Robert S. Dietz

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.

Collapsing Continental Rises: An Actualistic Concept of Geosynclines and Mountain-Building: A Reply

Conventional geosynclinal theory is not actualistic-modern examples are said not to exist. This deficiency is obviated if the continental terrace sedimentary wedges are modern miogeosynclines and if the subjacent continental-rise sedimentary prisms are modern eugeosynclines, together forming a couplet. Such equating is reasonable once the wave-built terrace concept of continental terrace development is discarded. Instead, the writer assumes construction of the terrace wedge (a miogeosyncline) by prograded wedges of paralic sediments on an isostatically downwarping continental margin. Most sediment, however, is assumed to be carried off the continental terrace to form the continental rise (a eugeosynclinal prism), an apron of turbidity current deposits of graywacke or flysch facies. The accumulation of the mio-eugeosynclinal couplet requires diastrophic quiescence except for isostatic depression. The orogenic or alpine mountain-building cycle is initiated by thrusting probably related to decoupling of a spreading sea floor so that the sima tends to slip beneath the sialic continental raft. The continental rise is then thrust, folded, and metamorphosed, forming a folded eugeosyncline; ultramafics are intruded and are technically incorporated in the folding. Since it lies inside and upon buoyant sial, the continental terrace wedge, or miogeosyncline, is folded and thrust to a lesser extent. Intrusion of plutons is post primary folding. Some implications of this actualistic concept of geosynclines are discussed. The eugeosyncline, for example, is oceanic and is composed of abyssal hemipelagic continental detritus. Orogeny generates new continental crust and causes continental accretion. The geosynclinal troughs are not formed by crustal compression. Island arcs, borderlands, trenches, and tectogenes play no part in this actualistic version.

Wave-Base, Marine Profile of Equilibrium, and Wave-Built Terraces: A Critical Appraisal

The concept of wave-base and its corollaries, the marine profile of equilibrium and the wave-built terrace, are largely erroneous. They do not dominantly control the development of the continental shelf and slope. The treatment of these concepts in geologic textbooks and sourcebooks needs revision. Wave-base, although a useful concept if applied generally to the surficial zone of the ocean of high-ambient agitation, does not control shelf deposition as the entire shelf is above any wave-base in this sense. Instead, the shelf is a drowned and relict surface, developed by oscillation of sea level and prograding and regressing paralic sediments—and here surf plays the dominant role. Sedimentation on the continental terrace is not controlled by wave-base but chiefly by topography; the continental slope is undergoing erosion rather than prograding, and ultimate deposition occurs on the continental rise. The wave-cut terrace is not cut at wave-base but at surf-base. Wave-built terraces are nonexistent both on a small scale and on a grand scale as an explanation of the continental slope. A marine profile of equilibrium is developed in the nearshore zone and is associated with a migrating lens of sand, but the outer-shelf profile is not a profile of equilibrium.

CLAY MINERAL COMPOSITION OF SOME SEDIMENTS FROM THE PACIFIC OCEAN OFF THE CALIFORNIA COAST AND THE GULF OF CALIFORNIA

This report presents the results of a study of the clay mineral composition of a series of bottom core samples collected by the staff of the Scripps Institute of Oceanography of the University of California in Pacific Ocean off the California coast and in the Gulf of California. The clay minerals were studied by X-ray diffraction, differential thermal, optical, chemical, and electron microscopic methods following particle size fractionation by sedimentation and supercentrifuge procedures. All of the samples from the Pacific Ocean and from the Gulf of California contained illite, montmorillonite, and kaolinite; generally illite was most abundant and kaolinite least abundant. A chloritic clay mineral could be identified definitely in some samples. The clay minerals were in very complex mixtures, including mixed crystallizations as well as mechanical mixtures of discrete phases. In general the crystallinity was lower, the individual size smaller, and the intergrowth more intimate than in ancient sediments which have been studied by the authors. Small amounts of quartz were associated with the clay minerals in the 1 to 0.1 and minus 0.1 micron fractions. Small amounts of another nonclay mineral crystalline phase that is probably a feldspar were also found in the finest size fractions of many samples. The analytical data suggest that kaolinite is slowly lost during diagenesis under marine conditions, perhaps being changed to an illite or chloritic clay mineral. The data also afford evidence that potash is taken up by the clay and suggest that it is taken up largely by partially degraded illite which is carried into the sea. Magnesium also appears to be taken up by the clay, perhaps by the illite. The widespread occurrence of montmorillonite indicates that this clay mineral is not lost quickly if at all during diagenesis under marine conditions.

Phosphorite deposits on the sea floor off southern California

Dredging operations on banks, on escarpments, and on walls of submarine canyons off southern California have shown that nodular phosphorite is the most abundant type of rock in these nondepositional environments. Approximately one-third of all the rock recovered is phosphorite. Petrographic and megascopic examination reveals that the nodules are largely formed by direct precipitation, but that they enclose some replaced material. Examination also shows that the phosphorite was probably deposited essentially in situ . Miocene Foraminifera have been identified in the nodules from many of the stations, but Recent or Pleistocene faunas have also been found in some of the phosphorite. Whereas the enclosed Miocene fauna suggests a Miocene age for most of the nodules, significant nonpaleontological data indicate that this fauna has probably been reworked into more recent deposits before enclosure in the nodules.

ALPINE SERPENTINES AS OCEANIC RIND FRAGMENTS

Accepting that alpine serpentines are tectonically emplaced in the solid state, the writer suggests that they may be fragments of the sea floor derived from the Oceanic Layer. This concept is based on the writer9s belief that eugeosynclinal graywackes may be equated with modern continental-rise turbidite prisms which are laid down on the deep-sea floor, abutting and overlapping, in part, the continental slope. When the sea floor thrusts toward the continent, the continental-rise prism is folded into a eugeosynclinal prism. The Franciscan graywackes prism of the California Coast Range would be an example. Pods of serpentine derived from the sea-floor sima underlying the eugeosyncline would be caught up in this folding, along with some deeper sub-M discontinuity ultramafic mantle rock. It is further supposed that the spilite-keratophyre suite characteristic of eugeosynclines is laid down in the deep ocean, Na metasomatism being caused by sea-water contact. On the floor of the open sea, away from the continental-rise turbidites, Layer 2 of sea-floor seismology probably is made up of spilite plus lithified eupelagic sediments altered to chert, ironstone, red argillite, and carbonate rock. This view has implications for the Mohole project.

Arctic Basin Geomorphology

The SSN Nautilus obtained the first continuously recorded echo-sounder profile across the center of the Arctic Basin during its 1958 cruise from Point Barrow to the North Pole and then to the Atlantic Ocean. The sounding lines, supplemented by lines from SSN SKATE reveal the following geomorphic features: (1) the Alaskan continental terrace and continental rise; (2) Canada Basin, a flat abyssal plain at the depth of 3850 m; (3) Chukchi Cap, a projection from the Chukchi Shelf with a summit elevation of about 900 m; (4) a continuation of Canada Basin, interrupted by a few sea knolls and having a depth of 3900 m; (5) Central Arctic Rise, a region of smoothly undulatory topography at depths between 2000 and 3700 m, with relief not exceeding 900 m; (6) Central Arctic Basin, an abyssal plain at 3940 m; (7) Lomonosov Ridge, rising abruptly from the basin to a summit at 1290 m: (8) beginning almost at the North Pole, the Eurasia Basin, an abyssal plain sloping south with a depth gradually increasing from 4090 m to 4500 m; (9) a region of rough topography containing numerous sharp peaks and having relief of as much as 1100 m and containing the deepest sounding of the Arctic Basin, 5335 m; (10) Nansen Ridge, a smoothly undulatory ridge rising to 1280 m where crossed.

MARINE GEOLOGY OF NORTHWESTERN PACIFIC: DESCRIPTION OF JAPANESE BATHYMETRIC CHART 6901

This paper is a geological discussion of the northwest Pacific, to accompany the Japanese Hydrographic Chart 6901 (a bathymetric chart of the northwest Pacific) which is presented as Plate 1. A special edition of this chart was published under an arrangement made by the writer between the Japanese Hydrographic Office and The Geological Society of America in the belief that it will provide useful topographic data in a previously little-known portion of the sea floor and that it will stimulate further interest in Pacific marine geology. A comparison of the land surface with the sea floor, both of which are contoured at 500-m intervals, emphasizes the variety of sea-floor features and the grand scale of this submerged topography where erosional leveling is subdued. The region covered by the chart can be divided into (1) the Pacific Basin proper, (2) the Philippine Sea Basin, including the arcuate ridges extending from Japan to Palau, and (3) the submerged portion of the Asia continental block, including the continental shelf and slope and the encompassed “intra-continental” basins. Large-scale undulations or swells and swales of low relief are the dominant structures of the Pacific Basin, but the five great groups of seamounts are the most striking features. A study of the original soundings shows some abrupt changes in level and grabenlike depressions which probably mark fracture zones in the earth9s crust. The seamounts are considered to be volcanoes, many of which are capped with coral. Smooth bottom characteristizes most of the Basin, suggesting a thick sedimentary blanket. Two of the seamount groups (the Emperor Seamounts and the Magellan Seamounts) are entirely submerged; another group pierces the surface at only two places (Marcus I. and Wake I.); the Caroline and the Marshall-Gilbert groups form numerous atolls. A large number of the seamounts have deeply submerged flat-topped summits and, thus, are guyots or tablemounts. However, because of the low seismicity and the rarity of raised islands, the Pacific Basin is considered a stable area, and the sinking of the guyots is thought to be a local isostatic subsidence in response to the load of the seamounts. The Philippine Sea Basin is bounded by the Asia continental block on the west and by the great geanticlinal ridges, extending from Japan to Palau, on the east. Trenches, presumably marking downbuckles in the earth9s crust, lie seaward of these ridges. These geanticlinal ridges are active belts surmounted by volcanoes and a string of raised islands. The Kyushu-Palau Ridge, another large geanticlinal ridge, bisects the basin. Soundings give little evidence of guyots on any of the ridges or in the Philippine Sea. The Daito Mountains, nested between the Kyushu-Palau Ridge and the Ryukyu Trench, are probably orogenic rather than volcanic. The Philippine Sea Basin appears to be non-sialic and a region with normal Pacific crustal structure which has been deformed by the same compressional forces that have affected the margin of Asia. The continental slope, marking the boundary of the Asia sialic block, extends from Kamchatka, along the Kurils, Japan, and the Ryukyus to Formosa, and probably along the east side of the Philippines. Extensive featureless shelves with shelf-breaks at normal depths fringe China and Siberia. The continental slopes are canyoned and irregular in detail. In the Okhotsk Sea there is a remarkable deep terrace which is probably a subsided normal shelf. The Kuril Basin, the Japan Sea Basin, and the Ryukyu Basin are similar abyssal pear-shaped basin within the continental framework. En bloc drifting of the associated island arcs, opening up simatic pools in the continental block, may account for these basins. Their smooth bottoms suggest that they are sediment traps for a great amount of sediment eroded from Asia.

TRANSPACIFIC DETECTION OF MYOJIN VOLCANIC EXPLOSIONS BY UNDERWATER SOUND

On 17 September 1952 a fishing boat, the MYOJIN MARU, discovered a submarine volcanic eruption 200 nautical miles south of Tokyo. The volcano appears to be the central cone of a caldera lying along the Fuji volcanic zone. The near-by Bayonnaise Rocks mark the highest, and the only subaerially exposed, portion of the caldera rim. A series of great explosions accompanied the extrusion of augite-hypersthene dacite. Since 26 September, when the main series of explosions ended, there has been only sporadic activity. Because of its remoteness, only a few visual observations of Myojin were made. Additional data were obtained from a tsunami recorder on Hachijo Island, 130 kilometers north of Myojin, which detected several tsunamis initiated by the volcanic eruptions, and by an atmospheric electricity recorder at Tokyo which detected atmospherics associated with the eruption. In contrast, a rather complete record of the Myojin activity was recorded by the U. S. Navy Sofar stations at Point Sur and Point Arena, California, about 8600 kilometers from the scene. More than 100 explosions were detected, one of which lasted for more than an hour. Explosions recorded on Sofar equipment agree in time with those observed visually or inferred from tsunami and atmospherics data. From the tsunami data and the Sofar records, it was independently concluded that the explosion which destroyed the Japanese Hydrographic vessel, No. 5 KAIYO MARU, occurred at about 1220 JST on September 24. All 31 people aboard the vessel were lost. This is believed to be the first time any signals on Sofar records have definitely been identified as of volcanic origin, and, since some of the signals were distinctive, installations similar to those of Sofar stations may prove of value for monitoring oceanic volcanic activity.

The Meteoritic Impact Origin of the Moon's Surface Features

The craters on the moon appear to have been formed by meteoritic impact rather than by volcanism. This mode of origin is suggested by their size, shape, distribution, associated features, and other considerations. Many of the aspects of lunar craters, such as their circular shape and the presence of central peaks, which have been considered as disproof of their impact origin, are the result of the craters being formed by explosion rather than by percussion. Variation of the physiographic form of these craters, with increase in their size from small, cup-shaped pits to large, walled plains, is mainly due to modification and melting by superheated molten lava, generated by the impact in the case of the larger craters. Maria are probably extensive lava plains generated by the impact of bodies of asteroidal dimensions and were formed relatively late in lunar history, as is indicated by the physiographic youth of the superimposed post-maria craters. Aside from the shape of the craters and the maria, the small mass of the moon and its low internal pressure make volcanism unlikely in the lunar environment.