Thomas H. Anderson

The evolution of Middle America and the Gulf of Mexico–Caribbean Sea region during Mesozoic time

A plate-tectonic model for the evolution of Middle America and the Gulf of Mexico-Caribbean Sea region is presented. The model, which is based upon the existence of the Mojave-Sonora megashear, incorporates into the Triassic Pangea reconstruction three microplates between North and South America, thus avoiding the overlap of the Bullard fit. These plates are the Yaqui, bounded on the north by the Mojave-Sonora megashear; the east and west Maya plates, bounded on the north by the Mexican volcanic zone and on the south by a predecessor of the Motagua fault zone; and the Chortis plate (parts of Guatemala and Honduras). During Late Jurassic time, as North America split away from Europe, Africa, and South America, shear, with left-lateral sense of displacement, occurred along the transform faults that bounded the micro-plates. If ∼800 km of left-lateral displacement along the Mojave-Sonora megashear, ∼300 km along the Mexican volcanic belt, and ∼1,300 km along a proto-Motagua megashear are restored, and if Yucatan and Cuba are rotated to fit against northern South America, then (1) a curvilinear belt of late Paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in Texas, eastern Mexico, nuclear Central America, and Colombia; (2) a Mediterranean-like sea is delineated that was a precursor of most of the present Gulf of Mexico; (3) correlation is implied between the distinctive quartzose San Cayetano Formation of Cuba and the Caracas and Juan Griego Groups of Venezuela. Geometric constraints suggest that probably shear initially occurred along the Mexican volcanic zone near the end of the Middle Jurassic. Subsequently, probably about 160 m.y. ago, displacements that total ∼800 km began along the Mojave-Sonora megashear. Contemporaneously, Yucatan and fragments of pre-Cretaceous rocks that compose parts of central and western Cuba migrated northward toward their present positions. Rotation of Yucatan was facilitated by considerable displacement along the proto-Motagua zone and along a zone that is probably coincident with the modern Salina Cruz fault. Accumulation of widespread major salt units of Late Jurassic (Callovian to early Oxfordian) age in the Gulf Basin probably occurred contemporaneously with the arrival of these blocks at their present positions. Clastic units that interfinger with some of the youngest salt units and rim the Gulf of Mexico have not recorded major recognized translations since their accumulation. Clockwise rotation of South America and the Chortis plate occurred during Early Cretaceous time. This movement, which was manifested by subduction of Jurassic ocean floor against the previously rifted precursor of the island of Cuba and under parts of Hispaniola and Puerto Rico, is recorded by circum-Caribbean orogeny. Abrupt changes in the relative motions between North and South America during Late Cretaceous time may have resulted in extension and outpourings of basalt upon the Jurassic rocks of the ocean floor of the Venezuelan Basin. West of Beata Ridge, sea-floor spreading formed the Colombian Basin. Related subduction occurred as the Chortis plate (including part of Central America, the Nicaraguan Rise, and southeastern Cuba) was sutured against the Maya East plate along the present Motagua fault and Cayman Trench. Our model is constrained by published geologic data, the relative positions of North and South America from Atlantic sea-floor magnetic anomalies, and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between North and South America. Paleomagnetic data from Middle America are sparse but do not conflict with the predicted motions of some of the microplates, especially Chortis.

Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region

The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.

Tertiary metamorphic core complexes in Sonora, northwestern Mexico

Several ranges encompassing more than 35,000 km 2 of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top-to-the-southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U-Pb and K-At ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene-early Miocene. Partitioning of extensional strain in Sonora was influenced by pre-Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60oW striking Mojave-Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic arc rocks and Upper Jurassic- Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex-related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic-Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two-mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre-Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25-18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest-British Columbia region but earlier than in the

The Mojave-Sonora megashear — Field and analytical studies leading to the conception and evolution of the hypothesis

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.

Jurassic volcanic rocks in northeastern Mexico: A possible remnant of a Cordilleran magmatic arc

Pre-Oxfordian Mesozoic subaerial volcanogenic rocks occur in a band extending northwest from Ciudad Victoria, Tamaulipas, to Santa Maria del Oro, Durango. These strata include Nazas, Rodeo, and Caopas Formations in Durango, Coahuila, and Zacatecas; La Boca Formation and its underlying volcanic basement at Canon de Huizachal, Tamaulipas; and volcanic units below La Joya Formation at Real de Catorce and Charcas, San Luis Potosi. Rocks at these localities have similar lithologies, stratigraphic positions, and paleontologic and isotopic ages. Field mapping in the Caopas-Pico de Teyra area, northern Zacatecas, and ancillary research provide insight into the nature of this suite. At least 3 km of abundant airfall tuff, tuffaceous siltstone, and uncommon ashflow tuff are present near Pico de Teyra; this sequence appears to belong to a more distal facies than the flows, breccias, and laharic conglomerates of the Nazas and Rodeo Formations exposed 25 km to the north. Porphyritic rhyolite (Caopas Formation) occurs within these volcanogenic rocks as a fault-bounded block and is interpreted as a cogenetic, subvolcanic pluton. A relatively undeformed portion of the Caopas has yielded a zircon U-Pb age of 158 ± 4 Ma. Petrographic and limited chemical data from these formations show that calc-alkaline andesite, dacite, and rhyolite are the most common compositions. The volcanogenic rocks in northeastern Mexico are south of the inferred trace of the Mojave-Sonora megashear. Their large volume, their lithologic and chemical characteristics, and their age suggest that these rocks may be a component of the Jurassic arc of western North America that was translated southeastward along the megashear.

Las Delicias basin: A record of late Paleozoic arc volcanism in northeastern Mexico

The Las Delicias basin in northeastern Mexico is defined by a thick sequence of mid(?)-Pennsylvanian through Permian marine strata, which forms most of the basement rocks of the Coahuila island, a terrane that stood above sea level during the Late Jurassic and Early Cretaceous. Most of the strata accumulated as mass-gravity deposits derived from an active volcanic arc. Principal components of these deposits are (1) andesitic and dacitic debris, (2) pelagic sediment that underwent postdepositional movement, and (3) limestone debris from the basin margin. Facies patterns and macroscopic soft-sediment structural relations indicate a source to the south; volcanic rocks at Canon Rosillo may be part of the arc. The Las Delicias area is probably not part of the Ouachita-Marathon system; stratigraphic relations and structural setting suggest that the Coahuila island may be an allochthonous stratigraphic terrane.

Analysis of light hydrocarbons in soil gases, Lost River region, West Virginia: Relation to stratigraphy and geological structures

Analyses of 471 near-surface soil-gas samples for light hydrocarbons, C1–C4, C2L, C3L, and H2 from the Lost River gas field in Hardy County, West Virginia, reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases, which occur directly over the faulted, eastern limb of the Whip Cove anticline. Compositional changes in the soil-gases data clearly define major changes in the maturity and locations of potential source beds. Grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators. Statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil-gas constituents. Large concentrations, coupled with high saturate-to-olefin ratios, further confirms that this active seepage is near macroseep levels. Variations in soil-gas compositional trends separate the soil-gas data into two domains, with oilier compositions to the west and gassier compositions to the east. Although the composition of the shallow soil gases above the Lost River gas field are oilier than the reservoir gases, they occur directly over the eastern, faulted limb of the producing anticlinal structure, suggesting that the dry gases from the Oriskany reservoir are probably mixed with oilier gases from organic-rich strata among Devonian shales. The eastern anomalies are much gassier and are very similar to the Oriskany gases produced by the Lost River gas field. The eastern anomalies directly overlie near-vertical beds of Devonian and older age formations that are likely conduits for deeper, mature thermal gases.

The Chixoy-Polochic fault and its associated fractures in western Guatemala

In western Guatemala, post-Cretaceous volcanic and sedimentary rocks locally cover the trace of the Chixoy-Polochic fault. No single throughgoing fault cuts these units. However, a complicated series of en echelon lineations identified from aerial photographs and field mapping represent the accommodations of displacements along the underlying fault. Comparison of fracture trends in the Chixoy-Polochic fault zone with models from experimental studies and field examples of strike-slip faults suggest that left-lateral displacement along the Chixoy-Polochic fault in this area is probably no more than a few kilometres, since the accumulation of the overlying beds of probable post mid-Tertiary age. Earlier lateral displacements of large magnitude are not precluded, although none have been documented by field study.

Mass-gravity deposits and structures in the Lower Cretaceous of Sonora, Mexico

Rocks that form the south flank of Sierra Azul, northern Sonora, Mexico, are correlative with the Jurassic(?) and Lower Cretaceous Bisbee Group of southern Arizona. We interpret them as basinal marine deposits featuring both mass-gravity deposition and deformation. Thick (hundreds of meters) bodies of sediment showing some internal disaggregation are separated by slide surfaces that cut down-section into footwalls. We believe that further mass movement formed these stacked slide masses into northwest-trending, southwest-vergent folds, though they contain many features normally associated with direct tectonism. Thus, the folds are not the product of Laramide crustal shortening, but rather they reflect earlier (Jurassic and Cretaceous) high-angle crustal movement that produced the paleo-upland (the Cananea high), the basin, and the slope that guided their development. We suggest their vergence shows paleoslope, not tectonic transport. This basinal marine sequence contains blocks of reefal limestone equivalent to the upper Mural Limestone (Bisbee Group) that have been widely used as evidence of an in situ carbonate bank; here, they are allochthonous. We suggest that they may have been derived from the margin waters of the Cananea high. Confusion of structures such as these formed by mass-gravity processes may be one reason that the pattern of Laramide deformation in northern Sonora is still poorly defined.

Pull-apart basins at releasing bends of the sinistral Late Jurassic Mojave-Sonora fault system

A 200–500-km-wide belt along the southwestern margin of cratonic North America is pervaded by northwestand east-trending faults that fl ank 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-fi ning sections. We argue that these basins share a common structural origin, based on: (1) the presence of faults, locally preserved, that generally defi ne 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 defi ne 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 structural anisotropies imposed upon crystalline basement northeast of the Mojave-Sonora megashear controlled or strongly infl uenced the regional distribution of the pull-apart basins as well as the orientation and style of younger structures and intrusions. Most Late Jurassic faults were modifi ed 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 igne*taco@imap.pitt.edu. 98 T.H. Anderson and J.A. Nourse spe393-03 page 98