ABSTRACT Kinematic analysis of faults in Trinidad reveals three main stages of the tectonic evolution of the southeastern Caribbean–South American plate boundary. During Stage 1, folding and thrusting occurred and are truncated by a Middle Miocene unconformity. This shortening event has been related by previous workers to the initial, oblique collision of the Great Arc of the Caribbean with the passive margin of South America. We propose that Stage 1 Middle Miocene east-northeast-trending compression documented in this study initially had a more northwest–southeast direction and has been rotated in a clockwise direction during this collision. This tectonic stage resulted in clockwise rotation of structures along the southeastern Caribbean plate margin within a broad, right-lateral, strike-slip zone. During Stage 2 in the late Miocene and middle Pliocene, south-southeast-trending shortening uplifted the Central Range, formed prominent north-dipping thrusts, bounded by oblique ramps such as the Los Bajos right-lateral strike-slip fault and formed piggy-back basins. This north-northwest– south-southeast trend of compression is compatible with coeval right-lateral shear on the El Pilar fault zone in Trinidad. We interpret this pattern of coeval thrusts and strike-slip faults of Stage 2 as the result of strain partitioning. In Stage 3 during the late Pliocene– Quaternary, the stress field rotated counterclockwise and east-southeast-trending compression reactivated previous thrusts as right-lateral, strike-slip faults, such as the Central Range fault. The rotation of the trend of compression deactivated previous, east–west-trending, strike-slip faults such as the eastward extension of the El Pilar fault zone into Trinidad. The polyphase tectonic evolution of Trinidad occurs in the regional context of the eastward motion of the Caribbean plate with propagation of the southern Subduction-Transform Edge Propagator (STEP) fault.

Abstract We compiled paleostress analyses from previous research works collected at 591 localities of striated fault planes in rocks ranging in age from Late Cretaceous to Quaternary in the circum-Caribbean and Mexico. The purpose of the study is to quantify a progressive clockwise rotation of the Caribbean plate during its Late Cretaceous to recent subduction of the proto-Caribbean seaway. Paleostress analysis is based on the assumption that slickenside lineations indicate both the direction and sense of maximum resolved shear stress on that fault plane. We have plotted directions of maximum horizontal stress onto plate tectonic reconstructions of the circum-Caribbean plate boundaries and infer that these directions are proxies for paleo-plate motion directions of the Caribbean plate. Plotting these stress directions onto reconstructions provides a better visualization of the relation of stress directions to blocks at their time of Late Cretaceous to recent deformation. Older, more deformed rocks of Late Cretaceous to Eocene ages yield a greater scatter in derived paleostress directions as these rocks have steeper dips, more pervasive faulting, and likely are affected by large rotations as known from previous paleomagnetic studies of Caribbean plate margins. Despite more scatter in measurements from older rock units, four major events that affected the Caribbean plate and the Great Arc of the Caribbean (GAC) are recognizable from changing orientations of stress directions: (1) Late Cretaceous collision of the GAC with southern Mexico and Colombia is consistent with a northeast direction of maximum compression in rocks of this age range in southern Mexico and east-west direction in Colombia, as the GAC approached the proto-Caribbean seaway; (2) Paleocene-Eocene collision of the GAC with the Bahamas platform in Cuba and Hispaniola and with the South American plate in Venezuela is consistent with clockwise rotation of stress directions in rocks of these ages in the northern Caribbean and counter-clockwise rotation of these rocks in the southern Caribbean; (3) Late Miocene collision and indentation of the Panama arc with northwestern South America is consistent with east-west directions in rocks of these ages; and (4) Oligocene to recent strike-slip faulting along the northern and southern boundaries of the Caribbean shows consistent directions for the northern (northeast) and southern (northwest) Caribbean. Stress directions document the progressive clockwise rotation of the Caribbean plate and the GAC motion from northeast in the Late Cretaceous, to east-northeast in the Paleogene, to east-west in the Neogene.

Abstract The Virgin Islands basin is a 4.5-km-deep passage that connects the Atlantic and Caribbean seas. A variety of models have been proposed to explain its tectonic origin, which range from right- and left-lateral pull-apart basins to a rotational-type basin or even a simple, orthogonal rift basin. This study integrates three data types to better understand the Miocene to recent kinematics of basin opening and its present-day tectonics known from a parallel zone of earthquakes and GPS results that span the basin from Puerto Rico to St. Croix (U.S. Virgin Islands). A grid of 400 km of 2D seismic lines provided courtesy of the Danish Galathea 3 expedition reveals the geometry of faults underlying the offshore basin to a depth of 7.5 seconds two-way time. The basin is asymmetrical having more throw along the southeastern normal fault than the normal fault along its northwestern edge. The island of St. Croix, is the uplifted footwall of the southeastern normal fault while the island of Vieques, Puerto Rico, is the uplifted footwall of the northwestern fault. Seismic data show that both bounding normal faults are listric and have associated rollover anticlines in the basin center. A linear, possibly strike-slip fault system can be traced for a distance of 4.7 km in the center of the basin. Fourteen normal fault planes have been measured in Miocene and younger rocks on the footwall blocks of Vieques and St. Croix and revealed dip-slip normal faults having fault planes oriented northeast to east-northeast and parallel to the long axis of the offshore basin. A GPS baseline between continuously recording sites in southwest Puerto Rico and St. Croix reveals that that the basin is presently opening in a direction of 100° - or roughly at right angles to its long axis - at a rate of 2.5 mm/yr. We conclude based on seismic data, striated fault planes, and GPS results that the present-day opening of the basin and perhaps its early evolution is the result of simple, orthogonal rifting in a northwest-southeast or west-northwest/east-southeast direction.

The Cerro Goden fault zone is associated with a curvilinear, continuous, and prominent topographic lineament in western Puerto Rico. The fault varies in strike from northwest to west. In its westernmost section, the fault is ∼500 m south of an abrupt, curvilinear mountain front separating the 270- to 361-m-high La Cadena de San Francisco range from the Rio Añasco alluvial valley. The Quaternary fault of the Añasco Valley is in alignment with the bedrock fault mapped by D. McIntyre (1971) in the Central La Plata quadrangle sheet east of Añasco Valley. Previous workers have postulated that the Cerro Goden fault zone continues southeast from the Añasco Valley and merges with the Great Southern Puerto Rico fault zone of south-central Puerto Rico. West of the Añasco Valley, the fault continues offshore into the Mona Passage (Caribbean Sea) where it is characterized by offsets of seafloor sediments estimated to be of late Quaternary age. Using both 1:18,500 scale air photographs taken in 1936 and 1:40,000 scale photographs taken by the U.S. Department of Agriculture in 1986, we identified geomorphic features suggestive of Quaternary fault movement in the Añasco Valley, including aligned and deflected drainages, apparently offset terrace risers, and mountain-facing scarps. Many of these features suggest right-lateral displacement. Mapping of Paleogene bedrock units in the uplifted La Cadena range adjacent to the Cerro Goden fault zone reveals the main tectonic events that have culminated in late Quaternary normal-oblique displacement across the Cerro Goden fault. Cretaceous to Eocene rocks of the La Cadena range exhibit large folds with wavelengths of several kms. The orientation of folds and analysis of fault striations within the folds indicate that the folds formed by northeast-southwest shortening in present-day geographic coordinates. The age of deformation is well constrained as late Eocene–early Oligocene by an angular unconformity separating folded, deep-marine middle Eocene rocks from transgressive, shallow-marine rocks of middle-upper Oligocene age. Rocks of middle Oligocene–early Pliocene age above unconformity are gently folded about the roughly east-west–trending Puerto Rico–Virgin Islands arch, which is well expressed in the geomorphology of western Puerto Rico. Arching appears ongoing because onshore and offshore late Quaternary oblique-slip faults closely parallel the complexly deformed crest of the arch and appear to be related to extensional strains focused in the crest of the arch. We estimate ∼4 km of vertical throw on the Cerro Goden fault based on the position of the carbonate cap north of the fault in the La Cadena de San Francisco and its position south of the fault inferred from seismic reflection data in Mayaguez Bay. Based on these observations, our interpretation of the kinematics and history of the Cerro Goden fault zone includes two major phases of motion: (1) Eocene northeast-southwest shortening possibly accompanied by left-lateral shearing as determined by previous workers on the Great Southern Puerto Rico fault zone; and (2) post–early Pliocene regional arching of Puerto Rico accompanied by normal offset and right-lateral shear along faults flanking the crest of the arch. The second phase of deformation accompanied east-west opening of the Mona rift and is inferred to continue to the present day.

Topography, bathymetry, regional structural observations, and fault slip measurements support the idea that the Mona rift is an active, offshore extensional structure separating a colliding area (eastern Hispaniola) from a subducting area (northwestern Puerto Rico). Near the city of Aguadilla in northwestern Puerto Rico, paleostress reconstruction through fault slip analysis demonstrates that the Mona rift is opening in an E-W direction. This fault slip analysis also indicates that the opening is oblique in the southern part of the Mona rift. We propose that oblique rifting results from accommodation of E-W extension by oblique right-lateral reactivation of previously mapped, NW-trending Eocene basement convergent structures (Aguadilla faults, Cerro Goden fault). The evolution of the stress field during the Miocene and the present E-W opening of the Mona rift support the assumption that the Miocene 25° counterclockwise rotation of Puerto Rico has stopped and that this island is presently moving to the east relative to the colliding Hispaniola.

Puerto Rico is located within a zone of tectonic transition between mainly east-west, North America–Caribbean strike-slip motion to the west in Hispaniola and east-northeast–oriented underthrusting to the east beneath the Lesser Antilles island arc. Various models and tectonic mechanisms have been proposed for the Neogene to present-day deformation of southern Puerto Rico, its island margin, and the Muertos trench by previous workers that include normal, thrust, and strike-slip faulting accompanied by large-scale rotations. In this study, we present the results of a regional study integrating onland mapping of striated fault surfaces in rocks ranging in age from Oligocene to possibly as young as earliest Pliocene, and offshore mapping of faults deforming the uppermost sediments beneath the seafloor. The tectonic geomorphology and distribution of late Quaternary marine terraces and beach ridges in south-central Puerto Rico suggest either stability or slow late Quaternary uplift along the south-central part of the coast. In contrast, the coastline of southwestern Puerto Rico exhibits no late Quaternary coastal sediments and a pattern of long-term drowning of coastal features. Fault striation studies of three formations composing the Puerto Rico–Virgin Islands carbonate platform of south-central Puerto Rico (Juana Diaz Formation basal clastic unit, Juana Diaz Formation upper carbonate unit, Ponce Formation) indicate two distinct extensional phases affecting the youngest formation (Ponce Formation of middle Miocene–early Pliocene age). The first event, a north-northeast–directed extensional event is accommodated by normal faults striking mainly to the west-northwest. A second, southeast-directed extensional event crosscut and reactivated faults formed during the first event and produced at least one northeast-trending Quaternary rift bounded by northeast-striking normal faults (Ponce basin). Offshore seismic profiling by previous workers and reported in this study support the presence of late Holocene seafloor-rupturing, northeast-striking normal faults that accommodate southeast extension of the southern margin of Puerto Rico. The post–early Pliocene extension direction is roughly perpendicular to the east-northeast–trending sections of the stable or slowly uplifting coastline along much of southern Puerto Rico. In addition to northeast-striking normal faults, offshore profiles confirm the presence of late Holocene, seafloor-rupturing left lateral strike-slip faults along the offshore extension of the Great Southern Puerto Rico fault zone. Where the Great Southern Puerto Rico fault zone curves to the northeast, the fault becomes less strike-slip and more normal in character and produces greater extensional and tilting effects in the linked Whiting half-graben. A neotectonic model for southern Puerto Rico to explain both directions of extension known from fault striation studies and the present tectonic geomorphology of the preserved Puerto Rico–Virgin Islands carbonate platform in south-central Puerto Rico involves late Miocene–early Pliocene oblique collision of the Bahama Platform with Hispaniola to the northwest of Puerto Rico and counterclockwise rotation and extension of the area of southern Puerto Rico. A later crosscutting extensional event during the post–early Pliocene involves left-lateral transtension of the southern margin of Puerto Rico with most strike-slip motion concentrated along the Great Southern Puerto Rico fault zone.