Carl Bowin

Continental Rise off Eastern North America

During mid-1967 two cruises of the Woods Hole Oceanographic Institutions R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Continental Margin Off Western Africa: Angola to Sierra Leone

About 30,750 line-km of geophysical traverses (seismic reflection and refraction, magnetics, and gravity) were made in the Gulf of Guinea and vicinity aboard R/V Atlantis II during 1972 and 1973 as part of the International Decade of Ocean Exploration program. These traverses, supplemented by about 50,000 line-km of previous ones by other ships, provide a basis for mapping and understanding the geologic structure, history, and origin of the region. The deep indentation of the outline of western Africa is paralleled by a similar bend of the Mid-Atlantic Ridge and by the prominent bulge of northeastern Brazil. These sharp bends are the result of left-lateral offsets by many transform faults in a belt of equatorial fracture zones. Some of the fracture zones continue eastward and intersect the entire length of the east-west coast of the Gulf of Guinea and penetrate the continent at the Benue trough or graben. The valleys of the fracture zones have been sites of sediment deposition, whereas the ridges have served as dams that force the sediment to move westward. Where enormous quantities of sediment have been delivered to the ocean by the Niger-Benue Rivers, a large delta has deeply buried the irregular topography of the fracture zones. In this entire belt of fractured ocean floor the structure, physiography, and stratigraphy have been controlled by lateral movement, or translation, of the ocean floor with respect to the continent. In contrast, the regions southeast and northwest of the belt of equatorial fractures have fewer large fracture zones, smoother topography, and simpler sediment wedges. These two regions owe their origin to simple divergence during sea-floor spreading, when new oceanic basement added at the Mid-Atlantic Ridge increased the distances between the African continent, the Mid-Atlantic Ridge, and the American continents. Deposition of sediments along the margins of the originally narrow Atlantic Ocean was dominated early by coarse-grained and largely nonmarine sediments. South of the Gulf of Guinea these deposits were followed by evaporites as products of restricted water circulation in a long narrow arm of the ocean. There was little flow of water across the equator because of the sliding-v lve nature of the region of translation between the two regions of divergence. As spreading continued, the ocean widened, and thick prisms of marine sediments were deposited on the continental margins. Large deltas in western Africa first began at the south, with the now submerged deltas of the Orange and the Congo Rivers being chiefly Mesozoic in age and having no present coastal projection. The Niger delta farther north is mostly Cenozoic in age. Petroleum prospects appear to be far greater in the Niger delta and the region of divergence south of it than in the entire region west of the delta. The favorable continental margin contains thicker sediments, large ancient and modern deltas, and salt and mud diapirs. End_Page 2209------------------------------

Plate Convergence and Accretion in Taiwan-Luzon Region

During the middle and late Miocene the northern continuation of the Luzon volcanic arc approached the Asian continental margin through subduction of oceanic crust on its western side. Collision deformation began in late Miocene time because middle Miocene marine fossils are present in the slate belt of Taiwan. South Taiwan, the crust-crust contact between the Philippine and Asia plates occurs at the large negative free-air gravity anomaly over the deformed accretionary wedge of the Manila Trench. The deformed sediment wedge has been uplifted to shallower depths toward Taiwan and forms part of the southernmost peninsula of Taiwan. The pattern of gravity anomalies suggests that the Luzon arc has underthrust the Central Range of Taiwan, to at least 40 km west of the longitud nal valley. Thus the west edge of the Philippine plate at Taiwan appears to be chisel shaped in cross section. Recent transcurrent faulting along the longitudinal valley is being superimposed on this collisional suture. The Ilan plain is a landward extension of the Okinawa Trough and forms the southwestern end of that extensional back-arc structure of the Ryukyu Island arc. The crust-crust contact between the Ryukyu arc and the Philippine plate occurs at the negative free-air anomaly of the accretionary wedge. On the west this contact abuts the northward end of the longitudinal valley fault. We speculate that the Luzon arc convergence zone in the region of Taiwan has been sealing southward and that concomitantly a new plate boundary may be developing on the east side of Luzon and progres ing northward. Magnetic anomalies trending N70°E in the China basin west of Luzon are associated with the crest of an extinct spreading ridge which formed the deep part of the China basin by separating older but shallower crust. Magnetic anomalies in the westernmost Philippine basin suggest the existence of north-trending anomalies. The consistency of the skewness, the small amplitude factor, and to a lesser extent the bathymetric cross sections in the westernmost Philippine basin compared with those of the Central Basin Ridge region on the southeast support the view that the Gagua Ridge may be an extinct spreading center and that the adjacent topography was formed by seafloor-spreading.

Arc-Continent Collision in Banda Sea Region

A 2-month marine geophysical study of the Banda arc region was conducted in late 1976 using the R/V Atlantis II of WHOI and the R/V Thomas Washington of SIO; 19 seismic refraction lines were successfully completed. Oceanic crust underlies the Banda Sea and Weber Deep. Continental crust 35 to 40 km thick underlies the Australian Shelf. Thick continental crust is also present beneath the Timor and Aru Troughs. Low-amplitude magnetic anomalies are present over the Australian Shelf and extend to near the western edge of the Banda outer arc and, together with the presence of metamorphic rocks, suggest that continental crust may extend to the eastern lip of the Weber Deep. Continuous seismic reflection profiling shows the Australian Shelf sedimentary sequence dipping beneath th accretionary prisms of the outer Banda arc at the Timor and Seram Troughs: the tectonic front of the subduction zone lies along the axis of these troughs. The bathymetric profile on the outer flank of the Timor and Seram Troughs is unusual in that the profile asymptotically approaches a shallow depth near sea level and no outer rise is present as at oceanic trenches. An elastic-flexure analysis of this topographic profile indicates that an elastic plate is an incorrect model for the lithosphere at this plate-convergence zone. The Aru Trough, although structurally on trend with the Timor Trough, is not presently a site of subduction and compression tectonics. Instead, it is now a place of crustal extension, and is an example of graben formation that is separating a block of Australian co tinental crust (beneath the Kai Islands) from the Australian platform. The present lack of structural continuity between the Seram Trough and the Aru and Timor Troughs is best demonstrated by the pattern of gravity anomalies. The discontinuity between the Seram and Aru Troughs supports the seismic evidence that the Seram subduction zone is separate from the Southern Banda subduction zone that is continuous with that beneath Java on the west. The Bouguer gravity anomaly pattern indicates a division of the Banda arc into four segments: a southern segment, Timor to Babar, with relative plate convergence possibly trending about N20°W between the Banda and Australian plates; a central segment from Tanimbar to about Kasiui beneath which the Java to Timor Benioff zone is bent to a northwar trend; a Seram segment that is converging with the Australian platform along approximately a S70°W trend; and a Buru segment that has rifted away from the Sulu Islands. The southern segment has been undergoing compressional deformation distributed across the width of the arc for the last 3 m.y. with attendant slowing of the differential-slip rate between the leading edge of the accretionary wedge and the underthrust Australian margin crust. Oblique convergence and the bend of the Benioff zone produce in the Tanimbar to Kasiui segment a slower convergence rate normal to the arc at the southern end which diminishes northward and changes to extension normal to the arc at the Aru Trough. Thrust focal mechanisms indicate that subduction is presently active at the Seram Trough. Seafloor- preading magnetic anomalies appear to have been found in the south Banda basin. The trend approximately N60 to 70°E, similar to Cretaceous anomalies in the eastern Wharton basin, suggests the possibility that the Banda Sea may be trapped oceanic crust. Water depths of 5 km and low heat flow (generally 1.5 HFU or less) are compatible with an old age (greater than 25 to 60 m.y.) for the Banda Sea crust. We conclude that the Outer Banda arc from Buru around to Timor, and possibly to Sumba, contained Australian continental crustal blocks and fragments prior to its collision with the Australian margin in the last 3 to 6 m.y. Continuous convergence following the addition of a thick Australian margin sedimentary sequence to the south Banda subduction zone has led to deformation being distributed over the width of the arc and not simply being taken up on a single thrust surface. This scenario helps reconcile the geologic relations on Timor, Seram, and Buru with the structural continuity of the Timor Trough with the Java Trench. End_Page 868------------------------------

Depth of principal mass anomalies contributing to the earth's geoidal undulations and gravity anomalies∗

Abstract To help decipher the distribution of the principal mass anomalies within the earth, an analysis is made of the earths potential field by simultaneously considering both gravity and geoid anomalies. Ancillary knowledge about the earth from astronomical, angular momentum, seismological, and plate tectonic studies aids in deciding between deep or broad shallow‐mass anomalies as the causative source of certain potential field anomalies. The geoid (or gravity) anomaly for an individual gravity potential field feature as a function of increasing spatial resolution (decreasing wavelength) is obtained by accumulating stepwise the contributions of spherical harmonic coefficients for each degree and order for any selected location. This is equivalent to displaying the spectral content of an anomaly. The ratio of gravity to geoid (g/N) for each individual harmonic degree is independent of the harmonic coefficient values, being determined only by the degree and values for normal gravity and the earths radi...

Continental Margin Off Western Africa: Senegal to Portugal

About 22,000 km of continuous seismic-reflection, magnetic, and gravity profiles, 118 radiosonobuoy recordings, 98,000 km of geophysical profiles from previous investigations, 15 deep-sea-drilling logs, and many dredge samples served to reconstruct the history of the continental margin and adjacent deep-ocean floor between Senegal and Portugal. Initial structures of the margin south of Morocco formed by divergence when Africa and North America separated 180 m.y. ago. The margin off western Portugal had a similar origin when the Iberian Peninsula and North America separated about 80 m.y. ago. Between these two divergent segments the area of the Strait of Gibraltar formed by a combination of translation from 180 to 72 m.y. ago and plate convergence from 63 m.y. ago to the p esent, with convergence becoming more intense during the past 10 m.y. Oldest sedimentary rocks atop basement include an evaporite of Late Triassic to Early Jurassic age. When the apron deposited by upbuilding and outbuilding became thick enough, mobility of the evaporites deformed the overlying sediments especially north of the Canary Islands. Except off Morocco and the Strait of Gibraltar and possibly off southern Senegal the sedimentary blanket is dominantly calcareous, reflecting the general lack of fluvial influx. Included is a middle-Late Jurassic algal reef that constitutes the lower continental slope off Morocco. During Aptian-Cenomanian time the deep ocean off much of northwestern Africa had only sluggish bottom circulation, recorded by organic-rich sediments. A major hiatus in deep-ocean sedimentary rocks and in three prominent sedimentary ridges (off Madeira, near Agadir canyon, and north of Conception bank) probably was caused by temporarily intensified circulation. Tertiary tectonics modified the divergent margin south of Morocco by folding of shelf strata off the western High Atlas, emplacement of the Canary Island Ridge, folding of slope strata off Spanish Sahara, and uplift of the Cape Verde plateau. These orogenies also may have uplifted oceanic basement beneath the upper rise and formed the volcanic seamounts along this ridge. Maximum modification by Tertiary diastrophism occurred on the margin of translation-convergence near the Strait of Gibraltar. There, the convergence phase caused uplift of Gorringe bank. Plate convergence also deformed sediments atop oceanic basement, aided by the mobility of Triassic-Jurassic evaporites. More recently, probably because of uplift of the Iberian Peninsula during the Pliocene, well-stratified Miocene and younger deposits atop the deformed lower unit slid oceanward away from the peninsula, with the megaslide coming to rest against the Moroccan continental slope. Associated folding also involved the lower deformed sequence.

Depth estimates from ratios of gravity, geoid, and gravity gradient anomalies

The utility of combining geoid, gravity, and vertical gravity gradient measurements for delineation of causative mass anomalies is explained and compared with spatial and spectral methods for depth estimation. Depth rules for various source geometries are reviewed and new rules developed for geoid, gravity, and vertical gravity‐gradient data. Both spatial and frequency‐domain methods are discussed. Simple ratios of single observations of different data types (e.g., geoid, gravity, or vertical gravity gradient) are shown to provide information comparable to the traditional spatial and frequency analyses of one data type alone.

Continental Margin Off Western Africa: Cape St. Francis (South Africa) to Walvis Ridge (South-West Africa)

Approximately 17,000 km of continuous gravity, magnetic, and seismic-reflection profiles were recorded to determine the structure of the continental margin from Cape St. Francis to Walvis Ridge, and of the adjacent Agulhas and Cape deep-ocean basins. These and previous sea-floor and land data suggest that basement structures are the result of the breakup of Gondwanaland and the dispersion of the fragments to their present positions. This breakup may have been initiated as early as the Carboniferous Period, but most of the dispersion has taken place since Middle Jurassic. Igneous activity during the early phase may have led to the emplacement of ridges along the continental margin. Later, block faulting and volcanism along the fracture zones that delineate the flow lines of the drifting continents produced Walvis Ridge, Cape Rise, and the Agulhas Plateau. One of these fracture zones, the Agulhas fracture zone, dominates the structural grain of the continental margin and deep-ocean floor off the African southern coast. Sediments as thick as 7 km buried the fragmented continental basement and adjacent oceanic basement off the west coast and formed a broad continental rise and abyssal plain within Cape basin. The source of much of this clastic debris is believed to be the Orange River. In contrast, sedimentation off the southern coast since the breakup of Gondwanaland has been very limited, mostly being restricted to the narrow St. Francis basin atop the shelf. The adjacent continental slope and Agulhas basin have only a thin sediment cover. Much of the sediment that was present on the slope has slumped into the narrow Agulhas fracture zone at the base of the slope. Numerous swells on the southwestern end of the Walvis Ridge, undulating topography of the ocean floor in the western part of the Cape basin, swells on the upper continental rise, and the rough topography of the Agulhas Plateau were formed by the movement of the South Atlantic Bottom Water that enters Cape basin on the west side, flows along the southern flank of Walvis Ridge, and then is deflected southward by the continental slope. Pleistocene eustatic changes in sea level considerably modified the shelf, upper slope, and the eastern end of Walvis Ridge by wave action, turbidity currents, and the Benguela Current.

The Geology of Hispaniola

At present we have a very imperfect picture of the evolution of an island arc. Linear chains of active volcanoes presently occur in many island arcs and are associated with active subduction of an oceanic lithospheric plate beneath the arc, as are deep-sea trenches. How lithospheric underthrusting begins is much in doubt. The proposed nascent island arc in the Indian Ocean (Sykes, 1970) outward from the Java Trench, suggested by a diffuse distribution of epicenters, has as yet not been supported by further studies in the area. The development of an island arc is poorly known, and the validity of interarc spreading (Karig, 1971a, 1971b) is debated. What happens to an island arc when underthrusting ceases is also a matter of conjecture. Obviously, detailed geologic studies of island arcs can and will contribute to a better understanding of these problems. This paper attempts to summarize the present geologic knowledge of the island of Hispaniola toward that undertaking.

THE TOKYO SURFACE SHIP GRAVITY METER: RECENT DEVELOPMENTS AND RESULTS OF COMPARISON MEASUREMENTS

The Tokyo Surface Ship Gravity Meter (TSSG) employs a single vibrating string accelerometer which is inherently nonlinear. This leads to errors unless its frequency is essentially constant over the sampling intervals. A new processor incorporating pulse train logic and employing a rapid sampling rate of about 50/sec has been designed to replace the digital computer which used about 2/sec.