PALEOSEISMIC ASSESSMENT OF THE NORTHERN TIJERAS-CAÑONCITO FAULT SYSTEM, CENTRAL NEW MEXICO

The Tijeras-Cañoncito fault system (TCFS) is a major structural element of the Rio Grande rift, and consists of the 41-km-long Canyon fault section south of the village of Golden and the 51-km-long Galisteo fault section north of Golden. We developed a preliminary 1:24,000 scale, 2-km-wide map along the fault from Golden to Lamy to define the late Quaternary behavior of the Galisteo fault section (GFS). The objectives of this research are to define the late Quaternary rupture behavior of the GFS and to help assess the segmentation of the TCFS. Through aerial photograph interpretation, aerial reconnaissance and field mapping, we delineated seven surficial map units along the GFS. These map units include Pleistocene and Holocene alluvial-fan and fluvial deposits, as well as piedmont deposits associated with the late Pliocene to early Pleistocene Ancha Formation and Tuerto Gravel. We observed unfaulted middle Pleistocene to Holocene fluvial and alluvial-fan deposits overlying the GFS north of Golden and unfaulted Tuerto Gravel overlying the GFS near Peach Springs in the Ortiz Mountains. These field relations indicate an absence of movement along the GFS since the late Pliocene to early Pleistocene. Based on evidence of late Quaternary surface-rupturing earthquakes along the Canyon fault section south of Golden and the absence of similar ruptures on the GFS, we infer a fault segment boundary for the TCFS at Golden. The segmentation may be related to a change in fault strike at Golden, differences in bedrock composition, and/or the distribution of strain to other faults in the eastern Rio Grande rift. We speculate that the regional extensional strain accommodated by displacement on the Canyon fault section is not transferred north eastward onto the Galsiteo section, but instead onto the La Bajada fault north of Golden.