Abstract Two different types of ‘transitional lithosphere’ have been documented along magma-poor rifted margins. One consists of apparently subcontinental mantle that has been exhumed, brittlely deformed, and serpentinized during late stages of rifting. A second is thinned (<10 km) continental crust, which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases, it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter, and thus that transitional lithosphere has relatively limited width. Here three representative cases of transitional lithosphere are examined: one in the Newfoundland–Iberia rift and one at Goban Spur (both exhumed mantle), and one off the Angola–Congo margin (thin continental crust flanked seaward by apparently exhumed lower continental crust±exhumed mantle). Considering the geological and geophysical evidence, it appears that depth-dependent extension (riftward flow of weak lower continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed. Transitional lithosphere in this wide zone may consist of subcontinental mantle, lower continental crust or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional lithosphere ceases to be emplaced (i.e. ‘final break-up’ occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage break-up: first, the rupture of the brittle continental crust; and, second, the eventual separation of the ductile subcontinental lithosphere which is coincident with emplacement of normal oceanic crust. Well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpentinized, exhumed mantle, and such anomalies therefore are not diagnostic of oceanic crust. Where present, the anomalies can be helpful in interpreting and dating the rifting history.

Abstract The idea that the Decade of North American Geology (DNAG) project should include preparation of a new geologic map of the continent was conceived early in the DNAG planning process. The minutes of a meeting of the Steering Committee chaired by L.T. Silver on January 29–30, 1980, record that: It was generally agreed that the geographic scope [of the DNAG project] would extend from the Arctic Ocean on the north to the southern limits of the Caribbean plate; from the Mid-Atlantic ridge on the east to the Pacifi c plate in the approximate vicinity of Hawaii. The emphasis would be placed on the geology of the continent; the adjacent sea fl oor would be carried as it is related to the continental story.… At the same meeting “the need for a new geologic map was discussed extensively with some disagreement.” However, at a meeting in May of the same year, a subcommittee appointed to examine the need for a new geologic map unanimously supported the proposal. It estimated that publication costs might be as much as $200,000, compilation costs might be $500,000, and the time required for compilation would be about 5 years. The Steering Committee agreed that a new geologic map covering the area of the DNAG project was needed, and placed compilation of the map on the list of official DNAG efforts. By May 1981, the compilers and principal cartographers had been selected, the base map chosen, the essential features of the explanation agreed upon, and

Abstract “The new Geologic Map of North America covers ~15% of Earth’s surface and differs from previous maps in several important respects: It is the first such map to depict the geology of the seafloor, the first compiled since the general acceptance of plate-tectonic theory, and the first since radiometric dates for plutonic and volcanic rocks became widely available. It also reflects enormous advances in conventional geologic mapping, advances that have led to a significant increase in the complexity of the map. The new map, printed in 11 colors, distinguishes more than 900 rock units, 110 of which are offshore. It depicts more than seven times the number of on-land units as are shown on its immediate predecessor, as well as many more faults and additional features such as volcanoes, calderas, impact structures, small bodies of unusual igneous rocks, and diapirs. When displayed at earth science institutions and libraries, this map is sure to impress viewers with the grand design of the continent and may inspire some to pursue the science of geology. The new Geologic Map of North America is also a “thinking map,” a source for new interpretations of the geology of North America, insights into the evolution of the continent, new exploration strategies for the discovery of mineral and energy resources, and the development of better ways to assess and mitigate environmental risks and geologic hazards.3 sheets (North, South, and Legend), approximately 74 x 40 inches.”

Abstract The Greater Antilles Outer Ridge, located north and northwest of the Puerto Rico Trench, is a deep (>5100 m), distal sediment drift more than 900 km long and up to 1 km thick. It has been isolated from sources of downslope sedimentation throughout its history and is formed of clay- to fine silt-size terrigenous sediments that have been deposited from suspended load carried in the Western Boundary Undercurrent, together with 0-30% pelagic foraminiferal carbonate. Because of the fine, relatively uniform grain size of the sediments, the outer ridge consists of sediments that are seismically transparent in low-frequency reflection profiles. Sediment tracers (chlorite in sediments and suspended particulate matter, reddish clays in cores) indicate that at least a portion of the ridge sediments has been transported more than 2000 km from the eastern margin of North America north of 40°N. The outer ridge began to develop as early as the beginning of Oligocene time when strong, deep thermohaline circulation developed in the North Atlantic and the trough initiating the present Puerto Rico Trench had cut off downslope sedimentation from the Greater Antilles. The fastest growth of the outer ridge probably occurred beginning in the early Miocene, about the same time that large drifts such as the Blake Outer Ridge were initiated along the North American margin. Since that time, the most rapid sedimentation has been along the crest of the northwestern outer ridge where suspended load is deposited in a shear zone between opposing currents on the two ridge flanks

Abstract A multichannel seismic-reflection survey of the Newfoundland basin southeast of the Grand Banks was conducted to investigate the rift-drift history of the basin and to examine the nature and location of the continent-ocean boundary. The data suggest that the continent-ocean boundary is marked by the “J” magnetic anomaly (M1-M0), which separates crust having a relatively smooth magnetic field to the west from higher amplitude anomalies to the east. Basement at the boundary is characterized by a set of large volcanic ridges that were constructed simultaneously with the adjacent Southeast New-foundland Ridge and the J-Anomaly Ridge. The ridges appear to have formed from magma derived from a mantle plume then located beneath the Southeast Newfoundland Ridge. The Newfoundland basin west of the interpreted continent-ocean boundary exhibits two distinct structural provinces in basement. A large (60 × 400 km) rift basin, the Salar basin, occurs beneath the continental slope and upper rise and is filled with evaporites that are probably Late Triassic to Early Jurassic in age. Seaward of the Salar basin, a zone of complexly faulted basement blocks with intervening, probably synrift, sedimentary and vol-canic fill extends to the J anomaly. Both the blocks and fill are capped by a high-amplitude, relatively flat seismic unconformity. The unconformity pinches out at the J anomaly and appears to corre-late with the “U,” or Avalon, breakup unconform-ity on the adjacent Grand Banks. Late Triassic rifting formed the Salar basin and caused an unknown amount of extension in adja-cent continental crust to the east. A second phase of Late Jurassic through Early Cretaceous rifting reac-tivated parts of the western margin of the Salar basin and probably caused substantial extension and thinning of continental crust to the east. The rifting culminated in broad uplift or doming of this extended crust, erosion of the breakup unconform-ity, and Barremian-Aptian emplacement of volcanic ridges marking the continent-ocean boundary at the J anomaly. All crustal elements (extended continental crust, J-Anomaly ridges, oldest oceanic crust) appear to have experienced unusually rapid subsidence, much faster than age/depth relationships of normal oceanic crust, following continental breakup.

Abstract In creating Volume M, the Western North Atlantic region (Vogt and Tucholke, 1986a) for the Geology of North America series, we deemed it best from both oceanographic and plate-tectonic viewpoints to deal with the entire North Atlantic spreading system from the equator to the Arctic (Figs. 1 and 2), rather than limiting treatment to the western half of the ocean basin. Even so, the scope in some places had to be expanded. The Atlantic, like other ocean basins, did not evolve in isolation from global changes in tectonic regime, oceanic circulation, or climate patterns (Fig. 3). The development of plate-tectonic theory since the late 1960s clearly has emphasized the importance of these large-scale linkages. The present chapter continues this philosophy, summarizing the geology of the North Atlantic but noting linkages to areas outside this ocean basin. The synthesis is based largely on material presented in Volume M. The citation or lack of citation of Volume Μ references here, however, reflects only the thematic fabric of the present synthesis, not the scientific merit of the chapters. We refer the reader to original sources in Volume Μ for more complete treatment. We begin this chapter by noting ties between Volume Μ and several other Geology of North America volumes, and we continue with some “vital statistics” that describe three basic components of the Atlantic in space and time: igneous crust, sediments, and ocean waters. This is followed by a discussion of scales of spatial and temporal variability, with emphasis on the latter. The chapter concludes with a summary of some of the important advances that have occurred in the three years since Volume Μ was published.

Abstract This volume deals with the geology and geophysics of the western North Atlantic, the basin between the Mid-Atlantic Ridge and the eastern margin of North America. Although the book’s focus is on the “North American Basin” (Fig. 1), individual chapters and charts extend the bounds beyond the “western North Atlantic.” These extensions were dictated by the geology, which cannot be synthesized meaningfully if constrained by artificial geographic or political boundaries. Generally speaking, the content of this volume is contained within the region surrounded by the continental margins of the Antilles, eastern North America, Greenland, and Iceland in the west and north, the Mid- Atlantic Ridge crest on the east, and approximately 10°–15° N latitude to the south. However, in dealing with the Mid-Atlantic Ridge, plate kinematic models, and Atlantic paleogeography, we have extended our scope north into the Arctic region and east into the eastern Atlantic in order to present a complete synthesis of this major plate boundary and its evolution through time. The significance of Iceland for oceanic geoscience often has been overlooked or underrated; to help remedy this, two chapters deal mainly with this remarkable subaerial exposure of the Mid-Atlantic Ridge. One topic, mid-plate seismicity and stress, also demanded a plate-wide treatment covering the entire “stable” lithosphere from the Mid-Atlantic Ridge to the western Cordillera. Placing this treatment in the North Atlantic synthesis presumes that mid-plate stress is closely related to absolute plate motion (which is calculated largely from oceanic data) and/or to “ridge-push” forces from the