Davis, George H.
Regents Professor, University of Arizona, Tucson, Arizona

Abstract: Amazing Displays of Deformation Band Shear Zones Within Structure-Tectonic Systems of the Colorado Plateau . . . From Zion to Bryce and Beyond

Mesozoic strata within the Colorado Plateau region of southern Utah bear the structural geologic impact of three major regional tectonic deformations: Laramide, Mid-Tertiary (Miocene), and Basin and Range. Laramide deformation associated with east-northeast/west-southwest compression created the Kaibab Uplift, the Circle Cliffs Uplift, and the San Rafael Swell, as well as the great monoclines that form a structural flank of each uplift. Moreover, Laramide compression created numerous broad anticlines and synclines, notably within the Kaiparowits Basin. Mid-Tertiary (Miocene) tectonism produced the laccoliths and associated domes of the Henry Mountains, the Marysvale volcanic field and associated laccoliths, and the Paunsaugunt thrust belt, which appears to have formed during emplacement, collapse, and spreading of the southern part of the Marysvale volcanic field. Basin and Range deformation is expressed in the form of three major discrete high-angle normal faults located within the Western High Plateaus: the Hurricane, Sevier, and Paunsaugunt.

Important aspects of the kinematics and mechanics of formation of each of the major regional structures are recorded in outcrop and larger scale semi-brittle shear zones that occur in porous sandstones in the form of deformation bands and zones of deformation bands. They occur abundantly in Navajo Sandstone (Jurassic), Entrada Formation (Jurassic), and Straight Cliffs Formation (Upper Cretaceous), especially where these formations crop out in proximity to the major regional structures. Deformation bands and zones of deformation bands of tectonic origin accommodate tiny ‘fault’ displacements through a shearing that involves the combination of pore-space collapse and grain-scale fracturing and cataclasis. Deformation banding is a strain-hardening deformation mechanism, which results in an abrupt ‘lock-up’ of the semi-brittle shear zone. In turn, the first-formed deformation band becomes a stress/strain concentrator for the next increment of deformation band shearing that will occur immediately alongside. In this way, zones of deformation bands evolve, and many of these can ultimately accommodate brittle faulting, including the formation of slickensided, slickenlined surfaces. Some of the most impressive zones of deformation bands display elegant Riedel geometries, even conjugate Riedel geometries. Because of localized loss of porosity, grain-scale fracturing, and selective precipitation of quartz in the very fine-grained crushed fractions, deformation bands and zones of deformation bands are quite resistant to weathering and erosion and thus project from outcrops as if they were resistant veins or dikes, in some cases creating fault-fin landscapes.

Zones of deformation bands associated with Laramide compression include normal faulting along the upper hinge zones of monoclines, thrust and reverse faults near the lower reaches of monoclinal folds, and strike-slip systems along transpressional zones. Those associated with Miocene thrusting include conjugate thrust deformation band shear zone systems. Zones of deformation bands associated with laccolith emplacement in the Henry Mountains reflect a flexural bending, a flexural slip, and a circumferential stretching strain. Zones of deformation bands associated with Basin and Range faulting are closely restricted to the fault zones proper, and consist typically of two systems of conjugate normal faults, one system striking parallel to the major fault zone, the other subordinate system striking perpendicular.

The deformation band shear zones have collectively imparted a macroscopic ductility to major regional lithostratigraphic units, especially the Navajo Sandstone. Slip-systems are innumerable, able to accommodate whatever regional and local strain must accrue to achieve structural compatibility. This contributes to the geometric elegance of the Colorado Plateau regional structures. Furthermore, the deformation band shear zones compartmentalize sandstone bodies in and around major regional structures. Because porosity is lost within deformation band shear zones, these structures seal the boundaries of compartments and dramatically influence fluid flow. Thus the structures are natural laboratories for characterizing reservoir-scale deformation.

AAPG Search and Discovery Article #90929©1998-1999 AAPG Distinguished Lecturers