|The neotectonic setting of Puerto Rico
D.G. Masson1 and K.M. Scanlon2
1Institute of Oceanographic Sciences,
2United States Geological Survey,
The island of Puerto Rico, in the north-east Caribbean, lies within a broad deformation zone between the Caribbean and North American plates. The simplest model for the tectonic setting of Puerto Rico has major strike-slip movement on nearly east-west lines in the vicinity of the Puerto Rico Trench coupled to a small anticlockwise rotation of a Puerto Rico block within the broader plate boundary zone. This simple model is attractive because it predicts the tectonic regime south of Puerto Rico, and provides an explanation for a possible component of extension across the Puerto Rico Trench west of 65.5°W. GLORIA long-range sidescan sonar data and seismic reflection profiles have been used to test this model by mapping the major tectonic features across the plate boundary north and south of Puerto Rico. To the north, the new data helps to resolve between conflicting models of underthrusting or strike-slip motion at the Puerto Rico Trench. No direct evidence of compression is seen, although evidence for normal and strike-slip movement is abundant. This combined with regional considerations, leads us to conclude that the main E.-W. trending part of the Puerto Rico Trench between 65.5° and 68°W lies within a strike-slip regime, although oblique convergence occurs both to the east and west where the plate boundary trends ESE. To the south of Puerto Rico, underthrusting of the Caribbean plate beneath the island decreases from west to east, and is ultimately replaced by extension in the Virgin Islands Basin east of 65°W.
Puerto Rico lies at the eastern end of the Greater Antilles island chain in the north-eastern Caribbean (Fig. 1). Since the end of the Eocene, the regional tectonics of Puerto Rico and the adjacent Virgin Islands have been dominated by left-lateral slip between the North American and Caribbean Plates (Burke and others, 1984). The islands lie within a complex deformation zone up to 300 km wide which occurs along the plate boundary, and which contains both extensional and compressional elements in addition to the dominant strike-slip movement (e.g. Sykes and others 1982; Mann and Burke, 1984; Burke and others, 1984; Byrne and others, 1985; Stein and others, 1988). The motion of the Caribbean Plate relative to neighbouring plates has been the subject of a lengthy debate. In their analyses of global plate motions, Jordan (1975) and Minster and Jordan (1978) calculated that Caribbean-North America motion at the Puerto Rico Trench should be almost pure strike-slip. This view, however, was challenged by Sykes and others (1982), who analyzed earthquake slip vectors from the north-eastern segment of the Caribbean-North American plate boundary. They concluded that oblique convergence was occurring along the Puerto Rico Trench segment of the plate boundary. A critical appraisal of the Jordan and Sykes and others models was presented by Stein and others (1988). By examining all available data from around the Caribbean Plate, these authors were able to demonstrate the superior all-round fit of the Jordan model, with the implication that no convergence occurs at the western part of the Puerto Rico Trench (Fig. 1, inset).
Within the broad plate boundary zone, most of the Caribbean-North America plate motion is thought to be accommodated north of Puerto Rico by strike-slip motion at the Puerto Rico Trench (e.g. Molnar and Sykes, 1969). However, some northward subduction of Caribbean ocean crust also occurs at the Muertos Trough to the south of Puerto Rico (Ladd and others, 1977; Byrne and others, 1985). Complex trans-tensional movement along Anegada Passage apparently links the eastern end of the Muertos Trough to the major strand of the plate boundary along the Puerto Rico Trench (e.g. Case, 1975; Case and others, 1984; Mauffret and others, 1986). Most authors argue that the Anegada passage has formed by pull-apart extension associated with left-lateral fault movement, in keeping with the overall left-lateral motion on the major plate boundary (see summaries in Case, 1975 and Case and others, 1984). However, Mauffret and others (1986) argue for a complex history involving both left and right-lateral displacement. They suggest that the most recent movement has been right-lateral, although an older left-lateral phase is also probable.
Aims of this study
In late 1985, the U.K. Institute of Oceanographic Sciences and the U.S. Geological Survey undertook a GLORIA long-range sidescan sonar survey of the entire Exclusive Economic Zone (EEZ) around Puerto Rico and the U.S. Virgin Islands (Figs. 1, 2; EEZ-SCAN 85 Scientific Party, 1987; Scanlon and others, 1988). The resulting mosaic covers an area in excess of 200,000 km 2, from 22°N to 15°N, straddling the Caribbean-North America plate boundary. We have used the sidescan data to divide the plate boundary zone into six geological provinces which form the framework for the interpretation presented in this paper (Fig. 3). North of Puerto Rico, three provinces can be distinguished: 1) oceanic crust of the North American Plate, 2) a basin and ridge province which includes the Puerto Rico Trench and the area of complex topography on its southern side, and 3) the steep island slope of Puerto Rico. South of the island, we can distinguish 4) a compressional tectonic province associated with the Muertos Trough, 5)an extensional province in the area of the Virgin Islands Basin, and 6)oceanic crust of the Venezuela Basin.
This paper describes the geological characteristics of each of the above provinces based on the sidescan data and associated seismic reflection profiles. We then examine our interpretations in the context of previous published work in the area. Finally, a regional tectonic model for the area is introduced, as we speculate on the tectonic setting of the entire area in the framework of Caribbean-North America plate interaction. The major points which we seek to make are that:
(1) Strike-slip motion is the dominant tectonic activity in the basin and ridge province immediately south of the Puerto Rico Trench, with no clear evidence for compression.
(2) The apparent eastward decrease in subduction of Caribbean oceanic crust at the Muertos Trough, and its ultimate transition of extension in Anegada passage, is compatible with a small anticlockwise rotation of Puerto Rico within the overall broad left-lateral plate boundary zone.
The GLORIA system
GLORIA is a long-range sidescan sonar system operating at a frequency of 6.5 kHz. Technical descriptions of the system have been published elsewhere (Somers and others, 1978; Somers and Searle, 1984). The data are recorded in digital form and processed to remove slant-range and aspect-ratio distortion and to correct for the attenuation of sound in water with increasing range (Chavez, 1986). The interpretation presented here was based on sonograph mosaics at a scale of 1:500,000 (EEZ SCAN 85 Scientific Staff, 1987).
GLORIA is the ideal regional mapping tool because its large swathe width (up to 45 km) and high tow speed (8 - 10 kts) allow large areas to be mapped quickly and efficiently. The present survey was conducted at a speed of 8 kts, with an airgun seismic profiling system, 3.5 kHz and 10 kHz high-resolution profiling systems, and a magnetometer towed simultaneously with the GLORIA fish. These profile data confirm the identity of sonograph targets crossed by the ship's track, as well as detailing their relationship with the sub-surface geology.
Oceanic crust of the North American Plate
The oceanic crust north of the Puerto Rico Trench is around 100 Ma in age (Rona, 1980). GLORIA shows a series of basement ridges with little or no sediment cover, trending 025 - 035°, throughout most of this area (Fig. 3). Although the surveyed area lies within the Cretaceous magnetic quiet zone, magnetic anomalies just to the west trend 030° (Rona, 1980; Fig. 1) and indicate that the 025 - 035° basement fabric is the original seafloor spreading fabric of the oceanic crust. Near the western edge of the survey area, near 20°N, a series of en echelon basement ridges, individually trending 150° but together defining a feature at 135°, are distinctly oblique to the spreading fabric (Fig. 3). This feature, which is also marked by a bathymetric high, has been identified as a north-westerly extension of a North Atlantic fracture zone ridge, Barracuda Ridge, by McCann and Sykes (1984).
The southern edge of the North American oceanic crust is strongly downfaulted into the Puerto Rico Trench, with the formation of some horst and graben structures as well as the predominant down-to-the-south normal faults (Figs. 4, 5). Such structures are commonly found where an oceanic plate bends into a trench, and result from tension in the upper part of the plate due to bending (e.g. Hilde, 1983). The trend of the normal faults is parallel or sub-parallel to the trench. The faults change trend abruptly (from 085° to 105°) at 65°45'W mirroring a change in trend of the trench which occurs just outside the eastern edge of our study area (Fig. 3). The zone of faulting also changes character at this point. To the west, it consists of a narrow zone made up to two major fault scarps (Fig. 3), while to the east, it is much broader and is made up of a complex horst and graben province (Fig. 3, and McCann and Sykes, 1984, their Fig. 3).
Puerto Rico Trench and the basin and ridge province
The flat, sediment covered floor of the Puerto Rico Trench is easily recognised as a band of low backscattering trending approximately 085° and crossing the surveyed area at 19°45'N (Fig. 6). Between the trench and the very steep lower continental slope north of Puerto Rico, between 19° and 19°30'N, lies a zone of ridges and sediment-filled basins up to 50 km wide (Figs. 3-6). As first noted by Ewing (1965), seismic profiles across the trench and the basin and ridge province show no clear evidence for recent compression. All of the sedimentary basins, including the trench, contain largely flat-lying, undeformed sediments, although the deeper strata within the trench exhibits a gentle southward tilt and, in places, very weak folding (Figs. 4, 5). Our profiles suggest a predominance of normal faulting south of the trench (Figs. 3-5), which may even indicate a component of extension across the plate boundary, although vertical movements associated with strike-slip faults are also likely to be important.
GLORIA data from the basin and ridge province show generally subtle backscattering contrasts, largely because the ship's tracks were orientated at a high angle to the major topographic features, thus minimising their apparent slopes. In addition, 3.5 kHz profiles collected with the GLORIA data show that the whole area is blanketed by a sediment veneer which much reduces expected backscattering contrasts between basement ridges and sedimentary basins. Despite this, major ridges and sedimentary basins have been mapped in considerable detail by careful integration of profile and sidescan data (Fig. 3). The major feature of these ridges and basins is that they have a dominant 95 - 105° trend, distinctly oblique to the axis of the Puerto Rico Trench which trends 086°. Although this has been noted in the case of the Main Ridge (Fig. 3; McCann and Sykes, 1984), it has not previously been recognised as a general feature of this region.
Within the basin and ridge province, a set of straight to slightly sinuous lineaments, trending 085 to 105°, can be traced, more or less continuously, for about 250 km across the entire width of the sonograph mosaic (Fig. 3). In the west, a single lineament occurs subparallel to and just south of the trench (Fig. 6). East of 66°45'W, this lineament turns to follow the trend of the Main Ridge. A second lineament, which parallels the trench from 66°45'W to the eastern edge of the study area, has a spur which runs along the southern edge of the trench floor for about 25 km (Fig. 3). Seismic profiles show no consistent topographic or geological feature which can be correlated with these lineaments; on various profiles they may correlate with apparent fault scarps or basement ridge crests or ridge flanks (Figs. 4, 5). No unequivocal interpretation of this set of lineaments is possible, but comparison with other areas where GLORIA and profile data have been collected strongly suggests that we have imaged a strike-slip fault system (e.g. Searle, 1981, 1986; Belderson and others, 1984; Parson and Searle, 1986). Straight to slightly sinuous lineaments of similar character are seen along large offset oceanic fracture zones, within which the fault zone may also show considerable variation in its topographic expression (e.g. Searle, 1981; Parson and Searle, 1986). No other geological feature is known to produce this combination of continuity and variability of topographic expression over distances of hundreds of kilometres. No sense of offset can be determined for the strike-slip faults on the basis of our data. However, given the left-lateral nature of the plate boundary, we assume that these are also left-lateral faults. In this context, a possible explanation for the trend of the Main Ridge is that it has formed at a restraining bend or compressional right-step of the left-lateral strike-slip fault system. However, confirmation of this interpretation must await the mapping of the continuation of the strike-slip faults outside our study area to the east of 65°, where at present we have no sidescan-sonar data.
Northern insular slope of Puerto Rico
Only in the extreme west of the survey area, around the head of Mona Canyon, do we see evidence for recent tectonic activity on the northern continental slope of Puerto Rico. Here, there is considerable evidence of faults trending NW-SE (Fig. 3), possibly on a northwesterly extension of the Great Southern Fault Zone seen on Puerto Rico (Gardner and others, 1980). Recent activity on the offshore faults is confirmed by seismicity data (McCann, 1985; Byrne and others, 1985). It is not at all clear how this NW fault trend relates to Mona Canyon structure. It seems likely that the box-shaped profile and extremely steep walls of Mona Canyon are fault controlled and that these faults have a N-S trend, only locally offset by NW-SE trending faults. However, no other N-S fault trends are seen in the area, with the possible exception of those controlling a small shelf-edge spur just east of Mona Canyon at 18°40'N, 67°06'W (Fig.3).
Farther east the upper slope is covered by a sequence of parallel-bedded, seaward-dipping sediments of probable middle Eocene to Recent age (Fox and Heezen, 1975; Perfit and others, 1980; Gardner and others, 1980; Meyerhoff and others, 1983; Moussa and others, 1987) cut by numerous canyons and large amphitheatre-shaped slide scars (Scanlon and Masson, in prep). Metamorphic rocks of pre-Eocene age, many of similar lithology to those exposed on north-west Hispaniola, outcrop on the very steep lower slope (Perfit and others, 1980).
Compressional regime associated with the Muertos Trough
It is now well established that Caribbean oceanic crust is being underthrust beneath Puerto Rico and eastern Hispaniola at the Muertos Trough, forming an accretionary wedge along the lower continental slope (Ladd and others, 1977; Byrne and others, 1985). GLORIA shows the deformation front at the base of the slope (Fig. 7), well-defined in the west but becoming less well-defined east of 66° and disappearing at around 65°, just to the south-west of St. Croix (Fig. 3). Within the accretionary wedge, fold axes trend E-W, parallel to the deformation front, along the lower slope. Higher on the slope, fold trends change to WNW-ESE, particularly towards the west of the survey area. Individual fold axes can rarely be traced for more than 20 km along strike, although one synclinal axis in the west of the study area, despite being much segmented by cross-faulting, can be followed for almost 100 km. The typical wavelength of folds is around 2 - 3 km near the deformation front. Overall, the accretionary complex gives a clear impression of becoming narrower and less well defined eastwards. At least 40 km of underthrusting has been documented at 68.5°W (Ladd and others, 1977), but this has decreased to zero at 65°W. We interpret this as indicating an eastward decrease in deformation rate, with important consequences in understanding Caribbean-North American plate motion (see discussion).
The Virgin Islands Basin Complex
The Virgin Islands Basin and adjacent smaller basins make up a structurally complicated province south-east of Puerto Rico (Fig. 3). The extensional nature of the basin is illustrated by an E-W seismic reflection profile across its western end (Fig. 8). GLORIA data show that fault trends are generally NE-SW to E-W (Fig. 9), although some NW-SE faults occur in a complex zone to the north-east of St. Croix (Fig. 3). North-easterly trending extensional faults also occur on the Virgin Islands Shelf (Donnelly, 1965) and in St. Croix; in the latter area they are dated as post-Oligocene in age (Whetten, 1966). Offshore, immediately to the west of St. Croix, faulting of late Miocene/early Pliocene age has been recognised (Holcombe and others, 1989), but seismic profiles clearly show that some offshore faults are active to the present day (Fig. 8). This is confirmed by active seismicity, particularly on faults associated with the northern margin of the Virgin Islands Basin (Murphy and McCann, 1979; Frankel and others,1980; McCann, 1985).
The Virgin Islands Basin appears to continue to the north-east through the Anegada Passage and to intersect with the Lesser Antilles compressional zone near 19°N, 63°W (Fig.1). To the south-west, however, the relationship between extension in the Virgin Islands Basin and compression in Muertos Trough is less obvious. Our seismic lines across the western end of the Virgin Islands Basin complex clearly show that extension decreases rapidly towards the south-west, with the throw of the major faults defining the extensional province decreasing from over 4 km to zero over a distance of less than 50 km. The final disappearance of the extensional province towards the south-west occurs within a few tens of kilometres of the eastern end of the Muertos Trough deformation front.
Venezuela Basin Ocean Crust
The oceanic crust of the Venezuela Basin is buried beneath a thick blanket of sediment and little structural information can be obtained from the GLORIA data. Normal faulting along the northern edge of the plate between 66° and 67°W, where it is bent down into Muertos Trough, is the only evidence for recent tectonic activity (Fig.7; Matthews and Holcombe, 1985).
Puerto Rico Trench: Strike-slip or oblique underthrusting?
Most authors agree that the main strand of the Caribbean-North America plate boundary lies along the Puerto Rico Trench. However, no such agreement exists regarding the relative plate motion and tectonic processes which occur at the plate boundary. Various authors have suggested oblique convergence (Sykes and others, 1982), strike-slip (Jordan, 1975; Minster and Jordan, 1978; Mann and others, 1984; Stein and others, 1988, or extension (Ewing and others, 1965). Several lines of apparently contradictory evidence have been presented in these papers, and it is easy to see why interpretational disagreements have persisted. In the following paragraphs we review the relevant published evidence and attempt to integrate it with our new data. The available evidence is most easily discussed in six categories, namely global plate motions, regional tectonics, seismicity, seismic reflection profiles, sidescan data, and gravity modelling.
(a) Global plate motions: These have been analysed by Jordan (1975), Minster and Jordan (1978) and Stein and others (1988). All agree that motion at the Puerto Rico Trench should be virtually pure strike-slip. The very small component of extension at the Puerto Rico Trench predicted by the Jordan model is not significant given the possible errors in the analysis.
Sykes and others (1982) took a more local view of Caribbean-North America plate motion, using only earthquake data from the north-eastern Caribbean in their calculations. Their model predicts oblique convergence at the Puerto Rico Trench, with the angle of convergence being 20 - 25°. However, their model may be seriously flawed because it ignores powerful constraints imposed on Caribbean plate motion by Caribbean-Cocos relative plate motion (Stein and others, 1988). Overall, global plate motion analysis would therefore appear to support strike-slip rather than convergent motion at the Puerto Rico Trench.
(b) Regional tectonics: The island of Puerto Rico shows scant evidence of recent active tectonics directly attributable to the major left-lateral plate boundary zone within which it is situated (Fig. 1). Little strike-slip or normal faulting has occurred since the early Miocene, although the occurrence of shallow water early Neogene limestone up to 5000 m below sea-level on the northern continental slope of the island demonstrates spectacular vertical movement since the early Pliocene (e.g. Moussa and others, 1987). In contrast, active tectonics associated with the plate boundary dominate the geology of the islands of Hispaniola and Jamaica further west (Fig. 1; Burke and others, 1980; Mann and Burke, 1984; Mann and others, 1984). Here, a consistent pattern of E-W trending strike-slip faults, NW-SE thrust faults and NE-SW normal faults is seen. This resolves to E-W strike-slip motion along the plate boundary (e.g. Burke and others, 1980; Mann and others, 1984) and consequently to no convergence at the Puerto Rico Trench.
(c) Seismicity: The most cited argument for Caribbean-North America convergence at the Puerto Rico Trench is the presence of a dipping zone of earthquakes which extends to depths in excess of 150 km beneath Puerto Rico and the Virgin Islands (Schell and Tarr, 1978; Frankel and others, 1980; Sykes and others, 1982; Fischer and McCann, 1984; McCann and Sykes, 1984). However, slip vectors for earthquakes in the Puerto Rico Trench area indicate motion sub-parallel to the trench, although fault planes are gently dipping rather than steep as would be expected in a typical strike-slip situation (Molnar and Sykes, 1969; Frankel and others, 1980; Frankel, 1982).
Detailed study of earthquake distribution shows that intermediate depth earthquakes are concentrated in two zones, one beneath the Virgin Islands east of 66°E (Frankel and others, 1980; Frankel, 1982; Fischer and McCann, 1984), the other beneath eastern Hispaniola between 67.5 and 71°W (Schell and Tarr, 1978; Byrne and others, 1985). Intermediate depth seismicity is considerably less intense in the intervening zone beneath Puerto Rico itself, although enough events have been recorded to define a dipping zone of earthquakes apparently continuous with that further east (Schell and Tarr, 1978; Sykes and others, 1982).
Overall, seismicity appears to support arguments for some convergence at the Puerto Rico Trench. However, slip vector analysis shows that the interpretation is not straightforward.
(d) Seismic reflection profiles: Seismic profiles across the Puerto Rico Trench have been interpreted to show either compression (McCann and Sykes, 1984) or a predominance of normal faulting (Ewing and others, 1965; this paper). However, none of the available profiles show direct evidence for compression, such as folding or reverse faulting. McCann and Sykes (1984) appear to have based their interpretation on (their) regional considerations and on the identification of various forearc components, such as forearc basins and outer arc ridges, in the basin and ridge province south of the Puerto Rico Trench. However, this identification cannot be carried to all profiles across this province, which show a considerable variety of basin and ridge structures (Fig. 5). More critically, we have been able to show that almost all of the tectonic elements of the basin and ridge province are distinctly oblique to the trench, a situation not seen in other forearcs, even in areas of highly oblique subduction such as off the western Aleutians (unpublished U.S. Geological Survey GLORIA data).
As noted earlier, our profiles are dominated by normal faulting (Figs. 4, 5) although we cannot differentiate between extension and vertical motion associated with strike-slip faulting. Certainly, some of the strike-slip faults identified from GLORIA data would appear to have vertical motion associated with them (Fig. 4).
(e) Sidescan data: Interpretation of GLORIA data has shown that most of the tectonic elements immediately south of the trench are distinctly oblique to it (discussed above), and that strike-slip faulting occurs in the same region. The discovery of strike-slip faulting is obviously compatible with the overall left-lateral nature of the plate boundary. However, it does not exclude the possibility of some convergence at the Puerto Rico Trench; strike-slip faulting associated with oblique convergence is well known from oblique subduction zones, such as the Aleutian Arc region.
(f) Gravity modelling: Molnar (1977) demonstrated that the gravity anomaly observed over the trench could be explained in terms of a "hanging flap of lithosphere" attached to the southern edge of the North American Plate and that the depth anomaly of the trench could be due entirely to the excess mass of this cold, dense flap. However, he was unable to distinguish between a model based on active subduction at the Puerto Rico Trench and a model where the flap was created further east at the Lesser Antilles subduction zone and then carried west by strike-slip motion. In this latter model, it is envisaged that material subducted beneath northeastern corner of the Caribbean plate would remain coupled to the North American plate and would be carried along beneath the northern edge of the Caribbean plate (Schell and Tarr, 1978).
In summary, all of the lines of evidence, with the exception of seismicity, favour strike-slip over convergent motion at the Puerto Rico Trench. However, as discussed in the next section, it may be possible to accommodate the earthquake evidence into a pure strike-slip model.
Plate motion at the Puerto Rico Trench - a strike-slip model
North of Puerto Rico, the Puerto Rico Trench trends 086°, the precise direction of strike-slip movement predicted by the pole of rotation given by Stein and others (1988). However, both east of 65.5°W and west of 68°, the plate boundary trends significantly south of east and some oblique convergence must occur (Fig. l, inset). This oblique subduction accounts for the dipping seismic zone beneath the Virgin Islands. However, as the North American plate moves relatively westward towards 65.5°W, the degree of convergence gradually decreases to zero. This could result in tearing of the North American plate as the subducting part of the plate detaches from the more northern part which continues to translate westward, giving a vertical, transcurrent plate boundary. Alternatively, if the subducting slab remains attached to the main plate to the north, the gradual northwest decrease in subduction would create a southward dipping slab beneath Puerto Rico, even though no convergence occurs at this part of the plate boundary (cf. Schell and Tarr, 1978). The plate boundary would then be a strike-slip fault of moderate dip. This scenario appears to fit all the evidence presented in the previous section, and is the interpretation preferred here. Clearly, it reconciles the apparently contradictory lines of evidence suggesting both a dipping seismic zone and a strike-slip plate boundary.
Strike-slip faulting in the basin and ridge province south of the Puerto Rico Trench, as observed in our GLORIA data, could be driven by incomplete de-coupling of the two major plates across the dipping plate boundary. Left-lateral strike-slip movement in this area would transfer westwards material accreted at the oblique subduction zone east of 65.5°W. If this is the case, then the dominant ESE structural trend of the basin and ridge province may be explained as an inherited trend from the zone of oblique subduction east of 65.5°W, where the ESE trend would parallel the subduction zone strike. Alternatively, the ESE trend may in part at least result from compression at a restraining bend in the strike-slip fault system.
Farther west again, the increase in intermediate-depth seismicity beneath Hispaniola may mark a further zone of oblique underthrusting (Fig. 1, inset). An abrupt change in the configuration of the subducting slab beneath eastern Hispaniola may also contribute to the zone of most intense intermediate depth earthquakes in this area (Schell and Tarr, 1978; Sykes and others, 1982; Westbrook and McCann, 1986).
Muertos Trough and Anegada Passage
Away from the main strand of the plate boundary, the major active tectonic features in the Puerto Rico area are Muertos Trough and Anegada Passage. At least 40 km of underthrusting has occurred at Muertos Trough, south of eastern Hispaniola at 68°30'W (Ladd and others, 1977), but our data suggests that this decreases eastward to zero at about 65°W. Regional modelling considerations lead us to prefer an interpretation based on an eastward decrease in deformation rate, because the spatial relationship between compression at Muertos Trough and extension in Anegada Passage is difficult to reconcile with significant compression zone propagation.
We have good evidence of extension along the south-western portion of Anegada Passage. This is confirmed by earthquake analysis, which shows that shallow earthquakes, particularly those associated with the north wall of the Virgin Islands Basin, appear to be associated with movement on the basin boundary faults (Murphy and McCann, 1979; Frankel and others, 1980). Bathymetric data show that the extensional trough extends north-eastwards through Anegada Passage to the deformation front of the Lesser Antilles Arc (Fig. 1).
Left-lateral strike-slip movement along Anegada Passage has long been suspected (see summary in Case and others, 1984) but significant strike-slip movement remains unproven. Mauffret and others' (1986) suggestion of recent E-W, right-lateral strike-slip is, however, more compatible with the rhomboidal shape of the basin which is defined by ENE and NW trending faults. This interpretation is also required by our model for the Puerto Rico area as discussed in the next section.
The above discussion clearly demonstrates that we do not yet fully understand the tectonic nature of Anegada Passage. Extension certainly occurs, and some component of strike-slip is possible. A pull-apart basin system associated with minor right-lateral slip appears to be the best interpretation. That this pull-apart system is linked southeastward to the eastern termination of Muertos Trough subduction appears to be an inevitable conclusion of our mapping of the two features. This link has previously been suggested by Byrne and others (1985), who mapped a "Puerto Rico Platelet" which had Muertos Trough and Anegada Passage as its southern and south-eastern boundaries respectively. Our recognition that deformation gradually decreases eastward along Muertos Trough, ultimately to be replaced by extension in Anegada Passage, allows us to examine how this Puerto Rico block is behaving within the broad Caribbean-North America plate boundary zone.
A model for the regional tectonic setting of Puerto Rico
A model for the formation of Muertos Trough and the Anegada Passage based on the rotation of a Puerto Rico block within a broad left-lateral shear zone is presented in Figure 10. This simple model is highly schematic, but it is attractive, not only because it reproduces the tectonic regime south of Puerto Rico, but because it also provides an explanation for a possible component of extension across the Puerto Rico Trench west of 65.5°W. We can offer no new evidence regarding the nature of the north-eastern boundary of the proposed rotating Puerto Rico block. The boundary will inevitably be characterised by oblique subduction, because of relative plate motions (e.g. Stein and others, 1988), but it is impossible to separate possible components of compression due to regional plate motion and local block rotation. Note that the model also predicts almost pure extension in Anegada Passage, with only a minor component of right-lateral strike-slip motion.
Anticlockwise rotation of crustal blocks within the Caribbean-North America plate boundary zone, similar to that suggested here, has previously been suggested on the basis of palaeomagnetic data by Fink and Harrison (1972), Vincenz and Dasgupta (1978) and Mann and Burke (1984). The age of possible rotation of Puerto Rico is constrained only to a small anticlockwise movement since the Cretaceous (Fink and Harrison, 1972), but igneous rocks possibly as young as Pleistocene in age have apparently been rotated on the adjacent island of Hispaniola (Mann and Burke, 1984, their Table 9).
Schell and Tarr (1978) speculated that anticlockwise rotation of Hispaniola could be driven by its oblique collision with the North American plate, and that such rotation could explain the extension in Anegada Passage and Mona Canyon as well as the compression at Muertos Trough. This raises the question of where the western boundary of a rotating Puerto Rico block might be? North-west of Puerto Rico, an obvious structural break occurs at Mona Canyon (Figs. 3, 10). On the basis of seismic activity, this was taken as the western boundary of their Puerto Rico microplate by Byrne and others (1985). However, no obvious structural or seismic boundary links Mona Canyon southward towards Muertos Trough, and the Muertos Trough compression zone itself extends much further west (Fig. 1). One possibility is that Puerto Rico and eastern Hispaniola are both undergoing anticlockwise rotation as (semi-) independent blocks with extension in Mona Canyon showing relative movement between them.
Long-range sidescan sonar images and seismic reflection data have been used to map the major tectonic features both north and south of Puerto Rico. To the north, we have been able to confirm that motion between North America and the Caribbean is almost pure strike-slip along the Puerto Rico Trench between 65°45' and 68°W, although a significant component of underthrusting occurs both to the east and west. This favours the model of Jordan (1975) and Minster and Jordan (1978) for Caribbean-North American plate motion. South of Puerto Rico, underthrusting of Caribbean ocean crust beneath Puerto Rico is seen in the west, but dies out eastward and is replaced by extension in the Virgin Islands Basin. A simple model involving rotation of a 'Puerto Rico tectonic block' within a broad strike-slip plate boundary satisfactorily accounts for the major observed tectonic features and is proposed as a working model for the area.
We would like to thank the officers and crew of the R.V. Farnella for their part in making a success of the GLORIA cruise. Reviews of various versions of this paper by K. Burke, W. P. Dillon, N. T. Edgar, D. R. Hutchinson, D. K. Larue, P. Mann, R. B. Whitmarsh and M. A. Winslow greatly improved its content and organization.
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