C h a p t e r 21

Neogene Intraplate Deformation of the Caribbean Plate at the Beata Ridge

A L A I N M A U F F R E T and S Y L V I E L E R O Y

We have studied all the seismic profiles existing in the Beata ridge area, in addition to Seabeam maps, to determine the tectonics of this structure and its relationships with the adjacent areas. The basis of this work is the multichannel Casis seismic survey carried out by the R/V Nadir in 1992. These seismic lines are migrated and clearly show evidence of compression and transpression. The Presqu'~le du Sud d'Hispaniola is an uplifted part of the volcanic igneous province that formed the Caribbean plate during Cretaceous time. This region was initially a part of the thick Beata volcanic plateau that collided with the central part of Hispaniola. A Seabeam map and single-channel seismic lines from the Seacarib 1 cruise show the active collision between the northeastern tip of Beata ridge and the western termination of the Muertos trough. Structural analysis indicates that the Muertos trough is an Eocene feature that has been reactivated in Recent times. The progressive emergence of the Muertos prism and its onland extension results from vertical stacking by thrusting of different parts of the prism. Several compressional structures can be seen on the eastern flank of the Beata ridge. The importance of these structures decreases systematically towards the south. One of these structures, the Taino ridge, was surveyed in detail during the Casis cruise. We describe reverse faults, pop-up and strike-slip faults. These tectonic features are compatible with a NE-SW compressive stress. A detailed site survey of the Aruba Gap was also performed during the Casis cruise. We show again compressional and wrench faults and an increase of the deformation towards the north. In contrast, we found no evidence of any compressional deformation on the western side of the Beata ridge. Here, steep NE-SW scarps shown by Seabeam maps, are predominant. We conclude from this tectonic framework that the Beata ridge has been deformed by compression and strike-slip faulting since the Early Miocene (23 Ma) by a NE-SW-oriented compressive stress. The Beata ridge is progressively uplifted from the south to the north up to the emergence of the Presqu'~le du Sud. However, the Beata ridge is a Cretaceous plateau and initial topography must be taken into account. The Beata ridge is placed in the regional tectonic framework and we show that the compression of the ridge is probably connected to the Sinu subduction zone in Colombia. We distinguish between the Colombian and the Venezuelan microplates separated by the Beata compressional zone. The former drifts towards the northeast faster than the latter. From our structural analysis we deduce 0.9 cm/yr of relative motion between the two plates.

INTRODUCTION The Caribbean plate is a large volcanic province probably formed, during the Cretaceous, in the Pacific O c e a n (Duncan and Hargraves, 1984). This plate is m o v i n g eastwards relative to the North A m e r i c a n ( N O A M ) and South A m e r i c a n ( S O A M ) plates (Pindell and Barrett, 1990). The C a r i b b e a n plate is d e l i m i t e d (Fig. 1) to the north by a left-lateral strike-slip fault zone ( C a y m a n - P u e r t o Rico fault system) and to the south by a c o m p l e x set of rightlateral faults. In addition, the N O A M and S O A M plates are slowly converging with a present pole of rotation located near the M i d - A t l a n t i c ridge (Pindell and Barrett, 1990; Mtiller and Smith, 1993; Mtiller et al., 1996). S e d i m e n t a r y d e f o r m e d belts (Ladd

and Watkins, 1978; L a d d et al., 1981, 1984, 1990) north of South A m e r i c a ( C o l o m b i a n and Venezuelan d e f o r m e d belts, Fig. 1) and south of Puerto Rico (Muertos trench, Fig. 1) m a y result from this c o m p r e s s i o n which increases towards the west. The D S D P results of Leg 15 (Edgar et al., 1973b) and the O D P results of L e g 165 (Scientific Party, Leg 165, 1996) indicated that the volcanic b a s e m e n t of the C a r i b b e a n plate was f o r m e d during a short period of the Cretaceous time (late Turonian to Campanian, 8 8 - 7 4 Ma). The same volcanic rocks outcrop in the Presqu'~le du Sud and B a h o r u c o Peninsula of Hispaniola (Maurasse et al., 1979) and Curaqao Island (Klaver, 1987) and these areas are considered to be uplifted pieces of the C a r i b b e a n plateau. In the central part of the C a r i b b e a n Sea, the Beata ridge,

Caribbean Basins. Sedimentary Basins of the World, 4 edited by E Mann (Series Editor: K.J. Hsti), pp. 627-669. 9 1999 Elsevier Science B.V., Amsterdam. All rights reserved.

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A. MAUFFRET and S. LEROY

Fig. 1. Plate tectonic frameworkof the Caribbean plate. The Gonave microplate (Rosencrantz and Mann, 1991) is indicated.

2-4 km deep, has a triangular shape and lies (Fig. 2) between the Colombian basin to the west and the Venezuelan basin to the east. The northern parts of the Colombian and Venezuelan basins have been named the Haiti and Dominican sub-basins, respectively, and we gave new names (from Indian tribes) to the main ridges that composed the Beata ridge. The Bahoruco Peninsula is the northern prolongation of the Beata ridge, whereas this ridge is separated from the deformed margin of the South American plate by the Aruba Gap. Many speculative models have been proposed for the formation of the Beata ridge: normal faulting (Fox et al., 1970; Fox and Heezen, 1975; Holcombe et al., 1990); a buoyant thick oceanic plateau which resists subduction (Burke et al., 1978) or reverse faulting related to a transpressive motion (Vitali, 1985; Mauffret et al., 1994). New multichannel seismic profiles, acquired during the Casis cruise performed in 1992 on the R/V Nadir in the Caribbean Sea (Fig. 2), demonstrate strong transpressive tectonics of a former volcanic plateau (Mauffret et al., 1994; Leroy, 1995; Leroy and Mauffret, 1996). The Beata ridge was studied in the framework of a comprehensive work on the geophysical and geological data of the Caribbean Sea (Leroy, 1995) to promote an ODP Leg on the deep structure of the Caribbean igneous province.

BATHYMETRY OF THE CENTRAL CARIBBEAN BASIN

Since 1980 (Case and Holcombe, 1980) no new bathymetric map of this region has been published. In 1985 a Seabeam survey was performed during the French Seacarib cruise (1985, R/V Charcot). Some data were published (Mercier de Lepinay et al., 1988; Jany et al., 1990; Mauffret and Jany, 1990) but the surveys of the Beata ridge as a whole are presented for the first time in this paper (Fig. 3). These data are integrated in the new map presented here (Fig. 3) that includes a compilation of previous data provided by the Marine Geophysical Data Center (Leroy, 1995). The Muertos trough lies in the northern part of the central Caribbean region between 5 and 4 km deep. This depression is delimited towards the north by a deformed slope. Towards the west the Muertos trough undergoes a prominent bend then disappears near Hispaniola. The Bahoruco Peninsula is bounded towards the east by a steep scarp from the coast to 3 km deep. The deep part of the scarp and the continental rise, below 3 km deep, are interrupted by several seamounts that trend north-south. The southern tip of the Bahoruco Peninsula extends southward by a spur from the coast to 2 km depth. The Beata ridge is bounded towards the northwest by a steep scarp that trends NE-SW. At the foot of

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

629

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Tairona Ridge,

Dominicansub basin 31 TainoRidge DSDP 151 153 Warao Rise

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Colombia Basin

Fig. 2. The northern parts of the Colombian and Venezuelan basins are renamed the Haiti and Dominican sub-basins, respectively. The Warao rise is a buried feature that bounds the Haiti sub-basin to the south. The backbone of the Beata ridge is formed by the DSDP 151 and Tairona ridges. Warao, Taino and Tairona are the names of Indian tribes. The position of the DSDP Sites (Legs 4 and 13) and ODP Sites (Leg 165) are indicated. The seismic tracks of the Casis cruise is also shown.

the scarp lies the Haiti sub-basin that is surrounded by the 4.2 km bathymetric contour. This basin is delimited towards the west by the Hess escarpment and towards the north by the steep continental slope of Haiti and the Haiti plateau that trends N W - S E . The eastern boundary of the Beata ridge with the Venezuelan basin is subdued, but two prominent features, the Taino ridge 3 to 4 km deep, and the Beata plateau surrounded by the 4-km-bathymetric contour outline the southeastern limit of the ridge. The central part of the Beata ridge, delineated by the 3-km-bathymetric contour, is formed by two prominent features: Tairona and DSDP 151 ridges that trend north-south. The Aruba Gap, 4 km deep, is located between the Beata ridge and the South

American deformed belt and forms a sill between the Colombian and Venezuelan basins.

SEISMIC CONTROLS In addition to the Casis cruise we examined all the MCS (multichannel seismic profiles) and SCS (single-channel seismic profiles): from the University of Texas at Austin, Shell, Institut Franqais du Pdtrole for the MCS profiles; from the Lamont Doherty Earth Sciences Observatory (Vema, R / V Conrad data and the monitors of the Ewing 9501 cruise); Texas A & M (Alamino cruise) and the Seacarib 1 cruise (SCS profiles) (Fig. 4). All these

630

A. MAUFFRET and S. LEROY

Fig. 3. New bathymetric map of the central part of the Caribbean Sea (from Leroy, 1995, modified). The Seacarib 1 Seabeam surveys (position indicated) were incorporated to the conventionalbathymetric data base (NGDC).

profiles were digitized and converted in depth with a velocity curve derived from DSDP results and velocity analysis of the Casis data.

SEISMIC STRATIGRAPHY

The seismic stratigraphy of the central Venezuelan basin was defined in the early work of Ladd and Watkins (1980). The DSDP results reveal the presence of four main seismic intervals (Fig. 5). An upper seismic interval in Hole 31 (Bader et al., 1970) and Holes 146 and 153 (Edgar et al., 1973b), corresponds to lithic unit 1, a chalk marl ooze and clay of Early Miocene to Recent age (eM, 23 Ma; Fig. 5). The second seismic interval corresponds to a Middle Eocene to Early Miocene radiolarian chalk and radiolarian chalk unit. Horizon A" (Fig. 5) forms the

base of this interval. The third seismic interval corresponds to lithified chalks, cherts, limestones and black shales which rest upon Santonian to Coniacian basalts. Horizon B" correlates with these basalts. In addition, we have identified in the Aruba Gap area a lower sedimentary unit between an equivalent of B" and a deeper horizon (V, Fig. 5B). In the volcanic crust several reflectors have been identified (sub-B" R reflectors and Moho, Fig. 5A), but a study of the deep crust is beyond the scope of this paper (Mauffret and Leroy, 1997). The upper seismic interval has a variable thickness, relatively thin in the flank of the Beata ridge, and increasing to 2 km thick in the southern part of the Venezuelan and Colombian basins where it fills a trench related to the South American deformed belt (Talwani et al., 1977; Biju Duval et al., 1982b). In the Colombian basin, the layer of Early Miocene to

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

631

Fig. 4. Seismic tracks used in this study.

recent sediments is thick in relation with the Magdalena deep-sea fan (Kolla et al., 1984). The DSDP Sites and the seismic profiles indicate that this interval consists of turbiditic sediments in basins, but a pelagic composition is inferred on the Beata ridge. The second seismic interval corresponds to a pelagic unit. The upper part, and in some places the entire interval (Fig. 5B), is chaotic. This layer is current-controlled, as confirmed by the presence

of several hiatuses in the DSDP holes that have been related to strong Early Miocene currents which were active as the Caribbean Sea opened towards the Pacific Ocean (Edgar et al., 1973a; Holcombe and Moore, 1977). The hummocky aspect of the Early Miocene reflector is widespread and the unit has been described as a prominent horizon by Houtz and Ludwig (1977). The Early Miocene to Middle Eocene seismic interval dips toward the South Amer-

Fig. 5. Seismic stratigraphy tied with the DSDP results. Except for (B) all the seismic profiles cross exactly the DSDP Site area. The main reflectors are Early Miocene (eM), Eocene (A ~I) and Santonian to Coniacian (B") in age, respectively. Note the thickening of the A"-B 'I interval from west (A, B) to east (C, D). The deep reflectors (V, sub-B", R and Moho) are described in another paper (Mauffret and Leroy, 1997). The location of profiles is indicated in inset.

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NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE ican deformed belt and this disposition indicates that compressional deformation along South America occurred since the Early Miocene epoch. At this time the floor of the Caribbean Sea was completely deformed, with a general tilting towards the south related to the formation of trenches (Biju Duval et al., 1982b). The structure of the young accretionary prism indicates a large amount of offscraping (Ladd et al., 1984) and tomography data indicate a long slab extending beneath northwest South America (Hilst and Mann, 1994). However, the south dip of the Venezuelan basin crust is partly the result of original construction of the Cretaceous volcanic plateau that thins from north to south (Diebold et al., 1999). The current-controlled layer cannot be identified on the top of the Beata ridge and the transparent facies of the layer overlying the top of the ridge suggests a pelagic environment (Fig. 5E). The A"-B" interval is evident in the Venezuelan basin and in the western part of the Colombian basin (Ladd and Watkins, 1980; Bowland, 1993). However, horizon A", which correlates with Middle Eocene chert, is not clearly identified in the eastern Colombian basin and on the Beata ridge (Fig. 5E). The A " - B " interval is thick in the Venezuelan (Fig. 5D) and Colombian basins and also on the eastern flank of the Beata ridge (Fig. 5C), but thin-

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ner in the Aruba Gap area where it was penetrated during drilling at DSDP Site 153 (Fig. 5A and B). A prominent hard ground is correlated with a late Maastrichtian hiatus at the DSDP Site 153. A similar hard ground was described at DSDP Site 151, but here the Paleocene directly overlies the Santonian sediments and basalts (Edgar et al., 1973b). In a basin located to the north of the DSDP Site 153, the A " - B " interval correlates in thickness and seismic facies with units encountered at the DSDP Site 153 (Fig. 5B). However, the acoustic basement (V, Fig. 5) is very different from the typical smooth B" reflector drilled at DSDP Site 153, and is overlain by a sedimentary layer with a compressional velocity of 3.9 k m / s (Fig. 5B).

NORTHERN BOUNDARY OF THE CENTRAL CARIBBEAN REGION

The present-day northern limit of the Caribbean plate is the Oriente fault Fig. 1), the extension of the Oriente fault in Hispaniola that lies in the Cibao valley (Fig. 6; Calais et al., 1992; Russo and Villasenor, 1995) and the Puerto Rico trench (Masson and Scanlon, 1991). A strain partitioning may occur and a component of compression may be absorbed in the

Fig. 6. Tectonic framework of Hispaniola. The main boundary between the Caribbean plate and the North American plate is located along the Oriente fault and the Cibao valley. However, a strain partitioning with a compressional boundary is possible north of Hispaniola. The central part of Hispaniola is occupied by island arc crust (Central Cordillera), deformed sedimentary belts (Peralta and Neiba) and basins (San Juan and Enriquillo). The Enriquillo basin bounds the Presqu'~le du Sud that is formed by Cretaceous volcanic rocks identical to that of the Caribbean basement (Maurasse et al., 1979). The Presqu'~le du Sud is split into two parts by the left-lateral strike-slip Enriquillo fault. This fault is related to the Navassa pull-apart basin (Mann et al., 1995). The southern offshore part of the Presqu'~le du Sud is severely deformed by compression and transpression (Bien-Aime Momplaisir, 1986). The Haiti sub-basin has probably a quasi oceanic crust. East of the Beata ridge the Seacarib Seabeam survey is indicated. The Muertos prism can be divided into three parts by recent reverse faults. The location of Figs. 7 and 8 is shown.

Fig. 7. The Haiti sub-basin has a thin crust as shown by the refraction data (refraction line 36W, Ewing et al., 1960). The sedimentary layers onlap a wedge at the base of the slope indicating old deformation. The bottom of the wedge is indicated by the white line and arrows (flat reflector). In contrast the upper slope is actively deformed with the Ile-a-Vache anticline and a deep syncline bounded by reverse faults (Bien-Aime Momplaisir, 1986). The location of the seismic profile is indicated in Fig. 6.

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NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE sedimentary deformed belt that lies north of Hispaniola (Fig. 6; Austin, 1984; Dillon et al., 1992). The Caribbean plate boundary has been located along the north of Hispaniola since the Miocene and before this boundary was probably placed along the Oriente fault, the Central Cordillera and Muertos trough (Mercier de Lepinay, 1987; Pindell and Barrett, 1990). Hispaniola can be divided into four main blocks. A Septentrional block separated from the Eastern and Central Cordilleras by the Cibao valley. These cordilleras underwent a Mesozoic island arc tectonism (Mercier de Lepinay, 1987; Lebron and Perfit, 1994). A central area, including the Peralta flysch, the San Juan basin, the Sierra de Neiba that is an accretionary prism since the Eocene up to the Pliocene (Mann and Lawrence, 1991; Mann et al., 1991a,b, 1995). This third block is delimited by the Enriquillo depression (Fig. 6). The southern block is formed by the Presqu'ile du Sud d'Haiti that is an uplifted portion of the Cretaceous Caribbean igneous province (Maurasse et al., 1979). This block collided with the northern block recently (Mercier de Lepinay et al., 1988). The Early Pliocene strata are folded in the western part of the Enriquillo basin and the collisional process is probably presently active Vila et al., 1990). The Presqu'ile du Sud is cut by the Enriquillo fault that extends towards the west to Jamaica and the Cayman spreading center (Sykes et al., 1982; Rosencrantz and Mann, 1991). The Navassa trough (Fig. 6) is a pull-apart basin located along the left-lateral Enriquillo fault (Mann et al., 1995). The margins of the Presqu'ile du Sud were studied in detail by Bien-Aime Momplaisir (1986) and she showed that the northern and southern margins are affected by a compressional or/and transpressional tectonics governed by a N40-45 compressive stress. The Presqu'*le du Sud can be assimilated to a giant positive flower structure. A seismic profile (Fig. 7) shows the deformation of the upper margin with a broad syncline and a thrust of the sedimentary cover of this basin on the Tle-~-Vache structure. A narrow anticline is located at the top of this feature. At the base of the steep slope lies the Haiti sub-basin. The correlation of the seismic profile with refraction results (Ewing et al., 1960) indicates that this basin has a thin oceanic crust. The Haiti plateau is a thick block related to the Cretaceous igneous province, whereas the thin crust of the Haiti sub-basin seems to be not affected by the Cretaceous volcanic event. However, the presence of some intra basement reflectors (sub-B"?; Fig. 7) may suggest a weak volcanic contamination of the thin crust in the Haiti sub-basin. A small sedimentary wedge lies above a flat reflector (d6collement?) at the base of the slope. However, the sedimentary layers of the basin onlap this wedge. Consequently

635

this wedge was formed during old compressional tectonics and the present deformation is restricted to the upper margin. Nevertheless the sedimentary wedge may also be formed by slope breccias at the base of the scarp and the chaotic aspect of the upper sedimentary layers suggests slumped sediments and erosional products. The presence of dipping reflectors (Fig. 8) into the acoustic basement of the Beata plateau indicates the volcanic formation of this feature. The deepest sedimentary unit onlaps the basement and this seismic configuration suggests that the basement relief is old. This basement and the lower sedimentary layers are tilted towards the NNE, whereas the thickness of the recent layers increases in the same direction (Fig. 8). A wedge of deformed sediments is evident at the base of the western slope of the Sierra de Bahoruco. Thickening of the recent sedimentary layers and tilting of the lower layer in the basin, reverse fault and d6collement suggest a compressional origin for the wedge. This compression is recent but probably inactive at the present day as shown by other seismic profiles crossing the southern part of the deformed wedge (see later). Consequently the Haiti plateau and the Haiti sub-basin had an eastwards motion relative to the Presqu'ile du Sud, but this motion is presently nonexistent. The Muertos trough has been described several times (Ladd and Watkins, 1978; Biju Duval et al., 1982a; Ladd et al., 1981, 1990). The seismicity shows a steep Benioff zone dipping to 125 km depth (Bryne et al., 1985; Russo and Villasenor, 1997). A Seabeam survey (Mercier de Lepinay et al., 1988; Ja W, 1989; Mauffret and Jany, 1990; Vila et al., 1990) of the western part of the Muertos trough shows the relationships between the front of the accretionary prism and the Sierra de Martin Garcia (Fig. 9A). However, this front is connected to the Peralta flysch belt in some publications (Mann and Lawrence, 1991; Mann et al., 199 la,b, 1995). In the Ocoa Bay (Fig. 9B) the interpreted boundary of the Muertos accretionary prism is located between a (piggyback?) basin and a deformed zone (written communication of S.R. Lawrence, co-author of Chapter 12) that in fact is located in the inner part of the Muertos prism. Moreover the 50-km contour of the Muertos Benioff zone is located near Ocoa Bay (Russo and Villasenor, 1997). The Muertos accretionary prism is divided into three parts: the San Pedro basin, an upper prism and a lower prism (Fig. 9A). Two out of sequence thrust zones separate the three units. A detailed study (Biju Duval et al., 1982a) of the San Cristobal basin, an onshore extension of the San Pedro basin, indicates that the Muertos trough is active since the Eocene, but the uplift of the San Cristobal basin is related to a Late Miocene-Recent compressional event. The Eocene

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NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE paleoprism (Peralta belt and Sierra de Neiba) has been shortened and uplifted by this recent event that is related to the collision of the Beata ridge with the northern block of Hispaniola (Biju Duval et al., 1982a). The uplift is presently occurring by the thrusting of the San Pedro basin over the upper prism that is underthrust by the lower prism (Fig. 10A, Biju Duval et al., 1982a). North of 18~ the lower prism disappears and the upper prism is directly adjacent to the undeformed basin (Figs. 9A, 10C). The slope off Bahoruco Peninsula is very steep from the coast to 2 km depth; then several hills, that trend northsouth, form an extension of the Beata ridge. The magnetic map contoured during the Seacarib cruise (Fig. 9C) shows that some of these hills are made of volcanic rocks. These seamounts are separated from the prism by the narrow Muertos trough (Fig. 10A, B). However, the Beata seamounts are never in contact with the prism and the easternmost seamount disappears abruptly (Fig. 10C). On the other hand the magnetic map (Fig. 9C) shows that N E - S W magnetic lineations of the Caribbean basement are visible beneath the prism and a positive anomaly (431 nanoteslas, Fig. 9C) may be correlated with a buried seamount. The northern tip of the Beata ridge may be cut by a right-lateral strike-slip fault (Fig. 9C). The Enriquillo basin that is the onshore extension of the Muertos trough is a ramp valley (Mann et al., 1991 a,b) bounded by two facing thrusts (Fig. 6). It is probable that the Enriquillo depression is floored by the Cretaceous volcanic basement of the Presqu'~le du Sud because volcanic Cretaceous basalts have been observed northwest of the Enriquillo basin (Pierre Payen anticline, Fig. 6; (Vila et al., 1988). Moreover, the magnetic map (Fig. 9C) shows the Caribbean basement below the Muertos prism. Consequently, the Cretaceous volcanic basement may collapse by normal faulting produced by flexural effects found in the bulge related to underthrusting and this basement finally subducts beneath the Neiba and Muertos prisms. On land these normal faults have been recently described along the northeastern flank of the Sierra de Bahoruco (Pubellier et al., 1999). The Seacarib profiles (Fig. 10) are not migrated and we cannot define if the faults that bound the Beata seamounts are reverse or normal. However, the southwards extension of these faults is clearly reverse (see next paragraph). In conclusion, the northern boundary of the Presqu'~le du SudBeata ridge may be a reverse fault, but the subduction of this block implies a final normal faulting. These normal faults and the strike-slip faults transfer large fragments of the Cretaceous volcanic plateau to the Gonave microplate (Fig. 1). This microplate was defined between the Cayman spreading center, the Oriente fault, the Plantain Garden-Enriquillo fault and the western coast of Hispaniola (Rosen-

637

crantz and Mann, 1991; Mann et al., 1995). The Enriquillo depression connects this microplate to the Dominican sub-basin. We propose that the Gonave microplate belongs to the Venezuelan plate separated from the Colombian plate by the Enriquillo fault and the Beata fault system (see later).

EASTERN BOUNDARY OF THE BEATA RIDGE

South of the Muertos-Beata collision zone previously described several hills trend north-south (Fig. 9A). On a seismic profile (Fig. 11A), already published in Ladd et al. (1981), the two western seamounts are conical but the third seamount is asymmetrical. The thinning of the A"-B" interval (old sedimentary fill, Fig. 11) away from structures suggest that these features are contemporaneous with the formation of the volcanic plateau. The Beata ridge has a thick crust that has been underplated (Mauffret and Leroy, 1997). This underplating induced an uplift and a coeval rifting (Diebold et al., 1999; Driscoll and Diebold, 1999). However, the sedimentary cover of the central basin is upturned and is now on the top of the structure (Fig. l lA). This configuration is abnormal for a tilted block resulting from a normal faulting. The configuration of the sedimentary layers suggests an old feature reactivated and recently uplifted (1 km, Fig. 11A). Although this profile is not migrated we suggest that a transpression is the cause of the uplift. The structure previously described forms a north-south ridge that is offset to the south (Fig. 11D). In the second seismic profile (Fig. 11B) an asymmetrical structure separates two regions of the Dominican sub-basin with a different seismic stratigraphy. The western region shows several sub-B" reflectors, whereas the basement of the eastern region is void of these reflections. Such a prominent contrast between the eastern and the western part suggests a lateral displacement. The western region seems to be transported from the south where the sub-B" reflections are also prominent (Fig. 11C), implying a right-lateral component of a transpressional fault. The vertical uplift of the fault is estimated to be 0.6 km. In addition this profile shows an inversion of basin related to a compressional or transpressional event. The third profile is a regional seismic section (Ewing 9501 cruise, profile 1321; Fig. 11C). A pop-up with a probable reverse fault dipping towards the NNW is identified. Moreover, westwards steep dipping reflectors can be seen in the crust. Although the vertical throw of the pop-up structure is only 0.25 km the probable reverse fault affects the whole crust. These transpressional features were also seen on several single-channel seismic lines. In conclusion, we identify a transpressional zone with a strong component

Fig. 9. Relationship between the Muertos prism and the geologic features of Hispaniola. (A) Seacarib Seabeam survey completed by conventional bathymetric data. The lower prism of Muertos is formed by elongated anticlines. The toe of the prism turns abruptly towards the north when the first seamounts of the Beata ridge appear and the Muertos trough is very narrow. Then the lower prism disappears as well as the Beata seamounts. The slope of the prism is steep and the toe is related to the recent deformed Sierra de Martin Garcia. The location of the seismic profiles illustrated in Figs. 10 and 11A is shown. (B) The toe of the Muertos prism cannot be located in the Ocoa Bay as presumed by Mann et al. (1991a,b, 1995), Mann and Lawrence (1991). The Muertos prism and its forearc (San Pedro and San Cristobal) were formed during the Eocene (Biju Duval et al., 1982a) but were reactivated recently by the collision of the Beata ridge. The San Pedro basin overthrusts the upper prism and the lower prism disappears beneath the upper prism. This successive stacking generates the uplift and the emergence of the prism. (C) Magnetic map performed during the Seacarib 1 cruise. The magnetic basement of the Caribbean basin can be seen beneath the Muertos prism. The southern Beata seamount shows a correlation with a positive magnetic anomaly, but the northern seamount is orthogonal to the magnetic grain. We cannot exclude a rotation of this seamount that is parallel to the Muertos trough (see A). A prominent magnetic anomaly (431 nanoteslas) is located below a reentrant of the prism. This anomaly maybe correlated with a seamount that subducts. A strike-slip fault may transport the tip of the Beata ridge beneath the Muertos prism.

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

639

Fig. 10. Single-channel seismic profile from the Seacarib 1 cruise. (A) The upper prism overthrusts the lower prism, the Muertos trough is narrow and deformed and is bounded by a Beata seamount. (B) The folds of the lower prism are particularly evident. (C) The lower prism disappears as well as the easternmost Beata seamount. This seamount may have subducted; see the magnetic map in Fig. 9C. The locations of the seismic profiles are indicated in Fig. 9A).

of compression. This transpression is presently active. The height of the d e f o r m e d features increases towards the north where the collision of the Beata ridge is in process.

BEATA PLATEAU

This 0 . 8 - k m - h i g h structure, n a m e d by Case and H o l c o m b e (1980), trends N N W - S S E and is isolated

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A. MAUFFRET and S. LEROY

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

641

Fig. 12. Seismic profile crossing the Beata Plateau. The Eocene A" horizon has been piston-cored (Talwani et al., 1966; Edgar et al., 1971). Although this profile is unmigrated the reverse motion of the faults is clear. See the inset map for profile location.

f r o m the B e a t a ridge (Fig. 12B). A s e i s m i c profile shows two steps that are p r o b a b l y reverse. H o r i z o n A" is e x p o s e d on the h i g h e s t scarp. E a r l y to Middle E o c e n e was i n d e e d piston c o r e d on this scarp (Talwani et al., 1966; E d g a r et al., 1971). 11 k m of s h o r t e n i n g (20%) is e s t i m a t e d for this 4 8 - k m - w i d e structure if w e a s s u m e that the B" reflector was initially flat.

TAINO RIDGE

T h e Taino ridge is l o c a t e d on the e a s t e r n flank of the B e a t a ridge (Fig. 13). T h e D S D P Site 31 ( B a d e r et al., 1970) allows us to c a l i b r a t e the s e i s m i c profiles and w e c o r r e l a t e the M i o c e n e - O l i g o c e n e b o u n d a r y with a p r o m i n e n t reflector (eM, 23 Ma; Fig. 5C and Fig. 14). A d e t a i l e d survey was perf o r m e d d u r i n g the Casis cruise. T h e s e i s m i c profiles

Fig. 11. (A) This seismic profile, already published by Ladd et al. (1981), shows two conical seamounts and a third western seamount that is asymmetrical. The sedimentary cover of the adjacent basin are tilted and uplifted on the top of the seamount that indicates a transpression. (B) UTIG (University of Texas at Austin) profile cruise courteously provided by J. Austin. The two basins separated by an asymmetrical seamount show a different seismic stratigraphy. The eastern basin is underlain by a basement with several internal reflections (sub-B"). If we compare this profile with the seismic profile illustrated in (C), this basin may be transported from the south along a right-lateral strike-slip fault. (C) Seismic profile shot during the Ewing 9501 cruise courteously provided by J. Diebold. The complete regional seismic profile is also shown in Driscoll and Diebold (1999). Pop-up related to a transpression. Observe the steep dipping reflector that may correspond to a reverse fault at a crustal scale. The Moho reflection is shown at 10 s two-way travel time. The thinning of the old sedimentary fill away from the structures (A, B) indicates that these features were formed during the construction of the Cretaceous volcanic plateau, but a recent reactivation is evident. The uplift of the hills can be estimated to be 0.25 km in the south and 1 km in the north. See the inset map (D) for profile location. This depth to basement (B") map shows the eastern flank of the Beata ridge. The north-south trend of the structures is evidenced by the seismic profiles presented and several other multichannel and single-channel seismic lines. These structures are offset in a dextral sense between the seismic profiles presented in (B) and (A), respectively.

642

A. MAUFFRET and S. LEROY 72~

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Fig. 13. Depth to basement (Bf~) in the Taino ridge area. The Casis survey and the Figs. 14-20 are indicated. Reverse faults, pop-up and strike-slip faults are compatible with a NE-SW compressive stress. DSDP Site 31 results (Bader et al., 1970) help to calibrate the seismic profiles. previously presented were unmigrated, whereas all the Casis profiles are migrated. The southern part of the Taino ridge is relatively large (Fig. 13). It is delimited on the western and eastern side by reverse faults with opposite vergence. On the western flank a clear reverse fault (see the seismic trace, Fig. 14) offsets the B fl, A I~ Early Miocene reflectors and the sea floor. The identical thickness west and east of

the reverse fault indicates recent activity of this fault. The throw of this fault is evaluated to 0.6 km. The second part of the Casis B01 (Fig. 15) seismic profile shows a flower structure and reverse faults with an eastern vergence that bound the Taino ridge to the east. The strike-slip fault probably delimits to the north the large plateau that forms the southern part of the Taino ridge (Fig. 13). A new flower structure

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

643

Fig. 14. Reverse fault in the southern part of the Taino ridge. The trace of the fault is visible on the profile. The throw of the fault is estimated to be 0.6 km. See the inset map (depth to basement: B") and Fig. 13 for profile location.

is identified on the next profile (Fig. 16). This structure is visible very deep in the crust and offsets a 10-km-deep horizon (R, Fig. 16). We propose that this major strike-slip fault connects the Taino ridge to the transpressive structure described in the Dominican sub-basin (Fig. 11). To the north the Taino ridge is narrow and is divided into two hills (Fig. 17) that are offset (Fig. 13). The transverse valley is formed by a strike-slip fault (Fig. 5C). The northern part of the Taino ridge is a typical pop-up delimited by reverse faults with opposed vergence (Fig. 18A). A 15-km-deep ddcollement merges with the western fault (Fig. 18B). The B " - A " and the A " - e M intervals are thicker on the west part of the structure than on the top. This difference in thickness indicates that this part of the Taino ridge was an old seamount that

was reactivated by recent faulting. The thickening of the crust below the Taino ridge is estimated to be 0.5 km. To the north the Taino ridge loses its elevation. A strike-slip fault may be responsible for the tilt of a block (Fig. 19). A positive magnetic anomaly is related to the block that could be a Cretaceous volcano. Another volcanic ridge lies west of this block (Fig. 20). We failed to detect any recent deformation of this structure that is probably an initial volcanic feature of the Beata igneous province. The shortening of the northern Taino ridge can be estimated to be 4.6 km for a 21.5-km-wide structure (20%; from 8100 to 8500, Fig. 18A). If we assume that the northern Taino ridge and the southern Taino ridge (Fig. 13) had the same width initially, we estimate the shortening to be 39 km per degree. The

Fig. 15. A flower structure indicates the presence of a strike-slip fault. See the inset map (depth to basement: B ' ) and Fig. 13 for profile location.

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NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

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Fig. 27. (A) Depth to basement map: B 't. (B) Seabeam map of the Seacarib 1 cruise. The position of the survey is indicated in Fig. 3. The northern tip of the DSDP 151 ridge, that trends north-south, terminates abruptly towards the north and a steep scarp shows a northeast-southwest orientation. The positions of the seismic profiles illustrated in Fig. 28 are shown.

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

657

Fig. 28. (A) Seacarib 1 seismic profile (see also Fig. 5E) showing the location of the DSDP Site 151 where an hard ground separates Paleocene sediments from the Santonian sandstone marl and chalk (Edgar et al., 1973b). (B) Seacarib 1 seismic profile showing the steep scarp that trends NE-SW. For location see Fig. 27. entation (N45 ~ of compression has been described in the Presqu'~le du Sud d'Hispaniola (Bien-Aime Momplaisir, 1986). This stress increases towards the north as shown by the structures described in the Venezuelan basin (Fig. 11) along the Taino ridge

(Fig. 13) and in Aruba Gap (Fig. 24). The shortening of the Beata ridge induces a shallowing towards the north. It is evident that the present topography results from an uplift linked to the compression, but it is difficult to evaluate the initial topography. We know

658

A. M A U F F R E T and S. LEROY 72~ t

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Fig. 29. (A) Depth to basement map (B"). (B) Seabeam map of the Seacarib 1 cruise showing the steep scarp that forms the boundary between the Beata ridge and the Haiti sub-basin. The position of the survey is indicated in Fig. 3. The position of the seismic profile illustrated in Fig. 30 is shown.

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

659

Fig. 30. Seacarib 1 seismic profile showing the steep scarp that bounds the Beata ridge. This is not the steepest portion of the scarp and the profile is not normal to the structure. The upper slope shows evidence of slumping and the basement outcrops in the lower slope as shown by a diving survey (Mauffret et al., in prep.). A recent deformation can be noted in the Haiti sub-basin (see also Fig. 8). For location see Fig. 29. that the Beata ridge is a 20-km-thick Cretaceous volcanic plateau. This great thickness is mainly caused by a volcanic underplating that initiated the uplift and rifting of the Beata ridge. Although a thickening by compression may have occurred during the Miocene, we think that the initial Cretaceous thickness was important. The Colombian and Venezuelan basins subduct in a normal way below the South American deformed belt. In contrast the northern part of the Beata ridge collides with the central part of Hispaniola in relation with the eastwards drift of the Caribbean plate relative to Hispaniola. This collision results from the buoyancy of thick volcanic crust that resists subduction beneath Hispaniola (Burke et al., 1978; Mercier de Lepinay et al., 1988). The seismic profile displayed in Fig. 27 indicates that a non-reactivated part of the DSDP 151 ridge is buried in the Colombian basin. This basin and the Haiti sub-basin have a thin crust and were consequently much deeper than the initial Beata volcanic plateau. The Colombian basin is depressed by the loading of the thick sedimentary cover and the subduction below the South American deformed belt (7.4 km, Fig. 32G). The 1.3-km-high step shown in Fig. 32G represents the boundary between the Beata ridge and the Colombian basin. This boundary extends from the Pecos fault zone to the southern tip of the DSDP 151 ridge (Fig. 31). 0.5 km of initial

topography and 0.5 km of uplift was estimated for the Pecos fault zone (Leroy and Mauffret, 1996). We suppose that the initial topography was the same and the reactivation is consequently 0.8 km high. This estimation is close to the 1-km-high regional uplift deduced from the DSDP results (Benson et al., 1970). The buffed part of the DSDP 151 ridge (Fig. 32A) was initially deeper than the northern part but the shape of the ridge is conserved (1.9 km high relative to the adjacent basement) except for the seamount located on the southern tip of the ridge (2.2 km, Fig. 32B). This seamount was probably uplifted (0.3 km) by the N E - S W fault that delimits this feature to the south (Fig. 26). The DSDP 151 ridge-Taino ridge area do not show any appreciable topographic step, but the region located between the DSDP 151 and Tairona ridges is 1.8 km uplifted (from 4 km to 2.2 km, Fig. 32G). The northern part of the Beata ridge, near the Bahoruco Peninsula, is 1 km uplifted relative to the south. The northernmost uplift is actually onland where the Sierra de Bahoruco is 2 km high. We conclude that 6.8 km of uplift may occur between the southern tip of the Beata ridge and Hispaniola. However, the results of a submersible survey (Mauffret et al., in prep.) indicate that the Beata ridge was shallow during the Late Cretaceous after the volcanic event and the 6.8 km uplift results from the constructional history of the

660

A. MAUFFRET and S. LEROY

Fig. 31. Depth to basement (B") of the central part of the Caribbean Sea. The eastern part of the Beata ridge is characterized by reverse faulting offset by some NE-SW faults. N-S highs dominate in the central part of the structure. The NE-SW faults are predominant in the western part of the Beata ridge.

volcanic plateau and a recent compressional event from the Miocene to the Present. We estimated about 40 km of shortening per degree of longitude. Thus, 170 km of shortening may have occurred at 18~ (Bahoruco Peninsula). If we assume that the Sylvie ridge was in strike with

the Marie Aimee ridge and the Bahoruco Peninsula, the Presqu'~le du Sud should be separated from the central Hispaniola by a 240-km gap (Fig. 33). The boomerang shape of the Presqu'~le du Sud-Beata ridge may be due to the progressive collision of the Beata plateau and the collage of fragments of

661

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

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Fig. 32. East-west cross-sections (A to F) and north-south cross section (G) of the Beata ridge. The depth to basement (B') is relative to the sea level. Except for section (B) (2.2 km) the DSDP 151 ridge has the same height relative to the adjacent basement (1.7 to 19 km), but we can see the rising towards the north of the ridge (G). For location see the inset map (depth to basement: B').

662

A. MAUFFRET and S. LEROY

Fig. 33. The DSDP 151 ridge the Tairona ridge and the Presqu'~le du Sud have been displaced to form a north-south structure. This reconstruction shows a 240-km-wide gap between the Presqu'~le du Sud and the central part of Hispaniola. The Presqu'ile du Sud may have underwent a counter-clockwise rotation (Mercier de Lepinay et al., 1988; Van Fossen and Channell, 1988; Jany, 1989). this plateau (Mercier de Lepinay et al., 1988; Jany, 1989). Therefore, the present east-west orientation of the Presqu'~le du Sud may be a neotectonic feature and a substantial counter clockwise-rotation of the Presqu'~le may have occurred (Van Fossen and Channell, 1988). If we assume that the compression occurred since the Early Miocene (23 Ma) then the motion rate is between 0.74 cm/yr (170 km) and 1.04 cm/yr (240 km). The eastern boundary is diffuse with an intraplate deformation, particularly along the Taino ridge. However, we do not have good seismic profiles to illustrate the easternmost deformation (Fig. l lA, B) and a deep thrust, dipping to the west, is suggested (Fig. 11C). The several thrusts observed on the eastern side of the Beata ridge imply a deep d6collement level. Fig. 18C shows that this d6collement is deeper than 15 km. It is clear that the Beata ridge forms a boundary between a Venezuelan microplate and a Colombian

microplate and the latter overthrusts the former. We suggest that the eastern boundary of the Colombian microplate lies along the easternmost thrust (Fig. 11) and the Beata plateau. The southern boundary of this microplate is the Colombian deformed belt. The northern part of this belt trends east-west and is narrow (Vitali, 1985; Vitali et al., 1985). North of the Santa-Marta Massif the deformed belt is segmented with N-S and E N E - W S W orientation of the toe of the prism (Fig. 34). After a new prominent bend (Vernette et al., 1992) the Colombian prism merges with the onshore Sinu belt (Duque-Caro, 1979, 1984; Vitali, 1985; Vitali et al., 1985; Toto and Kellogg, 1992) that is 10 km thick (Lehner et al., 1984). This prism was formed in the Early Miocene (Duque-Caro, 1979) along the buried Sinu trench that trends north-south. This active margin is confirmed by the seismicity that defines a 180-kmdeep Benioff zone. The length of the subducted crust is 350 km (Malav6 and Suarez, 1995). This Benioff

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE zone belongs to the Bucaramanga segment (Pennington, 1981) and cannot be related to the Nazca Plate as suggested by Van der Hilst and Mann (1994). We suggest that the Sinu subduction zone is related to the Beata deformed zone by fight-lateral strike-slip faults and short north-south segments of subduction zones (Fig. 34). The northern part of the Sinu belt is clearly (Vernette et al., 1992) offset by a strikeslip fault zone and the South Caribbean marginal fault illustrated by fig. 13 in Kellogg and Bonini (1982) is probably a transpressive feature. Westwards, the Colombian microplate is delimited by the Panamanian subduction zone (Adamek et al., 1988; Protti and Schwartz, 1994). This Panamanian block is delimited to the north by a strike-slip fault that is connected to the Middle American trench south of Nicoya Peninsula (Fisher et al., 1994; Marshall and Anderson, 1995). This peninsula may belong to the Colombian microplate and a fault north of the peninsula may connect the Middle American trench and the Hess escarpment (Dengo, 1985). The western part of the Hess escarpment is active (Bowland, 1993), but the eastern part of this feature does not show clear evidence of recent faulting. On the contrary, the Pedro escarpment, that delimits the upper Nicaragua rise, shows much evidence of recent faulting (Mascle et al., 1985; Holcombe et al., 1990). Moreover, the lower part of the Nicaragua rise has the same volcanic basement as the Colombian basin (Mauffret and Leroy, 1997) and we placed the northwestern boundary of the Colombian microplate along the Pedro escarpment (Fig. 34). The northern limit of the microplate is the Enriquillo strike-slip fault and the Bahoruco thrust. The Venezuelan microplate is delimited to the east by the Lesser Antilles subduction zone and to the south by the Venezuelan deformed belt. The northern boundary lies along the Anegada Passage (Jany et al., 1990) and Muertos trough. A Gonave microplate was defined (Rosencrantz and Mann, 1991; Mann et al., 1995) between the Cayman spreading center, the Oriente fault to the north, the Walton-Plantain Garden-Enriquillo fault to the south and the western coast of Hispaniola. We suggest that this microplate is related to the Venezuelan microplate through the narrow Enriquillo basin. A recent recompilation of the magnetic data (Leroy, 1995) in the Cayman trough determined 0.9 cm/yr of half-rate spreading between chron 8 (26 Ma) and chron 1 (Fig. 35A). The motion along the Walton-Plantain Garden fault is evaluated in Jamaica Island to 0.4 cm/yr during the Miocene to Quaternary (Rosencrantz and Mann, 1991). However, the slip rate of the Enriquillo fault in Hispaniola is evaluated to 0.8 cm/yr (Mocquet and Aggarwal, 1983) and we suggest a 0.9 cm/yr motion towards the northwest of the Colombian microplate (Fig. 35).

663

Therefore, a 0.5 cm/yr of additional motion can be estimated along the Pedro fault. The velocity vector diagram (Fig. 35A) suggests a velocity as high as 2.6 cm/yr of the Colombian microplate relative the North American plate. GPS measurements in the Presqu'~le du Sud determine 2.3 cm/yr for the relative velocity between this area and the North American plate (Farina et al., 1995). The relative motion between the central block of Hispaniola and the North American plate is 1.5 cm/yr according to the previous study. Therefore, the relative motion between the central block of Hispaniola and the Presqu'~le du Sud is 0.8 cm/yr, a value very close to our estimation (0.9 cm/yr). The plate motion of the southern Caribbean region is documented by the GPS studies (Freymueller et al., 1993; Drewes et al., 1995; Kellogg and Vega, 1995). The Colombian microplate subducts beneath the Panama prism with a rate of 1 cm/yr and the rate of convergence between the Colombian microplate and the North Andes block (Sinu trench) can be evaluated to be 1.7 cm/yr (Kellogg and Bonini, 1982). The differential motion between the two plates may result from the greater convergence of the NOAM-SOAM in the western part of the Caribbean zone than in the eastern part (Mtiller et al., 1996) or from the influence of the large convergent motion of the Cocos plate. In the eastern Colombian basin intraplate deformation has been observed northeast of the Panamanian prism (Bowland, 1993) and the results of the DSDP Site 154 (Edgar et al., 1973b) indicate that the uplift of the deformed structure is related to an Early Pliocene reverse fault with a southwest vergence clearly displayed in the seismic profiles shown by Bowland (1993). The negative buoyancy of the young Cocos oceanic crust may induce a compressional stress in the overriding Caribbean plate (England and Wortel, 1980; Meijer, 1992). We cannot decipher the dominant process (Atlantic or Pacific) to produce a higher displacement of the Colombian microplate than the Venezuelan plate. We observe that the chrons 7 (25 Ma) and 8 (26 Ma) correspond to an increase of convergence between the Nazca plate and the South American plate (Pargo-Casas and Molnar, 1987) and between the North and South American plates (Mtiller et al., 1996), respectively. At this time the Cayman spreading center was also reorganized (Leroy, 1995) and Hispaniola began to drift away from Cuba. A global reorganization of plate motion in the Pacific and Atlantic Oceans may have influenced the internal stress of the Caribbean plate. There is much controversy about the pole position of the North America-Caribbean plate either to the north (Sykes et al., 1982; Deng and Sykes, 1995) or the south (Stein et al., 1988; Calais and Mercier de Lepinay, 1993; Lundgren and Russo, 1996). We agree with Heubeck and Mann (1991), who placed

Fig. 34. (A) The Colombian and Venezuela-Gonave microplates have been differentiated with two patterns. (B) Sketch that shows the relationship between the Sinu trench and the compression along the eastern flank of the Beata ridge. Refer to the text for explanation.

7~ -]

NEOGENE INTRAPLATE DEFORMATION OF THE CARIBBEAN PLATE AT THE BEATA RIDGE

665

Fig. 35. (A) Early Miocene reconstruction. Refer to the text for explanation. (B) Sketch (from Leroy and Mauffret, in prep., modified) inspired from Heubeck and Mann (1991). We suggest two different poles for the North American plate-Venezuelan microplate and the North American plate-Colombian microplate motions. A pole for Venezuelan-Colombian microplates motion is proposed near the southern tip of the Beata ridge where the compressional deformation is weak.

a pole for the NOAM-Colombian microplate in the south and a pole for the NOAM-Venezuelan microplate in the north (Fig. 35B). The pole for the Venezuelan-Colombian microplate must be very close to the southern tip of the Beata ridge (Fig. 35B) where the compressional deformation is weak.

CONCLUSIONS

This study together with to investigate ticularly true

shows how detailed seismic surveys, a regional coverage, can be useful a complex tectonic area. This is parfor the Taino ridge and Pecos fault

666 zone where we showed reverse faults and pop-up offset by strike-slip faults. The tectonic framework of these areas is coherent with a N E - S W stress that increases towards the north. The orientation of this stress is incompatible with a simple squeeze of the Caribbean plate between the North and South American plates (Burke et al., 1978) although a westwards increase of convergence between the two major plates may have influenced the faster migration of the Colombian microplate towards the east than the Venezuelan microplate. A clear boundary between the Colombian and Venezuelan microplates cannot be traced, although we do not have enough seismic information to exclude a major structure along the easternmost thrust. However, the deformation seem to be diffuse and localized along several structures. These structures are probably pre-existent but it is very difficult to differentiate between the initial topography and the observed topography that results from the Miocene to Recent compression. Our shortening estimations, that are as high as 20%, can be biased by this poor knowledge of the pre-existent topography. The displacement of the Colombian microplate relative to the Gonave-Venezuelan microplate is evaluated between 170 and 240 km since the Early Miocene. This is much greater than the 50 km of offset along the Enriquillo fault in Presqu'~le du Sud (Mann et al., 1995), although an additional motion may occur south of the Presqu'~le du Sud. However, our rate of 0.9 c m / y r is compatible with the recent and preliminary GPS results (Farina et al., 1995). The present Caribbean plate is a composite, partly formed by continental crust (upper Nicaragua rise), island arc crust (Jamaica and north Hispaniola), but the bulk of this plate is the Cretaceous igneous province. The boundary migrates to the south and fragments of the former plate are abandoned and integrated into the North American plate (Mann et al., 1995). In addition we propose internal boundaries that divide the Caribbean plate in several blocks: upper Nicaragua block, lower Nicaragua block, Colombian and Venezuelan microplates, northern Hispaniola. In such a complex tectonic framework it is impossible to study just the main boundary, i.e. the Cayman spreading centerPuerto Rico trench-Lesser Antilles subduction zone, to deduce the motion between North America and the Caribbean plate and we suggest that the pole for the North America-Venezuelan microplate motion is different from that of the North America-Colombian microplate motion. The South American plate undergoes an internal stress due to the rapid subduction of the young oceanic crust of the Nazca plate (England and Wortel, 1980; Meijer, 1992). In the northern part of South America, the North Andes block undergoes an E - W stress and migrates towards the north relative to the South American main plate. A similar

A. MAUFFRET and S. LEROY stress can be applied to the Caribbean plate by the Nazca and the Cocos plates.

ACKNOWLEDGEMENTS

This work was supported by grants INSU ATP 733 and 780. We thank the officers and crew of the R / V Nadir for assistance in this project. We are especially indebted to the technical crew of GENAVIR who greatly helped in the acquisition of multichannel data. These data were processed at Institut de Physique du Globe de Strasbourg and we thank R. Schlich and M. Schaming who have facilitated access to the processing center and helped us to use the Geovector software. We thank J. Diebold from Lamont-Doherty Earth Sciences Observatory, E Mann and J. Austin from the University of Texas at Austin, A. Mascle from the Institut du P6trole, and E Lehner from Shell to have provided several seismic lines that completed our seismic coverage. We thank E Mann, T. Holcombe, N. Donnelly and E. Calais for their constructive reviews and helpful suggestions. Contribution of URA 1759.

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