Abstract Avalonia, defined by its distinctive uppermost Ediacaran–Ordovician overstep sequence, extends from New England through Atlantic Canada to Wales. It unconformably overlies: (1) parts of one cratonic Neoproterozoic arc that which records several pulses at: 760–730 Ma, 680–600 Ma and 580–540 Ma; (2) an 800–760 Ma passive margin sequence; and (3) c. 976 Ma isolated plutons, possibly basement. Comparisons with modern arc dimensions suggest the dip of the Benioff Zone ranged from c. 22° W in Newfoundland to c. 52–67° elsewhere. A 600–580 Ma hiatus in arc magmatism in Cape Breton Island is attributed to overriding an oceanic plateau, leading to a 15° decrease in the dip of the Benioff Zone. The Collector magnetic anomaly along the Grand Banks and the Minas Fault is inferred to mark the Neoproterozoic southern margin of the Avalon Plate consisting of leaky transform faults and trench segments characterized by magnetite serpentinite mantle wedge beneath forearcs. The Minas Fault/Collector Anomaly connects similar arc units in Cape Breton Island and southern New Brunswick, suggesting that they were already offset by the Minas transform fault in the late Neoproterozoic. Similar tectonic, palaeomagnetic and isotopic data in the Timan Orogen of Baltica suggest that Avalonia may correlate with the Kipchak arc.

Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern México reveal a geological evolution characterized by Upper Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, prior to the onset of magmatism in the Trans-Mexican Volcanic Belt. Orogenic deformation began in the Late Cretaceous and was coeval with the Laramide orogeny with structures of similar orientation. Deformation consisted of E-W shortening that migrated to the east with time and with a general easterly vergence. Models that relate the Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western México, cannot be applied in southern México because Paleocene to upper Eocene arc magmatism occurs near the inferred paleotrench. An alternative possible origin due to collision of an insular arc against the western margin of México suffers from an absence of features and petrogenetic associations indicating the closure of an oceanic basin. In light of recent geochronological data, the general pattern of magmatic extinction from Upper Cretaceous–Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca presents some variations inconsistent with a simple pattern of extinction toward the E-SE. Maastrichtian to lower Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporaneous with a magmatic episode that has some documented adakitic affinities in the central part of the Sierra Madre del Sur. Magmatism from the Paleocene to middle Eocene seems to be concentrated in the Presa del Infiernillo area, although isolated centers existed in areas such as Taxco or the eastern Jalisco block. Finally, the main axis of magmatism between the middle Eocene and Oligocene developed along what is the present-day continental margin and extends 200 km inland as a broad band. In general, the geochemical characteristics of this magmatism indicate a low degree of continental crustal assimilation. Two episodes of principally sinistral lateral faulting that activated NW-SE– and later N-S–oriented faults, with variations in time and space, have been documented during the Eocene and lower Oligocene. The N-S set of faults was active only in the north of the Sierra Madre del Sur, whereas the activity of the NW-SE set continued during the Oligocene along the Oaxaca continental margin. The recognition of these deformational episodes suggests that extensional directions related to lateral faulting changed from NNW-SSE to NE-SW, and locally produced normal displacements on preexisting discontinuities. Fundamental problems still exist in the interpretation of the plate tectonic processes that produced the stress regimes acting on the different sets of faults, as well as in the determination of the factors influencing the migration of magmatism. Some of the arguments used to postulate the presence of the Chortis block off the southwestern Mexican continental margin during the early Cenozoic are uncertain. On the other hand, models that explain restricted displacements of the Chortis block with respect to the Maya block—without juxtaposition with the southwestern margin of México—suggest that continental truncation was essentially caused by subduction erosion and leave open the interpretation of the observed magmatic migration.