Date of Award:

5-1996

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

James P. Evans

Committee

James P. Evans

Committee

Susanne U. Janecke

Committee

Peter T. Kolesar

Abstract

An integrated field and structural analysis of the Mitten Park fault-fold structure, northwestern Colorado and northeastern Utah, examines its structural origin. The Mitten Park structure is a modified fault-propagation-fold. This new model incorporates faulting, folding, and fracturing in one deformational event to produce the Mitten Park fault and associated monocline.

The largest structure in the study area is the Mitten Park fault and associated monocline. The Mitten Park fault has approximately 127 meters (415 feet) of net slip, strikes S28°W and dips 55°WNW. In the footwall, net shortening was accommodated by reverse and normal faulting. Faulting was the result of northwest-southeast directed shortening. Reverse faulting accommodated the majority of the fault-related strain along the fault's trace and resulted in net shortening. However, normal faults in the overturned limb of the footwall of the Mitten Park fault also accommodated northwest-southeast directed shortening.

Folds in the study area are asymmetrical and statistically cylindrical in both the footwall and the hanging wall. Folding facilitates northwest-southeast directed shortening. There is a direct correlation between changes in the strike and dip of the fault plane and changes in the trend and plunge of fold axis in the footwall.

Fracture orientations show no significant variation in geometry from hanging wall to footwall. Fracture intensity increases with proximity to the Mitten Park fault.

Balanced cross sections of the Mitten Park area use a modified fault-propagation- fold model and are also constrained by field observations and interlimb angles of folds. Total shortening in the study area is 13.5% and was accommodated by the hanging wall, the footwall, and the Mitten Park fault. The hanging wall accommodated 70.8% of total shortening, the footwall accommodated 14.9% of total shortening, and the Mitten Park fault accommodated 14.3% of total shortening. The significant amount of strain in the footwall of the fault is different from classical models of fault-propagation-folds, which depict a rigid undeformed footwall.

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