AHLGREN, STEPHEN G.
The University of Arizona, Tucson, AZ

Abstract: New Interpretations of the Sequential Development of ThreeDimensional Deformation Band Shear Zone Networks in the Navajo Sandstone, Southern Utah

Deformation Band Shear Zones (DBSZs) are exquisitely exposed in southern Utah at Sheets Gulch along the eastern edge of the Waterpocket Fold. The 2 sq km region contains extensive unvegetated exposures of DBSZs, including some tip-to-tip exposures, and serves as an ideal natural laboratory for Studying DBSZs with Riedel shear geometries in great detail.

DBSZs first develop as a zone of unconnected, spaced slip surfaces up to a few millimeters wide and tens of centimeters long, oriented at high angles to the trend of the zone. With increasing strain, these incipient surfaces ("External" R' surfaces) assume a sigmoidal form and are apparently ductily deformed, with a shear sense consistent with offset along the DBSZ. Further strain is accommodated on through-going, crosscutting slip surfaces (R surfaces), oriented at smaller acute angles to the trend of the zone. Subsequently, more closely spaced slip surfaces ("Internal" R' surfaces) develop and link overlapping R surfaces. Rotation of External R' surfaces within these overlap zones brings the External R' and Internal R' surfaces into sub-parallel orientations.

These complex relationships are best visualized using threedimensional digital models constructed at a small scale from serial cross sections through dissected field samples, and at a larger scale from field maps. The digital models demonstrate the self-similarity of DBSZs over scales of at least three orders of magnitude and elucidate the threedimensional interconnectivity of DBSZs within the penetratively deformed and compartmentalized Navajo Sandstone. The models serve as the basis for both strain analysis and fluid flow modeling, and may also be utilized as predictive models in similarly deformed regions.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas