continental drift, iv: the caribbean...

CONTINENTAL DRIFT, IV: THE CARIBBEAN "PLATE"1

A. A. MEYERHOFF AND HOWARD A. MEYERHOFF

American Association of Petroleum Geologists and GeoSurveys, Tulsa, Oklahoma 74101

ABSTRACT

According to "the new global tectonics," the Caribbean Sea and the marginal lands surrounding it comprise one "lithosphere plate." Its eastern margin includes the Barbados Ridge, Lesser Antilles arc, and Aves Ridge; the western margin is composed of Central America and the Middle America Trench. These two margins are interpreted as subduction zones by "new global tectonics" advocates. The northern and southern margins are the alleged North Caribbean and South Caribbean faults, respectively. According to recent JOIDES drilling data, the Caribbean "plate" may be underlain by Late Cretaceous (75-85 m.y.) basaltic crust, and therefore is younger than both the Americas plate on the north, east, and south and the Cocos plate on the west. From this, some workers have inferred that the Caribbean formed by sea-floor spreading which separated North and South America. However, there is no spreading center in the Caribbean. More- over, geologic and seismic data prove that the proposed North Caribbean and South Caribbean faults do not exist. La Desirade is a Jurassic or older island east of the Lesser Antilles. Barbados Ridge is an aseismic, pre-Late Jurassic salient of South America (that is, part of the so-called Americas plate) that parallels and lies just east of the alleged Lesser Antilles subduction zone. The stratigraphic units of Barbados are correlat- able with those of northern South America. Structural trends on Barbados strike east-northeast, are Late Cretaceous-early Eocene, and parallel those of South America. These facts show that the position of Barba- dos with respect to that of South America has not changed for more than 140 m.y. Yet "the new global tectonics" require that La Desirade and the Barbados Ridge must have been subducted more than 70 m.y. ago. On the basis of the facts presented here, the so-called Caribbean plate and the regions surrounding it have held the same positions relative to one another since pre-Jurassic (probably pre-Paleozoic) time. "The new global tectonics," therefore, are not applicable in the Caribbean area.

INTRODUCTION

The problem.-For more than 60 years geologists have tried their hand at inter- preting the Caribbean region as a whole. Their greatest handicap was ignorance of the geology of the vast expanses covered by water-a cover that made extrapolation from mainland to island, and island to island, extremely speculative. Much more recently, the research vessel has brought geophysicists into the scene and, as knowl- edge of the sea floor has been revealed, new interpretations have been ventured. Al- though there are not quite as many interpretations as there are workers in the region (H. A. Meyerhoff 1954, p. 149), confusion is compounded by the constantly recurring need that confronts advocates of "the new global tectonics" to change their interpretations as new dredge hauls, new cores, new seismic-refraction data, and new gravimet- ric profiles belie yesterday's concepts. Each

1 Manuscript received April 20, 1971; revised June 7, 1971.

[JOURNAL OF GEOLOGY, 1972, Vol. 80, p. 34-60]

© 1972. The University of Chicago. All rights reserved.

new ad hoc concept tends to outrun the available evidence and too commonly seems oblivious of the geology exposed on land. We have no illusions about the finality of our views, but several recent papers show the dire need of some accord between land and sea geology-geophysics, particularly in the eastern Caribbean. It is our purpose to bring the facts together and to offer an interpretation of their meaning.

A principal key in the tectonics of the eastern Antilles-in fact, of the entire Ca- ribbean-is the Barbados Ridge (fig. 1). Suess (1908, p. 700-708) appears to have been among the first to recognize this fact. He observed that the position of the Barba- dos Ridge with respect to the Lesser Antil- les is nearly identical with that of the Ba- hamas-Turks and Caicos Islands with respect to the Greater Antilles. Both the Barbados Ridge and the Bahamas-Turks and Caicos are salients from a continent on the outer, or Atlantic sides, of the two An- tillean arcs. Suess noted further that the older formations of Barbados are related genetically to correlative units on Trinidad.

Suess's is the first geologic synthesis of

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CONTINENTAL DRIFT, IV

FIG. 1.-Index map, eastern Caribbean Sea. Modified from Hess (1966) and Bunce et al. (1971). Locations of figs. 4, 8, 9, 10, 11, 12, 13, 17, 18, 19, and 20 are shown.

the Caribbean. Since his day, many sum- maries and syntheses of Caribbean geology have been published, but only a few have succeeded in explaining any part of the complex tectonics and stratigraphy of the area. Among the most important works are those of Woodring (1928, 1954), Rutten (1935), Schuchert (1935), Hess (1938, 1966), Wa- ters and Hedberg (1939), H. A. Meyerhoff (1946, 1954), Bucher (1947), Hess and Maxwell (1953), Eardley (1954), Butterlin (1956), Barr (1958), Weyl (1961, 1966), North (1965), Judoley and Furrazola (1967) A. A. Meyerhoff (1967), Tijomirov (1967), Ball and Harrison (1969), Dengo (1969), Molnar and Sykes (1969), Stainforth (1969), and MacGillavry (1970).

Most syntheses have treated the Carib-

bean Sea as a foundered landmass (Wood- ring, Schuchert, Bucher, Eardley, Butter- lin, Judoley and Furrazola, Tijomirov) or a permanent oceanic basin surrounded by island arcs and geosynclines (Hess, Waters and Hedberg, H. A. Meyerhoff, Hess and Maxwell, Barr, Weyl, North, A. A. Meyer- hoff, Dengo, Molnar and Sykes, MacGil- lavry). Among the former group, two hypotheses are favored: (1) foundering (Wood- ring, Schuchert, Bucher, Eardley, Butter- lin), and (2) oceanization (Judoley and Furrazola, Tijomirov). Among the latter, three hypotheses have been advanced: (1) contraction (Waters and Hedberg, H. A. Meyerhoff, A. A. Meyerhoff), (2) convection (tectogene of Hess 1938; Hess and Maxwell, Barr), and (3) some form of con-

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tinental drift-generally sea-floor spreading (Hess 1966; North, Ball and Harrison, Den- go, Molnar and Sykes, Stainforth, Mac- Gillavry).

Sea-floor spreading and plate tectonics.- A major question during recent years is the age of the Caribbean floor. Most sea-floor spreaders ("mobilists") and nondrifters ("fixists") have regarded the floor as ancient oceanic crust. However, the mobilists generally have maintained that the Carib- bean floor predates the present Atlantic and Pacific crusts. To explain the presence and preservation of this ancient Caribbean crust, Mattson (1969), Edgar et al. (1971a), and Malfait and Dinkelman (1971) proposed that the Caribbean is a remnant of pre-Mesozoic sea floor which once flanked the ancient Darwin Rise of the Pacific basin. After the Darwin Rise ceased to be a spreading center, the East Pacific Rise

formed, and all ancient ocean floor was de- stroyed, except (1) the Darwin Rise itself (which ultimately is to be consumed in the trenches of the western Pacific as the sea floor plods steadfastly west-southwestward); and (2) the Caribbean "child" of the Dar- win Rise which, caught between North and South America as they wended their way westward, was sealed off on the west by the newly formed Middle America Trench and its subduction zone and on the east by the Lesser Antilles Trench and its so-called subduction zone (fig. 2).

This mobilist concept has been modified radically by the 1970-1971 drilling results of JOIDES Leg 15 (Purrett 1971). A 762-m hole reached basalt 746 m below the ocean bottom at 14°47'11" N, 69019'36" W. Only 18 m of basalt was drilled. The oldest faunas above the basalt are middle Late Cretaceous (Edgar et al. 1971b). Edgar and his col-

FIG. 2.--Index map, Caribbean "plate." Shows locations of figs. 6, 7, and 14, plus major elements of Greater Antilles orthogeosyncline. Note absence of through-going faults.

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leagues (in Purrett 1971) then did an abrupt about-face, concluded that the Ca- ribbean Sea is a young ocean basin (younger than the Atlantic and Pacific), and de- clared that it is the product of the tearing apart of North and South America during Late Cretaceous time.

According to mobilists, the Caribbean is one of the many "lithosphere plates" (Isacks et al. 1968; Morgan 1968) which comprise the earth's crust (fig. 3). The Ca- ribbean was first shown as a discrete lithosphere plate by Morgan (1968), although, ironically-and for very different reasons- one of us (H. A. Meyerhoff 1946, 1954) first expressed in print the concept of the Caribbean plate as an independent tectonic unit.

Both the mobilists and we agree on one point: the so-called Caribbean plate is moving eastward relative to the Atlantic, North America, and South America; but on the following points we differ: (1) the mobilists

FIG. 3.-"Lithosphere plates" of Caribbean and surrounding areas. From Morgan (1968). Published with permission of W. J. Morgan and American Geophysical Union.

also move the Caribbean plate relatively westward, at a somewhat slower rate than that of the two American continents, as the new Pacific floor descends down the Middle America Trench; (2) the mobilists require that unbroken transform faults bound the Caribbean plate on the north (North Ca- ribbean fault) and on the south (South Caribbean fault); and (3) they believe that the Western Atlantic (Americas) plate is being subducted on the east beneath the Lesser Antilles arc. These mobilist specula- tions conflict with the geological and geophysical facts.

EASTERN CARIBBEAN MARGIN

The eastern margin of the Caribbean basin includes a geomorphically and geologically complex system of subparallel ridges which range from slightly to marked- ly convex toward the Atlantic Ocean (fig. 1). From west to east, these are: Aves Ridge, Lesser Antilles, and Barbados Ridge. H. A. Meyerhoff (1946, 1954) termed this ridge complex the "Lesser Antilles anticlinorium."

Except for Aves Island, 215 km west of Dominica, the Aves Ridge is submerged through its entire north-south length of 550 km. It is approximately 100 km wide at the 2,000-m isobath. It joins the South American continent just northeast of La Blanquilla Island and converges northward toward the Lesser Antilles arc which it joins near Saba Island.

The Aves Ridge is separated from the Lesser Antilles arc by the Grenada Trough, which has an average width of 150 km at the 2,000-m isobath. The width of the trough from the crest of the Aves Ridge to the axis of the Lesser Antilles volcanic arc is about 215 km.

The Lesser Antilles arc is about 750 km long, and includes the Leeward Islands on the north and the Windward Islands on the south. The arc is only 25-40 km wide in the south, but north of Dominica it bifurcates into two island groups which comprise a ridge system more than 100 km wide. The two groups consist of an outer chain of

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limestone islands (Limestone Caribbees) and an inner chain of volcanic islands (Vol- canic Caribbees). The volcanic chain extends from Grenada, about 150 km north of the Paria Peninsula, Venezuela, to Saba, due east of St. Croix. The limestone islands are limited to the northern half of the island arc-from Marie Galante, north of Domini- ca, to Sombrero Island, east of Anegada.

The Tobago Trough, about 150 km wide, separates the volcanic islands of the Lesser Antilles from the Barbados Ridge. At the 2,000-m isobath, the Trough is only 80 km wide.

The island of Barbados is 160 km due east of St. Vincent in the Volcanic Carib- bees. The island is almost precisely in the center of the Barbados Ridge, which parallels the Volcanic Caribbees, and extends nearly 500 km northward from Tobago to its northern end. The northern end of the Ridge is almost 170 km due east of Marie Galante, the southernmost of the Lime- stone Caribbees.

PRINCIPAL TECTONIC PROBLEMS

The principal tectonic problems in the eastern Antilles (fig. 1) are (1) the origin and age of the Aves Ridge, (2) the origin and age of the Lesser Antilles arc, (3) the abrupt southern termination of the Lime- stone Caribbees at Marie Galante Island, (4) the origin and age of the Barbados Ridge, (5) the abrupt northern termination of the Barbados Ridge east of Marie Ga- lante Island, (6) the nature(s) of the junc- tions of the three ridges with the Greater Antilles on the north and with South Amer- ica on the south, and (7) the position of La D6sirade Island in the Lesser Antilles arc. Solution of these problems must take into account the facts that Aves Island, the southern terminus of the Limestone Car- ibbees, and the northern terminus of the Barbados Ridge are at the same latitude- about 15°42' N.

We believe that a successful solution to the problem of the Lesser Antilles's origin hinges on the establishment of their position in the framework of the entire Carib- bean.

DESCRIPTION OF EASTERN CARIBBEAN

RIDGE SYSTEM

Numerous hypotheses, theories, and factual observations have been recorded for the Caribbean region and for the land- masses that surround it. The Lesser Antil- les alone generally are interpreted as a com- pressional feature. The Volcanic and Lime- stone Caribbees have been described as a "typical" island arc system belonging to the circum-Pacific belt, even though they face the Atlantic Ocean (Benioff 1954; Guten- berg and Richter 1954). However, earthquake hypocenters generally are above a depth of 200 km, in contrast to those of some Pacific-facing arcs where hypocenters locally reach depths as great as 720 km. H. A. Meyerhoff (1954) interpreted the Lesser Antilles as a series of crustal "waves" pushed upward by eastward rotation of the Caribbean plate as it was caught in a vise- like movement between North and South America. The underlying cause of Carib- bean deformation was attributed by Meyer- hoff to contraction.

Hess (1938) interpreted the Lesser Antil- les complex to be part of a tectogene formed by downbuckling between two converging convection currents. When seismic and gravity studies showed that the huge "roots" required by the tectogene concept are not present beneath arcs, convection currents still were invoked as the mechanism which provided the compressive stresses needed to create the island arc.

When the sea-floor spreading hypothesis of Holmes (1931) was revived by Hess (1960, 1962), the Lesser Antilles logically was interpreted as a zone where the westward-spreading Americas "plate" plunged beneath the Caribbean "plate." Evidence purporting to show that part of the so- called Americas plate actually does plunge into a Benioff (subduction) zone beneath the Lesser Antilles island arc was published by Chase and Bunce (1969) (fig. 4). Today the sea-floor spreading, or mobile plate, model for the origin of the Lesser Antilles is widely accepted. The purposes of this paper are to show that the geological and

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SW SECTION A NE LESSER ANTILLES TRENCH

FAULTS

SEDIMENTS MANTLE

CFROM CHASE AND BUNCE, 1969; BUNCE ET AL,1971)

FIG. 4.-Southwest-northeast seismic-reflection profile A. Location shown on fig. 1. From Chase and Bunce (1969) and Bunce et al. (1971). Pub- lished with permission of authors, American Geo- physical Union, and Wiley-Interscience.

geophysical facts contradict the sea-floor spreading-mobile plate model in this area, and that North America, South America, and the Caribbean region have held relatively permanent positions through time with respect to one another.

BARBADOS RIDGE

Nature and origin.--The only exposed part of this ridge is Barbados island, 275 km north of Trinidad, and 225 km south of the northern terminus of the ridge. South of Barbados, the ridge is divided in two branches (fig. 1) separated by the Barbados Basin (Lattimore et al. 1971; Weeks et al. 1971).

Sedimentary rocks exposed on Barbados island range in age from Paleocene through Pleistocene (Senn 1940, 1947; Beckmann 1953; Baadsgaard 1960; Kugler 1961). The oldest strata, the Scotland Formation which ranges in age from Paleocene through early middle Eocene, are strongly deformed. They are mainly terrigenous marine turbidites and synorogenic flysch (sensu Ter- cier 1947). The beds contain a heavy mineral suite consisting of abundant opaque minerals (mainly ilmenite); common leucox- ene, zircon, epidote, zoisite-clinozoisite, chloritoid, hypersthene, garnet, and augite; and scarce tourmaline, staurolite, silliman- ite, andalusite, topaz, glaucophane, rutile, anatase, brookite, titanite, and corundum (H. D. Hedberg, in Senn 1940, p. 1560- 1561). Pyroclastic pyroxenes are present, as are milky white pebbles of vein quartz.

The source of the mineral suite listed is a mixed terrane of low- to high-grade meta-

morphosed acidic rocks, and basic volcanic rocks cut by diorite and veins of milky quartz. A low-grade metamorphosed terrane is present below the Late Jurassic on the Araya-Paria Peninsulas of northeastern Venezuela (Gonzilez de Juana et al. 1968), but medium- to high-grade metamorphic rocks are not exposed. The grade of meta- morphism on the two peninsulas increases northward. Consequently, medium- to high- grade metamorphic rocks may underlie the offshore area. Basic to silicic volcanic rocks cut by dioritic intrusives and by milky quartz veins comprise much of Tobago, Margarita, and the remaining Venezuelan- Netherlands Antilles. A fresh diorite from Tobago yielded a K-Ar date of 113 + 6 m.y.2 The Paleocene-Eocene section thick- ens toward the south-southeast (a fact which indicates southern provenance), and contains numerous, large exotic blocks of Albian limestone bearing faunas identical with those of Trinidad and the Eastern Venezuelan basin (Vaughan and Wells 1945; Douglass 1961). Additional outcrops of the Scotland Formation have been dredged from the Barbados Ridge north of Barba- dos island (Hurley 1966).

The inference to be drawn is sufficiently plain, as Senn (1940, 1947), Barr (1958), and Baadsgaard (1960) have noted. The pre-Late Jurassic basement of the Araya- Paria Peninsulas area, and the Cretaceous volcanic sequence of the Venezuelan-Neth- erlands Antilles and Tobago underlie the Barbados Ridge. Therefore, the Barbados Ridge is a part of the North Venezuela- Trinidad fold belt of South America that projects north-northeastward from Trini- dad and Tobago. The seismic data of Bas- singer et al. (1971), Lattimore et al. (1971), and Weeks et al. (1971) also show clearly the geological and geophysical continuity between Trinidad-northern Venezuela and the Barbados Ridge.

Hurley (1966) emphasized the fact that

2 Age determination was carried out by the Isotope Geology Unit of the Institute of Geological Sciences, London, and is quoted by permission of the director of the IGS. We thank Norman J. Snelling and John F. Tomblin for this information.

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the east-northeast-trending structures of Trinidad and Barbados and the great distance separating the two islands preclude a simple relation between them. Hurley's observation is correct, but he overlooked the fact that the low- to high-grade metamorphic clasts of the Scotland Formation require the presence, between Trinidad and Barbados, of a broad area of metamorphic rocks of higher grade than any that are exposed on Trinidad or in northeastern Vene- zuela.

A fact which is well known to some Ca- ribbean geologists, but which seems to be unknown to most geophysical-geological oceanographers, is that the pre-middle Eocene structural trends of Barbados strike east-northeast-west-southwest, parallel with the structures of Tobago (200 km south), Trinidad (275-350 km south), and northern South America (Senn 1940, 1947; Baadsgaard 1960). In middle and late Eo- cene times, strong folding, directed toward the north-northwest, affected the entire early Eocene and older sequence. Over- turned sections, thrusts, isoclinal and re- cumbent folds, and large gravity-glided olistoliths and olistostromes are abundant (Baadsgaard 1960). Baadsgaard (1960) suggested that the mass of pre-late Eocene sediments on the island was emplaced by gravity gliding from a ridge in the south- southeast. If this interpretation is correct, bathymetric data suggest that the distance of gliding did not exceed 50-75 km (fig. 1). The important facts to keep in mind are that (1) the east-northeast strike of the older formations requires a north-northwest- south-southeast compressive stress field in early and middle Eocene times, when the Lesser Antilles arc supposedly was being formed by a west-to-east stress field; and (2) the Barbados Ridge geologically is not a part of the Caribbean or Lesser Antilles, but a part of the South American continent.

Therefore, the Barbados Ridge most certainly does not take "the place of the Puerto Rico [Antilles] trench east of the southern end of the Antilles arc," as Chase and Bunce (1969, p. 1419) claimed; nor is

it "a mass of sedimentary material that fills the southern part of the Antilles [Lesser Antilles] trench." According to these authors, "The thickening of the sediments overlying the oceanic basement is due to the increasing proximity of South America, the source for the material of the sedimentary wedge that constitutes the continental rise" (p. 1419). Chase and Bunce wrote further: "That there is no basic change in the major subsurface structural elements is shown by the southward continuation of the negative gravity anomaly, which characterizes the [Puerto Rico] trench, through Barbados to Trinidad (Talwani, 1966)." Chase and Bunce's statements require comment.

1. Talwani (1966, p. 177) wrote that "Eastwards [from Puerto Rico], the negative free-air anomaly belt of the Puerto Rico Trench departs from the topographic trench and continues into Trinidad and Venezuela." Talwani neither stated nor im- plied that "there is no basic change in the major subsurface structural elements . . .." The gravity trends of the eastern Caribbean are shown in figure 5.

2. A large negative free-air gravity anomaly will be produced by either a deep, sediment-filled oceanic trench or by a thick prism of continental crust. The fact that a broad negative anomaly band strikes from east of the Virgin Islands to the Barbados Ridge is not proof of a common origin for the Lesser Antilles Trench and the Barba- dos Ridge. We regard the Puerto Rico Trench, Lesser Antilles Trench, and Bar- bados Ridge as three distinct structural features. The Puerto Rico Trench is an east-west-trending graben, characterized by strike-slip and dip-slip movements. It continues eastward from Puerto Rico to a location north of the Barracuda fracture zone (fig. 1; see Hess 1966). The Lesser Antilles Trench intersects the Puerto Rico Trench northeast of the Virgin Islands.

Molnar and Sykes (1969, p. 1655) rec- ognized that the large gravity anomalies and the presence of intermediate-depth earthquakes beneath Puerto Rico suggest

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POSITIVE VALUES (MGAL) ONEGATIVE VALUES (MGAL)

FIG. 5.-Pratt-Hayford isostatic gravity map of eastern Caribbean. From Bush and Bush (1969). Black dots are control points. Published with permission of authors and Gulf Coast Association of Geological So- cieties.

that an island-arc structure extends east- west, parallel with the Puerto Rico Trench. However, in the absence of volcanism in this region since the Eocene (Monroe 1968), Molnar and Sykes (1969, p. 1658) concluded that "the present tectonics of the Puerto Rico trench should not be regarded as typical of island arcs."

3. Khudoley and Meyerhoff (1971) were able to corroborate the conclusion of Mol- nar and Sykes that the Puerto Rico Trench is not an island-arc-type trench. They ana- lyzed the earthquake epicenters reported by Sykes and Ewing (1965, p. 5071), who erroneously had combined the hypocenter data from the Dominican Republic with those from Puerto Rico, and thereby por- trayed a hypocenter distribution beneath Puerto Rico and the eastern Dominican Republic similar to that of a typical island arc. If the hypocenters beneath the eastern Dominican Republic and the Puerto Rico Trench are plotted separately (figs. 6, 7),

patterns result which are totally unlike that of an island-arc hypocenter distribution and resemble most closely those associated with actively subsiding grabens and broad strike-slip zones.

4. The gravity data from east of the Lesser Antilles are sparse. Bush and Bush (1969) showed that the Lesser Antilles Trench axis passes through Barbados (fig. 5). Hess (1966, p. 2) showed that the main part of the Lesser Antilles negative anomaly belt passes along the western side of the Barbados Ridge. Masson-Smith and An- drew (1965, in Hurley 1966) showed one negative trend to strike from southwestern Barbados island to a point midway between Grenada and Tobago, and another negative trend to pass east of Tobago toward central Trinidad.

5. The facts have been mentioned that the pre-late Eocene structural trends of Barbados island are east-northeast-west- southwest, and that the stresses which de-

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OUTER PUFRTO RICO PUERTO CARIBBEAN A RIDGE TRENCH RI SEA A

LOWER LIMIT OR EPICENTERS, 1950-1964 (AFTER SYKES AND EWING, 195)

100 KMG

VERT. ExAC.= 2x

A

FIG. 6.-Hypocenters of Puerto Rico. From Khudoley and Meyerhoff (1971). Location shown on fig. 2. Published with permission of The Geolog- ical Society of America.

formed the island were directed toward the north-northwest. Geophysical data collected by Bassinger et al. (1971) between Barbados and Tobago, and between To- bago and Trinidad, show that (a) structures become progressively steeper and more complex with increasing depth; (b) mag- netically and seismically determined structural trends of the submarine part of the Barbados Ridge strike east-northeast; (c) the structural similarity across the entire area from Trinidad to Barbados shows that the Ridge has been deformed as a single tectonic unit; (d) the Cretaceous basic igneous rocks of Tobago can be traced northeast toward a point not far from Barbados; and (e) the whole area from Trinidad to Barbados belongs structurally and stratigraphically to northern South America and the Venezuelan-Netherlands Antilles. These statements are borne out in full by Collette et al. (1969; see fig. 8). Figure 8 shows the basement rising to near the surface at 110 N.

N NAVIDAD PUERTO RICO DOMINICAN CARIBBEAN SEA S 3 BANK TRENCH REPUBLIC

B

FIG. 7.-Hypocenters of eastern Dominican Republic. From Khudoley and Meyerhoff (1971). Location shown on fig. 2. Published with permission of The Geological Society of America.

This basement is interpreted to be the pre- middle Eocene, and more probably is pre- Late Jurassic medium- to high-grade metamorphic rocks equivalent to those exposed on the Paria-Araya Peninsulas.

The present north-south strike of the Barbados Ridge, paralleling the trend of the Volcanic Caribbees, apparently was acquired by the compressive stresses which formed Aves Ridge and the Lesser Antilles. Its earlier structural history, however, was an integral part of the development of the Caribbean orthogeosyncline of northern South America. A pre-Jurassic metamorphic terrane presumably extended from the Paria-Araya Peninsulas on the mainland northward to within a few tens of kilometers of the island of Barbados. Deformation before Paleocene time, probably in Late Cre- taceous time, raised the Caribbean orthogeosyncline, and its Late Jurassic rocks were shed north-northwestward into a foredeep much like the trenches on the ocean-

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FIG. 8.--South-north seismic-reflection profile B, Barbados Ridge, and Lesser Antilles Trench. Location shown on fig. 1. South American shelf at 90; basement approaches surface at 110; junction of Lesser Antil- les and Puerto Rico Trenches between 180 and 190. From Collette et al. (1969). Published with permission of B. J. Collette and Elsevier Publishing Company.

ward side of modern volcanic arcs, as Hur- ley (1966) concluded.

The Paleozoic-early Eocene deep-water turbidites and flysch deposits of the Scot- land Formation that accumulated in the foredeep were themselves deformed prior to the deposition of the middle Eocene to early Miocene Oceanic Formation (Beck- mann 1953; Kugler 1961). The latter is bathyal marl which overlies the Scotland Formation with marked angular unconformity on Barbados. Cores recovered from the Barbados Ridge between Barbados and Tobago, although of late Miocene or Qua- ternary sediments, contain reworked faunas and detritus from shallow-water strata as old as middle Eocene (Ramsay 1968). The cores show that the Eocene was raised and eroded from 11040' N at least as far as 130 N.

The Oceanic Formation shows the first link with the Lesser Antilles. It was de- posited in at least 1,000-1,500 m of water (Beckmann 1953), and its late Eocene beds contain volcanic ash of the same composition as the ash of the modern Lesser Antil- lean volcanoes (Senn 1940, 1947). The water depth in which the Oceanic accumulated contrasts with the evidence of concur- rent shallow-water deposition south of Bar- bados (Ramsay 1968).

Conclusions.--The Barbados Ridge is a north-northeastward salient of the Carib- bean orthogeosyncline of northern South America that extends 550 km along the eastern side of the Lesser Antilles arc. The Barbados Ridge basement is pre-Late Juras- sic-possibly Paleozoic or older. The close

similarity between the stratigraphy of Bar- bados and that of Trinidad and the identity of their faunal zones indicate that the two islands belong to the same geologic province. Geophysical data and structures mapped in the field also show the geologic unity of the area between the mainland and Barbados island. The Barbados Ridge may have been part of an outer deep-sea trench during the development of the Late Juras- sic-early Eocene Caribbean orthogeosyncline of northern South America.

LESSER ANTILLES

Origin.-The Volcanic Caribbees and, from Marie Galante Island northward, the Limestone Caribbees have been discussed briefly. The oldest paleontologically dated beds are middle Eocene-a limestone inter- bedded with volcanigenic beds on the island of St. Barthelemy in the northeastern part of the Lesser Antilles (Christman 1953). Late Eocene fossils have been collected by Robinson and Jung (1972) from Carriacou Island, north of Grenada.

On the basis of the dated middle-Eocene rocks, almost all workers in the region have assumed that the Lesser Antilles formed during latest Cretaceous through middle Eocene time, and that this volcanic arc has remained active ever since.

Fink's data.-In 1970, L. K. Fink (1970a, 1970b) provided a major surprise for students of Lesser Antilles geology. On the island of La Desirade, east of Grand Terre, Guadeloupe (fig. 1), Fink discovered an igneous assemblage which to date is unique in

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the Lesser Antilles. The main volcanic islands of the Lesser Antilles consist mainly of andesite and dacite, with lesser amounts of basalt, basic (basaltic) andesite, bandaite, and rhyolite. In contrast, La D6sirade is underlain by a "basement" of easterly striking pillowed spilite, quartz keratophyre, and bedded chert of unknown ages that are intruded by a rock very similar to trondhjemite (Fink, written communication, January 11, 1971). Greenschists and related rocks have been dredged from an escarpment north of La Desirade (Johnston et al. 1971). Similar eugeosynclinal rocks are common in the Greater Antilles (Khudoley and Meyerhoff 1971) and also are known from the Barracuda fracture zone (Bonatti 1967, p. 481-482) east of La D6sirade. Several K- Ar and U-Pb ages for the La Desirade trondhjemite range from 139 to 143 m.y. (Tithonian, Late Jurassic). Fink (written communication, January 11, 1971) stated that, "The fundamental question at this point is whether the igneous rocks on La Desirade are the only known exposures of nascent island arc products or relict sea- floor crust."

Fink (1970b) suggested that the ancestral Lesser Antilles arc began to form in Late Jurassic time and was active until Eocene- Oligocene time. At this time, according to Fink, crustal subduction ceased, a new arc formed on the west (the present Lesser An- tilles arc), and sea-floor spreading and subduction began anew (see Fink 1971, and in press). Fink proposed that the renewed sea-floor spreading caused the rise and emergence of the Barbados Ridge. Thus Fink (1970b, 1971, and in press) interpreted the development of the entire Lesser Antilles arc and the Barbados Ridge in terms of "the new global tectonics."

Problems.-If the term may be stretched a little farther than it has been already, La Desirade may be labeled a "maverick island." It is east of the Limestone Caribbees, and its structural trend is east-west. Its keratophyre and spilite match none of the volcanic products found among the Volcan- ic Caribbees, but are much more closely

linked with rocks of unknown age dredged from the Barracuda fracture zone on the east (fig. 1, and Bonatti 1967). Northwest of La D6sirade the rocks also can be matched, not only in type but also in ap- proximate age, by keratophyre and spilite in the Greater Antilles (Khudoley and Meyerhoff 1971). The island of La Desirade surfaces above a salient which protrudes 50 km east of the scarp that separates the Limestone Caribbees from the Lesser An- tilles Trench (Hess 1966, bathymetric chart). The salient lies north of a structural zone of weakness (possibly a transverse fault, as proposed by Fink 1970b), along which the north end of Barbados Ridge, the southern end of the Limestone Carib- bees, and Aves Island are aligned. In figure 9 this zone is shown as the southern edge of the Greater Antilles orthogeosyncline, and we suggest the term "Dominica fault" for this transverse feature. Although long-distance projection has its hazards, there is much more logic in extending the east- southeast trend of the Greater Antillean Jurassic-through-Eocene structures as far as La Desirade than there is in erecting this single island into a nascent island arc, halting sea-floor spreading, and restarting it to form the younger Lesser Antilles arc, as Fink (1970b) has proposed.

In addition to the Greater Antilles structural trend, there are sound tectonic reasons for our proposal that La D6sirade is the easternmost outpost of the Greater Antil- les orthogeosyncline. The latter, like the Caribbean orthogeosyncline of northern South America, was compressed between the North and South American continents against the rigid Caribbean basin, which we interpret to be an ancient oceanic plate fixed in position, except for a small eastward rotation, between North and South America. The major stress components were from south-southwest for the northern orthogeosyncline and from north-northwest for the southern orthogeosyncline. A sec- ondary component was directed toward the east and is believed to have caused the ini- tial development of Aves Ridge. Both the

44


FIG. 9.-Interpretation of relative geologic ages of Greater Antilles and Caribbean orthogeosynclines (Late Jurassic-middle Eocene), Aves Ridge (Late Cretaceous-middle Eocene), and Lesser Antilles arc (middle Eocene-present). Location shown on fig. 1.

northern and southern orthogeosynclines originated during Late Jurassic or earlier time, and the compressive stresses were operative through middle Eocene time. On both the north and south, Caribbean-facing volcanic chains or arcs were formed during part or all of this time span. Donnelly's (1964) contention that the Greater Antilles orthogeosyncline faced the Atlantic, rather than the Caribbean, is unfounded, as dem- onstrated by Khudoley and Meyerhoff (1971).

Since post-middle Eocene time the major stress component has been eastward. Aves Ridge, intruded by granodioritic and related plutons, became part of the compe- tent Caribbean crust, and compression has been exerted against and through the par-

tial closure effected by the converging Bar- bados Ridge extension of the Caribbean orthogeosyncline and the La Desirade extension of the Greater Antilles orthogeosyncline. The result of continued compression is the Caribbean-facing Volcanic Car- ibbees arc on the west, and the raised marginal ridge supporting the Limestone Car- ibbees on the east, which is overriding the Lesser Antilles Trench. This new arc is superposed on salients of the older Greater Antilles and Caribbean orthogeosynclines (fig. 9). The rigid La Desirade salient has been raised sufficiently for part of it to surface and to elevate the Oceanic Formation of Barbados to its present position. It is difficult, if not impossible, to visualize the pro- cess by which the Oceanic Formation was

45


raised to its present position if subduction by the so-called Americas plate was the body producing the operative stress. If subduction is taking place, it must be far out (somewhere in the Atlantic?).

Scholl et al. (1968, 1970) arrived at similar conclusions regarding the Peru-Chile Trench. However, Scholl and his colleagues did not have the benefit of a dated ridge, such as the Barbados Ridge, west of the Peru-Chile Trench, to be able to establish definitely that underthrusting has not oc- curred.

A collateral effect has been to superim- pose on the original east-northeast structure of the Barbados Ridge its present north-south trend, its dual ridge structure with the intervening Barbados Basin, as well as the Tobago Trough separating the Ridge from the southern Volcanic Carib- bees. It is possible that we are witnessing the formation of a new eastern Antillean orthogeosyncline.

Conclusions.-La D&sirade, as interpreted by us, is genetically a part of the Greater Antilles orthogeosyncline, as the Barbados Ridge is genetically a part of the Caribbean orthogeosyncline. The modern Lesser Antilles are younger than the Aves Ridge, and it is likely that they formed during Paleocene and early Eocene times.

AVES RIDGE

Description.-Aves Ridge (fig. 10) long has puzzled geologists. Most students of the eastern Caribbean have interpreted the Aves Ridge to be an anticlinorium which formed by compression in an east- west stress field. The only island on the

ridge, Aves Island, shows some evidence of east-northeast-striking structure (Maloney et al. 1968; Weeks et al. 1971). If this observation is confirmed, the Aves Ridge must have been in existence when the Ca- ribbean orthogeosyncline, including Barba- dos, was folded. Recent dredge hauls have confirmed that the rocks which comprise the Ridge did exist during the latest Cre- taceous-early Eocene folding of the Carib- bean geosyncline, and the nature of the recovered rocks led Fox et al. (1972) to con- clude that Aves Ridge is underlain by granitic rocks.

Weeks et al. (1971) reported that the dredge hauls recovered basalt at some localities, and that at 50 localities, granitic intrusive rocks were recovered. Fox et al. (1969, 1972) reported the dredging of granitic rocks, unconsolidated Quaternary sediments, and shallow-water, lithified carbonates of late Eocene and early to middle Miocene ages. Late Miocene and Plio- Pleistocene samples which were recovered indicate deep-water deposition.

The granites have been dated, but none of the dates has been widely publicized. Joseph R. Curray (written communication, September 3, 1970; Curray et al., in preparation) wrote that, "Neil Maloney and I did succeed in dredging some granitic rocks off the southern part of the Aves Ridge... and have, with some of our colleagues, been slowly working away at that rock ever since. We . .. now do have some information and are preparing to publish a short note. The data... have been worked up in collaboration with Manuel Bass,... J. Hawkins, and ... D. Macdougall.... We

0 50

KM

FIG. 10.-West-east seismic-reflection profile C, Aves Ridge. Location shown on fig. 1. From Bunce et al. (1971). Published with permission of authors and Wiley-Interscience.

46


have dates both by K-Ar and by fission track. These dates range from 46 to 66 million years [late Eocene to earliest Paleo- cene] with some consistent variation by technique, and are a bit younger than those obtained by Heezen and his associates.3 Most of our samples are relatively rich in K-feldspar and merge on granodiorite."

Thus a partial history of the Aves Ridge can be deduced. A sequence of strata of unknown lithology and age-probably Cre- taceous and earliest Tertiary-was deposit- ed and deformed. H. A. Meyerhoff (1954, p. 153) suggested that the Late Cretaceous volcanogenic section (mainly pyroclastic rocks) of St. Croix may belong to the Aves Ridge structural province and, if so, the St. Croix section may typify the sequence underlying the late Eocene carbonates of the Aves Ridge.

During and after deformation, granodioritic rocks of latest Cretaceous to Eo- cene age intruded the Aves Ridge. The ridge was uplifted and eroded, and a shallow-water late Eocene through middle Mio- cene carbonate platform formed. During late Miocene time, the ridge began to sink, and a deeper water regime has since characterized much of the ridge.

Relation to Lesser Antilles arc.-The relations between the Aves Ridge and the Lesser Antilles arc are unknown. We believe that the Aves Ridge probably is a volcanic-arc precursor to the Lesser Antilles arc. A. A. Meyerhoff (1966) compared the Lesser Antilles-Aves system with the ridge- trough-ridge systems of the Bartlett Trough and the Marianas-Bonin arc. In both ex- amples, two ridges are separated by 180- 220 km of deep water. However, the Bart- lett system is associated with an active fault zone; hence, a comparison with the double ridge systems of the western Pacific seems more appropriate (Dietz 1954; Karig 1971).

As stated, we prefer to interpret the Aves

3 Dates ranging from 57 to 89 m.y. have been determined at Columbia University (Fox et al. 1972). Sample collections were made by B. C. Heezen and his colleagues from aboard the R/V Eastward.

Ridge as a precursor to the Lesser Antilles arc. As the Caribbean plate was thrust eastward between the Greater Antilles orthogeosyncline on the north and the Carib- bean orthogeosyncline on the south, the first volcanic arc to form (in middle Creta- ceous time) was the Aves Ridge. After it was deformed and intruded during the early part of the "Laramide" orogeny, a new arc -the present Lesser Antilles arc-formed east of the Aves Ridge during Paleocene- Eocene time. The Lesser Antilles arc in- volved a segment of the inactive Greater Antilles orthogeosyncline (for example, La Desirade Island). Thus, as postulated by H. A. Meyerhoff (1954, p. 155), the Lesser Antilles anticlinorium forms "a transverse range which can be simply explained as a product of compression resulting from rotation of the Caribbean basin." One major difference between our interpretation and that of H. A. Meyerhoff in 1954 is that the Barbados Ridge is now known to be a salient of northern South America, which was uplifted anew during the Tertiary as a result of eastward movement of the Carib- bean "plate."

PRINCIPAL BASINS

GRENADA TROUGH

The Grenada Trough is between the Aves Ridge and the Lesser Antilles arc. In the south, the bottom of the Trough is nearly flat (fig. 11), and the beds dip gen- tly toward the south and west (Bunce et al. 1971). A turbidite sequence characterizes the upper part of the section and overlies unconformably an older section of rocks. The total thickness to acoustical "basement" is about 1,500-2,000 m, although older sediments may underlie the acoustical "basement." Northward the turbidite section wedges out, and west of Guadeloupe Island, the Tobago Trough section and the acoustical "basement" are disturbed by faults, gentle folds, and slumps (Bunce et al. 1971).

TOBAGO TROUGH

The Tobago Trough separates the Less- er Antilles arc from the Barbados Ridge.

47


0 50

KM

FIG. 11.-West-east seismic-reflection profile D, Grenada Trough. Location shown on fig. I. From Bunce et al. (1971). Published with permission of authors and Wiley-Interscience.

In contrast to the Grenada Trough, the floor of the Tobago Trough is bowed upward on the eastern and western flanks (fig. 12), and there is considerable evidence for large-scale slumping on both of its flanks. Although the ages of the sediments in the Grenada and Tobago Troughs have not been determined, the upturn of the strata on both margins of the Tobago Trough, in contrast to their horizontal position in the Grenada Trough, lends strong support to the earlier origin which we as- sign to the Aves Ridge.

Several unconformities are present in the section, particularly along the basin margins. At least 2,000-3,000 m of section is present above acoustical "basement."

North of the Trough, the section is tilted eastward, and there is much evidence for gravity sliding from west to east (Bunce et al. 1971).

LESSER ANTILLES TRENCH

The Lesser Antilles Trench extends north from the Tobago Trough along the eastern

margin of the Lesser Antilles arc. The oceanic basement and overlying rocks dip westward beneath the volcanic arc (Bunce et al. 1971). Our figure 4, taken from Chase and Bunce (1969), was interpreted by them as evidence for underthrusting of the Carib- bean "plate" by the so-called Americas plate. In sections farther south along the eastern side of the Barbados Ridge, Chase and Bunce (1969) and Bunce et al. (1971) also postulated underthrusting (fig. 13). Chase and Bunce used the steep-angle re- verse faults shown on figure 13 as further support for their underthrusting interpretation. However, the seismic sections they have presented also support west-to-east overthrusting of the Caribbean "plate," accompanied by slumping of unconsolidated sediment masses from the Lesser Antilles arc and the Barbados Ridge. The Ridge was broken into hundreds of fault slices during latest Cretaceous-early Eocene orogeny, and many of the faults seen on figure 13 may be of Eocene age. Regardless of

0 50

KM

FIG. 12.-West-east seismic-reflection profile E, Tobago Trough. Location shown on fig. 1. From Bunce et al. (1971). Published with permission of authors and Wiley-Interscience.

48


o 50

KM

FIG. 13.-West-east seismic-reflection profile F, East Ridge of Barbados Ridge. Location shown on fig. 1. From Bunce et al. (1971). Published with permission of authors and Wiley-Interscience.

their age, they show a remarkable resem- blance to the imbricate faults along the Lewis and Clark overthrust in Montana.

Relations between Puerto Rico Trench and Lesser Antilles Trench.-We have mentioned the fact that, on the basis of Hess's (1966) bathymetric map, the Puerto Rico Trench is separate from the Lesser Antilles Trench. The latter strikes southward, on the western side of the Barbados Ridge (fig. 1). The main point is that the two trenches are just that-two trenches, the Puerto Rico Trench striking far eastward into the At- lantic and intersecting the Lesser Antilles Trench at an angle of nearly 600°.

ALLEGED PLATE TECTONICS

DATING

The sea-floor spreaders' struggle to in- sert a Caribbean plate between the North and South American continents has been negated by the emergence of implacable facts. Emplacement from the Pacific Ocean had to take place in Jurassic time, or certainly not later than very Early Cretaceous time, because the most recent dating of the North Central American orogen (fig. 2) indicates that the Middle America Trench would have become a subduction zone in pre-Late Cretaceous time (Barr and Esca- lante 1969). This fact precludes Mattson's (1969) and Malfait and Dinkelman's (1971) postulated early Tertiary date of insertion. The Pacific origin further assumed an ancient floor for the Caribbean plate (Edgar et al. 1971a), for it was supposedly a drifted relict from the inactive Darwin Rise. How- ever, JOIDES Leg 15 drilling found Late

Cretaceous diabase beneath 746 m of sediments, the oldest of which contained a middle Late Cretaceous fauna. Thereupon, Edgar et al. (in Purrett 1971) abandoned their earlier insertion hypothesis and postulated that the Caribbean plate formed in situ with the north-south separation of North and South America.

A weak case might have been made for the retention of the Pacific origin of the plate because sediments at the diabase contact are baked, and the diabase obvious- ly is an intrusion younger than the sediments;4 hence the drill, which penetrated only 18 m of diabase, did not reach the base of the sedimentary section. This straw at which some mobilists have grasped would be more attractive if only there were a mid- Caribbean ridge to serve as a spreading center, or if, somewhere, there were a linear pattern of magnetic anomalies. Neither is present (fig. 14). It also would help if the southern, western, and northern margins of the Caribbean were not flanked by sedimentary sequences that thin toward the center of the basin from marginal thick- nesses of 6,000-8,000 m (Edgar et al. 1971a). The presence of these thick marginal sedimentary basins is devastating to any concept of insertion or in situ origin.

JOIDES site 10, leg 2, western Atlantic, also supposedly reached basaltic "basement" in Cam- panian (80-m.y.-old) sediments. Dating of the "basement" (which also has baked contacts) revealed the "basement" to be 15.9 m.y. old (Macdougall 1971), as predicted by Kamen-Kaye (1970) for most JOIDES basaltic "basements," the majority of which have baked contacts.

49


FIG. 14.-South-north seismic-refraction profile across Caribbean Sea. From Officer et al. (1959). Note total absence of evidence for midocean ridge spreading center. Location shown on fig. 2. Reprinted with permission from authors and Pergamon Press Ltd. (@ 1959).

PRESUMED NORTH CARIBBEAN FAULT

Figure 3 shows the "lithosphere plates" required by the sea-floor spreading hypothesis. The presumed Caribbean plate is bounded on the west and east by subduction zones, and it is bounded on the north and south by through-going faults. On the north is the so-called North Caribbean fault, which is supposed to consist of the Bartlett fault on the west, the Cibao graben in the center, and the Puerto Rico Trench on the east.

Figure 2 shows the fault systems of the northern Caribbean.

Earthquake data show that the faults are not connected (fig. 15; Molnar and Sykes 1969, p. 1655); bathymetric data support this thesis; and geologic data are even more conclusive.

The Bartlett fault system lies along a Paleozoic zone of structural weakness

(Suess 1909, p. 524-525; A. A. Meyerhoff 1966; Anderson 1969; Bonis 1969). Al- though strike-slip movements take place along the fault system, dip-slip movement has predominated from early in the fault zone's history (Anderson 1969; Bonis 1969) This statement is substantiated not only by field studies of the fault in Guatemala (An- derson 1969; Bonis 1969), but also by extrapolation of Cretaceous structural trends between Cuba and Hispafiola (A. A. Meyer- hoff 1966). On the west the fault dies out in central Guatemala (Anderson 1969), and on the east it ends due south of Oriente Prov- ince, Cuba (A. A. Meyerhoff 1966). It never reaches the Cibao graben.

The Cibao graben is isolated and does not join either the Bartlett fault system on the west or another fault zone on the east (A. A. Meyerhoff 1966; Khudoley and Mey- erhoff 1971). Dip-slip movement has char-

FIG. 15.-Earthquake epicenters of Caribbean. From Sykes and Ewing (1965) and Molnar and Sykes (1969). Published with permission of authors, American Geophysical Union, and The Geological Society of America.

50


acterized most of its history. There is no measurable strike-slip component in excess of a few kilometers. The fault probably originated during the Paleocene-early Eocene, much later than the Bartlett fault system (Khudoley and Meyerhoff 1971).

The age of the Puerto Rico Trench is post-early Eocene (Van Voorhis and Davis 1964; Monroe 1968). The fault system dies out on the western and eastern ends. The system forms a graben, and predominant movement is dip-slip (Sykes and Ewing 1965; Molnar and Sykes 1969, p. 1657- 1658). The Puerto Rico Trench abruptly cuts the west-northwest-east-southeast early Eocene-Cretaceous trends of Puerto Rico and the Virgin Islands. These trends strike obliquely across the Puerto Rico Trench and have been mapped with the magnetom- eter (Van Voorhis and Davis 1964).

Thus, the northern Caribbean is characterized by three unconnected fault systems of different ages.

To circumvent the fact that the Bartlett,

Cibao, and Puerto Rico fault systems are separate, Donnelly (1967)-following Barr (1958)-postulated that the so-called North Caribbean fault crosses the Virgin Islands via the Anegada Trough, passes south of Puerto Rico to the Enriquillo graben of Hispaiola, and joins the Bartlett fault zone west of Haiti. This postulate encounters even more serious problems. Not only are the ages of the Bartlett and Enriquillo faults very different, but also there is no earthquake activity or fault zone south of the Dominican Republic and Puerto Rico (Sykes and Ewing 1965; Molnar and Sykes 1969). Ball and Harrison (1969) also have reviewed some of the difficulties in postu- lating a North Caribbean fault.

PRESUMED SOUTH CARIBBEAN FAULT

The so-called Caribbean plate also has to be bounded on the south by a through-going fault. Figure 16 shows the general structure of the Trinidad-Barbados Ridge area of the southeastern Caribbean. The El Pilar

CONTOURS IN FM.

4-ANTICLINE OR ARCH

SYNCLINE OR TROUGH

FAULT OR FAULT ZONE D PROMINENT UNCONFORMITY

(EDGE OF BASIN \0 20 40 60

(FROM BASSINGER ET AL.,19711

FIG. 16.-Major structural elements of southeastern Caribbean. Locations of figs. 17, 18, 19, and 20 are shown. From Bassinger et al. (1971). Published with permission of authors and The American Associa- tion of Petroleum Geologists.

51


fault truncates the east-northeast regional strike of Trinidad and Venezuela, and it may extend beneath the Cariaco Trench, to continue its east-west strike on the mainland as far as Valencia. West of Valencia, most sea-floor spreaders connect the El Pilar fault with the Oca fault north of the Gulf of Maracaibo (fig. 15), but there is no evidence of a connection across the 350-km stretch between the Gulf of Maracaibo and Valen- cia. Apart from its discordant trend relative to the El Pilar fault, the Oca fault is supposed to pass westward through northwestern Colombia and/or Panama into the Pa- cific. No surface faults and no epicentral zones exist in this area. The Oca fault dies out just west of the Guajira Peninsula of northeastern Colombia (fig. 15). The problems were summarized briefly by Ball and Harrison (1969), but were expressed very nicely by Molnar and Sykes (1969, p. 1660- 1661), in what may be described as classic understatement: "Hence, in northwestern South America, the present data do not ap-

o (0 KM (FROM BASSINGER ET AL 197,1

--SEDIMENTS +--I"BASEMENT"

FIG. 17.-North-south seismic-reflection profile G, east of Tobago, across El Pilar block and fault. Location shown on figs. 1 and 16. From Bassinger et al. (1971). Published with permission of authors and The American Association of Petroleum Geol- ogists.

pear to define a consistent pattern. There is some suggestion of underthrusting of the Pacific under South America from the seismicity, but other complexities are present. Moreover, the present mechanism solutions do not seem to clarify the tectonic pattern very much."

Surface and geophysical data from Trini- dad, northeastern Venezuela, and the areas offshore show that the El Pilar and Oca faults are characterized by minor strike- slip and large-scale dip-slip movements. Figure 17 depicts the south end of the Bar- bados Ridge far east of Tobago at the eastern end of the El Pilar fault, close to the point where it cannot be traced farther east by geophysical methods or by epicenter patterns. The gentle drag resulting from dip-slip movement and the total absence of evidence for strike-slip movement are note- worthy.

Figure 18 shows a section just east of Tobago, which also crosses the El Pilar fault. The evidence for dip-slip motion is clear, but evidence for strike-slip movement is negative. Figure 19, of the Dragon's Mouth between the Northern Range of Trinidad and the Paria Peninsula of Vene- zuela, shows the same type of evidence. North of the line of section of figure 19, the section is flat-lying and almost undisturbed -a typical post-early Eocene section of the Venezuelan continental shelf. The absence of any evidence for an east-west, through- going strike-slip fault on the shelf or across the southern part of the Lesser Antilles arc (fig. 20) eliminates Barr's (1958) suggestion that the South Caribbean fault passed south of Grenada and north of the shelf off

N S SECTION H

FIG. 18.-North-south seismic-reflection profile H, east of Tobago, across El Pilar block and fault. Location shown on figs. 1 and 16. From Bassinger et al. (1971). Published with permission of authors and The American Association of Petroleum Geologists.

52


FIG. 19.-North-south seismic-reflection profile I, between Paria Peninsula and Northern Range of Trinidad. Crosses El Pilar block and fault. Loca- tion shown on figs. 1 and 16. From Bassinger et al. (1971). Published with permission of authors and The American Association of Petroleum Geologists.

Venezuela. Geophysical data published by Lattimore et al. (1971) and Peter and Latti- more (1971) also eliminate the possibility of a South Caribbean fault. Therefore, as is true of the so-called "North Caribbean fault," there is no evidence for a through- going "South Caribbean fault."

EPICENTERS AND STRUCTURE OF

EASTERN CARIBBEAN

It is difficult to resist putting frosting on a cake, and in the eastern Caribbean, the frosting is available. Therefore, it should not be wasted.

Figure 4 shows the Chase and Bunce (1969) interpretation of a lithosphere plate sliding under the Lesser Antilles arc and the Barbados Ridge, which is east of the Lesser Antilles arc. If Chase and Bunce are correct, the epicenters in this area should be distributed east of and beneath the Barba- dos Ridge on the south. Figure 21 shows clearly that the epicenters are uniformly just west of, beneath, and just east of the Lesser Antilles arc, but do not underlie, or fall east of Barbados and the eastern Bar- bados Ridge. Moreover, because the epicenters do not underlie the Ridge or fall

east of the Ridge, the Chase and Bunce hypothesis, if correct, would record the only exception to island-arc epicenter distribu- tions in the world. In short, figure 21 shows that the Barbados Ridge is aseismic.

The importance of figure 21 cannot be overstated. This figure shows that, if subduction of the Caribbean by the so-called Americas plate is taking place, the Barba- dos Ridge (which is part of the South Amer- ican continent and of the alleged Americas plate) is not affected. This clearly is not possible. Nor can it be argued that the Bar- bados Ridge has been rotated into its present position, because the structures underlying the Ridge and exposed on Barbados island parallel those of Tobago, Trinidad, and northern South America.

A very similar situation exists in the southwestern Pacific, from Tonga-Kerma- dec on the north, onto North Island (New Zealand) on the south (Hatherton 1970, 1971). According to the seismic data (on the assumption that the sea-floor spreading hypothesis is correct), the eastern part of North Island is being subducted under the rest of North Island, a concept which, as in the Lesser Antilles, is ruled out by the geology and geophysics; eastern North Island is Permian or older.

In the Caribbean, other problems also arise.

1. The concept that the so-called Carib- bean plate was inserted from the Darwin Rise of the Pacific is invalidated by the facts that (a) the North and South Central American orogens (fig. 2) were in existence by Early to middle Cretaceous time (A. A. Meyerhoff 1967; Barr and Escalante 1969)

FIG. 20.-South-north seismic-reflection profile J, west of Grenada. Location shown on figs. 1 and 16. From Hurley (1966). No through-going faults are present. Hence, so-called South Caribbean fault does not pass north of Venezuelan shelf, as suggested by Barr (1958).

53


FIG. 21.-Distribution of epicenters in eastern Antilles. From Molnar and Sykes (1969). Published with permission of authors and The Geological Society of America.

and (b) the sediments in the Middle Amer- ica Trench are fiat-lying (Ross 1971).

2. The latest hypothesis-that the Ca- ribbean plate formed by sea-floor spreading during Late Cretaceous-early Tertiary time (Edgar et al., in Purrett 1971)-is negated by the fact that (a) there is no spreading center in the Caribbean; (b) the Cretaceous and younger strata are flat-lying adjacent to the Greater Antilles and Caribbean orthogeosynclines; (c) the Barbados Ridge, 550 km long, fronts two-thirds of the Lesser Antilles arc (thus, 550 km of the width of the Caribbean plate had to be in existence since pre-Late Jurassic time); and (d) the Aves Ridge does extend the full width of the Caribbean Sea and has existed since pre- latest Cretaceous time. These points also

eliminate the Ball and Harrison (1969) "extension" hypothesis. Edgar et al. (in Purrett 1971) brushed aside the absence of a spreading center and attributed the origin of the Caribbean ocean basin to the tearing of North America from South America. In effect, Edgar et al. had to propose another hypothesis of ocean-basin formation.

3. La Desirade Island also is of pre-Late Jurassic origin, but maintains its east-west structural grain and its position east of the postulated Lesser Antilles subduction zone.

4. According to all pre-drift reconstruc- tions (see Bullard et al. 1965), the area of the Lesser Antilles was adjacent to West Africa. Some credence might be given to this concept if there were correlative orthogeosynclines in West Africa, but there are none.

54


5. According to the sea-floor spreaders, the alleged Americas plate had moved more than 500 km between Late Jurassic and Paleocene times. This amount of movement means that La Desirade and the Barbados Ridge should have been subducted by Pa- leocene time. No hypothesis of sea-floor spreading explains the presence of these two ancient segments of older rocks east of the alleged subduction zone. According to the measurements of all sea-floor spreaders (the half rate of spreading is 1-1.5 cm per year), Barbados Ridge and La D6sirade Island now should be subducted, without trace, 200-400 km beneath the Lesser Antil- les arc.

The amount of supposed subduction which must have taken place, the aseismic nature of the Barbados Ridge, the necessity for a medium- to high-grade metamorphic terrane between the Araya-Paria Penin- sulas, the Collette et al. (1969) and Bas- singer et al. (1971) geophysical data from the Barbados Ridge, and the time-sequence implications of the structural trends on Barbados were not taken into account by Daviess (1971). Daviess' failure to consider these facts in his reconstruction negates his interpretation of Barbados in terms of sea- floor spreading.

6. If the presumed Americas plate is descending beneath the Caribbean, that part of the plate which is east of the Lesser Antilles has to be moving at a faster rate than the rest of the plate.

7. Finally, the Mohoroviid6 discontinu-

DEPTH

IN

KM

w E 0 0

5 5

10 10

15 15

20 20 W E

0 100 200 KM (FROM BUNCE ET AL., 1971 ) VELOCITIES IN KM/SEC

VERT. EYAG.= 14: I

FIG. 22.-West-east cross section of Aves Ridge-Lesser Antilles arc-Barbados Ridge, from Bunce et al. (1971). Note that Mohorovcid discontinuity is at same depth beneath the so-called Caribbean plate on west and Americas plate on east. Published with permission of authors and Wiley-Interscience.

55

ity is 14 km below sea level east of the Bar- bados Ridge and west of the Aves Ridge, as shown on figure 22. The same is true for the eastern Pacific and the western Carib- bean. These facts make no sense at all if the alleged Americas plate is moving westward beneath the Caribbean and if the presumed Cocos (part of the Eastern Pacific) plate is moving northeastward beneath the Carib- bean (L. Austin Weeks, oral communication, December 9, 1970).

SUMMARY AND CONCLUSIONS

All current efforts to fit the Caribbean rim and basin into "the new global tectonics" mold fail every geologic test and have meager, if any, geophysical support.

1. Insertion of a Caribbean "plate" between the North and South American continents from the Pacific Ocean, as proposed by Mattson (1969), Edgar et al. (1971a), and Malfait and Dinkelman (1971), is an untenable hypothesis unless the "plate" was inserted before Late Cretaceous time.

2. Although the JOIDES Leg 15 bore- holes were not drilled to the base of the sedimentary section, Mattson's (1969) and Malfait and Dinkelman's (1971) early Ter- tiary date of insertion is precluded by closure of the postulated zone of entry in Cen- tral America by the pre-Late Cretaceous age of the Middle America Trench and associated orogenic belt.

3. The mobile plate concept requires through-going boundary faults on the north


and south of the Caribbean, but such faults do not exist.

4. The conclusive geophysical and geological evidence that movement along the several discontinuous faults on the northern and southern margins of the Caribbean basin has been vertical, with only minor strike-slip displacement, precludes the long- distance lateral shifts postulated by the mobilists.

5. The alternative idea of in situ origin by Caribbean sea-floor spreading (Edgar et al., in Purrett 1971) fails to produce an active or dormant midocean ridge, a corre- sponding epicenter belt, or linear magnetic pattern.

6. The claim that the so-called Americas plate is underriding and subducting the eastern margin of the Caribbean basin along the Barbados Ridge-Lesser Antilles arc is vitiated (a) by the presence of pre- Late Jurassic structural elements (Barba- dos Ridge and La Desirade), which, according to the requirements of the concept, should have been subducted at least 70 m.y. ago; (b) by the extension of the Less- er Antilles Trench west, instead of east, of the Barbados Ridge; and (c) by the aseismic nature of the Barbados Ridge.

In contrast to the shifting hypotheses of the mobilists, the cumulative geophysical and geologic evidence combines to provide an integrated, if incomplete, interpretation of Caribbean history, at least since Late Jurassic time.

1. The presence of Jurassic (or older) orthogeosynclines on the north (Greater Antilles) and south (Caribbean) shows that the Caribbean basin had its present east-west length since the Jurassic Period.

2. The Jurassic date, the character of the rocks on La Desirade, and the east-west structural trend indicate that this island is the eastern outpost of the Greater Antilles orthogeosyncline, raised to its present elevation by overthrust at the edge of the Lesser Antilles Trench.

3. The Aves Ridge, interpreted to be a product of compression directed from west to east, and antedating the formation of

the Lesser Antilles arc, proves that the Ca- ribbean basin had its present north-south width in pre-Late Cretaceous time.

4. Barbados Ridge is a pre-Late Jurassic feature, genetically (stratigraphically and structurally) related to, and an integral part of, the Caribbean orthogeosyncline, whose tectonic history it has shared, at least into early Tertiary time. The east- northeast strike of the pre-middle Eocene structures and stratigraphic trends parallels those of Trinidad and Venezuela, and dem- onstrates that the Barbados Ridge has not changed position since Mesozoic time.

5. The Lesser Antilles arc probably is an early Tertiary feature, a successor to the Aves Ridge.

6. Continued compressive stresses from the west have given the Lesser Antilles arc its north-south orientation and have super- imposed a younger north-south geomorphic pattern on the older (Late Cretaceous-early Eocene) east-northeast structures of the Barbados Ridge.

7. The west-to-east compression, caused by compression against, and rotation of, the Caribbean basin, apparently caused the overthrust that has exposed the Oceanic Formation on Barbados and brought La D6sirade to the surface.

8. Overthrust of the eastern margin of Caribbean basin, rather than underthrust of the alleged Americas plate, is favored by the structures shown in Barbados Ridge by Chase and Bunce (1969) and Bunce et al. (1971), as well as by the salients in the Lesser Antilles arc and the elevation of the Limestone Caribbees, La Desirade, and Barbados island.

Few, if any, of the known geological features of the Caribbean are adequately ac- counted for, or even considered, by the ad- herents of "the new global tectonics." The geophysical data on which they depend are, at best, equivocal, and some interpretations even lack that support (e.g., the in situ origin of the Caribbean). The "instant re- interpretations" of Caribbean history advanced as new data become available make mobilist dicta about other island arcs sus-

56


pect. The data published by Scholl et al. (1968, 1970) in the southeastern Pacific, and by Hatherton (1970, 1971) in New Zealand, also challenge "the new global tectonics" concept.

We believe that our interpretation of the Caribbean presented here will not have to be changed after the next dredge haul or core is taken from the ocean floor.

ACKNOWLEDGMENTS.-Typing was done by Miss Amy Lee Brown, Mrs. Carol Thompson, and Miss Ernestine R. Voyles. Drafting was done by Mrs. Kathryn L. Meyerhoff.

We are grateful to scores of colleagues who have encouraged and aided us in this work. Among them are Elizabeth T. Bunce, S. A. Bush, Joseph R. Curray, John I. Ewing, L. K. Fink, Jr., Bruce C. Heezen, Norman J. Snelling, John F. Tomblin, and L. Austin Weeks, who supplied the writers with valuable data for

this paper. L. K. Fink, Jr., was particularly generous in supplying us with radiometric dates from La D6sirade. We also thank Dr. Fink's co-workers, J. Mattinson, C. A. Hopson, C. T. Harper, and J. J. Stipp. Norman J. Snelling and John F. Tomblin were most kind to provide us with the radiometric dates from Tobago. We are grateful to M. E. Bickford, Meade Cadot, H. A. Ireland, J. A. Peoples, and Curt Teichert for valuable suggestions and criticisms when this paper first was presented orally on February 18, 1971, at The University of Kansas, Lawrence, Kansas. We particularly appreciate the thorough reviews by Jules Braunstein, Peter Misch, and Benjamin A. Tator-reviews which strengthened this paper, a version of which was presented at the Sixth Caribbean Geological Conference, Margarita, Venezuela, in July 1971 under the title, "Tec- tonics of the Eastern Caribbean." These acknowledgments in no way imply agreement with our opinions and conclusions.

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