Paleomagnetism of the quaternary bandelier tuff: Implications for the tectonic evolution of the Española basin, Rio Grande rift

We present newly acquired paleomagnetic data from Bandelier Tuff exposures in the Jemez Mountains (New Mexico) that show no statistically signifi cant tectonic rotation over Quaternary time. Cooling units of the tuff were mapped in detail and correlated using new geochemical data, allowing us to confi dently sample isochronous units for paleomagnetic remanence directions. In total, 410 specimens were subjected to step-wise thermal and alternating fi eld demagnetization. Of the 40 accepted site means, 30 have α ₉₅ values ≤5° Analysis of the geographic distribution of the site-mean declinations of the data set reveals no statistically signifi cant tectonic rotation either across (northwest/southeast) the northeast-striking Jemez fault or across (east/west) the north-striking Pajarito fault zone. Similarly, our data do not record any measurable relative change in declination difference (-1.1° ± 1.6°) that could be interpreted as a rotation over the ~0.36 m.y. time duration between deposition of the two principal stratigraphic members of the Bandelier Tuff. The step-over discussed in this paper is an area of exceptional structural complexity and, as such, meets the defi nition of "accommodation zone." We propose the name "Jemez-Embudo accommodation zone" for this composite of structural and volcanic features in recognition of its regional importance in the evolution of the Rio Grande rift. In this part of the rift, where Proterozoic- and Laramide-age faults have preconditioned the crust, idealized relay ramps, prevalent locally, do not occur at the regional scale. Instead, transfer fault zones have developed between half grabens dominated by preexisting faults. The pattern of faulting and accommodation of strain in the right-relayed step-over of the rift has been more or less invariant since the onset of rifting. From a global perspective, the difference between areas of modest crustal extension dominated by distributed deformation and those regions that develop transfer fault zones may ultimately be diagnostic of crustal conditioning and fault strength, such that weak fault systems focus strain within narrow zones.