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 Abyssal currents strong enough to erode the sea floor and carry suspended sediment great distances have been detected in many of the world's oceans. In contrast to episodic turbidity currents that flow down-slope and deposit entrained sediments in expansive and level turbidites, these currents are the expression of continual thermohaline circulation that typically flows along bathymetric contours and deposits transported sediments as cryptically-bedded "contourites" (Heezen and Hollister, 1964). The Western Boundery Undercurrent is an example of such a current. Most of its water mass is derived from sources in the Norwegian-Greenland Sea, and in its equator-ward, anti-clockwise flow along the margin of the North Atlantic Basin, it controls the growth of numerous "drift" deposits such as the Feni, Hatton, Gardar and Erik ridges in the north and the Hatteras, Blake-Bahama, Caicos and Greater Antilles outer ridges in the south. Seismic profiles across these drifts show that some contourite deposits have grown in a complex and irregular manner, while in others the conditions of erosion, transport and deposition have remained surprisingly uniform for millions of years. The accompanying profile is an example of the latter. The profile is a portion of multichannel line 77 collected and processed by Lamont-Doherty Geological Observatory. Asingle-channel analog version of this same profile has been published elsewhere (Tucholke and Mountain, 1979; Tucholke and Laine, in press). The profile is located on the lower continental rise 300 km (186 mi) east of North Carolina where it crosses the crest of the Hatteras Outer Ridge (see accompanying location map). This sediment drift was first identified near its southern end at 330N where it has clear topographic expression (Rona, 1969). The ridge is roughly 550 km (341 mi) long by 150 km (93 mi) wide, and is oriented parallel to regional bathymetric contours. The ridge's western flank is buried by Pleistocene turbidites that form the lower continental rise terrace. The undulating swells of the exposed seaward flank of the ridge have been termed the "lower continental rise hills," and their origin has been variously attributed to gravity slides, abyssal current deposition, and erosion by turbidity currents (see Tucholke and Laine, in press). However, as can be deduced from this profile alone, the hills are the modern expression of large sediment waves that have been accreting for a significant length of time. They are oriented nearly east-to-west in the area of this profile (Asquith, 1979); thus their trend is inclined 300 to the direction of the prevailing current, much like sediment waves in other areas (Flood and Hollister, 1974; Embley et al, 1980). This profile was collected 24-fold and was recorded at a 4 msec sampling rate. The processing applied after stacking included spherical divergence correction, predictive deconvolution, time-varying filtering from 15 to 80 Hz, time-varying gain and trace equalization. These parameters were balanced to show the structures at moderate depth with maximum clarity; consequently the resolution in the uppermost 150 msecs of the sediment column is less than ideal. However, Asquith (1979) showed with 3.5 kHz echosounder records that the migrating sediment waves visible on the accompanying profile continue upward to the sea floor.

Abstract Tightly curved mountain belts are prominent features of global topography. Typically, these ‘oroclines’ occur in areas of regional compression but enclose basins where extension has been contemporaneous with outward directed thrusting in the orogens. Examples of such basin–orogen pairs include the Alboran Sea–Gibraltar Arc, Tyrrhenian Sea–Aeolian Arc, Aegean Sea–Hellenic Arc and Pannonian Basin–Carpathian Arc, all in the western Tethys but matched in the eastern Tethys by the Banda Sea and Outer Banda Arc. The development of the basins has been variously explained by gravitational collapse of rapidly elevated mountain blocks and by extrusion prompted by asthenopheric flows, but it is not even universally agreed that similar processes have operated in all cases. Critics have cited gross differences in volcanic activity (absent from the Gibraltar Arc, modest in the Carpathians but intense in other examples) and in the presence or absence of recognizable Wadati–Benioff Zones. The superficial similarities between the Caribbean Sea–Antilles Arcs and typical oroclinal basin–orocline pairs have recently been invoked in support of an in situ Caribbean evolutionary model, even though the disputed origins of oroclines limit their reliability as analogues. The Caribbean's considerably greater area further emphasizes the need for caution, while the most obvious objection to identifying it as a member of the oroclinal group is its very long history. Oroclinal basins typically pass from initiation to effective stabilization in a few tens of millions of years, whereas the original Caribbean oceanic crust, which is now bounded to the east and west by active subduction zones, was probably formed in the Jurassic. Rather than invoking an overall common origin for the Caribbean and the Tethyan basins, it is more useful to look for shared causes of specific individual similarities. The impact of a rigid block might be as effective in imposing curvature on a mountain belt as rapid expansion in an adjacent area. However, it does seem that the case for the crust of the Caribbean being typical of oceanic large igneous provinces (LIPs) may have been overstated and, in the light of oroclinal analogues, that some features of the still poorly understood Beata Ridge and Lower Nicaragua Rise may be most easily explained by east–west extension promoted by the convergence between North and South America.

The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. The free-air minimum east of the Lesser Antilles is related to underthrusting of the Caribbean plate by the Atlantic Ocean plate. Here the axis of the free-air minimum lies very close to the eastern border of a zone of epicenters which apparently marks the commencement of crustal faulting along the underthrust. The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. It is postulated that differential shifts between the Caribbean plate and the underthrust Atlantic plate have occurred at least twice, once in late Eocene to early Oligocene, during which the outer island chain of the northern Lesser Antillean arc was formed, and again sometime since the late Miocene, when the Barbados ridge and associated uplifted topography were formed. This latter deformation caused the disappearance of a trench opposite the southern Lesser Antillean arc and a displacement of the Puerto Rico trench axis oceanward away from the axis of the negative free-air anomaly belt near the northeast corner of the Caribbean plate.