CORRELATION OF MESOZOIC CALDERAS AND IMPLICATIONS FOR CENOZOIC EXTENSION, SOUTHERN ARIZONA

Geologic mapping in the Tucson area has identified two new Mesozoic caldera fragments, and refined the orientation and age constraints of a third. Each is filled with quartz-feldspar-biotite phenocryst-rich, rhyolite ash-flow tuff.

In the Roskruge Mountains, multiple lines of evidence indicate that a north-tilted, trap-door caldera fragment is the western continuation of the Tucson Mountains caldera, source of the 72 Ma Cat Mountain Rhyolite. Cenozoic extension of the intervening, 20km wide basin is probably minor.

In the Sierrita Mountains, 50 km to the south, Red Boy Rhyolite fills a caldera we redefine as a northwest-facing structure. Due to regional southeast tilting, the west-facing caldera wall now dips very gently to the west, and the caldera floor, previously interpreted as an east-facing wall, dips steeply to the southeast. Pre-caldera volcanics consist of a rhyolite-andesite-quartzite succession bounded abruptly to the southeast by a very thick pile of andesitic lava.

In the Santa Rita Mountains, 50 km to the east, Mt Fagan Rhyolite defines an east-tilted, 20 km wide, caldera fragment that overlies a complex Laramide fold belt bounded abruptly to the southwest by a graben filled with 2 km of subaqueous andesite.

Since discrete outflow sheets have not been identified, it is possible that all of these segments are part of one caldera. Moderate (10-20 km) restoration of displacement on the north-dipping Ajo Road-Helmet Peak detachment could bring the Sierrita fragment into the vicinity of the Tucson-Roskruge caldera complex. Restoration of the Mt Fagan fragment towards the Sierrita fragment, assuming correlation of the mafic-lava filled graben, would require up to 60 km of displacement along a vector of approximately 070. Although some of this displacement might be taken up by strike-slip faults, the presence of a major detachment fault in the intervening Santa Cruz basin, which would have to merge with the Catalina-Rincon detachment system and add to its total displacement, is implied.