ABSTRACT The mid-Cenozoic succession in the northeast limb of the Mount Diablo anticline records the evolution of plate interactions at the leading edge of the North America plate. Subduction of the Kula plate and later Farallon plate beneath the North America plate created a marine forearc basin that existed from late Mesozoic to mid-Cenozoic times. In the early Cenozoic, extension on north-south faults formed a graben depocenter on the west side of the basin. Deposition of the Markley Formation of middle to late? Eocene age took place in the late stages of the marine forearc basin. In the Oligocene, the marine forearc basin changed to a primarily nonmarine basin, and the depocenter of the basin shifted eastward of the Midland fault to a south-central location for the remainder of the Cenozoic. The causes of these changes may have included slowing in the rate of subduction, resulting in slowing subsidence, and they might also have been related to the initiation of transform motion far to the south. Two unconformities in the mid-Cenozoic succession record the changing events on the plate boundary. The first hiatus is between the Markley Formation and the overlying Kirker Formation of Oligocene age. The succession above the unconformity records the widespread appearance of nonmarine rocks and the first abundant appearance of silicic volcanic detritus due to slab rollback, which reversed the northeastward migration of the volcanic arc to a more proximal location. A second regional unconformity separates the Kirker/Valley Springs formations from the overlying Cierbo/Mehrten formations of late Miocene age. This late Miocene unconformity may reflect readjustment of stresses in the North America plate that occurred when subduction was replaced by transform motion at the plate boundary. The Cierbo and Neroly formations above the unconformity contain abundant andesitic detritus due to proto-Cascade volcanism. In the late Cenozoic, the northward-migrating triple junction produced volcanic eruptive centers in the Coast Ranges. Tephra from these local sources produced time markers in the late Cenozoic succession.

ABSTRACT The early Miocene Markagunt (MGS) and late Oligocene Sevier (SGS) gravity slides in southwestern Utah, USA, exhibit the full range of structural features commonly seen in modern landslides, but on a gigantic scale—they are among Earth’s largest terrestrial landslides. The MGS, discovered in 2013, consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment, (3) a ramp segment where the slide cut up-section and the basal fault “daylighted,” and (4) a former land-surface segment where the upper plate moved at least 35 km over the Miocene landscape. The MGS remained undiscovered for so long precisely because of its gigantic size (>5000 km 2 , >95 km long, estimated volume 3000 km 3 ) and initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGS basin-range tectonism. Preliminary mapping of the SGS, discovered in 2016, shows it to be smaller (>2000 km 2 ) and slightly older than the MGS. Both gravity slides are large contiguous sheets of andesitic lava flows, volcaniclastic rocks, source intrusions, and regional ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Oligocene to Miocene Marysvale volcanic field. Failure was preceded by slow gravitational spreading accommodated by the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporite-bearing strata at a depth of ~2 km; this thrust system deformed Middle Jurassic through lower Oligocene strata. MGS emplacement is presently constrained between ca. 23 and 21 Ma; SGS emplacement is presently constrained between ca. 25 and 23 Ma. The presence of basal and lateral cataclastic layers, injectites (clastic dikes), pseudotachylyte (frictionite), deformed clasts, and a variety of kinematic indicators suggests that each gravity slide represents a single catastrophic emplacement event from the north to the south; possibly the MGS comprises two gravity slides. The principal zone of failure was in mechanically weak, clay-rich sedimentary strata at the base of the volcanic section. The MGS and SGS are significant because they provide examples of lithified landslide structures so large that they may be mistaken for tectonic features. However, these gravity slides lie at right angles to regional compressional tectonic structures and are cut longitudinally by modern basin-range normal faults, and thus offer compelling case studies for how to differentiate features resulting from surficial verses tectonic processes. Here we offer a history of MGS and SGS discovery, our current understanding of the gravity slides as of late 2018 (which are the focus of ongoing research), and a guide to locations of particularly instructive exposures where we document our conclusions about size, distinctive structural features, emplacement ages, and interpreted emplacement mechanisms.

ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.