Elizabeth T. Bunce

Chain and romanche fracture zones

Abstract A series of left-lateral faults displace the axis of the Mid-Atlantic Ridge in the equatorial Atlantic. Two of the most prominent of these faults are the Romanche and Chain Fracture Zones. The ridge crest is offset 180 miles by the Chain Fracture Zone. The Vema Depth of 4106 fm is the maximum depth observed in the Romanche Trench. The floor of the trench is locally smoothed by sediments gravity transported from adjacent areas. Allochthonous fossils occur in cores from the trench floor. Scour by bottom currents is apparent in photographs and in the character of the sediments obtained by coring.

GEOPHYSICAL INVESTIGATION OF THE CONTINENTAL MARGIN BETWEEN CAPE HENRY, VIRGINIA, AND JACKSONVILLE, FLORIDA

Forty seismic-refraction and reflection profiles on the eastern continental shelf and adjacent deep-water areas of the Atlantic Ocean, from 29°39′ to 36°30′ N. Lat. and 73°30′ to 81°10′ W. Long, trace the transition from deep-oceanic to continental-type structures. The transitional area divides naturally into three parts: the continental shelf, the Blake Plateau, and the adjoining deep-water area. The results on the continental shelf are correlated with adjacent continental geology. The deepest horizon traced along the shelf is interpreted as granitic basement, which has compressional velocities of 5.82–6.1 km/sec. At the southern extremity it is at a dept of 6 km, shoals to 0.86 km near Cape Fear, and deepens north of Cape Hatteras to more than 3 km. North of Charleston, South Carolina, there is excellent depth correlation with granitic basement in coastal wells; to the south all deep wells are inland. Age correlations are based on well data near the coast, which indicate to us that most of the observed section is Cretaceous. On the Blake Plateau, several layers (1.83–4.5 km/sec.) are interpreted as sedimentary. A 5.5-km/sec. layer is found only south of a line from 30°30′N., 78°W. to Cape Canaveral. Velocities higher than 5.5 km/sec, have been measured on six profiles on the Blake Plateau. The 5.5-km/sec. layer and a 6.2-km/sec. layer appear to form a positive feature to the south of the above-mentioned line. Higher velocities, 8.0 km/sec, and 7.28 and 7.3 km/sec., which are probably not the same horizon, are found at markedly different depths. Possibly these represent the M layer and ultrabasic material, depending on relations not now known. The deep-water area is a continental slope and rise modified by the Blake Plateau and by a ridge trending southeastward from Cape Fear and deepening from about the 1500-fathom contour to more than 2000 fathoms (3657 m). The ridge is underlain by thick low-velocity layers (1.83–2.96 km/sec.), interpreted as sediments, and higher-velocity layers which form a distinct linear structure having the same general trend as the ridge. At its northwestern end this trend terminates against a thick lower-velocity section interpreted as a sediment-filled trough. South of the ridge profiles are similar to those of the ocean basins. Excellent seismic-refraction evidence of faulting indicates subsidence of the ridge relative to its surroundings. A hypothesis interprets the ridge as a former chain of islands and reefs on a structural trend colinear with the Cape Fear Arch. The structural pattern formed by the Piedmont crystallines and the Peninsular Arch of Florida and the Cape Fear Arch and the ridge resembles that formed by the Japanese archipelago, the Ryukyus, and the Bonin ridge. The pattern of the trough of the Blake Plateau and the deep-water area is somewhat similar to such modern features as Exuma Sound and the Tongue of the Ocean in the Bahamas. Such comparisons should not be regarded as strictly homologous but are suggested as possibly reflecting similarities in deep-lying tectonic activity. Similarly it would seem fruitful to consider similaritiesof deep structure beneath isolated seamounts, chains of seamounts, submerged ridges, island arcs, and mature mountain ranges.

Geophysical study of the northwest African margin off Morocco

Abstract Six geophysical profiles were taken over the northwestern African margin between the Canary Islands and Morocco. A magnetic smooth zone occupies the upper continental rise landward of lineated sea-floor spreading anomalies of Late Jurassic to Early Cretaceous age. This smooth zone is believed due in part to a Late Jurassic uniform polarity interval; its seaward boundary is correlated with a basement high and/or abrupt landward thickening of sediments. Spreading may have started over 200 × 10 6 yr ago at a rate of 0·55 cm/yr and accelerated 150 to 160 × 10 6 yr at the smooth zone boundary. No counterpart of the east coast magnetic slope anomaly was observed off Morocco while the distance between the shelf break and the smooth zone boundary is also less here than off northeastern America. This difference suggests a shift in the position of the accreting plate boundary from west to east, about 200 × 10 6 yr ago, after the formation of the slope anomaly. Gravity data show that the Moroccan margin is largely in isostatic equilibrium, and that the zone of transition from oceanic to continental crustal thickness is wider off the Canaries (400 km) than off Morocco (100–150 km). The sediment section of the seismic reflection profiles includes an upper acoustically transparent unit (usually restricted to the upper rise) a middle stratified unit, and a lower transparent unit. The upper unit is probably pelagic calcareous ooze. Other lithologies are unknown but the stratified unit may in part include an Early Tertiary unconformity. Sections of the shelf and slope exhibit deformation that have probably been caused by the Alpine orogeny.

Continuous Seismic Profiles of the Outer Ridge and Nares Basin North of Puerto Rico

Seismic-reflection profiles over the outer ridge and the Nares Basin north of Puerto Rico show that three principal structures can be traced from the north wall of the Puerto Rico Trench northward to 24° N. between 65°12′ W. and 67°36′ W. The shallowest structure in the Nares Basin consists of flat-lying strata, whereas the corresponding layer on the outer ridge is nearly everywhere acoustically transparent. The transparent layer is thickest near the center of this portion of the outer ridge and thins outward in every direction. The next-deeper structure consists of layers which in places are continuous, but elsewhere are more or less broken up. This series of layers is interrupted by the third structure, a basement having relief of several hundred meters. The basement surface lies about 1 km below the flat floor of the Nares Basin but rises to control the shape of sea floor over parts of the outer ridge. In a few locations where all structures have nearly flat surfaces and also where seismic-reflection data can be compared with older refraction data, the transparent layer corresponds with the 2.1-km/sec layer, the second series of layers corresponds with the 4.2-km/sec layer, and the basement corresponds with the 5.2-km/sec layer. Other possible correlations with refraction data are discussed. The distribution of these three structures suggests a basement high trending irregularly southwest-northeast across the ridge with flanking layered rocks to northwest and southeast. All three structures may have been sampled in the dredging of the north wall in 1962. It is suggested that the transparent layer is a remnant of a continental rise originally extending northward from Puerto Rico prior to the formation of the Trench.

Preliminary Results of the 1964 Cruise of R.V. Chain to the Indian Ocean

Geophysical investigations of the northern Somali Basin and the Seychelles-Mauritius Ridge conducted aboard R.V. Chain of the Woods Hole Oceanographic Institution are described and some results presented. Gravitational and total magnetic fields and bathymetry were measured continuously, and continuous seismic reflexion profiles were recorded over a major portion of the track. Cores, dredge samples, heat flow measurements, and underwater photographs were also obtained. It is considered that the northern portion of the Somali Basin is a deep sedimentary basin partially enclosed to the east by a submarine ridge from which alkaline gabbro has been dredged and to the south by partially buried abyssal hills. On the evidence from seven crossings of the Seychelles-Mauritius Ridge, it is proposed that the Ridge comprises two sections. The northern section, composed of nearly horizontally stratified rocks, extends from near the northern part of Saya de Malha Bank to the Seychelles Platform. The southern section is a linear, probably volcanic ridge that extends from north of Mauritius through Saya de Malha Bank, and may continue as a subsurface feature to the northeast. The two sections abut near Saya de Malha Bank, forming a continuous topographic feature.

Recent improvement in technique of continuous seismic profiling

Abstract This paper reports improvements in continuous seismic profiling techniques. modifications have been made in both source and receiver systems to obtain increased signal to noise ration, These include use of directional broad band arrays and the simple summation of the outputs of separately towed arrays.