Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

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.

Forty kilometers south of Tecate, Baja California Norte, is an extensive area of pre–medial Cretaceous, greenschist-facies flysch, poorly exposed, and of unknown thickness. These rocks are lithocorrelative with the Triassic(?) French Valley and Julian Schist Formations and the Upper Triassic-Middle Jurassic Bedford Canyon Formation, north of the international border. The flysch type strata of the area consist of tabular, thin- to thick-bedded subarkosic metasandstone interbedded with metapelite. Isotopic studies on detrital zircon suggest a mixed population of discordant Middle to Late Proterozoic zircon and late Paleozoic and Triassic zircon. The flysch strata, herein named the Rancho Vallecitos Formation, is divided into a predominantly sandstone lithofacies and a predominantly shale lithofacies. Vertical sequence patterns and associations of sedimentary structures (graded bedding, outsized clasts, meniscate and diffuse laminae, etc.) indicate that the sandstones of both lithofacies were deposited by high-density turbidity currents. Rare ripple-laminated contourites of the shale lithofacies indicate deposition by northerly and southerly oriented paleocurrents. Vertical sequence patterns of sandstone beds in the sandy lithofacies suggest deposition in the outer part of a submarine fan-like system. Features of the predominantly shale lithofacies suggest a basin-plain depositional environment. Subordinate pebbly mudstone and local olistostrome/mélange deposits of the shaly lithofacies indicate proximity to slope areas. The overall fine-grained, poorly sorted, and matrix-rich character of all the sandstones suggests deposition in a large submarine fan or cone system of unrestricted open ocean basins, characteristically fed by large river/delta systems. Modal analyses of the sandstone framework indicates sediment sources of recycled orogens and possibly craton interior.

An area near Hemet in Riverside County, California, has been studied in which the sedimentology and pre-intrusive metamorphism at the site of the Southern California Batholith are well preserved. Pelitic schists of the Jurassic (?) Bedford Canyon Formation are conformably overlain by a 13,000-foot thick section of quartzite, schist, and amphibolite of the French Valley Formation (new name). These rocks were formed from a series of shales, shale-clast conglomerates, poorly sorted feldspathic, calcareous, and arkosic sandstones, and rare basaltic extrusives. Relict sedimentary textures and structures preserved at low metamorphic grade indicate a marginal basin environment; the age of deposition, formerly thought to be Paleozoic, is unknown. The sedimentary rocks were intruded by basaltic sills, folded and regionally metamorphosed at low pressures (3–5 kb) and moderate-to-high temperatures, up to 750°C, producing andalusite, cordierite, sillimanite, and garnet in the pelitic schists, and converting the basic igneous rocks to garnet-diopside-amphibolites. Four metamorphic zones are distinguished. Their boundaries trend north-south, athwart the regional strike, and bear no relation to the distribution of plutonic intrusives of the Southern California Batholith. Ultra-basic magma was intruded following the kinematic phase of metamorphism but was metamorphosed at temperatures and hydrostatic pressures comparable to those affecting the enclosing gneiss. The metamorphism is thought to have antedated emplacement of the Southern California Batholith (Late Cretaceous) and is analogous to the low-pressure andalusite-type metamorphism described by Miyashiro (1961).

The Mojave-Sonora megashear, which bounded the Jurassic southwestern margin of the North America plate from 170 to 148 Ma, may be linked northward to Alaska via the previously recognized discontinuity between the Insular and Intermontane terranes and co-genetic regional elements such as transtensional basins, transpressional uplifts, and overlapping correlative magmatic belts. The longer, continental-scale fault thus defined, which is called the Mexico-Alaska megashear, separated the North America plate from a proto-Pacific plate (the Klamath plate) and linked the axis of ocean-floor spreading within the developing Gulf of Mexico with a restraining bend above which mafic rocks were obducted eastward onto Alaskan sialic crust that converged against the Siberian platform. The fault, about 8000 km long, lies among more than a dozen large basins (and numerous smaller ones) many of which formed abruptly at ca. 169 Ma. The basins, commonly containing Middle and Late Jurassic and Cretaceous clastic and volcanic units, distinguish a locally broad belt along the western and southwestern margin of the North America plate. The basin margins commonly coincide with easterly striking normal and northwesterly striking sinistral faults although most have been reactivated during multiple episodes of movement. The pattern of intersecting faults and the rarely preserved record of displacements along them suggest that the basins are structural pull-aparts formed at releasing steps of a sinistral continental margin transform and are therefore transtensional. The width of the zone delineated by the basins is a few hundred km and extends west-northwesterly from the Gulf of Mexico across northern Mexico to southern California where it curves northward probably coincident with the San Andreas fault. Principal basins included within the southern part of the transtensional belt are recorded by strata of the Chihuahua trough, Valle San Marcos and La Mula uplift (Coahuila, Mexico), Batamote and San Antonio basins (Sonora, Mexico), Little Hatchet and East Potrillo Mountains and Chiricahua Mountains basins (New Mexico), Baboquivari Mountains Topawa Group (Arizona), regional Bisbee basin (Arizona, New Mexico, and Sonora, Mexico), Bedford Canyon, McCoy Mountains, Inyo Mountains volcanic complex and Mount Tallac basin (California). The latter probably extend into Nevada as part of the Pine Nut assemblage. At the southern margin of the Sierra Nevada of California, the inferred fault steps west then north, roughly along the Coast Range thrust and into the Klamath Mountains. The Great Valley (California) and Josephine ophiolites (Oregon) record these two major, releasing steps along the Mexico-Alaska megashear. From the northwestern Klamath Mountains, the Mexico-Alaska megashear turns east where Jurassic contractional structures exposed in the Blue Mountains indicate a restraining bend along which transpression is manifest as the Elko orogeny. Near the border with Idaho the fault returns to a northwest strike and crosses Washington, British Columbia, and southern Alaska. Along this segment the fault mainly coincides with the eastern limit of the Alexander-Wrangellia composite terrane. West of the fault trace in Washington, the Ingalls and Fidalgo ophiolites record separate or dismembered, co-genetic, oceanic basins. Correlative sedimentary units include Nooksack, Constitution, and Lummi Formations and the Newby Group, within the Methow basin. In British Columbia, the Relay Mountain Group of the Tyaughton basin, and Cayoosh, Brew, Nechako, Eskay, and Hotnarko strata record accumulation from Bajocian through Oxfordian within a northwestward-trending zone. From southern Alaska and northwestward correlative extension is recorded in basins by sections at Gravina, Dezadeash-Nutzotin, Wrangell Mountains, Matanuska Valley (southern Talkeetna Mountains), Tuxedni (Cook Inlet), and the southern Kahiltna domain. The pull-apart basins began to form abruptly after the Siskiyou orogeny that interrupted late Early to Middle Jurassic subduction-related magmatism. Convergence had begun at least by the Toarcian as an oceanic proto-Pacific plate subducted eastward beneath the margin of western North America. As subduction waned following collision, sinistral faulting was initiated abruptly and almost synchronously within the former magmatic belt as well as in adjacent oceanic and continental crust to the west and east, respectively. Where transtension resulted in deep rifts, oceanic crust formed and/or volcanic eruptions took place. Sediment was accumulating in the larger basins, in places above newly formed crust, as early as Callovian (ca. 165 Ma). The belt of pull-apart basins roughly parallels the somewhat older magmatic mid-Jurassic belt. However, in places the principal lateral faults obliquely transect the belt of arc rocks resulting in overlap (southern British Columbia; northwestern Mexico) or offset (northern Mexico) of the arc rocks of at least several hundreds of kilometers. The trace of the principal fault corresponds with fault segments, most of which have been extensively reactivated, including the following: Mojave-Sonora megashear, Melones-Bear Mountain, Wolf Creek, Bear Wallows–South Fork, Siskiyou and Soap Creek Ridge faults, Ross Lake fault zone, as well as Harrison Lake, Bridge River suture, Lillooet Lake, and Owl Creek faults. Northward within the Coast Range shear zone, pendants of continental margin assemblages are interpreted to mark the southwest wall of the inferred fault. Where the inferred trace approaches the coast, it corresponds with the megalineament along the southwest edge of the Coast Range batholithic complex. The Kitkatla and Sumdum thrust faults, which lie within the zone between the Wrangellia-Alexander-Peninsular Ranges composite terrane and Stikinia, probably formed initially as Late Jurassic strike-slip faults. The Denali fault and more northerly extensions including Talkeetna, and Chilchitna faults, which bound the northeastern margin of Wrangellia, coincide with the inferred trace of the older left-lateral fault that regionally separates the Intermontane terrane from the Wrangellia-Alexander-Peninsular Ranges composite terrane. During the Nevadan orogeny (ca. 153 ± 2 Ma), strong contraction, independent of the sinistral fault movement, overprinted the Mexico-Alaska megashear fault zone and induced subduction leading to a pulse of magmatism.