New Mexico Geological Society Annual Spring Meeting — Abstracts

Three types of fissures have been identified in portions of the Pajarito fault system, which locally defines the active western margin of the Rio Grande rift in the area of Los Alamos, New Mexico. The fault system is approximately 50 km long and up to 5 km wide in places, and is primarily expressed at the surface as a complex series of normal faults, monoclines, faulted monoclines, and zones of distributed faulting. The fault system displaces the 1.2- million-year-old Bandelier Tuff more than 150 m in places, and paleoseismic studies suggest three surface-rupturing seismic events in the Holocene. Fissures have been identified at the upper hinges of monoclinal folds, as openings along faults, and in linear alignments of large holes and cracks where there is little or no other surface expression of faulting. Tensional fissures are commonly found at the upper hinges of monoclinal folds in the Pajarito fault system, and appear to be associated with seismic events. Such fissures are typically filled with large, angular blocks of adjacent lithologies (typically tuff) and finer-grained (commonly laminated) material that washes into the fissures. Paleoseismic trenching investigations across such tensional fissures in the Pajarito fault system indicate that they can be useful for dating paleoseismic events. Fissures associated with the Pajarito fault system accommodate significant opening on normal faults and fault-propagation folds, and are therefore important in assessing surface and shallow subsurface hydrology. The possibility that fissures opened during seismic events also has consequences for evaluating seismic surface rupture hazards.