ALONG-STRIKE VARIATION IN A FAULTED MONOCLINE, PAJARITO FAULT, LOS ALAMOS, NEW MEXICO

Field studies of the geometry of extensional fault-related folds provide key contextual data for paleoseismic investigations in the Española basin of the Rio Grande rift.  The Pajarito fault (PF) forms the active western boundary of the rift near Los Alamos, NM, where it strikes north to northeast along the western margin of Los Alamos National Laboratory. Along strike, the PF is expressed at the surface as a large normal fault, a faulted monocline, and a distributed zone of deformation with significant down-east vertical displacement of the 1.2 Myr-old Tshirege Member of the Bandelier Tuff (Qbt).  Stratigraphic and geochemical correlations of Qbt subunits allow us to estimate displacement on the PF, even where mass wasting extensively modifies the escarpment.   Detailed field studies of 7 km of the 40-km-long PF show that the geometry of the fault varies appreciably along-strike.

Maximum stratigraphic separation occurs south of the study area, where the PF is expressed at the surface as two down-east normal faults with >200 m of displacement on Qbt.  Farther north, within the study area, the PF is a narrow (300-m-wide) faulted monocline with >120 m stratigraphic separation, a prominent graben at the base of the escarpment, and hanging wall bedding that dips toward the fault—consistent with the fault tip having broken the surface.  Down-east displacement on the PF diminishes somewhat to the north, the zone of deformation broadens, and bedding dips along the monocline decrease.  Down-east displacement in this sector is accommodated by monoclinal folding and small-offset distributed normal faulting, including crestal grabens, with ~35 m maximum displacement on any single mapped fault. The along-strike variation in fault-zone structure suggests that the fault tip of the PF lies below the surface in the northern part of our study area.  The youngest dated surface ruptures along the PF (late Holocene) and recent small magnitude earthquakes occurred near the north end of our study area.

Previous paleoseismic investigations indicate that different fault traces along the PF have ruptured at different times.  This may be a function of the distributed nature of faulting associated with upward and lateral propagation of the PF. An improved understanding of the geometry of the PF will aid in assessing seismic hazards in this area.