New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping in the north-central to northeastern portion of LANL revealed small faults and broad zones of defonnation that have little potential for seismic surface rupture. The 7.5 km² area lies ~1 km east ofthe Pajarito fault system, ~2.5 km south of the Sawyer Canyon fault, and within a zone of infened, subsurface, pre-Bandelier Tuff faults. Mapped changes in welding and crystallization in the Tshirege Member of the Bandelier Tuff coincide with or parallel stratigraphic contacts, providing large-scale; planar markers for determining structural offset. Fault and fracture zones include: 1) An ~18-m-wide zone of abundant fractures and faults that range in width from 1-60 cm, and have a mean strike of N12E ± 26° and mean dip of 87 ± 5°; 2) A 60-m-wide fault zone with ~1.2 m of down-to-the-northwest displacement; 3) A 15-20 cm-wide fault that is oriented N16E, 86E and consists of densely spaced deformation bands; and 4) Three small (<1 m of vertical displacement) faults in the western portion of the map area. Fault zones 1 and 2 coincide with a zone of shallower dip that trends N10-15E, and could accommodate up to 8 m down-to-the-west displacement across ~500 m. No stratigraphic markers are present where fault 3 is exposed; however, 3-D structural analysis suggests that it may be associated with distributed down-to-the-east deformation accommodating 5-6 m of displacement over ~300m. Changes in dip of Tshirege Member units may result from distributed deformation or deposition over paleotopography.