Studies of oceanic tectonics utilizing detailed surveying and stratigraphy

Abstract

The sea floor off the coasts of Washington and southern British Columbia consists of two major features: the Cascadia Basin and the chain of seamounts to the west of it, called the Cobb Rise. The Basin is magnetically featureless and appears to be a syncline deeply filled by sediment. Clues to the tectonic history of the region are therefore more likely to be found on and beyond the Cobb Rise, which forms the western boundary of the Basin and interrupts the westward flow of terrigenous sediment. A program of detailed topographic and magnetic surveving, complemented by heat-flow coring and seismic profiling, is underway to attempt to determine the structure and history of the abyssal hills on the seaward side of Cobb Rise. Knowledge of the stratigraphy and the topography of the basement-sediment interface is expected to reveal the approximate age of the sea floor in this area and possibly the nature of the basement itself. The magnetic and geothermal data may provide direct evidence of the subcrustal structure associated with Cobb Rise, particularly if the transition zone between the high heat flow on the rise and the low heat flow west of it can be studied in detail. A small seamount has been discovered at 47°N, 132°W and has been intensively surveyed as the nucleus of the future survey area. It rises 630 meters above the surrounding bumpy plain and has extremely steep faces to the south and east. Heat flow near the seamount is somewhat below the oceanic average and the several long gravity cores obtained in the area show some correlable layering in the pelagic ooze.