The Eastern San Juan Mountains: Their Ecology, Geology, and Human History

3. Mineralization in the Eastern San Juan Mountains

Figure 2.6 Structures and intrusions in vicinity of the central caldera complex. Abbreviated names: SL = San Luis Peak. Calderas of the San Luis complex: RC = Rat Creek; CC = Cebolla Creek; NM = Nelson Mountain. Modified from Lipman 2000, figure 14. t e r t i a r y V o l C a n i s m i n t h e e a s t e r n s a n J u a n m o u n ta i n s 35 Pagosa Peak Dacite (Lipman 2000); some of these faults indicate localized and continued subsidence in the southern segment during subsequent eruptions of Fish Canyon Tuff. Several fault clusters are related to resurgent uplift of caldera floors. Especially conspicuous is the Deep Creek Graben along the keystone crest of the steep-sided Snowshoe Mountain Dome, within Creede Caldera (Steven and Ratté 1965). Other resurgent structures include the graben faults of the Creede Mining District that were also initiated as keystone faults on the elliptical resurgent uplift within the Bachelor Caldera (Steven and Lipman 1976), faults bounding trapdoor-style uplift of the San Luis Peak block within the caldera associated with eruption of the Nelson Mountain Tuff (plate 8), and probably also the faults that cut the intra-caldera Fish Canyon Tuff in the uplifted block of La Garita Mountains. Another group of structures includes linear grabens and other faults (figure 2.6) adjacent to calderas that appear largely to have been initially established during segmented subsidence of La Garita Caldera, then passively buried by younger tuff sheets and lava flows. These include the Los Pinos and Cochetopa Grabens that connect the northern segment of La Garita Caldera to the Cochetopa Caldera; some faults of the Clear Creek Graben, to the west of the central segment; and perhaps initial faulting along the Rio Grande Graben, to the southeast of the central segment. The southwestern bounding faults of the Clear Creek Graben appear to have controlled a subparallel segment of La Garita Caldera–wall unconformity, along which the Carpenter Ridge Tuff and younger units are depositionally banked against steep slopes without major later faulting. In contrast, northeastern bounding faults from Bristol Head to Spring Creek Pass had continued movement after eruption of the Nelson Mountain Tuff. To the southeast, the Pass Creek Fault Zone runs between the central caldera cluster and the Platoro Caldera complex, displacing younger than the Fish Canyon Tuff and associated with modest hydrothermal alteration. Rift-Related bimodal VolCaniSm During late stages of San Juan volcanism, the Rio Grande Rift Zone became active within the present-day San Luis Valley, to the east of the San Juan Mountains (figure 2.1). The style of volcanism changed to a bimodal assemblage of widely distributed flows of weakly alkalic basalt and basaltic andesite, accompanied by small lava flows of silicic rhyolite, especially in the vicinity of the older Platoro Caldera complex. Only a few north- and northwest-trending faults within the eastern San Juans have appropriate geometry and timing to reflect such regional tectonism. Northwesttrending faults of the Rio Grande Graben cut basaltic lava flows of the Hinsdale Formation dated at about 24 Ma, south of South Fork. Late movement along the Bristol Head master fault of the Clear Creek Graben suggests that faults localized by, and initially active during formation of, La Garita Caldera were also influenced Peter W. Lipman and William C. McIntosh 36 by regional stresses associated with initial southwestward-directed extension along the Rio Grande Rift Zone. North-trending normal faults along the east margin of the North Pass Caldera also displace lavas of the Huerto Andesite, thus documenting rift-related extension much more recent than the caldera-related structures. RefeRenCeS Bachmann, O., M. A. Dungan, and P. W. Lipman. 2002. The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvenation and Eruption of an Upper Crustal Batholithic Magma Chamber. Journal of Petrology 43:1469–1503. Bethke, P. M., and R. L. Hay, eds. 2000. Ancient Lake Creede: Its Volcano-Tectonic Setting, History of Sedimentation, and Relation to Mineralization in the Creede Mining District. Special Paper 346. Washington, DC: Geological Society of America. Colucci, M. T., M. A. Dungan, K. M. Ferguson, P. W. Lipman, and S. Moorbath. 1991. Precaldera Lavas of the Southeast San Juan Volcanic Field: Parent Magmas and Crustal Interactions. Journal of Geophysical Research 96:13, 412–434. Larsen, E. S., and W. Cross. 1956. Geology and...