George G. Shor

Middle America Trench: Seismic-Refraction Studies

Seismic-refraction profiles in the Middle America Trench show that the main crustal layer ranges from normal oceanic thickness (about 5 km) t o half again as thick. Sediments are thicker than in the Pacific Basin. Because of the increased depth of water and sediment at all stations and thickening of the main crustal layer at some stations, the M discontinuity bows down beneath the trench. At the outer edge of the continental shelf near Guatemala, the water is replaced by sediments and rocks which may be either consolidated sediments or volcanic rocks; the M discontinuity is at little greater depth than beneath the trench.

The Structure and Tectonic History of the Eastern Aleutian Trench

The tectonic character of the eastern Aleutian Trench and some major events in its geologic history can be estimated from nine continuous seismic reflection records. A section of pre-trench, deep oceanic sediments rests on the down-warped crust that forms the trench. Nearly horizontal undeformed strata that unconformably overlie this deep oceanic section partially fill the trench. The trench fill is thickest near present sediment sources. A Pliocene age for development of the eastern Aleutian Trench is estimated from the thickness of deep oceanic sediment that accumulated after the trench began to fill. The eastern Aleutian Trench thus appears younger than the central Aleutian Trench—a relation which helps to explain the distribution of sediment along the two trench segments. Depression of the eastern Aleutian Trench diminished or virtually ended soon after the trench began to fill. The undeformed fill provides no evidence for a large thrust fault zone at the base of the continental slope. Nor is there any evidence that oceanic sediments have disappeared beneath the continents in late Tertiary time. These observations are difficult to reconcile with the simple model of a continental margin advanced in the hypothesis of plate tectonics.

Marine seismic refraction studies near Kodiak, Alaska

Seismic refraction profiles near Kodiak, Alaska have provided data for construction of a cross‐section showing the crustal structure from the east end of Kodiak Island across the continental terrace and slope to the Aleutian Trench (Figures 1 and 2). Five reversed refraction stations and one unreversed station were available (Table 1); interpretation was aided by access to reflection profiles which cross the refraction lines (Shor, 1972).

Geological Interpretation of Seismic Profiles in Prince William Sound, Alaska

A series of seismic reflection profiles was made during the summer of 1964 in the northeastern part of Prince William Sound, Alaska. Three rock units recognized in the profiles are (1) the ice-sculptured metamorphic basement, overlain by (2) probable glacial drift, and (3) Holocene marine sediments. Channels in the basement may have been formed in part by faulting as well as by glacial scour; they have been extensively modified by fill of rock units (2) and (3), so that the present sea floor does not closely represent the effects of glaciation. The Holocene strata have been faulted, but the vertical separation is small, suggesting that sound reacts to regional tectonism without severe Recent faulting.

Reflexion studies in the eastern equatorial Pacific

Abstract A limited seismic reflexion survey has been made in a portion of the equatorial Pacific far from land and shielded from turbidity-current deposition. It covers the boundary between present-day clay deposition in the north and present-day carbonate deposition in the south. The profiles show that the sediment is decidedly thicker in the carbonate area and provide evidence for a greater rate of accumulation in valleys than on hills.

Sea-Floor Exploration

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