Calderas of the Sierra Madre Occidental Volcanic Field western Mexico

Abstract

Approximately 296,000 km2 of western Mexico is covered by a volcanic sequence that has an average thickness of 1 km and is mostly ash‐flow tuff. Although some 350 calderas are suggested by this tremendous volume, only a few have been located or described. In the state of Durango the only well documented examples are in the 30‐Ma old Chupaderos caldera complex near Durango City. Although the complex is mostly covered by Quaternary basalt and sediment, the exposed portion reveals nested calderas that produced two major rhyolitic ash‐flow tuffs. A third major tuff appears to have been erupted from the complex, but no caldera has yet been found. The caldera formed by the first eruption contains a resurgent dome and a thick sequence of moat sedimentary rocks and lava flows that are hosts for significant iron and tin deposits. Other possible Durango calderas exist at El Salto west of Durango City, at Temoaya south of Durango City, and in the Sierra de la Silla to the north. Calderas in Chihuahua have beep described from mountain ranges in the central part of the state and from the Sierra Madre Occidental to the west. A composite, resurgent caldefa, 35 Ma old and approximately 22 km in diameter, has been de scribed from the Sierra Pastorias immediately south of Chihuahua City. The main erupted unit is well exposed in the caldera's resurgent dome, and a second tuff found outside the caldera was emplaced from a trap door structure nested in the caldera's northwest corner. The area also contains a younger, smaller, resurgentIy domed caldera. Other possible calderas include a relatively large (35 km in diameter) structure in the Sierra del Nido 75 km north of Chihuahua City and a poorly defined structure related to a sequence f older (45 Ma) silicic volcanic rocks at Cerro Jesus Maria 25 km northwest of Chihuahua City. Calderas in the Sierra Madre plateau have been described from the Tomochic and Ocampo areas of western Chihuahua. The Tomochic area records volcanic activity from 34.5 to 28.9 Ma and contains at least three calderas. These include the Tomochic caldera, a strikingly well preserved, resurgent caldera approximately 20 by 25 km in diameter. The north structural margin of the Tomochic caldera cuts the intracaldera facies of an ash‐flow tuff related to the older Las Varas caldera. Epithermal precious‐metal fissure‐veins are located where the Tomochic structural margin cuts this older unit. A third structure, the Corallitos caldera, is located only a few kilometers to the northeast. Reconnaissance studies in far western Chihuahua indicate the existence of a large (40 km) resurgent caldera centered on the village of Ocampo. Landsat imagery shows a well developed arcuate drainage pattern, and maps of the area show a distribution of lithologies that is consistent with the caldera interpretation. Gold and silver deposits, hosted almost exclusively in precaldera rocks, are best developed where the older rocks are cut by faults of the caldera's ring‐fracture zone and resurgent dome. In general, the calderas thus far mapped in western Mexico follow the Valles‐type resurgent caldera model. Variations, however, of the ideal caldera cycle are common, as are repetitions of the cycle to form caldera complexes. All of the better‐known calderas have associated mineral deposits, although they may be best developed in precaldera rocks.