LATE QUATERNARY PALEOSEISMICITY OF THE CENTRAL HUBBELL SPRING FAULT AND IMPLICATIONS FOR STRAIN PARTITIONING IN THE CENTRAL RIO GRANDE RIFT, NEW MEXICO

The Hubbell Spring fault (HSF) is near the eastern margin of the Albuquerque-Belen basin in the central Rio Grande rift, and is one of the most active faults in the region. Recent mapping and geophysical studies indicate that the fault geometry is more complex and longer than previously thought, with two dominant west-dipping splays (western and central) extending for over 40 km south of Albuquerque. An eastern splay (also referred to as the Meadow Lake fault) is buried along its southern 2/3 and appears to be older than late Quaternary. Our paleoseismic investigation of the Carrizo Spring trench site on the central HSF revealed structural, stratigraphic, and pedologic evidence for at least 4, and probably 5, large earthquakes that occurred since deposition of piedmont deposits on the Llano de Manzano surface about 83 ± 6 ka. All of these events included warping across a broad deformation zone, whereas the 3 largest events also included discrete slip across five fault zones. Behavior appears non-characteristic, with preferred vertical displacements per event ranging from 0.4 to 3.7 m. Fault-related deposition was dominated by eolian rather than colluvial sedimentation, similar to previous trench studies in the area. The total down-to-the-west throw of piedmont deposits is 7.3 ± 0.5 m, yielding an average vertical slip rate of 0.09 ± 0.01 mm/yr. Luminescence ages suggest that the 3 largest faulting events on the central HSF may correlate to the three youngest events on the western HSF, but additional ages are needed to confirm this. Two smaller warping events on the central HSF do not appear to correlate to any events on the western HSF. We estimate that over 90% of the late Quaternary strain on the HSF occurred as coseismic rupture of the western and central splays. Additional investigation is needed to determine how this may relate to paleoseismicity on other adjacent, subparallel Quaternary faults along the eastern rift margin.