Abstract A three-dimensional (3D) thermal–kinematic modelling approach based on finite-element techniques is used to study lower-crustal viscosity at transform margins during the continent–ocean transform development stage and after the ridge has passed by. Nine modelling scenarios combining different equilibrium surface heat flows and lower-crustal rheologies are studied. Modelling results indicate that substantial parts of the lower crust at transform margins have the potential to flow at geologically appreciable strain rates, which can lead to uplift/subsidence, as well as lateral variations, in upper- and lower-crustal thicknesses and Moho depth. These low-viscosity zones (i.e. parts of the lower crust with effective viscosities of less than 10 18 Pa s) make up distinct ductility distributions that vary in space and time during margin evolution. Three basic ductility patterns and related thermal processes can be identified: reduced lower-crustal viscosities originating at the continental rift and the continent–ocean boundary (COB), respectively; reduced lower-crustal viscosities along the transform caused by the migrating ridge; and the background distribution of lower-crustal ductility resulting from the equilibrium temperature field. Superposition of all three ductility patterns and the complex interaction of the underlying perturbations of the temperature field result in distinct differences in the potential of lower-crustal flow both in space (parallel and perpendicular to the transform) and with time. Thus, modelling results provide templates for understanding lower-crustal flow at transform margins in general and await further studies comparing model predictions with actual field observations.

Abstract In recent years deep-water petroleum exploration has been booming. The Gulf of Mexico, some West African regions, and Brazil are leading this growing activity. Current deep-water plays focus on presalt, subsalt, stratigraphic pinch-out and deep-water folded belt targets. Comparisons of conjugated margins across the Atlantic are commonly used by explorationists to extend the prospectivity of known plays. However, what are deep-water plays? Are they only plays presently located in water depths more than 2000 m water depths or do they also include plays developed initially in relatively deep oceanic tectonic settings but are now underneath shallow water? For that matter, what about plays which developed in relatively shallow water but are now located in water depths greater than the continental slope. Present deep-water tectonic settings are mostly compressional toe-thrust regions of larger massive gravitational collapses related to (A) major deltas (B) allochthonous salt provinces, or (C) subduction-related accretionary wedges. Back-arc extensional basins in deep-water settings are underexplored, except for the Black Sea. Other plays presently in deep waters but from a geologic perspective initially formed in relatively shallow marine water and subsequently subsided are the pre-evaporitic plays of the Campos-Santos basins in Brazil and Angola offshore basins. Some elements of deep-water petroleum systems are still poorly understood. Although the presence of widespread deep-water source beds on oceanic crust is mostly known through DSDP/ODP wells, the role of tectonics and volcanic activity in the generation and maturation has yet to be adequately evaluated. Classical upwelling models intended to explain marine source beds may need to be refined. Note that most current deep-water-plays involve siliciclastics. There is no reason to preclude deep-water carbonate plays, the reservoirs of which are analogous to the outer platform, relatively deep-water pelagic carbonates producing in the Bay of Campeche in Mexico. Traditional seismic stratigraphy views unconformities as being caused by eustatic sea level changes. However, the origin of widespread deep-water unconformities needs to be further elucidated. An increasing number of deep crustal seismic surveys along West Africa show low-angle extensional detachments similar to those found on the Galicia margin of Spain and Portugal and comparable with tectonic styles of the Basin and Range Province of the western United States or else in the western Mediterranean ( e.g. , Western Alps or Betic Cordillera). Classical rifting models always need to be updated to be “on target” with new and often surprising seismic observations. In addition to the four principal types of deep-water plays currently being explored, additional plays should be found in deep-water carbonates, volcanic margins including hot spots or volcanic-lineaments, and subsided rifted systems; these may account for significant yet to be explored future plays in many parts of the “deep-water” world.

A variety of lithostratigraphic units may be outlined within the pre-Cretaceous crystalline basement of the Atlantic and Gulf Coastal Plains. An expanse of undeformed granite occurs within central peninsular Florida; biotite within the pluton yields 40 Ar/ 39 Ar plateau dates of ca. 525 to 530 Ma. The high-level character of the granite and its emplacement age are similar to post-kinematic granitic plutons within northern portions of the Rokelide orogen in Guinea, West Africa. A sequence of calc-alkaline, felsic volcanic-volcaniclastic rocks, together with associated hypabyssal granite, occurs throughout the basement terrane. These are variably altered and display markedly discordant 40 Ar/ 39 Ar age spectra. Crystallization ages are likely best represented by ca. 675 to 690 Ma Rb-Sr whole-rock and 40 Ar/ 39 Ar hornblende plateau ages previously reported for correlative calc-alkaline granites exposed in southeastern Senegal. A Lower Ordovician–Devonian sedimentary sequence with non-Laurentian paleontological characteristics occurs within the Suwannee basin of the northern Florida subsurface. Detrital muscovite from Lower Ordovician sandstones in the succession records 40 Ar/ 39 Ar plateau ages of ca. 500 to 510 Ma. The sequence is similar to that within the Bové basin in Senegal and Guinea. A small area of high-grade metamorphic rocks occurs in the east-central Florida subsurface. Hornblende from this terrane records 40 Ar/ 39 Ar plateau ages of ca. 485 to 495 Ma. The character and post-metamorphic cooling history of these rocks are similar to that of units within the Rokelide orogen in Sierra Leone and Liberia, West Africa. In the subsurface Wiggins Arch of southwestern Mississippi, hornblende and biotite from interlayered gneiss and amphibolite record similar 40 Ar/ 39 Ar plateau ages of ca. 300 to 310 Ma. Phyllite from a lower-grade metasedimentary sequence penetrated in the Wiggins Arch of southwestern Alabama records a 40 Ar/ 39 Ar whole-rock plateau age of ca. 315 Ma. Characteristics of the pre-Cretaceous basement units suggest that they represent an extension of the Mauritanide, Bassaride, and Rokelide orogens of West Africa. There is no apparent record of Paleozoic tectonothermal activity in central and southern portions of the Atlantic and Gulf Coastal Plain subsurface basement terrane. This is in marked contrast to complete late Paleozoic reworking of basement units penetrated in the area of the Wiggins Arch. This suggests that a major dextral transcurrent fault system was likely active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various units of the Atlantic and Gulf Coastal Plain basement are not correlative with any of the northern non-Laurentian terranes exposed in the southern Appalachian orogen (e.g., Carolina terrane of the eastern Piedmont) which had earlier accreted to exterior positions along the eastern margin of the North American craton. These were transported into their present structural positions along a basal décollement during late Paleozoic collision of Gondwana and Laurentia, and are separated from the stranded Gondwana units of the Coastal Plain basement by a suture approximately marked by the Brunswick-Altamaha magnetic anomaly.