THRUST FAULTS AS A MECHANISM OF ATTENUATION IN THE STEEP LIMBS OF FORCED FOLDS

East-dipping thrust faults in the steeply-dipping limb of the basin-bounding monocline along the east flank of the Colorado Front Range have been ascribed to a variety of origins:

·Backthrusting from the upper detachment of a triangle zone (Sterne and Raynolds, 2001; Sterne, 2002))

·Shortening from east-west compression (Siddoway, 2002)

·Rotated growth faults (Weimer, 2004)

Our analysis of this large displacement structure (associated with the Golden and Rampart faults), as well as several small-displacement folds, show that the east dipping faults are part of a conjugate thrust system. The conjugate faults are a primary mechanism of attenuation as the strata are folded over the edge of an uplifted basement block. Thinning of 19% by this mechanism was measured in a shale unit in one outcrop.

Analyzing the orientation of thrust faults in the steep limbs of a variety of structures along the Front Range and in Central Colorado indicates that this is a common phenomenon in extended steep limbs of forced folds. These thrust faults appear to be part of an orderly system of layer-parallel extension. Thrust faults that dip into the uplifted block will place older strata over younger strata and thrust faults that dip away from the uplifted block place younger strata over older strata.

This system appears to be scale-independent as it is observed operating on the sub-meter scale and the sub-kilometer scale. The process can create isolated, rhombic boudins or fault panels. These conjugate systems can tectonically isolate a sandstone boudin within a shale unit.

Our results suggest that the conjugate set may initiate when the strata have been tilted about 60 degrees from horizontal. With further rotation of the steep limb (even to overturned) the conjugate set is also rotated. With rotation, one of the sets becomes more favorably oriented to slip and becomes dominant.