SEISMIC SIGNIFICANCE OF FAULT-ZONE ARCHITECTURE IN GRANULAR POROUS MEDIA

The ~50 km long Pajarito fault system is a complex zone of deformation that displaces the 1.2 Ma surface of the Bandelier Tuff in the vicinity of Los Alamos, NM. The system records paleoseismicity associated with Rio Grande rift extension, with individual faults recording events as young as 2 ka. Both natural canyons and excavated trenches provide evidence that can be used to better understand the near-surface evolution of coseismic fault-zone structure and architecture.

The main faults of the Pajarito system exhibit clay-rich fault cores surrounded by damage zones that are dominated by minor faults and fractures. Damage zone character varies with the degree of welding of the ash-flow tuff (ignimbrite) cooling units that constitute the Bandelier Tuff. Of particular interest are glassy non-welded units, which exhibit porosities of ~30-55%, and are locally so poorly lithified that they can be disaggregated by hand. These have been compared mechanically to poorly lithified sands. Recent work indicates that structures developed in such sands depend on strain rate: seismic strain rates result in the formation of deformation bands, whereas aseismic creep is recorded by grain alignment through distributed particulate flow. In glassy non-welded units of the Bandelier Tuff, damage zone structures are deformation bands, consistent with the documented history of seismicity. In contrast, many faults in older poorly lithified sediments of the Rio Grande rift include both foliated zones formed through particulate flow, termed ‘mixed zones', and deformation bands. We hypothesize that the presence of mixed zones in these older fault zones may reflect a longer history that included episodes of aseismic creep.