ABSTRACT This field trip examines Paleoproterozoic basement, Neoproterozoic metasedimentary strata, and crosscutting Mesozoic intrusive rocks at Frazier Mountain, Placerita Canyon, and Limerock Canyon in the western San Gabriel Mountains block, California. We present new U-Pb zircon geochronology results that constrain the Proterozoic through Cretaceous tectonic and magmatic history. The excursion ends in San Antonio Canyon in the eastern San Gabriel Mountains where several large rock avalanche deposits are sourced from distinct basement rocks. 10 Be surface exposure ages and post-infrared infrared stimulated luminescence burial ages demonstrate late Pleistocene to Holocene movements for these landslides.

International field experiences offer exceptional opportunities for effective student learning in the geosciences. Over the 10 yr period between 1998 and 2008, more than 40 undergraduate students from 14 institutions participated in field research investigating active tectonics on the Nicoya Peninsula, Costa Rica. Three different project models were used: (1) a month-long summer research project, (2) a series of 1 to 2 wk independent field study projects, and (3) a week-long field research module. These projects shared a common research theme (active tectonics), field area (Nicoya Peninsula), and pedagogy (experiential learning), thus allowing for easy comparison of teaching methods, logistics, and learning outcomes. Each model has unique pedagogical benefits and challenges, and is therefore better suited for a different group size, student to faculty ratio, project duration, and budget. Collectively, these student research projects generated significant publishable data relevant to ongoing investigations of forearc tectonics and earthquake hazards along the Costa Rican Pacific margin. Individual student projects were carefully designed to provide a quality field learning experience, while adding a new piece to the larger research puzzle. Indicators of project success include levels of student engagement; gains in technical and cognitive field skills; and productivity of student-authored publications, reports, and presentations. Students commonly described these projects as instrumental in shaping their professional identity as geoscientists. Blending international field research with experiential learning pedagogy creates a powerful synergy that captures student imagination and motivates learning. By placing students beyond the comfort of their home learning environment, international field projects pique student curiosity, sharpen awareness and comprehension, and amplify the desire to learn. Experiential learning pedagogy encourages students to define their own research agenda and solve problems through critical thinking, inquiry, and reflection. The potent combination of international fieldwork and experiential learning helps students to develop the self-confidence and reasoning skills needed to solve multifaceted real-world problems, and provides exceptional training for graduate school and professional careers in the geosciences.

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.

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.

A 200–500-km-wide belt along the southwestern margin of cratonic North America is pervaded by northwest- and east-trending faults that flank basins containing thick deposits of locally derived conglomerate and sedimentary breccia. These deposits that crop out mainly in the northern part of mainland Mexico, or southern parts of Arizona and New Mexico are unconformable at their bases, have similar Upper Jurassic and/or Lower Cretaceous stratigraphic ages, and commonly preserve volcanic components in the lower parts of upward-fining sections. We argue that these basins share a common structural origin, based on: (1) the presence of faults, locally preserved, that generally define the basin margins, (2) similar basal units comprised of coarse conglomeratic strata derived from adjacent basement, and (3) locally preserved syntectonic relationships to bounding faults. Fault orientations, and our observation that the faults (and their associated basins) extend south to the inferred trace of the Late Jurassic Mojave-Sonora megashear, suggest that the basins formed in response to transtension associated with sinistral movement along the megashear. Northwest-striking left-lateral strike-slip faults that terminate at east-striking normal faults define releasing left steps at which crustal pull-apart structures formed. These faults, plus a less-developed set of northeast-striking right-lateral faults, appear to comprise a cogenetic system that is kinematically linked with the Mojave-Sonora megashear; that is, the maximum principal stress trends east and the plane containing maximum sinistral shear stress strikes northwesterly. Late Jurassic faults northeast of the Mojave-Sonora megashear controlled the regional distribution of the pull-apart basins and influenced the orientation and style of many younger structures and intrusions. Most Late Jurassic faults were modified during subsequent episodes of deformation. N60°E-directed contraction during the Late Cretaceous (Laramide) orogeny reactivated older east-striking normal faults as sinistral strike-slip faults; northwest-striking sinistral faults were reactivated as steep reverse faults. Some stratigraphically low units were thrust across basin margins as a result of inversion. Many of the pull-apart basins encompass outcrops of Late Jurassic igneous rocks and/or mineralized Laramide or Tertiary plutons. Some northwesterly faults appear to have influenced the position of breakaway zones for early Miocene detachment faults. Despite the common and locally strong structural and magmatic overprinting, remnants of the Late Jurassic faults are recognizable.

We utilize new geological mapping, conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fine-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of finely banded felsic, intermediate, and mafic orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), medium-grained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafic dikes. Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated medium- and coarse-grained biotite granite augen gneisses that yield mean 207 Pb/ 206 Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with southwestern Arizona. Later influence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial ε Nd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The Eastern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and ε Nd but display structures discordant to the southwesterly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may reflect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic Mojave-Sonora megashear or the Permian Coahuila transform). Crustal blocks with these specific petrologic, geochronologic, and isotopic characteristics can be found in south-central and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent.

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

In an effort to characterize the crustal structure of northwestern Mexico (and constrain the Mojave-Sonora megashear) we studied the Magsat magnetic anomalies from that area. Published anomaly maps covering this area include an extensive positive anomaly covering the southern United States, a positive anomaly over the southern half of the Baja California peninsula, and a magnetic low in between. We interpreted a magnetic profile over these anomalies, focusing on its tectonostratigraphic terrane nature. The profile was further constrained by crustal thicknesses from seismological studies and heat flow data. In our model the Cochimi terrane and the North American craton (Colorado Plateau and southern Basin and Range) are characterized by high magnetic susceptibility in agreement with the mafic nature of their corresponding crusts. The Yuma and the Seri terranes have lower magnetic susceptibilities as expected from their felsic to basic crustal nature. The oceanic crust from the Gulf of California is modeled with a low magnetic susceptibility value due to the high heat flow observed at the extensional basins. Our model satisfies the presence of a subvertical contact between crusts with contrasting magnetic signatures of the Seri terrane (comprising the Caborca subterrane) and the southwestern sector of the North American craton. The Mojave-Sonora megashear itself is below the resolution of the Magsat data. Nevertheless, our model implies that because of their different magnetic signatures, the crystalline basements from the southern United States and northern Mexico (Seri terrane) are of different nature, which does not support the continuity of the North American craton into northwestern Mexico.

Major successions as well as individual units of Neoproterozoic to Lower Jurassic strata and facies appear to be systematically offset left laterally from eastern California and western Nevada in the western United States to Sonora, Mexico. This pattern is most evident in units such as the “Johnnie oolite,” a 1- to 2-m-thick oolite of the Neoproterozoic Rainstorm Member of the Johnnie Formation in the western United States and of the Clemente Formation in Sonora. The pattern is also evident in the Lower Cambrian Zabriskie Quartzite of the western United States and the correlative Proveedora Quartzite in Sonora. Matching of isopach lines of the Zabriskie Quartzite and Proveedora Quartzite suggests ∼700–800 km of left-lateral offset. The offset pattern is also apparent in the distribution of distinctive lithologic types, unconformities, and fossil assemblages in other rocks ranging in age from Neoproterozoic to Early Jurassic. In the western United States, the distribution of facies in Neoproterozoic and Paleozoic strata indicates that the Cordilleran miogeocline trends north-south. A north-south trend is also suggested in Sonora, and if so is compatible with offset of the miogeocline but not with the ideas that the miogeocline wrapped around the continental margin and trends east-west in Sonora. An imperfect stratigraphic match of supposed offset segments along the megashear is apparent. Some units, such as the “Johnnie oolite” and Zabriskie-Proveedora, show almost perfect correspondence, but other units are significantly different. The differences seem to indicate that the indigenous succession of the western United States and offset segments in Mexico were not precisely side by side before offset but were separated by an area—now buried, eroded, or destroyed—that contained strata of intermediate facies.

Reconstructing the geological evolution of central and western Mexico during the end of the Paleozoic and the beginning of the Mesozoic is very difficult because of a lack of exposures. The few outcrops available, and indirect information obtained from geophysical and geochemical data suggests that Central and Western Mexico are made up of a mosaic of pre-Jurassic terranes, and that previously defined ter-ranes are mostly composites of basements of different origins. Most of those terranes are allochthonous with respect to North America, but some developed not far from their present position. It has been suggested that the Coahuila and Sierra Madre terranes (Oaxaquia block), part of Gondwana during Early Paleozoic, collided with North America by Late Paleozoic time. However, their Mississippian faunas of North American affinity suggest that the collision might have occurred earlier. The nature of the basement of the Central terrane is unknown, but it is inferred to be allochtho-nous because there is an accretionary prism at its NE boundary. The basement of the Parral and Tahue terranes is formed by a deformed volcano-sedimentary complex of Early Paleozoic age, whose origin and paleogeographic evolution remains unknown. The Caborca and Cortes terranes are formed by Proterozoic metamorphic complexes and an accreted eugeoclinal Paleozoic sedimentary wedge. The basement of the Zihuatanejo terrane is made up of Triassic ocean-floor continental-rise assemblages accreted in Early Jurassic time. An overview of new stratigraphic and geochronologic data indicates that a number of tectonic events occurred during Late Paleozoic to Early Mesozoic time. A continental arc with a paleo-Pacific, east-dipping subduction zone evolved from Carboniferous to Early Permian time in eastern Mexico (Oaxaquia), and it was in part contemporaneous to deformation in the Ouachita belt. This was followed by a period of volcanic quiescence during middle Permian. A more felsic arc, with a different distribution of the volcanic axis, developed along all the paleo-Pacific margin in the Permo-Triassic. Terranes in northwestern Mexico show a completely different geological evolution during the Carboniferous and Permian time. They were characterized by passive margin sedimentation and by folding and thrusting of eugeoclinal rocks in the Mississippian and Late Permian. By Late Triassic, a passive or rifting margin developed along the western margin of Oaxaquia, and thick successions of continent-derived sediments were accumulated on the paleocontinental shelf and slope (Potosi Fan) and in a marginal active oceanic basin (Arteaga Basin). Those rocks were deformed and accreted to nuclear Mexico by Late Triassic–Early Jurassic time, before the development of the Late Jurassic continental arc that was widespread along western and central Mexico.

The El Antimonio Group is herein proposed as a new lithostratigraphic unit that encompasses the Antimonio, Río Asunción, and Sierra de Santa Rosa Formations in a revised nomenclature from Lucas and Estep (1999b). The type section for the Antimonio, Río Asunción, and the lower part of the Sierra de Santa Rosa Formations is located in the Sierra del Álamo, whereas the representative upper part of the Sierra de Santa Rosa Formation is located in the mountains of same name in northwestern Sonora. The ∼4.5-km-thick sedimentary succession of this group is abundantly fossiliferous, and its biostratigraphic age is constrained between the Late Permian and Early Jurassic. The 3.4-km-thick section that crops out in the Sierra del Álamo is divided into 14 unconformity-bounded sequences that are tens to hundreds of meters thick and grade from the base upward from a fluvial to shallow marine conglomerate to open marine shale. The El Antimonio succession is correlated with several other Triassic and Jurassic sections that are known in Sonora, all of which are located south of the proposed trace of the Mojave-Sonora megashear. The closest Triassic and Lower Jurassic sections that are located north of the Mojave-Sonora megashear that we correlate with the El Antimonio are known in southern Nevada and southeastern California and include the Moenkopi, Virgin Limestone, Union Wash, Silverlake, and Fairview Valley Formations and the Kings sequence. On the basis of these proposed correlations, we suggest that the El Antimonio Group was deposited in an evolving shallow shelf (Upper Permian–Triassic) to fore-arc basin (Lower Jurassic) that was originally positioned adjacent to southern California and later translated to its present position, along with the Caborca block, by left-lateral Jurassic displacement of the Mojave-Sonora megashear. In this proposed paleogeography, a lower Mesozoic magmatic arc that accumulated volcanic, volcaniclastic, and shallow marine sedimentation in the Mojave Desert and along the California-Nevada border separated the El Antimonio basin from a shallow shelf that developed to the north. New U-Pb geochronology on detrital zircon and Sm/Nd isotope and petrographic data from terrigenous samples of the El Antimonio Group may help to elucidate its provenance and to support this paleogeography. Zircon grains from samples of the lower, middle, and upper parts of the El Antimonio Group yielded ages that cluster around 1.8, 1.6–1.7, 1.4, and 1.00–1.18 Ga and 340, 270–240, and 190 Ma. The Pro-terozoic zircons are interpreted to indicate provenance from the basement provinces of the southwestern United States, although a reworked source for these grains is also possible as they are present in the Cordilleran miogeocline and off-shelf assemblages of Nevada and California and in Proterozoic and Paleozoic strata in Sonora. The closest known sources for the Permian and Lower Triassic zircons are plutons and volcanic rocks that formed a lower Mesozoic magmatic arc extending from southeastern to northern California and western Nevada. Probable sources for the single zircon grain dated at 340 Ma are the Sierra-Klamath terranes, according to interpretation by other authors of grains of similar age in rocks of Nevada. Grains dated around 190 Ma in the youngest sample most probably reveal provenance from the Lower Jurassic magmatic arc of southeastern California or southern Arizona. The Sm/Nd isotopic data from three samples of the lower, middle, and upper parts of the El Antimonio Group indicate a progressive decrease in model ages, from the base upward (T DM = 1.9–1.8 to T DM = 1.13 Ga) of this succession, indicating a most probable derivation from the Yavapai and Grenville provinces in the southwestern United States. Sandstone and conglomerate clast composition in the El Antimonio Group indicate mixed sources of provenance from sedimentary and vulcanoplutonic origin. These most probably correspond to the Proterozoic and Paleozoic sedimentary successions of southwestern North America and to the Triassic-Jurassic magmatic arc of this same region, respectively.

Five Triassic and Jurassic tectonosequences recognized on the Colorado Plateau have been identified within the Caborca terrane in Sonora, Mexico. Piercing lines defined by truncation of tectonosequence boundaries constrain the pre-offset position of the terrane. Restoration of 1040 ± 290 km of Middle to Late Jurassic left slip along the Mojave-Sonora megashear places the Caborca block in a paleogeographic position that satisfies the constraints provided by truncated tectonosequences and yields predictable regional facies distributions for all tectonosequences. The restoration is consistent with previous estimates of displacements based upon offset of correlative (1) terrains of crystalline basement and (2) pre-Oxfordian cover.

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.

Among supracrustal sequences of the Jurassic magmatic arc of the southwestern Cordillera, the Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona, is remarkable for its lithologic diversity and substantial stratigraphic thickness, ≈8 km. The Topawa Group comprises four units (in order of decreasing age): (1) Ali Molina Formation—largely pyroclastic rhyolite with interlayered eolian and fluvial arenite, and overlying conglomerate and sandstone; (2) Pitoikam Formation—conglomerate, sedimentary breccia, and sandstone overlain by interbedded silt-stone and sandstone; (3) Mulberry Wash Formation—rhyolite lava flows, flow breccias, and mass-flow breccias, with intercalated intraformational conglomerate, sedimentary breccia, and sandstone, plus sparse within-plate alkali basalt and comendite in the upper part; and (4) Tinaja Spring Porphyry—intrusive rhyolite. The Mulberry Wash alkali basalt and comendite are genetically unrelated to the dominant calcalkaline rhyolite. U-Pb isotopic analyses of zircon from volcanic and intrusive rocks indicate the Topawa Group, despite its considerable thickness, represents only several million years of Middle Jurassic time, between approximately 170 and 165 Ma. Sedimentary rocks of the Topawa Group record mixing of detritus from a minimum of three sources: a dominant local source of porphyritic silicic volcanic and subvolcanic rocks, identical or similar to those of the Topawa Group itself; Meso-proterozoic or Cambrian conglomerates in central or southeast Arizona, which contributed well-rounded, highly durable, polycyclic quartzite pebbles; and eolian sand fields, related to Middle Jurassic ergs that lay to the north of the magmatic arc and are now preserved on the Colorado Plateau. As the Topawa Group evidently represents only a relatively short interval of time, it does not record long-term evolution of the Jurassic magmatic arc, but rather represents a Middle Jurassic “stratigraphic snapshot” of the arc. This particular view of the arc has been preserved primarily because the Topawa Group accumulated in deep intra-arc basins. These nonmarine basins were fundamentally tectonic and extensional, rather than volcano-tectonic, in origin. Evidence from the Topawa Group supports two previous paleogeographic inferences: the Middle Jurassic magmatic arc in southern Arizona was relatively low standing, and externally derived sediment was introduced into the arc from the continent (northeast) side, without appreciable travel along the arc. We speculate that because the Topawa Group intra-arc basins were deep and rapidly subsiding, they became the locus of a major (though probably intermittent) fluvial system, which flowed into the low-standing magmatic arc from its northeast flank.

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacific–North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacific margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fluvial, debris-flow, and lacustrine sediments. “Dry” eruptions of welded ignimbrite were replaced by “wet” eruptions of nonwelded, easily reworked ignimbrite and phreatoplinian fall. This Late Jurassic transition from hyperarid to more temperate climatic conditions may thus form a superregional “time line” that ties the Cordilleran plate margin to events in the interior of the continent.

In the western Bisbee Basin of southern Arizona, detailed mapping and sequence analysis of the Glance Conglomerate along the largest basin-bounding fault, the Sawmill Canyon fault zone, reveals interbedded clastic, volcanic, and volcaniclastic lithofacies and their relationship to intrabasinal faulting, unconformities, and basin-bounding faults. The basin fill is dominated by small polygenetic, multivent volcanic complexes ranging in composition from rhyolite to andesite typical of continental arc volcanism. Syndepositional basin-bounding faults, the Sawmill Canyon and Gringo Gulch fault zones, controlled subsidence within the basin and plumbed small batches of magma to the surface. Small intrabasinal faults show stratigraphically limited offsets that alternate between normal and reverse separation. Eight unconformable surfaces occur within the basin. Five are asymmetrical, with one very steep wall and one gradually sloping wall. They show extreme vertical relief (460–910 m) with very high paleoslope gradients (40°–71°) that dip away from the master fault. We interpret these as uplifted fault scarps or paleoslide scars. The other three unconformities are symmetrical, V-shaped surfaces that have less steep walls, with vertical relief of 200–600 m and paleoslope gradients of 20°–25°. We interpret the symmetrical surfaces to be walls of deep paleocanyons cut during basin uplift events or following large ignimbrite eruptions. Analysis of the unconformably bound stratigraphic sequences shows deposition to be related to subsidence along large basin-bounding faults modified by intrabasinal, high-angle, syndepositional normal and reverse faults. Erosion of the sequence-bounding unconformities took place during uplift associated with basin inversion. Alternation of uplift and subsidence and the juxtaposition of intrabasinal reverse and normal faults is typical of strike-slip basins. We interpret the Glance Conglomerate in the Santa Rita Mountains as the fill of an intra-arc strike-slip basin where strike-slip deformation was concentrated along the thermally weakened arc axis. We suggest a model for the Bisbee Basin of a strain-partitioned, obliquely convergent continental arc with backarc extension-transtension.

Huizachal Canyon, one of a series of generally east-west trending canyons in Tamaulipas, northeastern Mexico, exposes a pre–Late Jurassic sequence of fossiliferous pyroclastic and epiclastic rocks, the Huizachal Group. Heretofore considered a sedimentary package associated with either metamorphic or intrusive rocks, structural relationships and petrographic studies presented here indicate that the Huizachal Group was in fact deposited unconformably upon an older, undescribed, sequence of pyroclastic rocks. Four igneous units are recognized in the steeply dipping older volcaniclastic sequence (respectively, units A–D): a complex suite of pyroclastic flows, accretionary lapilli tuff(s), and lava flows (unit A); a homogeneous, finegrained felsitic rock (unit B); a sequence of conglomerates (unit C); and a mixed assemblage of rocks including mafic-to-intermediate composition lava flows and intercalated conglomerate and tuff (unit D). Most of these rocks have undergone extensive late-stage or postdepositional silicification, but relatively immobile trace elements demonstrate that these rocks range from subalkaline basalt to rhyolite. The younger, relatively flat-lying Huizachal Group overlies these rocks in angular unconformity. The fossil assemblage comes from a <10-m-thick sequence in the lower part of the Huizachal Group, which also is the result of pyroclastic volcanic deposition. Some of the organisms entrained within these tuff(s) were reworked by volcanic processes; others appear to have been actively trapped in a manner analogous to Pompeii. New U-Pb isotopic data from zircon in a volcaniclastic rock from the lowest part of the Huizachal Group (La Boca Formation) yields an age of 189 ± 0.2 Ma (analytical error). The sedimentary rocks immediately above this unit contain fossils considered to be Early Jurassic in age. Thus, the zircon isotopic age agrees with, and is supportive of, the age estimates based upon fossil vertebrates. Trace element geochemistry of the volcanic units is strongly suggestive of subalkalic ocean-continent Andean volcanism. Thus, the volcanic and sedimentary rocks of Huizachal Canyon were most likely deposited in a convergent plate margin setting instead of an extensional rift system as previously proposed. El Cañón del Huizachal, es uno de una serie de cañones con orientación este-oeste en Tamaulipas, en el Noreste de México; ahí aflora el Grupo Huizachal, una secuencia de rocas fosilíferas piroclásticas y epiclásticas pre-jurásicas tardías. Hasta ahora considerado como un paquete asociado con rocas ígneas intrusivas o metamórficas, las relaciones estructurales y los estudios petrográficos presentados aquí sugieren que el Grupo Huizachal estuvo, de hecho, depositado de manera discordante por encima de una secuencia de rocas volcánicas piroclásticas más vieja, no descrita. Se reconocen cuatro unidades ígneas en la secuencia vulcanoclástica más antigua (respectivamente unidades A–D): un conjunto complejo de flujos piroclásticos, tobas de lapili acrecional, y flujos de lava (unidad A); una roca homogénea félsica de grano fino (unidad B); una secuencia de conglomerados (unidad C); y un conjunto mixto de rocas que incluye flujos de lava de composición máfica a intermedia, así como una toba y un conglom-erado intercalados (unidad D). Casi todas estas rocas han sufrido una silicificación extensiva en su fase tardía, o post-deposicional, pero elementos traza relativamente inmóviles demuestran que estas rocas comprenden desde basalto subalcalino a riolita. El conjunto de fósiles proviene de una secuencia de <10 m de espesor de la parte inferior del Grupo Huizachal. Esta secuencia es también el resultado de un depósito volcánico piroclástico. Algunos de los organismos arrastrados dentro de estas tobas fueron retraba-jados por procesos volcánicos; otros parecen haber sido atrapados enérgicamente de una manera análoga a la de Pompeya. Nuevos datos de U-Pb isotópicos de zircón en una roca vulcanoclástica de la parte más inferior del grupo Huizachal (Formación La Boca) dio una edad de 189 ± 0.2 Ma (error analítico). Las rocas sedimentarias inmediatamente arriba de esta unidad contienen los fósiles considera-dos como de edad jurásica temprana. De este modo, la edad isotópica del zircón concuerda y apoya la edad estimada basada en los vertebrados fósiles. La geoquímica de elementos traza de las unidades volcánicas sugiere fuertemente un vulcanismo andino subalcalino océano-continente. Así, las rocas volcánicas y sedimentarias del Cañón Huizachal fueron depositadas probablemente en un ambiente de margen de placa convergente en vez de un sistema de rift extensional como se había propuesto anteriormente.

The Taray Formation is a mudstone-rich body of rock characterized by dismembered beds of sandstone and shale and fragments of a great range of sizes in a finer matrix. The formation is known only from exposures in northern Zacatecas, in the southern part of the San Julian uplift. Structurally, it underlies a thick Jurassic sequence that includes the volcanogenic Caopas, Rodeo, and Nazas formations and younger rocks of La Joya (volcaniclastic) and Zuloaga (limestone) formations. The base of the Taray is unknown. The texture and structures of Taray and its tectonostratigraphic association with Jurassic volcanic rocks suggest that it is formed of mélange of probable Jurassic age. Sections of interbedded sandstone and mudstone commonly show progressive dismemberment. Initial subtabular beds commonly pinch and swell. Further extension was accommodated by boudinage or by faults, which cut gently across bedding and dissect sandy layers into lenses and slivers. Progressive stratal disruption formed isolated, deformed inclusions of sandstone, commonly mixed with volcanic and cherty debris, encased in dark-gray, fine-grained, foliated, crenulated matrix. Some large boulders of quartzose sand that slid downslope, crumpling sediments at their bows, were subsequently encased by matrix and both were distinctly extended so that the boulders were fractured, whereas the weaker, pliant matrix responded ductilely. Taray also contains blocks (as much as hundreds of meters in maximum dimension) of diverse composition including: laminated and massively bedded, light-colored chert; fragmented volcanic flow rock; fine, quartzose sandstone; and carbonate beds, some of which contain fusulinids and crinoid debris. Some of these rocks show tight folds that formed, in some cases, while they were semiconsolidated; later they were incorporated in muddy matrix as lithified fragments. The unexposed contact between mudstone-rich Taray Formation and the volcanic and volcaniclastic beds of the Nazas Formation also marks changes in the attitude of planar structures. We interpret Taray as part of a Jurassic(?) accretionary prism. The tectonostratigraphic sequence at this latitude (west to east) of accretionary prism (Taray), arc (Nazas, Rodeo, Caopas Formations), and overlying clastic cover (La Joya Formation), which developed against and upon a margin of Paleozoic and Precam-brian crust, cannot be extended northward and must either turn abruptly westward along the southern edge of the Mesozoic Coahuila Island or be truncated and offset, probably along the Mojave-Sonora megashear.

Paleomagnetic studies of Paleozoic sedimentary and plutonic rocks demonstrate that the Yucatan Block did not lie between the North and South American plates in the Pangean assembly during the Permian. In the Middle Permian, the Yucatan Block lay in an inverted orientation on the western margin of Pangea at 6–10°S, probably forming part of the NW coast of South America. Subsequently the block rotated in a series of counterclockwise motions as the North and South American plates separated. By 230 Ma, Yucatan had rotated ∼47° counterclockwise and moved slightly northward, to the equator. Counterclockwise rotation continued through the Jurassic: ∼41° between 230 Ma and ca. Oxfordian time, and another ∼47° between the Oxfordian and Tithonian, at which time, the approximate present orientation with respect to North America was achieved. Passage of the Yucatan Block from NW South America into the gap created by the separation of North and South America is a motion consistent with the left-lateral translations along the Mojave-Sonora or similar megashear. The fact that Yucatan has exhibited a counterclockwise motion throughout its Mesozoic history suggests that the microplate may have acted in a ball-bearing fashion between the larger North and South American plates. The Permian sedimentary remanence appears to be carried dominantly by magnetite and maghemite in red bed strata deposited on the margins of Silurian plutons within the marine Santa Rosa basin of the Maya Mountains. This remanence decays exceedingly linearly to the origin of orthogonal-axes plots. Biostratigraphy indicates a late Pennsylvanian to Middle Permian sedimentary age for the Santa Rosa strata, and the presence of both polarities of remanence in a 110 m magnetostratigraphic sequence of four polarity intervals demonstrates a post-Kiaman, i.e., Middle Permian, magnetization age. Another remanence is exhibited by Silurian plutons, also dual polarity, but corresponding to a paleopole ∼60° counterclockwise of that recorded in the Middle Permian sedimentary rocks. Exceedingly uniform K/Ar ages of 231 ± 7 Ma characterize all Maya Mountains plutons and a volcanic complex, indicating a 230 Ma resetting of the K/Ar radiometric systems in plutons dated as Late Silurian by U/Pb (Steiner and Walker, 1996). Furthermore, metamorphic aureoles are developed in the Permian Santa Rosa strata that border the Silurian plutons, suggesting that the 230 Ma resetting was a postintrusion event that involved the margins of the plutons. The spatial relationship between the pluton and sedimentary poles is reminiscent of that between the North American Late Triassic and Permian reference paleopoles. Therefore the combination of 230 Ma reset K/Ar systems, metamorphic aureoles in strata younger than the plutons, and a magnetization resembling a Late Triassic remanence all suggest that a 230 Ma hydrothermal event remagnetized the igneous rocks and reset their radiometric systems. This probably was an event related to the initial breakup of Pangea. The Maya Mountains plutons yield a paleopole that is statistically identical to that of the Chiapas Massif to the south. Both plutonic complexes exhibit somewhat dispersed dual polarity magnetization populations, suggesting that both were remagnetized at ca. 230 Ma. Importantly, the identical remanences in these widely separated plutonic complexes indicate that the Yucatan Block (including the Chiapas Massif) has been a structural entity since at least 230 Ma.