TRANSVERSE ACCOMMODATION FAULTING IN THE PARADOX BASIN, COLORADO PLATEAU: MANAGING STRAIN COMPATIBILITY

The Paradox Basin contains three dominant NNW structural-geologic systems, each of which trends NNW: 1) salt-wall corridors that contain salt-cored anticlines and halokinetic features related to Pennsylvanian to Jurassic passive diapirism; 2) gentle upright folds (“Laramide structures”) produced by late Cretaceous through Paleocene ENE-WSW tectonic shortening; and 3) swarms of near-vertical normal faults representing mid-Cenozoic to modern collapse due to subsurface dissolution of salt. Less obvious at regional scale is a system of ENE-striking faults orthogonal to the dominant structural grain. These transverse faults (16 in total) have a subtlety of expression with short trace lengths (3 to 12 km) and relatively small offsets (10 to 100 m). Kinematics expressed by these faults are almost exclusively extensional (NNW/SSE stretching).

We interpret the ENE-striking faults as transverse accommodation faults, which evolved during the creation of the three basin-wide structural geologic systems. Most were active during more than a single window of time: Permo-Triassic and/or latest Cretaceous to early Cenozoic and/or Neogene. Transverse faulting operated to manage strain compatibility within dominantly brittle strata where strain compatibility could not be fully accommodated through smooth variations in deformation. Specifically, the faulting is an essential accommodation to changes in (1) breadth, height, and crestal geometry of salt walls and associated halokinetic features, (2) plunge and/or trend and tightness of Laramide folds, and (3) net volume of dissolution along the margins of salt walls. Both the earliest (passive diapirism and halokenesis) and latest (dissolution collapse) loading conditions were gravitational. ENE-WSW Laramide compressional tectonic loading not only formed folds but also generated syntectonic NNW-SSE stretching. Overall, the long-lived and periodically recurrent transverse faulting is the natural result of fault-slip adjustments geared to minimizing strain incompatibilities. Were it not for salt walls accommodating some bulk strain requirements through ductile behavior, transverse faults in the Paradox Basin may have been yet more plentiful. Fault-localized reduction bleaching and mineralization along transverse fault systems reveal that fluids were drawn into these dilational sites.