GEOPHYSICAL INVESTIGATIONS OF THE CASCADE SUBDUCTION ZONE, NORTH AMERICA

This study derives integrated tectonic, geologic, and geophysical models along two well-studied profiles across the contrasting northern and central segments of the Cascade subduction zone. The models are based on physical parameters such as seismic velocity, density and temperature, and on the knowledge of the lithologic composition of the rocks. A new tectonic interpretation is offered for the structure of the crust and upper mantle under the Cascade subduction zone, the key point of which is the detachment of the lower portion of the down-going oceanic slab ~42 Ma and the subsequent rearrangement of mantle convection flows, which violated the ordinary structure of the convergent zone. After the break of the slab, the edge of the still drifting North American continent rode over the hot oceanic mantle of the mid-ocean ridge. This caused tholeitic volcanic activity in the western Cascades 40-18 Ma and the subsequent renewal of magmatic and tectonic activity in the western third of North America, including the rise and extension of the crust, which resulted in the formation of the Basin and Range province. A short-lived upward flow of hot material above the break line of the oceanic slab produced the western Cascades complex of volcanic and volcanogenic sedimentary rocks at a distance from the oceanic trench half as large as is common for oceanic arcs, its tholeitic lavas being uncommon for volcanic arcs. Later, subduction resumed, but devoid of its cold eclogite portion, the slab sank into the mantle slowly and at lop-dipping angle, cooling the continental margin, reducing the volcanic flow of the western Cascades, and causing retreat of the volcanic front to the east. By the time the flow of new, hot material was shut off beneath the continent, the North American continent had already drifted quite far. As a result, a substantial portion of the hot oceanic asthenosphere of the mid-ocean ridge happened to underlie the edge of the continent behind the new subduction plane. Approximately 10 Ma, the Oregon margin again experienced the conditions typical of subduction zones and the high Cascade arc was formed. Thus, as a result of the Cenozoic tectonic activity, the upper mantle under the Oregon margin happened to contain magma sources generating basalts similar to the basalts of mid-ocean ridges and island arcs and to the magmas of intracratonic rifts.