H. W. Menard

Gulf of california: a result of ocean-floor spreading and transform faulting.

Ocean-floor spreading tore southern Baja California from mainland Mexico 4 million years ago and has subsequently rafted it 260 kilometers to the northwest along the Tamayo Fracture Zone. Magnetic-anomaly profiles indicate spreading at the mouth of the gulf at 3.0 centimeters per year and a rise-crest offset of 75 kilometers inside the gulf across the Tamayo Fracture Zone.

Sea Floor Spreading, Topography, and the Second Layer

Local sea floor topography and also the thickness of the second layer of the oceanic rise-ridge system appear related to the spreading rate in the region. Slow spreading, away from the ridge center at 1 to 2 centimeters per year, is associated with a thick second layer, a central rift, and adjacent rift mountains. Fast spreading, 3 to 4.5 centimeters per year, is associated with a thin second layer and subdued topography lacking any central rift. The volume of lava discharged in this layer per unit time and per unit length along the crest of the whole active system is relatively constant regardless of the spreading rate. Total second layer discharge of the system has been about 5 to 6 cubic kilometers per year during the last several million years.

Morphology and Tectonic Evolution of the East-Central Pacific

Mapping of the East Pacific Rise between the equator and 40° S. shows the location and trend of nine active fracture zones and the probable location of a few more. An area of relatively shallow sea floor west of Easter Island marks a second active spreading center discovered by Herron (1972b). New data indicate that segments of several fracture zones exist on either side of the rise and can be correlated across that feature. New segments of the fossil Galapagos Rise have been identified, indicating that its southern part is significantly deeper than the northern segments. At 15° S., 120° W., the topographic configuration of the East Pacific Rise is markedly asymmetric: the western part is shallower than 2,000 fm (3,750 m) and occupies twice the area of that part to the east of the rise crest.

Fragmentation of the Farallon Plate by Pivoting Subduction

At the beginning of Cenozoic time a large region between the Pacific and American plates consisted of a continuous Farallon plate. Beginning 55 m.y. ago, the Farallon plate has been complexly fragmented by the formation of ridge-trench transforms, paired ridges, leaky transforms and transverse spreading centers as well as by ridge jumps, and ridge and fracture zone reorganization. Many of these phenomena can be related to pivoting of fragments of the plate around points near ridge-trench-fault triple junctions. Pivoting appears to be a consequence of the shape of the subducted part of the plate which narrows to a point near the oblique intersection of a ridge and trench. Pivoting fragmentation produces two types of phenomena where plates converge. The rate of convergence varies with distance from the pivot and this determines many aspects of topography, structure, sedimentation, vulcanism, and metamorphism. Transverse phenomena which penetrate North America for hundreds of kilometers are associated with the pivoting Cocos plate. They lie over the tapering edge of the subducting plate.

Magnetic lineations in the Northeast Pacific

Abstract Magnetic data for the northeast Pacific are summarized and interpreted in terms of sea floor spreading. Unclear areas exist in the “disturbed zone” and west of the Juan de Fuca Ridge. The direction of spreading changed about 55 my ago. An intermediate plate was gradually destroyed as segments of the ridge neared the continent and the timing of this destruction appears related to the timing of the origin of the San Andreas fault system.

Hawaiian Swell, Deep, and Arch, and Subsidence of the Hawaiian Islands

Detailed echo-sounder fathograms were obtained in the vicinity of the Hawaiian Islands during the joint Scripps Institution of Oceanography-United States Navy Electronics Laboratory Expedition to the Mid-Pacific, 1950. These fathograms give the first clear picture of the bathymetry in the area. The Hawaiian Ridge is superposed on a broad low rise, the Hawaiian Swell, which is about 600 miles across. Along the base of most of the Hawaiian Ridge is a depressed area, the Hawaiian Deep. Three fathograms demonstrate that the Deep is especially well developed along the northeast side of the southeastern end of the archipelago. Outside the Hawaiian Deep there is a large arcuate arch, the Hawaiian Arch, which is more than 200 miles across and at least 600 miles long. A deep (180-fathom) terrace, which appears to be a drowned shelf, exists along the Mauna Kea and Kohala coasts of Hawaii. Other Hawaiian islands are fringed by terraces to depths as great as 700 fathoms. Three possible origins of the Hawaiian structure are examined: (1) that the structure is related to strike-slip faulting along which there has been great effusion of lava; (2) that the structure is related to crustal buckling and thrust-faulting; and (3) that it is related to vertical forces which have arched up the crust and produced tension fractures out of which lava has poured. It is concluded that the Hawaiian Swell may have been produced by arching of the crust above a zone of divergence between two thermal convection cells in the viscous subcrust. The drowned shelves fringing the Hawaiian Islands are thought to have been produced by isostatic subsidence related to the overloading of the crust. Because of the rigidity and elasticity of the crust, this subsidence has caused depression of the crust beyond the limits of the superposed load to form the Hawaiian Deep. The Hawaiian Arch may be related to an elastic bulge or to the outward displacement of viscous subcrustal rock.

DEFORMATION OF THE NORTHEASTERN PACIFIC BASIN AND THE WEST COAST OF NORTH AMERICA

Four great bands of unusually irregular topography named “fracture zones” have been discovered in the northeastern Pacific basin, and three have been traced into western North America. Individual zones range from at least 1400 to 3300 miles long and average 60 miles wide. The zones follow great circles for most of their lengths, and all are roughly parallel. Topography within the fracture zones is characterized by great seamounts, deep narrow troughs, asymmetrical ridges, and escarpments. Two escarpments are about 1 mile high and more than 1000 miles long. Two fracture zones separate regions which differ in depth by a quarter or half a mile. The fractured area includes 8,000,000 square miles (5 per cent of the earth9s surface), and the parallel trends of the fractures indicate a single origin. It is tentatively concluded that an annular convection current rising near the Hawaiian Islands and sinking near North America stressed the crust and produced the fracture zones by plastic deformation. The San Andreas fault marks a complementary zone of plastic deformation, and the California Coast Ranges lie within the zone. The Channel Islands and Transverse Ranges of California, and the Revilla Gigedo Islands and Volcanic province of Southern Mexico lie within continental extensions of the deep-sea fracture zones and parallel the zones. Some geomorphic provinces on the sea floor are bounded by fracture zones as are the Sierra Nevada, the Great Valley of California, the Baja California Peninsula (including the Peninsular ranges), and the Gulf Trough. Evidently the provinces were formed either with or after the fracture zones, and the stresses which formed them did not transect the zones. The zones appear to cut Mesozoic structure; they contain more active volcanoes and earthquakes than is normal for the northeastern Pacific area.

Elevation and subsidence of oceanic crust

Abstract A moving oceanic crustal plate is elevated where it is created at the traiting and it subsides as it grows older. The subsidence continues far beyond the obvious topographic boundaries of midocean ridges. The rate of subsidence averages 9 cm/103 yr for the first 10 million years; 3.3. cm/103 yr for the next 30 million years; and is estimated at 2 cm/103 yr for the next 30 million years. Even older crust in the Pacific subsided at a slower rate for an additional 25 million years. In the Atlantic, however, the rate was much faster during the same period. Subsidence is related to the interaction of mantle degassing, erosion, sedimentation, mantle counterflow, and sea floor spreading. “Midplate rises” exist in all the major ocean basins. Many are broad elevations characterized by thick pelagic sediment overlying faulted and uplifted turbidities. They show no signs of intense vulcanism or sea floor spreading center phenomena. They probably mark the locus of small transient convection cells which act under rather than at the edges of large crustal plates.

Acoustic and Other Physical Properties of Shallow‐Water Sediments off San Diego

This paper reports on a continuing study of the mass physical properties of surficial, shallow‐water marine sediments off San Diego, California. Sound velocity measurements in situ at 100 kc were made by pulsing between small transducers inserted into the bottom by diver. Density, porosity, and size analyses were determined on relatively undisturbed samples taken by diver. The following averaged representative values have been obtained: Velocity in sediment at Impedance Density Porosity Med. diam 60°F (105 g/Sediment type (g/cc) (%) (mm) (ft/sec) cm2 sec)Fine sand 1.93 46.2 0.19 5520 3.25Very fine sand 1.92 47.7 0.12 5435 3.18Silty very fine sand 1.68 61.3 0.05 5075 2.60Medium silt 1.69 60.9 0.03 4825 2.49Clayey fine silt 1.60 65.6 0.02 4800 2.34Laboratory measurements of velocity and attenuation 25 to 35 kc were made by a resonant‐chamber method using diver‐taken samples; average attenuation values of about 0.5 db/ft (silt) to about 5 db/ft (fine sand) were obtained. At three stations the sediment sound ...

Topography and Heat Flow of the Fiji Plateau

THE Fiji Plateau is an extensive region at a depth intermediate between continent and ocean basins. Above it rise the islands of Fiji where acidic plutonio rocks occur within an ocean basin in a tectonic setting wiiich can only be compared with the granitic Seychelles Islands in the deep western Indian Ocean. The results of the few seismic refraction lines run on the plateau indicate a variable crust of intermediate thickness1. Thus the Fiji Plateau is an unusually interesting and promising region for tectonic studies. In the spring and summer of 1967, the Scripps Institution of Oceanography expedition Nova studied this region. In this preliminary communication we discuss results in topography and heat flow and suggest a relationship between them.