Tectonic Summary of Leg 18

Abstract

INTRODUCTION On Leg 18, a group of sites were drilled to see if evidence relating to subduction could be found at the continental margins off Oregon and Alaska. Furthermore, any indication of the rate of subduction was sought for comparison with the rates proposed in north Pacific plate tectonic models (Atwater, 1970; Hayes and Pitman, 1970). Interactions between lithospheric plates—a simple theo-rtical concept—produce complex relations along the con-tinental margins of the north Pacific. As with other developing geologic theories, there is a need to check the theory with field observations, where discrepancies between theory and observation seem to occur. For instance, the 6 cm/yr subduction under western Alaska required by present north Pacific models is difficult to reconcile with the lack of large compressional structures in sediments filling the Aleutian Trench and the steep faults and open folds of Neogene age on the continental shelf (von Huene et al., 1972). The absence of a Benioff zone beneath the continental margin off Oregon and Washington seems to indicate that subduction is not taking place, yet some observations are enticingly suggestive of subduction, such as the folded Pliocene and Pleistocene sediments of the Oregon lower continental slope. Similarly, off Alaska, the deformation during the great 1964 earthquake and the results of focal-mechanism studies are most easily explained by underthrusting (Plafker, 1969; Stauder and Bollinger, 1966). Thus some field observations support the theory and others conflict with it. A strong attraction for favoring underthrusting is its association with the popular theory of plate tectonics, but from observations along most conti-nental margins the possibility of other tectonic mechanisms such as vertical uplift or massive slumping are not easily excluded. The apparent contradition between the proposed rates of subduction and the general absence of compres-sional structures prompted selection of the Leg 18 sites. Rapid sedimentation can bury many structures and rates of sedimentation are easily verified by deep sampling. Further-more, from studies of cores, we thought we might establish whether ocean floor sediments are incorporated into the continental slope. These were some of our thoughts as we began Leg 18. A group of sites (174, 175, and 176) were drilled to elucidate the geologic history of the Oregon continental margin and the adjacent oceanic Cascadia Basin. This history should reflect plate movements, because a subduc-tion zone is postulated to form the juncture between oceanic and continental areas (Atwater, 1970; Silver, 1969, 1971). From the sediment sequence at each site a part of the geologic history can be inferred, and a summary of the three holes provides information on the tectonic history.