**The neotectonic setting of Puerto Rico**

D.G. Masson^1^ and K.M. Scanlon^2^

^1^Institute of Oceanographic Sciences,\
Wormley, Godalming, Surrey, GU8 5UB, UK.

^2^United States Geological Survey,\
Woods Hole, Massachusetts, MA 02543, USA.\
\
*modified from GSA Bulletin, 1991. v. 103, no. 1, p. 144-154.*

Top of Form 1

Bottom of Form 1

**ABSTRACT**

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.

**INTRODUCTION**

**Regional Setting**

Puerto Rico lies at the eastern end of the Greater Antilles island chain
in the north-eastern Caribbean [(Fig. 1)](../../images/tect1_g.gif).
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)](#Burke). 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](#Sykes); [Mann and
Burke, 1984](#Mann); [Burke and others, 1984](#Burke); [Byrne and
others, 1985](#Byrne); [Stein and others, 1988](#Stein)). 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)](#Jordan) and [Minster and Jordan (1978)](#Minster) 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)](#Sykes), 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)](#Stein). 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)](../../images/tect1_g.gif).

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](#Molnar)). However, some northward subduction of Caribbean ocean
crust also occurs at the Muertos Trough to the south of Puerto Rico
([Ladd and others, 1977](#Ladd); [Byrne and others, 1985](#Byrne)).
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); [Case
and others, 1984](#Case); [Mauffret and others, 1986](#Mauffret)). 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](#EEZ);
[Scanlon and others, 1988](#Scanlon)). 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); 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)](#Chavez). The interpretation presented here was based on
sonograph mosaics at a scale of 1:500,000 [(EEZ SCAN 85 Scientific
Staff, 1987)](#EEZ).

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.

**INTERPRETATION OF [PUERTO RICO GLORIA DATA](../glorialo.htm)**

**Oceanic crust of the North American Plate**

The oceanic crust north of the Puerto Rico Trench is around 100 Ma in
age [(Rona, 1980)](#Rona). GLORIA shows a series of basement ridges with
little or no sediment cover, trending 025 - 035°, throughout most of
this area [(Fig. 3)](../../images/neo3g.gif). Although the surveyed area
lies within the Cretaceous magnetic quiet zone, magnetic anomalies just
to the west trend 030° (Rona, 1980; [Fig. 1](../../images/tect1_g.gif))
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)](../../images/neo3g.gif). 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)](#McCann).

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](../../images/neo4ag.gif), 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](#Hilde)). 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)](../../images/neo3g.gif). 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](#McCann), 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)](../../images/tect6_c.gif). 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](../../images/neo4ag.gif),
[5](../../images/tect5_g.gif)). 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](../../images/neo3g.gif)). 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](#McCann)), 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)](../../images/neo3g.gif). In the west, a single
lineament occurs subparallel to and just south of the trench [(Fig.
6)](../../images/tect6_c.gif). 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](../../images/neo4ag.gif), [5](../../images/tect5_g.gif)). 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](#Searle); [Belderson and others,
1984](#Belderson); [Parson and Searle, 1986](#Parson)). 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](#Searle)). 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)](../../images/neo3g.gif), possibly on
a northwesterly extension of the Great Southern Fault Zone seen on
Puerto Rico [(Gardner and others, 1980)](#Gardner). Recent activity on
the offshore faults is confirmed by seismicity data ([McCann,
1985](#McCann); [Byrne and others, 1985](#Byrne)). 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)](../../images/neo3g.gif).

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](#Fox); [Perfit and others,
1980](#Perfit); [Gardner and others, 1980](#Gardner); [Meyerhoff and
others, 1983](#Meherhoff); [Moussa and others, 1987](#Moussa)) 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)](#Perfit).

**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](#Ladd); [Byrne and others, 1985](#Byrne)).
GLORIA shows the deformation front at the base of the slope [(Fig.
7)](../../images/tect9a_c.gif), 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)](../../images/neo3g.gif). 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)](#Ladd), 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)](../../images/neo3g.gif). The extensional nature of the basin is
illustrated by an E-W seismic reflection profile across its western end
[(Fig. 8)](../../images/neo8g.gif). GLORIA data show that fault trends
are generally NE-SW to E-W [(Fig. 9)](#FIG), 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)](#Donnelly) and in St. Croix; in the
latter area they are dated as post-Oligocene in age [(Whetten,
1966)](#Whetten). Offshore, immediately to the west of St. Croix,
faulting of late Miocene/early Pliocene age has been recognised
[(Holcombe and others, 1989)](#Holcombe), 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](#Murphy); [Frankel and others,1980](#Frankel);
[McCann, 1985](#McCann)).

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)](../../images/tect1_g.gif).
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](#FIG);
Matthews and Holcombe, 1985).

**DISCUSSION**

**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)](#Sykes),
strike-slip ([Jordan, 1975](#Jordan); [Minster and Jordan,
1978](#Minster); [Mann and others, 1984](#Mann); [Stein and others,
1988](#Stein), or extension [(Ewing and others, 1965)](#Ewing). 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)](#Jordan), [Minster and Jordan (1978)](#Minster) and [Stein and
others (1988)](#Stein). 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)](#Sykes) 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)](#Stein). 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](#Moussa)). In contrast, active
tectonics associated with the plate boundary dominate the geology of the
islands of Hispaniola and Jamaica further west ([Fig. 1](#FIG); [Burke
and others, 1980](#Burke); [Mann and Burke, 1984](#Mann); 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[,]{.underline} 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](#Schell);
[Frankel and others, 1980](#Frankel); [Sykes and others, 1982](#Sykes);
[Fischer and McCann, 1984](#Fischer); [McCann and Sykes,
1984](#McCann)). 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](#Molnar);
[Frankel and others, 1980; Frankel, 1982](#Frankel)).

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](#Fischer)), the other beneath eastern Hispaniola
between 67.5 and 71°W ([Schell and Tarr, 1978](#Schell); [Byrne and
others, 1985](#Byrne)). 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](#Schell); [Sykes and others, 1982](#Sykes)).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](#McCann)) or a predominance of normal faulting ([Ewing
and others, 1965](#Ewing); 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)](#FIG). 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)](#FIG) 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)](#Molnar) 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)](#Schell).

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)](#Stein). 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)](#FIG). 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](#Schell)). 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)](#FIG). 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](#Schell); [Sykes and others, 1982](#Sykes);
[Westbrook and McCann, 1986](#Westbrook)).

**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)](#Ladd), 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](#Murphy);
[Frankel and others, 1980](#Frankel)). Bathymetric data show that the
extensional trough extends north-eastwards through Anegada Passage to
the deformation front of the Lesser Antilles Arc [(Fig. 1)](#FIG).

Left-lateral strike-slip movement along Anegada Passage has long been
suspected (see summary in [Case and others, 1984](#Case)) but
significant strike-slip movement remains unproven. [Mauffret and
others\' (1986)](#Mauffret) 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)](#Byrne), 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](#FIG). 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](#Stein)), 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 (1971)](#Fink), [Vincenz and Dasgupta (1978)](#Vincenz) and
[Mann and Burke (1984)](#Mann). The age of possible rotation of Puerto
Rico is constrained only to a small anticlockwise movement since the
Cretaceous [(Fink and Harrison, 1971)](#Fink), 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)](#Schell) 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)](#FIG). On
the basis of seismic activity, this was taken as the western boundary of
their Puerto Rico microplate by [Byrne and others (1985)](#Byrne).
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)](#FIG). 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.

**CONCLUSIONS**

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)](#Jordan) and [Minster and Jordan
(1978)](#Minster) 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.

**ACKNOWLEDGMENTS**

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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