Northern Lesser Antilles Research Articles

The parasites of Anolis lizards in the northern Lesser Antilles : I. Patterns of distribution and abundance.

The helminth communities from ten species of lizard on seven islands in the Caribbean were sampled by collecting one hundred specimens of each species. Nine genera of parasites were identified; these included six nematodes, two digeneans and an acanthocephalan. No relationship was discernible between parasite density or abundance and island area or altitude, although dry islands tend to have fewer species of parasites. Anolis lizards of the bimaculatus and wattsi series share similar parasites with four out of nine species common to both series. The parasite community of lizards on these islands is depauperate with respect to similar surveys on the larger islands of the Greater Antilles.On three of the islands lizards were sub-sampled by collecting from moist woodland and more xeric habitats. These data suggest that differences between habitats are as significant as differences between islands in determining parasite burdens. Worm burdens of the commonest parasite species, T. cubensis, increased monotonically with host body size and no evidence was found to suggest that these parasites affect either host survival or fecundity. The sex-ratio of this species correlated with mean abundance of the parasite, with females the dominant sex on islands or in habitats where the parasite was common. This pattern may reflect haplodiploid sexual determination in this species.

The parasites of Anolis lizards the northern Lesser Antilles : II. The structure of the parasite community.

The communities of parasitic helminths from ten species of lizards on seven islands in the Caribbean were examined to ascertain the relative importance of predictable deterministic factors and unpredicatable colonization or extinction events in determining the structure of the parasite community. A simple graphical model of community structure is used as a "null model" to describe the features of a community that are dependent only upon the size of the host population and features of the life histories of the constituent parasite species. This model predicts that parasite species will exhibit a nested pattern of local and global relative abundance. The observed data correspond fairly well to this pattern. The absences of individual parasite species from communities where they might be expected to be present emphasizes the role of stochastic colonization and extinction events in delineating the constituent members of the community on any island.Statistical analysis of the distribution of parasite species per host illustrates that this pattern is random in habitats where parasite species diversity is low, but decreasingly variable in habitats where more diverse parasite communities occur. Increased parasite diversity also leads to an increase in the proportion of hosts that contain mixed species infections. Comparisons of worm burdens from single and mixed species infections within individual hosts suggest that interactions between parasite species only rarely leads to reduced worm burdens.

A Critical Comparison of the Taxon-Cycle and Character-Displacement Models for Size Evolution of Anolis Lizards in the Lesser Antilles

revealed an interesting pattern: on most islands with only one species, that species is of intermediate size, but two-species islands usually contain both a large and a small species (Schoener, 1970; Table 1). The evolution of large and small size has often been interpreted as an example of character displacement (e.g., Schoener, 1988b; Williams 1972; Losos, 1990). Roughgarden and associates (e.g., Roughgarden et al., 1987; Roughgarden and Pacala, 1989; Rummel and Roughgarden 1985a) have proposed an alternative hypothesis; they argue that a taxon cycle is operating in the northern Lesser Antilles in which a larger species invades an island occupied by an intermediate-sized species, and both evolve smaller size until the former species is intermediate in size and the latter species is extinct (Fig. 1). Roughgarden and Pacala (1989) reviewed the pertinent data and concluded that the available evidence overwhelmingly favors the taxon-cycle model. Here I argue that much of this evidence is either inconclusive, incorrect, or more consistent with the character-displacement model.

Geologic evaluation of plate kinematic models for the North American-Caribbean plate boundary zone

Four published plate kinematic models for the present-day North American-Caribbean plate boundary predict significantly different fault behavior along this dominantly strike-slip plate boundary zone (PBZ). Using a computer graphics system, we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the PBZ from Central America to the northern Lesser Antilles. Our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire PBZ. However, all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the PBZ. We conclude that the present-day deformation within the North American-Caribbean plate boundary zone cannot be described by rotation about a single pole. Instead, the Caribbean “plate” appears to be segmented into at least three major “sub-plates” or blocks, each of which requires an individual pole of rotation to describe its motion relative to the North American plate. Sub-plate boundaries correspond to two poorly studied zones of diffuse deformation within the Caribbean plate (Honduras Depression of Central America and Beata Ridge of the Caribbean Sea).

Rb‐Sr and Sm‐Nd isotopic characterization of Eocene volcanic and volcaniclastic rocks from Puerto Rico

Sm‐Nd and Rb‐Sr data are presented for six volcanic and volcaniclastic samples from the Eocene belt and the Toa Baja drill hole of Puerto Rico in order to provide a preliminary isotopic description of Greater Antilles arc rocks. The lava samples have trace and rare earth element compositions typical of island arc rocks. The isotopic compositions of the volcaniclastic samples are displaced from those of the lavas, and suggest that they include an authigenic or detrital crustal component. The similarity of Sr and Nd isotopic ratios of Puerto Rican lavas and northern Lesser Antilles volcanic rocks suggests that they may have been produced from similar magma sources.

Development of a prehistoric interaction sphere in the northern Lesser Antilles

[First paragraph]INTRODUCTION This paper identifies the development of a prehistoric interaction sphere among a specific cluster of northern Lesser Antillean islands, by examining physicalgeography, natural resources and archaeological remains. The islands used for this study are Anguilla, St. Martin-St. Maarten (hereafter to be referred to simply as St. Martin), Dog Island, St. Barthelemew (St. Baits) and their adjacent cays. It is suggested that the processes of divergent evolution and adaptive radiation increased the isolation of economic exploitation and social adaptations, thus allowing for the emergence of a cluster specific interaction sphere.

A PHYLOGENETIC ANALYSIS OF CHARACTER DISPLACEMENT IN CARIBBEAN ANOLIS LIZARDS.

Twenty-seven islands in the Lesser Antilles contain either one or two species of Anolis lizards. On nine of the ten two-species islands, the species differ substantially in size; 16 of the 17 one-species islands harbor an intermediate-sized species. Two processes could produce such a pattern: size adjustment (or character displacement), in which similar-sized species evolve in different directions in sympatry; and size assortment, in which only different-sized species can successfully colonize the same island together. Previous analyses implicitly have assumed that size is evolutionarily plastic and determined solely by recent ecological conditions, and consequently have tested the hypothesis that character displacement has occurred on each of the ten two-species islands. Other studies have focused only on size assortment. By analyzing such patterns in a phylogenetic context, I explicitly consider historical effects and can distinguish between size adjustment and size assortment. Using a minimum evolution algorithm, I assess evidence for size adjustment by partitioning changes in size along branches of the phylogenetic tree. Size evolution appears rare (a minimum of 4-7 instances of substantial size evolution). In the northern (but not the southern) Lesser Antilles, size change was significantly greater when a descendant taxon occurred on a two-species island and its hypothetical ancestor occurred on a one-species island, thus supporting the size adjustment hypothesis, though size adjustment might have occurred only once. The relative rarity of size evolution suggests that size assortment might be responsible for nonrandom patterns. In both the northern and southern Lesser Antilles, a null model of no size assortment is convincingly rejected. Closely related taxa, however, are usually similar in size, and hybridization between species has been reported. Consequently, similar-sized species might not coexist because they interbreed and coalesce into one gene pool. A null model that only allows species from different "clades" to co-occur is rejected for the northern Lesser Antilles, but is ambiguous with regard to the southern Lesser Antilles. Thus, competitive exclusion is probably responsible for the pattern of size assortment in the northern Lesser Antilles; both competitive exclusion and interbreeding of closely related species of similar size might be responsible for the patterns evident in the southern Lesser Antilles.

Thin section petrography of northern Lesser Antilles ceramics

AbstractThin sections from 44 sherds representing eight prehistoric sites on four islands (Barbuda, Montserrat, Anguilla, St. Martin) in the northern Lesser Antilles (West Indies) were examined using a petrographic microscope. Point counting distinguished three temper associations: (1) exclusively volcanic,(2) volcanic and carbonate, and (3) volcanic, carbonate, and grog. Exclusively volcanic or dominantly volcanic (with low carbonate) temper associations occur in all Saladoid and many post‐Saladoid sherds, with plagioclase feld‐spar and volcanic rock fragments being most abundant. Thin sections with significant carbonate content or with grog were restricted to post‐Saladoid sherds from Barbuda and to a lesser extent Anguilla. the presence of volcanic grains in sherds from the limestone islands of Barbuda and Anguilla indicates that volcanic islands in the region served as sources of pottery (or volcanic temper) for these two islands.

Cenozoic Tectonic and Sedimentary Development of the North American-Caribbean Transform Boundary: ABSTRACT

The North American-Caribbean plate boundary consists of strike-slip deformation extending 3,200 km from the Middle American volcanic arc in western Guatemala to the northern Lesser Antilles. The authors have reconstructed the complex sedimentary and tectonic evolution of this zone over the past 65 m.y. by integration of onshore geologic data and offshore geophysical data. Onshore geologic data consists mainly of detailed structural maps and biostratigraphic studies from uplifted sedimentary basins in Hispaniola and Jamaica. Offshore data consists of single and multichannel seismic profiles and side-scan sonar maps. Both onshore and offshore data have been synthesized into a set of computer-based, paleogeographic maps for the following Cenozoic periods: late Paleocene, middle Eocene, late Oligocene, early Miocene, late Miocene, Pliocene, and Holocene. These data allow them to generalize the tectonic evolution of the boundary into three stages. Stage 1: Paleocene to early Eocene rifting and bimodal volcanism associated with formation of the Cayman Trough pullapart basin. Nonmarine to shallow marine sedimentation was controlled by northwest-northeast-striking normal faults formed at a high angle to the direction of plate motion. Stage 2: middle Eocene to early Miocene strike-slip faulting across a broad, California Borderlands-type margin. Shallow marine to marine sedimentation was controlled by strike-slipmore » faults parallel or at a low angle to the direction of plate motion. Stage 3: middle Miocene to present strike-slip faulting along a single major fault. Deep-marine sedimentation was controlled by a major strike-slip fault scarp formed approximately parallel to the direction of plate motion over most of the length of the boundary. Eustatic sea level effects are recognizable in all three stages.« less

Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles

Volcanic rocks from the island of Martinique in the Lesser Antilles island arc are highly diverse isotopically. Values of δ 18O (‰ SMOW) in lavas erupted over the last 5 Ma from five stratigraphically discrete and geochemically distinct volcanic centers are: Morne Jacob= +7.5to+7.9‰;Pitons du Carbet= +8.0to+8.9‰;Diamant= +6.3to+7.4‰;Piton Mt. Conil, +7.2to+7.3‰, andMt. Pelée= +6.5to+7.4‰. 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low-temperature hydration. The least 18O-rich andesites on Martinique have δ 18O values similar to those of the northern Lesser Antilles ( δ 18O = +6.0 to +7.3‰ ) and other intra-oceanic arcs like the Marianas ( δ 18O = +5.5 to +6.8‰ ). O-isotope ratios are compositionally dependent; correlations between δ 18O and radiogenic isotope ratios ( 87Sr/ 86Sr, 143Nd/ 144Nd, 206Pb/ 204Pb) and indices of fractionation, show that modification of isotopic compositions occurred during magmatic differentiation rather than at source. A two-stage model is proposed for the origin and differentiation of magmas on Martinique. Primary basaltic melts, which are broadly similar in composition to primitive island-arc magmas elsewhere (e.g. Marianas, Aleutians, South Sandwich Islands), are generated from the subduction-modified asthenospheric mantle wedge. These magmas ascend into the arc crust and differentiate. On Martinique, crystal fractionation is accompanied by contamination with isotopically distinct crust, including a component of terrigenous sediments. Thus, subduction-zone processes at the Lesser Antilles and other island arcs are similar, involving only a small (albeit significant isotopically) volumetric contribution from subducted sediment.

Avian Predation on Anolis Lizards in the Northeastern Caribbean: Inter‐Island Contrast

This paper presents results of a field study of avian predation on Anolis lizards on islands in the northeastern Caribbean, addressing the question of what causes annual fluctuation in lizard abundance. The two main potential avian predators, the Pearly—eyed Thrasher (Margarps fuscatus) and the American Kestrel (Falco sparverius), were studied on three adjacent islands in the northern Lesser Antilles: Anguilla, St. Martin, and St. Eustatius. Anoles on Anguilla were found to be quantitatively unaffected by avian predation; thrashers there did not eat vertebrates, and the kestrel density was too low to have significant impact on the lizard population. In contrast, thrashers in moist habitat on St. Martin ate vertebrates including anoles, and thrasher predation potentially accounted for the anole decline to the dry—season abundance low. Additionally, an unanticipated thrasher niche shift was discovered. Thrasher body size and diet appear to be related to the abundance of a congeneric species, M. fuscus. On Anguilla, where M. fuscus was absent, M. fuscatus was smaller and had a non—vertebrate diet. On St. Eustatius, M. fuscus was abundant while M. fuscatus was larger and preyed on anoles. M. Fuscus was rate on St. Martin, where M. fuscatus had an intermediate body size and a diet comparable to that on St. Eustatius.

General Patterns in West Indian Insects, and Graphical Biogeographic Analysis of Some Circum-caribbean Platynus Beetles (Carabidae)

The relationships of the Antillean terrestrial arthropodan and onychophoran taxa are reviewed using track analysis, in which tracks are considered the connected areas of ende- mism of cladistically unresolved monophyletic groups. The West Indian fauna is shown to be of complex affinities, with elements related to Central and South America, North America, Africa, Asia, and Australia. Numerous taxa in a variety of insect orders exhibit Antillean-African re- lationships, suggesting that resolved cladistic analyses of such groups will allow testing of vicariance hypotheses of Antillean geohistory. Relationships among taxa on Cuba, Hispaniola and Puerto Rico support a geologic reconstruction in which proto-Hispaniola is composed of eastern Cuba, central and northern Hispaniola and Puerto Rico during the early Tertiary. Cladistic analysis of a clade within the carabid beetle genus Platynus is used to generate taxon- area cladograms. The clade is composed of 32 species, 8 fronf the mainland, 14 from Hispaniola, 4 from Jamaica, 4 from the northern Lesser Antilles, and 2 from Cuba. Fitch optimization is used to generate area transformation series from the taxon-area cladograms. Of the 39 equally par- simonious character cladograms generated in the cladistic analysis, two taxon-area cladograms most parsimoniously account for the area relationships of the Platynus species. These are com- pared to two contrasting geologic hypotheses of area relationships: a mobilist geologic hypothesis and a stabilist geology/taxon dispersal hypothesis. Using component analysis and a graphical representation of the area transformation series, called closed biogeographic graphs, the mobilist geologic hypothesis is shown to better account for the area relationships of the Platynus clade. These area relationsips include: 1) a relatively earlier association of portions of the northern Lesser Antilles with the Greater Antilles; 2) a hybrid origin for Hispaniola, with the central and northern cordilleras related to eastern Cuba, and the southern Haitian peninsula related to Jamaica; 3) a relatively later relationship of Jamaica and northern Central America. The techniques of biogeographic graph analysis are compared to recent attempts at track analysis and to the methods of cladistic biogeography. The biogeographic graphs are shown to be a useful adjunct to component analysis, but suffer from a loss of directed component information. (Biogeography, Caribbean, Carabidae, Platynus, beetles, vicariance.)

Subduction and seismic hazard in the northern Lesser Antilles: Revision of the historical seismicity

An important set of French and British archives on the historical seismicity of the northern Lesser Antilles is analyzed. We present the most significant data, which help to constrain the 8 February 1843, the 16 May 1851 and the 29 April 1897 earthquakes. The 1843 event is the largest reported historical earthquake of the Lesser Antilles subduction arc, and produced intensity IX on a segment about 100 km long, between Antigua and Grande-Terre (Guadeloupe), which contradicts the 300 km long proposed by other authors, who mainly rely on Robson's catalog (1964). This suggests a magnitude 7.5 to 8.0. The absence of tsunami and noticeable vertical deformation of the coasts in the source area makes very unlikely a normal fault mechanism, but does not rule out the possibility of a subduction event, because of the low-dip angle of the interplate plane. The dominance of moderate instrumental thrust earthquakes in the source area suggests a subduction mechanism for the 1843 event. The northern and southern limits of the 1843 source area roughly coincide with the Barracuda ridge and the Tiburon ridge, respectively, which may have acted as barriers during the rupture process. The northwestern portion of the Tiburon ridge is associated with a lack of instrumental seismicity, near Grande-Terre, which could indicate a possible magnitude 7.5 to 8.0 event in the next future. Moreover, its mechanism could be normal, which would provide the additional risk of a damaging tsunami. The evaluation of seismic hazard must also take into account possible shallow, moderate earthquakes, as the 1851 and the 1897 events, which produced intensities VII to VIII in Guadeloupe, and may be related to observed major Plioquaternary faults in this island.

Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks

Isotopic and geochemical data are presented from the northern and central Lesser Antilles arc. Island arc tholeiites from the northern islands are restricted in 143Nd 144Nd and 87Sr 86Sr and show large enrichments in LILE relative to other trace elements, as exemplified particularly in high Ba/La ratios. These volcanic rocks represent typical island arc compositions, similar to those described from the Aleutians, Marianas and South Sandwich Islands. In contrast, samples from the central islands (Martinique and St. Lucia) are highly variable in isotopic and trace element ratios, and reflect overprinting of primitive island arc magma chemistry by subsequent crustal contamination. Such magmas therefore do not represent island arc mantle source characteristics. Northern Lesser Antilles arc mantle sources are displaced from MORB Pb isotopic compositions towards the higher 207Pb 204Pb ratios characteristic of sediments. Also, Sr and Nd isotopic compositions are highly restricted and distinct from those of MORB. As a result, mixing models between MORB source and subducted sediment are tightly constrained in terms of the allowable proportion of sediment. The characteristically high LILE REE and LILE HFSE ratios of island arc volcanics cannot be easily reconciled with sediment + MORB source mixing. Buffering of isotopic compositions together with considerable relative LILE enrichment might be achieved by mixing between mantle and a hydrous fluid which has equilibrated with not only sediment but also a large proportion of variably altered oceanic crust. The fluid has isotopic compositions closer to those of MORB, thereby permitting a greater range of mixing proportions. At the same time, the LILE characteristics of the modified mantle source are dominated by the very high concentrations of these elements in the fluid. High degrees of partial melting of mantle modified by a hydrous fluid reproduce the trace element pattern of the source in the derivative volcanic rocks and account for the depletion of HFSE and HREE relative to MORB.

Subduction of aseismic ridges beneath the Caribbean Plate: Implications for the tectonics and seismic potential of the northeastern Caribbean

Normal seafloor entering the Puerto Rico and northern Lesser Antillean trenches in the northeastern Caribbean is interrupted by a series of aseismic ridges on the North and South American plates. These topographic features lie close to the expected trend of fracture zones created about 80–110 m.y. ago when this seafloor was formed at the Mid‐Atlantic Ridge. The northernmost of the ridges that interact with the Lesser Antillean subduction zone, the Barracuda Ridge, intersects the arc in a region of high seismic activity. Some of this seismicity including a large shock in 1974, occurs within the overthrust plate and may be related to the deformation of the Caribbean plate as it overrides the ridge. A large bathymetric high, the Main Ridge, is oriented obliquely to the Puerto Rico trench and intersects the subduction zone north of the Virgin Islands in another cluster of seismic activity along the inner wall of the trench. Data from a seismic network in the northeastern Caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity. Magnetic anomalies, bathymetric trends, and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the Main and Barracuda ridges are parts of a formerly continuous aseismic ridge, a segment of which has recently been overridden by the Caribbean plate. Reconstruction of mid‐Miocene to Recent plate motions also suggest that at least two aseismic ridges, and possibly fragments of the Bahama Platform, have interacted with the subduction zone in the northeastern Caribbean. The introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long. These ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes. They also leave a geologic imprint on segments of the arc with which they have interacted. A 50‐km landward jump of the locus of island arc volcanism occurred in Late Miocene time along the northern half of the Lesser Antilles. We postulate that the subduction of a segment of seafloor of anomolously thick crust, being more buoyant than adjacent seafloor, resulted in a marked shoaling in the dip of the descending slab and, therefore, a shift of the locus of volcanism. In the region near western Puerto Rico and eastern Hispanolia, Plio‐Pleistocene interaction with a similar feature, in this case a part of the Bahama Platform, about 3–4 m.y. ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone. That segment of the Bahama Platform interferred with the subduction process and was subsequently sutured onto the Caribbean plate when the boundary jumped about 60 km to the northeast. The maximum size of historic shallow earthquakes along the Lesser Antillean arc varies from about 7.0–7.5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8.0–8.5 along the northern part of the arc where the dip is shallow. The interaction of anomalous seafloor, as along the northern Lesser Antilles, can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones. Major seismic gaps and regions of high seismic potential currently exist along the northern Lesser Antilles and to the north of Puerto Rico. Both gaps are bounded by anomalous features on the downgoing plate. The intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes. A great shock in 1787 may have ruptured an existing seismic gap north of Puerto Rico between 65° and 67°W. Thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes.

Donnees sur la structure de l'arc insulaire des Petites Antilles, entre Ste-Lucie et Anguilla

Geological and geophysical data were collected during a marine geological cruise (ARCANTE 1) carried out in 1977–1978 between St. Lucia and Anguilla, corresponding to about two thirds of the length of the Lesser Antilles archipelago. They allow of a reconsideration of the structural framework of this island arc. Considered as a whole, the Lesser Antilles ridge is characterized by a pattern contrasting sharply between its northern and southern halves. The major discontinuity axis runs to the south of the Guadeloupe archipelago. The basement of the northern half is supposed to be mainly Mesozoic and is connected with the evolution of the Greater Antilles, whilst the southern half was generated only since the Eocene. The NE flanks of the Lesser Antilles ridge are steep and dissected by large submarine valleys such as the Desirade sea trough; their recent sedimentary cover (post-lower Miocene) is lacking or very poor, enabling the outcropping of older formations: volcanic Mesozoic basement (around la Desirade), Upper Cretaceous, Eocene and Oligocene sediments. Conversely, the NW flanks are gently sloping and are draped by a rather thick overburden of Plio-Pleistocene deposits. To the south, the steep SW flanks are covered with a moderate thickness of recent sediments and display characteristics contrasting with those of the SE flanks, facing the Barbados accretionary prism. From the south up to Martinique, there is only one volcanic ridge, of Eocene to Present age. To the north of Martinique this ridge splits into two axes. The submarine landmarks of these two arcs have been disclosed during the present survey. The eastern one, or outer arc, shows a row of seamounts and islands often marked by strong magnetic (Dien Bien Phu Bank, Marie-Galante, Agouti Bank) or heavy gravimetric (Marie-Galante, Grande-Terre) anomalies. This outer arc was superimposed on the pre-Eocene basement, and is expressed in the Eocene and Oligocene volcanic terrains cropping out in the islands of Antigua, St. Bartholomew, St. Martin and Anguilla which rise from the wide northern insular shelves. A major (ca. 70 km) interruption in the continuity of this axis takes place between Dien Bien Phu Bank and Marie-Galante. La Desirade and Barbuda islands, although pertaining to the “Limestone Caribbees”, are not included in the outer arc. The western — or inner — arc, made up of upper Miocene to Present volcanics, begins with Dominica island and terminates at Luymes Bank, but an interruption of about 50 km is to be noted between Basse-Terre and Montserrat. The outer and inner arcs are separated by the narrow (ca. 50 km) and elongated (ca. 250 km) Kallinago depression. This intra-arc trough is generally filled by less than 1 km of recent sediments and is subdivided into three smaller basins; it was not initiated by extensional rifting related to back-arc spreading, as was previously assumed. It is possible that from Martinique to Guadeloupe an intermediate arc was in activity during the Miocene. Data from exploratory oil surveys provide some new insights into the geology and the structure of the wide Saba Bank, located to the west of the inner arc and near the northern end of the archipelago. Its volcanic substratum is of Upper Cretaceous—Paleocene age and is part of the pre-Eocene basement of the northern Lesser Antilles. The general characteristics of the NE flanks of the Lesser Antilles ridge may indicate some tilting toward the west of the northern part of the island arc. This may be ascribed to the arrival of Atlantic fracture zone ridges (Barracuda and Tiburon) in the subduction zone, during the Miocene. This event could explain the jump of the volcanic line from the outer to the inner arc.

Motion of Caribbean Plate during last 7 million years and implications for earlier Cenozoic movements

The direction and rate of movement of the Caribbean plate with respect to North America are determined from the slip vectors of shallow earthquakes and from the configuration of downgoing seismic zones in the Greater and Lesser Antilles. A calibration of the relative plate motion for the northeastern Caribbean using data from other subduction zones indicates an average rate of 3.7±0.5 cm/yr for the past 7 million years (Ma). The direction of plate motion inferred from focal mechanisms (ENE) is nearly the same as that deduced from the configuration of downgoing seismic zones going around the major bend in the arc. With respect to North America, the Caribbean plate is moving at an angular velocity of 0.36°/Ma about a center of rotation near 66°N, 132°W. Vector addition using those data and that for the relative motion of North and South America indicates that the Caribbean is moving at an angular velocity of 0.47°/Ma about a center of rotation near 60°N, 88°W with respect to South America. The presence of intermediate‐depth earthquakes beneath Puerto Rico and the Virgin Islands is ascribed to the curvature of the plate boundary and a component of underthrusting that has been going on for at least the past 7 Ma and is likely occurring today. The alternative hypothesis that earthquakes beneath those areas are occurring in materials that were subducted during the Eocene, the last major episode of magmatism, is not tenable from thermal considerations. The lack of recent magmatism in the eastern Greater Antilles is ascribed to the relatively small component of underthrusting. The 2 cm/yr rate of seafloor creation along the mid‐Cayman spreading center for the past 2.4 Ma does not appear to reflect the total Caribbean‐North American plate motion while the 4 cm/yr spreading rate from 6.0 to 2.4 Ma does. Between the mid‐Cayman spreading center and eastern Guatemala, the northern boundary of the Caribbean plate is narrow and follows the southern margin of the Cayman trough. Seismic activity between the spreading center and eastern Hispaniola, however, occurs over a zone about 250 km wide that extends from Cuba to Jamaica and across the entire width of Hispaniola. Individual faults within this broad plate boundary appear to have accommodated differing amounts of motion as a function of geological time while the cumulative plate motion across the zone remained nearly constant. The percentage of total plate motion accommodated near southern Hispaniola and Jamaica is inferred to have increased about 2.4 Ma ago. That change may have been caused by the collision of parts of the Bahama bank and northern Hispaniola. This explanation for the sudden decrease in seafloor creation along the mid‐Cayman spreading center is less catastrophist than the hypothesis that the entire Caribbean plate suddenly changed its velocity with respect to surrounding plates. The Caribbean plate may be regarded as a small buffer plate whose motion is now governed by the movement of the larger North and South American plates which bound it on three sides. The Caribbean plate is either at rest or moving eastward at a rate of no more than 1 cm/yr in the hot spot reference frame. Since the relative motion of the larger plates surrounding the Caribbean has been nearly constant for the last 38 Ma (anomaly 13 time) and since the forces on the Caribbean plate do not appear to have changed greatly during that interval, we extrapolate the motion of the last 7 Ma back to 38 Ma. A reconstruction for the late Eocene places the Caribbean plate about 1400 km west of its present position. The faster rate of plate motion we calculate makes it more likely that the lithosphere beneath the basins of the Caribbean originated in the Pacific. It also has implications for the seismic potential of the region, paleocirculation in the Atlantic Ocean and origin of sediments in the area. Our late Eocene reconstruction aligns the eastern continental margin of Yucatan with that along the southeast side of the Nicaragua rise. This 2500‐km‐long feature may have acted as an arc‐arc transform fault from the late Mesozoic to the late Eocene. Arc‐related rocks of those ages in the Greater Antilles and northern Lesser Antilles define a northwesterly trending subduction zone along the northeastern edge of the former East Pacific‐Caribbean plate. At least three fragments of anomalous seafloor have been sutured onto Hispaniola in the past 50 Ma.

A large normal-fault earthquake in the overriding wedge of the Lesser Antilles subduction zone: The earthquake of 8 October 1974

A large earthquake ( Ms = 7.6) occurred in the overriding wedge of the Caribbean plate on 8 October 1974. Hypocenters of locally recorded aftershocks suggest that the main shock rupture extended from about 40 km, immediately above the inferred thrust interface between the Caribbean and American plates, to shallow crustal depths of about 10 km. The focal mechanism solution and aftershock hypocenters taken together imply that the earthquake occurred on a southeast-dipping normal fault that strikes NNE to NE, transverse to the regional strike of the northern Lesser Antilles arc. The location and focal mechanism of the 1974 shock are thus similar to those of small earthquakes occurring in the Adak Canyon region of the central Aleutians and studied by LaForge and Engdahl (1979). Both regions, the northern Lesser Antilles and the central Aleutians, are zones of oblique plate convergence, and it is possible that wedge normal faulting, transverse to the island arc, is a consequence of this oblique convergence. Alternatively, the large normal-fault earthquake in the northern Lesser Antilles may have been a consequence of the resistance of the aseismic Barracuda Ridge, on the American plate, to subduction beneath the Caribbean plate. The focal mechanism of the 1974 earthquake indicates that the shock should not have released a significant amount of the elastic strain that is thought to have been accumulating along the thrust boundary of this segment of the Lesser Antilles arc since the great earthquake of 1843.

General distribution of water masses within the eastern Caribbean Sea during the winter of 1972 and fall of 1973

Quasi‐synoptic descriptions of the circulation and water mass distributions within the eastern Caribbean Sea are presented for winter 1972 and fall of 1973. Vertical sections of properties within these basins and along the axes of the major passages into this region, together with property distributions on isopycnal surfaces (chosen to be representative of the principal water masses), are used to describe the circulation. The flow paths of the various water masses into and out of the region are identified. The water masses identified within the eastern Caribbean Sea and their points of entrance into the region are as follows: Subtropical Underwater enters through the northern Lesser Antilles, 18°C Sargasso Sea Water enters through the passages of the Greater Antilles, Tropical Atlantic Central Water and Antarctic Intermediate Water enter through the southern Lesser Antilles, and North Atlantic Deep Water enters the Venezuelan Basin through Anegada‐Jungfern Passage and enters the Colombian Basin between Jamaica and Hispaniola. Water mass distributions and dynamic maps are used to compare and contrast the quasi‐synoptic current patterns present within the eastern Caribbean Sea during the winter of 1972 and fall of 1973. During both periods, a two‐stream westward current was observed in the Grenada Trough and Venezuelan Basin. Within the Colombian Basin, these streams combine to form one northwestward flow out of the region. The area where these streams combine is controlled by the intensity of a dynamic high located south of Hispaniola. The intensity of this high appears to be controlled by the curl of the local wind stress and tends to weaken during the fall when the wind stress curl reaches a minimum. Therefore ‘seasonal’ changes in the current pattern may actually be associated with seasonal changes in the atmospheric forcing. Geostrophic computations give a net westward transport for the Caribbean Current system of 29×106 m3 s−1 during both seasons. This net westward transport agrees quite well with transport estimates of flow into and out of this region by previous investigators.

The geochemical characteristics and conditions of petrogenesis of the volcanic rocks of the northern lesser antilles — A review

This paper is concerned with the islands of Montserrat Nevis, St. Kitts, St. Eustatius and Saba, which lie on the inner volcanic are at the northern of the Lesser Antilles. Andesites greatly predominate over basalts and dacites in this part of the arc. Generally the lavas from the northern Lesser Antilles contain low abundances of Ni, Cr and residual trace elements but lavas from Saba are enriched in these elements compared with the other islands in the group.