| Abstract |
Late Cenozoic volcanism in the Cascade Range and northwestern Basin and Range province is characterized by at least 4000 volcanic vents less than 16 Ma. Vent data were obtained from map compilations by R. G. Luedke and R. L. Smith (U.S. Geological Survey Misc. Investigations Series Maps I-1091-C, D; 1981, 1982) and include monogenetic and small polygenetic volcanoes in addition to major volcanic centers. Pronounced spatial, temporal, and compositional variations in volcanism are exhibited by distribution of vents 0-5 Ma, and no single section across the Cascade Range is representative of volcanism of the entire arc. Based on the distribution of 2821 vents less than 5 Ma, the Cascade Range can be divided into five segments, with vents of the High Lava Plains along the northern margin of the Basin and Range province in Oregon forming a sixth group. This segmentation model is different from models that emphasize offsets in alignment of recently active stratovolcanoes. Our northernmost segment contains the isolated stratovolcanoes Glacier Peak and Mount Baker, as well as the Cascade Range centers in British Columbia (Meager Mountain, Mount Cayley, and Mount Garibaldi). From Mount Raineir to Mount Hood, segment two encompasses a wide (about 150 km) zone of primarily basaltic vents and includes the occurance of basaltic fields east and west of a poorly defined andesitic arc. The third segment is a narrow, N-S trending belt that extends 370 km from south of Mount Hood to the vicinity of the Oregon-California border and is characterized by a dense distribution of predominantly andesite (including basaltic andesite) vents. The fourth segment includes Mount Shasta and Medicine Lake volcano in California. The Lassen area, spatially isolated from the Cascade Range to the north in terms of the distribution of both andesitic and basaltic vents, constitutes the fifth segment. This six-fold subdivision of 0-5-Ma vents also is applicable to the distribution of 0-1-Ma vents. |