The most striking feature of the eastern Peninsular Ranges batholith is the large volume of relatively homogeneous tonalite and low-K granodiorite distributed in a series of large zoned Late Cretaceous intrusive centers referred to as La Posta–type plutons. The Sierra San Pedro Mártir pluton in northern Baja California is an outstanding example, and this study was undertaken to test models for the origin of these large composite arc plutons as well as to investigate along-strike variability within this Late Cretaceous belt. The Sierra San Pedro Mártir pluton consists of a nested series of granitoids divided into hornblende, biotite, and muscovite zones that become progressively more felsic and younger inward to a slightly more mafic and lower-K muscovite core zone. Zircon and monazite U/Pb ages from each of the zones indicate composite assembly of the pluton over an ~7 m.y. time span (97–90 Ma), consistent with field evidence and internal compositional variability of the pluton. The Sierra San Pedro Mártir pluton consists of high-Na, high-Al calcic granitoids that contrast with high-K calc-alkaline granitoid intrusive suites typical of Sierra Nevada Late Cretaceous intrusive centers. Whole-rock major-element, trace-element, and rare earth element (REE) data from an ~20-km-long traverse from the margin to the core of the Sierra San Pedro Mártir pluton document compositions that closely match chemical characteristics of Archean high-Al tonalite-trondhjemite-granodiorite (TTG). REEs are in general strongly fractionated, with high (La/Yb) N ratios typical of high-Al TTG. However, large variations in heavy (H) REE abundances and light (L) REE/HREE abundance ratios within the Sierra San Pedro Mártir pluton are comparable to the total range of REE variability within the Peninsular Ranges attributed by previous workers to regional west to east variations across the batholith. High Sr contents and lack of strong Eu anomalies indicate a general lack of plagioclase in the source residue. Compositions are consistent with deep crustal or slab melting from a basaltic source region with residual garnet and amphibole. Hornblende-plagioclase thermobarometry indicates emplacement depths around 17 km and crystallization temperatures ranging from 650 °C to 700 °C. Unlike La Posta–type bodies to the north, which are exclusively ilmenite-series granitoids, the Sierra San Pedro Mártir pluton is partly magnetite-series rocks in the outer hornblende zone. Ilmenite-series rocks preferentially sequester Fe in biotite via Tschermak exchange. Relatively low 87 Sr/ 86 Sr initial isotopic compositions of 0.7038–0.7050 and δ 18 O whole-rock values of 8.5‰ in the Sierra San Pedro Mártir pluton are more typical of the western zone of the Peninsular Ranges batholith. The along-strike variation of La Posta–type centers may be correlated to progressive distancing from Proterozoic North American cratonal basement and/or diminishing contributions of subducted sediment and associated basement into the Cretaceous melt source region of the magmas.

Prebatholithic rocks are exposed near the town of San Felipe in northeastern Baja, California. Two spatially separated stratigraphic sections contain mature quartzite, marble, metaargillite, and interbeds of pelitic schist and micaceous quartzite. Eight mappable rock units of formational rank collectively measure at least 3,800 m. The depositional setting was in a shallow-marine environment, marginal to the craton. Based on lithology and stratigraphy, the sequence is provisionally correlated with miogeoclinal rocks of Late Proterozoic and Early Cambrian age in northwestern Sonora, Mexico. Unit G is correlated with the Proveedora Quartzite and units D, E, and F are correlated with the Puerto Blanco Formation of the Caborca area.

At Rancho San Marcos, halfway between Tecate and Ensenada in northwestern Baja California, a 1 km by 5 km group of giant olistoliths of Early Ordovician age occurs within phyllite and metasandstone of Mesozoic(?) age. This group of giant olistoliths is underlain by a mélange of olistolith-derived granule to boulder-size fragments in a foliated, phyllitic matrix. Granitic rocks and andesite/dacite dikes of the Cretaceous Peninsular Ranges arc and batholith intrude both autochthonous and allochthonous rocks. The olistoliths of Ordovician rock are resistant, moderately to well-sorted, blue-gray quartzite; brown, gray, and black, commonly argillaceous, bedded chert; medium to dark gray, finely to coarsely recrystallized, carbonate rock; minor amounts of brown to gray-green metaargillite; and clast-supported cobble conglomerate. North Atlantic and Midcontinent province conodonts from the carbonate rock indicate a medial Arenigian (Early Ordovician) age. Both the Ordovician (allochthonous) and Mesozoic (autochthonous) rocks have undergone low greenschist grade regional metamorphism of Cretaceous age. The quartzite has been openly folded, but argillaceous units are pervasively foliated and isoclinally folded with strikes N30 to N70°W, and dips to the northeast. Field relationships suggest that the mélange is sedimentary, not tectonic, in origin. Emplacement occurred at a time of tectonic unrest during which debris was shed westward off an unstable continental margin into flysch basins. The allochthonous rocks of Rancho San Marcos appear similar in age and lithology to portions of the eugeoclinal Valmy Formation of north-central Nevada. If these strata are correlative, palinspastic reconstruction appears to require large-scale left-lateral displacement. Proposed sinistral displacement on the medial Jurassic Mojave-Sonora megashear, plus northward translation on the San Andreas fault system in the Neogene, would place Valmy-equivalent rocks at roughly the same latitude as San Marcos.

The bedrock strata of the northern Sierra Las Pintas are approximately 520 m thick and are here divided into six informal units of formational rank: unit SP1, calcareous sandy siltstone and calcareous siltstone; unit SP2, siltstone interbedded with calcareous siltstones, crinoidal grainstone, and granule conglomerates; unit SP3, graded crinoidal grainstone; unit SP4, massive and normally graded beds of coarse sandstone and granule to cobble conglomerate; unit SP5, basalt flows with pillow breccia and hyalotuffs; and unit SP6, bedded chert and calcareous argillite in thrust contact with underlying units. Lophyophyllid corals and brachiopods from unit SP3 suggest a Carboniferous age. The bedrock strata of the southern Sierra Las Pintas are approximately 720 m thick and divided into four informal units of formational rank: unit AG1, bedded chert and argillite; unit AG2, sandstone, siltstone, and debris flow; unit AG3, limestone and shale; and unit AG4, pillow basalt and basalt flows. Based on conodonts recovered from unit AG1 and conodont fragments recovered from unit AG3, the section is bracketed between Early Devonian and Early Mississippian. Thin-section analysis suggests that the terrigenous rocks from the two areas were derived from similar terranes with cratonal source areas. Minor- and trace-element analyses of basalt indicates eruption during a rifting event, perhaps near a continental margin. The basement exposures of the northern and southern Sierra Las Pintas are separated by a cover of Miocene volcanic strata and can not be correlated on a unit-to-unit basis, but could have been deposited in the same basin. Rocks of similar provenance, age, and lithologic associations are found elsewhere in Baja California Norte, Sonora, Sinaloa, and the Havallah and Schoonover sequences of Nevada.

The boundary between the western Jurassic-Cretaceous arc terrane and the eastern, pre-Cretaceous cratonally derived metasedimentary rocks extends along the axis of the Peninsular Ranges. In the southern Sierra Calamajue (lat. 29°25′N), where this boundary is exposed, detailed stratigraphic and structural analyses of these lower greenschist grade rocks indicate that the western and eastern terranes are structurally interleaved in a 5- to 7-km-wide zone. Five northwest-trending tectonostratigraphic units are mapped in this area. The structurally lowest (southwesternmost) unit consists of 1,300 m of chert and fine-grained clastic rock, with minor limestone of Late Mississippian age. These rocks are structurally overlain by 2,600 m of Upper Jurassic basaltic to andesitic volcanic and volcaniclastic rock and Lower Cretaceous (Alisitos Group) volcaniclastic and epiclastic rocks. Medium- to thick-bedded quartz sandstones in the upper part of the Jurassic sequence appear to be redeposited in the overlying Alisitos Group, suggesting a depositional contact between these rocks. Two of the three structurally overlying units consist of variably tectonized limestone and fine- to medium-grained clastic rocks of unknown (Cretaceous?) age. These units are in sharp fault contact with, and are separated by, Lower Cretaceous volcaniclastic and pyroclastic rocks. Three phases of deformation have been identified in these rocks. The oldest deformation, D 1 , produced steeply east dipping reverse faults (which separate the rock units), tight to isoclinal folds, and an associated, northwest-trending axial plane cleavage. Progressive flattening strain during D 1 rotated fold hinge lines and the X-axis of deformed lapilli into the direction of tectonic transport, which is defined by a steeply plunging extension lineation. Strain analyses indicate that shortening perpendicular to D 1 foliation probably exceeded 60 to 70% in some areas. The timing of this deformation is constrained by a 103 ± 4-Ma Rb/Sr whole-rock age for deformed metavolcanic rocks (Alisitos Group), and a U-Pb zircon age of circa 100 Ma for an undeformed granite that crosscuts the early fabric. The second recognizable deformation, D 2 , produced a conjugate set of kink bands that indicate shortening parallel to the earlier formed fabric. A map-scale sinistral flexure, which occurs at the join between the Sierra Calamajue and the Sierra la Asamblea, deforms structures produced during D 1 and D 2 .

Mapping along the eastern coast of Baja California adjacent to the Canal de Las Ballenas (lat. 29°30′ to 29°40′) reveals over 4,000 m of complexly folded and faulted, metamorphosed sedimentary and volcanic strata, locally intruded by tonalite and gabbro of Cretaceous age. Protoliths include lime mudstone, coarse-grained bioclastic packstone, bedded black chert and shale, thin-bedded flysch-type sandstone-shale, thick boulder and cobble conglomerates, minor quartz arenite, and pillowed alkaline basalt. The depositional environment was anoxic slope to basin. The entire sequence is here named the Canal de Las Ballenas Group. Conodont fragments and favositid corals indicate a Devonian age, suggesting correlation with rocks in the Sierra Las Pinta to the northwest, and to southern Sonora to the east. The recognition of these rocks adds an important link to our understanding of the southwestern edge of North America in mid-Paleozoic time. The area has been pervasively deformed, first by tight, isoclinal folding and shearing, which produced large recumbent folds with axes dipping gently to the northeast and verging east-southeast; and second by large synforms with steep, east-trending axes.

Prebatholithic metasedimentary rocks east of El Marmol on the 30th parallel of Baja California include tightly folded Permian and Lower Triassic rocks metamorphosed to upper greenschist/amphibolite facies. These rocks are divided into one informal and three formal formations. The rocks of El Marmol (informal formation) have a minimum thickness of 2,000 m and consists of thin-bedded argillite, sandstone, and chert with sparsely fossiliferous carbonate rock interstratified with more thickly bedded lenses of calcareous quartzarenite, impure carbonate rock, and chert-quartzite-carbonate clast conglomerate. The rocks appear to have been variously deposited by sediment gravity flows and intervening quiet-water deposition in a hemipelagic setting. The rocks of El Marmol contains no datable fossil material but are Lower Permian or older. The Arroyo Zamora Formation has an estimated thickness of 500 m, conformably overlies the rocks of El Marmol, and is composed primarily of massive or crudely laminated silty metaargillite that locally exhibits fine-grained turbidite successions and pebbly mudstone. Slumps, boudined sheets, and channelized sediment, gravity-flow deposits composed of coarser carbonate and quartzose clasts, including carbonate blocks up to several meters, are scattered throughout. Fossils in both the matrix and clasts of these coarse-grained deposits include Lower Permian (Leonardian) fusulinids, bryozoa, brachiopods, and crinoid columnals. The Cerro El Volcan Formation has an estimated thickness of 1,400 m and is divided into three members. The lowermost member (A) consists of 1,000 m of rhythmically bedded, dark siliceous argillite that contains intervals of metamorphosed massive calcareous sandstone, sandy limestone, and thinly bedded carbonate rock with siliceous partings. The overlying member (B) consists of 50 m of metamorphosed conglomerate, pebbly quartzite, and calcareous sandstone. Angular blocks throughout this member commonly contain bioclastic debris. The uppermost member (C) consists of 350 m of laminated argillite containing widely spaced interbeds of metamorphosed argillaceous carbonate rock. It is considered Late Permian in age because it lies conformably on rocks of Early Permian age, and is overlain with possible disconformity by the Lower Triassic De Indio Formation. The De Indio Formation has an estimated thickness of 300 m and is divided into two members. The lower member (A) contains up to 50 m of metamorphosed chert-pebble conglomerate, quartzite, cross-bedded calcareous sandstone, and limestone. The upper member (B) consists of 250 m of carbonaceous, staurolite-bearing argillite. Ammonoids and conodonts from member A are of Early Triassic (Smithian) age. These four formations have lithologies and faunal assemblages similar to coeval rocks deposited along the outer edge of the Cordilleran miogeocline of western North America, and were probably deposited in a southern extension of this miogeocline.

Disarticulated Permian crinoid columnals and pluricolumnals, up to 9 cm in diameter, from exotic carbonate blocks in the base of the Zamora Formation, found along Arroyo Zamora near El Volcan, Baja California, Norte, are among the largest ever reported. The relatively inflexible columns that these columnals and pluricolumnals formed are interpreted to have lived in relatively strong currents along the edge of the shelf or upper slope. They are designated Apletoanteris bajaensis n. gen. and n. sp.

Prebatholithic clastic and minor carbonate and volcanic rocks are exposed along the 30th parallel of eastern Baja California Norte, Mexico. This formation has an apparent thickness of over 6,000 m and is subdivided into three informal members. The lower member (200 m) is comprised of quartz-rich, boulder-pebble conglomerate and sandstone, volcaniclastic sandstone, and minor limestone interpreted to have been deposited in a shallow-marine setting. The middle member (5,000 m) gradationally overlies the lower member, and consists of rhythmically thin-bedded, and laminated, black chert, fine-grained sandstone, and shale interpreted to represent a slope to basin-plain environment. The upper member (850 m) rests unconformably on the middle member and is composed of cobble conglomerate, sandstone, shale, and vesicular andesite. The sequence contains angular granules of black chert probably reworked from the middle member. The upper member is interpreted to have been deposited in a nonmarine environment. The age of the section is interpreted to be Aptian-Albian (possibly as young as Turonian) based on stratigraphic and intrusional relationships and nerineid gastropods from the lower member. The sequence is interpreted to have been deposited in a continental margin back-arc basin, contemporaneous with the Aptian-Albian Alisitos volcanic arc complex exposed 30 km to the west. The section was subsequently deformed into near-vertical, steeply plunging isoclinal folds, and metamorphosed to greenschist-facies grade.

The formation name Julian Schist is applied to prebatholithic metasedimentary wall rocks of the Peninsular Ranges batholith of central and eastern San Diego County. This study focuses on the type area of the Julian Schist located approximately 64 km east-northeast of San Diego, near Julian, California. Field mapping and petrographic analysis distinguish several types of andalusite-and sillimanite-bearing pelitic schist and gneiss and a sequence of calc-silicate–bearing feldspathic metasandstones. These rocks have a strong foliation that generally strikes northwest. The axes of tightly appressed isoclinal folds, boudins, and elongate minerals plunge steeply. Relict bedding and other sedimentary structures, such as graded laminations, channel scours, and rip-up clasts, preserved in a section of interbedded quartz-mica schist and feldspathic metasandstone, indicate that the protolith was a sequence of turbidites. Several turbidite facies are recognized and suggest deposition in the middle- to outer-fan lobes and fringe of a submarine fan. In the early Mesozoic these rocks were part of a large clastic wedge shed from the craton in the east into a marginal basin. The wedge contains shallow deltaic, turbidite-fan, and basin-plain deposits. These rocks were deformed and metamorphosed during mid- to late Mesozoic subduction, island-arc collision, and emplacement of the Peninsular Ranges batholith.

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.

Mesozoic sandstones from six rock units that underlie the Jurassic-Cretaceous arc of peninsular California appear to contain a mixture of late Paleozoic-early Mesozoic zircons and a mixture of Precambrian zircons averaging about 1,540 Ma. These data suggest that the detrital zircons were derived from the southwestern United States and northern Mexico during late Triassic-early Jurassic time. Two of six Paleozoic sandstones contain detrital zircons whose average U-Pb age is 1,500 Ma. This age is similar to the average age of Precambrian basement in adjacent Arizona and Sonora. However, three of the six Paleozoic sandstones contain zircons whose average U-Pb age is two billion years or greater. This age exceeds that of possible source rocks located in the southwestern Cordillera, but is similar to U-Pb ages for detrital zircon in the Shoo Fly Complex of the northern Sierra Nevada (Girty and Wardlaw, 1985; Miller and Saleeby, 1991).

The prebatholithic history of Baja California extends from latest Precambrian through medial Cretaceous time. Late Proterozoic and Paleozoic shallow shelf deposits of the Cordilleran geocline extend into the northeastern part of the northern peninsula. Continental slope and basinal deposits of Devonian to Permian age were laid down along the eastern side of the northern peninsula. Lower Triassic shelfal deposits were deposited on top of Permian slope-basin deposits. From Late Jurassic through medial Cretaceous time volcanic-arc rocks and arc-derived detritus interfingered with cratonally derived sediment on both sides of what is now the Gulf of California. East-vergent folds in middle Paleozoic strata along the Gulf side of the peninsula suggest that slope/basin rocks were thrust eastward over the miogeoclinal rocks perhaps in late Paleozoic or early Mesozoic time. In Early Triassic time the eastern peninsula became shelfal, then emergent, and then host to a continent-fringing arc. During the Jurassic and Cretaceous periods the arc extended, producing intra-arc basins of deep-water sedimentation. East over west contraction occurred about 100 Ma, closing and elevating the intra-arc basins.