G. Leonard Johnson

Recent plate motions in the Galapagos area

Recent accretion on the Cocos-Nazca spreading center has been asymmetric, with more material (along most of the rise) added to the Cocos than to the Nazca plate. There is evidence in the magnetic record that some of this asymmetric accretion has resulted from small discrete jumps of the rise axis to the south, forming and destroying transform faults. Relative and absolute models of instantaneous plate motions derived here provide an accurate representation of recent motions in the east Pacific. The existence of a self-consistent model that fits all the relative-motion data provides strong support for the hypothesis that plates behave rigidly. In addition, the exceptional agreement between the relative-motion and absolute-motion models provides strong support that the Wilson-Morgan hotspot hypothesis holds for the recent past. In particular, the agreement of the predicted instantaneous azimuths of the Cocos and Carnegie Ridges with their observed azimuths strongly suggests that at least the young parts of both aseismic ridges were formed by the motion of the Cocos and Nazca plates over a Galapagos hotspot.

Morphology and evolution of the Norwegian-Greenland Sea

Abstract Throughout the history of the Norwegian-Greenland Sea, the still active Mid-Oceanic Ridge has always been the dominant tectonic feature. It is cut by WNW-ESE fracture zones which originated early in the history of the basin and now bear evidence of the directions of early crustal movements.

Depositional ridges in the North Atlantic

Abstract Recent detailed surveys by the U.S. Naval Oceanographic Office have delineated three new sea floor ridges in the northern Atlantic. It is postulated that these ridges are sedimentary features formed by depositional processes which in turn are controlled by the flow of Norwegian Sea water.

The Gulf of Corinth floor

Abstract The Gulf of Corinth is a narrow, 65-mile-long, 470-fathom-deep trench. The canyons of the steep southern continental slope lead from major river mouths to coalescing sub-sea fans. A narrow abyssal plain underlain by turbidites occupies the deepest part of the gulf. Three telegraph cables were laid the length of the gulf and maintained for over fifty years. Two of the cables lay on the continental slope and one traversed the abyssal plain. The two cables on the continental slope were broken frequently by turbidity currents originating at the mouth of the major rivers. On the few occasions when cable repairs were made to the cable which crosses the abyssal plain, the cable was found buried.

OCEANIC AFFINITIES OF THE ALPHA RIDGE, ARCTIC OCEAN

Abstract Geological and geophysical information collected on the ice station CESAR have been integrated to develop an origin and history for the Alpha Ridge. The fossil assemblage recovered limits its age to no younger than Late Cretaceous. Fragmental alkaline volcanic rocks were dredged from the escarpments of the trough system. A high-velocity lower crust and depth of nearly 40 km to mantle is revealed by the refraction experiment. Cretaceous aseismic oceanic plateaus of the Pacific exhibit striking similarities to the Alpha Ridge. Plate reconstructions are presented that rotate the Arctic-Alaska plate away from the North American plate, with the Alpha-Mendeleev Ridge complex on a spreading centre being fed by a hot spot forming a feature similar to the Iceland-Faeroe Plateau.

North atlantic fracture zones near 53

Abstract Near 53°N the crest of the Mid-Atlantic Ridge is offset 370 km in a left lateral sense by a large fault. Smaller subsidiary and parallel fracture zones are found both to the north and south of the primary fault. There is no magnetic signature associated with the fracture zone.

Bathymetry of the north Greenland Sea

Abstract The Mid-Oceanic Ridge is a continuous tectonic feature over 40,000 miles in length. The portion of the ridge system which passes through the Greenland Sea is found to be well developed. The Mid-Oceanic Ridge in the north Greenland Sea is characterized by a deep rift valley, which lies just seaward of the Svalbard continental margin. The rift mountains are present to the west of the rift; however, perhaps due to burial, they are barely discernible to the east of the ridge. The Mid-Oceanic Ridge, between 79° and 81°N, is offset 300 miles to the northwest by the Spitsbergen Fracture Zone. This seismically active, complex zone apparently strikes to the northwest diagonally across the “Nansen Sill” and exists as a trench just seaward of the continental margin of northeast Greenland. Two northwest-southeast striking seismic-fracture zones, whose movement appears to be dominantly vertical, lie parallel and to the south of the Spitsbergen Fracture Zone. Two small abyssal plains, located at 75° and 77°N, are separated by the spectacular Greenland Fracture Zone. This has caused a 300-fathom depth difference in the plain levels.

The Arctic Mid-Oceanic Ridge

THE Mid-Oceanic Ridge, a broad fractured arch more than 40,000 miles long, is the largest tectonic feature on the surface of the Earth. Associated with the centre of the ridge over much of its length is an axial fracture or rift which is the locus of shallow earthquakes. An extension of this world encircling ridge into the Greenland Sea and Arctic Ocean was first hypothesized on the basis of a well defined pattern of earthquake epicentres and a few spot soundings1,2. The topographic form of the ridge was first revealed by precision depth records obtained during the pioneer polar crossings of the nuclear submarines Nautilus3 and Skate4. The Arctic Mid-Oceanic Ridge does not bisect the entire Arctic Basin but lies midway between the monolithic, aseismic Lomonosov Ridge and the Eurasian continental margin.

Morphology and structure of the Aegean Sea

Abstract During the summer of 1968, a detailed bathymetric survey of the Aegean Sea was conducted by the U.S. Naval Oceanographic Office. The Aegean Sea has many morphological features typical of island arcs and some which are not typical. On the basis of geomorphological interpretation, the Aegean Sea is discussed in three parts: the southern; the central; and the northern Aegean Sea. The southern portion is interpreted as being part of a typical island arc structure. The central Aegean is occupied by an elevated aseismic plateau. The northern Aegean contains a series of steep-sided troughs and ridges.

A moated knoll in the Canary Passage

A 100-fathoms high one-mile-wide elongate knoll approximately ten nautical miles long lies in 650 fms (1200 m) depth in the Canary Passage. The long axis is parallel to the axis of the passage. The knoll is flanked by a moat 5 to 20 fms deep which in turn is surrounded by a gentle arch or ridge of similar height. Although similar moats have been interpreted as a result of downward isostatic adjustments of islands and seamounts, this moat is considered the product of bottom scour by ocean currents.