Island Platform Research Articles

Gravity effects of volcanic islands

Abstract Gravity surveys of six volcanic islands in the Southern Cook Group (South-West Pacific Ocean) show that the radial anomaly gradient near the reef is much larger than the calculated effect for an island of uniform density. The gravity surveys also show that the difference between the modified Bouguer anomalies at the centre of each island and at the reef edge are much larger than the calculated effects for an island of uniform density. The observed values, however, are consistent with the calculated effects of an uncompensated island platform of density 2·35 g/cm3 which contains a core of density 2·87 g/cm3 and radius equal to the radius of the island at sea level. The gravity observations are incompatible with the theory of local isostatic compensation and therefore some form of regional compensation must apply. Gunn's isobaric hypothesis (lithospheric thickness 50 km) would give rise to a maximum depression below each island of less than 0·1 km and hence would have negligible gravitational effect. However, if the lithospheric thickness was only 10 km, the maximum depression would be 0·8 km and the densities of the island platform and core would be 0·14 g/cm3 larger than those computed on the basis of an uncompensated island.

Gravity survey in the Cook Islands

Abstract The observed Bouguer gravity anomalies on the four volcanic islands (Mangaia, Mauke, Mitiaro, and Rarotonga) in the Southern Cook Group and Manihiki Atoll in the Northern Group are consistent with the calculated gravitational effects of uncompensated island platforms of average density 2·32 g/cm3 containing volcanic cores of average density 2·88 g/cm3. However, the anomalies on Rakahanga Atoll in the Northern Group are consistent with an uncompensated island platform having a uniform density of 2·52 g/cm3 and no core.

A reconnaissance geophysical survey in the Andaman Sea and across the Andaman-Nicobar Island Arc

A marine geophysical study of the Andaman Sea has been conducted as part of the International Indian Ocean Expedition. A combination of magnetic, gravity, bathymetric, and sea-floor heat-flux measurements, seismic sparker reflection profiles, and bottom sediment samples has been used in a study of the seaward continuity of major subaerial tectonic trends. The data indicate positive continuity of the structural trend of the Barisan Range of northern Sumatra and the Burma Range. It was found that the central graben of the Barisan Range of northern Sumatra extends into the Andaman Sea north to latitude 10°N. A previously unreported interdeep has been observed between the outer sedimentary island arc and the inner igneous trend of the major primary arc which forms the western boundary of the Andaman Sea. Continental thickness of the crust is indicated under the sedimentary island platform. In the area of the backdeep, the north-northeast trends of the Malaysian peninsula are prominent.

Island-Arc Structure Interpreted from Aeromagnetic Data Near Puerto Rico and the Virgin Islands

Research Article| February 01, 1966 Island-Arc Structure Interpreted from Aeromagnetic Data Near Puerto Rico and the Virgin Islands ANDREW GRISCOM; ANDREW GRISCOM U. S. Geological Survey, Menlo Park, California Search for other works by this author on: GSW Google Scholar WILBURT H GEDDES WILBURT H GEDDES U. S. Naval Oceanographic Office, Washington, D. C. Search for other works by this author on: GSW Google Scholar Author and Article Information ANDREW GRISCOM U. S. Geological Survey, Menlo Park, California WILBURT H GEDDES U. S. Naval Oceanographic Office, Washington, D. C. Publisher: Geological Society of America Received: 28 Sep 1964 Revision Received: 23 Jul 1965 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1966, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1966) 77 (2): 153–162. https://doi.org/10.1130/0016-7606(1966)77[153:ISIFAD]2.0.CO;2 Article history Received: 28 Sep 1964 Revision Received: 23 Jul 1965 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ANDREW GRISCOM, WILBURT H GEDDES; Island-Arc Structure Interpreted from Aeromagnetic Data Near Puerto Rico and the Virgin Islands. GSA Bulletin 1966;; 77 (2): 153–162. doi: https://doi.org/10.1130/0016-7606(1966)77[153:ISIFAD]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A contoured aeromagnetic map of the Muertos Trench, Puerto Rico–Virgin Islands Platform, Puerto Rico Trench, and outer ridge provides new information concerning the structure of this region. The lithology, age, and structure of magnetic rocks north of the Puerto Rico Trench may be similar to those of the pre-Oligocene rocks of Puerto Rico. There appears to be no major fault on the north side of the Puerto Rico Trench. A major fault (possibly a thrust fault) with a vertical separation of at least 2 km is buried beneath the middle Tertiary and younger sedimentary rocks of the north side of the Puerto Rico–Virgin Islands Platform. The magnetic patterns are consistent with an origin by block faulting for the Anegada Passage and the Virgin Islands Basin. There is a possible chain of submarine volcanoes south of and parallel to the Puerto Rico–Virgin Islands Platform. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Virgin islands bathymetric survey

A bathymetric survey in the vicinity of the Virgin Islands showed that Anegada and Jungfern Passages, which connect the Atlantic Ocean with the Caribbean Sea between the Virgin Islands Platform and St. Croix Island, are the deepes charted passages between the two seas. The 1,072-fathom sill depth of Jungfern Passage is the limiting factor in the exchange of deep water between the Atlantic Ocean and the Carribean Sea. Furthermore, it was found that the Virgin Islands Basin, which lies between Anegada and Jungfern Passages, has a flat floor 2,400 fathoms deep. It is bounded on the north and south by sea scarps having apparent slopes of 9 to 43 degrees. The eastern end of the basin is divided into two arms which embrace a 420-fathom sea knoll. Both these arms terminate at sills which separate them from Anegada Passage and St. Croix Basin. The western end of the basin is connected with a smaller basin, 2,200 fathoms deep, which is bordered by Jungfern Passage on the south and by Grappler Bank on the west.

Marginal Sediments of Mississippi Delta

Marginal Sediments of Mississippi Delta

Bathymetry of the Puerto Rico Trench

The Puerto Rico Trench extends for about 400 mi in a generally east‐west direction 80 mi north of Puerto Rico and has a five to ten‐mile wide “flat” floor 4350 fm deep. The Trench is separated from the Nares Basin to the north by a 2900‐fm divide, and is roughly paralleled to the south by a series of seamounts which form an interrupted ridge from 2700 to 3700 fm deep. South of this ridge are basins, 4000 to 4100 fm deep, which may drain into the Trench through San Juan Canyon. The Trench floor has two kinds of sediment in alternating layers up to several meters thick: (1) layers of abyssal red clay with low carbonate content, which is “mormal” at this great depth, and (2) anomalous layers of highly calcareous sands showing excellent graded bedding and shallow water benthonic fossils. The layers of anomalous sand are attributed to the action of turbidity currents, which may have transported the sediment from neighboring island platforms down submarine canyons and deposited it in the Trench, rather than to former shallow water conditions in the Trench.

Electric Facilities and Operating Plan for the First Chicago Subway

The $64,000,000 subway project now being completed will provide long- needed rapid-transit terminal facilities in the Chicago downtown district. The State Street subway will be completely equipped and in operation early in 1943 and will constitute a major contribution to the problem of handling additional traffic resulting from a decrease in the use of passenger automobiles. This first subway route is five miles long and with fixed equipment will cost almost $34,000,000. The subways were built with track sections in tunnel-at low level. Platforms 500 feet long are provided at stations, except in the congested area where there is a continuous island platform 3,300 feet in length with access provided by mezzanine stations in each block. All fixed equipment is being installed by the city. Despite the delay in securing equipment because of present scarcity of critical materials, the entire project will be built and equipped in a period of four years. This high-speed construction program required expeditious planning of locations for all electric facilities. Power will be 600 volts direct-current. The 144-pound contact rail will be energized by existing substations and energy delivered by conventional positive and negative feeders, with contact rails reinforced with parallel feeders. Adequate sectionalization of the entire system is provided largely by automatic operation. Centralized supervisory control of the all-relay type includes most modern accessories, such as an illuminated diagram board and remote-control dispatching equipment. Train movements will be controlled throughout by a modern signal and electropneumatic interlocking system with automatic stops.