DISTRIBUTION OF FAILURE MODES OF THE COLORADO PLATEAU SANDSTONES IN SPACE AND TIME: IMPLICATIONS FOR INTERPLAY AMONG DEFORMATION MECHANISM/TEXTURE/ RHEOLOGY AND STRESS

Field studies spanning 30 years indicate that sandstones of the Colorado Plateau and its surroundings failed by two distinct mechanical processes: Localization of deformation into bands, and Discrete dilational fracturing. This presentation will provide an overview of the distribution of the structures (deformation bands and joints/sheared joints) produced by these failure modes in the Plateau in space and time. Three spatial domains are recognized: Regions of bands only, Regions of joints only, and Overlapping bands/joints. Bands-only domain is found in the San Rafael Desert in south-central Utah, where the bands occur in nearly flat-lying aeolian Entrada Sandstone of Jurassic age. Joints-only domain is delineated in the Book Cliffs of north-central Utah where progradational fluvio deltaic sandstones of the Mesaverde Group of Upper Cretaceous to Eocene age are intercalated with shales. There the joints, and faults which evolve by subsequent shearing of the joints, occur in the Castlegate Sandstone overlying the Mancus Shale along the base of the Book Cliffs for tens of km. Field relationships suggest that two parameters may characterize the differences between the two distinct structural domains: Textural and depositional architecture and/or mechanical loading.

Regions characterized by overlapping patterns of bands and joints, which reveal more about the conditions of their formation, are numerous. The Capitol Reef National Park, the Arches National Park and the San Rafael Swell in Utah and the Valley of Fire State Park in SE Nevada are excellent localities to see younger joints overlapping older bands, which may indicate change of rheology of the Entrada and Navajo (or Aztec) sandstones, or change of loading conditions. South of the Arches National Park, the Moab normal fault displays contemporaneous bands and joints localized at respective compressional and extensional domains of fault relays. The only place where younger bands are seen to occur within a system of older joints/sheared joints is that of the strike-slip faults in the Valley of Fire Park. These spatial and temporal interplays between the two major failure modes along a single fault is due to perturbation of stresses at normal fault relays or strike-slip stepovers, which provide limiting conditions for the transition from one mode to the other