Neotectonics and subsidence of the northern Puerto Rico–Virgin Islands margin in response to the oblique subduction of high-standing ridges

Abstract

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 km3/km of margin, equivalent to an erosion rate of 42–60 km3/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.