ABSTRACT Volcanic rocks from the Ebbetts Pass (ca. 6–4.6 Ma) and the Sierra Crest–Little Walker (ca. 12–9 Ma) volcanic centers provide a test of how structural setting can influence magma storage and transport. Both volcanic centers lie within well-exposed pull-apart basins within the ancestral Cascades arc. Prior studies show that highly potassic lavas at the Sierra Crest–Little Walker volcanic center erupted during periods of higher rates of Walker Lane transtensional faulting. In contrast, the Ebbetts Pass volcanic center, while erupting similarly evolved volcanics, has few high-K lavas, and the volcanics appear to have erupted during higher transtensional extension rates. Here, we present new mineral composition data (clinopyroxene, olivine, and plagioclase) that reveal contrasts between the Ebbetts Pass and Sierra Crest–Little Walker plumbing systems. We found that the high-K volcanics of the Sierra Crest–Little Walker volcanic center are the only volcanic materials to be erupted from a very wide range of depths, from just above the mantle to the shallow upper crust (~0.5–9 kbar). In contrast, every other eruptive unit in the Sierra Crest–Little Walker volcanic center records storage depths that are restricted to the upper half of the upper crust (<2 kbar). The Ebbetts Pass volcanic center also differs from the Sierra Crest–Little Walker volcanic center in that volcanics there record cooler temperatures and a much more restricted range of pre-eruption temperatures. For example, clinopyroxenes span a 250 °C temperature range (875–1125 °C) at the Sierra Crest–Little Walker volcanic center but just a mere 50 °C range (950–1000 °C) at the Ebbetts Pass volcanic center. For olivine and plagioclase, temperatures at the Ebbetts Pass volcanic center are not only more restricted than those at the Sierra Crest–Little Walker volcanic center, but they cluster only at the low-temperature range of that exhibited by some of the Sierra Crest–Little Walker volcanics. Plagioclase thermometry shows a trend of decreasing temperature with time, while olivine temperatures are similar across the Sierra Crest–Little Walker and Ebbetts Pass volcanic centers. We infer this to mean that the magmas entered the system equally hot, but that cooling was greater at the Ebbetts Pass volcanic center. Perhaps our most significant finding is, unlike other large, intermediate-composition volcanic centers, the Ebbetts Pass volcanic center is not transcrustal. We also measured olivine profiles for diffusion time scales, but we found no evidence of a contrast in such time scales between the two volcanic centers. Fieldwork and age dates show that the transtensional basin that holds the Ebbetts Pass volcanic center experienced subsidence rates 50% higher than the basin that holds the Sierra Crest–Little Walker volcanic center system (3000 m/m.y. vs. 2000 m/m.y.), indicating that higher transtensional strain rates were active in the Ebbetts Pass volcanic center. We posit that larger transtensional strain rates encouraged recharge magmas at the Ebbetts Pass volcanic center to quickly transit otherwise viable lower- and middle-crust magmatic staging regions. This implies that transtensional stresses may exert a profound control on volcanic center development. However, we acknowledge that magmatic flux could be a significant limitation in interpreting these findings, as varying magmatic input over time might influence magma storage.

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.

We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely inter-related new models for: (1) the paleogeography of the ancestral Cascades arc, (2) the stratigraphic record of uplift events in the Sierra Nevada, (3) the tectonic controls on volcanic styles and compositions in the arc, and (4) the birth of a new plate margin. Previous workers have assumed that the ancestral Cascades arc consisted of stratovolcanoes, similar to the modern Cascades arc, but we suggest that the arc was composed largely of numerous, very small centers, where magmas frequently leaked up strands of the Sierran frontal fault zone. These small centers erupted to produce andesite lava domes that collapsed to produce block-and-ash flows, which were reworked into paleocanyons as volcanic debris flows and streamflow deposits. Where intrusions rose up through water-saturated paleocanyon fill, they formed peperite complexes that were commonly destabilized to form debris flows. Paleo-canyons that were cut into Cretaceous bedrock and filled with Oligocene to late Miocene strata not only provide a stratigraphic record of the ancestral Cascades arc volcanism, but also deep unconformities within them record tectonic events. Preliminary correlation of newly mapped unconformities and new geochronological, magnetostratigraphic, and structural data allow us to propose three episodes of Cenozoic uplift that may correspond to (1) early Miocene onset of arc magmatism (ca. 15 Ma), (2) middle Miocene onset of Basin and Range faulting (ca. 10 Ma), and (3) late Miocene arrival of the triple junction (ca. 6 Ma), perhaps coinciding with a second episode of rapid extension on the range front. Oligocene ignimbrites, which erupted from calderas in central Nevada and filled Sierran paleocanyons, were deeply eroded during the early Miocene uplift event. The middle Miocene event is recorded by growth faulting and landslides in hanging-wall basins of normal faults. Cessation of andesite volcanism closely followed the late Miocene uplift event. We show that the onset of Basin and Range faulting coincided both spatially and temporally with eruption of distinctive, very widespread, high-K lava flows and ignimbrites from the Little Walker center (Stanislaus Group). Preliminary magnetostratigraphic work on high-K lava flows (Table Mountain Latite, 10.2 Ma) combined with new 40 Ar/ 39 Ar age data allow regional-scale correlation of individual flows and estimates of minimum (28,000 yr) and maximum (230,000 yr) time spans for eruption of the lowermost latite series. This work also verifies the existence of reversed-polarity cryptochron, C5n.2n-1 at ca. 10.2 Ma, which was previously known only from seafloor magnetic anomalies. High-K volcanism continued with eruption of the three members of the Eureka Valley Tuff (9.3–9.15 Ma). In contrast with previous workers in the southern Sierra, who interpret high-K volcanism as a signal of Sierran root delamination, or input of subduction-related fluids, we propose an alternative model for K 2 O-rich volcanism. A regional comparison of central Sierran volcanic rocks reveals their K 2 O levels to be intermediate between Lassen to the north (low in K 2 O) and ultrapotassic volcanics in the southern Sierra. We propose that this shift reflects higher pressures of fractional crystallization to the south, controlled by a southward increase in the thickness of the granitic crust. At high pressures, basaltic magmas precipitate clinopyroxene (over olivine and plagioclase) at their liquidus; experiments and mass-balance calculations show that clinopyroxene fractionation buffers SiO 2 to low values while allowing K 2 O to increase. A thick crust to the south would also explain the sparse volcanic cover in the southern Sierra compared to the extensive volcanic cover to the north. All these data taken together suggest that the “future plate boundary” represented by the transtensional western Walker Lane belt was born in the axis of the ancestral Cascades arc along the present-day central Sierran range front during large-volume eruptions at the Little Walker center.

The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km 3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1–15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal “point source” explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone–Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid migration of plume-head material, at ~20 cm/yr to the southwest, a direction not previously recognized.