HR: 13:55h
AN: T43E-02    [Abstracts]
TI: Two Types of Compaction Bands in Aztec Sandstone at Valley of Fire, NV
AU: * Eichhubl, P
EM: peter.eichhubl@beg.utexas.edu
AF: The University of Texas at Austin, Jackson School of Geosciences, Bureau of Economic Geology, Austin, TX 78758, United States
AB: Deformation in porous and granular media tends to localize along planar bands of finite width, referred to as deformation bands. Based on their dominant relative displacement they have been subdivided into shear, compaction, and dilation bands, with compaction bands reported from laboratory experiments and two field sites. At one of these sites, Valley of Fire, NV, two types of compaction bands are observed, with distinct orientations and structural attributes: Pure compaction bands (PCBs) are composed of single strands that are 5-10 mm thick, are rarely planar and more commonly wavy or zigzag. PCBs are inferred to form perpendicular to the maximum compressive principal stress. Shear-enhanced compaction bands (SCBs) are planar, often composed of multiple strands, and form conjugate sets that are bisected by PCBs. SCBs are oriented at 32-53° relative to the inferred direction of the maximum compressive principal stress. Based on field observations and textural image analysis, SCBs are characterized by approximately equal magnitudes of band-perpendicular shortening (compaction) and shear, with shear not exceeding 2 mm. Shear-enhanced compaction bands are distinguished from shear bands with macroscopic shear in excess of 1-2 mm.

Within the cross-bedded eolian sandstone, pure compaction bands, shear-enhanced compaction bands, and shear bands occur in two distinct styles within different parts of dune sets: Foresets contain one set of PCBs at high angle to bedding and two sets of SCBs in a strike-slip configuration, in addition to bedding-parallel SCBs. Bottom sets typically contain no PCBs, but two sets of SCBs in a reverse-slip configuration and bedding-parallel shear bands. Both styles can be shown to have formed concurrently. The absence of PCBs in bottom sets is attributed to slip along bedding-parallel shear bands in more clay-rich bottom sets.

These observations suggest that the occurrence of PCBs is limited to loading conditions that approach uniaxial stress state. Zigzagging of PCBs is interpreted to result from a local increase in triaxiality ahead of a propagating PCB. Regional structural relations indicate that the compaction bands formed at estimated 300 m of burial in the vicinity of an approaching thrust sheet suggesting high deviatoric stresses, high porosity, and low induration at the time of deformation.
DE: 4410 Bifurcations and attractors
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting