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Geological Society of America Special Paper 385 2005

Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data

Nancy R. Grindlay* Lewis J. Abrams Luke Del Greco Center for Marine Science and Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, North Carolina 28409, USA

Paul Mann Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78759-8500, USA

ABSTRACT

It has been postulated that the western boundary of the Puerto Rico–Virgin Islands microplate lies within the Mona Passage and extends onland into southwestern Puerto Rico. This region is seismically active, averaging one event of magnitude 2.0 or larger per day, and over 150 events of magnitude 3.0 or greater occurred during the past ﬁ ve years. Moreover, there have been at least 13 historical events of intensity VI (MM) or greater in the past 500 years. We conducted a high-resolution seismic and sidescan sonar survey of the insular shelf of western and southern Puerto Rico during May 2000 in an effort to identify Holocene faults and to further assess the seismic hazard in the region. We focus on an ~175 km2 part of the surveyed area offshore of western Puerto Rico, extending from Punta Higuero to Boquerón Bay. This area was targeted as a likely place to image recent faults, because multi-channel seismic proﬁ les offshore western Puerto Rico show numerous WNW-trending normal and strike-slip faults that offset Oligocene-Pliocene age carbonate rocks and underlying Cretaceous basement rocks. Analyses of these data identify three zones of active deformation within the survey area: (1) the Cerro Goden fault zone; (2) the Punta Algarrobo/ Mayagüez fault zone that lies offshore the city of Mayagüez; and; (3) the Punta Guanajibo/Punta Arenas fault zone. Two of the offshore fault zones, the Cerro Goden and Punta Algarrobo, show strong correlation with fault zones onland, Cerro Goden and Cordillera, respectively. Many of the mapped faults offshore appear to reactivate older WNW-trending basement structures and show evidence of some component of right-lateral motion that is consistent with geodetic measurements. The offshore deformation zones are also associated with headlands and linear NW-SE magnetization lows (serpentinite dikes?) mapped offshore. Elongate outcrops of serpentinite in western Puerto Rico are colinear with the fault zones we have mapped offshore,

*[email protected]

Grindlay, N.R., Abrams, L.J., Del Greco, L., and Mann, P., 2005, Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data, in Mann, P., ed., Active tectonics and seismic hazards of Puerto Rico, the Virgin Islands, and offshore areas: Geological Society of America Special Paper 385, p. 139–160. For permission to copy, contact [email protected]. © 2005 Geological Society of America. 139 140 N.R. Grindlay et al.

suggesting that either the presence of serpentinite has localized fault activity or that fault activity has remobilized serpentinite. This offshore study improves assessments of the seismic hazard in Puerto Rico by identifying targets for onshore paleoseismic studies and by better deﬁ ning the total length of offshore Holocene faults.

Keywords: Holocene faulting, tectonics, Puerto Rico, sidescan sonar, high-resolution seismic reﬂ ection.

INTRODUCTION generally WNW and E-W trends (Fig. 2; Western Geophysical Company and Fugro, Inc., 1973; Larue and Ryan, 1990; Grindlay The onshore and offshore region of western Puerto Rico is et al., 1997; van Gestel et al., 1998, 1999). one of the most seismically active regions beneath the island of Because most of the large historical earthquakes have been Puerto Rico. During the past fi ve years alone, over 150 earth- located far offshore and existing nearshore geophysical data is quakes with a magnitude of 3.0 or greater have been recorded sparse, an understanding of the distribution and nature of near- by the local seismic network in the western region of Puerto shore faults and the seismogenic potential on the insular shelf Rico (Puerto Rico Seismic Network, 2004). There is historical of western Puerto Rico is lacking. For this reason, we conducted evidence for at least 48 felt seismic events in the Mona Passage a high-resolution geophysical survey (SCS and sidescan sonar and western Puerto Rico between 1524 and 1958, 13 of which imaging) in May 2000 to characterize the structure of the seafl oor had estimated intensities of >VI (Modifi ed Mercalli) (Ascencio, and sub-bottom of the shallow insular shelf (Fig. 3). The primary 1980). Most notable is the 1918 (estimated 7.3 Ms) earthquake objective of this study was to search for youthful faulting (e.g., in the Mona Passage (Figs. 1 and 2) and the resulting tsunami, seafl oor offsets), and to correlate offshore faults with possible which together killed at least 116 people and caused four mil- Quaternary faults mapped onshore to better defi ne their total lion dollars in damage in northwestern Puerto Rico (Reid and length in western Puerto Rico. The delineation of nearshore fault Taber, 1919), at a time when the population of Mayagüez was systems provided by this study, combined with their direction approximately 17,000 people. Today, Mayagüez is the third and rate of slip from onland surveys, will assist in the improve- most populous city on the island of Puerto Rico with more than ment of hazard models. Until now, hazard models have relied 150,000 inhabitants. only on offshore source zones for hazard prediction, because of Because southwestern Puerto Rico is characterized by “basin- the lack of evidence for nearshore and onshore faulting. and-range” style topography and frequent shallow (<50 km) seismicity (McCann et al., 1987; Joyce et al., 1987; Ascencio, 1980; TECTONIC SETTING OF PUERTO RICO AND THE Puerto Rico Seismic Network, 2004), this region has been the VIRGIN ISLANDS focus of studies to locate Holocene faults onland. Two seismic refl ection lines across the southern margin of the Lajas valley The island of Puerto Rico is located within a diffuse and imaged displacements in Quaternary lacustrine sediments and complex plate boundary zone between the North America and Cretaceous basement rocks (Meltzer and Almy, 2000). Offsets Caribbean plates (Fig. 1). Seismicity and marine geophysical within the sediments and the basement rocks indicate Quaternary studies suggest that Puerto Rico and the Virgin Islands are the faulting, most likely of a transtensional nature (Meltzer and Almy, emergent part of a microplate that lies within the North Amer- 2000). Most recently, trenching studies of the South Lajas fault ica–Caribbean plate boundary zone (Byrne et al., 1985; Masson near the town of Boquerón showed evidence of Holocene activity and Scanlon, 1991; Mann et al., 1995). The Puerto Rico trench (Fig. 3). Displacements in Holocene alluvium show valley-side- and the Muertos trough form the northern and southern boundar- down, normal separation with a component of strike-slip motion ies, respectively, of the Puerto Rico–Virgin Islands microplate (Prentice and Mann, this volume). (Fig. 1). The eastern boundary of the microplate lies within the Marine geophysical studies of the Mona Passage indicate Anegada Passage. The western boundary of the microplate is several sets of youthful, large displacement, normal faults (Gard- poorly defi ned; its northern portion may be the Mona rift, while ner et al., 1980; Larue and Ryan, 1990; Grindlay et al. 1997; van its southern portion may lie beneath the Mona Passage or extend Gestel et al., 1998). The Mona rift, located at the northern end of into southwestern Puerto Rico. the Mona Passage (Figs. 1 and 2), is a N-S–trending graben with extremely large throws, in many instances over 2 km (Gardner et Phases of Deformation Impacting the Region al., 1980; Larue and Ryan, 1990; Grindlay et al., 1997; van Gestel et al., 1998). Recent tectonic activity in the area is suggested by It is likely that the style, geometry, and distribution of faults uneroded rocks on the fault scarps, and by normal faults and ver- within the Mona Passage and western Puerto Rico are a function tical fi ssures which cut tilted surface layers (Gardner et al., 1980). of at least three separate and sequential phases of deformation to South of the Mona rift, single-channel (SCS) and multichannel impact this region. The fi rst phase is an Eocene age transpressive seismic (MCS) data show an abundance of normal faults with event focused along two major shear zones the Great Northern o o o o o o o o 70 W 69 W 68 W 67 W 66 W 65 W 64 W 63 W o 21 N Constraints from high-resolution seismic and sidescan sonar data 141

North America Plate

o 20 N Puerto Rico Trench

Bunce FZ SFZ 1946 1946 Bowin FZ 1943 o 19 N Hispaniola ûû¡¡ Mona Rift PRVI Microplate 19.5 16.9 1918 Puerto Rico GSPRFZ GNPRFZ o a Passage 18 N Mon 20.4 Anegada Passage

St. Croix

o Muertos Trough 17 N

Caribbean Plate N 10 mm/yr o 16 N Figure 1. Tectonic setting of the northeastern Caribbean region. Plate vectors relative to a ﬁ xed North America plate from Mann et al., 2002. Focal Mech- anisms from Harvard CTM database, Molnar and Sykes (1969) and Huérfano (1994). Epicenters of recent large magnitude earthquakes in the region also labeled with year of occurrence (U.S. Geological Survey National Earthquake Information Center [NEIC] PDE database). GNPRFZ—Great Northern Puerto Rico fault zone; GSPRFZ—Great Southern Puerto Rico fault zone; PRVI—Puerto Rico–Virgin Islands; SFZ—Septentrional fault zone.

o o o o 68 W 67 W 66 W 65 W Mona Rift

o 18.5 N 1918 Desecho Is. San Juan Mona Passage Cerro Goden F ault Zone GNPRFZ

Mona Is. Mayagüez GSPRFZ o Lajas Valley Ponce 18 N Fig. 3 ge Parguera Shelf assa ada P Aneg

-6000 -4000 -2000 0 1000 Depth (m) Figure 2. Shaded relief map of the island of Puerto Rico and offshore region. Major structural lineaments and faults shown by heavy black lines. Faults in Mona Passage identiﬁ ed by Grindlay et al. (1997) and van Gestel et al. (1998). Structural lineaments onland from Glover (1971). Digital elevation model (DEM) created from U.S. Geological Survey 3 arc second DEM for Puerto Rico and National Oceanic and Atmospheric Administration (NOAA) hydrographic data. Location of the 1918 earthquake shown by ﬁ lled circle. Boxed area shows coverage of Figure 3. Inferred axis of E-W–trending arch shown as dashed thick gray line. GNPRFZ—Great Northern Puerto Rico fault zone; GSPRFZ—Great Southern Puerto Rico fault zone. o o o 142 67 15'W N.R. Grindlay67 10'W et al. 67 05'W

Puerto Rico Desecheo Ridge

Punta Higuero

o 18 20'N La Cadena de San Francisco Mountains

Fig. 5 Punta Cadena Cerro GodenFault Zone Añasco Bay

Añasco Rio Añasco Basin

o 18 15'N

Mayagüez Punta Algarrobo Basin

Fig. 8 ? Mayagüez ? ? ? Monte del Estado Serpentini Mayagüez Bay ?

o 18 10'N Punta Guanajibo te B

elt ID-131-D (FIg. 4) (FIg. ID-131-D Punta Arenas Fig. 10 Rio Guana Serp

entinite Belt jibo ûû¡¡

o 18 05'N 600

450

300

150

Punta Ostiones 0 Lajas Valley -150 Boquerón Bay South Lajas Fault D -300

o -450 18 00'N U Fig. 12 -600

-750 Depth (m) Cabo Rojo

Figure 3. Shaded relief map of western Puerto Rico and offshore region. Ship tracks for May 2000 survey shown in thin red lines. Trackline for a portion of multichannel seismic (MCS) line ID-131-D (Fig. 4) is shown as dashed line. The four boxes represent coverage area of maps shown in Figures 5, 8, 10, and 12. Location of the Cerro Goden fault zone determined from McIntyre (1971) and Mann et al. (this volume, Ch. 6) and location of South Lajas fault from Glover (1971), Meltzer and Almy (2000), and Prentice and Mann (this volume). Dashed line with question marks is the hypothesized eastward extension of the Cordillera fault. Serpentinite bodies mapped onland are shown as areas with striped ﬁ ll pattern. Inferred axis of E-W–trending anticline formed during early Pliocene N-S–shortening event shown as dashed thick black line. Constraints from high-resolution seismic and sidescan sonar data 143

Puerto Rico fault zone and the Great Southern Puerto Rico fault in areas where relatively thick accumulations of unlithifi ed allu- zone (Figs. 1 and 2; Erikson et al., 1990, 1991). vium were present. The second phase of deformation of N-S–shortening occurred in response to post–Eocene-Neogene convergence Existing Seismic Data between the North America and Caribbean plates (Dillon et al., 1996; van Gestel et al., 1998). The episode of deformation MCS data were acquired in 1972–1973 by Western resulted in a broad, 120-km-wide arch that has an axis that Geophysical Company and Fugro for the Puerto Rico Water extends from eastern Hispaniola through the Mona Passage to Resources Authority (Western Geophysical Company of Amer- the western side of the island of Puerto Rico (van Gestel et al., ica and Fugro, Inc., 1973). MCS profi les separated by 5–10 km 1998) (Figs. 2 and 3). Localized extension at the crest of the arch surround the island and were collected in a site assessment for presumably caused many small faults with a range of orienta- a nuclear power plant. The MCS data were acquired with a 24- tions, including many E-W–trending normal faults. channel streamer with 67 m group spacing and were digitized The third and ongoing phase of deformation is characterized and recorded using a SDS 1010 system. After acquisition, these by extension in the Mona Passage and possibly western Puerto data were processed using a single deconvolution operator, lim- Rico. The extension is believed to be caused by the differential ited velocity analyses, and stacking. The MCS data set is owned ENE relative motion of the Puerto Rico–Virgin Islands micro- by, and stored at, UPR as large-scale paper and Mylar records. plate with respect to eastern Hispaniola (Grindlay et al., 1997; We digitally scanned and examined the limited number of MCS van Gestel et al., 1998; Jansma et al., 2000; Mann et al., 2002). lines that cross or coincide with our shallow-water survey of the The extension could be accommodated by reactivation of WNW- inner shelf. In addition, we have used one MCS line that has trending normal faults or shear zones developed during previous been reprocessed to include migration (Fig. 4; Detrich, 1995). deformational phases. Recently published GPS geodetic mea- Initial reports provided by Western Geophysical Company of surements collected during a 10-year period in the region suggest America and Fugro, Inc. (1973) indicate an abundance of faults that the amount of differential motion between Hispaniola and that cut deeply into or through the Oligocene-Pliocene carbonate Puerto Rico is ~5 mm/yr (Lopez et al., 1999, Jansma et al., 2000; platform off western Puerto Rico. We have used mapped traces Mann et al., 2002). Geodetic studies also suggest that Puerto Rico of these faults, projected to the surface as originally proposed and the Virgin Islands are currently behaving as part of the stable by Western Geophysical Company of America and Fugro, Inc. Caribbean plate and are moving in an ENE direction (~070°) at a (1973), where they could be verifi ed by re-inspection of the rate of 19–20 mm/yr relative to North America (Fig. 1; Jansma et original MCS data. al., 2000; Mann et al., 2002). SEISMIC STRATIGRAPHY AND GEOLOGY GEOPHYSICAL DATA COLLECTION AND OFFSHORE WESTERN PUERTO RICO PROCESSING U.S. Geological Survey marine geologic maps based on We conducted a 10-day marine geophysical survey using the grab and dive samples and 3.5 kHz seismic records show surface University of Puerto Rico’s R/V Isla Magueyes in May 2000. We sediment thickness ranging from <1 to >10 m (Grove, 1983; surveyed extensively (728 line km of data) the insular shelf off of Schlee et al., 1999) on the western insular shelf of Puerto Rico. western Puerto Rico, from Punta Higuero to Cabo Rojo (Fig. 3). Seafl oor sediment composition is mixed terrigenous and skeletal Sidescan sonar and high-resolution sub-bottom profi ler data were sand and mud nearshore, and mostly carbonate mud farther off- acquired simultaneously and merged with Differential Global shore, punctuated by carbonate hardgrounds and shallow reefs Positioning Satellite (DGPS) navigation (±5 m resolution). Sur- (Grove, 1983; Schlee et al., 1999). Presently, the Añasco River veying extended very close to shore in water depths as shallow deposits siliciclastic material in the nearshore environment, as 2 m, increasing our ability to correlate with known onshore resulting in relatively thick (up to100 m) local accumulations of faults. Trackline spacing was ~300 m. The sidescan sonar system generated 300–400-m-wide swaths of 100 kHz and 500 kHz seabed refl ectivity data, which were later assembled into mosaic images with 1 m pixel resolution. All of the sonographs have TABLE 1. PROCESSING STEPS FOR SINGLE-CHANNEL SEISMIC DATA been fi ltered, slant range corrected, bottom corrected, destriped, and beam-angle corrected before being placed into a georefer- Convert fi les from SEG-Y format 2 enced digital mosaic. The SCS boomer system operated at 280 Spherical Divergence Gain (t ) Bandpass Filter: 100-150-1000-1100 Hz Joules and 2 shots per second. At the average ship speed of 5 kts, Spectral Whitening: 150-1000 Hz this resulted in a shot spacing of ~1.3 m. The SCS data have been Digitization of the seafl oor processed as shown in Table 1. Vertical resolution is estimated at Application of seafl oor mute 1 m, and penetration ranged from only imaging the seafl oor in Trace Mixing: 3 traces, not weighted reefal areas to over 100 milliseconds below the seafl oor (msbsf) Application of AGC: 50 ms 144 N.R. Grindlay et al. B A km

1 18.14°N 67.23°W ed in this study that .5 V.E.=6.0

19 km PAFZ 0 PAFZ V.E.=2.2 oor features identiﬁ ta Arenas fault zone (PAFZ). (B) Three- (B) zone (PAFZ). Arenas fault ta location of seismic show g. 14D). Boxes uth–shortening episode post–early Pliocene. 995). This MCS line parallels the west coast of 995). Ch. 6, ﬁ rom the northwest.

with seaﬂ PGFZ PGFZ Mayagüez Mayagüez Basin 20 km

MCS Line ID-131-D Mayagüez Basin PA/MFZ Fig. 9 ectors truncated by high-angle, normal faults (dashed lines), forming several structural basins. The north-dip- structural basins. (dashed lines), forming several ectors truncated by high-angle, normal faults PA/MFZ Dipping Reflectors

Añasco Valley Añasco Basin

67.05°W

CGFZ Oligocene-Pliocene Carbonate 18.32°N

Pliocene?-Holocene Sediment Añasco Basin Mtns. Cadena La Mayagüez N Rio Añasco N CGFZ Fig. 7 0

0.5 1.0 1.5 2.0 MCS ID-131-D MCS Two-Way Time (s) Time Two-Way le shown is located along the north-dipping limb of a regional arch developed during Phase 2 (e.g., Mann et al., this volume, during Phase 2 (e.g., Mann et al., this volume, arch developed is located along the north-dipping limb of a regional le shown ectors are interpreted to be the Oligocene-Pliocene carbonate platform that experienced deformation associated with a north-so ectors are interpreted to be the Oligocene-Pliocene carbonate platform that experienced les shown in Figures 7 and 9 that parallel this MCS line and image similar features. The large normal faults appear coincident normal faults The large in Figures 7 and 9 that parallel this MCS line image similar features. les shown ne the Cerro Goden fault zone (CGFZ), Punta Algarrobo/Mayagüez fault zone (PA/MFZ), Punta Guanajibo fault zone (PGFZ), and Pun Punta Guanajibo fault zone (PA/MFZ), Algarrobo/Mayagüez fault zone (CGFZ), Punta ne the Cerro Goden fault profi defi combined with MCS line ID131-D. Illumination is f of western Puerto Rico looking eastward dimensional shaded relief perspective Figure 4. (A and B) Western Geophysical multichannel seismic (MCS) line ID131-D reprocessed including migration from Detrich (1 Western Figure 4. (A and B) Puerto Rico (see inset for location) and clearly shows north-dipping refl Puerto Rico (see inset for location) and clearly shows ping refl The profi Constraints from high-resolution seismic and sidescan sonar data 145

siliciclastic sediments. The age of the unlithifi ed material imaged faulted carbonate platform and/or linear disruptions of the sedi- by SCS data is unknown, but is inferred to be primarily alluvium ment at the seafl oor observed on the sidescan sonar mosaics. deposited since the last sea-level lowstand (i.e., Holocene) on an erosional (often hardground) surface. OBSERVATIONS AND INTERPRETATIONS The insular shelf to slope break within the study area is marked by an almost continuous line of submerged coral reefs In this section, we present the observations and interpretations that rise to water depths of 15–20 m. The entire insular shelf of the sidescan sonar and SCS data collected during the May 2000 and upper slope were exposed to subaerial erosion from before cruise. Data are presented in four geographic regions (Fig. 3): the 15,000 years until after 10,000 years ago (Morelock et al., 1994). North Añasco Bay region, the North Mayagüez Bay region, the In the southwest along the Parguera shelf (Fig. 2), the Holocene South Mayagüez Bay region, and the Boquerón Bay region. shelf-edge reefs began growing 8,000–9,000 yr ago on the newly submerged Pleistocene surface (Morelock, 1994; Hubbard et al., North Añasco Bay Region 1996). These coral died off 6,000 yr ago and the surface was bar- ren until massive coral began to grow a few hundred years ago The sidescan sonar mosaic of the northern Añasco Bay (Fig. 5) (Morelock, 1994; Hubbard et al., 1996). shows a low refl ectivity area on the ~3-km-wide insular shelf near Underlying the Holocene sediments is a thick (up to 1500m) the mouth of the Añasco River. The areas of low refl ectivity are Oligocene–early Pliocene age sequence of shallow-water lime- interpreted to be deposits of fi ne-grained sediments, mainly mud stone that lies unconformably over Cretaceous-Eocene volcanics and sands (Grove, 1983). The areas of high and chaotic refl ectivity (Moussa et al., 1987). It is assumed that these carbonates are near the shelf edge are interpreted as unsedimented reefs. The shelf contiguous with the tilted carbonate sequences that extend off- becomes narrower (<1 km) and more reef- and reef debris–domi- shore on the northern and southern insular slopes of the islands nated to the north, indicated by more and larger areas of high and (Moussa et al., 1987; van Gestel et al., 1998). The tilted platforms chaotic refl ectivity. The shelf edge parallels the coastline, and south of opposing dip are interpreted to be limbs of a roughly east- and west of Punta Cadena, the seafl oor drops off rapidly to depths west–trending, 120-km-wide arch that resulted from a north- in excess of 300 m. Two parallel, curvilinear, high-refl ectance fea- south–shortening event (Phase 2 described in section II) from tures that are separated by 200–300 m are clearly visible in the sid- post Eocene to late Neogene (Dillon et al., 1996; van Gestel et escan sonar mosaic (Figs. 5 and 6). These linear features parallel al., 1998). The MCS profi le shown in Figure 4 images the north- the coastline as it changes trend from northeast-southwest, north of ern limb of this arch. Existing MCS profi les offshore western Punta Cadena to roughly east-west, south of Punta Cadena. Seis- and southern Puerto Rico, collected by Western Geophysical mic profi les (Fig. 6) show two areas of steeper slope ~10–15 m Company and Furgo, show numerous E-W– and WNW-trending high, at ~80 m and ~40 m depth, which correspond to the location normal and strike-slip faults offsetting the carbonate platform of the highly refl ective linear features seen on the sidescan sonar strata and the underlying Cretaceous-Eocene volcanic basement mosaic. It has been suggested that during the last transgression, sea (Larue and Ryan, 1990). level rise either stopped, slowed dramatically, or even dropped to The primary limitation of these MCS data for this study is form paleo-shores (e.g., mid-Atlantic continental shelf; Emery and the lack of resolution of structure in the uppermost sediments and Uchupi, 1984). We hypothesize that the two “steps,” or terraces, the limited coverage in shallow water. The high-resolution SCS imaged on the slope represent stillstands or slow downs in sea- profi les from our survey, therefore, complement the existing MCS level rise where reefal growth was able to keep pace with sea-level profi les collected by Western Geophysical Company and Fugro, rise. These submerged terraces are comparable to the submerged Inc. SCS profi ling of carbonate hardgrounds and reefs resulted in reefs found at 20 m water depths at the edge of the insular shelf. high-amplitude seafl oor refl ections with very limited sub-seafl oor Most importantly, offsets in these linear features can be used to penetration along with signifi cant seafl oor multiples obscuring pinpoint locations of youthful faults offshore and potentially deter- any possible sub-bottom structure. Sediment thicknesses of <1 m, mine sense of relative motion along the fault trace. which is often the case, were not resolvable. In other locations, Shore-parallel SCS lines just seaward of the Añasco River sediment stratigraphy is well imaged above a relatively high- reveal the thickest sections (>100 ms) of continuous fl at-lying amplitude refl ection(s) forming acoustic basement, interpreted to refl ections that onlap and bury reef-like pinnacles, sometimes be the top of the carbonate platform strata. Acoustic basement on display cut and fi ll geometries and occasionally are severely SCS data often appears as a uniformly dipping refl ection or series attenuated, presumably the result of gas-charged sediments. This of refl ections beneath onlapping and/or draping, semi-continuous, seismic facies is interpreted as well-stratifi ed alluvium from the relatively fl at-lying refl ections to hummocky clinoforms. In other Añasco River. These deposits thin and extend to the northern, areas, stratigraphic relationships indicate acoustic basement was east-west–trending shore, where they appear as relatively thick affected by cut and fi ll processes. (25 m) accumulations between shallow reef heads. Several of our For the purposes of this study, youthful faulting is identi- high-resolution SCS profi les (Fig. 7) clearly show displacements fi ed by displacements of the seafl oor and within the uppermost, of the seafl oor and these alluvial sediments that we interpret to unlithifi ed (Holocene?) sediment, lying unconformably over the be Holocene age. 146 N.R. Grindlay et al. oor. These oor. gure. Dashed gure. F3 11W' o 67 lled circles. Bathym- Rio Añasco les projected to the seafl

Añasco Bay as ﬁ gnitudes >2.5 shown ea is enlarged and shown as an inset. Reefs enclosed and shown ea is enlarged continuous fault traces within the CGFZ. Earthquake traces within the CGFZ. Earthquake continuous fault 12W' o F3 67 Silty Clay

10 Cerro Goden Fault Zone Fault Goden Cerro 30 Gas Charged Sediments REEFS les indicated by thick solid lines. Multichannel seismic (MCS) tracks are F2 les ID-131-D and I-129-D extend beyond southern boundary of ﬁ beyond les ID-131-D and I-129-D extend F1 of deeper structures in MCS proﬁ les and offsets 50 I-129-D

13'W

o 100 L41

ed on SCS lines is not apparent MCS XI-108-D (Fig. 7) or ID-131-D 4). 150 Sand / Mud Fig. 7 in SCS proﬁ oor offsets

100 L37

ID-131--D (Fig. 4) 10 50 L33 200 14'W 67 o Punta Cadena REEF

Fig. 6 XI-108-D (Fig. 7) 250 400 Añasco Basin 0 Terraces F2

400 800 m 300 15'W 67 o 67 18'N 16'N 17'N o o o 18 18 18 epicenters (U.S. Geological Survey National Earthquake Information Center [NEIC] PDE database 1973–2000) of earthquakes with ma Information Center [NEIC] PDE database 1973–2000) of earthquakes National Earthquake epicenters (U.S. Geological Survey features are interpreted as offshore extensions of the Cerro Goden fault zone (CGFZ). F1, F2, and F3 mark the location of semi- of the Cerro Goden fault extensions features are interpreted as offshore in Figure = 10 6 and dashed box ar area is shown m, and 1 etry contour interval resolution mosaic. Imagery within boxed m pixel by ~350–400 m right-laterally (F2). F3 identifi by dashed lines appear to be offset lines mark location of linear features visible in sidescan sonar mosaic, seafl ID-131-D, XI-108-D and I-129-D; single-channel seismic (SCS) tracks are L37 L41. MCS profi Figure 5. Sidescan sonar mosaic of North Añasco Bay (Fig. 3 shows location). Location of seismic profi 3 shows Añasco Bay (Fig. 5. Sidescan sonar mosaic of North Figure Stillstand Paleo-ShoresConstraints from high-resolution seismic and sidescan sonar data 147 67o14'15"W 67o14'W

18o17'30"N

F1 Cerro Goden Fault Zone L33

A 18o17'15"N

100 0 100 m A

Shotpoint 9100 9200 9300 9400 9500 N 0 0 SCS Line 33 F2 Punta Cadena A 40 30 F1 A F2

Terraces F3 B Cerro Goden Fault Zone 80 60 B L33 C Depth (m)

120 90 Two-Way Time (ms) Two-Way

160 120

0 100 200 V.E.= 4.6 m 200 B 150

Figure 6. (A) Sidescan sonar enlargement (Fig. 5) showing terraces labeled “A” and “B” and a portion of the Cerro Goden fault zone (CGFZ), including F1 and F2 as in Figure 5. (B) Single-channel seismic (SCS) Line 33 showing the two terraces at ~80 m and ~40 m formed by sea-level stillstands or slow downs during the most recent transgression (depth scale assumes V = 1500 m/s). (C) Map view showing SCS line 33 (solid line), terraces as dotted lines and fault traces making up the CGFZ as dashed lines. 1400 1300 1200 1100 1000 S 900 148 0 Shotpoint N.R. Grindlay et al. 0 F1 SCS Line 37 0 100 200 MCS XI-108-D meters V.E.= 9.3 20 15 Punta Cadena Depth (m) 37 41 F2 CGFZ

Reef 40 30 D Two-Way Time (ms) Two-Way 60 Acoustic Basement Tilted 45 Reflections A

1500 1400 1300 1200 1100 1000 900 800 S 0 Shotpoint 0 F1 SCS Line 41 V.E.= 9.3 0 100 200 F2 meters

20 Reef 15 Depth (m) Reef

40 30

Two-Way Time (ms) Two-Way Tilted Reflections Acoustic Basement 60 B 45

0 1 2 km MCS Line XI-108-D V.E.= 4 SE 0 0 F2 200 150 400 300

600 450 Depth (m) Pliocene-Holocene? 800 Sediment 600 1000 750 1200 900 Two-Way Time (ms) Two-Way 1400 1050 Oligocene-Pliocene 1200 1600 Carbonate C Figure 7. (A and B) Single-channel seismic (SCS) profi les across the Cerro Goden fault zone (CGFZ) (depth scale assumes V = 1500 m/s). Ap- proximately 20–25 m of sediment fi lls depressions between exposed reef heads. Acoustic basement in these profi les is interpreted as the boundary between alluvium (Holocene?) and well-indurated sediments and sedimentary rocks. Offsets (dashed lines) of seafl oor and offsets and tilting of sediments indicate recent fault displacement. (C) Multichannel seismic (MCS) Line XI-108-D shows offset (dashed line) of the Oligocene- Pliocene carbonate platform and overlying section which is assumed to be Pliocene and younger in age. The trace of this normal fault projected to the seafl oor aligns with fault (F2). Note that fault F3 is not associated with any deep structure imaged. (D) Map view showing MCS and SCS tracklines (solid lines) and the fault segments within the CGFZ (dashed lines). Constraints from high-resolution seismic and sidescan sonar data 149

On the basis of the SCS, sidescan sonar and bathymetry of fault motion can be seen on the MCS profi les (Figs. 4 and data, we have identifi ed parallel, semi-continuous fault traces 9B); however, due to their low resolution, it is not clear whether (Figs. 5, 6, and 7) that trend ENE. One of these fault traces (F3, the seafl oor or uppermost sediments are disrupted. The SCS Fig. 5) cuts across the submarine terraces at nearly right angles profi les also show the north-dipping refl ectors truncated by and appears to offset them in a right-lateral sense. The offset the south-facing scarp near the seafl oor and only a thin (<2 m) in the reef at 20 m water depth is most clearly distinguishable veneer of sediment at the ridge (Fig. 9A). The thin Holocene and is ~400 m (Fig. 5, inset). Fault traces can be traced close to (?) sediments on the top of the ridge are consistent with a recent shore, but on SCS lines closest to the shore, no displacements are uplift history, but are too thin to enable offset strata to be seen. observed in the well-stratifi ed and relatively thick sediments. Additionally, the lack of piercing points makes determination MCS profi les XI-108D (Fig. 7) and ID-131-D (Fig. 4, of the direction and magnitude of strike-slip motion (if any) north end) cross the narrow shelf south of Punta Cadena and impossible to determine. reveal a high-angle normal fault offsetting the Oligocene-Plio- Sidescan sonar imagery shows areas of lower refl ectiv- cene carbonate strata. It is not clear from the MCS profi les if ity “streaking” down the south face of the scarp (fi lled arrows, this fault displaces sediments on the seafl oor. The map view Fig. 8) and, along with the SCS data, indicates sediment transport projection of these deep faults to the seafl oor lies within the through breaks in the reef. SCS profi les show a thick wedge of zone of parallel and semi-continuous fault traces that displace sediments accumulated at the base of the slope (Fig. 9). Since the seafl oor and Holocene sediments. This narrow band of no river empties directly into Mayagüez Bay, the source of these faults is interpreted as an offshore portion of the Cerro Goden sediments is interpreted to be the Añasco River to the north. Off- fault zone (Figs. 3 and 4). sets of this alluvium and seafl oor at the base of the scarp align with displacements and terminations of linear seafl oor features North Mayagüez Bay Region observed closer to shore in SCS and sidescan sonar data. The set of offsets that projects from the head of the scarp landward The sidescan sonar mosaic of the North Mayagüez Bay toward Punta Algarrobo and those that project toward Mayagüez region (Fig. 8) shows a wider (3–5 km) section of the insular shelf Bay are referred to as Punta Algarrobo and Mayagüez fault that is dominated by low-refl ectivity seafl oor that is determined zones, respectively (Fig. 4). from grab samples to be mainly silty sand and mud (Schlee et al., 1999). Two large areas of chaotic and high refl ectivity at the South Mayagüez Bay Region mouth of the bay are interpreted to be reefal areas. The western edges of the reefs form a semicircle that is cut at its midpoint by The sidescan sonar mosaic of South Mayagüez Bay a narrow NW-SE–trending channel (Fig. 8). The sidescan sonar (Fig. 10), on a relatively wide section of the insular shelf (8– mosaic shows an abrupt change in refl ectivity along the southern 10 km), is dominated by highly refl ective seafl oor interpreted edge of the channel that corresponds to small offsets, down to the to be large expanses of coral reefs. The highly refl ective area south, in the SCS data that project northwestward to a very steep, is truncated to the north by a NW-SE–trending lineament that south-facing slope (Fig. 9). West of the shelf edge the seafl oor marks an abrupt change to lower refl ectivity seafl oor (muds and deepens, forming a bowl-shaped basin called the Mayagüez basin sands). Corresponding to this boundary and visible on adjacent (Figs. 4, 8, and 9). In the sidescan sonar imagery, we observed at SCS profi les is a narrow (400-m-wide) trough, with a nearly least two linear and parallel, high-refl ectivity features that follow vertical 10 ms offset (down to the south) at the seafl oor that contours along the southern edge of this basin, which we interpret trends NW-SE (Figs. 10 and 11). An ~20 ms interval of fl at- to be submarine terraces comparable to those observed in North lying, continuous refl ections overlying dipping and diverging Añasco Bay (Fig. 8). These terraces also appear to terminate at refl ections terminate abruptly at the north wall of this trough the southern edge of the NW-SE–trending channel. (Fig. 11). This seismic image is interpreted as a relatively MCS lines running north-south offshore Mayagüez Bay thick section (~15 m) of well stratifi ed alluvium (Holocene?) clearly show north-dipping refl ectors truncated by high-angle, that has fi lled a fault-bounded depression that has continued normal faults forming a signifi cant scarp (Figs. 4 and 9). The to experience a component of vertical motion since infi lling north-dipping refl ectors are interpreted to be the Oligocene- began. In addition, contours marking the submerged reef at Pliocene carbonate platform and the scarp separates the clastic the shelf edge (20 m depth) appear to be offset ~750 m in the sediment depocenters of Mayagüez Bay and Añasco Bay into right-lateral sense across the trace of this fault (Fig. 10). Other, two sub-basins: the Mayagüez and Añasco basins (Fig. 4). SCS smaller vertical offsets of the seafl oor and surface sediments are profi les collected in the Mayagüez Bay area parallel to, and also imaged on parallel and adjacent SCS profi les. The set of overlapping these MCS lines (Fig. 9) show this scarp as a steep parallel and semi-continuous fault traces, trending NW-SE, that WNW-ESE–trending slope, with 50–100 m of relief. Several project toward Punta Guanajibo, defi ne the offshore section of parallel, high-amplitude refl ectors, which dip north at ~6°, the Punta Guanajibo fault zone (Figs. 4, 10, and 11). terminate at or near the surface beneath a thin section of fl at- Offshore from Punta Arenas, the reefal areas are cut by lying continuous refl ections. The dip and normal component several discontinuous NW-SE–trending lineaments (Punta Are- 150 N.R. Grindlay et al. ection 0 10'W o of deeper les and offsets 400 400m

Mayagüez Bay Punta Algarrobo 10 in SCS proﬁ oor offsets REEF thquake Information Center [NEIC] PDE database thquake tracklines indicated by thick solid lines and single- Silty Clay Sewage Outfall Pipe 11'W 67 o r mosaic, seaﬂ 67

10 10

REEF

12'W Channel o

REEF 10

Silty Clay

Sand/Mud

lled circles. Bathymetry contour interval = 10 m, and 1 m pixel resolution mosaic. = 10 m, and 1 m pixel lled circles. Bathymetry contour interval

I-129-D (Fig. 9) (Fig. I-129-D L104

Terraces

13'W

indicate location and dir Short black arrows zones (PA/MFZ). Algarrobo and Mayagüez fault These features locate the Punta oor.

100

Basin

100

Mayagüez 4) (Fig. ID-131-D

150

200

les projected to the seaﬂ 200 Mayagüez Fault Zone

Punta Algarrobo Fault Zone 230 14'W o 67 14'N 13'N 15'N o o o 18 18 18 channel seismic (SCS) line 104 shown as dotted line. Dashed lines indicate location of linear features visible in sidescan sona channel seismic (SCS) line 104 shown Figure 8. Sidescan sonar mosaic of the North Mayagüez Bay area (Fig. 3 shows location). Location of multichannel seismic (MCS) 3 shows 8. Sidescan sonar mosaic of the North Mayagüez Bay area (Fig. Figure structures in MCS proﬁ National Ear epicenters (U.S. Geological Survey Añasco Basin into the Mayagüez Basin. Earthquake of sediment transport from the as ﬁ with magnitudes >2.5 shown 1973–2000) of earthquakes 0 150 300 mConstraints from high-resolution seismic and sidescan sonar data 151 SCS Line 104 V.E.=11.2 N Shotpoint 2700 3000 3300 3600 3900 Mayagüez Basin

100 75 Punta Algarrobo Fault Zone Dipping Reflectors Depth (m)

Mayagüez Fault Fig. 8 150 113 Two-Way Time (ms) Time Two-Way

Multiple 200 150

A MCS I-129-D Rio Añasco

Añasco Basin

C Punta Algarrobo Fault Zone SCS Mayagüez Fault Zone L104

Mayagüez Basin

0 500 1000 m MCS Line I-129-D V.E.=2.9 N Mayagüez Basin Añasco Basin

200 150 Pliocene?-Holocene 400 Sediment 300 Depth (m)

600 450 Dipping Reflector 800 600 Two-Way Time (ms) Time Two-Way Oligocene-Pliocene 1000 Punta Algarrobo Carbonate 750 Fault Zone 1200 900 B

Figure 9. Coincident single-channel (SCS) and multichannel seismic (MCS) profi les across the Punta Algarrobo and Mayagüez fault zones (PA/MFZ) in Mayagüez Bay. (A) SCS line 104 shows that the high-angle normal fault imaged in MCS creates south-facing scarps and truncates north-dipping refl ectors, which outcrop at the seafl oor or are thinly sedimented. Boxed area is enlarged and reveals an offset in the seafl oor and near surface alluvium accumulating in the Mayagüez Basin. (B) MCS line I-129-D clearly shows north-dipping refl ectors truncated by high- angle, normal faults (dashed lines). The north-dipping refl ectors are interpreted to be the Oligocene-Pliocene carbonate platform (also see Fig. 4). The boxed area shows the area imaged by SCS line 104. (C) Map view showing tracklines of MCS (thin solid line) and SCS (thick solid line) profi les and the fault segments comprising the PA/MFZ (dashed lines). o o o o 67 13'W 67 12'W 67 11'W 15218 12N' N.R. Grindlay et al. 30 20 10 10 750m REEF Silty Clay

Fig.11

30 L134

18 11'N

Mayagüez Bay

Punta Guanajibo

Fault Zone

REEF ID-131-D ( Fig. 4)

o 18 10'N Punta Guanajibo

400 0 400 m

10 Rio Guanajibo Serpentinite Belt

o 18 9'N

Sand /Mud Punta Arenas Fault Zone Punta Arenas

o Silty Clay 18 8'N

Figure 10. Sidescan sonar mosaic of South Mayagüez Bay (Fig. 3 shows location). Thick black lines indicate locations of single-channel (SCS) and multichannel seismic (MCS) profi les. MCS profi le ID131-D extends beyond the northern border of fi gure (Fig. 4). Dashed lines indicate location of linear features visible in sidescan sonar mosaic, seafl oor offsets in SCS profi les and offsets of deeper structures in MCS profi les projected to the seafl oor. These features locate the Punta Guanajibo and Punta Arenas fault zones. Filled pattern locates serpentinite. Earthquake epicenters (U.S. Geological Survey National Earthquake Information Center [NEIC] PDE database 1973–2000) of earthquakes with magnitudes >2.5 shown as fi lled circles. Bathymetry contour interval is 10 m, and 1 m pixel resolution mosaic. Imagery within boxed area is shown in Figure 11. Constraints from high-resolution seismic and sidescan sonar data 153

Depth (m) B 1 0 15 30 45 A NE ed and tilted sediments 100 12'W o 3 200 67 200 0 200 400 m 1 300 correspond with SCS. (C) Map view showing SCS showing correspond with SCS. (C) Map view Tilted Reflections the Serpentinite belt mapped on shore. scan sonar records. Well-stratiﬁ scan sonar records. 400 L134 2 sediments that are also apparent on adjacent oor and/or surface 12'30"W o 500 67 1 4 600 3 700 3 SCS Line 134 800 11'N o 11'30"N 18 o 18 900 4 C 1000

Rio Guanajibo Serpentinite Belt 1100 Multiple V.E.= 12.3 V.E.= 1200 1 3 2 m 1300 Punta Guanajibo L134 5 4

5 Fault Zone 0 100 200

20 40 60 Shotpoint Two-Way Time (ms) Time Two-Way lineaments numbered to area of sidescan sonar mosaic from Figure fault bounded depression (B) Enlarged ll the fault 10 showing SCS lines are indicated by numbered arrows. Filled arrows number 1, 3, and 4 are offsets that are apparent on both SCS and side number 1, 3, and 4 are offsets Filled arrows SCS lines are indicated by numbered arrows. ﬁ within the PGFZ (dashed lines). Note that trend of projects to segments trackline (solid lines) and numbered fault Figure 11. (A) single-channel seismic (SCS) line 134 across the Punata Guanajibo fault zone (PGFZ). Offsets in seaﬂ zone (PGFZ). Offsets Figure 11. (A) single-channel seismic (SCS) line 134 across the Punata Guanajibo fault 154 N.R. Grindlay et al.

nas fault zone, Fig. 10). The lineaments extend southward into fault activity, including offset terrace risers and upslope-fac- the areas of muds and sands (dark-gray) and silty clays (light ing faults. The projection of the mapped onland trace of the gray), but do not correspond to clear offsets of the seafl oor in Cerro Goden fault zone offshore lies along the northern edge of the SCS data. This is not to say that offsets do not exist. Sedi- Añasco Bay and appears as a set of semi-continuous fault traces ment thicknesses in this area are extremely thin (<1 m, below indicating Holocene activity (Figs. 5, 6, and 7). To the north system resolution) and the hardbottom nature of the seafl oor and west of our survey area, MCS profi les indicate that the fault resulted in high-amplitude seafl oor refl ections with very limited zone extends to the Desecheo Ridge, the southern limit of the sub-seafl oor penetration along with signifi cant seafl oor multiples Mona rift (Figs. 3 and 15). obscuring any possible sub-bottom structure. 2. The second zone of deformation is the Punta Algarrobo/ Mayagüez fault zone (Figs. 14 and 15). The slope and truncated Boquerón Bay region dipping refl ectors that indicate the position of the Punta Algar- robo fault zone are not visible on the SCS profi les nearest to Extensive surveying of the area offshore of the Lajas valley shore due to the thickness of the sediment column resulting did not reveal any evidence of the South Lajas fault surveyed from rapid accumulation of alluvium, therefore it cannot be on land by Meltzer and Almy (2000) and Prentice and Mann directly linked to faults mapped onshore. However, the similar- (this volume). The sidescan sonar mosaic shows areas of very ity in trend, location, and type of motion suggest that the fault fi ne silty clays (light gray) close to shore in Boquerón Bay and zone imaged in the northern Mayagüez Bay area is the offshore in the northern half of the survey area (Fig. 12). Seafl oor sedi- extension of the Cordillera fault mapped by Moya (1994), who ments consisting of sands and muds (Schlee et al., 1999), seen proposed activity within the Quaternary. The offshore fault as dark gray areas in the sidescan sonar mosaic, dominate the zone projects into the WNW-ESE–trending Monte del Estado southern portion of the survey area. Within Boquerón Bay and at peridotite/serpentinite belt onland (Figs. 3 and 14). the mouth of the bay, SCS profi les show south-dipping refl ectors, 3. The third zone of deformation is the Punta Guanajibo/ which we interpret to be the Oligocene-Pliocene carbonate plat- Punta Arenas fault zone that projects to Punta Guanajibo and form, overlain by the relatively fl at-lying refl ections interpreted Punta Arenas (Figs. 14 and 15). These areas are characterized as Pliocene-Holocene sediments (Fig. 13). The presence of by the NW-SE–trending Rio Guanajibo serpentinite belt onland dipping refl ectors on adjacent profi les with dissimilar azimuths (Figs. 3 and 14). In addition, Moya (1994) identifi ed a WNW- enabled us to calculate a true dip of 3° to the SSW. The dipping trending fault with vertical offsets along the southern fl ank of refl ectors extend to the seafl oor where they appear to be truncated the Cordillera de Saban Alta, the seawardmost unit of the ser- along an erosional surface, on top of which lies a thin covering of pentinite belt (Figs. 3 and 14). Moya (1994) suggests that this sediment (Holocene?). The dip of the refl ectors abruptly changes fault shows Quaternary activity. to the north on the north side of an E-W–trending bathymetric Both the Punta Algarrobo/Mayagüez and Punta Guanajibo/ high at 18°06′N. We interpret this change in dip to mark the axis Punta Arenas fault zones project to, and are colinear with, the of the regional E-W–trending arch (Figs. 3 and 12). trend of onland serpentinite bodies. The serpentinite bodies are associated with magnetization lows that extend offshore, sug- DISCUSSION gesting that the serpentinite also extends offshore (Fig. 14). The faults mapped offshore in this study correspond to the magnetiza- Evidence for Holocene Faulting Offshore Western Puerto tion lows. This relationship suggests that either the presence of Rico and Onland Extensions serpentinite has localized fault activity or that fault activity has remobilized serpentinite. In either case, the relatively weak and The systematic sidescan sonar and high-resolution SCS ductile serpentinite may be accommodating part of the strain asso- survey of the western insular shelf of Puerto Rico has revealed at ciated with the regional stress regime by many frequent, small least three regions of active deformation. magnitude events, thus accounting for the intense, low magnitude 1. The fi rst region includes the offshore extension of the seismic activity that characterizes southwestern Puerto Rico. Cerro Goden fault zone (Figs. 14 and 15). Onland this fault par- The lack of evidence in the offshore for young faulting associ- allels an abrupt, linear mountain front separating the 270–361- ated with the South Lajas fault does not necessarily mean there is m-high La Cadena de San Francisco mountain front from the no Holocene offshore displacement on this structure. This fault has alluvial Añasco valley (Figs. 3 and 4). Previous workers have at least a component of strike-slip displacement and is only known postulated that the Cerro Goden fault zone continues to the to have had two displacement events in the Holocene (Prentice southeast as the Great Southern Puerto Rico fault zone and to and Mann, this volume). With such a small amount of Holocene the west, merges with the normal fault forming the eastern wall displacement, it is possible that it is not resolved even with a of the Mona rift (Garrison and Buell, 1971; McCann, 1985) high-resolution seismic system, especially if most of the motion (Figs. 1 and 15). Onland ~500 m south of the prominent La is strike-slip. However, the sidescan sonar imagery also failed to Cadena mountain front, Mann et al. (this volume, Ch. 6) found detect evidence of the fault, suggesting that if a young fault does geomorphic features suggestive of right-lateral Quaternary exist, recent sedimentation has masked its surfi cal expression on 67o15'W 67Constraintso14'W from high-resolution 67o13'W seismic and67o 12'Wsidescan sonar data67 o11'W 67o10'W155 10

18o6'N Silty Clay

18o5'N REEF10 400 0 400 800 m

10 20

18o4'N 20 Silty Clay

18o3'N

Punta Ostiones 20

o 18 2'N L139 Boquerón Boquerón Bay REEF Sand / Mud 10 10

o 18 1'N REEF 10 Silty Clay

REEF L64

18oN 10

Serpentinite

Figure 12. Sidescan sonar mosaic of Boquerón Bay area. Thick solid lines indicate location of single-channel seismic (SCS) lines 139 and 64 shown in Figure 13. Axis of E-W–trending arch shown as dashed thick black line. South of the axis, refl ectors in SCS profi les dip SSW; north of the axis refl ectors dips NNE. Filled pattern locates serpentinite. Earthquake epicenters (U.S. Geological Survey National Earthquake Information Center [NEIC] PDE database 1973–2000) of earthquakes with magnitudes >2.5 shown as fi lled circles. Bathymetry contour interval is 10 m, and 1 m pixel resolution mosaic. Sidescan sonar data were not able to resolve any seafl oor displacements offshore of the Lajas valley. 156 N.R. Grindlay et al. Depth (m) Depth (m) ec- 0 30 75 15 45 60 15 45 60 0 30 75 A B 600 N N 3800 800 les offshore of the Lajas les offshore 3600 1000 3400 South Dipping Reflectors South Dipping Reflectors 1200 3200 1400 aged in SCS and MCS indicating location of dipping refl Holocene Sediment ? 3000 1600 2800 1800 Multiple SCS Line 139 cial sediment offsets are not observed in any of the SCS profi in any are not observed cial sediment offsets 2600 les. An angular unconformity is apparent between the dipping carbonate strata and les. ections are interpreted as the uppermost portion of Oligocene-Pliocene carbonate Oligocene-Pliocene Carbonate 2000 oor or surfi 2400 2200 SCS Line 64 2200 2400 2000 Shotpoint 0

20 80 60 40 Two-Way Time (ms) Time Two-Way 100 les of Boquerón Bay. South-dipping reﬂ les of Boquerón Bay. 1800 m C 400 1600 Multiple 200 ections that are interpreted as Holocene in age. Seaﬂ 1400 0 oor.

Boquerón Bay 3°

1200 L139

Shotpoint at-lying, continuous reﬂ

1000 V.E.= 13.2 V.E.= L64 0

20 80 40 60 Two-Way Time (ms) Time Two-Way 100 tors projected to the seafl Figure 13. (A and B) Single-channel seismic (SCS) profi Figure platform. This dip is consistent with structure imaged by offshore multichannel seismic (MCS) profi This dip is consistent with structure imaged by offshore platform. uppermost fl valley. (C) Map view showing SCS trackline (solid lines). Series of strike and dip symbols for top of the carbonate platform im SCS trackline (solid lines). Series of strike showing (C) Map view valley. Constraints from high-resolution seismic and sidescan sonar data 157

67o18'W 67o12'W 67o06'W 18o24'N

+250 +250

+400

+25 0

18o18'N

CGFZ +250

| |

PA/MFZ

| ? Mayagüez ?

18 12'N

Cordillera

+250

| |

Fault

Monte del Estado Serpentinite? Belt

| | CSA PG/PAFZ Rio Guanajibo Serpentin 0

+250

+250 o 18 6'N ite B elt

Figure 14. Residual magnetic intensity map for western Puerto Rico (after Bracey, 1968). Contour interval = 50 gammas. Filled pattern locates serpentinite and offshore faults identiﬁ ed by this study and faults with proposed Quaternary activity onland shown by dashed lines. Dashed line with question marks locates the projection of the cordillera fault along the serpentinite belt to the western coast. Note the correspondence of the Punta Algarrobo/Mayagüez fault zone (PA/MFZ), the Monte del Estado peridotite/serpentinite belt onland and shaded zone of low magnetic intensity. CSA—Cordillera de Sabana Alta; PG/PAFZ—Punta Guanajibo and Punta Arenas fault zones.

the seaﬂ oor. Alternatively, the South Lajas fault could be one ingly apparent that the deformational history of Puerto Rico and of many short fault segments that, when taken together, form a its vicinity is very complex. One result of our offshore survey regional fault system within southern Puerto Rico. and the coordinated onland trenching and mapping studies (Prentice and Mann, this volume; Mann et al., this volume, Ch. Implications for Neotectonics of Western Puerto Rico and 6) is to show the presence of several Quaternary faults intersect- the Mona Passage ing the west coast, and that many of these faults appear to be reactivated, older, WNW-trending basement structures. On the basis of previous marine geophysical studies as well The youthful faults we mapped offshore appear to be as the recent onland and marine studies, it is becoming increas- largely conﬁ ned to the northern and central areas of the western 158 N.R. Grindlay et al.

N. Dipping Carbonate Lajas Valley N

Platform VE= 6X Z

Mayagüez SLF Aguadilla

Mona Cerro Goden FZ Rift Cabo PA/MFZ PG/PAFZ Rojo

Insular Shelf Muertos Trough

Desecheo Ridge Mona Passage

Puerto Rico Trench

Mona Rift PRVI Microplate DR

CGFZ PA/MFZ Puerto Rico PG/PAFZ Mona Passage Anegada Passage Post-early Pliocene-Late Quaternary? Rifting and Right-lateral Shear

MR PRVI Muertos Trough PINNED B Figure 15. (A) Three-dimensional perspective of western Puerto Rico looking eastward. Illumination is from the northwest. Faults identiﬁ ed during this survey are marked as black lines and are tentatively traced to Quaternary faults onland. Faults onland in the vicinity of Aguadilla from Hippolyte et al. (this volume). Also shown is the axis of the E-W–trending arch. Note the change in insular shelf morphology south to north from broad, ﬂ at and undeformed to relatively narrow, deformed and dissected. PA/MFZ—Punta Algarrobo/Mayagüez fault zone; PG/PAFZ—Punta Guanajibo/Punta Arenas fault zone; SLFZ—South Lajas fault zone. (B) Conceptual model for the neotectonics of the Puerto Rico region. Much of the Quaternary faulting is primarily right-lateral transtension. This extension is the result of differential relative motions (shown by the GPS vectors) both in direction and velocity between Hispaniola, Puerto Rico–Virgin Islands (PRVI) microplate and the Caribbean plate. Constraints from high-resolution seismic and sidescan sonar data 159

Puerto Rico insular shelf. The rugged seafl oor morphology in Christa von Hilldebrant, Victor Huérfano, and other members of this region refl ects this wide zone of deformation (Fig. 15). In the Puerto Rico seismic network and the University of Puerto marked contrast, the southwestern portion of the insular shelf, Rico–Mayagüez Geology Department generously provided time where we found little or no evidence of youthful faults, is wide, and logistical support for the fi eld work conducted in Puerto Rico. smooth, and fl at (Fig. 15). The offshore sections of the Cerro We thank Carol Prentice and Tom Pratt for their helpful reviews Goden, Punta Algarrobo/Mayagüez, and Punta Guanajibo/Punta of this manuscript. This research was funded by U.S. Geological Arenas fault zones are all observed to have normal fault geom- Survey Grant no. 00HQGR0010. University of Texas at Austin etries, and in the case of the Cerro Goden and Punta Guanajibo Institute for Geophysics (UTIG) contribution no. 1700. fault zones, some evidence suggesting right-lateral strike-slip component is observed. The onshore sections of both of these REFERENCES CITED faults also exhibit right-lateral motion of probable Quaternary age (Mann et al., this volume, Ch. 6; Moya, 1994). If the faults Ascencio, E., 1980, Western Puerto Rico seismicity: U.S. Geological Survey observed offshore are Holocene, then by their coincidence Open-File Report 80-192, 135 p. Bracey, D.R., 1968, Structural implications of magnetic anomalies north of the in location and style of motion with older structures (faults Bahama-Antilles islands: Geophysics, v. 33, p. 950–961, doi: 10.1190/ in the Oligocene-Pliocene carbonates) they likely represent 1.1439989. reactivation or continued motion on those structures (Fig. 4). Byrne, D.B., Suarez, G., and McCann, W.R., 1985, Muertos Trough subduction— Microplate tectonics in the northern Caribbean: Nature, v. 317, p. 420–421. The overall stress regime suggested by motion on these faults Detrich, C., 1995, Characterization of faults located off the western coast of therefore is extension aligned roughly NE-SW. (Fig. 15B). This Puerto Rico using seismic refl ection profi les [M.S. Thesis]: Lehigh Uni- is consistent with geodetic studies that suggest ENE extension versity, 122 p. Dillon, W.P., Edgar, T., Scanlon, K., and Coleman, D., 1996, A review of the is occurring in the Mona Passage and is largely accommodated tectonic problems of the strike-slip northern boundary of the Caribbean by the opening of the Mona rift and associated faults (Jansma et plate and examination by GLORIA, in Gardner, J.V., Field, M.E. and al., 2000; Mann et al., 2002), (Fig. 15B). Twichell, D., eds., Geology of the United States’ Seafl oor; The View from GLORIA: Cambridge University Press, p. 135–164. Emery, K.O., and Uchupi, E., 1984, The Geology of the Atlantic Ocean: New CONCLUSIONS York, Springer, 1050 p. Erikson, J., Pindell, J., and Larue, D.K., 1990, Tectonic evolution of the south- central Puerto Rico region: Evidence for transpressional tectonism: Jour- In summary, both the offshore and onland studies have nal of Geology, v. 98, p. 365–386. documented for the fi rst time evidence suggesting Quaternary Erikson, J., Pindell, J., and Larue, D.K., 1991, Fault zone deformational con- fault activity associated with a broad zone of deformation in the straints on Paleogene tectonic evolution in southern Puerto Rico: Geo- physical Research Letters, v. 18, p. 569–572. Mona Passage and western Puerto Rico. Three fault zones have Gardner, W.O., Glover, and L.K., Hollister, C.D., 1980, Canyons off northwest been identifi ed offshore western Puerto Rico: the Cerro Goden, Puerto Rico: Studies of their origin and maintenance with the nuclear the Punta Algarrobo/Mayagüez, and the Punta Guanajibo/Punta research submarine NR-1: Marine Geology, v. 37, p. 41–70. Garrison, L.E., and Buell, M.W., 1971, Sea-fl oor structure of the Eastern Greater Arenas fault zones. These faults appear to coincide with large Antilles, in Symposium on Investigations and Resources of the Caribbean WNW-trending offsets in the underlying Cretaceous volcanics Sea and Adjacent Regions, p. 241–245, UNESCO, Paris, France. and Oligocene-Pliocene carbonate sequences, suggesting that Glover, L., III, 1971, Geology of the Coamo area, Puerto Rico, and its relation to the volcanic arc-trench association: U.S. Geological Survey Profes- many of these faults represent zones of weaknesses that are sional Paper 636, 102 p. being reactivated in the most recent phase of NE-SW exten- Grindlay, N.R., Mann, P., and Dolan, J., 1997, Researchers investigate subma- sional deformation. Two of the fault zones, the Cerro Goden rine faults north of Puerto Rico: Eos (Transactions, American Geophysi- cal Union), v. 78, p. 404. and Punta Algarrobo, show strong correlation with fault zones Grove, K., 1983, Marine Geologic Map of the Puerto Rico insular shelf, north- onland, Cerro Goden and Cordillera, respectively. Our studies, western area—Rio Grande de Añasco to Rio Camuy: U.S. Geological coupled with the onland studies of Prentice and Mann (this Survey Miscellaneous Investigations Series Map I-1418, scale 1:40,000. Huérfano, V., 1994, Crustal structure and stress regime near Puerto Rico [M.S. volume), Mann et al. (this volume, Ch. 6), and Hippolyte et al. thesis]: Mayagüez, University of Puerto Rico, 89 p. (this volume) suggest that these faults pose a seismic hazard Hubbard, D.K., Gill, I.P., Ruebenstone, J.L., and Fairbanks, R.G., 1996, Paleo- and must be considered in future seismic hazard models for climate and paleoceanographic controls of shelf-edge reef development in Puerto Rico and the Virgin Islands: Geological Society of America the island of Puerto Rico, one of the most densely populated Abstracts with Programs, v. 28, no. 7, p. A-489. regions of the western hemisphere. Jansma, P., Mattioli, G., Lopez, A., DeMets, C., Dixon, T., Mann, P., and Calais, E., 2000, Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy: Tectonics, v. 19, p. 1021– ACKNOWLEDGMENTS 1037, doi: 10.1029/1999TC001170. Joyce, J., McCann, W.R., and Lithgow, C., 1987, Onland active faulting in the We gratefully acknowledge Captain Hector and the crew of the Puerto Rico platelet: Eos (Transactions, American Geophysical Union), v. 68, p. 1483. R/V Isla Magueyes for their skill and enthusiasm in navigating Larue, D.K., and Ryan, H.F., 1990, Extensional tectonism in the Mona Passage, the shallow reef-infested waters of western and southern Puerto Puerto Rico and Hispaniola: A preliminary study: Transactions, 12th Rico. John “super tech” Murray ensured the smooth opera- Caribbean Geological Conference, p. 223–230. Lopez, A., Jansma, P., Calais, E., DeMets, C., Dixon, T., Mann, P., and Mattioli, G., tion of the geophysical gear during the cruise. We thank Osku 1999, Microplate tectonics along the North American–Caribbean plate Backstrom and Daniel Lao for watchstanding during the cruise. boundary: GPS geodetic constraints on rigidity of the Puerto Rico-Northern 160 N.R. Grindlay et al.

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