During the past decade many geo-scientific discoveries suggest that continental margins are regions of extensive thrusting. Seismologic data establish that earthquake hypocenters are contained in a tabular volume of rocks that plunges steeply landward below the seaward edges of continents. First-motion studies of these earthquakes indicate a convergence of continental and oceanic crusts. On land, coastal areas of extensive thrust faulting are becoming known. These data support the hypothesis of ocean-floor spreading which requires extensive thrusting of oceanic crust beneath the continents. This hypothesis and the seismologically defined thrust zone imply that profound compressional deformation should take place at the base of continental slopes.

Structures produced by compressional forces are not observed in seismic-reflection records of the sediments filling marginal trenches or in sediments tilted against the continental slope during development of the trenches. Continental rises also consist of undeformed strata. Only deformation from subsidence and slumping has been seen at the foot of the continental slope from southern Chile to the outer Aleutian Islands. The observations of little or no thrusting at the juncture of the upper continental and oceanic crusts are now numerous and well established. These data must also be

End_Page 747------------------------------

considered in hypotheses that explain the development of continental margins.

End_of_Article - Last_Page 748------------