Plate Motion, Sedimentation, and Seismicity at a Restraining Bend

Home , Puerto Rico Trench

Earth and Planetary Science Letters, 70 (1984) 311-324 311 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[31

Neotectonics of Hispaniola: plate motion, sedimentation, and seismicity at a restraining bend

Paul Mann 1, Kevin Burke 2 and Tosimatu Matumoto 1 Institute for Geophysics, University of Texas at Austin, Austin, TX 78712 (U.S.A.) 2 Lunar and Planetary Institute, 3303 NASA Road 1, Houston, TX 77058; and Department of Geosciences, University of Houston/Central Campus, Houston, TX 77004 (U.S.A.)

Received November 28, 1983 Revised version received July 9, 1984

The pattern of Neogene faulting,volcanism, and sedimentation in Hispaniola helps to resolve a problem that arises in attempting to determine the direction of Caribbean plate motion from earthquakes alone, namely: How well do earthquake mechanisms define plate motion? Hispaniola occupies a compressional north-step or restraining bend in the generally east-west-trending Motagua-Cayman Trough-Puerto Rico Trench fault system which marks an active plate boundary zone (PBZ) of left-lateral strike-slip motion between the North America and Caribbean plates. Four areas are distinguishable in Hispaniola from field mapping and from the interpretation of satellite imagery and conventional aerial photographs. Two of these areas consist of active left-lateral strike-slip fault systems that roughly define the northern and southern coasts of the island. The structure of both fault systems suggest a roughly east-west-trending direction of relative plate motion consistent with previous findings. The intervening area consists of en echelon mountain ranges thrust up at the restraining bend from the early Miocene. A Pleistocene volcanic province within this area is interpreted as defining a diffuse extensional fault termination of the southern strike-slip fault zone. The fourth area in eastern Hispaniola is interpreted as being underthrust by discontinuous fragments of lithosphere in response to convergence at the restraining bend. It appears possible that thinner, young island arc lithosphere may be more prone to subduction than to "escape" or "flaking", processes that are more important in older, more buoyant lithosphere.

1. Introduction active motions of adjacent plates and tectonic reconstructions. Present-day Caribbean motion is Recorded seismic events enabled Molnar and assumed by some workers [4] to have been steady Sykes [1] to establish the identity of the Caribbean and has been extrapolated into the past to form a as a rigid lithospheric plate defined on its northern basis for reconstructions as old as Eocene. and southern margins by strike-slip plate boundary Various methods of determining North zones or "PBZ's" [2] and on its east and west by America-Caribbean relative plate motion have convergent boundaries and volcanic arcs (Fig. 1). yielded significantly different results (Table 1). Most of our knowledge of Caribbean plate motion Caribbean-South America relative plate motion is has come from the study of PBZ earthquakes over generally inferred indirectly from the relative mo- the last two decades and from marine geologic tions of Caribbean-North America and North surveys of the 1400 km long Cayman Trough America-South America plate pairs. Variation in pull-apart [3] (Fig. 1). The precise distribution, these results shown in Table 1 and Fig. 1 can be direction and rate of Caribbean plate motion is attributed to regional inferences based on local important in earthquake prediction and in identi- areas of PBZ deformation. In particular, slip-vec- fying areas of seismic risk as well as constraining tor analysis of earthquakes may reflect local inter-

/ IHISPANIOLAI 20

& CARIB

coco \ 10 600 km

80 6O Fig. 1. Plate tectonic setting of Hispaniola (CARIB = Caribbean plate; NOAM = North America; SOAM = South America; COCO = Cocos). Dashed lines represent direction of North American left-lateral strike-slip motion about a pole of relative motion suggested by Jordan [7] and Minster and Jordan [8] (lines labelled 1 ) and Sykes et al. [4] (line labelled 2). Slip lines labelled 1 are based mainly on earthquake slip vectors and fault strikes from the Cayman Trough pull-apart (shown in grey) and Central America and Suggest Mountains of Hispaniola (hatched area) result from plate convergence at a north-step or restraining bend in our east-west left-lateral strike-slip plate boundary zones. Slip lines labelled 2 are based mainly on earthquake slip vectors in the Hispaniola-Puerto Rico (PR) area and suggest northeasterly to east-northeasterly plate convergence in this area. Black thrust symbols indicate subduction zones with volcanic arcs; uncolored thrust symbols indicate convergent zones without associated volcanic arcs.

hal plate deformation rather than interplate mo- is a compressional north-step or restraining bend tion because of the interaction of active faults with in the principal displacement left-lateral strike-slip older surfaces in the easily deformed quartz and fault zone of the North American-Caribbean PBZ feldspar-rich island arc and continental crust that (Motagua- Swan-Oriente-Septentrional- Puerto constitutes much of the lithosphere in the northern Rico Trench Fault Zones) (Fig. 1). The island Caribbean. provides the largest onshore area of this PBZ We here describe Neogene deformation in deformation system outside Central America (Fig. Hispaniola as providing a way of resolving prob- 1). We discuss the late Ncogene geologic record lems inherent in interpreting a seismic record that within the framework of the new seismic data is short and contains substantial earthquakes only presented here and draw comparisons to similar indirectly related to plate motions. Hispaniola is strike-slip restraining bends in California, New shared between Haiti (western 30,000 km2) and Zealand and Lebanon. the Dominican Republic (eastern 50,000 km2) and 313

TABLE 1 Determinations of present-day Caribbean motion relative to North America

Reference Rate Direction Method of calculation Molnar and Sykes [1] a 0.5 cm/yr at empirical relationship between Lesser Antilles Arc seismicity and slip rate (Brune [5]).

Molnar and Sykes [1] b 2.2 cm/yr at N80 o E rate from closure of velocity Lesser Antilles Arc triangle for NOAM, COCOS, CARIB: direction from earthquake mechanisms

Isacks et al. [6] c 2 cm/yr rate based on downdip length of seismic zone in Lesser Antilles arc

Jordan [7] angular rate pole at 60 o N, 116 o E combination of earthquake of 0.20 _+ 0.07 °/Ma (see Fig. 1) slip vectors, Cayman Trough spreading rate and closure of velocity triangle for NOAM-CARIB-COCOS

Minster and Jordan 1.94 cm/yr essentially same pole combination of earthquake [8] ~ as Jordan [7] (see Fig. 1) slip vectors, Cayman Trough spreading rate

Sykes et al. [4] ~ 3.7 + 0.5 N70 ° E in rate from configuration of cm/yr over 7 Ma northeastern Caribbean; seismic zone in Puerto Rico pole at 66° N, 132°W and Lesser Antilles; direction (see Fig. 1) from earthquake slip vectors

a Authors note that rate does not include aseismic slip or seismic moments for thrust faults, b Principal source of error is the small angular separation between Cocos Pacific and Cocos-Americas plates. c Rate calculation assumes constant plate motion over 10 Ma. d Slip vectors used by Jordan [7] from Hispaniola, Puerto Rico Trench and area south of possible NOAM-CARIB-SOAM triple junction are excluded. Five out of seven earthquake slip vectors used to determine direction of plate motion arc from Puerto Rico Trench and northern Lesser Antilles; rate calculation assumes constant plate motion over 10 Ma.

2. Four structural areas in Hispaniola island is relatively much lower than the rest of the island and does not appear too affected by active The physiography of Hispaniola consists of al- faulting (Fig. 2A). We first describe the neotectonic ternating valleys and mountain ranges which are characteristics of each of the four zones and then mostly defined by active reverse or sinistral strike- attempt an overall interpretation of the structure slip or oblique-slip faults (Fig. 2A). Cretaceous to and seismicity of the island. Eocene island arc and platform carbonate rocks are typically found in the cores of mountain ranges at higher (> 1000 m) elevations while the valleys 3. Strike-slip faulting in northern Hispaniola contain thick sequences of Tertiary and Quaternary sedimentary rocks [9]. The northern zone of strike-slip faulting con- Field mapping and interpretation of LAND- sists primarily of the Camu and Septentrional SAT, SEASAT-SAR radar images and conven- Fault Zones of the northern Dominican Republic tional aerial photographs indicate two areas of and constitutes a 50 km wide, 300 km long land- active, through-going left-lateral strike-slip faults ward extension of the Oriente strike-slip fault zone that roughly follow the north and south coasts of of the Cayman Trough and the oblique-slip faults the island (Fig. 2A). The west-central part of the of the Puerto Rico Trench (Fig. 1). Both the Camu island consists of high ground bounded by domi- and Septentrional Faults are convex to the south nantly reverse faults along the edges of the up- and change strike from 090 ° to 115 ° (Fig. 2A). lifted mountain ranges. The eastern part of the The two faults bound the highest topographic 314

NEOTECTONICS OF HISPANIOLA

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Fig. 2. Summary of neotectonic structures of Hispaniola which have developed since the Miocene and their relation to historical seismicity: A. Topography, recent faults, and recent volcanic centers; B. Historical seismicity from Kelleher et al. [11]; C. Strain ellipse showing types of structures predicted in a left-lateral strike-slip plate boundary zone with east-west relative plate motion; D. Schematic map of major structural features produced since the Miocene in Hispaniola by North America-Caribbean relative plate motion. Mountain ranges approximate anticlines but are thrust bound at their edges. Motion is transformed between the two strike-slip fault zones by northwest-southeast extensional "pull-apart" structure across the west-central part of the island. We interpret this as a bypass response to locking across the northern convergent strike-slip fault zones. block of the Cordillera Septentrional where eleva- istic of active strike-slip faults such as shutter tions reach 1249 m along a ridge parallel and 2-3 ridges, aligned drainage, vegetation contrasts, en km north of the Septentrional Fault Zone (Fig. echelon folds, beheaded and skewed alluvial fans 2A). The Camu Fault Zone is much less topo- and uphill facing scarps [10]. Side-look radar graphically distinct than the Septentrional which imagery reveals low scarps in alluvium where the forms a fault line scarp as steep as 400 m over 2 fault crosses alluvial plains. The single fault trace km in the central section. The Septentrional Fault splays into at least two sub-parallel traces in the Zone is marked by a single prominent fault scarp western Dominican Republic (Fig. 2A). The north- along its central and eastern trace and forms a ern splay contains a 10 km long "push-up" area of remarkably straight contact between Quaternary compressional deformation at a 2.5 km right-step alluvium and Tertiary sedimentary rocks of the in the strike-slip faults trace (Fig. 2A). The sharp- southern Cordillera Septentrional (Fig. 2A). The ness of the fault trace near the push-up (area fault trace exhibits geomorphic features character- indicated in Fig. 2A) suggests the possibility of a 315 historical rupture perhaps during the 1842 earth- of its extreme landward segments in eastern quake [11] (Fig. 2B). The southern splay trends Jamaica and central Hispaniola. Parts of this fault almost due west along the flood-plain of the Rio zone have been recognized from historical [11] and Yaque del Norte and projects westward along the present-day [4] earthquakes. coast of northern Haiti (Fig. 2A). This splay is In south-central Dominican Republic and east- marked in the alluvium by a strong vegetation ern Haiti, the Enriquillo-Plantain Garden Fault contrast where the faulting has acted as a down- Zone is a remarkably straight and narrow zone of slope barrier to groundwater migration. deformation which trends due east. The fault trace The concentration of high ground on the north cannot be followed into eastern Hispaniola (Fig. sides of the Camu and Septentrional Fault Zones, 2A) and is locally obscured by alluvium in the the occurrence of push-up structure, and a locally sub-sealevel Enriquillo Valley of the Dominican thick (possibly 4 km or more) Plio-Pleistocene Republic (Fig. 2a) whose floor is dynamically de- clastic section south of the Septentrional Fault pressed by en echelon secondary thrusts associated Zone [9] all suggest that west-northwesterly sec- with east-west strike-slip displacement [15] (Fig. tions of these faults are "transpressional": that is, 2C). At higher elevations, the fault trace exhibits they have a significant component of convergence typical strike-slip geomorphic features such as (Fig. 2C). Northeast-trending, secondary extension travertine mounds, fault springs, uphill facing has occurred north of the Camu Fault Zone and scarps, and offset drainage. In eastern Haiti, the formed at least two Quaternary grabens near Bahia fault trend changes abruptly from 090 o to 099 o, a Maimon and Rio Yasica [12,13] (Fig. 2A). Conver- strike it maintains to eastern Jamaica (Figs. 1, 2A). gence across the westernmost section of the Sep- The west-southwesterly fault trace in southern Ha- tentrional Fault Zone may have resulted in the iti is interrupted by two Quaternary pull-apart splaying and in the progressive abandonment of basins at Clonard and Mirogoane Lakes and is the northern splay. locally marked by one very prominent fault valley The age and offset of the Septentrional and near Leogane [16]. En echelon extensional struc- Camu Fault Zones are poorly known. Both faults tures are also inferred from bathymetric maps of truncate rocks of Miocene age [12]. Overthrusting the offshore continuation of the fault in the along the Septentrional Fault Zone appears to Jamaica Passage, the marine strait separating east- have been initiated in the Pliocene based on the ern Jamaica and southwestern Haiti [16] (Fig. 1). evidence of the age of sediments accumulated on Right-steps of the fault trace in southwestern Haiti the overthrust valley block south of the fault [9]. and eastern Jamaica, are marked by local topo- Eberle et al. [13] have suggested a left-lateral offset graphic uplifts caused by convergence at "push- of 100 km on the Septentrional Fault Zone to ups" or restraining bend segments (Fig. 2A). In match areas of alluvial gold deposits in the these compressional segments, the single strike-slip Cordillera Septentrional and eastern Hispaniola. fault splays into a series of thrust and reverse faults which are topographically less prominent. Local elevations reach 2-3 km (Fig. 2A). 4. Strike-slip faulting in southern Hispaniola The existence of pull-aparts and the fault trough structure of the 099°-trending section of the En- Recently a major throughgoing strike-slip fault riquillo-Plantain Garden Fault Zone in Haiti and zone has been identified stretching from south- the Jamaica Passage suggest that this west-south- central Hispaniola to southeastern Jamaica where westerly section of the fault is "transtensional", displacement appears to be relayed northward at a that is, it has a significant component of diver- major restraining bend [14]. The fault extends to gence [16] (Fig. 2C). A northeast-trending graben western Jamaica and may merge with the southern in Port-au-Prince Bay north of the fault [17] and in edge of the Cayman Trough (Fig. 1). We have Azua Bay south of the fault (Fig. 2A) as well as named the continuous part of this fault the En- northwest-trending restraining bend or "push-up" riquillo-Plantain Garden Fault Zone on the basis segments are all consistent with this hypothesis. 316

The total offset on the Enriquillo-Plantain Garden east-west-trending segments of the fault zone have Fault Zone appears to be about 10 km based on a large strike-slip component whereas more north- the offset of Cretaceous rocks in Jamaica. The age westerly trending segments are reverse faults asso- of initial fault movement appears to be early Plio- ciated with overturned and intensely thrusted beds cene when secondary deformation associated with [20]. North-south seismic profiling across the the fault completely disrupted a later Miocene southern edge of the San Juan Basin between two deep marine basin in the Enriquillo-Cul-de-Sac of the ranges indicates a synclinal structure for the Valley of the Dominican Republic and Haiti basin and the presence of imbricate reverse faults [18,19]. dipping southwards at 45 ° [21] (Fig. 3; line of section shown in Fig. 2A). Uplift of the northwest- ern margin of the Cordillera Central occurs along 5. Deformation of the west-central mountain ranges at least three mapped faults all of which are con- of Hispaniola vex to the northeast (Fig. 2A). The Bonao Fault Zone forms the most prominent topographic scarp The west-central part of Hispaniola consists of and delineates the northeastern boundary of the en echelon, sinuous mountain ranges with eleva- highest region of the Cordillera Central (Fig. 2A). tions reaching a maximum of 3 km in the Cordillera The Hatillo Fault Zone (Fig. 2A) is a thrust dip- Central of the Dominican Republic (Fig. 2A). ping at approximately 30 ° to the southwest and Uplift of the ranges has occurred along northwest- carrying what are thought to be older metamor- erly trending reverse and thrust faults which over- phic rocks northwestwards over Cretaceous to early thrust the floors of neighboring valleys and gener- Tertiary [22]. The Hispaniola Fault Zone is an ally occur near the 500 m topographic contour Oligocene age strike-slip fault which juxtaposes (Fig. 2A). Mapping along the major fault zone, metamorphosed mafic volcanic rocks on its south which separates Cretaceous rocks of the Cordillera from quartz-albite-sericite-epidote-chlorite meta- Central from sedimentary rocks of the San Juan morphosed rocks on its north over a distance of Basin to the south (Fig. 2A), has shown that 170 km [22]. The west-northwesterly segment of

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Fig. 3. Interpretation of north-south Vibroseis seismic reflection line across the San Juan Valley (from Nemec [21]). Over 2 km of high-energy fanglomerates of Mio-Pleistocene age have filled the synclinal basin and record the uplift of the Cordillera restraining bend to the north. 317 the Hispaniola Fault Zone north of the Cordillera eating limited regional uplift and is relatively unaf- Central is marked by a narrow graben of folded fected by recent faulting (Fig. 2A). High ground Oligocene elastic rocks believed to have been de- exposing Cretaceous rocks is found only in the posited in an asymmetric depression along an ac- north and may reflect secondary compressional tive strike-slip fault [23]. The northwesterly strik- uplift along the southern edge of the South Samana ing segment of the Hispaniola Fault Zone is Bay Fault Zone (Fig. 2A). occupied by a wedge-shaped ultramafic body with reverse faults on both sides dipping eastward at 75-80 o and westward at 50-65 o [24]. The ultra- 8. Neotectonic synthesis mafic rocks are interpreted as the upfaulted base- ment of the Oligocene strike-slip basin which is We interpret structural pattern of strike-slip preserved to the north. This hypothesis is con- faults, grabens, and thrust-bound mountain ranges sistent with the observation that northwesterly (shown in Fig. 2A and schematically in Fig. 2D) as faults are thrusts and that more easterly trending indicating that Hispaniola occupies a 250 km wide faults have more of a strike-slip component (Fig. belt of left-lateral shear deformation resulting from 2c). east-west relative motion of the North America and Caribbean plates (Fig. 2C). East-west faults, such as the eastern part of the Enriquillo-Plantain 6. Recent volcanic province of Hispaniola Garden Fault Zone (Fig. 2A) are strike-slip faults (Fig. 2C). West-northwesterly trending faults, such Four areas of recent volcanic rocks have been as those in the Septentrional and Camu Fault identified in central Hispaniola (Fig. 2A). The Zones, are transpressional or convergent strike-slip easternmost volcanic center consists of a north- faults (Fig. 2C). West-southwesterly trending east-trending belt of domes and valley fillings of faults, such as those in the western part of the andesites, rhyolites, and basalts. K-Ar ages range Enriquillo-Plantain Garden Fault Zone (Fig. 2A), from 2 Ma B.P. to less than 0.5 Ma B.P. (latest are transtensional or divergent strike-slip faults Pliocene to late Pleistocene) [25]. Most of the (Fig. 2C). Northwesterly trending faults, such as volcanic eruptions appear to have occurred along the HatiUo, Hispaniola, and Bonao Fault Zones northeast-trending, presumably normal, faults (Fig. 2A), are reverse or thrust faults (Fig. 2C). which are orthogonal to the trend of the reverse The trend of recent volcanics is northeasterly and faults bounding the southern edge of the Cordillera Central (Fig. 2A). they are locally associated with northeasterly The western three volcanic centers consist trending normal faults (Fig. 2C). Recent grabens mostly of basalt flows [25] (Fig. 2C). Basalt flows also trend northeast (Fig. 2A). The dominance of reverse and convergent erupted at the eastern center overlie sedimentary strike-slip faults in northern and west-central rocks of Plio-Pleistocene age. The outcrop pattern Hispaniola suggests that this part of the island of two of these mafic volcanic centers is also constitutes an asperity or restraining bend in the elongate to the northeast and again suggests erup- east-west North American-Caribbean plate tion along northeast-trending normal faults (Fig. boundary zone (Fig. 1). The origin of the asperity 2C). The center near the west coast of Haiti occurs is attributed to the interaction of Miocene and along the landward extension of a recent north- younger east-west strike-slip faults with west- east-trending graben in Port-au-Prince Bay (Fig. 2C). northwesterly striking rocks formed in island arcs of Cretaceous to Eocene age. Oligocene strike-slip faults of which the Hispaniola Fault Zone (Fig. 7. Deformation of eastern Hispaniola 2A) is the most prominent represent an early stage in the evolution of the North America-Caribbean Most of eastern Hispaniola is low in elevation, PBZ. We interpret the Enriquillo-Plantain Garden covered by Neogene marine shelf sediments indi- Fault Zone, which more closely parallels the east- 318 west direction of plate motion (Fig. 2A), as a idea of sudden energy release along a 300 km bypass fault which is relieving the locking and segment following a period of locking across the uplift across the northern and central parts of the fault plane. Recent scarps in the western Domini- island (Fig. 2D). Motion is transformed between can Republic may have formed during this event the two strike-slip fault zones by northwest-south- (Fig. 2A). east extension across the west-central part of the A second belt of smaller, historical rupture island. This interpretation suggests a pull-apart zones of the 18th and 19th centuries coincides with origin for the Pleistocene volcanic province of the Enriquillo-Plantain Garden Fault Zone [11] central Hispaniola (Fig. 2D). Superposition of the and is compatible with reduced frictional slip on a northeast-trending volcanic centers on northwest- fault zone that is parallel or slightly oblique to the trending thrusts and reverse faults bounding the direction of plate motion (Fig. 2C). The younging Cordillera is predicted in an east-west strike-slip of ruptures eastward suggests a time-space migra- system (Fig. 2C) and is consistent with the cross- tion of seismic slip, which began in October of cutting relationships of mountain fronts and 1751 in the Dominican Republic, moved westward volcanic centers (Fig. 2A). An intriguing relation- across Haiti in the 18th and 19th centuries, and ship exists between active extension indicated by perhaps culminated in Jamaica during large earth- recent volcanism and grabens and active compres- quakes there in the 20th century (Fig. 2B). sion indicated by mountain ranges and seismicity.

10. Deep seismicity and its possible relation to 9. Shallow seismicity and its relation to crustal crustal structures in Hispaniola structures in Hispaniola A belt of intermediate depth earthquakes (70 < The present-day seismicity of Hispaniola con- h < 140 km) extends from the Lesser Antilles arc sists of a broad belt of earthquakes which range in as far west as the outcrop of the Bonao Fault Zone depth from 0 to 115 km. Two bands of shallow in central Hispaniola and events in this belt are seismic activity (< 70 km) are present: a northern particularly concentrated in eastern Hispaniola [4] band roughly parallels the Orient-Septentrional- (Fig. 2A). Detailed seismic work has shown that Camu-Puerto Rico Trench Fault Zones and events intermediate depth earthquakes from the northern on it have been concentrated in northeastern Lesser Antilles arc to the western end of Puerto Hispaniola and the Puerto Rico Trench [4]. A Rico are associated with a continuous underthrust more diffuse southern band of activity is devel- slab [4]. Intermediate earthquakes from the west- oped from Jamaica to southern Hispaniola and ern end of Puerto Rico to central Hispaniola are roughly parallels the Enriquillo-Plantain Garden associated with the deformation of discontinuous Fault Zone. The two bands of shallow earthquake slab fragments and slip vectors of earthquakes activity appear to merge in eastern Hispaniola and change orientation from approximately easterly extend as a single belt to the Lesser Antilles island (northeast of Puerto Rico) to southwesterly and arc. southerly (in eastern Hispaniola) (arrow in Fig. The record of historical earthquakes in 2B). Historical rupture zones in eastern Hispaniola Hispaniola and Puerto Rico dates back to the 16th and Puerto Rico are subcircular [11] (Fig. 2B) and century and closely corresponds to the present-day are consistent with intermediate-focus earthquakes strike-slip fault pattern [4,11] (Fig. 2B). A zone of presently occurring above subducted slabs in these ruptures in the 19th and 20th centuries extends areas. roughly parallel to the Septentrional-Puerto Rico A seven-station seismograph network installed Trench Fault Zones. The extended rupture zone of by one of us (T.M.) in the north-central Domini- the 1842 earthquake corresponds closely with the can Republic in December 1979 suggests two dif- west-northwesterly convergent section of the Sep- fuse zones of intermediate-depth earthquakes that tentrional Fault Zone and is compatible with the dip steeply to the southwest beneath the Cordillera 319

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Fig. 4. Comparison between restraining bends (A) along the San Andreas Fault system between the Pacific and North American plates (the Transverse Ranges) and (B) along the Dead Sea Fault system between the Sinai (Levant and Arabia plates (the Lebanon Ranges))• Slip lines about published poles of rotation are indicated: 1 = Minster and Jordan [8]; 2 = Woodford and McIntyre [40]; 3 = Le Pichon and Francheteau [43]; 4 = Garfunkel [41]. Pole positions determined from fault strikes from only one side of the restraining bend (e.g., 2, 4) yield directions of plate motion which are inconsistent with much of the geology of the plate boundary zone.

Central (area west Bonao and La Vega) to depths most Puerto Rico Trench near Samana (Fig. 4). of 115 km [26,27]. Although both seismic zones The western zone is better defined by a more have not yet been well defined, preliminary data dense occurrence of events that suggests a fairly suggests that both zones are between 30 and 50 km uniform seismic zone from the surface to maxi- thick and are separated by an approximately 30 mum depths of 120 km. The upper part of the km wide aseismic zone. Both zones extend to western zone approximately coincides with mapped depths of 115-120 km. The upper 50 km of the southwesterly dipping reverse and thrust faults eastern zone is relatively aseismic and cannot be (Hatillo, Hispaniola, Bonao Fault Zones) (Fig. confidently correlated to any surface structure, 2A). Above and to the west of the western zone in although it is in the general vicinity of the western- the Cordillera Central is a relatively dense zone of 320

hypocenters that range in depth from 30 to 0 km 11.1. Direction of Caribbean plate motion (Fig. 4). The two seismic zones are speculated to be Several tectonic models have been proposed for slabs of relatively young (Cretaceous-Eocene) is- the Neogene tectonics of Hispaniola. None have land arc lithosphere which are being subducted reconciled the apparent convergence of the Carib- along northwest trending thrust and reverse faults bean and North America plates suggested by inter- that accommodate shortening in the restraining mediate-depth earthquakes in eastern Hispaniola bend. Although more seismic work and geologic and the east-west strike-slip motion predicted from mapping will be necessary to better define the plate calculations [7] and from east-west Neogene slabs and their surface extensions, subduction ap- strike-slip offset in the Cayman Trough [3] (Fig. pears to occur in a more or less southwesterly 1). Most models have emphasized convergence over direction perpendicular to the strikes of known strike-slip because of: (1) lack of firm evidence for reverse faults in central Hispaniola (i.e., strike-slip faulting in the northeastern Caribbean Hispaniola, Hatillo, and Bonao Fault Zones) and (with the exception of the en echelon pattern of the westernmost Puerto Rico Trench. Earlier Cordillera in central Hispaniola) [28,29]; (2) the workers have previously pointed out the "island superficial resemblance of the east-west-trending, arc" structure of eastern Hispaniola (e.g., [1] and landward-dipping Puerto Rico and Muertos [31]) but have not interpreted the structure as a Trenches to active Pacific convergent margins [30]; product of convergence at a strike-slip restraining (3) the strong southerly component of earthquake bend. The latter interpretation is supported by two slip vectors suggesting north-south plate conver- observations: (1) earthquake slip vectors appear to gence [1,4,31] (Fig. 2B); and (4) closure of the locally diverge from an approximately east-west North America-Africa-South America relative direction north of Puerto Rico to a southwest plate motion circuit which suggest Neogene rela- direction (Fig. 2B) perpendicular to the strike of tive convergence between the Americas across the mapped reverse and thrust faults in eastern Caribbean [32,33]. Hispaniola (Fig. 2A); and (2) the areas above the Our observations suggest that convergence in underthrnst slabs are topographically low lying Hispaniola is related to secondary northwest- (Fig. 2A) and are not characterized by a large southeasterly compression resulting from the inter- number of shallow earthquakes. Low topography action of east-west relative plate motion with older is consistent with the idea of local convergence west-northwesterly trending island arc structures being accommodated by underthrusting rather (Fig. 2D). Neogene interaction has produced an than lithospheric shortening and thickening--the asperity or restraining bend in northern Hispaniola deformation style found farther west in the topo- which is being partially bypassed by a new, more graphically higher parts of central Hispaniola (Fig. slip-parallel strike-slip fault that was initiated 2A). sometime in the Pliocene (Fig. 2D). Shortening in eastern Hispaniola is accommodated by subduction of what is thought to be island arc lithosphere 11. Discussion to depths of at least 115 km (Fig. 4). Slip vectors from earthquakes in this zone trend perpendicular Structural and seismic data suggests that to local northwest-trending thrusts and arc struc- Hispaniola occupies an active restraining bend or tures established by the Eocene and are therefore asperity in the east-west North America-Carib- probably not reliable indicators of the overall di- bean strike-slip plate boundary zone (Fig. 1). The rection of Caribbean-North America plate motion restraining bend structure of the island has im- (Fig. 2B). Better long-term indicators of plate mo- portant implications for plate interactions in this tion are convergent and divergent strike-slip fault area as well as for tectonic processes occurring at systems and associated secondary structures (Fig. restraining bends. 2D) which have developed since the Miocene (that is over the last 6 Ma) and indicate an almost 321 exactly east-west direction of relative plate motion early Oligocene [34]. The fans consist of well- (Fig. 2C). The east-west direction of plate motion rounded to sub-rounded clasts of tonalite, green- indicated by these geologic structures agrees well stones, felsic tufts and vein quartz that suggest a with slip lines about the pole of relative motion deep level of late Miocene erosion of the Cordillera calculated by Jordan [7] and Minster and Jordan Central. [8] using a combination of earthquake slip vectors On the southern edge of the Cordillera Central, and fault strikes mostly from margins of the Cay- an influx of clastic sediments derived from the man Trough (Fig. 1). Our study of plate motion in erosion of an arc terrane occurred in: (1) early Hispaniola has independently arrived at the same Miocene on the north flank of the central Plateau conclusion as Minster and Jordan [8] who in Haiti [19] and its eastward continuation into the eliminated earthquake slip vectors from the San Juan Valley of the Dominican Republic [20]; Hispaniola and Puerto Rico Trench area because (2) late medial Miocene (Tortonian-Messinian) in "the data show internal scatter and the stress and the Enriquillo-Cul-de-Sac Valley [18,19,36] (Fig. strain fields are complex". 2A). This depositional history is consistent with Our data suggests that N70°E direction of and reflects the progressive post-early Miocene Caribbean relative motion suggested by Sykes et uplift of a restraining bend centered on the pres- al. [4] (Fig. 1) may partly reflect intraplate defor- ent-day Cordillera Central. The slightly younger mation at the Hispaniola restraining bend. How- age for the onset of clastic sedimentation on the ever, the number of local mechanisms in the south flank of the Central Plateau of Haiti and the Hispaniola and Puerto Rico areas is still quite Enriquillo-Cul-de-Sac Valley is consistent with limited and further progress can be expected in southward progradation of a clastic wedge derived determining the direction of plate motion using from the Cordillera Central. A very rapid sedi- earthquakes. mentation rate of 60 cm/1000 years can be derived from detailed paleontologic on the late 11.2 Early development of the Hispaniola restrain- Miocene clastic succession of the eastern En- ing bend riquillo-Cul-de-Sac Valley [36].

The history of uplift related to restraining bend 11.3. Comparison of Hispaniola with other restrain- formation in Hispaniola is recorded by the strati- ing bends graphic relationships and compositions of coarse clastic sediments eroded from uplifted areas and Restraining bends or compressional asperities preserved in surrounding basins. Moreover, bend are found along most active strike-slip fault sys- formation was accompanied by a rapid accelera- tems and occur on a variety of scales. Major bends tion of clastic sedimentation rates. (that is, fault separations of roughly 100 km) have There is good stratigraphic evidence for the been described in the Transverse Ranges of south- early uplift of the Hispaniola bend from both the ern California [37], the southern Alps of New northern (Cibao Basin) and southern flanks (San Zealand [38], and the Lebanon Ranges of Lebanon Juan Basin--cf. Fig. 3--and Enriquillo Basin) of [39] (Fig. 4). It is interesting to briefly compare the the Cordillera Central. Two large, apparently neotectonics of Hispaniola, which lies entirely undeformed Miocene alluvial fan complexes are within island arc lithosphere with the active found along the northern edge of the Cordillera tectonics of these other bends which are better Central in the Cibao [34]. The triangular shape studied and lie within continental lithosphere. and fining northwards of both fans into the Cibao All of the bends result in major deflections Basin indicates the early uplift of the northern (15-30 o) of the principal displacement strike-slip edge of the Cordillera Central by at least the late fault zone of their associated PBZ and result in Miocene, the approximate age of the basal clastic crustal shortening and topographic uplift of area rocks [35]. The basal unconformity of the eastern measured in thousands of square kilometers (Fig. fan clearly truncates sediments as young as late 4). Some workers have made regional inferences 322 on the direction of plate motion, which have failed aseismic subduction [45]. Mechanisms proposed for to recognize the importance of the restraining bend the Southern Alps restraining bend include: (1) a fault segment within the framework of the entire steady-state balance between erosion and uplift on PBZ [40,41]. Fault strikes from only one side of a reverse faults [46]; (2) subduction [47]; and (3) major restraining bend yield pole positions which flaking [48]. are much closer to the PBZ and are inconsistent Rapid erosion and acceleration of sedimenta- with much of the PBZ geology (cf. [42,43] for tion rates frequently occurs during uplift of re- discussions) (Fig. 4). straining bends and has become particularly well A variety of deformational mechanisms have understood for the Transverse Ranges of Cali- been suggested to occur at active bends; all may fornia [49]. lead to faulting and substantial earthquakes which Data presented here suggest that three of the are only indirectly related plate motion (Fig. 5). above processes, bypassing, uplift, and subduction, Different mechanisms, which have been suggested dominate at the Hispaniola restraining bend, which to occur at the Transverse ranges bend (Fig. 5), unlike the above examples, lies wholly within rela- include: (1) sideways escape of triangular con- tively young (Cretaceous to latest Eocene) island tinental fragments [44]; (2) bypassing of the bend arc lithosphere. It appears possible that thinner, area by more rectilinear faults [37]; (3) flaking of younger island arc lithosphere may be more prone sheetlike masses or crustal flakes or less than to subduction than to escape or flaking processes lithospheric thickness [45]; and (4) incipient that are more important in older more buoyant lithosphere.

A. ESCAPE -% Acknowledgements This work was supported by the NASA Geody- namics Program (NAG 5155 to Burke). We greatly appreciate the assistance and cooperation we have B. BYPASSING received from government and university authori- ties in Haiti and the Dominican Republic. We have benefitted from discussions with C. Cooper, G. Draper, J. Lewis, F. Maurrasse, M. Nemec, and F. Taylor. We thank M. Nemec for providing us with information on the San Juan Basin and T. C. FLAKING/ROTATION Dixon for kindly donating SEASAT-SAR radar imagery. A part of the research was done while K.B. was a visiting Scientist at the Lunar and Planetary Institute. This paper is LPI Contri- bution 536 and University of Texas Institute for Geophysics Contribution 591. D. SUBDUCTION

References

Fig. 5. Proposed deformational mechanisms which have been 1 P. Molnar and L.R. Sykes, Tectonics of the Caribbean and suggested to occur at restraining bends; all may lead to faulting Middle America regions from focal mechanisms and and earthquakes related to internal plate deformation at the seismicity, Geol. Soc. Am. Bull. 80, 1639-1684, 1969. bend and not to relative plate motion. Mechanisms B and C 2 K. Burke, J. Grippi and A.M.C. Sengor, Neogene structures appear to be occurring in the relatively young (Creta- in Jamaica and the tectonic style of the northern Caribbean ceous-Eocene) island arc lithosphere of Hispaniola. plate boundary zone, J. Geol. 88, 375-386, 1980. 323

3 T.L. Holcombe and G.F. Sharman, Post-Miocene Cayman 22 C.O. Bowin, Geology of the central Dominican Republic, Trough evolution: a speculative model, Geology (in press). Mem. Geol. Soc. Am. 98, 11-84, 1966. 4 L.R. Sykes, W.R. McCann and A.L. Kafka, Motion of 23 P. Mann and K. Burke, Post-Eocene NOAM-CARIB rela- Caribbean Plate during last 7 million years and implications tive plate motion and strike-slip offset of the Greater An- for earlier Cenozoic movements, J. Geophys. Res. 87, tilles island arcs, Geol. Soc. Am., Abstr. Prog. 13, 503, 1981. 10656-10676, 1983. 24 E.G. Haldemann, S.B. Bouwer, J.H. Blowes and W.E. Snow, 5 J.N. Brune, Seismic moment, seismicity and slip rate along Lateritic nickel deposits at Bonao Falconbridge Dominicana major fault zones, J. Geophys. Res. 73, 777-784, 1968. C. por A, Field Guide, Caribb. Geol. Conf. 9, 69-78, 1980. 6 B. Isacks, J. Oliver and L.R. Sykes, Seismology and the new 25 P. Vespucci, Preliminary account of the petrology of the global tectonics, J. Geophys. Res. 73, 5855-5900, 1968. late Cenozoic volcanic province of Hispaniola, Trans. 7 T.H. Jordan, The present day motion of the Caribbean Caribb. Geol. Conf. 9, 379-389, 1980. plate, J. Geophys. Res. 80, 4433-4499, 1975. 26 T. Matumoto and R. Saenz, Computerized seismic monitor- 8 J.B. Minster and T.H. Jordan, Present day plate motions, J. ing system for a small network, EOS 60, 888, 1979. Geophys. Res. 83, 5331-5354, 1978. 27 T. Matumoto, Technical report, Dominican seismograph 9 C. Bowin, The geology of Hispaniola, in: The Ocean Basin network, submitted to Corporacion Dominicana de Electri- and Margins, 3. The Gulf of Mexico and the Caribbean, cidad, Santo Domingo, 1981. A.E.M. Nairn and F.G. Stehli, eds., pp. 500-552, Plenum, 28 J. Matthews and T.L. Holcombe, Possible Caribbean under- New York, N.Y., 1975. thrusting of the Greater Antilles along the Muertos Trough, 10 P. Mann, K. Burke and T. Matumoto, Tectonic geomor- Trans. Caribb. Geol. Conf. 7, 235-242, 1976. phology and seismicity of the Septentrional Fault Zone, 29 J.W. Ladd, T.C. Shih and C.J. Tsal, Cenozoic tectonics of Dominican Republic, Geol. Soc. Am., Abstr. Prog. (in central Hispaniola and adjacent Caribbean Sea, Bull. Am. press). Assoc. Pet. Geol. 65, 466-489, 1981. 11 J.L. Kelleher, L.R. Sykes and J. Oliver, Possible criteria for 30 J.W. Ladd and J.S. Watkins, Active margin structures within predicting earthquake locations and their application to the north slope of the Muertos Trench, Geol. Mijnbouw 57, major plate boundaries of the Pacific and the Caribbean, J. 255-260, 1978. Geophys. Res. 78, 2547-2585, 1973. 31 B.A. Schell and A.C. Tarr, Plate tectonics of the northeast- 12 F. Nagle, Geology of the Puerto Rico Plate area, Domini- ern Caribbean Sea region, Geol. Mijnbouw 57, 319-324, can Republic, in: Hispaniola: Tectonic Focal Point of the 1978. Northern Caribbean, B. Lidz and F. Nagie, eds., pp. 1-28, 32 J.W. Ladd, Relative motion of South America with respect Miami Geological Society, Miami, Fla., 1979. to North America and Caribbean tectonics, Geol. Soc. Am. 13 W. Eberle, W. Hirdes, R. Muff and M. Pelaez, The geology Bull. 87, 969-976, 1976. of the Cordillera Septentrional, Trans. Caribb. Geol. Conf. 33 J. Pindell and J.F. Dewey, Permo-Triassic reconstruction of 9, 619-632, 1980. western Pangea and the evolution of the Gulf of 14 P. Mann, K. Burke, G. Draper and T. Dixon, Tectonics and Mexico/Caribbean region, Tectonics 1, 179-211, 1982. sedimentation at a Neogene strike-slip restraining bend, 34 H.C. Palmer, Geology of the Moncion-Jarabacoa area, Jamaica, EOS 62, 1051, 1981. Dominican Republic, in: Hispaniola: Tectonic Focal Point 15 P. Mann, F.W. Taylor, K. Burke and R. Kulstad, Subaeri- of the Northern Caribbean, B. Lidz and F. Nagle, eds., pp. ally exposed Holocene coral reef, Enriquillo Valley, 29-68, Miami Geological Society, Miami, Fla., 1979. Dominican Republic, Geol. Soc. Am. Bull. (in press). 35 J.B. Saunders, P. Jung, J. Geister and B. Biju-Duval, The 16 P. Mann, M.R. Hempton, D.C. Bradley and K. Burke, Neogene of the south flank of the Cibao Valley, Dominican Development of pull-apart basins, J. Geol. 91, 529-554, Republic: a stratigraphic study, Trans. Caribb. Geol. Conf. 1983. 9, 151-160, 1980. 17 W.P. Woodring, J.S. Brown and W.S. Burbank, Geology of 36 A. Mascle, B. Biju-Duval, G. Bizon, C. Muller, J. Saunders, the Republic of Haiti, 631 pp., The Lord Baltimore Press, P. Jung and J. Geister, Tertiary sequences south of the 1924. Cordillera Central, Field Guide, Caribb. Geol. Conf. 9, 18 J.C. Cooper, Geology of the Fondo Negro region, Domini- 107-138, 1980. can Republic, 145 pp., M.S. Thesis, State University of New 37 J.C. Crowell, The San Andreas Fault through time, J. Geol. York at Albany, 1983 (unpublished). Soc. London 136, 293-302, 1979. 19 W.A. van den Bold, Neogene biostratigraphy (Ostracoda) of 38 R.I. Walcott, Present tectonics and late Cenozoic evolution southern Hispaniola, Bull. Am. Paleontol. 66, 599-639, of New Zealand, Geophys. J. R. Astron. Soc. 52, 137-164, 1975. 1978. 20 R. Michael, Geology of the southwestern flank of the 39 P.L. Hancock and M.S. Atiya, Tectonic significance of Cordillera Central, Dominican Republic, M.S. Thesis, mesofracture systems associated with the Lebanese segment George Washington University, 1978 (unpublished). of the Dead Sea Transform fault, J. Struct. Geol. pp. 21 M.C. Nemec, A two phase model for the tectonic evolution 143-153, 1979. of the Caribbean, Trans. Caribb. Geol. Conf. 9, 24-34, 40 A.O. Woodford and D.B. McIntyre, Curvature of the San 1980. Andreas Fault, California, Geology 4, 573-575, 1976. 324

41 A. Garfunkel, Internal structure of the Dead Sea leaky 46 J. Adams, Contemporary uplift and erosion of the Southern transform (rift) and its relation to plate kinematics, Alps, New Zealand, Geol. Soc. Am. Bull. 91 (Part II), Tectonophysics 80, 81-108, 1981. 1-114, 1980. 42 G.P. Citron and D.L. Turcotte, Curvature of the San 47 R.G. Allis, Continental underthrusting beneath the South- Andreas Fault, California: comment, Geology 5, 324-325, ern Alps of New Zealand, Geology 9, 303-307, 1981. 1977. 48 H.W. Wellman, An uplift map for the South Island of New 43 X. Le Pichon and J. Francheteau, A plate-tectonic analysis Zealand and model for uplift of the Southern Alps, in: of the Red Sea-Gulf of Aden area, Tectonophysics 46, Origin of the Southern Alps, R.I. Walcott and M.M. Cress- 369-406, 1978. well, eds., R. Soc. N.Z. Bull. 18, 13-20, 1979. 44 D. McKenzie, Active tectonics of the Mediterranean region, 49 R.S. Yeats, Neogene acceleraton of subsidence rates in Geophys. J. R. Astron. Soc. 30, 109-185, 1972. southern California, Geology 6, 456-460, 1978. 45 R.S. Yeats, Quaternary flake tectonics of the California Transverse Ranges, Geology 9, 16-20, 1981.