VERTICAL-AXIS ROTATIONS ASSOCIATED WITH RELAY RAMPS IN THE RIO GRANDE RIFT, NEW MEXICO

While vertical-axis rotations have been incorporated into models of relay ramp evolution (Ferrill & Morris, 1999), the occurrence of rotations in natural relay ramp systems have yet to be fully identified. The Rio Grande Rift is comprised of temporally and spatially distributed basins and bounding extensional/transtensional faults. For example, as the Española basin developed, displacement shifted from the Cañones fault to the Pajarito fault and motion along the Embudo fault changed from dominantly dip slip to dominantly strike slip. Previous studies in the Española basin concluded that vertical-axis rotations occurred since the middle Miocene (Brown and Golembek,1997; Salyards et al.,1994), and work by Harlan (2004) and Hudson (2004), documented ~9° CCW vertical-axis rotations in Pliocene basalt flows and Oligocene volcaniclastic rocks since late Pliocene. Our re-evaluation of data by Doell et al. (1960) and McDonald and Palmer (1990) suggests that sites located near the axis of the rift (and north of the Valles Caldera) show ~9° of CCW rotation compared to sites on the Colorado Plateau. In addition, we are collecting new paleomagnetic and structural data from the Quaternary Bandelier Tuff. Two stratigraphic sections have been completed: in the Seven Springs (~260 m thick) section (located on the Colorado Plateau), 7 sites were drilled, and in the Sawyer Mesa (~39 m thick) section (located south of the caldera) 3 sites were drilled. At each site, 10 samples were collected and were demagnetized either with Alternating Field or Thermal methods depending on the extent of sample welding. All sites show reverse polarity; the AF demagnetization occurred at 180 mT and the thermal demagnetization analyses show decay at 620°C, suggesting that the carrier has a relatively high coercivity. We obtained no significant deviation in declination for any of the sites except the highest and youngest part of the Bandelier Tuff in the Seven Springs Section, which shows an 18° CCW difference from the reference direction. Along with the sections we are currently sampling in the Bandelier Tuff, our detailed kinematic analysis of faults in the stepover will allow us to characterize the total deformation field in this portion of the Rio Grande Rift.