Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murciélagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murciélagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia , are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the “San Miguel Formation” at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran miogeocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the “Los Murciélagos Formation” in northwest Sonora represent the southwesternmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods ( Dzieduszyckia sonora ) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600–1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora.

U-Pb isotopic dating of detrital zircons from a conglomeratic barite sandstone in the Sonora allochthon and a calciclastic sandstone in the Mina México foredeep of the Minas de Barita area reveals two main age groups in the Upper Devonian part of the Los Pozos Formation, 1.73–1.65 Ga and 1.44–1.42 Ga; and three main age groups in the Lower Permian part of the Mina México Formation, 1.93–1.91 Ga, 1.45–1.42 Ga, and 1.1–1.0 Ga. Small numbers of zircons with ages of 2.72–2.65 Ga, 1.30–1.24 Ga, ca. 2.46 Ga, ca. 1.83 Ga, and ca. 0.53 Ga are also present in the Los Pozos sandstone. Detrital zircons ranging in age from 1.73 to 1.65 Ga are considered to have been derived from the Yavapai, Mojave, and Mazatzal Provinces and their transition zones of the southwestern United States and northwestern Mexico. The 1.45–1.30 Ga detrital zircons were probably derived from scattered granite bodies within the Mojave and Mazatzal basement rocks in the southwestern United States and northwestern Mexico, and possibly from the Southern and Eastern Granite-Rhyolite Provinces of the southern United States. The 1.24–1.0 Ga detrital zircons are believed to have been derived from the Grenville (Llano) Province to the east and northeast or from Grenville-age intrusions or anatectites to the north. Several detrital zircon ages ranging from 2.72 to 1.91 Ga were probably derived originally from the Archean Wyoming Province and Early Paleoproterozoic rocks of the Lake Superior region. These older detrital zircons most likely have been recycled one or more times into the Paleozoic sandstones of central Sonora. The 0.53 Ga zircon is believed to have been derived from a Lower Cambrian granitoid or metamorphic rock northeast of central Sonora, possibly in New Mexico and Colorado, or Oklahoma. Detrital zircon geochronology suggests that most of the detritus in both samples was derived from Laurentia to the north, whereas some detritus in the Permian synorogenic foredeep sequence was derived from the evolving accretionary wedge to the south. Compositional and sedimentological differences between the continental-rise Los Pozos conglomeratic barite sandstone and the foredeep Mina México calciclastic sandstone imply different depositional and tectonic settings.

The Mojave-Sonora megashear model, which implies left-lateral strike-slip motion of northern México in Jurassic time, remains one of the most influential ideas concerning the geology of México. A comprehensive review of the literature related to this topic does not yet allow resolution of the controversy over the validity of this hypothesis. A clear conclusion is that the original hypothesis was based on a relatively simplistic model of the geology of Sonora, as the basement of the Caborca terrane is not simply a fragment of the Mojave Precambrian basement province of eastern California. Attempts to use quantitative techniques in testing the model have yielded results contrary to the hypothesis, such as clockwise rotations indicated by paleomagnetic data, and the diversity and complexity of the basement of Caborca indicated by geochemical and geochronological data. Other quantitative methods such as zircon provenance studies in quartzites of the sedimentary cover yield inconclusive results. The main conclusion of the studies of detrital zircons is that Grenvillean zircons are relatively abundant, but that their presence cannot be attributed solely to sources in the Grenville province in a fixist model. Stratigraphic correlations of upper Paleozoic and Mesozoic rocks in Caborca with similar sequences in California and Nevada do not provide convincing arguments of large displacement, but should be evaluated in more detail. Elements that have the potential to test the hypothesis with greater certainty include detailed studies of basement rocks, a refined stratigraphy of the Jurassic volcanic and volcaniclastic arc rocks south of the inferred fault trace, and an increased understanding of depositional trends in the miogeoclinal sequence. Structural studies are sparse in this region. It is particularly important to gain a better understanding of the effects in time and space of Late Cretaceous–Tertiary contractional deformation. A tectonic evolution model that does not conflict with the existing data is the proposal that displacement of a para-autochthonous Caborca terrane may have occurred in the late Paleozoic. Nonetheless, available data and geologic relations in the Caborca region do not require Late Jurassic slip of several hundred kilometers. El modelo de la megacizalla Mojave-Sonora, el cual implica desplazamiento lateral izquierdo en el norte de México durante el Jurásico, permanece como una de las ideas más influyentes en la geología del país. Una revisión general de la literatura relacionada con el tema no permite aún resolver la controversia sobre la validez de la hipótesis, pero una conclusión clara es que la hipótesis original estaba basada en un modelo relativamente simplista de la geología de Sonora, ya que el basamento del terreno Caborca no es un simple fragmento de la corteza Mojave del este de California. Intentos de utilizar métodos cuantitativos han dado resultados contrarios a la hipótesis, como el de las rotaciones horarias indicadas por el paleomagnetismo y la diversidad de basamentos en Caborca que sugieren la geocronología y geoquímica; otros métodos producen resultados indeterminados, como la proveniencia de circones en las cuarcitas de la cobertura del terreno Caborca. La conclusión más relevante de esos estudios es la abundancia de circones de edad Grenvilleana, pero su presencia no puede simplemente atribuirse a fuentes en la Provincia Grenville en un modelo fijista. Las correlaciones estratigráficas entre secuencias Paleozoico tardío y Mesozoico en Caborca y secuencias similares en California y Nevada no producen argumentos convincentes a favor de grandes desplazamientos, pero deben considerarse con datos más detallados. Elementos que podrían evaluar la hipótesis con mayor contundencia son estudios más detallados del basamento, una estratigrafía fina del arco volcánico Jurásico y de las rocas volcanoclásticas al sur de la traza inferida de la falla y un mejor conocimiento de la secuencia miogeosinclinal. Son pocos los estudios estructurales en la región y en particular un problema importante es resolver en tiempo y espacio los efectos de la deformación compresional Cretácico-Terciario. Un modelo que no entra en conflicto con la evidencia existente es la propuesta de que el desplazamiento del terreno parautóctono Caborca haya ocurrido en el Paleozoico tardío. Sin embargo, los datos existentes y las relaciones geológicas en la región de Caborca, no requieren de un desplazamiento de cientos de kilómetros en el Jurásico Tardío.

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 Caribbean Plate margins are assemblages of terranes located, since the Mid-Cretaceous, along transform boundaries between the Caribbean, North and South America and the Pacific and Atlantic oceans. Litho-stratigraphic, petrological and metamorphic features of the main units and their regional correlations allow definition of the main geotectonic elements (continental margins, oceanic basins, subduction zones, magmatic arcs) involved in the evolution of Caribbean Plate margins. They provide valuable constraints on plate evolution since the Jurassic. This involved proto-Caribbean ocean opening, thickening into an oceanic plateau, beginning of convergence in the Early Cretaceous, atypical evolution of a supra-subduction system during the Mid-Cretaceous, subduction of rifted continental margins, Late Cretaceous convergence related to eastward migration of two opposite triple-junctions and strike–slip tectonics. Using these data, we compare different models and suggest improvements.

Abstract Oceanic plateaus are areas of elevated and anomalously thick oceanic crust that are believed to form by enhanced partial melting in a mantle plume that is hotter than ambient upper asthenosphere. They are regarded as the oceanic equivalent of continental flood-basalt provinces. Because of the continual subduction of oceanic crust, the oldest known oceanic plateaus occurring in situ are Cretaceous in age. In order for oceanic plateaus to be preserved in the geologic record, they must be accreted onto continental margins. This process, involving their preservation as tectonic slices, depends on the fact that oceanic plateaus are more buoyant than normal ocean floor; thus, they are not easily subducted. If these plateaus encounter an oceanic arc, subduction polarity reversal may occur, and/or the locus of subduction may step back behind the trailing edge of the advancing plateau. At a continental subduction zone, only subduction back-step occurs. Geochemical evidence shows that basaltic and picritic rocks exposed in the thickened part of the Caribbean plate and around its margins (including northern South America) are parts of an accreted oceanic plateau that originated in the Pacific Ocean during the middle-to-late Cretaceous. Cretaceous subduction-related rocks also occur around the Caribbean margins and possess geochemical signatures (e.g., lower Nb and Ti) that are distinct from those of the oceanic plateau rocks. This arc material represents the remnants of the subduction-generated rocks with which the plateau collided at 80–90 Ma. Both island arc tholeiite and calc-alkaline magmatism occurred in these Cretaceous arcs, but the changeover between the two types appears to be gradual and cannot be used to determine the timing of subduction polarity reversal. Many Cretaceous tonalitic batholiths around the Caribbean margins appear to have formed during or shortly after accretion of the plateau rocks. In addition to the arc and oceanic plateau assemblages, Jurassic to Early Cretaceous fragments of the preexisting oceanic crust also occur around the region. The environmental impact of oceanic plateau volcanism around the Cenomanian-Turonian boundary and its link to the formation of organic-rich black shales is discussed in this paper.