The Cosoltepec Formation consists of unfossiliferous phyllites and psammites with tectonic slices of basalt and forms a major part of the Acatlán Complex of southern México. It has traditionally been interpreted as part of either a Cambro-Ordovician accretionary prism or a passive margin deposited in the Iapetus Ocean. Although no reliable age data are available from the basaltic slices, their widespread tectonic interleaving with the Cosoltepec Formation suggests that some of the sedimentary rocks were originally deposited directly on the ocean-floor basalts. The age of the Cosoltepec metasediments ranges from Ordovician to uppermost Devonian (oldest unconformably overlying sediments: uppermost Devonian). The mafic rocks have been affected by greenschist- to sub-greenschist-facies metamorphism. The geochemistry indicates that they are mainly mid-ocean ridge basalt (MORB) tholeiites with flat or depleted rare earth element (REE) patterns and resemble MORB derived from heterogeneous sources. Another minor group consists of ocean island basalts and andesites, which have distinctly fractionated REE patterns, whereas their mantle-normalized trace element patterns do not show Nb-Ta or Ti negative anomalies. Thus, these mafic rocks appear to represent oceanic lavas that were tectonically incorporated into the clastic rocks of the Cosoltepec Formation during deformation that started immediately prior to deposition of late Fammenian sedimentary rocks and continued into the Mis-sissippian. Tectonic juxtaposition of these rocks with eclogitic rocks suggests that the deformation was related to exhumation following subduction: the Cosoltepec Formation and its mafic lenses derived from the overriding plate, whereas the eclogitic rocks represent parts of the subducting plate. The probable Middle Ordovician–Middle Devonian age of the oceanic rocks together with their initial deformation during the Devonian–Carboniferous suggest that they represent vestiges of the Rheic Ocean.

The Acatlán Complex of southern México comprises metasedimentary and metaigneous rocks that represent the vestige of a Paleozoic ocean. Juxtaposed against granulite-facies gneisses of Mesoproterozoic (ca. 1 Ga) age, the complex has previously been related to the Iapetus Ocean and interpreted to preserve a tectonostratigraphic record linked to that of the Appalachian orogen: (1) Cambro-Ordovician deposition of a trench or forearc sequence (the Petlalcingo Group: the Magdalena, Chazumba, and Cosoltepec Formations) and an oceanic assemblage (the Piaxtla Group), (2) polyphase Late Ordovician–Early Silurian deformation (the Acatecan orogeny) during which the Piaxtla Group underwent eclogite-facies metamorphism synchronous with megacrystic granitoid emplacement, (3) deposition of the arc-related Tecomate Formation and intrusion of megacrystic granitoid plutons during the Devonian, and (4) deformation under greenschist-facies conditions during the Late Devonian Mixtecan orogeny. However, recent structural, geochronological, and geochemical studies have shown that (1) the Cosoltepec Formation is bracketed between ca. 455 Ma and the latest Devonian and may be part of a continental rise prism with slivers of oceanic basalt; (2) the Magdalena and Chazumba Units represent a clastic wedge assemblage of Permo-Triassic age; (3) the eclogitic metamorphism is locally Mississippian in age; (4) the Tecomate Formation is an arc complex of latest Pennsylvanian–Middle Permian age; (5) the megacrystic granitoid rocks span the Ordovician and have a calc-alkaline geochemistry, whereas accompanying mafic units have mixed continental arc–tholeiitic affinities and are locally as young as the earliest Silurian; (6) the greenschist-facies tectonothermal event occurred in the Permo-Triassic; and (7) the complex records a Jurassic tectonothermal event that resulted in local high-grade metamorphism and migmatization. This revised geological history precludes any linkage to Iapetus, but is consistent with that of the Rheic and paleo-Pacific Oceans and is interpreted to record (1) development of a rift or passive margin on the southern flank of the Rheic Ocean in the Cambro-Ordovician, (2) formation of either an arc or an extensional regime along the formerly active northern margin of Gondwana throughout the Ordovician, (3) ocean closure documented by subduction-related eclogite-facies metamorphism and exhumation during the Late Devonian–Mississippian, (4) Permo-Triassic convergent tectonics on the paleo-Pacific margin of Pangea, and (5) overriding of a Jurassic plume.

The Granjeno Schist of northeastern México is the oldest component of the Sierra Madre terrane and comprises polydeformed, pelitic metasedimentary and metavolcaniclastic rocks that enclose lenses of serpentinite-metagabbro. This low-grade Paleozoic assemblage is exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt where it is tectonically juxtaposed against ca. 1 Ga granulites of the Novillo Gneiss. Silurian strata that unconformably overlie the Novillo Gneiss are unmetamorphosed and contain fauna of Gondwanan affinity. LA-ICPMS (laser ablation inductively coupled plasma mass spectroscopy)U-Pb ages for detrital zircons from a Granjeno phyllite yield age populations that cluster in the ranges ca. 1375–880 Ma, ca. 650–525 Ma, and ca. 460–435 Ma and slightly discordant grains with individual ages of ca. 1435 Ma, ca. 1640 Ma, ca. 2105 Ma, and ca. 2730 Ma. The youngest detrital zircon indicates a maximum depositional age for the Granjeno Schist of ca. 435 Ma (Lower Silurian). Detrital zircons of Neoproterozoic–Cambrian age suggest a provenance in the Maya terrane beneath the Yucatan Peninsula or the Brasiliano orogens of South America, and a source for the detrital zircons of Ordovician–Silurian age is present in the Acatlán Complex of southern México. Provenance of the Mesoproterozoic detrital zircons is likely to have been the adjacent Novillo Gneiss, which has yielded ages of ca. 990–980 Ma, ca. 1035–1010 Ma, and ca. 1235–1115 Ma. These detrital ages closely match those recorded from the Cosoltepec Formation of the Acatlán Complex and support correlation of the two units, which are both interpreted to be vestiges of the southern margin of the Rheic Ocean.

Abstract Detrital zircon age populations from Palaeozoic sedimentary and metasedimentary rocks in Mexico support palinspastic linkages to the northwestern margin of Gondwana (Amazonia) during the late Proterozoic–Palaeozoic. Age data from: (1) the latest Cambrian-Pennsylvanian cover of the c . 1 Ga Oaxacan Complex of southern Mexico; (2) the ?Cambro-Ordovician to Triassic Acatlán Complex of southern Mexico's Mixteca terrane; and (3) the ?Silurian Granjeno Schist of northeastern Mexico's Sierra Madre terrane, collectively suggest Precambrian provenances in: (1) the c . 500–650 Ma Brasiliano orogens and c . 600–950 Ma Goias magmatic arc of South America, the Pan-African Maya terrane of the Yucatan Peninsula, and/or the c . 550–600 Ma basement that potentially underlies parts of the Acatlán Complex; (2) the Oaxaquia terrane or other c . 1 Ga basement complexes of the northern Andes; and (3) c . 1.4–3.0 Ga cratonic provinces that most closely match those of Amazonia. Exhumation within the Acatlán Complex of c . 440–480 Ma granitoids prior to the Late Devonian–early Mississippian, and c . 290 Ma granitoids in the early Permian, likely provided additional sources in the Palaeozoic. The detrital age data support the broad correlation of Palaeozoic strata in the Mixteca and Sierra Madre terranes, and suggest that, rather than representing vestiges of Iapetus or earlier oceanic tracts as has previously been proposed, both were deposited along the southern, Gondwanan (Oaxaquia) margin of the Rheic Ocean and were accreted to Laurentia during the assembly of Pangaea in the late Palaeozoic.

The Piaxtla Suite of the Acatlán Complex (southern México) has previously been considered a vestige of the Iapetus Ocean that underwent eclogite-facies metamorphism during Late Ordovician subduction and exhumation. Study of granitoid, mafic, and metasedimentary rocks of the Asis Lithodeme of the Piaxtla Suite reveals a complex tectonothermal history involving: (1) eclogite-facies syntectonic metamorphism preserved as aligned omphacite in mafic lenses dated at 346 ± 3 Ma (concordant U-Pb zircon age), which is inferred to result from subduction; (2) polyphase deformation involving WSW-ENE tectonic transport under amphibolite-facies conditions accompanied by migmatization due to decompression melting dated at ca. 347–330 Ma (SHRIMP [sensitive high-resolution ion microprobe] zircon ages); (3) continued deformation under greenschist facies; and (4) development of several phases of late folds and crenulation cleavage. Pressure, temperature, and time ( P-T-t ) data suggest rapid isothermal decompression from eclogite to upper amphibolite facies during the Visean (Middle Mississippian) followed by cooling. In the absence of age data for the latter stage, nearby unmetamorphosed latest Upper Devonian sedimentary rocks contain metamorphic, Piaxtla Suite clasts suggesting either diachronism in the exhumation process or synchronous exhumation and subsidence in adjacent areas. Such rapid exhumation of eclogites is typical of continent-continent collision zones, and the subhorizontal, WSW-ENE kinematics is compatible with either lateral thrust ramping or extension in the orogen. Devonian–Carboniferous subduction and exhumation are incompatible with an origin within the Iapetus Ocean, because that ocean had closed by Silurian times. However, they are consistent with oblique subduction of the leading edge of Gondwana along the southern flank of the Rheic Ocean during the amalgamation of Pangea.

Abstract The Rheic Ocean formed during the Late Cambrian–Early Ordovician when peri-Gondwanan terranes (e.g. Avalonia) drifted from the northern margin of Gondwana, and was consumed during the collision between Laurussia and Gondwana and the amalgamation of Pangaea. Several mafic complexes, from the Acatlán Complex in Mexico to the Bohemian Massif in eastern Europe, have been interpreted to represent vestiges of the Rheic Ocean. Most of these complexes are either Late Cambrian–Early Ordovician or Late Palaeozoic in age. Late Cambrian–Early Ordovician complexes are predominantly rift-related continental tholeiites, derived from an enriched c. 1.0 Ga subcontinental lithospheric mantle, and are associated with crustally-derived felsic volcanic rocks. These complexes are widespread and virtually coeval along the length of the Gondwanan margin. They reflect magmatism that accompanied the early stages of rifting and the formation of the Rheic Ocean, and they remained along the Gondwanan margin to form part of a passive margin succession as Avalonia and other peri-Gondwanan terranes drifted northward. True ophiolitic complexes of this age are rare, a notable exception occurring in NW Iberia where they display ensimatic arc geochemical affinities. These complexes were thrust over, or extruded into, the Gondwanan margin during the Late Devonian–Carboniferous collision between Gondwana and Laurussia (Variscan orogeny). The Late Palaeozoic mafic complexes (Devonian and Carboniferous) preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by derivation from an anomalous mantle which displays time-integrated depletion in Nd relative to Sm. Devonian ophiolites pre-date closure of the Rheic Ocean. Although their tectonic setting is controversial, there is a consensus that most of them reflect narrow tracts of oceanic crust that originated along the Laurussian margin, but were thrust over Gondwana during Variscan orogenesis. The relationship of the Carboniferous ophiolites to the Rheic Ocean sensu stricto is unclear, but some of them apparently formed in a strike-slip regimes within a collisional setting directly related to the final stages of the closure of the Rheic Ocean.

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.