ABSTRACT Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley, Nevada, USA. TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits record hydrologic changes that occurred here in a dynamic and temporally congruent response to abrupt climatic oscillations over the last ~300 ka (thousands of years). The deposits also entomb the Tule Springs Local Fauna (TSLF), one of the most significant late Pleistocene (Rancholabrean) vertebrate assemblages in the American Southwest. The TSLF is both prolific and diverse, and includes a large mammal assemblage dominated by Mammuthus columbi and Camelops hesternus. Two and possibly three distinct species of Equus, two species of Bison, Panthera atrox, Smilodon fatalis, Canis dirus, Megalonyx jeffersonii, and Nothrotheriops shastensis are also present, and newly recognized faunal components include micromammals, amphibians, snakes, and birds. Invertebrates, plant macrofossils, and pollen also occur in the deposits and provide important and complementary paleoenvironmental information. This field compendium highlights some of the classic stratigraphic sequences of the Las Vegas Formation within TUSK, emphasizes the significant hydrologic changes that occurred in the area during the recent geologic past, and examines the subsequent and repeated effect of rapid climate change on the local desert wetland ecosystem.

ABSTRACT The geology, stratigraphy, and paleontology of the Santa Ana Mountains of Southern California span 150 m.y. of subduction and 30 m.y. of transform faulting, producing complex geologic, stratigraphic, and paleontological settings. The mountains are bounded by the Elsinore fault zone on their east side, uplifting the mountains and tilting them westward, where sediments eroded from them were deposited in a variety of marine to terrestrial environments; most of these formations yield fossils so that a rich history of life can be reconstructed. The most recent geologic history includes the continued transform faulting with displacements of many kilometers northwesterly, juxtaposing separate blocks and biotas. The modern sediments are dominated by the Santa Ana River, which flows westerly at the northern end of the Santa Ana Mountains onto the coastal plain of Orange County. It is the primary aquifer supplying significant amounts of water to the residents. Humans have occupied the region for the last 12,000 yr, developing large, sophisticated populations, which, in the most recent years, have impacted the geology significantly. This field-trip guide starts north of the mountains in Ontario, California, and describes the Elsinore fault zone, the east side of the Santa Ana Mountains, and the ascent of the steep eastern side of those mountains. Extensive vistas of the geology to the east of the mountains can be seen from stops along the way. In the mountains themselves, the guide describes the granitoids of the Peninsular Ranges batholith, sedimentary rocks of the Jurassic Bedford Canyon Formation, rocks of the Cretaceous Santiago Peak Volcanics, and overlying sedimentary rocks of Mesozoic and Cenozoic age. At Ronald W. Caspers Wilderness Park, stops show the early Tertiary Silverado and Santiago formations preserving terrestrial environments that rest unconformably on the marine Cretaceous Williams Formation. On the west side of the mountains, stops at Cretaceous to Miocene conglomerates through mudstones reveal abundant marine mollusks, foraminifera, and vertebrate faunas among others, and a wide variety of sedimentary structures. Younger sediments, faults, and river courses occur along the final leg of the trip from the northern Santa Ana Mountains back to Ontario. Humans have interacted with the geology and its resources for possibly the last 12,000 yr, in ancient times utilizing rock resources and in modern times dealing with geological hazards in developmental and infrastructural construction.

ABSTRACT The Coyote Mountains of southern California lie at the interface of the Peninsular Ranges and Colorado Desert geomorphic provinces, along the active right-lateral Elsinore fault, just north of the Mexican border. This field-trip guide explores the southeastern part of the range and examines the field evidence for (1) the early Miocene passing of the edge of the slab window underneath the area as the subduction zone west of the area shut down, (2) the inception of the West Salton detachment fault between 8 and 4.5 Ma, and (3) the uplift and right-lateral distributed shearing of the range since ca. 1.2 Ma.

ABSTRACT Welcome to our three-day field trip aimed at exploring Mesozoic magmatism and tectonism in southern California. The broader southern California area is a great place to study Cordilleran-style continental margin arcs built across diverse basements ranging from thinner oceanic to thicker continental crust. We will weave together several themes during this trip: (1) spatial variations, including in the vertical dimension, of arc magmatic and tectonic systems; (2) volcanic-plutonic links; (3) pluton sizes and shapes; (4) emplacement and tectonic/plutonic mass balance issues; and (5) arc magmatic and tectonic tempos. While doing so we plan to discuss/ examine the main components of southern California Mesozoic magmatic systems while discussing temporally and spatially variable (1) melt sources, (2) processes in crustal columns, and (3) tectonic activity, all of which lead to the final preserved arcs. Bedrock and detrital zircon age patterns in a broad corridor across southern California define magmatic flare-up maxima in the Mojave area at ca. 250 Ma in a SE migrating arc, at ca. 170 Ma in a westward migrating arc, at ca. 100 Ma in the eastward migrating Peninsular Ranges Batholith (PRB), extending from just south of the Transverse Ranges and into Baja California, and additional smaller maxima at 81 and 73 Ma in the Transverse Ranges and Joshua Tree regions that may reflect the western edge of a broad region of melting and magmatism throughout the Basin and Range area. On this trip we will integrate studies from different areas of these Mesozoic arcs including Baja California (Paterson), the northern PRB (Clausen), the San Gabriel Mountains (Schwartz), and the Joshua Tree National Park area (Memeti and Paterson) with a wealth of new data being published by other groups. Day 1 will examine aspects of the Cretaceous PRB in the Perris block where the edge of the western belt and transition zone is exposed. On Days 2 and 3 we will examine parts of two tilted crustal sections, one in the eastern San Gabriel Mountains and the other in the Little San Bernardino Mountains and Joshua Tree National Park. While visiting these crustal sections we will discuss Late Cretaceous to Early Tertiary events to explore why crustal melting and magma emplacement occurred both inboard and post the main magmatic flare-ups in regions that usually are thought to have transitioned into cold fore arc.

ABSTRACT The Sierra Nevada batholith is an ~600-km-long, NNW-trending continental arc generally exposed from epizonal to mesozonal levels and showing a distinct strike-perpendicular zonation in structural, lithologic, petrologic, geochronologic, and isotopic patterns. South of 35.5° N, in the southern Sierra Nevada–northern Mojave Desert region, the depth of exposure increases markedly and a tectonostratigraphy consisting of three distinct, fault-bounded assemblages is observed. From high to low structural levels, these units are (1) fragments of shallow-level eastern Sierra Nevada batholith affinity rocks, (2) deeper-level western to axial zone rocks, and (3) subduction accretion assemblages (e.g., the Rand schist). This multi-tiered core complex is the product of shallow subduction that occurred over ~500 km of the plate margin in Late Cretaceous time. Slab shallowing was accompanied by intense contractile deformation within the crust and along the subduction megathrust; crustal thickening, uplift, and denudation of the residual arc to midcrustal levels; removal of the forearc and frontal arc by subduction erosion; and replacement of sub-batholithic mantle with underplated subduction assemblages. As the slab reverted to a “normal” trajectory, previously thickened crust no longer compensated at depth by a shallowly dipping slab became gravitationally unstable and underwent a profound phase of extensional collapse. Two subparallel shear zones, one separating assemblages 1 and 2 (the southern Sierra detachment) and the other juxtaposing units 2 and 3 (the Rand fault), comprise an integrated Late Cretaceous detachment system that accommodated extensional collapse. These Late Cretaceous events preconditioned the southern California crust for imprints of subsequent tectonic regimes. For example, subduction of the Pacific-Farallon slab window in early Neogene time created an extensional stress regime in the overriding plate, facilitating high-angle normal faulting across the previously extended region and volcanism associated with upwelling astheno-spheric material. The invasion of hot and buoyant asthenosphere destabilized dense sub-batholithic root material still affixed beneath the central Sierra Nevada batholith, leading to Pliocene–Quaternary delamination of the high-density rocks. Replacement of dense sub-batholithic root materials with asthenosphere has led to ~1 km of uplift across the southern Sierra Nevada and into the eastern San Joaquin Basin. The purpose of this trip is to highlight structural and petrologic records of multiple phases of tectonism in the southern Sierra Nevada–Mojave Desert region, illustrating the profound and lasting effect that shallow subduction may have on a continental margin.

ABSTRACT The Cemex Inc. limestone quarries are located in the Sidewinder Mountain–Black Mountain area northeast of Victorville in the Mojave Desert, California. Bedrock in this area includes Neoproterozoic and Paleozoic miogeoclinal carbonate-dominated metasedimentary roof pendants, and is the type location of the Jurassic Fairview Valley Formation, an overlying unnamed quartzite unit, and the Jurassic Sidewinder volcanic rocks. A variety of Triassic, Jurassic, and Cretaceous intrusive rocks and dikes also are abundantly exposed. Complex geologic structure in the area resulted from a prolonged geologic history that includes multiple Permian–Triassic and Mesozoic contractional, metamorphic, intrusive, and extensional deformation events, and younger Cenozoic deformation that has continued into recent time. Permian–Triassic deformation includes complex polyphase deformation and metamorphism of Paleozoic rocks, and intrusion of Triassic monzonite, followed by uplift and erosion. Lower Jurassic Fairview Valley Formation unconformably overlies Paleozoic rocks and Triassic monzonite and was largely derived from them. The Fairview Valley Formation was deformed and eroded prior to juxtaposition with overlying quartzite. Overlying the quartzite are Middle Jurassic Lower Sidewinder Volcanic rocks (181–165 Ma). During and following Lower Sidewinder volcanism, faulting related to regional caldera collapse events occurred. Extensional deformation occurred prior to and after intrusion of late Middle Jurassic granitic rocks. Northwest-trending dikes correlated with the Late Jurassic Independence dikes cut previously juxtaposed Fairview Valley Formation, quartzite, and Lower Sidewinder Volcanics. During Cretaceous time, intrusion of quartz monzonite plutonic rocks occurred. The Helendale fault, one of several active major northwest-striking faults of the Eastern California shear zone, has been traced for ~90 km and passes within 2 km of the Cemex operations. The location of the Helendale fault zone is thought to be a long-lived zone of structural weakness. Recurrent surface rupture along the modern Helendale fault in Holocene and recent time (200–10,000 yr) has been documented. Data suggest that right-stepping en echelon strands formed a pull-apart basin as much as 3 km wide in the Fairview Valley area, with up to 3 km of right lateral offset. The Cemex Inc. cement production facility is the largest in California. Deposits mined include the Reserve (White Mountain) quarry and the Black Mountain quarry. The Reserve quarry produced an estimated 25 million tons of calcite marble between the early 1940s and late 1970s. The deposit located on the south slope of Black Mountain was formed from metamorphosed Upper Paleozoic limestones. The Black Mountain quarry was opened in the early 1950s and originally formed a mountain. Currently, the quarry is an open pit ~1200 m long, 800 m wide, and >60 m deep. Production is estimated at 4 million tons per yr. Cement-grade ore mined at the Black Mountain quarry is limestone-cobble conglomerate of the upper member of the Jurassic Fairview Valley Formation, which was derived from the Pennsylvanian–Permian Bird Spring Formation, and only occurs in the quarry area. The deposit is folded into a tight northwest-trending syncline and bounded to the south, east, and north by faults. This unique deposit is suitable as a long-term (>100 yr) source of limestone for cement manufacture.

Abstract Our goals today are several-fold. We have now spent five days examining different parts of the Mesozoic Sierran arc, and hopefully discussions are already under way attempting to integrate both the shared and distinct characteristics of these individual magma plumbing systems and synchronous tectonics. We will briefly continue these discussions below. Our main focus will be to consider the arc as a whole and introduce a number of new regional data sets related to the tectonic and magmatic components of this arc. By the end of the day, we hope that our discussions have evolved to a consideration of the overall petrologic evolution of the arc, the tectonic and magmatic arc tempos, and their potential links. Without an airplane, or satellite, or Hollywood earth coring machine, it is difficult to take you to field locations where we can observe large sections of the arc. Instead, as we travel west back across the arc, we have selected a number of scenic overview stops, where we will introduce and discuss these new data sets while looking at gorgeous views of the arc…

Abstract Today’s trip introduces Field Forum participants to the earliest stages of magmatism in the Cretaceous section of the Sierran arc. Despite the paucity of fresh rock exposed in the western foothills, we have determined that the Fine Gold Intrusive Suite is a massive (>3100 km 2 ) and long-lived (ca. 19 million years, Lackey et al., 2012) intrusive complex, preserving a record of magmatism that provides considerable insight into the evolution of arc magma systems intruding across major crustal terrane boundaries. The primary units of the Fine Gold Intrusive Suite that will be viewed during the field excursion include the Bass Lake Tonalite, Ward Mountain Trondhjemite, and Knowles Granodiorite ( Fig. 2-1 ). The low-K 2 O, 87Sr/86Sr, high-Sr/Y, high- ?18O, and peraluminous composition of the Fine Gold Intrusive Suite rocks are geochemically distinct from the younger rocks of the central and eastern Sierra (Fig. 2-2 )…