TERTIARY VOLCANISM AND LANDSCAPE EVOLUTION IN THE SAN JUAN MOUNTAINS, COLORADO

Details of the scenic mountain landscape in the San Juan region result from river and glacial erosion during the last few million years, but many large-scale features are related to an intense pulse of volcanic activity from 35 to 23 Ma. The San Juan region was a major locus of the hemispheric-scale plate-convergence magmatism distributed discontinuously along the eastern margin of the American cordillera from Alaska to Chile. In southwestern Colorado, this mid-Tertiary magmatic pulse emplaced a composite upper crustal batholith, 75 by 125 km across, that is recorded by the largest negative Bouguer gravity anomaly in the United States. Associated with the rise and crystallization of batholithic magmas were clusters of dominantly intermediate composition central volcanoes (mainly 35-30 Ma), followed by enormous explosive eruptions of ash-flow tuffs, associated caldera collapse, and continuing eruption of intermediate-composition lavas. At least 20 large ash-flow sheets (>100 km³) were erupted from 17 calderas during the interval 32-23 Ma. Several individual ash-flow eruptions exceeded 1,000 km³ in erupted volume, and the 28-Ma Fish Canyon Tuff at >5,000 km³ associated with the enormous La Garita caldera (75x30 km) was produced by perhaps the largest well-documented explosive eruption on Earth. Some caldera areas were sites of recurrent collapse during sequential eruptions, and cumulative depths of subsidence are inferred to have been 10-17 km, requiring recycling of just-erupted volcanic rocks to provide space for the growing upper crustal batholith. The San Juan region is topographically high today because of buoyancy of the low-density batholith associated with the mid-Tertiary volcanism, and many individual peaks are erosionally modified remnants of constructional volcanic edifices or resurgently uplifted cores of Oligocene calderas. Although modified by inception of the Rio Grande rift along the east margin of the volcanic field late during its growth, virtually every major present-day valley and many smaller basins in the region are inherited from the Oligocene volcanic landscape.