PALEOMAGNETIC EVALUATION OF THE RESURGENT DOME AT THE QUATERNARY VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO

The Valles Caldera in the Jemez Mountains of north-central New Mexico has long been considered as the type example of a caldera with a well-defined resurgent dome (Smith and Bailey, 1968). The Valles Caldera formed at ~1.25 Ma, producing the Tshirege member of the Bandelier Tuff. The elongated structural resurgent dome, with over 1000 m of uplift, and a medial graben now occupied by Redondo Creek, trend northeast in alignment with the northeast orientation of the Jemez fault zone, a key structure of the Rio Grande rift. Uncertainty exists regarding the role and magnitude of post-caldera tectonism on the formation of the resurgent dome. We are using paleomagnetic data from exposures of the Tshirege member of the Bandelier Tuff throughout the Redondo dome area to quantify the magnitude of post caldera tilt of these rocks. Previous studies of the tuff conducted on outflow facies away from the caldera (Doell et al., 1968; Sussman et al., 2011) provide a reference frame for this study. A total of 375 samples were obtained from 27 sites around Redondo Peak near the Redondo Creek graben where the post-caldera volcanic and sedimentary infill units are exposed and well preserved. Overall, progressive demagnetization behavior is consistent with previously reported data from outflow facies, and there is little evidence of pervasive remagnetization related to hydrothermal activity attending dome formation. Further comparison of the data from the Tshirege member in the Redondo Peak area with those from outflow facies will facilitate estimates of the contribution of tectonic tilt to the formation of the Redondo dome and thus illuminate the structural history of the Valles Caldera.