Abstract The geological evolution of Avalonia was fundamental to the first application of plate tectonic principles to the pre-Mesozoic world. Four tectonic phases have now been identified. The oldest phase (760–660 Ma) produced a series of oceanic arcs, some possibly underlain by thin slivers of Baltica crust, which accreted to the northern margin of Gondwana between 670 and 650 Ma. Their accretion to Gondwana may be geodynamically related to the break-up of Rodinia. After accretion, subduction zones stepped outboard, producing the main phase (640–570 Ma) of arc-related magmatism and basin formation that was coeval with the amalgamation of Gondwana. Arc magmatism terminated diachronously between 600 and 540 Ma by the propagation of a San Andreas style transform fault, followed by the Early Paleozoic platformal succession used by Wilson to define the eastern flank of the proto-Atlantic (Iapetus) Ocean. This implies the ocean outboard from the northern Gondwanan margin survived into the Cambrian. Avalonia is one of several terranes distributed obliquely with respect to the adjacent cratonic provinces of Gondwana and Baltica, implying that these terranes evolved on different cratonic basements. As a result, their ages and differing isotopic signatures can be used to reconstruct their respective locations along the ancient continental margin.

Abstract The geology of southwestern Mexico (102–96°W) records several synchronous events in the Late Oligocene–Early Miocene (29–19 Ma): (1) a hiatus in arc magmatism; (2) removal of a wide ( c . 210 km) Upper Eocene–Lower Oligocene forearc; (3) exhumation of 13–20 km of Upper Eocene–Lower Oligocene arc along the present day coast; and (4) breakup of the Farallon Plate. Events 2 and 3 have traditionally been related to eastward displacement of the Chortís Block from a position off southwestern Mexico between 105°W and 97°W; however at 30 Ma the Chortís Block would have lain east of 95°W. We suggest that the magmatic hiatus was caused by subduction of the forearc, which replaced the mantle wedge by relatively cool crust. Assuming that the subducted block separated along the forearc–arc boundary, a likely zone of weakness due to magmatism, the subducted forearc is estimated to be wedge-shaped varying from zero to c . 90 km in thickness; however such a wedge is not apparent in seismic data across central Mexico. Given the 121 km/Ma convergence rate between 20 and 10 Ma and 67 km/Ma since 10 Ma, it is probable that any forearc has been deeply subducted. Potential causes for subduction of the forearc include collision of an oceanic plateau with the trench, and a change in plate kinematics synchronous with breakup of the Farallon Plate and initiation of the Guadalupe–Nazca spreading ridge.

Abstract Plate tectonics provide a unifying conceptual framework for the understanding of Phanerozoic orogens. More controversially, recent syntheses apply these principles as far back as the Early Archaean. Many ancient orogens are, however, poorly preserved and the processes responsible for them are not well understood. The effects of processes such as delamination, subduction of oceanic and aseismic ridges, overriding of plumes and subduction erosion are rarely identified in ancient orogens, although they have a profound effect on Cenozoic orogens. However, deeply eroded ancient orogens provide insights into the hidden roots of modern orogens. Recent advances in analytical techniques, as well as in fields such as geodynamics, have provided fresh insights into ancient orogenic belts, so that realistic modern analogies can now be applied. This Special Publication offers up-to-date reviews and models for some of the most important orogenic belts developed over the past 2.5 billion years of Earth history.

Abstract The Middle Miocene, thin-skinned, Chiapas fold-and-thrust belt (Gulf of Mexico–southeastern Mexico–Belize) consists of WNW-trending folds and thrusts, and East–West sinistral transcurrent faults resulting from N60°E shortening. Balanced cross-sections indicate that shortening varies from 48% (SW) to c . 8% (NE) with a total shortening of 106 km, and that thrusts merge into a basal décollement in the Callovian salt horizon. The Middle Miocene age of the deformation is synchronous with collision of the Tehuantepec Transform/Ridge with the Middle America Trench off Chiapas. The presently exposed Tehuantepec Transform/Ridge varies from a transform fault across which the age of the oceanic crust changes producing a step (down to the east) to a ridge resulting from compression following a change in plate motion and a series of seamounts. On the other hand, the earthquake data show that the part of the Tehuantepec Transform/Ridge subducted during the past 5 Ma is a step with no accompanying ridge. Whereas collision of a ridge segment with the trench is inferred to be responsible for the 13–11 Ma deformation in the upper plate, its termination at 11 Ma suggests an along-strike transition to a step. Collision of the Tehuantepec Transform/Ridge also appears to have terminated arc magmatism along the Pacific coast of Chiapas. The similarity between the petroleum-producing, Cantarell structure in the Sonda de Campeche and the buried foldbelt in the Sierra de Chiapas suggests there is considerable further hydrocarbon potential.

Abstract The volcanic Triassic Takla Group constitutes a significant part of Stikinia and Quesnellia, two major terranes of the Canadian Cordillera that are separated by high-pressure rocks of the Cache Creek terrane containing Asian fauna. The geochemical and isotopic characteristics of the Takla Group in Quesnellia and Stikinia are similar, that is, tholeiitic basalts characterized by low abundances of strongly incompatible trace elements, negative Nb anomalies, +6 to +8 ɛ Nd values, the low initial Sr isotopic ratios, and relatively horizontal chondrite-normalized heavy REE patterns, all features typical of relatively primitive arcs with little or no continental crust involvement. These similarities have led to several geometric models: post-Middle Jurassic duplication by strike-slip faulting, and oroclinal or synformal folding. However, they are all inconsistent with either palaeomagnetic or faunal data, and the presence of a Triassic overstep sequence, which indicates amalgamation c . 50 ma before emplacement of the youngest oceanic rocks of the Cache Creek terrane. An alternative model is proposed: oblique eastward subduction of the Cache Creek accretionary prism and fore-arc producing high-pressure metamorphism, followed by extrusion into the arc and exhumation by the Middle Jurassic. This model implies that these high-pressure rocks, rather than marking an oceanic suture between disparate arc terranes, support a para-autochthonous origin.

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.

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 Within the Appalachian–Variscan orogen of North America and southern Europe lie a collection of terranes that were distributed along the northern margin of West Gondwana in the late Neoproterozoic and early Palaeozoic. These peri-Gondwanan terranes are characterized by voluminous late Neoproterozoic ( c . 640–570 Ma) arc magmatism and cogenetic basins, and their tectonothermal histories provide fundamental constraints on the palaeogeography of this margin and on palaeocontinental reconstructions for this important period in Earth history. Field and geochemical studies indicate that arc magmatism generally terminated diachronously with the formation of a transform margin, leading by the Early–Middle Cambrian to the development of a shallow-marine platform–passive margin characterized by Gondwanan fauna. However, important differences exist between these terranes that constrain their relative palaeogeography in the late Neoproterozoic and permit changes in the geometry of the margin from the late Neoproterozoic to the Early Cambrian to be reconstructed. On the basis of basement isotopic composition, the terranes can be subdivided into: (1) Avalonian-type (e.g. West Avalonia, East Avalonia, Meguma, Carolinia, Moravia–Silesia), which developed on juvenile, c . 1.3–1.0 Ga crust originating within the Panthalassa-like Mirovoi Ocean surrounding Rodinia, and which were accreted to the northern Gondwanan margin by c . 650 Ma; (2) Cadomian-type (e.g. North Armorican Massif, Ossa–Morena, Saxo-Thuringia, Moldanubia), which formed along the West African margin by recycling ancient ( c . 2.0–2.2 Ga) West African crust; (3) Ganderian-type (e.g. Ganderia, Florida, the Maya terrane and possible the NW Iberian domain and South Armorican Massif), which formed along the Amazonian margin of Gondwana by recycling Avalonian and older Amazonian basement; and (4) cratonic terranes (e.g. Oaxaquia and the Chortis block), which represent displaced Amazonian portions of cratonic Gondwana. These contrasts imply the existence of fundamental sutures between these terranes prior to c . 650 Ma. Derivation of the Cadomian-type terranes from the West African craton is further supported by detrital zircon data from their Neoproterozoic–Ediacaran clastic rocks, which contrast with such data from the Avalonian- and Ganderian-type terranes that suggest derivation from the Amazonian craton. Differences in Neoproterozoic and Ediacaran palaeogeography are also matched in some terranes by contrasts in Cambrian faunal and sedimentary provenance data. Platformal assemblages in certain Avalonian-type terranes (e.g. West Avalonia and East Avalonia) have cool-water, high-latitude fauna and detrital zircon signatures consistent with proximity to the Amazonian craton. Conversely, platformal assemblages in certain Cadomian-type terranes (e.g. North Armorican Massif, Ossa–Morena) show a transition from tropical to temperate waters and detrital zircon signatures that suggest continuing proximity to the West African craton. Other terranes (e.g. NW Iberian domain, Meguma) show Avalonian-type basement and/or detrital zircon signatures in the Neoproterozoic, but develop Cadomian-type signatures in the Cambrian. This change suggests tectonic slivering and lateral transport of terranes along the northern margin of West Gondwana consistent with the transform termination of arc magmatism. In the early Palaeozoic, several peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Ganderia, Meguma) separated from West Gondwana, either separately or together, and had accreted to Laurentia by the Silurian–Devonian. Others (e.g. Cadomian-type terranes, Florida, Maya terrane, Oaxaquia, Chortis block) remained attached to Gondwana and were transferred to Laurussia only with the closure of the Rheic Ocean in the late Palaeozoic.

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.