Abstract The Equatorial Atlantic evolved from a transform margin to an oblique-passive margin from the early rifting to early drifting stages. The entire Equatorial region of the South Atlantic behaved as a global-scale accomodation zone linking the evolving Central and Southern Atlantic ocean basins. Lithospheric keels and the roots of thick, stable Proterozoic cratons worked as an anchor, preventing and postponing the continental rupture. The transition, from a continental transform margin to an oblique-passive margin, lasted approximatelly 10 m.y. Once oceanic lithosphere started to be created at the main transtensional segments, large offset transform faults developed. Remarkable differences are observed between adjacent basins. Deformation partitioning ocurred at the equatorial margin due to the coaxiality of the progressive deformation. Diachronous deformation occurred as a function of the degree of obliquity of each basin at a specific time. Individual segments of the margin are associated with individual strike-slip basins at early rifting stages and have different amounts of obliquity with a decrease in obliquity from south to north. Because, basement faults form and develop during rifting their geometry gets locked at the time of first emplacement of oceanic crust. Therefore, the geometry of the basement and basement faults can be used to reconstruct the geometries of the original strike-slip basins prior to oceanic spreading.

Abstract Isotopic ages in volcanic arc igneous and subduction complex rocks in Venezuela and on the offshore island of Aruba are consistent with the finding that the ages of arc igneous activity and high pressure–low temperature metamorphism in both of those areas are restricted to times between ca 150 Ma and ca 70 Ma. That age range, and the restriction of fossil ages in the subduction complexes to between mid-Jurassic (ca 170 Ma) and late Cretaceous (ca 70 Ma) times, reveals a match to the ages of volcanic arc rocks that were involved in collision with the Andean Margin of Ecuador more than 2000 km (1242.7 mi) away. The similarity of ages can be explained if the rocks in both areas are those of the Great Arc of the Caribbean, which has been considered to have collided with the west coast of South America during the late Cretaceous. Synthesizing results from Ecuador and Colombia shows that in those areas the Great Arc was involved in collisions, first with the Caribbean–Colombian Oceanic Plateau (CCOP) and then with the Andean Margin of South America. By 70 Ma a ca 200-km(124.2-mi)-wide Ribbon Continent consisting of fragments of both the Great Arc of the Caribbean and the CCOP was traveling to the north in a transpressive plate boundary zone (PBZ) along the Colombian coast. By 65 Ma, as the CCOP began to enter the Atlantic Ocean and a newly formed Caribbean plate (CARIB) separated from the Farallon plate, parts of the Ribbon Continent began to be carried in a southern CARIB transform PBZ eastward along the north coast of South America. We characterize three W–E-trending belts in that part of the Ribbon Continent: (1) a Northern Belt consisting largely of Great Arc of the Caribbean intrusions and subduction complex rocks; (2) a Central Belt, very well known in Venezuela, consisting of Great Arc of the Caribbean subduction complex rocks; and (3) a fold-and-thrust belt in the Serrania del Interior and Lara nappes of Venezuela. A receiver function (seismic) study has shown where rocks of the Great Arc of the Caribbean abut the South American continent along an E–W-trending line in Venezuela. We find that rocks of both the subduction complex of the Great Arc and rocks of the Serrania del Interior have been thrust across that boundary in secondary thrusts as the Ribbon Continent has propagated to the east in the South Caribbean transform PBZ. The structure of the north coast of South America is being radically altered by the northward movement of the Maracaibo Block as it escapes from deformation related to the collision of the Panama Arc with Colombia. Restoration of movements within that block during the past 15 My has been essential to reconstruct the structure and history of the Ribbon Continent on the north coast of South America.

This outline of the topographic evolution of Africa tied to the history of the African Surface illustrates how a unique geomorphic history over the past 180 million years reflects the continent's distinctive tectonics. The African Surface is a composite surface of continental extent that developed as a result of erosion following two episodes of the initiation of ocean floor accretion around Afro-Arabia ca. 180 Ma and 125 Ma, respectively. The distinctive tectonic history of the African continent since 180 Ma has been dominated by (1) roughly concentric accretion of ocean floor following those two episodes; (2) slow movement of the continent during the past 200 m.y. over one of Earth's two major large low shear wave velocity provinces (LLSVPs) immediately above the core-mantle boundary; (3) the eruption during the past 200 m.y. of deep mantle plumes that have generated large igneous provinces (LIPs) from the core-mantle boundary only at the edge of the African LLSVP; and (4) two episodes during which basin-and-swell topography developed and abundant intracontinental rifts and much intra-plate volcanism occurred. Those episodes can be attributed to shallow convection resulting from plate pinning, i.e., arrested continental motion, induced by the successive eruption of the Karroo and Afar plumes. Shallow convection during the second plate-pinning episode generated the basins and swells that dominate Africa's present relief. By the early Oligocene, Afro-Arabia was a low-elevation, low-relief land surface largely mantled by deeply weathered rock. When the Afar plume erupted ca. 31 Ma, this Oligocene land surface, defined here as the African Surface, started to be flexed upward on newly forming swells and to be buried in sedimentary basins both in the continental interior and at the continental margins. Today the African Surface has been stripped of its weathered cover and partly or completely eroded from some swells, but it also survives extensively in many areas where a lateritic or bauxitic cover has accordingly been preserved. Great Escarpments, which are best developed in the southern part of the continent, have formed on some swell flanks since the swells began to rise during the past 30 m.y. They separate the high ground on the new swells from low lying areas, and because they face the ocean at some distance from the African coastline, they mimic rift flank escarpments at younger passive margins. The youthful Great Escarpments have developed in places where the original rift flank uplifts formed at the time of continental breakup. Their appearance is therefore deceptive. The African Surface and its overlying bauxites and laterites embody a record of tectonic and environmental change, including episodes of partial flooding by the sea, during a 150-million-year long interval between 180 Ma and 30 Ma. Parts of African Surface history are well known for some areas and for some intervals. Analysis here attempts to integrate local histories and to work out how the surface of Afro-Arabia has evolved on the continental scale over the past ∼180 m.y. We hope that because major landscape development theories have been spawned in Africa, a review that embodies modern tectonic ideas may prove useful in re-evaluation of theory both for Africa itself and for other continents. We recognize that in a continental-scale synthesis such as this, smoothing of local disparities is inevitable. Our expectation is that the ambitious model constructed on the basis of our review will serve as a lightning rod for elaborating alternative views and stimulating future research.

Abstract The Red Sea—Gulf of Aden rift System provides a superb example of the formation of passive continental margins. Three phases are well represented: (1) continental rifting (Gulf of Suez); (2) rift-to-drift transition (northern Red Sea); and (3) sea-floor spreading (Gulf of Aden and southern Red Sea). Recently published radiometric and biostratigraphic ages, outcrop studies, and reflection seismic profiles more tightly constrain the evolution of this rift system. The principal driving force for separation of Arabia from Africa was slab-pull beneath the approaching Urumieh-Dokhtar volcanic arc on the north side of Neotethys. However, the rifting trigger was impingement of the Afar plume beneath northeast Africa at ~31 Ma. Rifting followed quickly thereafter, initiating in the Gulf of Aden, perhaps in the area between Socotra Island and southern Oman. Extension occurred in the central Gulf of Aden by ~29 Ma. Shortly thereafter, at ~27 Ma, rifting jumped to Eritrea, east of the Danakil region. Rifting then spread from Eritrea to Egypt at ~24 Ma, accompanied by a major dike-emplacement event that covered more than 2,000 km in possibly less than 1 Ma. At ~14 Ma, the Levant transform boundary formed, largely isolating the Gulf of Suez from later extension. Constriction of the Suez-Mediterranean and Red Sea-Aden marine connections resulted in widespread evaporite deposition at this time. Sea-floor spreading began in the eastern Gulf of Aden at ~19 Ma, the western Gulf of Aden at ~10 Ma, and in the south-central Red Sea at ~5 Ma. Propagation of the oceanic ridge has taken much longer than the propagation of its continental rift predecessor. Therefore, the rift-to-drift transition is diachronous and is not marked by a specific “breakup” unconformity. The Red Sea sub-basins are each structurally asymmetric during the syn-rift phase and this is seen in the geometries obtained when its present paired conjugate margins are palinspastically restored. During the Late Miocene and Pliocene, regional-scale, intrasalt detachment faulting, salt flowage, and mass-movement of the post-Miocene salt section toward the basin axis masked the deeper fault block geometry of most of the Red Sea basin. This young halokinesis has enormous consequences for hydrocarbon exploration.

Abstract Studies of active plate boundaries have shown that geodetic and earthquake records from historic periods are insufficient for completely understanding fault behavior and regional patterns of crustal movements. Several pioneering studies have demonstrated that geologic studies of the Holocene and late Pleistocene epochs (Sieh, 1981) can provide valuable information on crustal processes, such as seismogenic fault slip and aseismic uplift, which often exhibit long-term temporal and spatial variations. Realization of the importance of the longer, more extensive geologic record of crustal movements has led to many recent advances in neotectonics, a multidisciplinary field emphasizing the relationship between Neogene relative plate motions and structural, sedimentary, volcanic, and earthquake processes. We have previously defined the time span of the "neotectonic" phase of Caribbean development as Neogene (Mann and Burke, 1984a). The selection of the Miocene for the beginning of the period during which neotectonic structures form is consistent with most Caribbean tectonic models that show the approximate configuration of present-day plate boundaries established by the Miocene (see Pindell and Barrett, this volume). Our purpose is to review the neotectonics of the Caribbean Plate with particular emphasis on recent results from geologic studies of major strike-slip fault systems along the northern and southern edges of the plate and subaerial fault systems within arc systems at the eastern and western edges of the plate. Caribbean tectonic studies have traditionally focused on either earthquakes (e.g., Molnar and Sykes, 1969; Sykes and others, 1982) or mapping Paleogene and Cretaceous rocks (e.g., volume edited by Donnelly, 1971).

The Kara impact structure, on the Kara Sea coast of Russia, consists of two adjacent impact craters, the Kara and the Ust-Kara craters. The Kara crater is located on land and has a pre-erosion diameter of about 65 km, whereas the Ust-Kara crater is mostly submarine and has only limited onshore exposure. The diameter of the Ust-Kara crater was earlier suggested to be about 25 km, but recent morphological studies indicate a diameter >70 km. This is not incompatible with Seasat and Geosat data. It has been suggested that the Kara impact event may be associated with the K/T boundary event. Previously reported K-Ar ages showed wide margins of uncertainty and clustered around 60 Ma, while more recent K-Ar determinations seemed to support an association with the K/T boundary. Our own analyses of several 40 Ar- 39 Ar age spectra, however, indicate an age of >70 Ma for the Kara impact event, perhaps close to the age of the Campanian-Maastrichtian boundary. Even if there is no association with the K/T boundary event, a double impact leading to craters of approximately 65 and 80 km diameter must have been an important geologic event with possible global significance. The record of this large-scale impact event may have been preserved in deep-sea sediment cores.

Abstract The pattern of faulting and fault-related deformation in Cretaceous to Neogene rocks, together with the distribution of Neogene sediments, suggest that Jamaica is the site of two complex, right-stepping restraining bends along the strike-slip plate boundary between the North American and Caribbean plates. Block convergence at the eastern bend between the Plantain Garden and Duanvale fault zones is manifest by topographic uplift (>2 km), rapid erosion, and northwest-southeast shortening of Cretaceous and Paleogene metamorphic, volcanic and sedimentary rocks in the Blue Mountains and Wagwater Belt. Limited data on the age of faulting in Jamaica suggest that deformation and uplift related to bends in the faults probably began in the middle to late Miocene and was roughly contemporaneous with initial strike slip along the eastern extension of the Plaintain Garden fault zone in southern Hispaniola. Uplift and deformation at the western bend is less prominent, and for the most part involves carbonate rocks that are cut by numerous west-facing fault scarps thought to be formed by east-dipping high-angle reverse faults. Both bends appear to have nucleated on northwest-striking normal faults that bounded Paleogene rifts. Maps of historic and recorded earthquakes on Jamaica indicate a close spatial association between the two restraining bends and the largest-magnitude events. In Jamaica, as in other active and ancient strike-slip zones, it is unclear how observed compressional deformation relates to the following three mechanisms: (1) restraining bend development or interaction of two parallel, overstepping strike-slip faults; (2) simple shear adjacent to a single strike-slip fault; or (3) end effects caused by termination of a single strike-slip fault.

Alternative fits of the continents around the future site of the Caribbean about 200 Ma ago and alternative relative motions since then of North and South America and of Africa with respect to each other allow a wealth of information, including data tabulated here on the distribution of rift systems; early ocean floor; obducted ocean floor fragments and dated plutons to be assessed in relation to a history of Caribbean development. After an early rift phase, the Gulf of Mexico formed by divergence mainly before the Caribbean itself. Convergence on what are now the northern and southern Caribbean margins during the Cretaceous produced arc-systems and carried the present Caribbean ocean floor, which represents an oceanic plateau, out of the Pacific. Cenozoic convergence in the Lesser Antilles and Central America has been contemporary with more than 1000 km of roughly eastward motion, distributed in wide plate boundary zones, of the Caribbean with respect to both North and South America. Moderate internal deformation of the Caribbean plate is perhaps attributable to its oceanic plateau character because it behaves mechanically in a way that is intermediate between that of normal ocean floor and continent. Although numerous problems remain in Caribbean geology, a framework into which many of them can be accommodated is beginning to emerge.

Plate tectonic interpretations are commonly used for Phanerozoic tectonic features, but there are still differences of opinion regarding the best model for Proterozoic tectonic features. We suggest that it will be more fruitful to apply the plate model as used in Phanerozoic examples than to build a special model based only on Proterozoic data, or to decide ad hoc what modifications of the plate model may be necessary. As a stimulus for discussion and further work we present plate tectonic interpretations for three widely discussed problems in the Proterozoic terranes of eastern North America: the search for a Grenville suture, the relationship between the Grenville orogeny and Keweenawan rifting, and possible relationships between the Labrador Fold Belt and the Canadian Southern Province. We emphasize the separate stages of the Wilson Cycle of ocean opening and closing; examine some of the available data appropriate for plate tectonic interpretations, particularly isotopic dates; and point out new avenues of investigation suggested by the model.

Abstract Aulacogens are long-lived deeply subsiding troughs, at times fault-bounded, that extend at high angles from geosynclines far into adjacent foreland platforms. They are normally located where the geosyncline makes a reentrant angle into the platform. Their fill is contemporaneous with, as thick as, and lithologically similar to the foreland sedimentary wedge of the geosyncline but in addition has periodically erupted alkalic basalt and fanglomerate. Although many aulacogens have suffered mild compressional deformation, tectonic movement within them is mainly vertical; large-scale horizontal translations are rare. Aulacogens are known throughout the Proterozoic and Phanerozoic, and incipient aulacogens occur at reentrants on modern continental margins. The 1700-to-2200-million-year-old Athapuscow Aulacogen of Great Slave Lake began as a deeply subsiding transverse graben during the early miogeoclinal stage of the Coronation Geosyncline. During the orogenic stage of the geosyncline, the aulacogen became a broader downwarp that received abnormally thick exogeosynclinal sediments from the orogenic belt. The aulacogen was compressed mildly, prior to a final stage involving transcurrent faulting, one-sided uplift, and continental fanglomerate sedimentation. The aulacogen is distinguished from the foreland sedimentary wedge of the geosyncline by having paleocurrents parallel rather than transverse to its structural trend, by having high-angle faults rather than low-angle thrusts, by its alkalic basalt volcanism, and by the lack of metamorphism. It is hypothesized that deep-mantle convective plumes produce three-armed radial rift systems (rrr triple junctions) in continents stationary with respect to the plumes. If only two of the arms spread to produce an ocean basin, the third remains as an abandoned rift extending into the continental interior from a reentrant on the new continental margin. For example, the Benue Trough, located in the Gulf of Guinea reentrant on the west coast of Africa, may be such an abandoned rift arm formed during the Cretaceous period at the time of initial rifting of Africa and South America. Inasmuch as new continental margins are predestined to become geosynclines, such abandoned rift arms are juvenile aulacogens. In this model, aulacogens and geosynclines have a common origin but differ in the extent of rifting.