The Aconchi-El Jaralito area is 100 km northeast of Hermosillo, and about 200 km southwest of Douglas, Arizona. The intrusive rocks of the region are divided into three units on the basis of their field relations, morphology, mineralogy, chemical composition, and isotopic ages. These units are two granitic batholiths and a group of porphyritic rhyolite stocks. The oldest batholith is the granitic-granodioritic El Jaralito batholith, which has the largest exposure area. Lithologically it varies from a true granite to quartz monzonite, quartz diorite, and granodiorite. The rocks of this batholith are considered I-type or Cordilleran granites. The K/Ar ages reported for EI Jaralito batholith range from 51.8 to 69.6 Ma. The second batholith is the Aconchi batholith, which has a smaller exposure area. It is lithologically simple, consisting almost exclusively of alkali granites, including abundant pegmatites. This is considered an S-type granitoid, with primary muscovite and red garnet. One K/Ar date for this unit is 35.96 ± 0.70 Ma. Chemical analyses show no clear separation between the two batholiths. There is an overlap in the values of some of the oxides. The youngest intrusive rocks in the area are two porphyritic rhyolite stocks, which have quartz, feldspar, and biotite phenocrysts set in a quartz-feldspar matrix. The chemical composition of the stocks is similar to the alkali granites of the Aconchi batholith. Cataclastic deformation was observed within both batholiths in isolated localities; however, the structure of these deformed rocks was not studied in detail. Chemical and isotopic data for these batholiths remain incomplete; it is highly recommended that future chemical data include trace-element concentrations.

ABSTRACT We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.

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.