Abstract: Deformation of the Footwall by "Shortcut Faults" During Inversion: Examples from Europe and the Sangre de Cristo Mountains, Colorado

Charles F. Kluth

Inversion structures form when an earlier basin is incorporated into a later uplift. In some cases, the fault zones are reactivated by later movement in the opposite sense of the earlier movement. In the case of steep earlier normal faults, later reverse movement is efficient at transferring shortening into vertical uplift, but not very efficient at accommodating large amounts of shortening. In the case where the inversion episode involves more shortening than the pre-existing fault geometry can accommodate, faults often form that cut the earlier footwall ("shortcut faults") at lower angles. In this manner, part of the earlier footwall is transferred to the later hanging wall and incorporated into the uplift, and more shortening can be accommodated. Seismic data from inversion structu es in Europe illustrate footwall shortcut reverse faults that cut the footwall of pre-existing normal faults. Exposures in the Sangre de Cristo Mountains in south-central Colorado indicate that the footwall of the Pennsylvanian Central Colorado trough was incorporated into the hanging wall of Laramide inversion. The hanging walls of lowthrusts of Laramide age carry lower Paleozoic rocks resting in depositional contact with the Precambrian basement. The lower Paleozoic rocks were eroded from the Pennsylvanian uplifts adjacent to the trough. Thus, these preserved contacts were derived from the footwall of the earlier structure. Stacking relationships in the thrust plates indicate that some of the thrusts formed out of sequence, but the details of the thrust timing are still not completely nown. Seismic and field data in that same area indicate that the Late Tertiary normal faulting, associated with development of the Rio Grande rift has cut into the footwall of the Laramide thrust system during reinversion of the Laramide uplift.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994