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.

The Early to early Late Jurassic magmatic arc of the lower Colorado River region of southern California and southwest Arizona spanned ∼30 m.y., from ca. 190 to 158 Ma. The arc-type volcanic and plutonic rocks interacted extensively with the Proterozoic Mojave Province crust and show evidence for geographic-based age and compositional changes. The region lies adjacent to an amagmatic gap in the Jurassic arc of the southwest United States, near the western terminus of proposed Late Jurassic basins formed in conjunction with the opening of the Gulf of Mexico, and near, but to the north of, the projection of the trace of the sinistral Late Jurassic Mojave-Sonora megashear where it crossed from northern Mexico into the United States. Quartz-phyric dacitic to rhyolitic pyroclastic and locally hypabyssal rocks of the Dome Rock sequence were emplaced in two broad time periods, one between 190 and 185 Ma and the second between 173 and 158 Ma. Three compositionally expanded pluton units constituting the Kitt Peak–Trigo Peaks superunit were emplaced in the mid- to upper crust between 173 and 158 Ma, broadly contemporaneous with the younger phase of explosive volcanism. The compositionally expanded plutonic rocks consist of three informally named temporally and compositionally distinct magmatic units, from oldest to youngest, the Araz Wash diorite, the Middle Camp porphyritic granodiorite, and the Gold Rock Ranch granite. Each unit was emplaced over 4–6 m.y. periods of time. Dioritic rocks dominate the older Araz Wash diorite unit (173–169 Ma), granodiorite dominates the Middle Camp porphyritic granodiorite unit (167–163 Ma), and granite dominates the Gold Rock Ranch granite unit (163–158 Ma). Shortening and regional metamorphism throughout the lower Colorado River region accompanied emplacement of the Gold Rock Ranch granite unit. A Late Jurassic(?) mafic-felsic dike swarm forms the youngest magmatic unit in the region. The Jurassic magmatic history in the lower Colorado River region ended in the early Late Jurassic at ca. 158 Ma. Termination of magmatism in the Late Jurassic in the lower Colorado River region is distinct from adjacent parts of the arc to northwest in the Mojave Desert region or to the southeast in southern Arizona, where Late Jurassic magmatism continued to at least 146 Ma. At this time in the Late Jurassic, the Mojave-Sonora megashear had cut through the arc to the south of the lower Colorado River region, where degradation of the arc is recorded in sedimentary rocks now composing the lower parts of the McCoy Mountains Formation, the Winter-haven Formation, and informally named rocks of Slumgullion.

Early Mesozoic arc magmatism of the southern Sierra Nevada region records the onset of plate convergence–driven magmatism resulting from subduction initiation near the end of Permian time along a prior transform margin. We provisionally adopt the term California-Coahuila transform for this complex boundary transform system, which bounded the southwest margin of the Cordilleran passive margin, its offshore marginal basin, and fringing island arc. In Pennsylvanian–Early Permian time, this transform cut into the arc-marginal basin and adjacent shelf system, calved off a series of strike-slip ribbons, and transported them differentially southward through ∼500–1000-km-scale sinistral displacements. These strike-slip ribbons constitute the principal Neoproterozoic–Paleozoic metamorphic framework terranes for the superposed Mesozoic batholithic belt in the Sierra Nevada and Mojave plateau regions. The southern Sierra Nevada batholith intruded along the transform truncation zone where marginal basin ribbons were juxtaposed against the truncated shelf. Strike-slip ribbons, or blocks, liberated from the truncated shelf occur today as the Caborca block in northwest Mexico, and possibly parts of the Chortis block, farther south. The oldest arc plutons in the Sierra region were emplaced between 256 and 248 Ma, which matches well with ca. 255 Ma high-pressure metamorphism recorded in the western Sierra Foothills ophiolite belt, interpreted to approximate the time of subduction initiation. The initial phases of arc plutonism were accompanied by regional transpressive fold-and-thrust deformation, kinematically marking the transition from transform to oblique convergent plate motion. Early arc volcanism is sparsely recorded owing to fold-and-thrust–driven exhumation having accompanied the early phases of arc activity. By Late Triassic time, the volcanic record became quite prolific, owing to regional subsidence of the arc into marine conditions, and the ponding of volcanics in a regional arc graben system. The arc graben system is but one mark of regional suprasubduction-zone extension that affected the early SW Cordilleran convergent margin from Late Triassic to early Middle Jurassic time. We interpret this extension to have been a dynamic consequence of the subduction of exceptionally aged Panthalassa abyssal lithosphere, which is well represented in the Foothills ophiolite belt and other ophiolitic remnants of the SW Cordillera. Middle and Late Jurassic time was characterized by important tangential displacements along the SW Cordil-leran convergent margin. In Middle Jurassic time, dextral impingement of the Insular superterrane intra-oceanic arc drove a migrating welt of transpressional deformation through the SW Cordillera while the superterrane was en route to its Pacific Northwest accretionary site. Dextral transtensional spreading in the wake of the obliquely colliding and translating arc opened the Coast Range and Josephine ophiolite basins. In Late Jurassic time, a northwestward acceleration in the absolute motion of the North American plate resulted in an ∼15 m.y. period of profound sinistral shear along the Cordilleran convergent margin. This shear is recorded in the southern Cordillera by the Mojave-Sonora megashear system. Late Jurassic intrusive units of the southern Sierra region record sinistral shear during their magmatic emplacement, but we have not observed evidence for major Late Jurassic sinistral displacements having run through the Sierran framework. Possible displacements related to the megashear in the California to Washington regions are likely to have: (1) followed preexisting transforms in the Coast Range ophiolite basin and (2) been accommodated by oblique closure of the Josephine ophiolite basin, and the northern reaches of the Coast Range ophiolite basin, proximal to the southern Insular superterrane, which in Late Jurassic–earliest Cretaceous time was obliquely accreting to the inner Cordillera terranes of the Pacific Northwest.

Abstract Basement outcrops are rare in northeastern Mexico, but the effect of older basement structures on the current structural plan is clear. Two large structures, the San Marcos fault and the northern edge of the Sierra Madre Oriental, are oriented roughly east-west, and have been postulated as sites of the Mojave-Sonora megashear. Both structures were probably active in the late Jurassic. The San Marcos fault continued to be active into the Cretaceous, while the northern edge of the Sierra Madre Oriental became inactive. Both structures were strongly reactivated during the Laramide orogeny. Potential fields data integrated with regional geologic mapping suggests a series of horsts and grabens were created during Middle-Late Jurassic rifting of the basement, but that relative displacements are far less than those proposed for the Mojave-Sonora megashear system. The northern edge of the Sierra Madre Oriental, with its large left-step along the Monterrey salient, is kinematically more compatible with the postulated left-lateral offset along the Mojave-Sonora megashear, while the San Marcos fault has numerous right-stepping offsets, which is kinematically incom-patible with the Mojave-Sonora megashear hypothesis. In general, however, these data do not support the concept of a large-offset left-lateral megashear in northeastern Mexico.

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.

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.