ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California.

ABSTRACT The Sierra Madre fault zone is a south-vergent, active reverse fault that accommodates shortening between basins on the northern margin of the Los Angeles region and the San Gabriel Mountains. The preservation of late Quaternary alluvial fill and fan surfaces in the hanging wall of the fault provides evidence of long-term uplift. Surface rupture from the 1971 Mw 6.6 San Fernando earthquake and evidence of large prehistoric displacements from trenching investigations emphasize the ongoing hazard posed by the fault system to the region. This one-day field trip visits some of the key locations near Pasadena and San Fernando, California, where slip rates have been determined from cosmogenic and luminescence dating of abandoned surfaces dating to 50–70, ca. 30, and ca. 12 ka and surface offsets measured from lidar and pre-development topographic maps. Another stop is the site of a paleoseismic trench, which provided key evidence on the timing and displacement of past ruptures on the fault. In combination, results from these field investigations converge on a slip rate for the eastern ~100 km of the fault zone of 1–2 mm/yr, which matches or exceeds the rates for other reverse faults in southern California. This rate, in combination with trenching data that show no evidence of post–mid Holocene ruptures along the central and eastern portions of the fault, indicate the fault zone poses a significant seismic hazard to the region.

ABSTRACT Montecito, California, has a complicated Quaternary history of debris flows, the most recent being the Montecito debris flows of 9 January 2018, which were wildfire-debris flow–linked events that took 23 lives and damaged or destroyed several hundred homes. Relative flow chronology, based on boulder weathering, incision rates, and soil dates with limited numerical (radiocarbon and exposure) dating, is used to identify paths of prehistoric debris flows. Topography of debris flow fans on the piedmont is significantly affected by the south-side-up reverse Mission Ridge fault system. Examination of weathering rinds from Pleistocene debris flows confirms that the Rattlesnake Creek–Mission ridge debris flows are folded over the ridge, and that lateral propagation linked to uplift of marine terraces (uplift rate of ~0.5–1 m/k.y.) significantly altered debris flow paths. As communities continue to rebuild and live in these hazard-prone areas, disaster risk reduction measures must take into account both spatial and temporal components of vulnerability. This field guide includes four stops from Montecito to Santa Barbara. The first stop will be to observe debris flow stratigraphy over the past ~30 ka beneath an earthquake terrace and a prehistoric Chumash site on the beach near the Biltmore Hotel in Montecito. The second stop will be at San Ysidro Creek in San Ysidro Canyon, the site of the largest Montecito debris flow that occurred on 9 January 2018. We will discuss source area and processes of the debris flow, and take a short hike up the canyon to visit the debris flow basin and a ring net designed to reduce the future hazard. The final two stops will explore the debris flow chronology of Santa Barbara over the past ~100 ka. Figure 1 shows the location of the field-trip stops. There is no road log as field sites can be found with a search on a smartphone.

ABSTRACT This field trip provides a rare opportunity to visit outcrops and structures that highlight the geology, history, and natural beauty of Santa Cruz Island, a remnant of prehistoric California isolated off Santa Barbara. Santa Cruz Island provides some of the most southwestward positioned subaerial outcrops of the North American landmass, while displaying a rare glimpse of prehistoric coastal southern California and picturesque and seldom accessible exposures of Tertiary strata. Most of the stops are difficult to reach and many are closed to public access. Stops 1, 9, 9B, 9C, 13, and 13B are within the Channel Islands National Park, and access to the park portion of the island is by public boat transport (Island Packers) from Ventura Harbor to Prisoners Harbor. Stop 1 is near the pier at Prisoners Harbor and easily accessible; however, the other stops require roundtrip hikes of at least 10 miles from the pier. One of the goals of this four-day trip is to visit as much of the island’s varied geology as possible. A significant body of widely recognized geologic research has been done on the island from late Quaternary sea-level and climate changes to the tectonic evolution of the western North American plate boundary, and in particular the transformation of a subduction to transform plate boundary along a continental margin. Discovery that SCI and the western Transverse Ranges have rotated ~90° clockwise since the early Miocene (Kamerling and Luyendyk, 1979, 1985; Luyendyk et al., 1980) brought on an intense period of research on the island from the late 1970s through the 1990s. Much of this work has been published in both the formal and informal literature. Two decades later, this field trip is an opportunity to review much of these additions to geologic understanding with the advantage of gains in knowledge since then. The guide will emphasize each stop’s importance, offer questions for future research, and showcase the island’s earth science educational opportunities. This four-day trip requires 4WD vehicles and includes some 3–6 km (~2–4 mile) hikes. Dedicated to Dr. Lyndal Laughrin, Santa Cruz Island Reserve Director, Emeritus, The Sage of Santa Cruz Island

ABSTRACT This field trip examines Paleoproterozoic basement, Neoproterozoic metasedimentary strata, and crosscutting Mesozoic intrusive rocks at Frazier Mountain, Placerita Canyon, and Limerock Canyon in the western San Gabriel Mountains block, California. We present new U-Pb zircon geochronology results that constrain the Proterozoic through Cretaceous tectonic and magmatic history. The excursion ends in San Antonio Canyon in the eastern San Gabriel Mountains where several large rock avalanche deposits are sourced from distinct basement rocks. 10 Be surface exposure ages and post-infrared infrared stimulated luminescence burial ages demonstrate late Pleistocene to Holocene movements for these landslides.

This volume includes five geologic field-trip guides in the Los Angeles region associated with the 2020 GSA Cordilleran Section Meeting that was scheduled for May 2020, in Pasadena, California. The guides are organized in a generally counterclockwise order around the Los Angeles Basin. The first guide by Burgette et al. provides new slip rates, age constraints, and observations of the active Sierra Madre fault zone that borders the northern side of the San Gabriel and San Fernando Valleys. The Nourse et al. guide takes a new look at the San Gabriel Mountains from a basement and geomorphologic perspective. Further west, Keller et al. provide one of the first published field-trip guides focused on the 9 January 2018 Montecito debris flows that caused 23 deaths. The volume then moves south to Santa Cruz Island, where Davis et al. provide an updated review of the island’s geology within the California borderlands. The final guide returns to the east, where Platt et al. present the unique geology of Santa Catalina Island with a focus on the subduction-related Catalina Schist.

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.