A MAGMATIC LINEAMENT OBLIQUE TO THE CASCADE RANGE

A magmatic lineament is an alignment of three or more magmatic manifestations that define a narrow zone or belt, e.g., a parallel dike swarm extending 3 km or more in length, a fissure with several eruptive centers, or adjacent stratovolcanoes whose deposits overlap. Such a lineament occurs in the central Cascade Range of Washington, north of Mount Rainier. Here five basaltic to andesitic volcanoes, two calderas, a cluster of rhyolite plugs, a large rhyolitic intrusive-volcanic complex, an andesite sill complex, and four granitoid plutons, two of which are batholiths, form a N60°W striking belt about 80 km long and 15 km wide. The centers are spaced 3 to 22 km apart, range from 26 to 14 Ma, and young to the northwest. Many other magmatic manifestations occur irregularly disposed in the area, but none forms this number of features in this length of alignment. In addition, an elongated, 7 by 2 km wide, hydrothermally altered and mineralized zone, 6.3 Ma, lies in the middle of the belt across the Cascade crest, indicating that magmatic activity was later renewed in the lineament. The origin of the lineament is uncertain, and at present can only be speculated. No faults, fracture zones, or parallel dike swarms occur within the lineament. The supracrust in the lineament appears to be unaffected, except by the magmatic centers. However, a series of post-15 Ma NNW-striking faults cross the lineament, some offsetting the intrusions and volcanic deposits short distances. The lineament is probably controlled by tectonic activity in the subcrust, where a small amount of extensional displacement, possibly a kilometer or less, may have reduced lithostatic pressure, causing magma generation at the crust-mantle interface and the formation of the magmatic lineament.