Chronology and structural control of Late Cenozoic volcanism in the Loma Creston quadrangle, southern Jemez volcanic field, New Mexico
— Richard M. Chamberlin and William C. McIntosh

Geologic mapping and 29 new ⁴⁰Ar/³⁹Ar age determinations of seven major volcanic units confirm several episodes of mafic and silicic volcanism in the southern Jemez field in late Cenozoic time. Volcanism began with eruption of the aphyric basalt of Chamisa Mesa at approximately 9.9± 0.9 Ma. Overlying lava domes of the Canovas Canyon Rhyolite yield ages of 9.7 to 9.4 Ma. A second mafic pulse, the basalt of Bodega Butte (new name) is precisely dated at 9.14± 0.12 Ma (n= 4). A separate mapping project shows this 9.1 Ma olivine-augite basalt fills SW-descending paleovalleys near the base of a composite volcano of the Paliza Canyon Formation centered on Ruiz Peak, 10 km north of Loma Creston. We interpret the basalt of Bodega Butte as a late-stage flank eruption from the 9.5 to-9.1 Ma volcano at Ruiz Peak. Biotite from a Paliza Canyon trachydacite lava flow at Loma Creston is dated at 9.44± 0.16 Ma. The basalt of Bodega Butte apparently flowed southward around the low hill of dacite at Loma Creston. The trachyandesite of Mesita Cocida (new name), dated at 7.09± 0.21 Ma (n=3), is a microporphyritic, plagioclase-hornblende flow that unconformably overlies tilted volcaniclastic sediments at Mesita Cocida. Plinian pulses of the Bearhead Rhyolite at 7.0, 6.8 and 6.3 Ma produced the overlying Peralta Tuff and thin coeval ash beds. The final eruption in the study area was a xenocrystic basaltic andesite from a fissure vent on northwest Santa Ana Mesa at 2.4 Ma. Two episodes of hydrothermal alteration (ca. 9-8 and 7-5.3 Ma) are linked to crustal intrusions that fed ENEtrending vent zones of the Canovas Canyon and Bearhead rhyolites. Adularia from the red jasperoidal zone at Mesita Cocida is precisely dated at 5.31± 0.08 Ma. An en echelon belt of jasperized dextral-oblique normal faults at Mesita Cocida is aligned with a narrow transverse graben near Bodega Butte. The ENE-trending Bodega-Cocida shear zone (BCSZ) is interpreted as a preexisting basement structure, locally reactivated as a zone of distributed sinistral shear within the Rio Grande rift. We suggest that the BCSZ formed as the relatively rigid ENE-trending margin of the Colorado Plateau was dragged obliquely westward away from mobile lithosphere under the rift to create a focus of dilatant sinistral shear, which in turn controlled the ascent of late Miocene rhyolite intrusions under the southern Jemez volcanic field.