The Lake Mead fault system is a northeast-striking, 130-km-long zone of left-slip in the southeast Great Basin, active from before 16 Ma to Quaternary time. The northeast end of the Lake Mead fault system in the Virgin Mountains of southeast Nevada and northwest Arizona forms a partitioned strain field comprising kinematically linked northeast-striking left-lateral faults, north-striking normal faults, and northwest-striking right-lateral faults. Major faults bound large structural blocks whose internal strain reflects their position within a left step-over of the left-lateral faults. Two north-striking large-displacement normal faults, the Lakeside Mine segment of the South Virgin–White Hills detachment fault and the Piedmont fault, intersect the left step-over from the southwest and northeast, respectively. The left step-over in the Lake Mead fault system therefore corresponds to a right-step in the regional normal fault system. Within the left step-over, displacement transfer between the left-lateral faults and linked normal faults occurs near their junctions, where the left-lateral faults become oblique and normal fault displacement decreases away from the junction. Southward from the center of the step-over in the Virgin Mountains, down-to-the-west normal faults splay northward from left-lateral faults, whereas north and east of the center, down-to-the-east normal faults splay southward from left-lateral faults. Minimum slip is thus in the central part of the left step-over, between east-directed slip to the north and west-directed slip to the south. Attenuation faults parallel or subparallel to bedding cut Lower Paleozoic rocks and are inferred to be early structures that accommodated footwall uplift during the initial stages of extension. Fault-slip data indicate oblique extensional strain within the left step-over in the South Virgin Mountains, manifested as east-west extension; shortening is partitioned between vertical for extension-dominated structural blocks and south-directed for strike-slip faults. Strike-slip faults are oblique to the extension direction due to structural inheritance from NE-striking fabrics in Proterozoic crystalline basement rocks. We hypothesize that (1) during early phases of deformation oblique extension was partitioned to form east-west–extended domains bounded by left-lateral faults of the Lake Mead fault system, from ca. 16 to 14 Ma. (2) Beginning ca. 13 Ma, increased south-directed shortening impinged on the Virgin Mountains and forced uplift, faulting, and overturning along the north and west side of the Virgin Mountains. (3) By ca. 10 Ma, initiation of the younger Hen Spring to Hamblin Bay fault segment of the Lake Mead fault system accommodated westward tectonic escape, and the focus of south-directed shortening transferred to the western Lake Mead region. The shift from early partitioned oblique extension to south-directed shortening may have resulted from initiation of right-lateral shear of the eastern Walker Lane to the west coupled with left-lateral shear along the eastern margin of the Great Basin.

Puerto Rico and the northern Virgin Islands define the eastern terminus of the Greater Antilles, which extend eastward from offshore eastern Central America to the Lesser Antilles volcanic arc and mark the boundary between the Caribbean and North America plates. In Hispaniola, Puerto Rico, and the northern Virgin Islands, the Puerto Rico trench and the Muertos trough define the northern and southern limits of the plate boundary zone, respectively. Three microplates lie within the boundary zone: (1) the Gonave in the west; (2) the Hispaniola in the center; and (3) the Puerto Rico–northern Virgin Islands in the east. Results from Global Positioning System (GPS) geodesy conducted in the region since 1994 confirm the presence of an independently translating Puerto Rico–northern Virgin Islands microplate whose motion is 2.6 ± 2.0 mm/yr toward N82.5°W ± 34° (95%) with respect to the Caribbean. Geodetic data are consistent with east-west extension of several mm/yr from eastern Hispaniola to the eastern Virgin Islands. Extension increases westward with the most, 5 ± 3 mm/yr, accommodated in the Mona rift, confirming earlier GPS geodetic results. East-west extension of 3 ± 2 mm/yr also is observed across the island of Puerto Rico, consistent with composite focal mechanisms and regional epicentral distributions. Although the loci of extension are not known, similarity of GPS-derived velocities among sites in eastern Puerto Rico suggests the active structures lie west of the San Juan metropolitan area. Reactivation of the Great Northern and Southern Puerto Rico fault zones as oblique normal faults with right-lateral slip is a possibility. East-west extension of 2 ± 1 mm/yr also must exist between eastern Puerto Rico and Virgin Gorda, which likely is attached to the Caribbean plate. These extensional belts allow eastward transfer of slip between North America and the Caribbean from the southern part of the plate boundary zone in the west to the northern segment in the east. Motions along or across any of the individual subaerial structures of Puerto Rico are ≤2 mm/yr. The Lajas Valley in the southwest, where microseismicity is greatest, is the locus of highest permissible on-land deformation. Northwest-southeast to east-west extension of 2 ± 1 mm/yr is also observed across the Anegada Passage.

High-resolution single-channel seismic reflection profiles, bathymetry and sides-can sonar imagery from the Puerto Rico trench document the present-day and post-collisional effects of the obliquely subducting southeastern extension of the Bahama Province and the Main Ridge fracture zone on the northern Puerto Rico–Virgin Islands margin. In contrast to an orthogonal system, where it is unlikely that two high-standing ridges will impact the same section of margin, along the Puerto Rico trench convergence is highly oblique and the deformational effects of the two ridges are superimposed and often difficult to isolate. A middle–upper Miocene margin-wide unconformity in the Oligocene–lower Pliocene shallow-water carbonate platform of the Virgin Islands, Puerto Rico, and eastern Hispaniola provides an excellent horizontal reference frame for the timing and impact of the high-standing ridges on the margin. During the past 10 m.y., trenchward tilting (4–6°) of the carbonate platform including the margin-wide erosional surface to >5000 m water depths provides evidence that the margin has experienced significant subsidence. In this paper we present a model of accelerated subduction erosion and diachronous margin subsidence triggered by the ridges sweeping from east to west beneath the Puerto Rico forearc. Evidence for ongoing and past subduction erosion include zones of enhanced seismicity, oversteepening and mass wasting of the forearc slope, and landward migration of the inner trench-slope break. We document over 3 km of Neogene subsidence presumed to be the result of rapid crustal thinning associated with the tunneling of the buoyant, thick (∼20-km-thick crustal/sediment section) southeastern Bahama Province beneath the forearc. During the past 3.5–5 m.y., the volume of material eroded from the overriding plate is estimated to be ∼210 km 3 /km of margin, equivalent to an erosion rate of 42–60 km 3 /m.y./km of margin. A significant reduction in the rate of margin subsidence in the eastern part of the survey area suggests that the Main Ridge fracture zone has had a relatively small erosional impact on the margin, and that under normal conditions, this highly oblique convergent boundary is characterized by relatively slow rates of subduction erosion. Three strike-slip fault zones are imaged in the forearc: the East Septentrional fault zone, the Bunce fault zone, and the Bowin fault zone. The Bunce and Bowin fault zones trend N80°–90°E, subparallel to the predicted North America–Caribbean relative plate motion vector (N70°E) determined by global positioning system (GPS) studies. Seismic reflection profiles across the Puerto Rico trench in the western section of the survey area, where the Bunce fault zone is <10 km from the deformation front, reveal thick (0.75–1.5 km) accumulations of relatively undeformed trench sediment fill. This lack of shortening deformation suggests that convergence approaches pure strike-slip and that strain within the forearc is largely accommodated by the strike-slip fault zones. Strike-slip faulting appears to be progressively partitioned to the east where deformation in the trench is more clearly contractional. Although not a typical plow mark, the Bowin fault zone appears to represent the long-term (∼10 m.y.) track of the underthrusting and colliding southeastern Bahama Province across the forearc area that is consistent with the convergence direction exhibited by GPS results.

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.