Crustal thickness variations in Venezuela from deep seismic observations

Tectonophysics 459 (2008) 14–26

Contents lists available at ScienceDirect

Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Crustal thickness variations in Venezuela from deep seismic observations M. Schmitz a,⁎, J. Avila a, M. Bezada a,b, E. Vieira a,c, M. Yánez a,d, A. Levander b, C.A. Zelt b, M.I. Jácome e, M.B. Magnani b,f the BOLIVAR active seismic working group a

FUNVISIS, Prol. Calle Mara, El Llanito, Caracas 1070, Venezuela Rice University, Houston, Texas, USA now at: PDVSA, Puerto La Cruz, Venezuela d now at: Western Geco, Caracas, Venezuela e USB, Caracas, Venezuela f now at: University of Memphis, Tennessee, USA b c

A R T I C L E

I N F O

Article history: Received 17 July 2006 Received in revised form 22 January 2007 Accepted 15 November 2007 Available online 4 April 2008 Keywords: Crustal thickness Deep seismic observations Venezuela Caribbean South America

A B S T R A C T The Caribbean–South America plate boundary zone is a complex zone of plate interactions, forming thrust belts and foreland basins in northern Venezuela. Within the framework of the BOLIVAR and GEODINOS projects, the geodynamics of plate interactions is being investigated using interdisciplinary geological and geophysical methods. Here, we focus on the results of the land based active seismic observations done in 2004 along four deep seismic wide angle profiles, acquired perpendicular to the Caribbean–South America plate boundary in northern Venezuela between longitudes 63° W and 70° W, and ranging from about latitudes 12 °N to about 9 °N. The mostly unreversed profiles provide information on the crustal structure from the oceanic-transitional crust on the southern border of the Caribbean plate to the continental crust of the Caribbean Mountain System and their associated foreland basins, which are bordered to the south by the Guayana Shield, which corresponds to stable South America plate. The derived crustal thickness oscillates around 35 km along the coastline, corresponding to the Caribbean Mountain System, and decreases only slightly towards the Leeward Antilles. To the south, in the area of the Eastern Venezuela Basin, crustal thickness reaches 40 km, increasing towards the Guayana Shield to 45 km. Nevertheless, there are two regions of anomalous crustal thickness, proven by arrivals from the lower crust and the Moho discontinuity. In the eastern part of the Eastern Venezuela Basin, crustal thickness reaches up to 50 km, with high velocity anomalies within the lower crust, which are interpreted as reworked lower crustal and upper mantle material, associated to the plate interactions of the South American and the Caribbean plates. The second anomalous zone is a remarkable crustal thinning from 35 km to 27 km in the Falcón Basin in western Venezuela, which extends eastwards into the Bonaire Basin, as documented by PmP reflections derived from land shots, and observations of the air gun blasts on the stations of the Venezuelan seismological network. © 2008 Published by Elsevier B.V.

1. Introduction Interdisciplinary geophysical and geological studies are being carried out within the framework of the BOLIVAR (Broadband Ocean–Land Investigations of Venezuela and the Antilles arc Region) and GEODINOS (Geodinámica Reciente del Límite Norte de la Placa Sudamericana — Recent Geodynamics of the Northern Limit of the South American Plate) projects in order to investigate the geodynamics of the complex Caribbean–South America (CAR–SA) plate boundary zone. As part of these investigations, active seismic measurements were done in 2004 in northern Venezuela and in the southeast Caribbean (Levander et al., 2006). ⁎ Corresponding author. Fax: +58 212 2579977. E-mail address: [email protected] (M. Schmitz). 0040-1951/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.tecto.2007.11.072

Information on the crustal structure of the area is available for the Caribbean plate, where thickened oceanic crust is interpreted as an oceanic plateau (e.g. Edgar et al., 1971; Case et al., 1990). The origin of this thickened oceanic crust has been intensely discussed during the last decades (e.g. Pindell and Dewey, 1982; Sykes et al., 1982; Meschede and Frisch, 1998; Pindell and Kennan, 2001; Kerr and Tarney, 2005; James, 2006). Widely accepted is an origin west of its actual position with an eastward migration of the Caribbean plate of 2 cm/year with respect to the South American continent (e.g. Weber et al., 2001), accommodated along mayor strike slip fault systems on the northern edge of the continent (e.g. Schubert, 1984; Audemard et al., 2000). In contrast, little has been known about the continental crustal structure and Moho depths in Venezuela, as evidenced in a compilation of crustal thickness at a global scale, presented by Mooney

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

et al. (1998). First results of deep wide-angle seismic measurements in Venezuela were obtained for the east coast of Maracaibo Lake in western Venezuela (Gajardo et al., 1986; Guédez, 2003), where a crustal thickness of 40–43 km was derived for this sedimentary basin. Results of seismic studies done on the Guayana Shield in southern Venezuela indicate a crustal thickness of 45 km for the craton (Schmitz et al., 2002), which decreases towards the north, as it enters into the Eastern Venezuela Basin (Schmitz et al., 2005), reaching a thickness of about 35 km near the coast. In the central offshore area, records of airgun shots on the stations of the Venezuelan Seismological Network allow to derive first information on the crustal thickness of the CAR–SA transition zone (Guédez, 2003). In this contribution, we focus on the land based active seismic observations done in April/May 2004 in northern Venezuela between 63° W and 70° W along four mayor seismic transects (Fig. 1). Along each of the main profiles, two land shots were recorded on portable stations. Records of the air gun lines on the stations of the Venezuelan Seismological Network allow deriving to some extent lateral variations of the crustal structure and thickness. As a result, we are able to derive a map of crustal thickness in Venezuela, which comprises northern and eastern Venezuela from the Caribbean Sea in the north to the Guayana Shield in the south. 2. Geotectonic setting Northern Venezuelan thrust belts and foreland basins were formed as the result of tectonic interaction between the Caribbean and South American plates. It is therefore important to understand the geology,

15

age and formation hypotheses of the main structures within the southern Caribbean Plate. First, we present here a regional view of the tectonic evolution in the area, including the structure and geology of the Coastal Cordillera and the Eastern Serranía del Interior thrust belts, the Guárico and Maturín basins, the Guayana Shield, the Falcón Basin and the Leeward Antilles (Fig. 1). 2.1. Caribbean and South American plates The present day lateral displacement of the Caribbean Plate is 1.5–2.0 cm/yr with respect to the South American Plate, with a sinistral sense of shear in the north and a dextral sense of shear in the south (Mann et al., 1990; Weber et al., 2001). Wide areas of compressional, extensional and strike–slip tectonic regimes characterize the margins of the Caribbean Plate. The location of the Caribbean Plate boundary is more clearly defined along the western and eastern margins, other boundaries are more complex and still subject to debate. The Lesser Antilles and Middle America Arc systems are predominately convergent margins with well-developed seismic Benioff zones and volcanic arcs, with convergence rates of 4 cm/yr and 7.4 cm/yr respectively (Westbrook and McCann, 1986; DeMets et al., 1994). The northern and southern boundaries of the Caribbean Plate have experienced Neogene strike–slip, compression and extension, across a broad plate boundary zone. The evolution of the southeastern margin of the Caribbean Plate has been debated controversly. For many years, the South American– Caribbean boundary has been interpreted as a narrow zone of pure strike–slip movement (Molnar and Sykes, 1969; Pérez and Aggarwal,

Fig. 1. Location map with airgun lines (red lines with line numbers), seismological stations from the Venezuelan Seismological Network (inverted triangles with station codes; sections displayed in this paper from stations MONV = Monte Cano, TURV = Turiamo, PCRV = Puerto La Cruz, CRUV = Carúpano) and land shots (stars with shot point name) with the respective recording lines (black), used in this study. Simplified tectonic units after Stephan (1985) and Ysaccis et al. (2000): a) Mérida Andes, b) deformed Mesozoic passive margin and Lara nappes, c) Falcón Basin, d) Bonaire and other minor marine basins, e) Southern Caribbean Deformed Belt with subduction in the north and Leeward Antilles in the south (including the Aruba–Curaçao Basin), f) Caribbean nappes and Coastal Cordillera Thrust Belt, g) Eastern Venezuela Basin, sub-divided into the Guárico Basin in the west and the Maturín Basin in the east, h) Tuy–Cariaco Basin, i) Blanquilla Basin, j) Carúpano Basin, k) Serranía del Interior Thrust Belt, l) Guayana Shield, m) Aves high. Location of seismic profiles obtained during previous experiments indicated with yellow lines (recording lines), red circles (seismological stations) and blue stars (shot points): COLM (Gajardo et al., 1986), MAR and TIERRA (Guédez, 2003), ECOGUAY (Schmitz et al., 2002), ECCO (Schmitz et al., 2005); M = Maracaíbo, C = Cacacas, B = Barcelona, CG = Ciudad Guayana.

16

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

Fig. 2. Seismic record sections (reduction velocity for all record sections is 6 km/s) and ray-tracing model along the westernmost profile along Longitude 70° W: A) records from BOL3 at MONV; B) records from Aracua shot point; C) records from Barquisimeto shot point. Arrivals from the upper crust (Pg), reflections from the Moho (PmP) and intracrustal reflections (PiP) can be distinguished in the record sections. A second Moho reflection (PmP2) is observed at the Montecano record section. D) Observed and calculated travel times (top), ray tracing (center) and velocity model (bottom). A crustal thinning from about 35 to 27 km north of Aracua shot point and reflections from the subducting Caribbean slab north of Monte Cano station are the most prominent features along this profile; MONV = Monte Cano seismological station, A = Aracua shot point, B = Barquisimeto shot point.

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

1981). More recent interpretations consider subduction of the oceanic Caribbean Plate in western Venezuela (i.e. under the Falcón Basin) under the continental South American Plate (Van der Hilst and Mann, 1994). Towards the east of Venezuela (i.e. under the Eastern Serranía del Interior Thrust Belt) a change in subduction polarity is observed. In this area the continental South American lithospheric mantle is subducting under the oceanic Caribbean Plate (Russo and Speed, 1994; Jácome et al., 2003). In this model, strike– slip movement constitutes a minor component of oblique convergence. In support of this interpretation minor strike–slip movement (less than 150 km) has been reported by Audemard, and Giraldo (1997), in a regime of strain partitioning with oblique convergence between the Caribbean and South American plates. Marine refraction and gravity data have located the Moho discontinuity under the Venezuelan Basin in the Caribbean Plate at a depth of between 15 and 20 km. This oceanic crust is thicker than average (Officer et al., 1959; Edgar et al., 1971; Biju-Duval et al., 1978; Diebold et al., 1981; Case et al., 1990; Diebold et al., 1999). Based on seismic reflection data, drilling results and field investigations on obducted mafic igneous complexes, Donnelly et al. (1990) and Donnelly (1994) have suggested that this anomalous thickness was produced by a large flood-basalt eruption that took place near Beata Ridge (in the center of the Caribbean Plate, between Hispañola and Colombia), around 85 Myr ago, and is responsible for the buoyancy of the Caribbean Plate. Several interpretations have been proposed for the origin of the Caribbean Plate, one of the models proposes that this plate was formed in the Pacific region and moved into its present position by a system of strike–slip faults that involved more than 4000 km of lateral displacement (Pindell and Dewey, 1982; Burke et al., 1984; Pindell and Barrett, 1990; Stephan et al., 1990; Pindell and Kennan, 2001). Another hypothesis is based on recent evidence and proposes that the Caribbean Plate was formed nearer to its present-day position between the two Americas (Sykes et al., 1982; Donnelly, 1985; Meschede, 1998; Meschede and Frisch, 1998). New magnetic and stratigraphic data (Barckhausen et al., 1998) confirm that the basaltic eruption event that generated the Caribbean crust took place on the Cocos Ridge (Meschede, 1998). However, whether the plateau was formed in the Pacific or near its present location is not yet clear. Recent studies based on palaeomagnetic and geochemical data support that the Caribbean Plate was located to the west of its present position but near the Americas and not in the Pacific. The difference between the two hypotheses is based mainly in different interpretations of the palaeomagnetic and geochemical evidence. 2.2. Northern Venezuela At least four different geodynamic events have contributed to the formation of the structures found in northern Venezuela: 1) Paleozoic orogenic event; 2) Rifting phase associated with the break-up of Pangea during Jurassic and the earliest Cretaceous time; 3) Diaochronous development of a passive margin from Cretaceous (in Western Venezuela) to Paleogene (in Eastern Venezuela); 4) Oblique collision between the Caribbean and South American plates, which generated the Coastal Cordillera Thrust Belt and the Guárico Foreland Basin in the central region of Venezuela, and the Eastern Serranía del Interior Thrust Belt and the Maturín Foreland Basin in the East. 2.3. Coastal Cordillera Thrust Belt and Guárico Foreland Basin During Middle Eocene the Caribbean Plate collided with the northcentral portion of Venezuela emplacing the Coastal Cordillera (Fig. 1). The tectonic load associated with the thrust belt flexed the lithosphere forming the Guárico Basin, with 5 km of Eocene–Oligocene sediments in the west and 7 km in the east (Erlich and Barrett, 1990; Jácome et al., 2008-this volume).

17

The Coastal Cordillera Thrust Belt is formed by different geological provinces which contain (from north to south): Late Jurassic to Early Cretaceous basic and ultrabasic rocks, Precambrian and Paleozoic basement rocks, Jurassic to Cretaceous lower crust–upper mantle fragments, volcano-sedimentary sequences and basaltic to rhyolitic rocks, and Late Cretaceous–Paleocene molassic sediments and flysh sequences (Beck, 1986; Bellizzia, 1986; Navarro et al., 1988; Donnelly et al., 1990; Ostos, 1990; Giunta et al., 2003). 2.4. Eastern Serranía del Interior Thrust Belt and Maturín Foreland Basin The Serranía del Interior Thrust Belt is a southeast verging N70Etrending fold-and-thrust belt (Rossi, 1985). It is bounded by four principal tectonic features, which are: to the north, the dextral strike–slip El Pilar fault; to the south the deformation front; and to the east and west Los Bajos and San Francisco strike–slip faults, respectively (for location of the faults refer to Audemard et al., 2000). This thrust belt principally comprises Cretaceous and Tertiary rocks deposited at a passive margin (Lilliu, 1990). These rocks were uplifted during northwest–southeast Oligocene–Present oblique convergence at the boundary between the Caribbean and the South American plates. The Maturín Foreland Basin extends from the deformation front in the north to the Orinoco River in the south (Fig. 1). The Orinoco River marks the northern limit of the basement outcrops of the Guayana Shield (González de Juana et al., 1980). The basin is bounded to the west by the Guárico sub-basin and to the east by the Atlantic Ocean. The foreland basin is filled with 8–12 km of Neogene sediments, accommodated as the result of thrust sheet loading and continental subduction-dynamic topography, which forced the American continental lithosphere to flex downward in between the Guayana Shield to the south and the Coastal Cordillera Trust Belt to the north (Roure et al., 1994; Jácome et al., 2003). The sources of sediments lie in the south (Guayana Shield), the north (the rising Serranía del Interior Thrust Belt) and the west. 2.5. Guayana Shield South of the Maturín and Guárico foreland basins, the Guayana Shield outcrops as sialic Precambrian continental crust. This shield is composed mainly of metasedimentary and metaigneous rocks at amphibolite to granulite facies that have been intruded by granites (FeoCodecido et al., 1984). The reported ages of these crystalline rocks range from 3600 Ma to 800 Ma (González de Juana et al., 1980). The crustal thickness at the northern edge of the shield is 45 km (Schmitz et al., 2002). 2.6. Falcón Basin and Leeward Antilles The Falcón Basin is located in the northwestern part of Venezuela over an area of approximately 36,000 km2 (Fig. 1). The basement of the basin is formed by rocks emplaced during the collision between the Caribbean and the South American plates during Paleocene–Lower Eocene time (Audemard, 1995). During the Oligocene, extension produced the opening of the Falcón Basin, which had originally a NE–SW elongate shape and communicated to the east with the deep Bonaire Basin (Fig. 1). Along the Falcón Basin axis igneous intrusions of basaltic composition are observed (Muessig, 1978). These intrusions, as well as recent gravimetric information, suggest that some crustal thinning occurred in association with the opening of the basin (Muessig, 1984; Rodríguez and Sousa, 2003). The origin of the basin is still unclear. Early theories suggest that it was formed as a pull-apart basin in a dextral strike–slip fault system (Muessig, 1984) with the islands of Aruba, Curaçao and Bonaire (i.e. Leeward Antilles) forming an almost contiguous area during the Eocene. They were separated due to an east–west trending extensive regime that was also responsible for the

18

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

opening of the Falcón and Bonaire basins (e.g. Boesi and Goddard, 1991; Macellari, 1995). Despite the existing dextral strike–slip fault systems in the area (Oca-Ancón fault system, etc.), the total fault displacement is estimated to be less than 150 km (Audemard and Giraldo, 1997). This is not congruent with the creation of these faults as a pull-apart basin of dimensions similar to those of the Falcón Basin. Therefore, Audemard (1995) proposes a backarc origin for the opening of the basin, with north–south rather than east–west trending extension in Early Eocene. The Falcón basin was inverted after Middle Miocene by a compressional tectonic regime that turned it into a structural high (Audemard, 1995). 3. Seismic data acquisition and processing A combined seismic refraction/wide-angle reflection study was conducted in April/May 2004 (Levander et al., 2006) with airgun sources at sea and land shots as energy sources. In this paper, we analyze the recordings of the airgun blasts at the stations of the Venezuelan Seismological Network (Guralp CMG-40T three-component, 30 s seismometers), as well as the recording of the land blasts done with temporary stations (a total of 550 REFTEK 125-01 recorders connected to 4.5 Hz vertical geophones; courtesy of IRIS-PASSCAL Instrument Center) along profiles perpendicular to the coastline (Fig. 1). Shot point spacing for the airgun pulses, using a tuned airgun array with a combined volume of over 170 cubic meters, varied regarding the type of lines shot. For the seismic reflection lines, where multi channel seismic (MCS) recordings were done, 20 s time interval between shots was used, corresponding to a shot point interval of 50 m. Along the main profiles, shooting was repeated with a 60 s time interval between shots in order to enable ocean bottom seismometer (OBS) recordings along the refraction lines, resulting in a shot point interval of 150 m. Shots from both, reflection and refraction lines, were recorded by broadband stations of the Venezuelan National Seismological Network. Recordings along the main profiles were continued from the coast inland along 150–200 km long lines with a receiver spacing of 300 to 500 m. Installation in the field was carried out by 10–12 teams of volunteers from U.S. and Venezuelan institutions. Positioning of the receiver locations was done using Garmin GPS V handheld units with an accuracy of about 10 m. The data were sampled with a frequency of 100 Hz. Two shots, spaced between 50 and 100 km, using chemical explosions between 600 and 1000 kg (2/3 Pentolite and 1/3 Anfo, placed in 37 m deep boreholes of 6″ diameter) were fired along each land profile. From the continuous records on the seismological network stations, 30 s intervals where extracted at each shot time in SEG-Y format for the offshore lines. These individual 30 s traces were assembled to make common receiver gathers. Due to the low energy received at the stations and to the large amount of traces available in each section with a relatively small distance between them, consecutive traces were stacked in groups of 4 (for the 60 s refraction lines) and 10 (for the 20 s reflection lines) traces to enhance the signal to noise ratio. As a result, separation between traces was increased to 600 m for refraction lines and 500 m for reflection lines. Offsets were calculated using the UTM coordinates of each shot and the recording station, and the geometry was then used to generate files in SEG-Y format, which were processed using the SU package (Cohen and Stockwell, 1994). For the mobile sections, pre-processing (time corrections and generations of SEG-Y files) was carried out by the staff of the IRIS-PASSCAL instrument center. The preliminary sections were then edited to elimi-

19

nate defective traces. Geometry was calculated in the same manner as the offshore sections. The resulting sections were band-pass filtered using corner frequencies of 1 and 30 Hz, and trace balancing was applied to all seismic sections within the SU-package. All data are displayed in reduced travel-time using a reduction velocity of 6 km/s. The data quality is good for the recordings of the land shots, as well as for the recordings of the airgun sources at the seismological stations. 4. Analysis of seismic record sections and modelling We display the record sections from the land shots and the most important record sections from the air gun blasts for each of the 4 main profiles from west to east located approximately at longitudes 70° W, 67° W, 65° W and 64° W. Record sections from the air gun blasts are identified with the respective number of the BOLIVAR offshore line (BOL) and the recording station code. Record sections from the land shots are identified by the respective name of the land shot. The picks obtained from the seismic phases were correlated, and for the Moho reflections, the crustal thickness and the average crustal velocity were estimated using the Zmax formula (Giese, 1976). The 1-D estimates were taken as input for 2-D forward modelling using the ray tracing program RAYINVR (Zelt and Smith, 1992) in an iterative procedure, starting with the structure of the sedimentary basin and the upper crust, and then including the Moho reflections in order to model the whole crust. Satellite bathymetry data (Smith and Sandwell, 1997) were used as constraints on the marine parts of the models. The forward modelling approach was used due to the low coverage of data, which results in relatively poorly constrained 2-D models from mostly unreversed data, obtained from only a few shot points or stations along each line. The 2-D models are consistent with the picks from the data used, nevertheless they must be considered non-unique and interpretative. 4.1. Profile along longitude 70° W in western Venezuela In the northernmost record section of the BOL3 line, recorded at the Monte Cano seismological station (BOL3-MONV) (Fig. 2A), the Pg phase is identified at offsets between −42 and −120 km, at reduced travel times between 1.6 and 2.6 s. A 0.8 s delay in the arrival times is observed at offsets between −60 and −94 km, associated to the Aruba– Curaçao sedimentary basin (Fig. 2D), located between Aruba and Curaçao. Arrivals corresponding to the PmP phase (reflections from the Moho discontinuity) were identified between −74 and −118 of offset, with the critical reflection at −74 km and 4.3 s. Another phase, identified as PmP2 can be clearly followed at offsets between −97 and −190 km with the critical reflection at −97 km and 5.8 s. This phase is interpreted as reflections on the crust–mantle boundary of the subducting Caribbean Plate. In the record section from the Aracua shot, Pg and PmP arrivals are clearly recognizable to distances of up to 100 km (Fig. 2B). First arrivals were identified with an apparent velocity of approximately 4.6 km/s, interpreted as sedimentary Pg, at offsets of up to 20 km to the south and −18 km to the north at reduced times between 1 and 1.7 s. Arrivals corresponding to the crystalline Pg phase are observed to the north of the shot point at offsets between −20 and −36 km, and to the south of the shot point at offsets between 20 and 65 km, in both cases at times between 1.8 and 2 s. PmP arrivals are observed at offsets between 50 km and 100 km to the south with critical distance at 50 km and 6.3 s. To the north, the PmP can be identified at offsets between −32 and −50 km with a critical reflection at −32 km and 5.4 s. On the

Fig. 3. Seismic record sections and ray-tracing model from the profile in central Venezuela along Longitude 67° W: A) records from BOL13 at TURV; B) records from Ortiz shot point; C) records from Calabozo shot point. Arrivals from the upper crust (Pg), reflections from the Moho (PmP) and intracrustal reflections (PiP) can be distinguished in the record sections. D) Observed and calculated travel times (top), ray tracing (center) and velocity model (bottom). The continental crust is thinning towards the north, and the crust / mantle boundary is not well defined north of Ortiz, due to strong scattering of seismic waves in the Ortiz — north record section. TURV = Turiamo seismological station, O = Ortiz shot point, C = Calabozo shot point.

20

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

Fig. 4. Seismic record sections and ray-tracing model from the profile in eastern Venezuela along Longitude 65° W: A) records from BOL20 at PCRV; B) records from San Mateo shot point; C) records from Cantaura shot point. Arrivals from the upper crust (Pg) and reflections from the Moho (PmP) can be distinguished in the record sections. Differences in the arrival times of the Pg phase indicate varying sedimentary thickness along the Eastern Venezuela Basin. D) Observed and calculated travel times (top), ray tracing (center) and velocity model (bottom). PCRV = Puerto la Cruz seismological station, SM = San Mateo shot point, C = Cantaura shot point. Rays from shot points ET = El Tigre, L = Limo and CP = Ciudad Piar correspond to data displayed in Schmitz et al. (2002, 2005). The derived velocity model indicates an increased crustal thickness to the south with about 50 km, decreasing to 40 beneath PCRV and slight thinning towards the Caribbean Sea in the north.

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

records to the north, PmP arrivals are observed earlier than to the south, with a critical distance of only ~ 30 km, which suggest a crustal thinning in that region. In the record section corresponding to the Barquisimeto shot point (recorded to the north), three seismic phases were identified (Fig. 2C). The Pg phase is observed at offsets between −22 and −77 km and at 1.1 to 2.4 s, and the PmP phase is observed between −57 and −107 km of offset with a critical distance at −57 km and at 7.2 s. A third phase, interpreted as an intra-crustal reflection, is observed at offsets from −55 to −102 km with a critical reflection at −55 km with 4.8 s. As derived from the ray-tracing model (Fig. 2D), the BOL3-MONV section provides information on the Moho depth, as well as the maximum depth and the velocity structure of the basin located between Aruba and Curaçao (offshore sedimentary structure was based on a preliminary tomographic model, courtesy of Gail Christeson). Another interesting result is the reflection from the subducting Caribbean slab, derived from the PmP2 phase observed in that section, which can be traced down to 45 km. On land, the information from the two seismic sections help determine the velocity structure of the inverted Falcón Basin, and define an anomalous Moho topography, providing for the first time seismic evidence of the crustal thinning in the Falcón Area, previously inferred from gravimetric modelling (Rodríguez and Sousa, 2003). 4.2. Profile along longitude 67° W in central Venezuela In the marine record section of the BOL13 line, recorded at the Turiamo seismological station (BOL13-TURV) (Fig. 3A), the Pg phase is identified at offsets between 50 and 55 km, at a reduced travel time of 2.0 s. The PmP reflection is observed between −70 and −130 km offset with 2.5 to 4.0 s. On the record sections from the land shots (Fig. 3B and C), arrivals from the crystalline basement (Pg) are well defined in both sections, with less delay in the Guárico basin (south) with respect to the Coastal Cordillera Thrust Belt (north). To the north, Pg vanishes at about 60 km distance from the Ortiz shot point. In the same record section, PmP arrivals are very scattered compared to the observations Ortiz — south and Calabozo — north. Critical distances for the Moho reflections of about 70 km at 4–5 s reduced travel time lead to a crustal thickness of about 35 to 40 km south of Ortiz (Fig. 3B), decreasing towards the north. An intracrustal reflection, observed between 80 and 100 km distance and 2 s on the Ortiz — south record section (PiP) is associated with reflections from the top of the lower crust. The BOL13-TURV section provides information on the Moho depth, shallowing towards north (Fig. 3D). From the Ortiz record section (Fig. 3B), no clear response is observed from the crustal base in the north, whereas towards the south, crustal thickness increases, modeled by PmP reflections from Ortiz and Calabozo. The derived crustal thickness varies between 35 and 40 km beneath the Coastal Cordillera Thrust Belt and the Guárico Basin, thinning towards the Caribbean Sea in the north. An intracrustal reflection (PiP) form the Ortiz — south record section indicates the top of the lower crust at 20 km in depth. The upper layer in the model shows velocities between 4.2 and 5.2 km/s, which are interpreted as passive margin sediments of Late Cretaceous age. On land, the model presents a thinning of sedimentary thickness toward Calabozo shot in the south. The location of this structure is correlated with the position of a forebulge at Early Miocene (17 my) time (Pindell et al., 1988). This former forebulge is a response of the lithospheric loading, generated by thrust belts of the Caribbean Plate, specifically the Coastal Cordillera Thrust Belt. 4.3. Profile along longitude 65° W in eastern Venezuela In the record section of the BOL20 line, recorded at the Puerto la Cruz seismological station (BOL20-PCRV) (Fig. 4A), the Pg phase is recorded between −30 and −70 km offset, at reduced travel times between 1.5 and 3.5 s. A strong delay of about 1 s is observed in the

21

arrival times between −40 and −60 km offset, corresponding to the response of the sediments of the Tuy–Cariaco basin (see Fig. 1). PmP arrivals were identified between −55 and −130 km of offset, with the critical reflection at −55 km and 4.5 s. In the record section from the San Mateo shot, Pg can be traced until offset of 80 km and PmP arrivals are clearly recognizable to distances of up to 150 km (Fig. 4B). First arrivals were identified with an apparent velocity of up to 4.9 km/s, interpreted as sedimentary Pg, at offsets of up to 30 km to the south and −20 km to the north at reduced times between 1.5 and 3.3 s. Arrivals corresponding to the crystalline Pg phase are observed to the south of the shot point at offsets between 40 and 80 km, at times between 3.5 and 3.8 s. Various multiples, associated to the sedimentary layers of the Eastern Venezuela Basin, can be observed from shot point San Mateo to the south at distances up to 40 km and 8 s, as well as on the Cantaura north and south record sections (Fig. 4C), up to 55 km distance. PmP arrivals are observed at offsets between 80 and 150 km to the south with critical distance at 80 km and 6 s. In the record section corresponding to the Cantaura shot point, the delay in the Pg arrivals is larger to the north (up to 3.5 s at 60 km distance) than to the south (2.5 s at 75 km distance), which reflects a decrease in the sedimentary thickness towards the south. PmP reflections are only observed towards the south at distances between 80 and 100 km with the critical reflection at 80 km offset and 4.2 s, whereas no coherent signals from a Moho reflection are visible in the northern part of the section. The ray-tracing model (Fig. 4D) includes information from previous deep seismic observations in the southern part of the profile (Schmitz et al., 2002, 2005) from the shot points El Tigre, Limo and Ciudad Piar, located at the northern edge of the Guayana Shield. A thickened crust to values of 40–45 km is evidenced by observations from San Mateo — south (Fig. 4B), and Ciudad Piar — north (Schmitz et al., 2002). Reflections from the depth range 38 to 40 km from the shot points El Tigre, Limo and Ciudad Piar, previously interpreted as PmP reflections (Schmitz et al., 2005), seem to be derived from lower crustal levels. Information of the sedimentary thickness of the Eastern Venezuela Basin is provided by records from four shot points within the basin (Fig. 4D; Schmitz et al., 2005), allowing for a detailed morphology of the basin with a maximum sedimentary thickness of 13 km close to San Mateo shot. The velocities between 4.2 and 5.2 km/s are correlated with sedimentary layers. They have a thickness up to 5 km at north of Puerto la Cruz. In the deepest part of the basin (north of San Mateo shot) a thickness of 11 km is observed. This thickness decreases to the south to about 300 m at the edge of the basin (Schmitz et al., 2005). The deeper part of the thick sediments located north of San Mateo shot, which represent passive margin sequences, might be related to the Espino Graben of Jurassic age (Salazar, 2006), and to Cretaceous passive margin sediments. 4.4. Profile along longitude 64° W in eastern Venezuela In the easternmost marine record section of the BOL28 line, recorded at the Carúpano seismological station (BOL28-CRUV) (Fig. 5A), the Pg phase is observed at offsets between −50 and −105 km at reduced travel times between 0.5 and 1.6 s. Arrivals corresponding to the PmP phase were identified between −63 and −173 km of offset with the critical reflection at −63 km and 3.4 s. A 3 s delay in the arrival times of this phase is observed between −110 and −150 km, which is associated with the sediment accumulation in the eastern part of La Blanquilla Basin, where the basement is observed at a depth of about 10 km. There are some indications for Pn arrivals (refractions from the upper mantle) at distances between −255 and −280 km and reduced travel times of 7.0–7.2 s (reduction velocity 8 km/s, Fig. 5B). The resulting upper mantle velocity is 8.1 km/s, which is therefore used in eastern Venezuela for modelling upper mantle velocities.

22 M. Schmitz et al. / Tectonophysics 459 (2008) 14–26 Fig. 5. Seismic record sections and ray-tracing model from the profile in eastern Venezuela along Longitude 64° W: A and B) records from BOL28 at CRUV (for reduction velocities of 6 and 8 km/s, respectively); C) records from Jusepín shot point; D) records from Pericoco shot point. Arrivals from the upper crust (Pg), reflections from the Moho (PmP), intracrustal reflections (PiP) and arrivals from the upper mantle (Pn) can be distinguished in the record sections. A reflection (Pm ⁎ P), which is associated with a strong impedance contrast at lower crustal levels, is observed 1 s previous to the Moho reflection at the Pericoco record section; E) Observed and calculated travel times (top – different time scales for marine and land travel times), ray tracing (center) and velocity model (bottom). CRUV = Carúpano seismological station, J = Jusepín shot point, P = Pericoco shot point.

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

23

Fig. 5 (continued ).

In the land record section of the Jusepín shot, Pg arrivals are clearly observed toward the north at offsets up to −70 km and reduced travel times up to 3 s (Fig. 5C). Towards the south, the Pg phase is identified at distances up to 50 km, with an even higher delay of up to 4.2 s. This phase has an apparent velocity of 6.4 km/s and an additional delay of 1.2 s (with respect to the arrivals from the Serranía del Interior) that could be related to the sediments of the Maturin Sub-Basin (see Fig. 1 for location). Towards the north, an intracrustal phase (PiP) is identified with a good signal/noise ratio at offsets between −63 and −73 km and 5 s, and a PmP phase with a critical distance of −63 km at a reduced travel time of 7.0 s. In the land record section of the Pericoco shot (Fig. 5D), the arrivals of the Pg phase are identified at offsets up to 130 km at reduced times between 2 and 3.9 s. A 1.5 s delay is observed between −10 and −50 km offset, which is associated with the sediments of the Maturin Sub-Basin (same as on the reversed record section). The PmP phase is clearly observed from offsets −110 to −150 km, with the critical reflection at −105 km and 7.5 s. About 1 s earlier than this PmP reflection, a reflection with less amplitude is observed in the distance range −105 to −150 km and reduced times between 2.8 and 5.8 s. This phase, with a critical distance of −105 km, is interpreted as a reflection from lower crustal levels and is called here Pm ⁎ P (Fig. 5D). As derived from the ray-tracing model (Fig. 5E), the BOL28-CRUV section provides information on the Moho depth, as well as on the sediment thickness of up to 10 km of the eastern part of La Blanquilla Basin. Another interesting result is the reflection from the

lower crustal levels, derived from the Pm ⁎ P phase observed in Pericoco land section. The derived velocity model indicates crustal thickness to the south with about 50 km, decreasing to 40 km beneath CRUV and only slightly thinning towards the Caribbean Sea in the north. On land, the information from the two seismic sections helps determine the velocity structure of the basement and sediment thickness of Eastern Venezuela Basin, and to define a crustal thickening towards the Guayana Shield, observed on the Pericoco land section. 5. Discussion of crustal thickness in northern Venezuela The crustal models derived from the four main north–south profiles, together with results from previous deep seismic refraction studies for the east coast of Maracaibo Lake (COLM; Gajardo et al., 1986), the central offshore area (Mar and Tierra; Guédez, 2003), the Eastern Basin (ECCO; Schmitz et al., 2005) and the Guayana Shield (ECOGUAY; Schmitz et al., 2002) (for location, see Fig. 1), allow to derive a map of the crustal thickness, covering large areas of northern Venezuela, up to the Orinoco River (Fig. 6). In general, the crustal thickness in Venezuela decreases from about 40 km south of the Caribbean Mountain System (composed by Coastal Cordillera Thrust Belt and Serranía del Interior Thrust Belt), to less than 35 km along the coastline. Values of more than 35 km at the coast are derived only in the central Coastal Cordillera Thrust Belt (Guédez, 2003) and in western Venezuela. A pronounced crustal thinning is observed

24

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

under the Falcón Basin along the profile at 70° W (Figs. 2 and 6), which extends eastward into the Bonaire Basin, as documented by PmP reflections derived from land shots, and observations of the air gun blasts on the stations of the Venezuelan seismological network (Bezada et al., 2008). The crustal thinning under the Falcón Basin, interpreted from the seismic data, is consistent with previous interpretations based on gravimetric modelling along the same profile (Rodríguez and Sousa, 2003). This thinning dates back to the Oligocene–Miocene boundary, as documented by basaltic intrusions (Muessig, 1978; McMahon, 2001), and has been explained by extensional subsidence (Audemard, 1995). In the Falcón Basin, Oligocene–Miocene sediments show P-wave velocities of 3.0 to 3.4 km/s and early Eocene sediments show velocities of 4.1– 5.0 km/s. An interface between the upper and lower crust is evidenced by the intracrustal phase PiP (reflection) observed in the Barquisimeto section (Fig. 2C). Nevertheless, intracrustral phases are not present in any other section in this area. Considerable crustal thickness is observed in the southern part of the Caribbean Sea with values ranging from 25 to 35 km. The main sedimentary basins along the profiles are clearly evidenced by delays in the first arrivals and can be modeled along all the profiles. North of the Leeward Antilles, crustal thickness decreases to 15–20 km, evidenced by seismic refraction data (e.g. Edgar et al., 1971; Case et al., 1990; Diebold et al., 1999), which is still thicker than average oceanic crust (Mooney et al., 1998). The maximum crustal thickness of 30– 35 km, observed in the east along the profile at longitude 64° W, is

attributed to the structure of the Paleo-Antillean volcanic arc, the Aves Ridge. In the southeast of the Eastern Venezuela Basin, there is strong evidence for a crustal thickening from 45 to more than 50 km in depth, located east of longitude 65° W. This observation is based on late arrivals from Moho reflections (Figs. 4D and 5E) along the profiles at longitudes 65° W and 64° W. Deep phases along the two profiles give hints of a layered lower crust, interpreted as a pre-existing Moho discontinuity between 35 and 40 km in depth, evidenced by weak reflections in the Limo and El Tigre sections along profile 65° W (Schmitz et al., 2005; Fig. 4), and in the Pericoco section along profile 64° W (Fig. 5). Lower crustal velocities are modelled with 7.0–7.2 km/s, based on observations from the northern edge of the Guayana Shield (Schmitz et al., 2002). PmP reflections from deeper levels are observed on the San Mateo — south (Fig. 4B) and Ciudad Piar — north (Schmitz et al., 2002) record sections along profile 65° W, and at the Pericoco — north (Fig. 5D) record section along profile 64° W, resulting in a Moho depth of more than 50 km beneath the Maturín Basin. The deeper part of the thickened crust might be considered lower crustal material, underplated during Caribbean — South America plate interactions. Candidates for the underplated material might be material from processes related to the subduction of the South America continental plate beneath the Caribbean (e.g. Russo and Speed, 1994; Jácome et al., 2003), subducting Atlantic slab in the north east (e.g. Molnar and Sykes, 1969), or from the Paleo-Caribbean plate subducted

Fig. 6. Map of crustal thickness in northern Venezuela derived from seismic wide angle observations. The surface gridding algorithm (Wessel and Smith, 1991; tension factor T = 0.07) is used for the interpolation of Moho depths derived from the deep seismic observations. The bold lines indicate the portion of the profiles that are modelled with Moho reflections. For location of shot points and recording lines see Fig. 1; M = Maracaíbo, C = Cacacas, B = Barcelona, CG = Ciudad Guayana.

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

from the north (J. Pindell, personal communication). Detailed information on upper mantle structures will be available within the near future from seismological observations (Levander et al., 2006). The results will hopefully help to understand the complex configuration of subducting slabs in the area and their implication in the geodynamics of the region. Deep seismic reflections at 12.7 s from an experiment along a 6 km long line done in 1991 (about 50 km south of Pericoco shot point of profile 64° W; see Fig. 1) are interpreted as Moho reflections (Marquez et al., 1992). Nevertheless, considering the features obtained in this study, these reflections might correspond to lower crust interface or the pre-existing Moho discontinuity observed in the deep seismic sections (Figs. 4 and 5). 200 km further south, a crustal thickness of 45 km is observed on the northern edge of the precambrian Guayana Shield (Schmitz et al., 2002). 6. Conclusions The deep seismic refraction data presented in this paper allow us to model the main crustal features of northern Venezuela. Seismic data were acquired in 2004 within the framework of the BOLIVAR and GEODINOS projects along the plate boundary zone between the South American continent and the Caribbean plate. These data were obtained along four main north–south profiles, located approximately from west to east at longitudes 70° W, 67° W, 65° W and 64° W. Along each profile, two land shots were recorded on the portable stations deployed along 150–200 km long lines and air gun line shots were recorded on the stations of the Venezuelan Seismological Network, close to the coastline. Additionally, results from previous deep seismic observations (Gajardo et al., 1986; Schmitz et al., 2002; Guédez, 2003; Schmitz et al., 2005) were integrated to these data generating a low resolution map of crustal thickness covering great part of northern Venezuela (Fig. 6). The principal results along the four north-south profiles can be given as follows: 6.1. The 70° W profile in western Venezuela The section generated by BOL3 line and seismological station MONV provides information on the Moho depths and the velocity structure of the Aruba–Curaçao Basin. Another important structure revealed by this section is the subducting Caribbean slab, which is evidenced by reflections from about 50 km depth beneath Aruba (Fig. 2A and D). The two sections from the Aracua and Barquisimeto land shots allow us to determine the velocity structure of the Falcón Basin and the respective Moho topography. They provide seismic evidence for the crustal thinning beneath this basin (Bezada et al., 2008), which is consistent with previous interpretations based on gravimetric modelling along the same profile (Rodríguez and Sousa, 2003). 6.2. The 67° W profile in central Venezuela In the central Profile 67° W (Fig. 1), we observe very little variation in crustal thickness along the continental part of the profile (Fig. 3), because no good resolution is obtained beneath the Coastal Cordillera Thrust Belt (northern part of the Ortiz shot record). The sediments of the Bonaire Basin can be identified in the observations of line BOL13 at station TURV. Thin sedimentary coverage in the Guárico Basin, observed in the Calabozo north–south record sections, is interpreted as effects of a Miocene forebulge in that area (Pindell and Barrett, 1990). 6.3. The 65° W and 64° W profiles in eastern Venezuela The seismic line BOL20, recorded at station PCRV (Figs. 1 and 4) provides information on the Moho depth at Serranía del Interior Thrust Belt and the velocity structure of the Tuy–Cariaco Basin, north of PCRV. Further east, the observations from BOL28 at CRUV allow us to determine the depth of the La Blanquilla Basin to about 10 km (Fig. 5).

25

Pg arrivals from the land shots San Mateo and Cantaura, together with previous records (Schmitz et al., 2005) along profile 65° W, help to determine the velocity structure and sediment thickness of the Maturín Basin, more specifically the sediments deposited in the Espino Graben (Salazar, 2006). Along profile 64° W, differences in the Pg arrivals from the land shots Jusepín and Pericoco (Figs. 1 and 5) indicate the transition from the Serranía del Interior passive margin sediments in the north (small delay) to the Neogene sediments of the Maturín Basin further south (big delay). The seismic sections from the land shots at the two easternmost profiles (65° W and 64° W) provide information about a crustal thickening, proven by arrivals from the lower crust and the Moho discontinuity (San Mateo and Pericoco shot points). In the eastern part of the Eastern Venezuela Basin, crustal thickness reaches up to 50 km, with high velocity lower crust, which is interpreted as underplated lower crustal and upper mantle material. Acknowledgements We are thankful for the contribution of volunteers, other field personnel from various US and Venezuelan universities and institutions (FUNVISIS, USB, UCV, PDVSA, DHN, Rice University), as well staff of IRIS-PASSCAL Instrument Centre (M. Fort, B. Greschke, E. Gutierrez, P. Miller, W. Zamora), and the crews from the research vessels R/V Maurice Ewing and R/V Seward Johnson II, for the realization of the seismic measurements. We thank the local communities and land owners for the support and permit for installation of recording equipment and drilling and blasting of land shots. We acknowledge the contributions of G. Housepian (Geointer) and L. Pregitzer (CAVIM) and their respective crews for drilling and blasting, respectively. Recording instruments at land were provided by IRIS Instrument Centre, New Mexico, which is gratefully acknowledged. We thank J.E. Soares and E. Flüh for detailed reviews, which helped to improve considerably the quality of the paper. Further members of the BOLIVAR active seismic working group are: D. Sawyer, (Rice); V. Rocabado, J. Sánchez (FUNVISIS); G. Christeson, P. Mann, A. Escalona (UTIG); N. Nevado (PDVSA-INTEVEP). Contribution to projects FONACIT G-2002000478, PDVSA-INTEVEP — FUNVISIS — 04–141, and NSF — Continental Dynamics Program grant EAR-003572. References Audemard, 1995. La Cuenca Terciaria de Falcón. Venezuela Noroccidental: Síntesis Estratigráfica, Génesis en Inversión Tectónica, IX Congreso Latinoamericano de Geología, Caracas, Venezuela (in digital format). Audemard, F., Giraldo, C., 1997. Desplazamientos dextrales a lo largo de la frontera meridional de la Placa Caribe, Venezuela Septentrional. VIII Congreso Geológico Venezolano, Tomo I, Maracaibo, pp. 101–108. Audemard, F.A., F.A.udemard, M.F.A., Machete, M.N., Cox, J.W., Dart, R.L., Haller, K.M., 2000. Map and database of Quaternary faults in Venezuela and its offshore regions. US Geological Survey, Open-file report 00-018, 72 pp. + map. Barckhausen, U., Roerser, H., Huene, R., 1998. Magnetic signatures of upper plate structures and subduction seamounts at the convergent margin of Costa Rica. J. Geophys. Res. 103, 7079–7093. Beck, C., 1986. Caribbean colliding. Andean drifting and the Mesozoic–Cenozoic geodynamic evolution of the Caribbean, VI Congreso Geológico Venezolano, Caracas, Venezuela, 10, pp. 163–182. Bellizzia, A., 1986. Sistema Montañoso del Caribe, una cordillera alóctona en la parte norte de América del Sur. VI Congreso Geológico Venezolano, Caracas, Venezuela, 10, pp. 6657–6836. Bezada, M., Schmitz, M., Jácome, M.I., Rodríguez, J., Audemard, F., Izarra, C. and the BOLIVAR Active Seismic Working Group, 2008. Crustal structure in the Falcón Basin Area, Northwestern Venezuela, from seismic and gravimetric evidence. J. Geodyn. 45, 191–200. doi:10.1016/j.jog.2007.11.002. Biju-Duval, B., Mascle, A., Montadert, L., Wanneson, J., 1978. Seismic investigations in Colombia, Venezuela and Grenada Basin, and on the Barbados Ridge for future IPOD drilling. Geol. Mijnb. 57, 105–116. Boesi, T., Goddard, D., 1991. A new geologic model related to the distribution of hydrocarbon source rocks in the Falcón Basin, Northwestern Venezuela. In: Biddle, K.T. (Ed.), Active Margin Basins. American Asociation of Petroleum Geologists Memoir, 52, pp. 303–319. Tulsa, USA. Burke, K., Cooper, C., Dewey, L.F., Mann, P., Pindell, J., 1984. Caribbean tectonics and relative plate motions. In: Bonini, W.E., Hargraves, R.B., Shagan, R. (Eds.), The

26

M. Schmitz et al. / Tectonophysics 459 (2008) 14–26

Caribbean–South American Plate Boundary and Regional Tectonics. Geological Society of America, Boulder Colorado, pp. 31–61. M.162. Case, J.E., MacDonald, W., Fox, P.J., 1990. Caribbean crustal provinces, seismic and gravity. In: Dengo, G., Case, J.E. (Eds.), The Caribbean Region. Geological Society of America, Boulder Colorado, pp. 15–36. H. Cohen, J., Stockwell, J., 1994. The user's manual center for wave phenomena. Colorado School of Mines. 126 pp. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1994. Effects of recent revisions to the geomagnetic time scale on estimates of current plate motions. Geophys. Res. Lett. 21, 2191–2194. Diebold, J.B., Stoffa, P.L., Buhl, P., Truchan, M., 1981. Venezuela basin crustal structure. J. Geophys. Res. 86, 7901–7923. Diebold, J.B., Driscoll, N., EW-9501 Science Team, 1999. New insights on the formation of the Caribbean basalt province revealed by multichanel seismic images of volcanic structures in the Venezuelan Basin. In: Mann, P. (Ed.), Caribbean Basins. Sedimentary basins of the world, Amsterdam, pp. 561–589. Donnelly, T.W., 1985. Mesozoic and Cenozoic plate evolution of the Caribbean region. In: Stehli, F.G., Webb, S.D. (Eds.), The Great American Biotic Interchange, pp. 89–121. Donnelly, T.W., 1994. The Caribbean Sea Floor. In: Donovan, K., Jackson, T.A. (Eds.), Caribbean Geology, and Introduction, pp. 41–64. Donnelly, T.W., Beets, D., Carr, M., Jackson, T., Klaver, G., Lewis, J., Maury, R., Schellenkens, H., Smith, A., Wadge, G., Westercamp, D., 1990. History and tectonic setting of Caribbean magmatism. In: Dengo, G., Case, J.E. (Eds.), The Caribbean Region. Geol. Soc. Amer., Boulder, Colorado, pp. 339–374. H. Edgar, N.T., Ewing, J.I., Hennion, J., 1971. Seismic refraction and reflection in the Caribbean Sea. Am. Assoc. Petrol. Geol. Bull. 55, 833–870. Erlich, R.N., Barrett, S.F., 1990. Cenozoic plate tectonic history of the northern Venezuela–Trinidad area. Tectonics 9, 161–184. Feo-Codecido, G., Smith, F.D., Aboud, N., Di Giacomo, E., 1984. Basement and Paleozoic rocks of the Venezuela Llanos Basin. In: Bonini, W.E., Hargraves, R.B., Shagan, R. (Eds.), The Caribbean–South American Plate Boundary and Regional Tectonics. Geol. Soc. Amer., Boulder, Colorado, pp. 175–187. M.162. Gajardo, E., Nicolle, J.L., Castejon, B., Marquez, C., Urbáez, M., 1986. Modelo de corteza en la Costa Oriental del Lago de Maracaibo. III Congr. Venez. de Geofísica, Caracas, pp. 102–111. Giese, P., 1976. Depth calculation. In: Giese, P., Prodehl, C., Stein, A. (Eds.), Explosion Seismology in Central Europe — Data and Results. Springer, Berlin, pp. 146–161. Giunta, G., Marroni, M., Padoa, E., Pandolfi, L., 2003. Geological constraints for the geodynamic evolution of the southern margin of the Caribbean plate. In: Bartolini, C., Buffer, R.T., Blickwede, J. (Eds.), The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon habitats, basin formation, and plate tectonics. Amer. Ass. Petrol. Geol., M 79, pp. 104–125. González de Juana, C., Arozena, J., Picard Cadillat, X., 1980. In: Foninves (Ed.), Geología de Venezuela y de sus Cuencas Petrolíferas, 2, pp. 95–994. Caracas. Guédez, R., 2003. Estudio cortical en el área centro-norte y noroccidental de Venezuela a partir de datos de sísmica de refracción. Undergraduate thesis, Universidad Central de Venezuela, Venezuela, 124 p. Jácome, M.I., Kusznir, N., Audemard, F., Flint, S., 2003. The formation of the Maturín Foreland Basin, Eastern Venezuela: thrust sheet loading or subduction dynamic topography. Tectonics 22, 1–17. Jácome, M.I., Rondón, K., Schmitz, M., Izarra, C., Viera, E., 2008. Integrated seismic, flexural and gravimetric modelling of the Coastal Cordillera Thrust Belt and the Guárico Basin: North-Central Region, Venezuela. Tectonophysics 459, 27–37 (this volume). doi:10.1016/j.tecto.2008.03.008. James, K., 2006. Arguments for and against the Pacific origin of the Caribbean Plate: discussion, finding for an inter-American origin. Geol. Acta 4, 279–302. Kerr, A., Tarney, J., 2005. Tectonic evolution of the Caribbean and northwestern South America: the case for accretion of two Late Cretaceous oceanic plateaus. Geology 33, 269–272. Levander, A., Schmitz, M., Avé Lallemant, H.G., Zelt, C.A., Sawyer, D.S., Magnani, M.B., Mann, P., Christeson, G., Wright, J., Pavlis, D., Pindell, J., 2006. Evolution of the Southern Caribbean Plate Boundary. EOS 87 (Nr. 9), 97, 100. Lilliu, A., 1990. Geophysical interpretation of Maturín Foreland, Northeastern Venezuela, Msc. Thesis, Houston University, Houston, Texas, USA. 124 pp. Macellari, C.E., 1995. Cenozoic Sedimentation and Tectonics of the Southwestern Caribbean pull-apart Basins of South America. Memoir, 62. American Association of Petroleum Geologist, pp. 757–780. Mann, P., Schubert, C., Burke, K., 1990. Review of Caribbean neotectonics. In: Dengo, G., Case, J.E. (Eds.), The Caribbean Region. Geol. Soc. Amer., Boulder, Colorado, pp. 307–338. H. Marquez, C., De Almeida, M., Capello, M.A., Lugo, L., Yibirin, R., Del Pino, E., Hernández, A., Lilliu, A., Acosta, J., Passalacqua, H., 1992. Determinación de eventos sísmicos profundos en la Cuenca Oriental de Venezuela: pruebas experimentales. VI Congreso Venezolano de Geofísica, Caracas, Memorias, pp. 353–359. McMahon, C., 2001. Evaluation of the effects of oblique collision between the Caribbean and South American plates using geochemistry from igneous and metamorphic bodies of northern Venezuela. Ph.D. thesis, University of Notre Dame, USA, 227 pp. Meschede, M., 1998. The impossible Galapagos connection: geometric constrains for a near-American origin of the Caribbean Plate. Geol. Rundsch. 87, 200–205. Meschede, M., Frisch, W., 1998. A plate-tectonic model for the Mesozoic and Early Cenozoic history of the Caribbean Plate. Tectonophysics 296, 269–291.

Molnar, P., Sykes, L., 1969. Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity. Geol. Soc. Amer. Bull. 80, 639–1684. Mooney, W.D., Laske, G., Masters, G., 1998. CRUST 5.1: a global crustal model at 5° × 5°. J. Geophys. Res. 103, 727–747. Muessig, K., 1978. The Central Falcón Igneous Suite, Venezuela: Alkaline Basalt intrusions of Oligocene–Miocene Age. Geol. Mijnb. 52, 261–266. Muessig, K., 1984. Structure and Cenozoic tectonics of the Falcón Basin, Venezuela and adjacent areas. Geol. Soc. Amer. Mem. 162, 217–230. Navarro, E., Ostos, M., Yoris, F., 1988. Revisisón y definición de unidades litoestratigráficas y síntesis de un modelado tectónico para la evolución de la parte NorteCentral de Venezuela durante el Jurásico Medio-Paleógeno. Acta Cient. Venez. 39, 427–436. Officer, C.B., Ewing, J.I., Hennion, J.F., Harkrider, D.G., Miller, D.E., 1959. Geophysical investigations in the Eastern Caribbean; summary of 1955 and 1956 cruises. In: Ahrens, L.H., Press, F., Rankama, K., Runcorn, S.K. (Eds.), Physics and Chemistry of the Earth, 3, pp. 17–109. Ostos, M., 1990. Tectonic evolution of the south-central Caribbean based on geochemical and structural data. Ph.D. Thesis: Houston, Rice University, 411 pp. Pérez, O.J., Aggarwal, Y.P., 1981. Present-day tectonics of the southeastern Caribbean and northeastern Venezuela. J. Geophys. Res. 86, 10791–10804. Pindell, J.L., Dewey, J.F., 1982. Permo-Triassic reconstruction of western Pangea and the evolution of the Gulf of Mexico/Caribbean region. Tectonics 1, 179–211. Pindell, J.L., Barrett, S.F., 1990. Geological evolution of the Caribbean region; a plate tectonic perspective. In: Dengo, G., Case, J.E. (Eds.), The Geology of North America: The Caribbean Region. Boulder, vol. H. Geological Society of America, pp. 405–432. Pindell, J., Kennan, L., 2001. Kinematic evolution of the Gulf of Mexico and Caribbean, Gulf Coast. Asociation of Geological Societies Conference. Pindell, M.I., Cande, S.C., Pitman III, W.C., Rowley, D.B., Dewey, J.F., LaBrecque, J., Haxby, W., 1988. A plate-kinematic framework for models of Caribbean evolution. Tectonophysics 155, 121–138. Rodríguez, J., Sousa, J.C., 2003. Estudio geológico-estructural y geofísico de la sección cabo San Román-Barquisimeto, estados Falcón y Lara. Undergraduate thesis, Universidad Central de Venezuela, Venezuela. Rossi, T., 1985. Contribution a l'etude geologique de la frontiere Sud-Est de la plaque Caraibes Etude geologique de la Serranía, La Serranía del Interior Oriental (Venezuela) sur le transect Cariaco-Maturin, Syntheses Paleogeographique et Geodynamique, Ph.D. Thesis, Universite de Bretagne Occidentale, France. 340 pp. Roure, F., Carnevali, J.O., Gou, Y., Subieta, T., 1994. Geometry and kinematics of the North Monagas thrust belt (Venezuela). Mar. Petrol. Geol. 11, 347–362. Russo, R.M., Speed, R.C., 1994. Spectral analysis of gravity anomalies and the architecture of tectonic wedging, NE Venezuela and Trinidad. Tectonics 13, 613–622. Salazar, M.B., 2006. Evolución estructural e implicaciones tectónicas del Graben de Espino. Master thesis, Universidad Simón Bolívar, Caracas, Venezuela, 197 pp. Schmitz, M., Chalbaud, D., Castillo, J., Izarra, C., 2002. The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data. Tectonophysics 345, 103–118. Schmitz, M., Martins, A., Izarra, C., Jácome, M.I., Sánchez, J., Rocabado, V., 2005. The major features of the crustal structure in north-eastern Venezuela from deep wideangle seismic observations and gravity modelling. Tectonophysics 399, 109–124. doi:10.1016/j.tecto.2004.12.018. Schubert, C., 1984. Basin formation along the Bocono–Moron–El Pilar fault system, Venezuela. J. Geophys. Res. 89, 5711–5718. Smith, W.H.F., Sandwell, D.T., 1997. Global seafloor topography from satellite altimetry and ship depth soundings. Science 277, 1957–1962. Stephan, J.F., 1985. Andes et Chaine sur la Transversale de Barquisimeto(Venezuela). In: Mascle, A. (Ed.), Evolución geodynamique. Symposium geodynamique des Caraibes, pp. 503–529. Paris. Stephan, J.F., De Lepinay, B.M., Calais, E., Tardy, M., Beck, C., Carfantan, J.C., Olivet, J.L., Vila, J.M., Bouysse, P., Mauffret, A., Borgois, J., Thery, J.M., Tournon, J., Blanchet, R., Decourt, J., 1990. Paleogeodynamic maps of the Caribbean: 14 steps from Lias to Present. Bull. Geol. Soc. France 8, 915–919. Sykes, L.R., McCann, W.R., Kafka, A.L., 1982. Motion of Caribbean Plate during last 7 million years and implications for earlier Cenozoic movements. J. Geophys. Res. 87, 10656–10676. Van der Hilst, R., Mann, P., 1994. Tectonic implications of tomographic images of subducted lithosphere beneath northwestern South America. Geology 22, 451–454. Weber, J.C., Dixon, T.H., DeMets, C., A., Ambeh, W.B., Jansma, P., Mattioli, G., Saleh, J., Sella, G., Bilham, R., Pérez, O., 2001. GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela. Geology 29, 75–78. Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data. EOS Trans. AGU 72 (441), 445–446. Westbrook, G.K., McCann, W.R., 1986. Subduction of Atlantic Lithosphere beneath the Caribbean. In: Vogt, P.R., Tucholke, B.E. (Eds.), The Western North American Region: Boulder Colorado. Geol. Soc. Amer., The Geology of North America, Boulder, Colorado, M. , pp. 341–350. Ysaccis, R., Cabrera, E., Del Castillo, H., 2000. El sistema petrolífero de la cuenca de la Blanquilla, costa afuera Venezuela. VII Simposio Bolivariano de Exploración Petrolera en Cuencas Subandinas, Caracas, pp. 411–425. Zelt, C.A., Smith, R.B., 1992. Seismic traveltime inversion for 2-D crustal velocity structure. Geophys. J. Int. 108, 16–34.