“Evolved” Triangle Zones along the Eastern Flank of the Colorado Front Range

Sterne, Edward J. ¹ (1) Petro-Hunt LLC, Denver, CO

Faults along the eastern flank of the Colorado Front Range display a diversity of attitudes and juxtaposed age relationships. More than half of the measured fault planes dip east or toward the foreland and of these approximately half show younger-over-older stratal juxtapositions. Also, some of the faults exhibit strange map patterns: they closely parallel younger strata along their eastern side and ramp across older strata on their western side.

This talk proposes a modified triangle zone model, one that allows backthrusts within the intercutaneous wedge, to explain the observed fault data. The “evolved” triangle zone model predicts a variety of characteristic fault types including: 1) foreland-dipping roof thrusts which either show no stratigraphic separation or anomalous younger-over-older relationships; 2) foreland-dipping intercutaneous backthrusts which exhibit both older-over-younger and younger-over-older bedding relationships; and 3) hinterland-dipping floor thrusts that show older-over-younger bedding relationships.

Evolved triangle zones are found at multiple stratigraphic and structural levels along the range. These stacked detachment levels accommodate displacement transfer along the range and give rise to some surprisingly complex but restorable structures. Examples will show how this model helps in understanding apparently anomalous thrust relationships in a variety of tectonic settings.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana