Download image (jpg, 50 KB). Mo, Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140 km eastnortheast of Puerto Rico. Office of Ocean Exploration and Research | National Oceanic and Atmospheric Administration | Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none have gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. This shows that the low elevation of ocean basins and high elevation of continents is compensated by the thickness of the crust at depth. The Puerto Rico Trench is also associated with the most negative gravity anomaly on earth, -380 milliGal, which indicates the presence of an active downward force. The Puerto Rico Trench is atypical of oceanic trenches. [27], At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. Earthquakes and tsunamis in Puerto Rico, the Virgin Islandsand adjacent islands are mostly caused by the convergence of the North American tectonic plate with the Caribbean tectonic plate on which the islands are located. ) or https:// means youve safely connected to the .gov website. Then, at 3.5 Ma, the carbonate platform was tilted by 4 toward the trench over a time period of less than 40 kyr, such that its northern edge is at a depth of 4000 m and its reconstructed elevation on land in Puerto Rico is at +1300 m (Figures 1 and 2). Abstract The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. Local measurements of the gravity of Earth help us to understand the planet's internal structure. Well-documented slab tears that are associated with high rates of intermediate-depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Figure 4. Determining wh, Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. Download image (jpg, 44 KB). Report an Error, Site Index At a minimum, these include the tidal correction gtid, the terrain correction gT, and the free air correction gFA. Careful analysis of the gravity data allows geologists to make inferences about the subsurface geology. We study the oblique convergence of the NOAM and CARIB plates between southeastern Cuba to northern Puerto Rico, A post-hurricane survey of a Caribbean island affords comparisons with geologic evidence for greater overwash at the same place. Astrophysical Observatory. Gravity anomalies and the origin of the Puerto Rico Trench P. Molnar Geology 1977 The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. The Puerto Rico Trench is also associated with the most negative gravity anomaly on Earth, -380 milliGal, which indicates the presence of an active downward force. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. The gravity anomaly is the difference between the observed acceleration of an object in free fall (gravity) near a planet's surface, and the corresponding value predicted by a model of the planet's gravitational field. During Leg 1 of this expedition, the ship will transit from her home port in North Kingstown, Rhode Island, to the U.S. We use accounts of historical earthquakes to assign intensities and the intensity assignments for the 2010 Haiti earthquakes to derive an intensity attenuation relation for Hispaniola. Two-thirds of this is from the Moon. At the center of a level plateau, it is approximately equal to the free air anomaly. Examples include a magnitude 7.5 earthquake centered northwest of Puerto Rico in 1943, and magnitude 8.1 and 6.9 earthquakes north of Hispaniola in 1946 and 1953, respectively. The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure 1). The hazard from tsunamis is also apparent. To explore along with the expedition team, visit this page. The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. Bathymetry collected by Okeanos Explorer and University of New Hampshire Law of the Sea Project is shown in the background and gray boxes show the extent of existing Okeanos Explorer mapping surveys in the Atlantic. ago over a time period as short as 14-40 kyr. Over 4 million U.S. citizens live on these islands, mostly along the coast. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. The main topographic features of this area include: the Lesser Antilles volcanic arc, the old inactive volcanic arc of the Greater Antilles (Virgin Islands to Hispaniola), and the Caribbean and North American oceanic plates. Subsequent cooling thickens the lithosphere again and subsidence takes place. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The trench is less deep where the component of subduction is larger. Figure 1. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different) and that the Atlantic Ocean floor seaward of the outer rise, Puerto Rico, and the Caribbean Sea floor are each in isostatic equlibrium. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. Widespread damage in Puerto Rico and Hispaniola from an earthquake in 1787 was estimated to be the result of a magnitude 8 earthquake north of the islands. The crustal thickness and the free-air anomaly computed for sinkers of different sizes and depths are then compared with the measured freeair anomalies and seismic depths of the Puerto Rico trench and the mid-Atlantic rise. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. This brings the free-air anomaly, which omits the correction terms for either, close to zero. Figure produced in Google Earth Pro. The complex dates in part from the Eocene, and elements of the arc system. II - The Puerto Rico trench and the mid- Atlantic rise. Puerto Rico, the Virgin Islands to its east, and eastern Hispaniola to its west, are located on an active plate boundary zone between the North American plate and the northeast corner of the Caribbean plate (figure 1). [24], Gravity anomalies provide clues on other processes taking place deep in the lithosphere. The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S. Virgin Islands, where 4 million U.S. citizens reside. Other corrections are added for various gravitational models. By determining the likely hazards and their causative mechanisms and providing this information to government agencies and the public, the USGS can aid in such activities as improvement of building codes, encouraging safer zoning, and assisting public education for response to hazards. Rapid gravity computation for two dimensional bodies with application to the Mendocino submarine fracture zone, A crustal section across the Puerto Rico trench, Comments on paper by W. Jason Morgan, 'Gravity anomalies and convection currents, 2. [10], The next correction is the free-air correction. The Puerto Rico Trench is also associated with the most negative gravity anomaly on earth, -380 milliGal, which indicates the presence of an active downward force. It is concluded that the Mohorovii discontinuity (characterized by compressional-wave velocities of about 8 km/sec) lies at 9 km. Because the island lies on an active plate boundary, earthquakes are a constant threat, and the densely populated coastal areas are vulnerable to tsunamis (Uri ten Brink, 2003). Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Location of earthquakes as a function of depth and size in the northeastern Caribbean. The Bouguer, free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. This glider will be recovered several months later by NOAA Ship Nancy Foster when she is operating in the area. ROV dives will mostly be conducted during the day, while CTD casts, and multibeam, singlebeam, and sub-bottom acoustic mapping will occur at night when the ROV is on deck. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The ridge and its flanks appear to be fully isostatically compensated. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none has gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. Newton showed that the measured value of gravity was affected by the rotation of the Earth, which caused the Earth's equator to bulge out slightly relative to its poles. This platform was horizontally deposited over Cretaceous to Paleocene arc rocks starting in the Late Oligocene. [8], The local topography of the land surface affects the gravity measurement. The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. This belt of positive anomalies is called the Outer Gravity High. This is consistent with seismic data and suggests the presence of a low-density magma chamber under the ridge axis. The trench is less deep where the component of subduction is larger. Click image for larger view and image credit. This takes into account the fact that the measurement is usually at a different elevation than the reference ellipsoid at the measurement latitude and longitude. The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. Its northward edge is at a depth of 4,200 meters, and its southern edge can be found on land in Puerto Rico at an elevation of a few hundred meters. below sea level under the ocean basin, 12 km under the Caribbean Sea, at about 16 km under the trench, and at slightly shallower depth under Puerto Rico. Secure .gov websites use HTTPS [1] Typically the model is based on simplifying assumptions, such as that, under its self-gravitation and rotational motion, the planet assumes the figure of an ellipsoid of revolution. A lock () or https:// means youve safely connected to the .gov website. We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc region using 3-D models of static Coulomb stress change. Further refinements of the model field are usually expressed as corrections added to the measured gravity or (equivalently) subtracted from the normal gravity. Search for other works by this author on: Some aspects of the gravity field and tectonics of the northern Caribbean region, Trans, Fifth Carib. The USGS has an ongoing program to identify and map the faults in this region using various geophysical and geological methods in order to estimate the location and magnitude of potential earthquakes. The geologic settings of Puerto Rico and the Virgin Islands have created or contributed to several pressing societal issues related to human safety, environmental health, and economic development. The origin of the steep platform edge and the amphitheaters has been attributed to: (1) catastrophic failure, or (2) localized failures and progressive erosion. Bathymetry of the northeast corner of the Caribbean plate. Its northward edge is at a depth of 4,200 m, and its southern edge can be found on land in Puerto Rico at an elevation of a few hundred meters. The crustal thickness and the free-air anomaly computed for sinkers of different sizes and depths are then compared with the measured freeair anomalies and seismic depths of the Puerto Rico trench and the mid-Atlantic rise. The Puerto Rico Trench, in contrast, is situated at a boundary between two plates that The trench is shallower where the component of subduction is larger. The normal gravity accounts for the bulk gravitation of the entire Earth, corrected for its idealized shape and rotation. The isostatic anomaly includes correction terms for both effects, which reduces it nearly to zero as well. of Earth and Planetary Sciences, 1981. In the 20th century alone there have been several very large earthquakes north of Puerto Rico (Ms 7.3 in 1918; Ms 7.8 in 1943; Ms 8.0 in 1946 and four major aftershocks of Ms 7.6, 7.0, 7.3, 7.1 between 1946 and 1953). (or is it just me), Smithsonian Privacy Van Houten, C. A. Burk, H. D. Holland, L. C. Hollister. The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. Share sensitive information only on official, secure websites. In effect, the terrain correction levels the terrain around the measurement point. An official website of the United States government. A lock ( Share sensitive information only on official, secure websites. Both terrain higher than the measurement point and valleys lower than the measurement point reduce the measured value of gravity. Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause ten Brink, Uri Abstract The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. For a measurement point above the reference ellipsoid, this means that the gravitational attraction of the bulk mass of the earth is slightly reduced. E-mail Updates | User Survey | Contact Us | Report Error On This Page | Privacy Policy | Disclaimer | Site Info | Site Index Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. A crustal section across the Puerto Rico trench, J. geophys. Agreement NNX16AC86A, Journal of Geophysical Research (Solid Earth), Is ADS down? Other Puerto Rican cities also have substantial risk.The hazard from tsunamis is also apparent. The unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The transit area is in the Western Atlantic Ocean from Rhode Island straight to Puerto Rico. Download image (jpg, 44 KB). Gravity anomalies and convection currents. [2] Gravity on the surface of this reference ellipsoid is then given by a simple formula which only contains the latitude. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. The overwash, after crossing a fringing coral reef and 1.5 km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Plate convergence is highly oblique (2010), resulting in a complex crustal boundary with along-strike segmentation, strain partitioning and microplate tectonics. The trench is less deep where the component of subduction is larger. The rate at which these plates come together is similar to the rate at which a human fingernail grows. Here the axis of the free-air minimum lies very close to the eastern border of a zone of epicenters which apparently marks the commencement of crustal faulting along the underthrust. 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