Downloaded from gsabulletin.gsapubs.org on May 1, 2015 The neotectonic setting of Puerto Rico D. G. MASSON Institute of Oceanographic Sciences, Wormley, Godalming, Surrey, GU8 5UB, United Kingdom K. M. SCANLON U.S. Geological Survey, Woods Hole, Massachusetts 02543 ABSTRACT left-lateral slip between the North American and keeping with the overall left-lateral motion on Caribbean plates (Burke and others, 1984). The the major plate boundary (see summaries in The island of Puerto Rico, in the northeast islands lie within a complex deformation zone as Case, 1975, and Case and others, 1984). Mauf- Caribbean, lies within a broad deformation much as 300 km wide, which occurs along the fret and others (1986), however, argued for a zone between the Caribbean and North plate boundary, and which contains both exten- complex history involving both left- and right- American plates. The simplest model for the sional and compressional elements in addition to lateral displacement. They suggest that the most tectonic setting of Puerto Rico has major the dominant strike-slip movement (Sykes and recent movement has been right lateral, al- strike-slip movement on nearly east-west others, 1982; Mann and Burke, 1984; Burke and though an older left-lateral phase is also lines in the vicinity of the Puerto Rico Trench others, 1984; Byrne and others, 1985; Stein and probable. coupled to a small counterclockwise rotation others, 1988). The motion of the Caribbean of a Puerto Rico block within the broader plate relative to neighboring plates has been the Aims of This Study plate boundary zone. This simple model is subject of a lengthy debate. In their analyses of attractive because it predicts the tectonic re- global plate motions, Jordan (1975) and Minster In late 1985, the U.K. Institute of Oceano- gime south of Puerto Rico, and provides an and Jordan (1978) calculated that Caribbean- graphic Sciences and the U.S. Geological Survey explanation for a possible component of ex- North America motion at the Puerto Rico undertook a GLORIA long-range sidescan sonar tension across the Puerto Rico Trench west Trench should be almost pure strike-slip. This survey of the entire Exclusive Economic Zone of 65.5°W. GLORIA long-range sidescan view, however, was challenged by Sykes and (EEZ) around Puerto Rico and the U.S. Virgin sonar data and seismic reflection profiles others (1982), who analyzed earthquake slip Islands (Figs. 1, 2; EEZ SCAN 85 Scientific have been used to test this model by mapping vectors from the northeastern segment of the Staff, 1987; Scanlon and others, 1988). The re- the major tectonic features across the plate Caribbean-North American plate boundary. sulting mosaic covers an area in excess of boundary north and south of Puerto Rico. To They concluded that oblique convergence was 200,000 km2, from 22°N to 15°N, straddling the north, the new data help to resolve be- occurring along the Puerto Rico Trench segment the Caribbean-North America plate boundary. tween conflicting models of underthrusting or of the plate boundary. A critical appraisal of We have used the sidescan data to divide strike-slip motion at the Puerto Rico Trench. the Jordan and Sykes and others' models was the plate boundary zone into six geological No direct evidence of compression is seen, presented by Stein and others (1988). By exam- provinces which form the framework for the although evidence for normal and strike-slip ining all available data from around the Ca- interpretation presented in this paper (Fig. 3). movement is abundant. This, combined with ribbean plate, these authors were able to North of Puerto Rico, three provinces can be regional considerations, leads us to conclude demonstrate the superior all-round fit of the distinguished: (1) oceanic crust of the North that the main east-west- trending part of the Jordan model, with the implication that no con- American plate, (2) a basin and ridge province Puerto Rico Trench between 65.5°W and vergence occurs at the western part of the Puerto which includes the Puerto Rico Trench and the 68°W lies within a strike-slip regime, al- Rico Trench (Fig. 1, inset). area of complex topography on its southern side, though oblique convergence occurs both to Within the broad plate boundary zone, most and (3) the steep island slope of Puerto Rico. the east and west where the plate boundary of the Caribbean-North America plate motion South of the island, we can distinguish (4) a trends east-southeast. To the south of Puerto is thought to be accommodated north of Puerto compressional tectonic province associated with Rico, underthrusting of the Caribbean plate Rico by strike-slip motion at the Puerto Rico the Muertos Trough, (5) an extensional province beneath the island decreases from west to Trench (Molnar and Sykes, 1969). Some north- in the area of the Virgin Islands Basin, and east, and it is ultimately replaced by extension ward subduction of Caribbean ocean crust, (6) oceanic crust of the Venezuela Basin. in the Virgin Islands Basin east of 65°W. however, also occurs at the Muertos Trough to This paper describes the geological character- the south of Puerto Rico (Ladd and others, istics of each of the above provinces based on INTRODUCTION 1977; Byrne and others, 1985). Complex trans- the sidescan data and associated seismic reflec- tensional movement along Anegada Passage ap- tion profiles. We then examine our interpreta- Regional Setting parently links the eastern end of the Muertos tions in the context of previous published work Trough to the major strand of the plate boundary in the area. Finally, a regional tectonic model for Puerto Rico lies at the eastern end of the along the Puerto Rico Trench (Case, 1975; Case the area is introduced, as we speculate on the Greater Antilles island chain in the northeastern and others, 1984; Mauffret and others, 1986). tectonic setting of the entire area in the frame- Caribbean (Fig. 1). Since the end of the Eocene, Most authors have argued that the Anegada Pas- work of Caribbean-North America plate inter- the regional tectonics of Puerto Rico and the sage has formed by pull-apart extension asso- action. The major points which we seek to make adjacent Virgin Islands have been dominated by ciated with left-lateral fault movement, in are the following. Geological Society of America Bulletin, v. 103, p. 144-154, 10 figs., January 1991. 144 Downloaded from gsabulletin.gsapubs.org on May 1, 2015 NEOTECTONIC SETTING OF PUERTO RICO 145 70° 65° 60° Figure 1. Location map showing the extent of GLORIA long-range sidescan sonar coverage. Inset depicts the major tectonic elements of the northeastern Caribbean plate margin, including a highly schematized interpretation of the tectonics of Hispaniola (modified from Rona, 1980; and Mann and others, 1984). (1) Strike-slip motion is the dominant tec- graph mosaics at a scale of 1:500,000 (EEZ throughout most of this area (Fig. 3). Although tonic activity in the basin and ridge province SCAN 85 Scientific Staff, 1987). the surveyed area lies within the Cretaceous immediately south of the Puerto Rico Trench, GLORIA is the ideal regional mapping tool magnetic quiet zone, magnetic anomalies just to with no clear evidence for compression. because its large-swathe width (up to 45 km) the west trend 030° (Rona, 1980; Fig. 1) and (2) The apparent eastward decrease in sub- and high tow speed (8-10 kts) allow large areas indicate that the 025°-035° basement fabric is duction of Caribbean oceanic crust at the to be mapped quickly and efficiently. The present the original sea-floor-spreading fabric of the Muertos Trough, and its ultimate transition of survey was conducted at a speed of 8 kts, with oceanic crust. Near the western edge of the sur- extension in Anegada Passage, is compatible an airgun seismic profiling system, 3.5-kHz and vey area, near 20°N, a series of en echelon with a small counterclockwise rotation of Puerto 10-kHz high-resolution profiling systems, and a basement ridges, individually trending 150°, but Rico within the overall broad left-lateral plate magnetometer towed simultaneously with the together defining a feature at 135°, are distinctly boundary zone. GLORIA fish. These profile data confirm the oblique to the spreading fabric (Fig. 3). This identity of sonograph targets crossed by the feature, which is also marked by a bathymetric The GLORIA System ship's track, as well as detailing their relationship high, has been identified as a northwesterly ex- with the subsurface geology. tension of a North Atlantic fracture zone ridge, GLORIA is a long-range sidescan sonar sys- Barracuda Ridge, by McCann and Sykes (1984). tem operating at a frequency of 6.5 kHz. Tech- INTERPRETATION OF The southern edge of the North American nical descriptions of the system have been PUERTO RICO GLORIA DATA oceanic crust is strongly downfaulted into the published elsewhere (Somers and others, 1978; Puerto Rico Trench, with the formation of some Somers and Searle, 1984). The data are re- Oceanic Crust of the North American Plate horst and graben structures as well as the pre- corded in digital form and processed to remove dominant down-to-the-south normal faults slant-range and aspect-ratio distortion and to The oceanic crust north of the Puerto Rico (Figs. 4, 5). Such structures are commonly correct for the attenuation of sound in water Trench is -100 Ma in age (Rona, 1980). found where an oceanic plate bends into a with increasing range (Chavez, 1986). The in- GLORIA shows a series of basement ridges with trench; they result from tension in the upper part terpretation presented here was based on sono- little or no sediment cover, trending 025°-035°, of the plate due to bending (Hilde, 1983).