William J. Ludwig

Sediments and Structure of the Japan Sea

Seismic reflection (profiler) traverses of the Japan Basin, Yamato Basin, and intervening Yamato Ridge reveal horizontally stratified sediments over weakly stratified sediments. The basement surface is rough in some places and smooth in others and rises with the topography of Yamato Ridge and the lower continental slopes of Siberia and Japan. Compared to the Japan Basin, Yamato Basin has a shallower sea floor and thinner sediments. In each basin, the sediments decrease in thickness outward from a center. Wide-angle reflection and refraction data from 65 sonobuoy stations made en route give velocities in the sediments that range from 1.6 to 3.2 km/sec. Smooth oceanic basement (or layer 2) has two refracting layers, 3.5 and 5.8 km/sec; rough oceanic basement is typified by the 5.8 km/sec velocity alone. Layer 2 is thicker in the Yamato Basin than in the Japan Basin because of a greater amount of 3.5-km/sec capping material. Layer 3, of velocity about 6.8 km/sec, lies at nearly the same depth beneath the basins and Yamato Ridge. The results of profiler-sonobuoy measurements combined with the results of earlier two-ship seismic refraction measurements indicate that the Japan Basin and Yamato Basin are underlain by oceanic crust which in turn is covered by sediments (and volcanics?) that have built a shallower sea floor than that in the western North Pacific basin. Yamato Ridge appears to be mainly a pile of volcanics resting on an oceanic layer at normal depth. The crust of Yamato Basin may also have been modified to the extent that it has a thicker than normal layer 3 and a low-velocity mantle. Other profiler-sonobuoy data, gathered in the strait between Japan and Korea and across the Japan Sea margin of Southwest Japan and Northeast Japan, are presented. A number of profiler crossings of Toyama Channel indicate its formation by turbidity currents. Turbidity flows in this channel and in other channels transport sediment to the abyssal plain.

The Manila Trench and West Luzon Trough—I. Bathymetry and sediment distribution☆

Abstract Data from bathymetric (PDR) and seismic-reflection (profiler) traverses made across the island margin west of Luzon, Philippines, are here reported. A new contour chart of sea floor topography is presented for the region. A shallow arcuate depression approximately 2700 fms (4940 m) deep, herein called the Manila Trench, extends as a narrow, nearly continuous furrow sub-paralleling the west coast of Luzon from 16° 40′N south to about 13°N. The Manila Trench is divided into two deeps at about 15° 50′N, 119° 15′E by a submarine ridge trending southwest from the coast near the Lingayen Gulf. The north end of the trench is bounded by a submarine ridge trending east-west from the coast along latitude 16° 50′N. The south end of the trench extends southeastward as a major re-entrant into Mindoro Straits, west of Mindoro Island. A sediment-filled trough, herein called the West Luzon Trough, forms a terrace and trough approximately midway between the floor of the Manila Trench and the continental shelf. The trough is bounded on the north by the southwest-trending ridge from the Lingayen Gulf vicinity and on the south by a ridge trending northwest-to-north from Lubang Island. The latter ridge forms the seaward border of the West Luzon Trough. A group of seismic-reflection profiles illustrates the variation in sea floor topography and sub-bottom structure along the length of the trench and the trough. With the exception of the West Luzon Trough, no detectable amount of sediment is present eastward of the trench floor. A fairly uniform thickness of acoustically transparent sediment overlies a rough oceanic basement layer in the adjacent South China Sea basin. These sediments are continuous along the west wall of the trench and end abruptly as a perched ledge just west of the trench floor. The steep slopes of the perched ledges and evidence of fairly recent step-faulting along the west wall of the trench indicate post-depositional subsidence or extension of the sea floor near the axis of the trench.

Mantle reflections beneath the crestal zone of the East Pacific Rise from multi-channel seismic data

Abstract Two 24-fold, stacked multi-channel seismic-reflection profiles show the position of the M-discontinuity nearly continuously across the East Pacific Rise near 9°N. In the past, the position of this important boundary in the oceans has been determined almost exclusively by seismic-refraction measurements. The mantle reflection events are enhanced by the process of migration which diminishes the masking effect of the sea-floor diffractions. In one crossing of the Rise crest, the mantle reflections do not parallel the topographic relief in the time section, while in the other the mantle reflections follow the sea-floor topography. The presence of mantle reflections beneath the axial zone of an active spreading center indicates that the M-discontinuity is a feature which forms nearly contemporaneously with the oceanic crust.

Structure of Continental Margin off Punta del Este, Uruguay, and Rio de Janeiro, Brazil

The results of 33 seismic refraction profiles recorded along two lines, one west-east between Punta del Este and the Rio Grande rise and the other southeast-northwest between the Rio Grande rise and Rio de Janeiro, are presented in two structure sections, with continuous seismic reflection profiles recorded parallel with the refraction lines. Plots of magnetic and gravity data recorded along the reflection lines are shown with the seismic data. These data, supplemented by previously reported measurements on the northern Argentine shelf and in the Argentine basin, are compiled in a generalized structure map. The seismic section across the Uruguayan continental margin is similar to one previously measured eastward of Rio de la Plata. The continental margin south of Rio de Janeiro contains a sediment-filled shelf embayment and a marginal plateau which extends 450 km out to sea. Possible velocity-age correlations are given for the seismic sections.

Geophysical Investigations in the Submerged Argentine Coastal Plain

This report describes the results of seismic-refraction measurements conducted on the submerged coastal plain of the Buenos Aires province. Two elongated asymmetrical sediment-filled depressions in the Precambrian basement complex, the Rio Salado basin and Bahia Blanca basin (which includes the Rio Negro basin), were traced seaward. The two basins, separated by the Tandil high, trend approximately southeast. Each is about 150–200 km wide and at least 700 km long. No indication of closure was observed at the seaward end although both basins were traced well beyond the edge of the continental shelf. Four seismic layers overlying the basement were determined in an unconsolidated layer (A), a semi-consolidated layer (B), and two zones in a consolidated layer (C) and (D). These layers are identified as Quaternary and Pliocene-Miocene, Lower Miocene and Upper Cretaceous, Upper Paleozoic, and Lower Paleozoic to a depth of about 8.0 km based on well logs and seismic-refraction data made available by Yacimientos Petroliferos Fiscales. A sub-basement layer (6.84 km/sec) was measured under the continental rise at an average depth of 10–12 km. The Mohorovicic discontinuity was measured on two profiles at a depth of 26.5 km under the outer portion of the continental shelf. The basement configuration and stratigraphic sequence along several traverses are shown by seismic sections. A 10-km interval basement-contour map of the area shows the outline of the basement as determined by marine seismic data together with the correlation to YPF seismic-refraction measurements on land.

Structure of Vema Fracture Zone

Abstract Unreversed seismic refraction profiles were recorded over the axial trough portion of Vema Fracture Zone and over the crust on opposite sides, using short- and long-range sonobuoys. Travel time—distance analysis of refracted arrivals indicates that south of the axial trough there is a 600 m thick layer of (assumed) velocity 3.6 km/sec overlying a 4 km thick 6.3-km/sec crustal layer between it and the upper mantle. Beneath the axial trough a 2.2 km thick 4.3-km/sec layer on a 2.6 km thick 5.9-km/sec layer overlies the mantle. The main crustal layer of velocity 5.9–6.3-km/sec may correspond to layer 3 of the deep-ocean basin, but its velocity and thickness are significantly less beneath the axial trough. Oceanic basement (layer 2) north of the axial trough is approximately 2 km thick and has a 3-component-velocity structure; deeper crustal layers were not detected due to insufficient profile length.

Sediment Thickness and Depth to Basement in Western North Atlantic Ocean Basin

More than 200,000 km of single-channel and multi-channel seismic reflection data are used to construct an isopach map of sediment thickness and a map of depth to basement in the western North Atlantic Ocean basin, including the continental margins of eastern North America. Velocity regression equations and velocity estimates determined statistically from 17 provinces in the basin are used to convert reflection time in the sediments to thickness. The region between 15°N and 45°N lat. and between 40°W and 85°W long. is portrayed in Mercator projection at the scale of 1.0 in. per degree longitude (about 1:4,383,000 at equator). In this paper we outline the methods used for map preparation and discuss the most prominent features observed in basement struct re and in sediment thickness distribution.

The Manila Trench and West Luzon Trough—III. Seismic-refraction measurements☆

Abstract Data from 13 seismic-refraction profiles recorded west of central and northern Luzon in the South China Sea are presented in two schematic structure sections. The eastern edge of the South China Sea Basin has a velocity-structure similar to that of normal oceanic crust, except that the average thickness of the main crustal layer (layer 3) is only about half normal thickness, and the M-discontinuity is shallower than normal by about 2–3 km. The intermediate level West Luzon Trough off central Luzon contains approximately 4 km of sediment with velocities of 2·1 and 2·6 km/sec overlaying a 4·4 km/sec basement layer which presumably constitutes the outer ridge of the trough. Measurements made west of northern Luzon indicate that in physiography and velocity-structure it is similar to the region off central Luzon, except there is no topographic trench along the foot of the island margin. Instead there is a subtle depression in the sea floor with sediments much thicker than are found off central Luzon. This indicates that if a trench is forming in the area, it is being filled contemporaneously with sediments transported from the north. There is an intermediate level sediment-filled depression off North Luzon that is not as well developed as its southern counterpart, but which nevertheless constitutes a major linear feature. The outer ridge consists mainly of material with velocity about 3·6 km/sec and is interpreted as sedimentary. The outer ridge follows the western convex side of the north and central Luzon arc and is broken only by a promontory complex off the coast near the Lingayen Gulf. The ridge may represent the outer non-volcanic component of a double island-arc system, similar to the outer non-volcanic ranges situated along the eastern convex side of the Mindanao Arc.

Structure of Argentine Continental Margin

Data from 100 seismic-refraction profiles recorded on the extra-Andean part of the Argentine continental margin are presented in ten schematic structure sections and two generalized structure maps. The results are correlated with previously reported measurements in the offshore Buenos Aires province and the Argentine basin, and with seismic-refraction measurements, bore holes, and surface geology on land. Between Buenos Aires and the southern end of Tierra del Fuego four of the major subsurface elements of Argentina extend eastward under the continental shelf; from north to south they are: (1) the Rio Salado basin, (2) the Bahia Blanca basin, (3) the San Jorge basin, and (4) the Magellan basin. Another sediment-filled depression, the Malvinas basin, was discovered lying between the Falkland Islands and Tierra del Fuego. The Rio Salado, Bahia Blanca, and Malvinas basins extend out to the edge of the continent and have been filled without barriers to sediment transport at the seaward ends. The overall effect of bending appears to be well demonstrated by the arcuate trends of the main structural elements and the tectonic lines of extra-Andean Patagonia, for the most part NNW-SSE with a complementary W-E direction. The data are consistent with the supposition of a mass drifting westward with some clockwise rotation.

Structure of the northern Brazilian continental margin

Results from two-ship seismic refraction profiles and supplementary data from several sonobuoys show that typical oceanic crust underlies both the landward and seaward sides of the North Brazilian Ridge. The Ceara Rise is also an oceanic structure, probably formed by local tectonic activity. Sediments of the upper Amazon Cone are more than 11 km thick, but they thin both landward and seaward. Owing to the great sedimentary thickness, refraction arrival times from oceanic basement could not be identified, but basement is assumed to extend under the cone.