The Caribbean or Lesser Antilles

The Caribbean area is set between the continents of N and S America and, in the west, is currently separated from the Pacific by the platelets of Central America. The Caribbean did not start to form in the east until the development of Tethys began to separate Africa from the 'New World'. The current geology, as represented in the tectonic map of the area prepared on a scale of 1:2,500,000 by Case and Holcombe (1980), is clearly complex and is, as one may infer from this map and articles in the literature, to some extent conjectural and not well understood.

The general geography of the Caribbean Sea, its main islands and adjacent landmasses, is shown schematically in Figure 7.21, together with the structural elements of the region. There are belts of deformed sediments completely surrounding the Caribbean Plate. Active external subduction is occurring at the Middle American Trench on the Pacific sea-board, and outside the Lesser Antilles Arc where the Atlantic lithosphere is being subducted. Internally, subduction is taking place at the Muertos Trough (MT) to the north, and at the northern limits of the South Caribbean Deformed Belt (SCDB) and North Panama Deformed Belt. Intense seismic and volcanic activity occur at both E and W ends of the region, while zones, or small areas, of seismic activity also occur along the northern and southern margin of the region.

The Colombian and Venezuelan Basins are two main structural elements of the Caribbean, where thicknesses of sediments have accumulated, which range from a few hundred metres to several kilometres of relatively undeformed pelagic sediments and turbidites. In these two basins, the age of the sediments is

Figure 7.21 Major tectonic-geomorphic elements of the Caribbean region, showing structural trends and zones of seismic activity.

usually younger than 90 Ma. In addition, there are three areas (the Nicaraguan Rise, Beata and Aves Ridges) whose early sediments, which are moderately deformed at intermediate to shallow depths of water, usually date from between 90 and 73 Ma.

It is important to note that, because magnetic anomalies over the Caribbean are of low amplitude, it is not easy to identify sea-floor spreading patterns. Indeed, Burke (1988) points out that, for the Caribbean ocean floor proper, there is, as yet, no wholly persuasive identification of magnetic anomalies.

There is debate as regards the extent of the arcuate feature which is so noticeable at the eastern limit of the Caribbean. Some refer to a variety of small segments of the Greater and Lesser Antilles (Pindell and Barrett, 1988). Burke (1988), however, suggests that the whole of the Lesser Antilles, and even part of the Greater Antilles, constitute one huge feature related to the penetration of the Pacific into the Caribbean (Figure 7.22). Just how a near semicircular arc could have developed from this intrusion of Pacific units between N and S America requires special pleading.

A reconstruction of the evolution of the western part of the Caribbean area based on the Atlas 3.3 program is shown in Figure 7.23. It will be seen that, since about 60 Ma, the migration of the component elements has been mainly from south to north. From this, one may infer that the Beata Ridge probably marks the most eastward incursion of the Pacific Plate. The main body of this ridge is over 1000 km away from the northern portion of the Antilles Island Arc.

The structural incursion along the north coast of S America extended much further to the east (Figure 7.24). However, as may be inferred from Figure 7.25, this structural incursion disrupted an arcuate system which was already in place. It will be seen from this latter figure that the Lesser Antilles islands are not currently a perfect arc, in that they comprise three sets.

The development of one main arc and two minor ones is attributed by Bouysse and Westercamp (1990) to modifications of the original arc, as the result of collision of two, three or even four elongated ridges in the Atlantic. These authors refer to an ancient arc, older than 50 Ma, and recent arcs aged less than about 25 Ma, with an intervening hiatus of volcanic activity that lasted about 10 Ma. As regards the age of initiation of the ancient arc, they are understandably vague, and suggest that it could have been active from the early Cretaceous. It is, of course the initiation and development of the ancient arc which most concerns us here. Let us now define the limits of what we consider the original Caribbean Arc to be. It will be seen from Figure 7.25 that the only well-developed trench in the eastern Caribbean occurs north of Puerto Rico, and that this trench only curves around the northern isles of the Great Antilles as far as Antigua and Barbuda. However, because of the seismicity and volcanic activity that occur in the vicinity of this island arc, it is

Letchworth State Park Camping Map

Figure 7.22 Schematic representation of the Caribbean 80 Ma ago as the result of penetration of the Pacific plate (after Burke et al.).

Backtracks relative to Mexico

Figure 7.22 Schematic representation of the Caribbean 80 Ma ago as the result of penetration of the Pacific plate (after Burke et al.).

Backtracks relative to Mexico

A Yucatan 160 - 140 Ma SE to NW: 140 - 0 Ma No movement

B Nicaragua 160-61 Ma Mainly Wto E . 60 - 0 Ma. S lo N

C Panama 160 - 0 Ma Continental movement: U-lum 60 Ma

A Yucatan 160 - 140 Ma SE to NW: 140 - 0 Ma No movement

B Nicaragua 160-61 Ma Mainly Wto E . 60 - 0 Ma. S lo N

C Panama 160 - 0 Ma Continental movement: U-lum 60 Ma

Backtrack Defenition
Figure 7.23 Using the 'backtrack' of platelets forming the Central American Isthmus. In the last 60 Ma movement in tracks C and D has been northward.

obvious that it is associated with an active, down-going slab that exists at least as far south as Grenada. These islands define an almost perfect circular arc through 110°, the radius R of which is 400 km, while the radius of the trench is 550-600 km. The geometry of these features, albeit modified by later events, is consistent with what one would expect to be generated by a major impact.

Figure 7.24 Inferred movement of the Caribbean-South America plate boundary zone (PBZ) from early Paleocene (60 Ma) to the present day.

7.5.1 The impact event

Figure 7.24 Inferred movement of the Caribbean-South America plate boundary zone (PBZ) from early Paleocene (60 Ma) to the present day.

7.5.1 The impact event

At what date did the impact occur? We can reasonably put an upper limit on impact at about 120 Ma, which represents, in this area, the age of 'old', normal Caribbean floor.

We have argued that impact events cause noticeable changes in the track, usually regarding direction and rate of plate movement. The track of the easternmost cape of Puerto Rico is shown in Figure 7.26. The track between 76 and 64 Ma is shown in Figure 7.26a. It will be seen that there was a modest change of direction and pace at about 66.5 Ma. This, we have already seen, relates to the Chicxulub impact event.

The other, earlier event, occurred at about 73.1 Ma, where there was a much more marked, sudden change in the direction of motion, and a 30 per cent reduction in rate of motion following the impact (Figure 7.26b). This event is taken to be the impact that gave rise to the Antilles arcuate features. One may infer from the greater angular change and the greater increase in rate of motion, that this event was probably more energetic than the Chicxulub impact, from which it is not greatly separated in either distance or time.

The Aves Ridge is unique, in that Coffin and Eldholm (1992) are in doubt as to this feature's origin. These authors suggest that this submarine ridge may be either a 'drowned' continental flood basalt, or an ocean basin flood basalt. It has already been argued that ocean basin flood basalts can result from the jetting of melt generated by a major impact. It is interesting to note, therefore, that corroboration of the impact date comes from the palaeontological evidence, which indicates that the Aves Ridge is younger than about 73 Ma.

We have already noted that the depth of water was moderate to shallow. In shallow water, the peak impact-stress of a major, fast moving, cometary body, would be capable of generating a significant volume of melt. Hence, jetting would be a distinct possibility. Alternatively, in somewhat deeper water, a large impacting body could cause an anticlinal, 'down-range', bulge of oceanic lithosphere, especially if the

Puerto Rico

ß Tobago Trinidad

Puerto Rico

ß Tobago Trinidad

0 200 km

Figure 7.25 The Caribbean Arc revealing complexities of the subduction zone which developed some 40-50 Ma ago as the result of the influence of a series of approximately E-W ridges. It also shows morphological regions within the island arc, with indications of the minimum age of the sediments together with the inferred position of the impact crater.

lithosphere was relatively thin. Fractures would develop in such a structure, thereby permitting the intrusion of asthenospheric material to enter the ridge and even to develop extrusions on the sea-floor.

The Banda Arc is a relatively small structure. Nevertheless, it has a diameter of about 450 km, so that it is approaching Alpine in scale. This arc is a puzzle surrounded by an enigma, in that it has received relatively little structural study in the far from well-understood complexities of the larger area which comprises the East Indies (Figure 7.27).

Harris (1989) concludes that 'the Banda orogen is an arcuate mountain system comparable in scale and structural style to other arcuate orogenic belts, such as the Alpine, Carpathian, Aegean, and Caribbean. These, and other examples of arcs, form loops where a mountain chain changes in strike by up to 180°. Whether the orogenic processes that give rise to these features are the result of the relative motions of larger bounding plates, or the produce of body forces which act independent of plate kinematics is controversial' (Wezel, 1988; Dewey, 1988). Thus, conventional mechanisms as applied to the Banda Arc are vague.

The fundamental reasons for the disposition of the main units within this Indonesian complex are understood in terms of the main plate movements at the southern and eastern limits of the area. The magnificent chain of islands that extends from Sumatra eastward to Timor and beyond, with an attendant trench, is obviously the consequence of the Indo-Australian plate, which moves generally northward and is subducted beneath the islands and shallow seas that form the southern East Indies. Australia has already made contact with the eastern area at Timor, and has caused the development of the mountain range that forms the spine of New Guinea. This continental unit is now probably at the stage which India had reached some 30-40 Ma ago.

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