Abstract The Sierra de Juárez Complex (SJC) of southern Mexico contains an extensive geological record from Precambrian to Cenozoic, involving Rodinia, NW Gondwana, western equatorial Pangaea, and eastern peninsular Mexico. It is thus critical for palinspastic reconstructions and lithotectonic correlations, mainly between the Mexican and NW South America terranes. In this contribution, we investigate the tectonic evolution of the northern SJC from Silurian to the Lower Cretaceous on the basis of fieldwork, petrography, and zircon U–Pb geochronology by laser ablation–inductively coupled plasma mass spectrometry. Our results allow us to constrain five main geological events: (1) Middle Paleozoic sedimentation along NW Gondwana during transtensional tectonics; (2) volcanosedimentary activity between 292 and 281 Ma in NW Gondwana during Rheic Ocean closure; (3) early-middle Permian metamorphism related to flat-slab subduction postdating Pangaea assembly; (4) Lower–Middle Jurassic anatexis and magmatism coeval with regional shearing at c. 175 Ma influenced by transtensional tectonics along eastern peninsular Mexico during Pangaea tenure; and (5) intermediate to acid magmatism between c. 136 and 129 Ma, correlated with the Zongolica continental arc in southern Mexico, followed by deep-crustal shearing related to either the formation of the extensional Chivillas basin or the Upper Cretaceous–Cenozoic contractional episode documented in the Cuicateco Terrane.

ABSTRACT The Oaxacan Complex is the largest outcrop of Grenville-age rocks in Mexico, constituting the main crustal fragment in the backbone of Oaxaquia. It is mainly composed of scarce metasediments, intruded by arc, alkalic, and tholeiitic magmas (ca. 1.3 to ca. 1.01 Ga) and later affected (ca. 0.99 Ga) by granulite-facies metamorphism. A detailed study, combining U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry, with in situ Hf isotopes in zircon grains, allowed comparison of the age and isotopic patterns of the Oaxacan Complex granulite rocks with those from other similar outcrops in Mexico (Huiznopala and Novillo Gneisses, Guichicovi Complex) and with the neighboring orogens such as the Grenville Province of the eastern United States and Canada, the Sveconorwegian orogen of SW Baltica, and some of the localities in which Mesoproterozoic rocks border the Amazonian craton of South America (Colombia, Peru, Brazil). Detrital zircon ages show that most metasedimentary rocks are younger than 1.4 Ga (only three samples contained zircon grains between 1.6 and 1.4 Ga), whereas U-Pb dating of igneous rocks (1245–1161 Ma) confirmed previous findings. Hf isotopes of dated zircon grains show that few crystals have negative ε Hf( t ) values, indicating a recycling component from an older crust, but most of them are moderately primitive, with ε Hf( t ) values of up to +12, and linear arrays parallel to the 176 Lu/ 177 Hf average crustal evolution model. Those Hf values are indicative of partial assimilation of an older crustal component, with Hf model ages of ca. 1.65–1.50 Ga. Comparison of these data helps to constrain possible Mesoproterozoic conjugate margins of Oaxaquia and propose a paleogeographic model in which Oaxaquia acted as the leading edge of Amazonia, together with the Colombian terranes, and received sedimentary input from different sources such as the southern Sveconorwegian orogen, the U.S.–Canada eastern Grenville Province, and some of the Mesoproterozoic belts bordering the Amazon craton.

ABSTRACT Sandstone petrography, detrital zircon geochronology, and sedimentology of Lower Cretaceous to Paleocene strata in the Cuicateco terrane of southern Mexico indicate an evolution from extensional basin formation to foreland basin development. The Early Cretaceous extensional basin is characterized by deposition of deep-marine fans and channels, which were mainly sourced from Mesoproterozoic and Permian crystalline rocks of the western shoulder of the rift basin. Some submarine fans, especially in the northern Cuicateco terrane, record an additional source in the Early Cretaceous (ca. 130 Ma) continental arc. The fans were fed by fluvial systems in updip parts of the extensional basin system. The transition from middle Cretaceous tectonic quiescence to Late Cretaceous shortening is recorded by the Turonian–Coniacian Tecamalucan Formation. The Tecamalucan Formation is interpreted as pre-orogenic deposits that represent submarine-fan deposits sourced from Aptian–Albian carbonate platform and pre-Mesozoic basement. The foreland basin in the Cuicateco terrane was established by the Maastrichtian, when foredeep strata of the Méndez Formation were deposited in the Cuicateco terrane, Veracruz basin, and across the western Gulf of Mexico, from Tampico to Tabasco. In the Zongolica region, these strata were derived from a contemporaneous volcanic arc (100–65 Ma) located to the west of the basin, the accreted Guerrero terrane (145–120 Ma), and the fold belt itself. By the Paleocene, sediments were transported to the foreland basin by drainages sourced in southwestern Mexico, such as the Late Cretaceous magmatic rocks of the Sierra Madre del Sur, and the Chortis block.

ABSTRACT The Oaxacan Complex represents the largest outcrop of Grenvillian basement in Mexico. Broadly, it consists of pelitic gneisses, quartzofeldspathic gneisses, metasomatic calc-silicates, orthoamphibolites, and marbles, all intruded by anorthosites, orthocharnockites, and orthogneisses. The entire assemblage underwent granulite-facies metamorphism ca. 1 Ga. We studied for the first time the ultramafic rocks of the Oaxacan Complex, represented by six different samples, all corresponding to ultramafic granulites. Their igneous equivalents are orthopyroxenites, websterites, and clinopyroxenites, and they occur as metric-scale lenses or centimetric layers in paragneisses, or in mingling textures with anatectic marbles. We studied their petrography, geochemistry, geochronology, and geothermobarometry to elucidate their genesis and tectonic implications. Our samples have enriched mid-ocean-ridge basalt and oceanic-island-arc affinities, both tholeiitic and calc-alkaline. Rare earth element patterns normalized to chondritic uniform reservoir from whole rock or single minerals define two or three main groups related to their origin and metamorphic history. Based on their protoliths, these rocks can be divided into: (1) ortho-derived pyroxenites (pre–Grenvillian orogeny), the origin of which was a magmatic cumulate or mafic melt or a mantle rock that had undergone metasomatism; and (2) para-derived pyroxenites (syn- or post-Grenvillian orogeny), the origin of which was a calc-silicate rock undergoing pervasive anatectic and metasomatic processes. The geothermobarometry revealed different stages in the syn- and post-Grenvillian granulitic metamorphic history of the Oaxacan Complex. The high temperature calculated from one sample (~945 °C), in the ultrahigh-temperature metamorphic field, is probably closer to the granulitic metamorphism peak than those obtained in previous studies, although a relict igneous temperature cannot be ruled out with the present data.

ABSTRACT The Gulf of Mexico is best understood as a subsidiary basin to the Atlantic, resulting from breakup of Pangea. The rifting process and stratigraphy preceding opening of the gulf are, however, not fully understood. We present new stratigraphic, sedimentologic, and provenance data for the Todos Santos Formation (now Todos Santos Group) in southern Mexico. The new data support a two-stage model for rifting in the Gulf of Mexico. Field and analytical evidence demonstrate that strata assigned to the Todos Santos Group in Mexico belong to two unrelated successions that were juxtaposed after rotation of the Yucatán block. An Upper Triassic fluvial siliciclastic succession in the western Veracruz basin is intruded by the San Juan del Río pluton (194 Ma, U-Pb) along the Valle Nacional fault. We refer to this succession as the Valle Nacional formation (informal) of the Todos Santos Group, and correlate it with El Alamar Formation of northeast Mexico and the Eagle Mills Formation of the northern Gulf of Mexico. Triassic red beds register an early rifting phase in western equatorial Pangea. Sandstone composition indicates that the Valle Nacional formation is mostly arkoses derived from multiple sources. Paleocurrent indicators in fluvial strata of the Valle Nacional formation are S-SW directed, but restoration of paleomagnetically determined counterclockwise rotation indicates a W-SW–flowing fluvial system. Triassic rifting in the Valle Nacional formation and the Central Cordillera of Colombia Triassic extensional event, the record of which is preserved in mid-crustal levels, may represent conjugate margins. The Early–Middle Jurassic Nazas continental volcanic arc predated the Jurassic rifting phase that led to opening of the gulf. A record of arc magmatism is present in eastern Mexico underlying Middle Jurassic synrift successions, and it is present in La Boca and Cahuasas formations in the Sierra Madre Oriental and La Silla Formation north of the Chiapas Massif. These units have a similar age range between ca. 195 and 170 Ma. Arc magmatism in eastern Mexico is correlated with the Jurassic Cordilleran arc of Sonora, California, and Arizona, as well as the Jurassic arc of the Central Cordillera of Colombia. La Boca and La Silla units record intra-arc extension driven by slab rollback. The Jurassic rifting phase is recorded in the Jiquipilas formation of the Todos Santos Group and is younger than ca. 170 Ma, based on young zircon ages at multiple locations. The informal El Diamante member of the Jiquipilas formation records the maximum displacement rift stage (rift climax). Coarse-grained, pebbly, arkosic sandstones with thin siltstone intercalations and thick conglomerate packages of the Jericó member of the Jiquipilas formation are interpreted as deposits of a high-gradient, axial rift fluvial system fed by transverse alluvial fans. These rivers flowed north to northeast (restored for ~35° rotation of Yucatán). The Concordia member of the Jiquipilas formation records the postrift stage. Thick synrift successions are preserved in the subsurface in the Tampico-Misantla basin, but they cannot be easily assigned to the Triassic or the Jurassic rifting stages because of insufficient study. The Todos Santos Group at its type locality in Guatemala marks the base of the Lower Cretaceous transgression. Overall, three regional extensional events are recognized in the western Gulf of Mexico Mesozoic margin. These include Upper Triassic early rifting, an extensional continental arc, and Middle Jurassic main rifting events that culminated with rotation of Yucatán and formation of oceanic crust in the gulf.