This paper was published in Marine Geology, no. 161, p. 257-286, 1999, Copyright 1999 by Elsevier Science B.V.  This paper is also available in a pdf-version.

Three-phase tectonic evolution of the northern margin of Puerto Rico as inferred from an integration of seismic reflection, well, and outcrop data.

Jean-Paul van Gestela, *, Paul Mann1, b, Nancy R. Grindlay2, c and James F. Dolan3, d

a Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, Austin, TX 78712, USA
b Institute for Geophysics, The University of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500, USA
c Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, NC 28403-3297, USA
d Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA


Abstract

Integration of seismic reflection, well, and outcrop data from the Oligocene-Pliocene Puerto Rico-Virgin Islands platform (PRVI platform), north of Puerto Rico, indicates that three major tectonic phases characterize this seismically-active region within the North America-Caribbean plate boundary zone. Tectonic phase 1: Cretaceous to Eocene formation and sedimentary infilling from a southern source area into a forearc basin, formed between down-to-the-north normal faults near the present-day coast of Puerto Rico and an outer-arc ridge near the present-day shelf break. Tectonic phase 1 concluded the last volcanic arc activity in Puerto Rico produced by the subduction of oceanic crust of the North America plate beneath the Caribbean arc system. Formation of the outer arc high defining the northern limit of this elongate basin may have been related to the accretion of off-scraped sedimentary rocks beneath the northern margin of Puerto Rico in a manner similar to that observed in active arcs. The end of tectonic phase 1 is related to initial collision between the Caribbean arc and the Bahama carbonate platform. Tectonic phase 2: Oligocene to Pliocene formation of a ~1600 m thick, northward-thickening PRVI platform, predominately formed by carbonate rocks. This phase started with a period of non-deposition and erosion, resulting in an Latest Eocene¯Oligocene unconformity. The base of the PRVI platform is formed by a ~400 m thick middle to late Oligocene, basal siliciclastic sequence that prograded northward across the forearc basin. Depositional thicknesses of sedimentary layers deposited during phase 2 are controlled by two large arches: the NNW-trending Guajataca arch appears to have formed as the result of tectonic activity in the Mona Passage area; the northeast-trending San Juan arch cannot be related to any adjacent structure or plate boundary feature. Onlap relations between carbonate rocks of the platform and both arches suggest that the arches were most active in the period from middle Oligocene to early Miocene. Rocks deposited during Tectonic phase 2 are surprisingly devoid of faults and folds given their central position within the North America-Caribbean strike-slip plate boundary zone that is known from regional studies to have been active throughout the deposition of the platform rocks. Tectonic phase 3: Pliocene to Holocene northward tilting of the PRVI platform, submerged the northern edge of the platform to a depth of 4 km and elevated the southern edge of the platform to several hundred meters above sea level on Puerto Rico. Northward tilting of this area occurred on the northern limb of a large arch or anticline formed parallel to the long axis of the island of Puerto Rico and its shelf areas. The arch formed in response to a post-Pliocene convergence between the North America and Caribbean plates.

Author Keywords: Puerto Rico; tectonics; stratigraphy; carbonate platforms; forearc basins

Article Outline

1. Introduction
2. Tectonic and geologic setting of the Puerto Rico-Hispaniola microplate
3. Previous work
3.1. Onland stratigraphic studies
3.1.1. Cretaceous-Eocene arc rocks
3.1.2. Post-island arc Oligocene-Pliocene rocks
3.2. Onshore wells along the north coast of Puerto Rico
3.3. Previous multi-channel seismic results
4. Methods
4.1. EW96-05 survey and study area
4.2. Conversion of seismic reflection data into thickness data
5. Seismic reflection definition of three seismic megasequences along the northern margin of Puerto Rico
5.1. Megasequence PR1
5.1.1. Reflectors within PR1
5.1.2. Top reflector of PR1
5.1.3. Geologic interpretation of PR1 and its upper contact
5.2. Megasequence PR2
5.2.1. Reflectors within PR2
5.2.2. Top reflector of PR2
5.2.3. Geologic interpretation of PR2 and its upper contact
5.3. Megasequence PR3
5.3.1. Reflectors within PR3
5.3.2. Top reflector of PR3 (sea floor)
5.3.3. Geologic interpretation of PR3
5.4. Sequences within PR3
6. Discussion
6.1. Tectonic controls on seismic megasequences of the northern margin of Puerto Rico
6.2. Structural contour and isopach maps
6.3. Tectonic phase 1 during Cretaceous to Eocene time: formation and infilling of forearc basin developed on arc basement
6.4. Tectonic phase 2 during Oligocene to Pliocene time: formation of stable, northward-thickening PRVI platform
6.5. Formations within the PRVI platform
6.5.1. Structure of the San Sebastian Formation
6.5.2. Structure of the Lares Formation
6.5.3. Structure of the Cibao Formation
6.5.4. Structure of the Los Puertos Formation
6.5.5. Structure of the Aymamon and Quebradillas formations
6.6. Tectonic phase 3 during Pliocene to Holocene time: northward tilting of PRVI platform
7. Conclusions
7.1. Tectonic phase 1 during Cretaceous to Eocene time: infilling of forearc basin developed on arc basement
7.2. Tectonic phase 2 during Oligocene to Pliocene time: formation of stable, northward-thickening PRVI platform
7.3. Tectonic phase 3 during Pliocene to Holocene time: northward tilting of PRVI platform
Acknowledgements
References

1. Introduction

Most previous studies of tectonic events related to the interactions of the Caribbean and North America plates in Puerto Rico have focused on late Jurassic to Eocene volcanic, sedimentary, and metamorphic rocks that record mainly a pre-Eocene island arc history (e.g., Perfit et al., 1980; Schellekens, 1991) (Fig. 1). Far fewer studies have been devoted to the Oligocene to Holocene rock record that shows a mainly non-volcanic, strike-slip and oblique subduction history between the two plates (e.g., Dolan et al., 1991; Dillon et al., 1996 and Dillon et al., 1998). One reason for the lack of study of the post-island arc history is that most rocks of this age range are either restricted to narrow belts of generally low-dipping carbonate rocks along the north and south coasts of Puerto Rico or are offshore (Fig. 1). Except for coastal fringes of Pleistocene reef rocks and Quaternary alluvium, the Virgin Islands east of Puerto Rico are devoid of exposures of rocks younger than Eocene.


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Fig. 1. Bathymetric and tectonic map of the Puerto Rico and Virgin Islands area. Striped area near eastern Hispaniola, Puerto Rico and the Virgin Islands represents the PRVI platform of Oligocene to Pliocene age. Puerto Rico-Hispaniola microplate is bounded by the Puerto Rico trench, the Anegada Passage fault zone and the Muertos trough and continues westward to central Hispaniola. Bathymetric map with contour interval of 500 m is based on an integration of EW96-05 Hydrosweep bathymetric data, ETOPO-5 digital terrain map, and digitized National Ocean Survey bathymetric maps compiled by Mercado (1994). Box shows location of study area in Fig. 2. The offshore edge of the PRVI platform, shown by solid line, is interpreted from EW96-05 and Gulf seismic lines and MR-1 sidescan data. Arrow shows direction and rate of Grand Turk on the North America plate relative to Isabela on Puerto Rico from Dixon et al. (1998). Key to abbreviations: NPRSFZ=North Puerto Rico Slope fault zone, SPRSFZ=South Puerto Rico Slope fault zone, SFZ=Septentrional fault zone (from Grindlay et al., 1997).

Two belts of Oligocene-Pliocene carbonate rocks in Puerto Rico and a larger area of coeval carbonate rocks in the eastern Dominican Republic (Mann et al., 1991) represent a small fraction (~1900 km2) of a much larger (~47,200 km2) Oligocene to Pliocene Puerto Rico-Virgin Islands platform (PRVI platform) that covers much of the shelf and slope of Puerto Rico, the eastern Dominican Republic, and the Virgin Islands (Fig. 1 and Fig. 2). The objective of this paper is to use marine seismic reflection profiles from a ~9800-km2 survey of the PRVI platform north of Puerto Rico to reconstruct major Cenozoic tectonic events that have affected this part of the North America-Caribbean plate boundary zone (Fig. 2). These new seismic reflection data not only reveal the structure and stratigraphy of the platform rocks but also image older units beneath the PRVI platform that are related to the Eocene and older arc history of the area. These data are significant because they provide insights into the arc strike-slip transition that affected this part of the plate boundary as well as the evolution of the Puerto Rico-Hispaniola microplate (Van Gestel et al., 1998) (Fig. 1).


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Fig. 2. Bathymetric map with contour interval of 250 m based on the same compilation of bathymetric data described in the caption of Fig. 1. Light gray area indicates extent of PRVI platform along the northern margin of Puerto Rico. Dark gray area indicates landmass of Puerto Rico. Track lines of EW96-05 are indicated as dashed lines. The parts of the lines used in this paper are shown by thicker, dashed lines. Southward limit of PRVI platform on Puerto Rico and different mapped formation boundaries of the PRVI platform are from geologic mapping by Monroe (1980). Key to abbreviations of onland formations: SA=San Sebastian Formation; LA=Lares Formation; CI=Cibao Formation; LP=Los Puertos Formation; AY=Aymamon Formation; and QU=Quebradillas Formation.

2. Tectonic and geologic setting of the Puerto Rico-Hispaniola microplate

The island of Puerto Rico is an emergent part of the Puerto Rico-Hispaniola microplate, a stable block within the broad zone of strike-slip and oblique subduction between the North America and Caribbean plates (Byrne et al., 1985; Masson and Scanlon, 1991) (Fig. 1). The northern edge of this microplate is defined by oblique-slip and strike-slip faults in the Puerto Rico trench, the deepest area of the Atlantic Ocean (8395 m) (Masson and Scanlon, 1991; Grindlay et al., 1997; Larue and Ryan, 1998) (Fig. 1). GPS-based geodetic studies in the Dominican Republic west of Puerto Rico and in Puerto Rico itself show that the direction of North America plate motion relative to a fixed Caribbean is in a westward direction (Fig. 1) (Dixon et al., 1998). Earthquake data show that there is oblique subduction at the northern edge of the Puerto Rico-Hispaniola microplate at the Puerto Rico, because a Benioff zone extends to depths of 150 km beneath the island of Puerto Rico (McCann and Sykes, 1984; Dillon et al., 1996; Dolan et al., 1998). Seismic reflection profiling and side-scan sonar studies across the Puerto Rico trench confirm the thrust and oblique-slip character of active faults parallel to the axis of the trench (Masson and Scanlon, 1991; Grindlay et al., 1997; Larue et al., 1998).

The southern edge of the Puerto Rico-Hispaniola microplate is defined by the 5500 m deep Muertos trough and flanking accretionary prism (Matthews and Holcombe, 1974; Masson and Scanlon, 1991) (Fig. 1). Like the Puerto Rico trench to the north, this trough is associated with a Benioff zone that has been traced to depths of 75 km beneath the Puerto Rico-Hispaniola microplate (Byrne et al., 1985; Dillon et al., 1996; Dolan et al., 1998). Subduction at the Muertos trough appears to die out eastward where the trench becomes a less prominent bathymetric feature at about 66°W longitude (Masson and Scanlon, 1991) (Fig. 1). To the east, the Puerto Rico-Hispaniola microplate is bounded by the Anegada Passage fault zone, a zone of right-lateral transtensional slip that runs from the Muertos trough through the Virgin Islands Basin, the Anegada Passage and the Sombrero Basin (Fig. 1). To the west, the microplate extends to central Hispaniola, where it ends (Dillon et al., 1996). However, extensional faulting in the Mona Passage area and different eastward motion rates based on GPS measurements between Hispaniola and Puerto Rico (Dixon et al., 1998) indicate that the Puerto Rico-Hispaniola microplate is breaking up in two portions. In this situation the boundary of these two portions has to be drawn in the Mona Passage area, with its main extension in the Mona and Yuma Rift areas (Fig. 1).

3. Previous work

3.1. Onland stratigraphic studies

3.1.1. Cretaceous-Eocene arc rocks

Most previous studies of the geologic history of the PRVI area are based on outcrop studies (Fig. 1). There have been many stratigraphic, structural and petrologic studies of the Cretaceous to Eocene island arc rocks of the region (cf. review by Schellekens, 1991; Larue et al., 1998). These studies show that the rocks in the Cordillera Central of Puerto Rico and the Virgin Islands are the deeply eroded core of an island arc that was active from the Cretaceous through Eocene time (Jolly et al., 1998).

3.1.2. Post-island arc Oligocene-Pliocene rocks

The Cretaceous-Eocene arc basement rocks are unconformable overlain by Oligocene-Pliocene PRVI platform, that is mainly confined to two narrow belts along the north and south coasts of the island (Fig. 2). Early stratigraphic studies of the exposed PRVI platform section consist of measured sections and biostratigraphic studies by Hubbard (1923), Zapp et al. (1948) and Monroe (1973) and Monroe (1980). In this paper, we follow the more recently proposed terminology of Moussa et al. (1987).

The lower part of the section consists of siliciclastic rocks of the San Sebastian Formation of middle to late Oligocene age whereas the upper part consists of mainly upper Oligocene to lower Pliocene shallow-water carbonate rocks of the Lares, Cibao, Los Puertos, Aymamon, and Quebradillas formations (Monroe, 1980) (Fig. 3). Along the northern coast, these rocks are conformable and dip northward with a remarkably uniform dip of 4° (Moussa et al., 1987) (Fig. 2). Regional north-south cross sections by Briggs (1961) show northward stratigraphic thickening of all Oligocene to early Pliocene formations and eastward and westward thinning in only the formations of middle Oligocene through early Miocene age. Along the southern coast, a coeval carbonate section, which consists of the Juana Diaz and the Ponce formations, exhibits more complex structures and steeper dips (Monroe, 1980). The model that best explains the Pliocene to Holocene tilting of the carbonate section on both sides of Puerto Rico is late Neogene arching (Dillon et al., 1996 and Dillon et al., 1998; Van Gestel et al., 1998) (Fig. 1).


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Fig. 3. Correlation between well logs from the CPR-4 well from Briggs (1961) and the Toa Baja well on the San Juan arch from Gonzalez and Ruiz (1991). Correlation of seismic sequences (PR1 and PR3) proposed in this study, ages, depositional system and the eustatic sea level curve of Haq et al. (1987) are shown on the left and right of the two wells. See Fig. 2 for exact locations of both wells.

3.2. Onshore wells along the north coast of Puerto Rico

Two wells, the CPR-4 well and the Toa Baja well, have been drilled through the PRVI platform section along the north coast of Puerto Rico in an unsuccessful search for hydrocarbons (Fig. 3). The stratigraphy determined from the wells provides the most complete stratigraphic sections of the Oligocene-Miocene carbonate section because the low, uniform 4° dip of surface sections produces outcrop exposures of limited thickness. The wells are also useful for correlating the onland carbonate section to offshore seismic lines, as done by Briggs (1961), Meyerhoff et al. (1983), Larue and Berrong (1991), Larue et al. (1998), and in this paper.

CPR-4 well was drilled to a depth of 2144 m by Kewanee Interamerican Oil in 1960 near the town of Arecibo on the north coast of Puerto Rico (Briggs, 1961) (Fig. 3A). Briggs (1961) recognized 1706 m of the PRVI platform section in the CPR-4 well that ranged from the basal, siliciclastic San Sebastian Formation through the youngest carbonate unit of the PRVI platform, the Pliocene Quebradillas Formation (Moussa et al., 1987). Thicknesses of formations in the well were thicker than formations measured in outcrops landward of the well. This observation supports the northward thickening of the PRVI platform as seen in the regional north-south cross sections by Monroe (1980).

The second well on the north coast of Puerto Rico, the Toa Baja well, was drilled to a depth of 2704 m by Henley Drilling 16 km west of the city of San Juan (Fig. 3B). The lithologic and biostratigraphic well log has been described by Larue (1991), Montgomery et al. (1991) and Larue et al. (1998). The well encountered 579 m of middle Tertiary rocks of the San Sebastian, Cibao and Aymamon formations, unconformable underlain by mostly volcaniclastic and volcanic rocks. These strata are similar to Eocene strata exposed in the northern flank of the Central Mountains of Puerto Rico. Dating of these rocks is based on biostratigraphic studies (Montgomery et al., 1991) and argon laser fusion dating (Larue et al., 1998). The biostratigraphic information shows a latest Paleocene to latest Eocene age for the sedimentary rocks below the Latest Eocene-Middle Oligocene unconformity, but is questionable below 885 m. At a depth of 2584 m rocks have been dated as 48±2 Ma (Middle Eocene) using argon laser fusion dating of single hornblende crystals.

We question this age as it is only based on one data point. The age of the outcrops, 7 km SE from the Toa Baja well are of Cretaceous age (Larue et al., 1998), and from the seismic reflection profile (Fig. 4B) they seem similar to the deeper section of the Toa Baja well. In conclusion the age of the deeper strata penetrated by the Toa Baja well remains unclear, with a possible age range from Cretaceous to latest Eocene.


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Fig. 4. (A) Interpretation modified from Meyerhoff et al. (1983) of MCS line T-111D from the offshore area of northeastern Puerto Rico (to right) and MCS line P from the onshore area of Puerto Rico (to left). Note thick section of inferred middle Eocene-middle Oligocene sedimentary rocks controlled by down-to-the-north normal faults and pinching out before reaching the onshore CPR-4 well. Inset shows location map. (B) Interpretation modified from Larue et al. (1998) of MCS line T-101D to the right and MCS line 2 to the left. In the interpretation of Larue et al. (1998) the intermediate strata of Eocene age extend to onland Puerto Rico.

3.3. Previous multi-channel seismic results

In the early 1970s, several thousand kilometers of multi-channel seismic (MCS) data were collected along the north coast of Puerto Rico by Western Geophysical as part of an earthquake hazards study for siting a nuclear power plant along the north coast of Puerto Rico (Western Geophysical, 1974; Larue et al., 1998). Meyerhoff et al. (1983) interpreted one of the longer regional, north-south lines, line T-111D (Fig. 4A). This line crosses the PRVI platform, which dips at an average dip of 4° to the north with no change in stratal thickness or dip. This 4° dip is identical to the average dip measured from onland outcrops. Line T-111D also reveals the sub-PRVI platform stratigraphy of arc basement and an overlying fault-bounded basinal sequence that separates the arc basement and the overlying PRVI platform (Fig. 4A). The fault-bounded basinal sequence was inferred to be of late Middle Eocene-Middle Oligocene age by Meyerhoff et al. (1983) based on the known Cretaceous-Middle Eocene age of volcanic arc basement and Middle Oligocene-Pliocene age of the PRVI platform section. Meyerhoff et al. (1983) proposed that the unit is restricted to the offshore area of northern Puerto Rico and pinches out before it reached the coast, because large normal faults separate the thick offshore depocenter from a much thinner onshore onlap section (Fig. 4A).

In contrast, Larue and Berrong (1991) and Larue et al. (1998) interpreted a late Eocene basin sequence on line T-111D and T-101D (Fig. 4) and in the lower depths of the Toa Baja well, which they also interpreted as Eocene in age (Fig. 3B). Moreover, these proposed Eocene rocks cropped out as a thin strip of Eocene rocks south of the PRVI platform outcrops in Puerto Rico (Fig. 4B).

When we compare the Meyerhoff and Larue interpretations for the subsurface distribution of Eocene rocks, we favor the interpretation of Meyerhoff et al. (1983), rather than the interpretation of Larue et al. (1998). The most compelling observation in favor of the Meyerhoff interpretation is the presence of large normal faults on Line I-111D (Fig. 4A). In the Larue interpretation, these normal faults may be present in the 2-km wide data gap separating the two lines. The lack of shortening in the offshore part of the intermediate strata (Fig. 5 and Fig. 6) suggests that they are not similar to the folded strata, found beneath the PRVI platform sequence onshore. Furthermore, in the offshore section we observe an angular unconformity between the intermediate strata and the underlying arc basement and not between the intermediate strata and the overlying PRVI platform. In the onshore part this unconformity can be traced in between the lower Cretaceous-Eocene basement and the PRVI platform, which supports the Meyerhoff interpretation.


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Fig. 5. (A) Uninterpreted section of EW96-05 SCS line 21 (see inset map in B for location). (B) Interpretation of three main seismic megasequences of the northern margin of Puerto Rico from EW96-05 SCS line 21. PR1 is correlated to Cretaceous-Eocene island arc basement, PR2 is correlated to Cretaceous-Eocene forearc sedimentary rocks that downlap onto arc basement of PR1, and PR3 is correlated to the Oligocene to Pliocene PRVI platform. A pull-down effect related to the steep margin of the platform is indicated on the interpretation. Dashed line indicates the sea floor multiple.


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Fig. 6. (A) Uninterpreted section of EW96-05 SCS line 8 (see inset map in B for location). (B) Interpretation of three main seismic megasequences of the northern margin of Puerto Rico from EW96-05 SCS line 8. PR1 is correlated to Cretaceous-Eocene island arc basement, PR2 is correlated to Cretaceous-Eocene forearc sedimentary rocks that downlap onto arc basement of PR1, and PR3 is correlated to the Oligocene to Pliocene PRVI platform. Outer ridge confines forearc sedimentation of PR2. Dashed line indicates the sea floor multiple.

4. Methods

4.1. EW96-05 survey and study area

The main data set used in this study was collected by the authors on the EW 96-05 cruise of the RV Maurice Ewing in May and June, 1996. Data types included University of Hawaii MR1 sidescan sonar and Hydrosweep bathymetry (Grindlay et al., 1997), magnetics (Muszala et al., in press), gravity, and single-channel seismic reflection data. The 36 mostly NNE-SSW oriented shiptracks provided ~5600 km of single-channel seismic reflection data, which cover most of the PRVI platform, north of Puerto Rico (Fig. 2).

Although the collected data were only single fold, the quality of the seismic reflection profiles was excellent. Several artifacts were present in the data, which made the interpretation harder. First, a strong sea floor multiple was present in the data, which could not be removed due to the single channel nature of the data. Second, a pull-down effect caused by the difference in velocities in the water and the carbonate rocks. Finally, there were several side reflections from channels subparallel to the collected lines. These artifacts are indicated on the seismic reflection profiles shown in this paper.

The seismic reflection data have been processed through migration and interpreted. We were able to divide the seismic stratigraphy off the north coast of Puerto Rico into three main units, or megasequences (Fig. 5 and Fig. 6). Our SCS data are not of sufficient quality or penetration to subdivide the lower two megasequences into their component sequences. However, we have subdivided the youngest megasequence, PR3, into five component sequences: PR3a, PR3b, PR3c, PR3d, and PR3e, based on their lateral continuity, positions in the CPR-4 and Toa Baja wells (Fig. 3), and seismic facies characteristics. We have correlated each of these sequences to the formations identified on the basis of their lithologic composition by Monroe (1980) and Moussa et al. (1987) from the Oligocene-Pliocene PRVI platform (Fig. 3).

4.2. Conversion of seismic reflection data into thickness data

Two sources of velocity data for the study area were considered in the estimate of an average velocity value for the rocks of the PRVI platform: (1) Western Geophysical (1974) calculated an average velocity of 2.7 km/s for the PRVI platform based on their MCS data set that included line T-111D, shown in Fig. 4A; and (2) Anderson (1991) calculated a velocity of 2.93 km/s for the platform sedimentary rocks using seismic reflection data collected over the Toa Baja well. For this study, we averaged these two values and use a value of 2.75 km/s to convert TWT data to thickness data for the platform strata.

5. Seismic reflection definition of three seismic megasequences along the northern margin of Puerto Rico

5.1. Megasequence PR1

5.1.1. Reflectors within PR1

The oldest unit seen on existing MCS line T-111D in Fig. 4 (Meyerhoff et al., 1983) and several of the EW96-05 lines from different parts of the study area is interpreted as PR1. This unit has few internal structures. Discontinuous reflectors of PR1 are generally discordant to the overlying reflectors of PR2. Discontinuous reflectors are present for 0.5 s TWT beneath the top reflector of PR1 on line 21 (Fig. 5) and line 8 (Fig. 6). On all three of these lines, these reflectors are discontinuous, undulating, and cannot be followed for more than a few kilometers.

5.1.2. Top reflector of PR1

The boundary between PR1 and the better layered and overlying PR2 is a prominent seismic reflection event. On line 21 in Fig. 5, the reflector consists of several strong positive and negative amplitude reflectors found to a depth of 5.8 s. The presence of this reflector at depths of 5.5-6 s on line T-111D (Fig. 4A) and line 8 (Fig. 6) suggests that the reflector marks a major impedance contrast between PR1 (mainly volcanic rocks) and PR2 (mainly sedimentary rocks).

A slightly angular contact between PR1 and PR2 may contribute to the prominence of this reflector on the lines shown in Fig. 5 and Fig. 6. Reflectors within PR1 on Line 21 in Fig. 5 exhibit a gentle open folding and do not uniformly dip northwards as the overlying reflectors of PR2 and PR3.

5.1.3. Geologic interpretation of PR1 and its upper contact

Drilling at both the CPR-4 (Briggs, 1961) and Toa Baja wells (Larue, 1991) has confirmed that PR1 corresponds to volcaniclastic and volcanic arc basement rocks of Cretaceous-Eocene age. In the seismic reflection profiles off the coast of Puerto Rico, the upper contact of PR1 was observed to be a major angular unconformity separating the gently folded rocks of PR1 from the monoclinally-dipping overlying section of PR2. A similar angular unconformity has been described onshore separating the Cretaceous-Eocene arc basement rocks from overlying shallow water rocks of the San Sebastian Formation of Oligocene age that would correlate with the basal reflectors of PR3.

5.2. Megasequence PR2

5.2.1. Reflectors within PR2

PR2 is defined by a minimum of 1500 ms TWT packet of continuous reflectors (Oligocene-Pliocene PRVI platform). The reflectors are mainly low frequency and show few vertical variations within PR2. Reflectors within PR2 are fairly consistent along strike and can be traced on all seismic reflection profiles.

5.2.2. Top reflector of PR2

The reflector defining the boundary with the overlying PR3 is well defined on most lines in the survey area. The reflector consists of the lowest positive reflector of the well-layered PR3. Reflectors of PR2 beneath this horizon are more discontinuous and lower in frequency-content than the more continuous and higher frequent reflectors of PR3 (Fig. 5 and Fig. 6).

5.2.3. Geologic interpretation of PR2 and its upper contact

Geologic and age characteristics of PR2 remain speculative because the unit appears to pinch out before reaching the north coast of Puerto Rico (Fig. 4A). The age of the PR2 can be bracketed by the ages of PR1 and PR3, which crop out on Puerto Rico (Fig. 3). The age of PR2 is pre-Oligocene (age of basal part of PR3, the San Sebastian Formation), but older than the formation of the arc basement rocks. PR2 appears to consist of forearc basin sedimentary rocks formed between the outer arc ridge and the island of Puerto Rico. Therefore, PR2 strata most probably have been deposited during Cretaceous-Eocene arc activity in Puerto Rico (Jolly et al., 1998) and might even be younger in the probable case that the basin continued to be filled in by sediments after the arc activity stopped. The top of the PR2 does not show any unconformity with the overlying PRVI platform but has a smooth transition zone between the deposition of the two megasequences.

5.3. Megasequence PR3

5.3.1. Reflectors within PR3

PR3 is subdivided into five individual sequences that are conformable with reflectors in the underlying PR2 (Fig. 5 and Fig. 6). Sequence boundaries are placed at major reflectors that are continuous and correlatable across the study area. These boundaries are also verified by comparing the estimated thickness of each sequence with the known thickness of these formations in the CPR-4 and Toa Baja wells (Fig. 3). The lowest part of PR3 shows strong downlap onto the top of PR2 on most lines in the study area (Fig. 7). The sigmoidal geometry of these clinoforms at the base of PR3 indicates northward progradation of clastic sediment from Puerto Rico into the deeper water area of the Puerto Rico trench. Sequences above PR3a are conformable, parallel-sided, and show no evidence for northward propagation.


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Fig. 7. (A) Uninterpreted section of EW96-05 SCS line 3 (see inset map in B for location). (B) Interpretation of seismic megasequences PR3 from line 3. Note northward prograding clinoforms of the lower sequence PR3a (San Sebastian Formation) onto PR3 (late Eocene forearc basin). A side reflection related to a nearby channel in the platform is indicated on the interpretation. Dashed line indicates the sea floor multiple.

5.3.2. Top reflector of PR3 (sea floor)

The top section of PR3 is exposed at the sea floor and is eroded by submarine canyons fed by rivers on Puerto Rico (Masson and Scanlon, 1991) (Fig. 7). The location of these channels may be controlled by high angle faults with north-south strikes. Unfortunately, the presence of these channels obscures the lower part of the seismic reflection profiles, which makes it impossible to document any potential fault displacements.

5.3.3. Geologic interpretation of PR3

Because of the very good agreement in thickness, dip and dip direction, the megasequence PR3 has been interpreted as the Oligocene-Pliocene PRVI platform, as described along the northern coast of Puerto Rico.

5.4. Sequences within PR3

Age and lithologic correlations of offshore sequences were made through comparisons between the CPR-4 well and the seismic reflection profile of line 4 (Fig. 8). This reflection profile contains a complete section of the PR3 megasequence (Fig. 8). The southern end of line 4 is 22 km to the north of the CPR-4 well and is the closest seismic line of the EW96-05 survey to the well (Fig. 2). Interpreted sequences established on line 4 were then correlated with all adjacent seismic lines of the survey by detailed comparison of reflector spacing and seismic characteristics. We attempt to show these correlations by using an enlargement of representative line 4 close to the location of the CPR-4 well (Fig. 8).


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Fig. 8. Correlation of sequences within PR3 using a representative part of EW96-05 line 4 and onshore lithologic formations identified by Briggs (1961) in the CPR-4 well. This seismic line shows the seismic attributes of the different sequences and their boundaries as also their correlation to the lithological units. Inset shows location map.

Five sequences are recognized in the PR3 and are correlated to the formations that have been found onshore (Monroe, 1980; Moussa et al., 1987). In Table 1, a summary of the seismic character, the boundaries in between the different units and the geologic interpretation of the five different sequences can be found.

Table 1. Seismic characteristics of the different sequences of PR3
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6. Discussion

6.1. Tectonic controls on seismic megasequences of the northern margin of Puerto Rico

The thickness, facies and geometries of the three megasequences of the northern margin of Puerto Rico are controlled by tectonic events that have affected this segment of the North America-Caribbean plate boundary (Puerto Rico-Hispaniola microplate, Fig. 1). In the following sections, three main tectonic events along the northern margin of Puerto Rico have been inferred using isopach and structural contour maps of each of the main three megasequences along with their constituent sequences.

6.2. Structural contour and isopach maps

Structural contour maps have been compiled for each major boundary between the three seismic megasequences, PR1, PR2 and PR3, as well as the individual sequences of the PRVI platform, together with isopach maps of the megasequence or sequence overlying each of these surfaces. These map pairs show changes in accommodation space and the consequent sedimentary infill. The effects of tectonic deformation, subsidence, and differential compaction must also be considered when interpreting these maps.

6.3. Tectonic phase 1 during Cretaceous to Eocene time: formation and infilling of forearc basin developed on arc basement

From Cretaceous to Eocene (phase 1), the outer ridge and the arc basement rocks onland Puerto Rico are developed, which form the boundaries of a forearc basin that is filled in with sediments derived from the Puerto Rico arc to the south (Fig. 9). Basin infilling from southern sources can be concluded from the observed onlap of the basin sedimentary rocks on the underlying arc basement rocks (Fig. 5 and Fig. 6).


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Fig. 9. (A) TWT map to the top of PR1 surface (Cretaceous-Eocene arc rocks) with a contour interval of 200 ms. Identification of the top of PR1 in the shallow-water southwestern and southeastern parts of the study area was not possible because the reflection profiles are obscured by the sea floor multiple. The main structural features affecting the top of PR1 include the San Juan high and the outer ridge, which is shown in cross section on Line 8 in Fig. 6. Dip symbols indicate main dip direction of PR1 surface. (B) Isotime map with 100 ms contour interval of PR2. The main depocenter of this sequence in the forearc basin appears to be in the shallow-water southwestern corner of the study area, where EW96-05 was insufficient to resolve the thickness of the megasequence, because of the sea floor multiple.

The outer ridge forms the northern boundary of the forearc basin and has an east-west trend (Fig. 9). The formation of the outer ridge may be related to a period of subduction and frontal accretion of sediments offscraped from the downgoing North America plate at the current location of the Puerto Rico trench.

In the eastern part of the survey area, almost no sedimentary rocks are found north of the outer ridge. As a result of the northward thinning of the forearc basin sedimentary rocks, the PRVI platform almost immediately overlays the arc basement on the outer ridge. Locally the dip of the top of PRVI platform becomes conformable with the PRVI platform.

The San Juan high (Fig. 9) forms the east side of the basin. The boundaries in the southern and western part of the study area, near Puerto Rico, are not clearly defined (e.g., line 21 in Fig. 5). On line T-111D from Meyerhoff et al. (1983) (Fig. 4A), the forearc basin sedimentary rocks can be seen thinning abruptly from over 3 s TWT to less than 0.25 s across a series of down-to-the-north normal faults. The main depocenter of the forearc basin is northwest of Puerto Rico (Fig. 9B), where the basin reaches a depth of at least 1500 milliseconds TWT. This thickness cannot reliably be depth converted, because accurate velocities are not available for the sedimentary rocks in the forearc basin.

According to Larue and Berrong (1991) and Larue et al. (1998), this period of Cretaceous-Eocene forearc sedimentation was followed by a period of basin inversion and uplift. In their interpretation, this uplift event did not influence the northern part of the basin, but did produce basin inversion and uplift in the southern part of the basin. They support their conclusion by interpreting an angular discordance between the PRVI platform and the underlying forearc basin rocks seen in the onshore seismic reflection profiles, which cross the Toa Baja well (Larue et al., 1998; Fig. 4B). However, we propose that this angular unconformity instead separates arc basement and the PRVI platform in an area where the forearc basin strata were not deposited. We conclude that the PRVI platform overlies the upper Eocene unit in the forearc basin conformably, which was observed by Larue et al. (1998) in the northern part of the basin.

6.4. Tectonic phase 2 during Oligocene to Pliocene time: formation of stable, northward-thickening PRVI platform

In the second tectonic phase, the Oligocene to Pliocene PRVI platform forms and covers the previously deposited forearc sediments and the outer ridge. The extent and lateral uniformity of the PRVI platform indicates that the Hispaniola-Puerto Rico microplate provided a tectonically stable substrate in which the platform could develop during Oligocene to Pliocene time. Only slow subsidence was occurring at this time, probably related to sediment loading and thermal contraction of the underlying crest and lithosphere (Birch, 1986). This situation led to a platform with a maximum thickness of 1578 m, as can be seen in the isopach map of the PRVI platform (Fig. 10B).


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Fig. 10. (A) TWT map to the top of PR2 surface (Cretaceous-Eocene forearc basin rocks), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR2 surface. (B) Isopach map with 100 m contour interval of PR3 (PRVI platform). The main depocenter of this sequence is in the center of the study area between the San Juan and Guajataca arches.

The main depocenter during this period was located in the central part of the study area and has been called the North Coast basin by Larue et al., 1998 (Fig. 10B). The North Coast basin is about 180×50 km2 in extent, with its maximum depocenter of 1578 m at 18°50'N66°40'E (Fig. 9). Comparison of all lines in the area indicates that PR3 is wedge-shaped and thins in a landward direction (Fig. 9). The PRVI platform also thins over the Guajataca arch to the west (Fig. 11) and the San Juan arch to the east (Fig. 12). The Guajataca arch on the west side of the North Coast basin has a west-northwest trend, and curves eastward (Fig. 10), while the San Juan arch, which marks the eastern boundary of this basin, has a northwest-southeast trend, and curves westward (Fig. 10). Thinning of the platform across the San Juan arch can also be seen by comparison of the well logs of the Toa Baja well, which is close to the axis of the arch, with the well logs of the CPR-4 well, which is located near the thicker part of the North Coast basin west of the arch (Fig. 3).


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Fig. 11. (A) Uninterpreted section of EW96-05 SCS line 22 (see inset map in B for location). This line has been flattened along the top of the PR3d sequence to remove the 4° northward dip and to provide a pre-tilt, northeast oriented cross section. (B) Interpretation of seismic megasequences PR2 and PR3 from line 22. Note eastward thinning and onlap of the three middle Oligocene-early Miocene, lower sequences within megasequence PR3 (PR3a, PR3b, and PR3c) onto the San Juan arch. Dashed line indicates the sea floor multiple.


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Fig. 12. (A) Uninterpreted section of east-west oriented EW96-05 SCS line 5 (see inset map in B for location). (B) Interpretation of PR2 and PR3 from line 5. Note eastward thinning and onlap of the three middle Oligocene-early Miocene, lower sequences within PR3 (PR3a, PR3b, and PR3c) onto the San Juan arch. The Toa Baja well was drilled into the axis of the San Juan arch which is located about 20 km east of this line (cf. Fig. 2). Dashed line indicates the sea floor multiple.

The northward thickening and prograding clinoforms of the siliciclastic sediments of the San Sebastian formation indicates a source area near present-day Puerto Rico (Fig. 7). The origin of the sedimentary rocks of the youngest four formations (Cibao-Quebradillas formations) and parts of the older sequences (San Sebastian and Lares formations) is carbonate shelf material formed in shallow, tropical environments.

6.5. Formations within the PRVI platform

In Table 2, a summary can be found of the thicknesses of the various Oligocene to Pliocene formations as observed in the CPR-4 and the Toa Baja well, the outcrops and the isopach maps.

Table 2. Observed thicknesses of the different sequences of PR3
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6.5.1. Structure of the San Sebastian Formation

The thinnest section of the San Sebastian Formation occurs near the Guajataca arch to the west and the San Juan arch to the east (Fig. 13B). The San Sebastian Formation forms the transgressive base for the PRVI platform. Sediment was derived from the erosion of folded and faulted arc-related basement rocks on Puerto Rico. As terrigenous source areas were reduced to sea level, the supply of eroded siliciclastic rocks waned upward through the section until carbonate rocks became dominant in the overlying Lares Formation. Low eustatic sea level during the deposition of the San Sebastian Formation may have contributed to the erosion of exposed land areas at the time the Lares Formation was deposited (Fig. 3).


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Fig. 13. (A) TWT map to the top of PR2 surface (Cretaceous-Eocene forearc basin rocks), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR2 surface. (B) Isopach map with 25 m contour interval of PR3a (middle to late Oligocene San Sebastian formation). This unit marks the base of the PRVI platform. Its depocenter is in the center of the study area. The Guajataca and San Juan arches to the east and west of this depocenter are characterized by a much thinner PR3a.

6.5.2. Structure of the Lares Formation

The Lares Formation appears to have the same structure as the San Sebastian Formation with thinning near the Guajataca arch to the west and the San Juan arch to the east. However, the Lares Formation is divided into two smaller depocenters by the Arecibo high in the center of the North Coast basin (Fig. 14B).


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Fig. 14. (A) TWT map to the top of PR3a surface (middle to late Oligocene San Sebastian Formation), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR3a surface. (B) Isopach map with 25 m contour interval of PR3b (late Oligocene Lares Formation). The main depocenter of this sequence is in the center of the study area. The Guajataca and San Juan arches to the east and west of this depocenter are characterized by a much thinner PRVI platform.

The Lares Formation, records reefal deposition in clear water on the PRVI platform (Monroe, 1980). Onshore studies indicate that the eastern outcrop area of the Lares Formation was characterized by siliciclastic sedimentation that may have represented the final stages of the erosional event that produced the underlying San Sebastian Formation.

6.5.3. Structure of the Cibao Formation

The thickest sections of the Cibao Formation are located in the center of the North Coast basin and the thinnest sections located near the Guajataca arch to the west and the San Juan arch to the east (Fig. 15B). The structure of this sequence is similar to the structure of the underlying Lares Formation, with its depocenter limited by highs to the east and west and the Arecibo high dividing it in the center of the basin. The divided depocenters are shifted to slightly different locations than those of the underlying Lares Formation. The Cibao Formation in outcrop records shallow marine deposition on a clear water carbonate platform (Monroe, 1980).


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Fig. 15. (A) TWT map to the top of sequence PR3b surface (late Oligocene Lares Formation), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR3b surface. (B) Isopach map with 25 m contour interval of megasequence PR3c (lower Miocene Cibao Formation).

6.5.4. Structure of the Los Puertos Formation

In contrast to the underlying sequences, the isopach map of the Los Puertos Formation reveals few highs or lows and no thinning of PR3d towards the Guajataca or San Juan arches. The Los Puertos Formation records shallow marine deposition on a slowly subsiding coastal shelf.

6.5.5. Structure of the Aymamon and Quebradillas formations

The base of the Quebradillas Formation lies unconformably on the Aymamon Formation in outcrop. The deposition of both formations is very uniform and no depocenters or arch activity can be recognized on the isopach map (Fig. 17B). Both the Quebradillas and the Aymamon formations records shallow marine deposition on a clear water carbonate platform (Monroe, 1980).

6.6. Tectonic phase 3 during Pliocene to Holocene time: northward tilting of PRVI platform

After the deposition of the Quebradillas Formation, which has been well dated as Pliocene age (Monroe, 1980), the third and final tectonic phase began. During this stage the whole platform subsided and tilted approximately 4° northward. This 4° northward dip can be found in all units of the PRVI platform. The uniformity of the 4° dip can be observed both in the contour map of the top of the forearc basin sedimentary rocks (Fig. 10A) and the contour maps of the different sequences (Fig. 14AFig. 15AFig. 16AFig. 17A). This time period was marked by non-deposition and erosion of the top of the Quebradillas Formation. The whole platform tilted in a remarkably uniform way with no evidence for faulting, except for some gravity-driven slumps at the edge of the platform (Schwab et al., 1991). Tilting occurred over an extensive area of 308 km by 71 km, that included the North Coast basin and the area north of the Virgin Islands (Fig. 1).


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Fig. 16. (A) TWT map to the top of PR3c surface (early Miocene Cibao Formation), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR3c surface. (B) Isopach map with 25 m contour interval of PR3d (middle Miocene Los Puertos Formation). This unit is marked by a uniform deposition with equal distribution over the area.


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Fig. 17. (A) TWT map to the top of PR3d surface (middle Miocene Los Puertos Formation), with a contour interval of 500 ms. Dip symbols indicate main dip direction of PR3d surface. (B) Isopach map with 25 m contour interval of megasequence PR3e (middle Miocene Aymamon and late Miocene to early Pliocene Quebradillas formations). This unit is marked by a uniform deposition with a equal distribution over the area.

7. Conclusions

7.1. Tectonic phase 1 during Cretaceous to Eocene time: infilling of forearc basin developed on arc basement

We have documented three distinct, Cenozoic tectonic phases that occurred during development of the northern margin of Puerto Rico. During the first tectonic phase, the forearc basin was formed and later filled in by Cretaceous to Eocene sediments with a maximum thickness of at least 1500 ms TWT time (Fig. 18A). The main depocenter of this basin lies beneath the northern shelf of Puerto Rico and is bounded to the north by the outer ridge, which is an east-west oriented feature parallel to the present-day coast line of Puerto Rico. During this phase the Greater Antilles arc was still intact and formed a subduction zone with almost straight westward subduction at the location of the Puerto Rico trench. This activity continued until Eocene when the Bahamas platform collided with the island arc which started the second tectonic phase (Erikson et al., 1990; Jolly et al., 1998).


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Fig. 18. Summary of the three main Cenozoic basin phases along the northern margin of Puerto Rico based on data presented in this paper. (C) Shows present-day bathymetry of north-south profile at longitude 67°W. (A) Tectonic phase 1: Cretaceous to Eocene formation and infilling of an east-west trending forearc basin with large normal faults along the southern edge of the basin and onlap of the forearc basin sediments onto the outer ridge. (B) Tectonic phase 2: Oligocene to Pliocene period of tectonic quiescence and lack of large-scale folding or faulting of the 70-km wide extinct arc upon which the PRVI platform has been formed. (C) Tectonic phase 3: Pliocene to Holocene northward tilting of the platform along the northern margin of Puerto Rico, which has resulted in drowning of the platform to a depth of 4 km to the north and subaerial exposure of the platform to an elevation of about 100-200 m in the north coast area of Puerto Rico.

7.2. Tectonic phase 2 during Oligocene to Pliocene time: formation of stable, northward-thickening PRVI platform

The first tectonic phase was followed by a period of non-deposition and erosion, which generated the unconformity as can be seen in outcrops in north Puerto Rico and in the two onland wells. In the middle Oligocene the second tectonic phase started (Fig. 18B). During this second phase, the Oligocene to Pliocene PRVI platform was deposited, and covered the forearc basin and outer ridge. This period of tectonic quiescence resulted in uniform and horizontal deposition of six different PRVI platform formations. During phase 2, slow subsidence occurred as a result of sediment loading and thermal contraction (Birch, 1986), resulting in platform carbonates that reach a maximum thickness of 1578 m.

During tectonic phase 2, two arches are active: the Guajataca and the San Juan arch (Fig. 10B). As can be observed in both the seismic reflection profiles (Fig. 11 and Fig. 12) and the different contour maps (Fig. 13, Fig. 14 and Fig. 15), both arches were active from late Oligocene to early Miocene time, when the oldest three sequences of the PRVI platform were deposited.

The Guajataca arch may be related to tectonic activity in the Mona Passage area. Other evidence for this tectonic activity in this area during the deposition of the platform is the difference in thickness and structure of the carbonate platform on both sides of the Mona Rift.

The San Juan arch, which marks the eastern boundary of the North Coast basin, has a northeast trend (Fig. 10B). The origin of this arch is unclear, but is probably a pre-existing structure that formed during tectonic phase 1.

The Lares and Cibao formations also show the effect of the Arecibo high, which divides the basin into eastern and western subbasins and controlled depocenters during sedimentation of the Lares and the Cibao formations. After early Miocene time deposition changed to become more uniform, as seen in the isopach maps of the youngest three formations (Fig. 16 and Fig. 17) and in the seismic reflection profiles (Fig. 11 and Fig. 12).

This second tectonic phase occurred after the collision between the Bahamas platform and the island arc. During this tectonic phase the direction of motion changes from subduction to a strike-slip movement. Paleomagnetic studies of the Oligocene-Pliocene sedimentary rocks have shown a 24° counterclockwise rotation of Puerto Rico (Reid et al., 1991). Because of the presence of the Bahamas platform the northern part of the Puerto Rico-Hispaniola microplate was pinned and started rotating around the southern most point of the Bahamas platform extension. The tectonic activity in the Mona passage area may be related to this rotation of Puerto Rico. The Mona Passage area might have been the pivoting point of a more immobile Hispaniola and a rotating Puerto Rico.

Except for this rotation the tectonic activity during this time was very limited as can be seen from the stable uniform PRVI platform which was formed during this phase. This phase continued until the Bahamas platform was past Puerto Rico and the motion in the Puerto Rico trench had completely changed into a strike-slip boundary (Erikson et al., 1990; Jolly et al., 1998).

7.3. Tectonic phase 3 during Pliocene to Holocene time: northward tilting of PRVI platform

After Pliocene deposition of the youngest formation of the PRVI platform, the whole platform has undergone a uniform 4° tilt over a very wide area (Fig. 18C). This regional tilt is explained by the influence of the subduction in the nearby Puerto Rico trench. Previously proposed models, which attempt to explain the regional tectonic picture of this area, include: (1) large-scale north-south divergence (Speed and Larue, 1991); (2) extension caused by counterclockwise rotation (Masson and Scanlon, 1991); (3) pinning and localized divergence related to collision of the Bahama platform against Hispaniola (Vogt et al., 1976; Dolan et al., 1998); (4) interaction of the North America and Caribbean plates at depth, which results in large scale arching (Dillon et al., 1996 and Dillon et al., 1998; Van Gestel et al., 1998); and (5) tectonic erosion related to oblique subduction of the Bahama platform extension (McCann and Sykes, 1984). We support the idea of plate interaction at depth and regional arching as proposed by Dillon et al. (1996) and Dillon et al. (1998) and Van Gestel et al. (1998), because of the clear Benioff zones and arching effects that are observed in seismic reflection profiles. The uniform tilt of the PRVI platform both east and west of Puerto Rico must be caused by a continuous feature instead of having a time-transgressive, lateral varying origin.

Acknowledgements

This work was supported by National Science Foundation grants NSF-OCE-9504118 and NSF-OCE-9796189 to Grindlay, Mann and Dolan. Van Gestel thanks the UTIG student cruise fund for support to participate in the EW96-05 cruise and for its continued support to work on these data following the cruise. We thank the captain and crew of the R/V Maurice Ewing and the University of Hawaii marine technical group for their technical support at sea. UTIG contribution number 1427. CMSR contribution number 226.

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*Corresponding author. Tel.: +1-512-232-3207; fax: +1-512-471-8844; E-mail: jpges@utig.ig.utexas.edu

1Tel.: +1-512-471-0452; fax: +1-512-471-8844; E-mail: paulm@utig.ig.utexas.edu.

2Tel.: +1-910-962-7421; fax: +1-910-962-7077; E-mail: grindlayn@uncwil.edu.

3Tel.: +1-213-740-8599; fax: +1-213-740-8801; E-mail: dolan@coda.usc.com.


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