Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ≥1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleoproterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, “A-type” granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States

Abstract La Herradura is a structurally controlled gold deposit located in northwest Sonora, México. Minera Penmont, S. A. de C. V., a joint venture between Servicios Industriales Peñoles, S. A. de C. V., and Newmont Gold Company manage the project. In 1989 regional exploration led the Penmont Venture to claim the area ofthe deposit. Geologically the deposit is situated on the southwest edge of the Sonora-Mojave Megashear, a left-lateral 300 ×3 40-km northwest-trending regional structure that hosts several gold occurrences. At La Herradura, Tertiary age reactivation of the Jurassic age Sonora-Mojave megashear faults juxtaposed gneissic rocks against greenschist-grade meta volcanics. The geology ofthe oxidized deposit is dominated by strongly sheared biotite and quartzofeldspathic gneisses that are present in northwest trending lithotectonic slices (20-50 m wide) bounded by high angle faults. Mineralization is hosted mostly in the quartzofeldspathic gneiss and consists of high-angle, gold-bearing, mesothermal style, quartz-iron oxide veins and stockworks that parallel the northwest trend. Each mineralized block generally consists of a principal quartz-iron oxide vein with a sericite-iron oxide-iron carbonate halo of alteration with quartz veinlets, silicified tectonic breccia and lower grade gold values. Exploration and development drilling using both core and reverse circulation, has delineated a gold deposit that will produce an average of 150,000 ounces of gold per year from an open-pit heap-leach operation for more than eight years. La Herradura is scheduled to start production in 1998 and will become the largest gold producer ofthe Sonora Desert region. This paper summarizes the discovery, geology and development of the deposit.

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.

The megashear hypothesis is based upon reconnaissance geologic and geochronologic studies conducted principally from 1968 until 1974 in northwestern Sonora, Mexico. Our research incorporated U-Pb isotopic analyses of more than 70 zircon populations separated from 33 Precambrian rock samples with field relations and maps based upon structural and stratigraphic measurements. The results delineate a region known as the Caborca block and further reveal that the block is a principal element of an unexpected, discordant pattern of Proterozoic basement provinces. The Mojave-Sonora megashear was conceived in an effort to explain: (1) the unexpected pattern of two Proterozoic crystalline provinces with distinct chronologic histories of crust formation (1.8–1.7 Ga, Caborca block versus 1.7–1.6 Ga, Pinal Province); (2) the distribution of contrasting cover rocks overlying these basement blocks, (3) the abrupt northeastern limit of the Caborca block (terrane) against which volcanic and plutonic rocks of mid-Jurassic (mainly 180–160 Ma) age are juxtaposed, and (4) the distribution of Jurassic magmatic units that intervene between the provinces of Proterozoic crust. The similarities that exist between crystalline crust and overlying pre-Jurassic cover in northwestern Sonora, Mexico, and units in the Inyo Mountains–Death Valley region are attributed to the offset of correlative units along a Late Jurassic left-lateral strike-slip fault postulated to extend from the Gulf of Mexico to California and beyond. This large fault or megashear is a principal structure that accommodated 800–1000 km of left-lateral displacement among a set of transforms related to the opening of the Gulf of Mexico. The fault is compatible with Late Jurassic plate motion. The inferred trace of the Mojave-Sonora megashear is obscured by contractional and extensional deformation and extensive plutonism. These processes, concentrated along the fault, commonly obfuscate and displace fault zone rocks along the inferred trace as well as the rocks adjacent to it. However, the fault zone is exposed in Sierra de Los Tanques near the international boundary between Mexico and the United States, where mylonitic rocks that comprise three aligned, discontinuous, segments crop out 1 for ∼25 km. The zone of mylonitic rocks, which crosses Route 8, 13 km SW of Sonoita, is locally almost 5 km wide and separates Triassic granitoids and Precambrian gneiss from Jurassic volcanic and clastic rocks. The limited exposure of the fault zone is a principal concern of those who object to the Mojave-Sonora megashear hypothesis. Studies of paleomagnetism, structure, stratigraphy, crustal geochemistry, and detrital zircons do not refute the megashear concept; commonly they reinforce existing evidence in support of the hypothesis.

Abstract The Mojave Sonora megashear hypothesis proposes that the Caborca terrane, northwest Sonora, arrived at its present position with respect to the North America craton via left-lateral (southeastward) displacement along a strike slip fault system. Nonetheless, clear stratigraphic and paleomagnetic links exist between rocksof the Sonoran segment of the Jurassic Cordilleran arc (JCA), and lower Mesozoic strata of the Caborca and Antimonio terranes supporting an alternative Jurassic paleogeography for northwest Mexico. The characteristic “J” magnetizations in Jurassic rocks of the JCA givea (tilt corrected) mean of D=15.0°, 1=4.0° (a95=14.3°;k=12.4; N=10 sites). Magnetizations pass fold, conglomerate, and reversal tests and are interpreted to be primary in origin. The age of these rocks is roughly bracketed between about 190 and 160 Ma. Secondary “J*” magnetizations in Neoproterozoic and Jurassic rocks southof the f - megashear give an overall (in situ) mean of D=15.0°, 1=10.0° (a95=5.8°; k=23.0; N=28 sites). “J*” magnetizations fail a fold test but timing of acquisition is bracketed between about 120 and 190 Ma, based on the youngest age of remagnetized Jurassic strata andthe fact that acquisition must predate the Cretaceousnormal polarity superchron. The overall means of rocks south and north of the MSM are statistically indistin uishable, arguing against the existencer of a crustal discontinuity along the proposed locat on of the MSM. For the interval that brackets acquisition of secondar “J*” and primary “J*” magnetizations, rotation of both the JCA and the Caborca terrane with respect to North America is 12° to 50° clockwise, depending on the age assumed. Estimates oflatitudinal displacement vary from as little as l-250 km southward, for a Sinemurian age of the magnetization (195 Ma), toas much as -800 km northward for a 1' Callovian-Oxfordian age (155 Ma). If the Sonoran results are compared with high-latitude Middle Jurassic poles for North America, larger estimates of northward displacement(>1400 km) result. Although the timing of magnetization acquisition is based on reasonable geological arguments, an Early Cretaceous age for “J*” magnetizations is permissible. Such an interpretation would indicate significantly larger northward displacement (>2000 km) with respect to cratonic [North America. Together, the lack of clear evidence for large southward displacement, the observed clock-wise rotation, and the similarity of the Jurassic magnetizations in the Cordilleran arc with those ofthe Caborca block are not consistent with the Mojave-Sonora megashear model of significant Late Jurassic southeast motion of northern Mexico along a left-lateral strike-slip fault system.

Shallow-marine Triassic red sedimentary rocks and diabase intrusives were investigated on the Caborca Block in Sonora, Mexico. The lower 212 m half of the sequence was sampled as a magnetostratigraphic section. Samples exhibit exceedingly linearly decaying remanent magnetization and pass a fold test. Unblocking temperatures suggest that the remanence is carried by magnetite. The beds are inferred to be Early Triassic in age because they overlie Permian strata and are overlain by late Early Triassic (Spathian) Tirolites -bearing strata. The red bed samples exhibit an apparently reversed polarity (northern-hemisphere) remanence. Comparison of this polarity to a global compilation of Early Triassic magnetostratigraphy, combined with the age of the superposed beds and the sequence stratigraphic framework, suggests that the age of these beds and their magnetization may be middle Early Triassic (Dienerian). The remanence suggests a paleolatitude of magnetization of 21° N (±4°), so that in the Early Triassic, the Caborca Block may have lain off of western North America near the present location of Seattle, Washington. The overlying red sedimentary rocks containing Spathian ammonites have been remagnetized in a recent geomagnetic field direction. The entire sedimentary section has been intruded by diabase sills; yet oddly, diabase samples gave only widely scattered directions. The sampling site and Caborca Block are bordered by the left-lateral Mojave-Sonora megashear, but the paleopole is rotated clockwise relative to the North America Early Triassic reference pole, compatible with transport of the terrane in conjunction with right-lateral strike-slip faulting. Many terranes along the western North American margin have been shown to exhibit a history of Jurassic left-lateral transport followed by Cretaceous-Tertiary right-lateral movement (Beck, 1991). The current location of Caborca relative to its inferred Early Triassic paleolocation and the clockwise displacement of the Early Triassic paleopole may stem from a Jurassic left-lateral transport as postulated for the Mojave-Sonora megashear, followed by post-Early Cretaceous right-lateral motion, as observed in numerous other western North American terranes. The important point is that because of the multiplicity of terrane histories, e.g., northward then southward motion relative to cratonic North America, the inference of right-lateral transport for the Caborca Block does not, and cannot, disprove the existence of the left-lateral Mojave-Sonora megashear.

The Mojave-Sonora megashear constitutes a regional boundary between lithologically distinct Jurassic assemblages of different ages. North of the Mojave-Sonora megashear, arc-related volcanic, volcaniclastic, and clastic rocks, intruded by plutons (175–160 Ma) compose part of the Middle Jurassic (commonly ca. 175 Ma) igneous province, previously recognized in Arizona and California. Distinct domains among Jurassic igneous rocks in northern Sonora are: (1) southern Papago, a region where pre-Jurassic rocks are unknown, (2) Nogales-Cananea-Nacozari, where Jurassic rocks are underlain by 1.7–1.4 Ga crystalline basement, and (3) Mojave-Sonora, where strata, including Oxfordian beds, along the north side of the Mojave-Sonora megashear are commonly strongly deformed, as recorded by thrust faults, mylonitic foliation, and recumbent folds. The Mojave-Sonora domain extends across the southwestern margins of the southern Papago and the Nogales-Cananea-Nacozari domains. Strong deformation that distinguishes the zone markedly declines within a few tens of kilometers northward. South of the Mojave-Sonora megashear, in central and southern Sonora, Lower Jurassic clastic and volcaniclastic rocks distinguish the Caborca domain. Upper Jurassic sedimentary rocks, commonly conglomeratic, are abundant north of Mojave-Sonora megashear; a single occurrence is known south of the Mojave-Sonora megashear. Waning of subduction-related Middle Jurassic magmatism was followed by the abrupt formation, ca. 165 Ma, of Coast Range, Josephine, Great Valley, and Devil's Elbow ophiolites and the Smartville Complex within oceanic pull-aparts west of the margin of the North America plate. The formation of ophiolitic rocks signaled the beginning of transtensional faulting. Almost contemporaneously (ca. 163 Ma) the lowest volcanic units and overlying coarse sedimentary beds began to accumulate in fault-bounded continental pull-apart basins such as the McCoy Mountains basin. Other transtensional basins, formed at releasing steps where pull-aparts formed, are well developed within the Papago domain and other parts of southwestern United States and northern Mexico. From Sonora northward into California the Mojave-Sonora megashear fault zone, developed generally within the Middle Jurassic arc-parallel to the former continental margin, is inferred to link with strands of the Melones and Bear Mountain faults of the Foothills fault system, the Wolf Creek fault, and the Big Bend fault. A protuberance of Proterozoic basement (the Caborca block) that was truncated from the continental margin records ∼800–1000 km of left-lateral offset. The displacement of the Caborca block took place south of a major releasing step along the Big Bend fault with the result that a regional pull-apart that coincides with the Great Valley of California developed. Inboard of the Mojave-Sonora megashear Late Jurassic magmatic rocks crop out near faults at some releasing steps and within floors of some pull-apart structures. The distribution suggests that magma rose along faults and into areas of thin crust. In southern Arizona these igneous rocks are included as part of the Artesa layered sequence and the Ko Vaya plutonic suite. Oxfordian and younger beds, which crop out north of the Mojave-Sonora megashear may contain exotic blocks and contractional structures that are contemporaneous with the Nevadan orogeny. The variation in the style and intensity of deformation of Middle and Upper Jurassic strata, and Upper Jurassic conglomerate rich in clasts derived from rocks of the Caborca domain, are postulated to record transpression near the Mojave-Sonora megashear that locally overlapped the more widespread transtensional structures in time and space. The cessation of strike-slip faulting locally began ca. 150 Ma, as shown by undeformed intrusive bodies that cut older deformed Middle Jurassic rocks. By the time that the Independence dikes and correlative rocks were emplaced at 148 Ma, scant evidence of lateral faulting is known. Intrusions, young volcanic cover, transecting strike-slip faults, and multiple generations of low-angle extensional and contractional faults obscure Jurassic structures in Sonora and southern California. Despite these complications, removal of the effects of superposed structures reveals a viable trace for an inferred Late Jurassic left-lateral fault linking the Mojave-Sonora megashear and more northerly fault segments. The position of this major inferred fault is constrained by distinctive tectonostratigraphic domains. The Middle and Late Jurassic and earliest Cretaceous plate tectonic history includes (1) subduction (175–165 Ma), (2) coupling (ca. 165 Ma), (3) rifting, transtension, lateral faulting, transpression, and contraction (165–145 Ma), and (4) renewed subduction (ca. 135 Ma) along the western margin of the North America plate and terranes (e.g., Wrangellia) to the west. The structures that record the diverse plate processes and that are preserved best in the overriding North America plate are compatible with a consistently maintained easterly directed maximum compressive stress.