range front epeirogenic uplift as an expression of active mantle lithosphere delamination.