Western Caribbean intraplate deformation - GeoScienceWorld

Western Caribbean intraplate deformation: Defining a continuous and active microplate boundary along the San Andres rift and Hess Escarpment fault zone, Colombian Caribbean Sea Luis Carlos Carvajal-Arenas and Paul Mann

ABSTRACT The San Andres rift (SAR), located on the lower Nicaraguan Rise, is a previously poorly studied, active, 015°-trending, bathymetric, and structural rift basin that is 11–27 km (7–17 mi) wide and extends for 346 km (215 mi) across the western flank of the Caribbean plate. In this study, we integrate bathymetric maps, potential field data, and high-resolution, two-dimensional (2-D) seismic lines to understand the crustal structure, tectonic history, and tectonic origin of the SAR, which is one of the active areas within the otherwise stable Caribbean plate. We compiled regional gravity and magnetic data that revealed a negative gravity anomaly and positive magnetic anomaly that we interpret as a result of crustal thinning and an elevated Moho along the main rift axis of the SAR. Forward models of gravity data show four possible interpretations for the origin of the crust underlying and surrounding the SAR. Interpretations of 2-D seismic reflection data show structural features within the upper crust and sedimentary sections typical of other active rift systems including a SAR-parallel, north–south alignment of earthquakes with the larger events showing normal and strike-slip focal mechanisms. Sequential kinematic restorations based on 2-D seismic profiles reveal three major phases of SAR opening: (1) the initial early Eocene rifting stage; (2) middle Eocene extension; and (3) a rapid middle Miocene to early Pliocene extension accompanied by emergence of the San Andres Island as a rift shoulder. We propose

Copyright ©2018. The American Association of Petroleum Geologists. All rights reserved. Manuscript received May 12, 2017; provisional acceptance July 13, 2017; revised manuscript received October 4, 2017; final acceptance December 8, 2017. DOI:10.1306/12081717221

AAPG Bulletin, v. 102, no. 8 (August 2018), pp. 1523–1563

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AUTHORS Luis Carlos CarvajalArenas ~ Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Road, Houston, Texas 77004; [email protected]; carvajalaja@ gmail.com Luis Carlos Carvajal-Arenas received his B.S. degree in geology from the National University of Colombia in 2007. In 2017, he completed his Ph.D. in geology from the University of Houston. Luis Carlos developed his research in the subsurface structure, stratigraphy, and petroleum potential of the Nicaraguan Rise and Colombian Basin. During his professional career, he has worked as an exploration geologist for Prospex Energy, Petroleum Geo-Services (PGS), Chevron, and Statoil. Paul Mann ~ Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Road, Houston, Texas 77004; [email protected] Paul Mann is currently the Robert E. Sheriff Endowed Professor of Geology at the Department of Earth and Atmospheric Sciences of the University of Houston. He obtained his B.A. degree in geology from Oberlin College, and his Ph.D. at the State University of New York. Paul is the co-principal investigator of the Caribbean Basins, Tectonics, and Hydrocarbon (CBTH) Project, where his main research interest is the tectonics and petroleum geology of sedimentary basins. ACKNOWLEDGMENTS We thank Mike Saunders at Spectrum AS and Rick Roberson at Petroleum Geo-Services (PGS) who kindly provided the data set used in this study and gave us permission to publish the data in this paper. We thank the industry sponsors of the Caribbean Basins, Tectonics, and Hydrocarbon (CBTH) Project for their continued support, and for allowing us to have access to the Oasis Montaj (Geosoft), 2D MOVEä (Midland Valley), and PETRELä (Schlumberger) software. We thank the reviewers of this paper—Joan F. Flinch, Christian Brandes, and Frances P.

Whitehurst—for their constructive comments, and give special thanks to Lucia Torrado, Luis Carlos Carvajal Calderon, and Lulu Fritz for their encouragement and valuable comments during the study.

slab rollback and intraplate extension as main tectonic mechanisms to explain all rift phases and Neogene volcanism found in the western Caribbean region.

INTRODUCTION Molnar and Sykes (1969) used the distribution of earthquake epicenters and their focal mechanisms to define for the first time the existence of the rigid Caribbean lithospheric plate bounded to the north and south by seismically active strikeslip plate boundaries and to the east and west by inwardly facing subduction zones. Interpretation of the increasing number of seismic reflection profiles collected from the Caribbean plate by various groups from the United States and France during the 1980s and 1990s indicated the presence of active faults that cut the sea floor in locations hundreds of kilometers from the more active Caribbean plate boundaries. Some of these faults were interpreted as continuous zones of active faulting that defined intraplate boundaries and subdivided the Caribbean plate into several, less seismic “subplates” or blocks (Holcombe et al., 1990; Heubeck and Mann, 1991; Leroy and Mauffret, 1996). Global Positioning System (GPS) studies that spanned the Caribbean plate during the early 2000s concluded that the interior of the Caribbean plate was tectonically stable within the 1–3 mm/yr margin of GPS measurement error, which is coincidentally a common range of motion along a weakly active, intraplate fault (DeMets et al., 2000; Calais and Mann, 2009; Mattioli et al., 2014). More detailed GPS studies in the Jamaica area by Benford et al. (2012) proposed various geometries of deforming intraplate blocks south of Jamaica but there was no attempt to substantiate the geodetically proposed plate boundary with the actual, offshore fault pattern known from geophysical mapping (Figure 1A). This study focuses on the active, 015°-trending, bathymetric, and structural San Andres rift (SAR) that extends for 346 km (215 mi) across the Nicaraguan platform and varies in bathymetric width from 11 to 27 km (7–17 mi) and has water depths ranging from 1250 to 2500 m (4100–8200 ft) (Figure 1). Sykes et al. (1982) used well-located earthquakes to conclude that the line of seismicity parallel to the SAR near 82°W was the only persistent zone of earthquakes within the interior of the Caribbean plate; they interpreted the north–south zone as a reactivated, Eocene transform fault. Other workers relying more on marine geophysical data have proposed the possible origin of the SAR as a sigmoidally shaped pull-apart basin along the left-lateral, Pedro Bank fault zone (Milliman and Supko, 1968; Mann and Burke, 1984; Geister, 1992; Leroy et al., 1996; Carvajal-Arenas et al., 2015). Holcombe et al. (1990) proposed that the entire area of the 1524

Western Caribbean Intraplate Deformation: Defining a Microplate Boundary

Figure 1. (A) Location map of the San Andres rift (SAR) in the western Caribbean Sea and Nicaraguan Rise showing major tectonomorphologic features including faults, active rifts, oceanic spreading ridges, accretionary prisms, and subduction zones. Study area is shown by a black box. The Global Positioning System (GPS) vectors are shown using both the North American reference frame. Oil and gas occurrences are shown from both natural seeps and as reported in exploration wells (Carvajal-Arenas et al., 2015). (B) Zoom of SAR study area showing two-dimensional (2-D) seismic survey grid (red lines) and well data used in this study; dashed lines show 2-D seismic lines (lines C and D) used for structural restorations; line CC9 is used for 2-D forward gravity and magnetic modeling. Neogene volcanism reported from outcrops and well data are from an unpublished report by CanOcean Resources on the Nicaraguan hydrocarbon study of geology and hydrocarbon potential prepared for the exclusive use of the Instituto Nicaraguense de Minas e Hidrocarburos, accessed with permission of the Nicaraguan Ministry of Energy and Mines (1980), an unpublished geochemical data set by Beicip-Franlab accessed with permission of the Nicaraguan Ministry of Energy and Mines (1983), and Wadge and Wooden (1982). CCT = central Chortis terrane; ECT = eastern Chortis terrane; HB = Honduran borderlands; LNR = lower Nicaraguan Rise; SCT = southern Chortis terrane; SIUNA= Siuna terrane; UNR = upper Nicaraguan Rise. 100 km = 62 mi.

lower Nicaraguan Rise between the Pedro Bank fault zone to the north and the Hess Escarpment fault zone to the south contained north–south-striking rift structures that transferred motion between the two bounding, left-lateral, strike-slip fault zones. The objectives of this paper include (1) using two-dimensional (2-D) seismic data to describe the structural features of the 346-km-long (215-mi-long) length of the southern Pedro Bank fault zone in the area of the SAR and its junction with the Hess Escarpment fault zone to the south; this paper’s description complements the structural description of the northeastern Pedro Bank fault zone by Ott, Mann, Sanchez, and Carvajal-Arenas (under review) in its northeastern area near Jamaica (Figure 1); (2) using

well ties to the 2-D seismic data and well burial plots to infer the age of origin and main extensional phases for the SAR; (3) using gravity and magnetic data to define the crustal types in the area of the SAR and 2-D forward models to understand the tectonic origin of the underlying crustal blocks in the region of the SAR (i.e., arc, oceanic plateau, oceanic, continental); (4) describing the geology exposed on the uplifted islands of San Andres and Providencia along the eastern rift shoulder of the SAR (Figure 1); (5) restoring the Eocene to Holocene stratigraphic units prior to the main phases of rifting; and (6) using plate tectonic reconstructions of the Caribbean plate to recreate the SAR tectonic origin during its early opening in the Eocene and during its later pulses of extension. CARVAJAL-ARENAS AND MANN

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PREVIOUS GEOLOGIC STUDIES OF ISLANDS ON THE UPLIFTED, EASTERN SHOULDER OF THE SAN ANDRES RIFT Two islands are exposed on the eastern shoulder of the SAR: San Andres and Providencia. Both islands were fortified as naval bases for the Spanish Armada, Dutch smugglers, and English pirates and privateers during the sixteenth through eighteenth centuries (Newton, 1914). One of the famous stories describing this region is attributed to Captain Henry Morgan, an English pirate who used San Andres Island to attack and “sack” the city of Panama in 1670 (Newton, 1914; Parsons, 1956). In 1851, English naturalist and geologist Charles Darwin mentioned Providencia Island (Old Providence Island) as an extensive reef complex in his discussion of the West Indian reefs (Darwin, 1851).

Geology of San Andres Island Adjacent to the San Andres Rift San Andres Island is located on the footwall and 10 km (6 mi) to the east of the main bounding normal fault of the SAR and is composed entirely of Cenozoic ¨ biogenic calcareous deposits (Pilsbry, 1931; Burgl, ¨ 1959; Quintero and Burgl, 1960; Weyl, 1966; Milliman and Supko, 1968; Geister, 1972a, b, 1992). Schuchert (1935) and Mitchell (1955) mistakenly described San Andres Island as an exposure of igneous rocks without ever having visited the island themselves (Milliman and Supko, 1968) (Figure 2). The stratigraphy of San Andres includes (1) the lower Miocene San Andres Formation composed of shaly limestone and truncated by the upper Miocene unconformity (Kocurko, 1974); (2) upper Miocene detrital sandy limestone; and (3) the Pleistocene San Luis Formation composed of reefal limestones and organic-rich mudstone (Vargas-Cuervo, 2004; Servicio Geologico Colombiano, 2015) (Figure 2B).

Geology of Providencia Island Adjacent to the San Andres Rift Located 98 km (61 mi) to the northeast of San Andres, Providencia Island is also located on the eastern footwall of the main normal fault bounding the SAR 15 km (9 mi) to the east and is composed of volcanic rocks that include basalt and trachytic lava intruded by dioritic dikes (Mitchell, 1955; Parsons, 1956; 1526

Pagnacco and Radelli, 1962; McBirney and Williams, 1965; Pacheco-Sintura et al., 2014) (Figure 2B). Kerr (1978) and Geister (1992) summarized the main Miocene–Pliocene stages of the formation of volcanic rocks of Providencia: (1) sub-Miocene, subalkaline pillow basalt (Wadge and Wooden, 1982) and dacite formed in a tectonic setting of volcanic-arc basalt and subduction zones (Concha and Macia, 1993, 1995); (2) early Miocene quiescence; (3) lower Miocene intermediate- to high-silica volcanic rocks, dikes, and breccia deposited in a subaerial setting with calc-alkaline affinities (Wadge and Wooden, 1982) and within an extensional tectonic setting of withinplate basalts (Concha and Macia, 1995); (4) late Miocene brecciation and uplift followed by diking and erosion. Concha and Macia (1995) conclude that the Miocene volcanic rocks of Providencia can be interpreted as a mix of (1) subduction-related volcanic rocks supported by the lack of negative anomalies in Nb, Ta, and Tl in the normalization diagrams found in dacite (Figure 3A, B) and (2) intraplate volcanism characterized by alkali basalts plotted in the geotectonical setting as within-plate basalts (Figure 3C, D).

REGIONAL TECTONIC SETTING Previous tectonic models for the Caribbean region including the Nicaraguan Rise have been proposed by Pindell and Dewey (1982), Burke (1988), Pindell and Barrett (1990), Pindell and Kennan (2009), and Dickinson (2009). All of these previous models suggest that the Caribbean was formed in the Pacific and later moved through the gap created between North and South America during the opening of the proto-Caribbean Sea, and southwestward prolongation of the central Atlantic Ocean. The model for the Caribbean evolution can be summarized into seven tectonic stages. 1. Rifting between North America and South America began during the Middle to Late Jurassic and led to northwest-to-southeast sea-floor spreading of the Gulf of Mexico and proto-Caribbean Sea by the Late Jurassic (Pindell and Dewey, 1982). Much of the proto-Caribbean seaway would be subducted during the entry of Cretaceous arc systems into the Caribbean (Burke, 1988).


Figure 2. (A) Basement rock types of the western Caribbean Sea based on wells that penetrated basement (unpublished report by CanOcean Resources on the Nicaraguan hydrocarbon study of geology and hydrocarbon potential prepared for the exclusive use of the Instituto Nicaraguense de Minas e Hidrocarburos, accessed with permission of the Nicaraguan Ministry of Energy and Mines [1980]; and unpublished geochemical data set by Beicip-Franlab accessed with permission of the Nicaraguan Ministry of Energy and Mines [1983]) and onland Neogene volcanoes (Wadge and Wooden, 1982). (B) Geologic map of San Andres Island modified from Servicio Geologico Colombiano (2015). (C) Geologic map of Providencia Island modified from Servicio Geologico Colombiano (2015). (D) East–west geologic cross section of San Andres Island modified from Vargas-Cuervo (2004).

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Figure 3. Geochemical and tectonic setting discrimination diagrams using samples from Providencia Island (modified from Concha and Macia, 1993, 1995). (A) Total alkali silica diagram for volcanic igneous rocks from Le Bas et al. (1986). (B) Normalization diagram for dacite samples from Concha and Macia (1993, 1995) showing negative values in Nb, Ta, and Tl suggesting that Providencia basalts are genetically correlated with subduction processes. (C, D) Diagrams (Ti/100-Zr-Y*3) and (Hf/3-Th-Ta) from Concha and Macia (1995) showing values reserved for samples formed in within-plate basalts. Hf = hafnium; Nb = niobium; Ta = tantalum; Tl = thallium; Th = thorium; Ti = titanium; Zr = zirconium.

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2. The sedimented continental fragments of Yucatan and Chortis reached their present position after North America separated from South America during the Triassic–Jurassic breakup of Pangaea by the earliest Cretaceous (145 Ma) (Rogers et al., 2007). 3. An eastward-dipping subduction zone occupied the western margin of the proto-Caribbean Sea and formed the intraoceanic island arc called the Great Arc of the Caribbean, which extended the length of the entire North and South American Cordilleran margin during the Campanian (83–72 Ma) (Burke, 1988; Hildebrand, 2013). During this time, the Hess fault zone likely formed as a large, left-lateral strike-slip fault zone that was more east–west in its orientation prior to its translation and rotation to its present position. The collision of the Siuna Island arc complex with the continental Chortis terrane of northern Central America caused northwest–southeast shortening of the Colon fold-thrust belt (Rogers et al., 2007) and in the northeastern offshore extension of this same foldthrust belt on the northern Nicaraguan Rise (Sanchez et al., 2015) and on the eastern Nicaraguan Rise (Ott, Mann, Sanchez, and CarvajalArenas, under review). The Cuban and Great Antilles segment separated from the Jamaican segment of the arc and continued to migrate to the northeast to form the Yucatan Back-arc Basin (Rosencrantz, 1990). 4. The Paleocene–Eocene collision of the Great Arc of the Caribbean in Cuba with the Bahama platform caused the Caribbean plate to shift from its northeastward direction to a more eastwardly direction during the middle Eocene (43–37 Ma) (Mann et al., 1995). The Nicaraguan Rise underwent a widespread east–west extensional event as the Nicaraguan Rise rotated counterclockwise around the Yucatan block (Ott, Mann, Sanchez, and Carvajal-Arenas, under review). 5. Widespread extension of the northern Caribbean plate was manifested in north–south-trending rifts across the northern Nicaraguan Rise (Arden, 1969; Mann and Burke, 1990). This period of diffuse and regional east–west extension culminated with ultraslow (7.5 mm/yr) sea-floor spreading in the Cayman Trough pull-apart basin in the early Eocene that continues to the present day as the Caribbean plate moves unimpeded into the central Atlantic Ocean (Mann and Burke, 1990).

6. During the middle Miocene (16–13 Ma), the central core of the Caribbean plate composed mainly of the Caribbean large igneous province was transported to the northeast along the northeast-striking, leftlateral Pedro Bank fault zone, and a right-lateral, transpressional zone along the Beata Ridge located east of the study area (Leroy et al., 1996; Mauffret and Leroy, 1997). The trailing edge of this block was marked by rifting along the SAR. The leading edge of the block was the transpressional zones of Jamaica and southern Hispaniola (Ott, Mann, Sanchez, and Carvajal-Arenas, under review). 7. During the late Miocene (13–5.3 Ma) to Holocene, continued east–west extension in the SAR area led to widespread, alkaline volcanisms of Miocene–Pliocene age that include the islands of Providencia and Corn Islands, and Roncador, Quitasueño, and Serrana cays (Wadge and Wooden, 1982; Geister, 1992; MacMillan et al., 2004).

Tectonic Evolution of the San Andres Region Few authors have postulated the tectonic evolution for the SAR area. In general, there are two conceptual ideas. (1) The volcanic origin of the SAR found in early reports indicates that the SAR area emerged due to volcanism caused by subduction Schuchert (1935). Mitchell (1955) suggested the trend of San Andres and Providencia Islands is an “embryo island-arc formed similarly to the Lesser Antilles.” Similarly, Arden (1969) proposed that the Nicaraguan Rise is genetically related to the Antillean island chains of the Caribbean in that both originated as belts of crustal mobility. (2) The tectonic concept was introduced by Milliman and Supko (1968) indicating that the region of the SAR formed without the formation of magnetic anomalies. Similarly, Mills and Hugh (1974) suggested that Providencia and San Andres Islands could be remnants of an old foreland area that behaved similarly to the tectonic history of the northern Caribbean Sea. Fifteen Cenozoic rifts besides the SAR have been identified in the Nicaraguan Rise (Mann and Burke, 1984). The distribution, orientation, and relative ages are compatible with the hypothesis that the rifts formed as a result of intraplate extension of the northern Caribbean as it moved eastward relative to North America about continental promontory in Central America (Mann and Burke, 1984; Holcombe et al., 1990; Leroy et al., 1996). CARVAJAL-ARENAS AND MANN

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DATA SET USED IN THIS STUDY Bathymetric, Gravimetric, and Magnetic Data Sets All maps shown in this paper were created using a projection of Universal Transverse Mercator zone 17N (UTM-17N) with the following parameters: datum, World Geodetic System 1984; units, degree; latitude origin, 0°; central meridian, -81°W; false east, 500000; false north, 0. We used several topographic and bathymetric data sets to make the maps for this paper, including (1) digital elevation models (DEMs) from the Shuttle Radar Topography Mission-30 Plus (Becker et al., 2009), which has a resolution of 30 arc seconds; (2) 14 bathymetric profiles obtained from ship tracks from the Nicaraguan Rise from a Simrad EA500 multibeam system (these bathymetry data were also used to constrain the 2-D forward gravity and magnetic models); (3) DEMs with a resolution of 25 · 25 m obtained from a Google Earth Pro used to show the topography of San Andres and Providencia Islands. Two sources of gravity data were used in this study: (1) publicly available gravity grids from Sandwell et al. (2014), used to display the free-air anomaly and its corresponding filters. The Sandwell et al. (2014) data set provides new radar altimeter measurements from satellites CryoSat-2 and Jason-1 combined with previous gravity data sets to provide maps that are two times more accurate than previous maps and (2) proprietary gravity data acquired by Petroleum Geo-Services (PGS) in 2008 and made available for this publication. The PGS gravity ship tracks cover 980 km (609 mi) and were acquired with a LaCoste and Romberg marine gravimeter. We used two sources of magnetic data for this paper: (1) gridded magnetic data from the Decade of North American Geology (DNAG) and the Earth Magnetic Anomaly Grid (EMAG2), published by Zeitz (1982) and Maus et al. (2009), respectively, and (2) a second magnetic survey provided by PGS used for areas that lacked coverage from either the DNAG and EMAG2 data sets; this second set of data was interpolated with different statistical methods. According to Nabighian et al. (2005), the boundaries of this second PGS survey may have shifted from 100 to 300 nT in some areas. For that reason, our study relies on raw data directly obtained from ship tracks 1530

acquired by PGS in 2008 for the purpose of forward modeling of 2-D gravity and magnetic data. The PGS magnetic ship tracks cover 980 km (609 mi), and were acquired with a SeaSPY marine magnetometer. Refraction stations used in this paper were mainly taken from published academic studies (Ewing et al., 1960; Ludwig et al., 1975; Houtz and Ludwig, 1977; ten Brink et al., 2002; Garnier-Villarreal, 2012). Additionally, refraction stations were used as a qualitycontrol mechanism to constrain the main layers of the 2-D gravity and magnetic forward models (i.e., sedimentary section, upper crust, lower crust, and Moho surface).

Seismic Data and Well Data The structural and sedimentary interpretations of seismic data from the SAR are based on 980 km (609 mi) of 2-D seismic lines with high resolution and depth penetration (10 s) that are tied to 26 wells drilled in offshore Nicaragua (Figure 1).

METHODOLOGY This study is based on a workflow and methodology consisting of (1) regional analysis of potential field data; (2) interpretation of seismic profiles; and (3) structural modeling based on kinematic restorations.

Regional Analysis of Potential Field Data We processed all of the gravity and magnetic grids using Geosoft Oasis Montaj software. The free-air anomaly map was reprocessed from raw data obtained from Sandwell et al. (2014) (Figure 4A) and was used to generate two more gravity filters (Figure 4B, C). The Bouguer anomaly offshore was processed to remove the effects of carbonate platforms on the freeair anomaly (Figure 4B). To generate the Bouguer anomaly offshore, we added a constant density correction of 0.97 g/cm3 to the water column layer. This correction improved the resolution of deep crustal structures in the SAR area. Similarly, we used the tiltderivative filter (or analytic signal) to enhance and highlight fault and crustal discontinuities identified from the offshore Bouguer anomaly. Short wavelengths are enhanced by the calculation and integration of the first derivative calculated in X, Y, and Z direction


(Verduzco et al., 2004). The resulting map improved the detection of the shape and edges of the gravity anomalies (Figure 4C). Magnetic grids were calculated from raw data provided by Maus et al. (2009). The total magnetic field was displayed using a lighting layer shade of 45° (Figure 5A). To remove the inclination effect caused by the nonvertical satellite acquisition (Figure 5B), reduction to the pole (RTP) was applied to the total magnetic field. This process removes the inclination effect from total-field magnetic anomalies by transforming the total-field anomaly into the vertical component of the field produced as if the source were at the North magnetic pole -90° inclination (Bajgain, 2011). The inclination and declination used in the RTP was calculated from the International Geomagnetic Reference Field subroutine in Oasis Montaj based on the latitude and longitude. As with the gravity data, the tilt-derivative grid was processed from the total magnetic field to better enhance the detection shapes and edges of magnetic anomalies (Figure 5C). The isostatic Moho depth was calculated based on information from the Moho geometry gravity inversion experiment (MoGGIE) done by Aitken (2010) and Steffen et al., (2011). The MoGGIE calculates the Moho depth based on the Airy model of isostasy where the depth is calculated from Archimedes’ principle depending on the crustal loading (positive topographic surface–above sea level) or the lack of loading (negative bathymetric surface–below sea level). The density model used in the gravity inversion used the following densities of crustal layers: offshore crust (2.55 g/cm3), onshore rock (2.35 g/cm3), and water (1.03 g/cm3); the zero topography (sea level) is assumed to be in isostatic equilibrium with a Moho at a depth of 31 km (19 mi). We used the GM-SYSÔ module of Oasis Montaj for the 2-D gravity and magnetic forward modeling. This calculates the gravity based on the method developed by Talwani (1965) and the magnetic responses of the synthetic models. From the seismic interpretation of line C, we proposed four scenarios to model 2-D gravity and magnetic profiles along the SAR (Figure 6). Additionally, five 2-D gravity and magnetic forward models were carried out based on the most likely crustal structure interpreted from line C. The 2-D forward models were interpreted from the free-air and total magnetic field anomalies acquired by PGS,

and processed with standard values of density and magnetic susceptibilities (Lowrie, 2007). Each profile is composed of several blocks that represent an interpretative view of subsurface structures. Each block emulates physical characteristics of subsurface rocks to have the best fit of observed profiles (gravity, magnetic) with an inferred curve (calculated curve) derived from the geometry and distribution of each block. Physical properties used in each block are based on well data and outcrop descriptions summarized in Figure 7. The contacts between each block are constrained by the interpretation of seven seismic horizons (Figure 6), refraction stations (Figure 7), and the surface grid of the Moho obtained from the calculation of the isostatic Moho depth (Figure 7).

Seismic-Data and Well-Data Interpretation We used 980 km (609 mi) of 2-D seismic data from five individual seismic lines crossing the axis of the SAR (Figures 8–12). The interpretation of nine horizons was based on (1) the correlation of 26 wells available in the Nicaraguan Rise (Abrams and Hu, 2000; Carvajal-Arenas et al., 2015; Carvajal-Arenas, 2017; Torrado, Carvajal-Arenas, Sanchez, Mann, and Silva-Tamayo, under review); (2) seismic interpretations compiled in more than 13,000 km (8077 mi) of available 2-D auxiliary seismic data not shown in this paper; and (3) previous authors (Holcombe et al., 1990; Bowland, 1993; Kerr et al., 2003; Carvajal-Arenas et al., 2013) (Figure 1A).

Structural Modeling Based on Kinematic Restorations Prior to making structural restorations of the seismic lines crossing the SAR, we converted seismic lines A to E from time domain to depth using the layer-cake method for depth conversion (Marsden, 1989) (Figure 1B). The velocity model used for depth conversion is based on stacking velocities obtained from the seismic profiles. The stacking velocities are used to correct offset seismic traces to zero offset prior to the common midpoint stacking and poststack migration (Bell, 2002). These velocities increase with depth and do not recognize sedimentary packages as single units. Using the 2-D depth conversion module of Midland Valley 2D MOVEÔ software, we were CARVAJAL-ARENAS AND MANN

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able to differentiate the interval velocities using Dix formulae (Dix, 1955). The resultant depth profiles were then used for kinematic restorations. We restored five, 2-D seismic lines across the strike of the SAR with a gridding resolution of 500 m (1640 ft) (Figure 1B). Nine horizons were structurally restored on the lines: (1) top igneous basement–Late Cretaceous; (2) top Late Cretaceous–lower Paleocene; (3) top upper Paleocene; (4) top lower Eocene; (5) top upper Eocene; (6) top lower Miocene; (7) top middle Miocene; (8) top upper Miocene; and (9) top sea floor. Structural restorations used a simple shear algorithm along active faults for each time period, relating the geometry of the deformational features found in the hanging wall to the fault’s shape (Verrall, 1981; Gibbs, 1983; Withjack and Peterson, 1993). For the fault movement step, we modified the shear vector to avoid creating gaps between the hanging wall and footwall blocks (0°–40°). Decompaction was calculated after each retro-deformed step using the Sclater and Christie (1980) algorithm. Lithological properties used in the decompaction process correspond to lithologic values found in 26 available wells along the Nicaraguan Rise (Figure 1), and standard petrophysical properties such as porosity, depth coefficient, and Poisson ratio were taken from Allen and Allen (2013). As an end result, five kinematic restorations along the SAR were calculated from the Late Cretaceous to the Holocene and their resultant offsets are displayed in Table 1.

RESULTS Crustal Architecture of the Nicaraguan Rise and the San Andres Rift Systematic exploration of continental and oceanic lithosphere over the past decades by gravity and

magnetic methods has dramatically improved our geological understanding of the deep crust and upper mantle and its tectonic evolution (Castro et al., 2011). To constrain the crustal structure underlying the Nicaraguan Rise and the SAR, we interpret the gravity and magnetic grids shown in Figures 4 and 5 based on variations in amplitude, wavelength character, lineament distribution, texture, and structural discontinuities.

Regional Gravity Data Positive-, intermediate-, and low-gravity anomalies as seen on free-air, Bouguer, and tilt-derivative gravity grids can be used to classify the various crustal blocks of the western Caribbean. Areas with positive anomalies ranging from 151 to 214 mGal (red and magenta tones) are present in the Cayman Trough, in areas of the Colombian Basin, and across the eastern Pacific Ocean and are interpreted as areas of normal oceanic crust with average densities around 2.95 g/cm3 (Ewing et al., 1960; Edgar et al., 1971) (Figure 4A, B). Varying textures of the gravity field within these areas of positive anomalies include strong parallel lineaments associated with spreading anomalies generated at oceanic-spreading ridges such as at the midCayman spreading center in the Cayman Trough (Leroy et al., 2000; ten Brink et al., 2002) and spreading anomalies generated by the East Pacific Rise and other spreading centers in the eastern Pacific Ocean (Barckhausen et al., 2001). Areas of positive anomalies located in the middle of the Colombian Basin are also seen without strong parallel lineaments and with a smooth texture, such areas of the Caribbean large igneous province (CLIP) were formed by the construction of an igneous plateau by multiple layers of volcanic flows rather than at a single spreading center (Kerr et al., 2003). In contrast, some northwest-to-southeast lineaments present in the

Figure 4. Regional gravity grids of the western Caribbean Sea showing tectonic features and crustal provinces. (A) Free-air gravity anomaly map (Sandwell et al., 2014) of the western Caribbean Sea showing major tectonic features delineated by abrupt gravity gradients; the San Andres rift (SAR) is expressed by low anomalies (blue tones) in areas with high sediment accumulation whereas areas of shallow, crystalline basement show higher anomalies (magenta tones). (B) Bouguer gravity anomaly calculated for the offshore area to remove bathymetric effects that include the extensive, Cenozoic carbonate platform covering much of the Nicaraguan Rise. Major structures buried by recent carbonates are revealed along the upper and lower Nicaraguan Rise and include Eocene rifts on the upper Nicaraguan Rise, and buried volcanoes on the lower Nicaraguan Rise. (C) Tilt derivative derived from Bouguer gravity anomaly map; three major provinces are identified in the tilt derivative: upper Nicaraguan Rise (UNR), lower Nicaraguan Rise (LNR), and Colombian Basin (CB); the SAR separates the two, contrasting gravity patterns. CARVAJAL-ARENAS AND MANN

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Figure 5. Regional magnetic grids of the western Caribbean Sea. (A) Total magnetic field (TMF) from Maus et al. (2009) of the western Caribbean Sea with overlay of major tectonomorphologic features; TMF reveals long and short wavelengths indicating deeper and shallower magnetic sources, respectively; longer wavelengths are related to a thicker, carbonate platform covering the upper Nicaraguan Rise. (B) Reduction to the pole (RTP) is calculated from the total magnetic field and is used to improve the spatial position of magnetic anomalies; RTP defines tectono-morphologic provinces and terrane boundaries. (C) Tiltangle derivative calculated from the total magnetic field; this method identifies structural trends associated with terrane boundaries and intrabasement features; spreading anomalies of Eocene–Holocene age are observed in the Cayman Trough and of Miocene age on the Cocos plate. LNR = lower Nicaraguan Rise; UNR = upper Nicaraguan Rise.

CLIP are identified close to the surface of the Mono Rise in the Colombian Basin (Figure 4B, C). A distinctive family of intermediate-gravity wavelengths between 61 and 150 mGal (orange and yellow tones) is observed on the Honduran borderlands along with east–northeast-trending lineaments parallel 1534

to left-lateral strike-slip trends of the North America– Caribbean plate boundary zone and is known to correspond to transtensional faults and basins of the Honduran borderlands (Rogers et al., 2007; Sanchez et al., 2015) (Figure 4B). Gravity anomalies between -79.5 and 60 mGal (green and blue tones) characterize


the upper Nicaraguan Rise (Figure 4B); crustal affinities based on refraction profiles indicate continental crust with average crustal velocities around 7 km/s (22,970 ft/s) (Ewing et al., 1960; Edgar et al., 1971; Ludwig et al., 1975; Mauffret and Leroy, 1997; Ott, Mann, Sanchez, and Carvajal-Arenas, under review). The upper Nicaraguan Rise shows a smooth gravity surface with the exception of two areas of lineaments southwest of Jamaica (Figure 4C). Lineaments near Jamaica may reflect the transpressional reactivation of the island since the late Miocene (Arden, 1975; Mann et al., 1984; 2007; Ott, Mann, Sanchez, and Carvajal-Arenas, under review). The lower Nicaraguan Rise and some areas of the SAR show a mixture of high-gravity (206 mGal) and medium-gravity (95 mGal) anomalies corresponding to thickened CLIP, island arc, or thinned continental crust (Figure 4B, C). The SAR displays a northeast trend of high- to medium-gravity amplitudes (~116 mGal) that parallel the Pedro Bank fault zone (Figure 4A). We observe a linear pattern of positive, free-air, and Bouguer anomalies parallel to the north–south-trending SAR indicative of a thick-skinned rift structure and not a surficial artifact related to carbonate platforms overlying the basement (Figure 4B, C). As previously described by Christofferson and Hamil (1978), we observe discontinuous, radial lineaments emanating from the area of the SAR (Figure 4B, C). These radial lineaments have lengths of less than 50 km (31 mi), have high positive wavelengths, and their radial pattern suggests a plume origin as described from radial fracture and dike patterns from other rifted areas above plume heads as described by Ernst and Buchan (2001).

Regional Magnetic Data Because the total magnetic intensity map responds to the sum of magnetic fields contributed by different crustal structures, we cannot quantitatively classify the anomalies of the western Caribbean Sea using the total magnetic field (Figure 5A). For this reason, we interpret the regional maps of the total magnetic field based on their textural patterns. As with the gravity data, the western Caribbean can be subdivided into major textural patterns in the magnetic grid: areas with short wavelengths with parallel trends that are associated with oceanic spreading centers such as those seen in the Cayman Trough and on the Cocos and Nazca plates in the

eastern Pacific Ocean (Figure 5C). Short-wavelength signatures correspond to areas where the magnetic sources in the basement are located close to the surface. For example, magnetic lineations in the Cayman Trough were formed when oceanic spreading occurred along a short, 100-km-long (62-mi-long) spreading center active since the early Eocene at spreading rates of approximately 7.5–15 mm/yr (Rosencrantz et al., 1988; Rosencrantz, 1995; Leroy et al., 2000). Although the region of the lower Nicaraguan Rise similarly displays short wavelengths, the uneven lineament distribution indicates processes different from a single spreading ridge. Instead, we interpret the short wavelengths and uneven lineaments in the lower Nicaraguan Rise as areas affected by east–west extension accompanied by volcanism and igneous intrusion. Areas with longer-wavelength magnetic anomalies are well expressed on the tilt-derivative map (Figure 5C). Areas with signatures of longer-magnetic wavelengths correspond to areas where the magnetic basement (magnetic source) is located farther from the surface, or the magnetic rock intensity in the basement is weak. The upper Nicaraguan Rise displays long wavelengths matching with previous observations of continental crust origin that is consistent with previous seismic refraction measurements (Ewing et al., 1960; Arden, 1969, 1975; Edgar et al., 1971) (Figure 5C). Additionally, the tilt derivative shows an unusual behavior in the Colombian Basin where the central part of the Colombian Basin shows long wavelengths indicative of a magnetic source that is much deeper than the surrounding areas and that is more typical of the CLIP lithologies (Figure 5C). Possible explanations for this unusual magnetic signature in the central Colombian Basin include (1) the crustal type underlying the area and emitting longer wavelengths is continental rather than CLIP or oceanic; (2) the anomalously sedimentary overburden of 3–5 km thick (2–3 mi thick) related to the Magdalena submarine fan reduces the basement magnetic intensity; and (3) the spacing between magnetic ship tracks from this area of the Colombian Basin is wide and therefore introduced a sampling bias for comparing this area to other more closely sampled areas. The northeast trend of the SAR is more apparent on gravity data (Figure 4) than it is on magnetic data (Figure 5). However, reduction to the pole (Figure 5B) and the tilt-derivative grid (Figure 5C) show that the SAR forms a major boundary between two magnetic, CARVAJAL-ARENAS AND MANN

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Figure 6. Two-dimensional forward modeling of gravity and magnetic data showing four possible interpretations of the crustal structure underlying the San Andres rift (SAR) along line C (see Figure 10). Models 1–4 show our preferred representation of crustal geometry and crustal composition along line C with a good match between calculated and observed data. Crustal thinning is observed for all the scenarios for crustal structure beneath the SAR. Crustal extension occurs beneath the rift because the area between rift flanks shows negative Bouguer gravity anomaly (Allen and Allen, 2013). (A) Gravity and magnetic (Grav/Mag) model 1 represents the SAR as a boundary between continental crust to the west and the Caribbean large igneous province (CLIP) to the east. (B) Gravity and magnetic model 2 depicts the SAR as a boundary separating continental crust to the west and an intruded CLIP to the east. (C) Gravity and magnetic model 3 shows the SAR as extended continental crust; the Hess fault forms a major boundary between extended continental crust to the west and CLIP to the east. (D) Gravity and magnetic model 4 shows (1) the SAR within an extended continental crust that has been intruded in the lower Nicaraguan Rise, and (2) the Hess Escarpment as the boundary between an extended continental crust to the west and the CLIP to the east.

crustal provinces: a longer wavelength, magnetic province of presumed continental origin to the northwest on the upper Nicaraguan Rise and a shorter wavelength, magnetic province of presumed oceanic or CLIP origin on the lower Nicaraguan Rise (Figure 5B, C).

Gravity and Magnetic Two-Dimensional Forward Modeling of the San Andres Rift Four 2-D gravity and magnetic forward models were tested along seismic line C (Figures 1B, 6) to determine the crustal type and origin of the western Caribbean Sea and to understand the effect of the SAR and its related magmatism on the surrounding crust. The known, static parameters used for the four gravity and magnetic models include (1) the gravity and magnetic profiles extracted along 195 km (121 mi); (2) the geometry of the sedimentary layers known from the seismic profiles; (3) the sedimentary densities; and (4) refraction data from previous workers. Variable parameters used for the four gravity and magnetic models include (1) crustal structure; (2) crustal densities; and (3) crustal susceptibilities. An isostatic Moho profile was extracted from a regional grid onto each of the four gravity and magnetic models to compare crustal thicknesses predicted by each model to the actual refraction data (Figure 7).

Gravity and Magnetic Profile of Crustal Interpretation 1 Crustal model 1 interprets the SAR as a major, crustal boundary separating continental crust to the northwest and CLIP to the southeast (Figure 6A). Along the profile, crustal thicknesses varies from 22 km (14 mi) within the continental crust in the northwest to 18 km (11 mi) in the CLIP. An anomalous thickness of 27 km (17 mi) is interpreted in the middle crust of the CLIP. The effect of the SAR extension over the crustal model 1 is observable in a rapid decrease of crustal thickness from 22 km (14 mi) to 19 km (12 mi) in areas where a major extension takes place (Figure 6A). Although this model fits with observed gravity data (error 7.856%), the magnetic response shows an error of more than 41.258% for the anomalously thick section of the CLIP. In summary, crustal model 1 can be discarded because of its large magnetic error and its deviation from the isostatic Moho profile, and its deviation from previous refraction observations (Ewing et al., 1960; Edgar et al., 1971; Ludwig et al., 1975). CARVAJAL-ARENAS AND MANN

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Gravity and Magnetic Profile of Crustal Interpretation 2 Crustal model 2 interprets the SAR as a major, crustal boundary between continental crust to the northwest and an intruded CLIP province to the southeast (Figure 6B). Crustal model 2 reduces the anomalously thick, middle section of crust by replacing it with CLIP (average density, 2.95 g/cc) that is intruded by lower-density rocks (average density, 2.75 g/cc). Crustal thicknesses varies from 24 km (15 mi) to 21 km (13 mi) within the continental crust, and from 20 km (12 mi) to 18 km (11 mi) for the CLIP. The effect of the SAR extension over the continental crust and the CLIP is shown as a thinner crustal thickness of 16 km (10 mi) (Figure 6B). The Moho calculated in crustal model 2 approximates the isostatic Moho profile and the refraction stations, along with providing a better fit to the calculated magnetic profile (error, 38.569%). However, the crustal structure represented in model 2 of the lower Nicaraguan Rise suggests that its density and crustal velocities decrease with depth in a way not seen from nearby, seismic refraction stations. For example, previous gravity profiles across the lower Nicaraguan Rise report velocities of 7 km/s (22,310 ft/s) at depths of 10 km (6 mi) (Ewing et al., 1960; Edgar et al., 1971; Ludwig et al., 1975). These velocities correspond to density values suitable for continental crust of 2.7 g/cc but not for density values of CLIP crust of 2.95 g/cc that were calculated for model 2. Gravity and Magnetic Profile of Crustal Interpretation 3 A different approach for crustal model 3 presents the SAR as part of an extended continental crust and the Hess Escarpment fault zone is the major crustal boundary separating the extended continental crust to the northwest from CLIP to the southeast (Figure 6C). Model 3 is similar to model 2, but the boundary between the upper and lower continental

crust varies depending on the calculated gravity and magnetic profiles. For model 3, crustal thickness varies from 25.5 km (15.8 mi) to 19 km (12 mi) in the continental crust section to the northwest, and 18 km (5 mi) for the crustal thickness of the CLIP to the southeast (Figure 6C). Similar to models 1–3, extension along the SAR thins the crust from an average thickness of 25 km (16 mi) outside of the rift area to thinned, thickness of 19.5 km (12.1 mi). Although the gravity profiles show a good match between the observed and calculated profiles for model 3, the boundary of the upper and lower crust in section I and section II displays an irregular magnetic tendency with higher errors (error, 23.534%) (see Figure 6C). For example, the top of the lower crust is anomalously shallow in sections I and II to compensate for the observed gravity and magnetic profiles used for model 3.

Gravity and Magnetic Profile of Crustal Interpretation 4 Model 4 depicts the SAR as a rift within previously extended, continental crust of the lower Nicaraguan Rise that has also been intruded by magmatic bodies in the lower and middle crust (Figure 6D). Model 4 also shows the Hess Escarpment fault zone as the major crustal boundary that separates the extended continental crust to the northwest and from the CLIP to the southeast (Figure 6D). Crustal thickness varies from typical continental crust thickness to the northwest with an average thickness of 20 km (12 mi), and to the southeast the characteristics of the CLIP with an average of 17.5 km (10.8 mi). Moreover, the calculated Moho profile fits with the isostatic Moho for both the upper and lower Nicaraguan Rise (Figure 6D). The effect of the SAR by crustal thinning is from 22 km (14 mi) outside of the SAR area to 18 km (11 mi) along the SAR axis. To fit the magnetic calculated

Figure 7. (A) Isostatic Moho depth (IMD) was calculated using the Airy model of isostasy and Archimedes’ principle that includes parameters of crustal loading (positive topographic surface–above sea level shown as magenta tones) or the lack of loading (negative bathymetric surface–below sea level shown as blue tones). Refraction stations were used to control the quality of the IMD; most of the observed refraction stations yielded a good match with the IMD. Main tectonomorphologic features are overlain and coincide with crustal provinces and tectonic boundaries inferred from gravity and magnetic maps. (B) Schematic crustal sections based on two-dimensional (2-D) gravity and magnetic forward modeling. Using the same modeling parameters from gravity model 4 (see Figure 6D), we calculated five 2-D gravity and magnetic forward models; several changes in crustal type are observed along the length of the San Andres rift (SAR). Areas of extreme crustal stretching in the SAR are observed in the centers of lines C and D; the end points of these lines are unextended zones outside the area of the pull-apart basin defined by the Pedro Bank and Hess Escarpment fault zones. CARVAJAL-ARENAS AND MANN

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Figure 8. (A) Uninterpreted, northwest-trending, seismic profile AA9 in two-way time (TWT) from the northernmost part of the San Andres rift (SAR) showing the area of rift initiation on the lower Nicaraguan Rise. (B) Interpreted seismic profile from the northernmost part of the SAR showing the Pedro Bank strike-slip fault zone transitioning onto normal faults of the SAR. Normal slip-dip faults frame the symmetrical graben and dip inward to converge on the rift axis.

profile with the magnetic observed profile, we include intrusive bodies beneath the lower Nicaraguan Rise with densities of 2.95 g/cc and susceptibilities of approximately 7000 centimeter–gram–second system of units (u-cgs). We interpret that intrusive events have taken place in the lower Nicaraguan Rise since the middle Miocene to have been focused by the presence of the SAR whose normal faults provided pathways for both intrusive and extrusive volcanism as observed in the volcanic history of Providencia Island located on the eastern edge of the SAR (Kerr, 1540

1978; Wadge and Wooden, 1982; Geister, 1992; Concha and Macia, 1995) (Figure 6D).

Isostatic Moho Depth and Crustal Thickness Variations along the San Andres Rift We calculated the isostatic Moho depth as a method to extrapolate the results from scattered refraction stations present in the western Caribbean to other areas that lack any refraction coverage to have a more regional Moho map for 2-D gravity and magnetic


Figure 9. (A) Uninterpreted, west–southwest-trending, seismic profile BB9 in two-way time (TWT) crossing the northern part of the San Andres rift (SAR). (B) Interpreted seismic profile from the northern section of the SAR. The northern section records the initial formation of the SAR during the Miocene along vertical-dipping, normal faults aligned with left-lateral strike-slip faults of the Pedro Bank fault zone. Line B shows 60°-dipping, normal slip-dip faulting, and growth strata along normal faults within Miocene sections that record ongoing, active deformation since the early Miocene and rapid subsidence associated with regional extension as recorded by upper Miocene pinnacle reefs that are covered with recent clastic sediments.

forward modeling (Figure 7A). The isostatic Moho depth demonstrates that gravity inversion based on isostatic equilibrium and Archimedes’ principle can provide an excellent fit with the plotted refraction stations (Aitken, 2010) (Figure 7A). Examples of western Caribbean areas where the gravity inversion results fit well with the Moho known from previous refraction surveys include (1) isostatic Moho depth identified in the Cayman Trough shows depths

roughly at 6.5 km (4.0 mi) that corresponds to observed refraction values reported by ten Brink et al. (2002); (2) refraction measurements of continental crust on the upper Nicaraguan Rise by Ewing et al. (1960) match well with observed values of the isostatic Moho depth ranging from 22 km (14 mi) to 26 km (16 mi); (3) refraction stations reported by Holcombe et al. (1990) from the Colombian Basin as thick oceanic plateau with an average thickness of CARVAJAL-ARENAS AND MANN

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Figure 10. (A) Uninterpreted, northwest-trending seismic profile CC9 from a more extended area of the central San Andres rift. (B) Interpreted seismic profile showing filling of Upper Cretaceous–lower Paleocene sedimentary rocks into major depocenters; gravitational collapse of upper Paleocene–lower Eocene sedimentary rocks; and uplift and erosional truncations by the late Eocene. TWT = two-way time.

17 km (11 mi) match well with what is observed from the isostatic Moho depth. In summary, the error observed between refraction stations and the calculated, isostatic Moho depth for model 4 is roughly 9% with an exception of two anomalous areas calculated for the lower Nicaraguan platform that have an error of 35%. The results obtained from the isostatic Moho depth for model 4 have shown to have solid a correlation with field-based refraction measurements for determining crustal thickness (Steffen et al., 2011). Additional 1542

refraction stations will provide further tests for the accuracy of the isostatic Moho depth method. Of the four 2-D gravity and magnetic forward models tested along line C (Figure 6A, D), we propose that model 4 is the most accurate because (1) model 4 fits observed and calculated gravity and magnetic profiles; (2) model 4 is consistent with regional geologic data; and (3) model 4 shows consistency with both the observed Mohorovicic discontinuity known from previous, refraction surveys and the calculated, isostatic Moho profile (Figure 6D).


Figure 11. Interpreted seismic profile DD9 from the south section of the San Andres rift (SAR) showing the maximum amount of extension observed along the entire rift. Toward the northwest, an undeformed carbonate platform marks the western boundary of the SAR whereas the eastern boundary shows a broad zone of tilted normally faulted blocks. Main structural and stratigraphic features are indicated and include normal faults, Upper Cretaceous–lower Paleocene unconformities, and wide rift between uplifted rift shoulders of the eastern normal fault of the SAR. These footwall blocks expose deeper crustal rocks on the islands of Providencia and San Andres including Miocene volcanic rocks on Providencia Island. TWT = two-way time.

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Figure 12. (A) Uninterpreted, southwest to northeast-oriented seismic profile EE9 shown in two-way time (TWT) crossing the southernmost section of the San Andres rift (SAR). (B) Interpreted seismic profile from the southernmost section of the SAR displaying normal faults, lower Eocene pinnacle reefs. The northern extension of San Andres Island shows similar subsurface, sedimentary dips to those measured from outcrops in rock exposures on San Andres Island (see Figure 2).

Using the same parameters as used for model 4 (Figure 6D), we calculated five 2-D gravity and magnetic forward models along the SAR (Figure 7B). These five models for the SAR were based on seismic lines A to E (Figure 1B) and display the lower Nicaraguan Rise as an extended continental crust with localized intrusions. East–west extension of the SAR is shown by a shallowing of the Moho accompanied by crustal extension of the crust lower Nicaraguan Rise that provided pathways for extrusive and intrusive Miocene volcanism (Figure 7B). 1544

Imaging the Structure of the San Andres Rift Using Two-Dimensional Seismic Data Five high-resolution and deep-penetration 2-D seismic lines oriented perpendicular to the Pedro Bank fault zone reveal the subsurface structure of the SAR (Figures 8–12). In map view, the SAR shows the characteristic, “lazy-S” shape of a pull-apart basin with a basin length from north to south of 346 km (215 mi) (Molnar and Sykes, 1969; Mann and Burke, 1990). The northwestern end of the SAR forms


Table 1. Total Extension Calculated from Sequential Two-Dimensional Kinematic Restorations from the Late Cretaceous to the Holocene

Stage 1

2

3 4 5

Period of Extension

Line A: Figure Line B: Figure Line C: Figures 8, m (ft) 9, m (ft) 10–14, m (ft)

Late 610 (2001) 50.8 Cretaceous Early -57.2 (188) 309 Paleocene Early 801 (2628) 160 Eocene Late Eocene 172 (564) 268 Early 343 (1125) 872 Miocene Late 496 (1627) 692 Miocene Pleistocene 1221 (4006) 1625

Line D: Figures 11–14, m (ft)

Line E: Figure Average of 12, m (ft) Extension, m (ft)

Velocity of Extension, mm/yr

(167)

1982 (6500)

532 (1745)

671 (2201)

770 (2526)

0.15

(1014)

2234 (7329)

1144 (3753)

615 (2017)

849 (2785)

0.21

(525)

3672 (12,047)

1011 (3317)

1230 (4035)

1375 (4511)

0.11

(879) (2861)

2469 (8100) 1772 (5814)

1490 (4888) 1330 (4363)

615 (2018) 1454 (4770)

1003 (3291) 1154 (3786)

0.07 0.06

(2270)

2722 (8930)

825 (2707)

2461 (8074)

1439 (2526)

0.12

(5331)

2578 (8458)

4495 (14,747)

4978 (16,332)

2980 (9777)

1.19

Five stages are identified along the San Andres rift (SAR) showing (1) Late Cretaceous–Paleocene early rifting, steady subsidence due to crust cooling; (2) early Eocene–middle Eocene development of the Verolania rift captured in the SAR as an early extension stage; (3) late Eocene–Oligocene steady subsidence impressed by a regional unconformity; (4) early–late Miocene reactivation of the Pedro Bank fault zone increases subsidence and carbonate platform drowning triggered by the collision of the Panama arc; (5) late Miocene–Holocene rapid subsidence by the reactivation of the SAR.

a narrow submarine valley in the Serranilla cays that expands in width to the Albuquerque cays at the southwestern end of the basin (Figure 1B). Characteristic rift features of the SAR that are observed on the seismic lines include (1) high-angle (>60°) normal faults; (2) domino-style, rotated fault blocks; and (3) wedge-shaped Cenozoic growth strata located along rift-bounding normal faults as seen in the northern part of the SAR (Figure 8). The dip of normal faults varies from north to south along the axis of the rift. (1) Normal faults with higher dips in a dip range of 55°–85° are observed at the northern and southern ends of the rift (Figures 8, 9). Additionally, on these areas, the rift basin is narrow (23 km [14 mi]) bounded by fault blocks with least amount of block rotation and a minimum total amount of extension (Figure 12). (2) The central region displays low dips of normal faults ranging from 35° to 55° where the total amount of extension due to block rotation is greatest (Figures 10, 11) and displaying the widest part of the SAR (53 km [33 mi]).

Northern San Andres Rift Transitioning to the Pedro Bank Fault Zone: Line A In this northern area of the rift, the rift axis is narrow and the rift has a half-graben morphology as compared

to the full-graben morphology in the more extended central part of the SAR (Figure 8). The half-graben shows a growth fill ranging in age from early Miocene to Holocene (Figure 8). Other observations in line A include (1) intrabasement reflectors observed along seismic lines A to E are characterized by a remarkable layered acoustic stratification within the basement. We suggest that this layered sequence might correspond to rocks of continental affinity in the upper and lower Nicaraguan Rise, and oceanic affinity units in the Colombian Basin (western side of the Hess Escarpment). (2) Two major pulses of extension are recorded in surrounding areas of the SAR. From the Late Cretaceous to early Paleocene, the CLIP moved between Mexico and South America along transform faults allowing regional extension nearby the Hess Escarpment fault zone (Burke et al., 1984). Growth strata along normal faulting is identified to the eastern part of this section (Figure 8). The identification of growth strata along the sedimentary record is an important process because knowing the thickness and age of the growth strata, we can calculate the vertical component of the structural growth (Schneider et al., 1996). During the middle–late Miocene, eastward motion of the Caribbean plate is CARVAJAL-ARENAS AND MANN

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accommodated by left-lateral strike-slip faults that bound the northern and south Caribbean plate boundary zones (Burke, 1988). (3) Regional extension observed along the SAR and the lower Nicaraguan Rise allowed the sedimentation of growth strata over major normal faults (Figure 8). In contrast, areas without enough sediment supply showed normal fault displacement with no-growth strata.

Beginning of Block Rotation in the San Andres Rift: Line B Line B shows brittle extension of the crust, extensional fault arrays, and fault-controlled subsidence that are typical features of rift basins (Figure 9) (Allen and Allen, 2013). The northern section of the SAR displays a 60° normal slip-dip faulting that starts to converge into a rift axis. Rift flanks start to emerge bounding active areas from inactive areas. Some observations of line B include (1) domino faulting affecting sediments from the Late Cretaceous to the middle Miocene suggests that, by the middle Miocene, a pulse of extension deformed the lower Nicaraguan Rise (Figure 9). Domino faulting involves crustal block rotation and thick skin deformation (Terres and Sylvester, 1981) attributed as the most likely process that affect the lower Nicaraguan Rise. (2) Pinnacle reefs are present on this line and are localized on rift-related structural highs that by the early–middle Miocene failed to keep up with sea-level rise (Mutti et al., 2005) and were subject to regional drowning (Cunningham, 1998) (Figure 9). (3) The upper Miocene regional unconformities are identified along the upper and lower Nicaraguan Rise (Figure 9). Sea-level change due to glacioeustacy (Miller and Mountain, 1996; Miller et al., 1998) is the most likely process for upper Miocene erosional surfaces. Even in deep areas of the Nicaraguan Rise such as in the SAR, super–upper Miocene sediments cover this regional unconformity. Toward the south in the Colombian Basin, the upper Miocene unconformity becomes conformable in structural lows (Arden, 1975; Holcombe et al., 1990; Bowland, 1993; Carvajal-Arenas et al., 2013). Central, Deep Section of the San Andres Rift as Seen on Seismic Line C The most representative cross section of the most extended part of the central SAR is shown by line C 1546

(Figure 10). The SAR at this latitude shows a graben morphology with elevated, rift-flank topography bordering a depositional basin where the distance between rift flanks varies from 40 km (25 mi) to 50 km (31 mi). Oblique normal dip-slip faulting ranges between 45° and 60° forming a negative flower structure. Block rotation and growth strata is also observed between rift flanks. Active tectonism in the SAR is observed in onlapping syndepositional sediments filling the rift since the late Miocene. Some observations of line C include the following. (1) Based on seismic character interpretation (Bowland, 1993; Abrams and Hu, 2000), we suggest the deposition of Upper Cretaceous–lower Paleocene sediments present in major depocenters in the lower Nicaraguan Rise (Carvajal-Arenas et al., 2015; Torrado, CarvajalArenas, Sanchez, Mann, and Silva-Tamayo, under review) (see Cayos Basin; Figure 10). (2) East to the SAR, Eocene carbonate complexes are identified on the shoulders of the Cayos Basin, suggesting that, by the Eocene, this region was close to the surface (Figure 10).

Maximum Extension of the San Andres Rift: Line D Similarly to the previous line, line D shows the SAR as a wide pull-apart basin with a maximum extension of 57 km (35 mi) taking place between latitudes 13.5°N and 12.5°N (Figure 11). Mann and Burke (1984) described the SAR based on bathymetry as a deeper trough surrounded by shallow carbonate banks. Normal dip-slip faulting within a structural framework of a negative flower structure ranges between 45° at rift flanks and 55° at the rift axis (Figure 11). Block rotation is strong and accumulates rotations of more than 25° between the undeformed Nicaraguan platform and the eastern rift flank. Some observations of line D include (1) Upper Cretaceous–lower Paleocene unconformities overlain by Cenozoic sequences are identified in the upper Nicaraguan Rise. These unconformities are triggered by convergent events that uplifted the Honduran and Nicaraguan region by the Late Cretaceous (Mills et al., 1967; Rogers et al., 2007; Sanchez et al., 2015). (2) We attribute the noticeable thickness variation of Cenozoic sediments on the flanks of the SAR to reflect a rebound of the western flank of the SAR following the pulse of middle to late Miocene extension (Figure 11). During the Miocene, the San Andres Island also emerged on the eastern flank of the rift (Figure 11, see Discussion section).


Southernmost San Andres Rift and Nutibara Rift: Line E The southern section of the SAR is characterized by narrowing of the SAR to a width of 23 km (14 mi) and by counterclockwise rotation and drag of the Pedro Bank fault zone into the left-lateral Hess Escarpment fault zone (Figure 12). The Nutibara rift 15 km (9 mi) to the east of the SAR also appears near the Hess Escarpment fault zone intersection. Based on bathymetric profiles, the Nutibara rift runs parallel to the Pedro Bank fault zone and intersects the Hess Escarpment at the Albuquerque cays (Figures 1B, 2A). Some observations of line E include (1) pinnacle reefs localized in depocenter areas of the lower Nicaraguan Rise overlying upper Paleocene sediments (Figure 12). By the Late Paleocene, most of the region of the Nicaraguan Rise was subject to a prolonged surface exposure (Sanchez et al., 2015); depocenters and low areas in the lower Nicaraguan Rise exhibit transitional environments that were appropriate for the deposition of carbonate complexes (Torrado, Carvajal-Arenas, Sanchez, Mann, and Silva-Tamayo, under review). Bounding the SAR to the west and the Nutibara rift to the east, the northern extension of San Andres Island shows an extrapolation of the surficial geology of San Andres Island (Figures 2B, 12) mimicking tilted blocks dipping to the northeast (Figure 2D). Similarly, an analogous example of tilted blocks affected by an active tectonism is observed to the east of San Andres Island (see east region; Figure 12). In this case, this region was (1) uplifted, (2) tilted by the middle Miocene, and (3) exposed during the late Miocene.

STRUCTURAL ANALYSIS AND KINEMATIC EVOLUTION OF THE SAN ANDRES RIFT FROM EOCENE TO HOLOCENE Postulated hypotheses for the deformation of the SAR and its seismic activity can be found in (1) the observation of regional lineaments in the lower Nicaraguan Rise published in the preliminary bathymetric and geologic map of the Caribbean (Case and Holcombe, 1977). Based on Case’s bathymetric lineaments, Christofferson and Hamil (1978) postulated that San Andres Island was the focus of a geologic event that resulted in radially and concentrically disposed lineaments about the island. They proposed that radially discontinuous bathymetric lineaments

were caused by meteorite impacts, mantle plumes (Morgan, 1971; Christofferson, 1973), and mantle blobs (Schilling and Noe-Nygaard, 1974); (2) Wadge and Burke (1983) proposed that deformation in the lower Nicaraguan Rise may be related to counterclockwise rotation of the Chortis block relative to the Colombian Basin; and (3) faulting, small-displacement deformation, and recent volcanism observed in the surface of the lower Nicaraguan Rise points to slow rifting and translation (Holcombe et al., 1990) is the most likely process for the formation of the SAR.

Earthquakes and Focal Mechanisms of the San Andres Rift Since the first seismic events were recorded in 1972, the SAR has recorded more than 183 earthquakes with magnitudes of 3.95 in a crustal depth range between 5 km (3 mi) and 26.9 km (16.7 mi) (Figure 13). More than 80% of these earthquakes are located in areas where the Pedro Bank fault zone bends toward the southwest onto the Pedro Bank fault zone in its northernmost area (area A), and toward the west onto the Hess Escarpment fault zone in its southernmost area (area B) (Figure 13). Earthquakes along the northern section in area A delineate the central rift axis with events with magnitudes around 4.1 and depths of 26.2 km (16.2 mi) (Figure 13). Calculated earthquake focal mechanisms from the Global Centroid-Moment-Tensor (CMT) Cata¨ log (Ekstrom et al., 2012) and the International Seismological Centre (2015) along the SAR show a variety of focal mechanism solutions (Figure 13). Events 2, 3, 4, 5, and 6 in Figure 13 all record transtensional events associated to left-lateral strikeslip faulting (Fernandez et al., 2007; GuzmánSpeziale, 2010; Symithe et al., 2015). Events 1 and 7 in Figure 13 record local transpressional events. Additionally, points 3, 4, and 5 located on the western margin occur at depths of 10 km (6 mi) with magnitudes of 5.2, whereas points 2 and 6 located on the eastern margin have depths of 30 km (19 mi) and magnitudes of 5.3 (Figure 13). Moment magnitudes are similar in both rift flanks but earthquake depths are shallower on the western margin of the SAR than the eastern margin. We interpret the deeper earthquakes on the eastern margin as active faulting along the west-dipping normal fault bounding the SAR. CARVAJAL-ARENAS AND MANN

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Figure 13. Earthquake activity and focal mechanisms from the area of the San Andres rift (SAR) taken from the Global Centroid-Moment¨ et al., 2012) and the International Seismological Centre (2015). Earthquakes recorded since 1974 show Tensor (CMT) Catalog (Ekstrom active, seismogenic deformation along the SAR at the intersection of the SAR with the southwestern Hess Escarpment. Areas A and B represents the most seismically active areas of the SAR. Seven focal mechanisms calculated from major tectonic events (magnitude ~5) indicate left-lateral strike-slip motion of the Pedro Bank fault zone along with localized areas of normal faulting of the SAR that is consistent with its proposed origin as a 325-km-long (202-mi-long) pull-apart basin between the Pedro Bank left-lateral strike-slip fault zone to the north and the Hess Escarpment left-lateral strike-slip fault to the south. The Hess Escarpment fault zone becomes less seismically active toward the east and more seismically active toward in the west where focal mechanisms also record localized compression.

Scattered focal mechanisms along the SAR show normal dip-slip solutions that correspond to normal faulting and block rotation (point 7, Figure 13). Thrust focal mechanism events at a depth of 33.7 km (20.9 mi) are present at the junction of the Hess Escarpment and Pedro Bank fault zone (point 1, Figure 13). 1548

INTEGRATING RIFT STAGES AND SEDIMENTATION OF THE SAN ANDRES RIFT The SAR reveals typical features of an active rift based on its bathymetry (Figure 1A), 2-D gravity and magnetic profiles (Figures 4–6), 2-D seismic data (Figures 8–12), and distribution of earthquakes (Figure 13).


According to the classification of Huismans and Beaumont (2011), the SAR can be classified as a nonvolcanic rifted margin with a depth-dependent extension resulting in (1) narrow regions (