ABSTRACT: Fault Propagation Folds: Geometry, Kinematic Evolution, and Hydrocarbon Traps

Shankar Mitra

Fault-propagation folds constitute an important trap style in fold and thrust belts. In these structures, propagating thrust fault loses slip upsection by transferring its shortening to a fold developing at its tip. Area-balanced theoretical models show that for any given footwall cutoff angle (q), small changes in thickness (g*) can result in a wide range of interlimb angles (< 10%). The progressive evolution of fault-propagation folds is commonly characterized by the tightening of the fold hinge and steepening of the front limb. This requires thickening of stratigraphically higher units in the early stages of folding, followed by localized thinning of the front limb in the late stages of folding.

A fault-propagation fold can be modified by subsequent translation on propagating thrusts. Depending on the tightness of the fold, the thrust fault may propagate through the undeformed footwall, the synclinal axial plane, or the front limb of the anticline. Deeper thrusts commonly are abandoned upsection, and the slip transferred to steeper imbricates, producing triangular fault slivers in the footwall, and the listric geometries commonly observed in many natural thrust faults. The fold can also be translated onto an upper detachment, producing a mode II ramp-related fold.

Structural traps in fault-propagation folds include primary traps in the anticlinal crest, and secondary traps in the folded footwalls, and adjacent to forelimb and backlimb imbricate thrusts. Some examples of hydrocarbon-producing fault-propagation folds are the Turner Valley anticline (Alberta Foothills), the Garzan-Germik oil field (southeast Turkey), and the Digboi oil field (Naga Hills thrust belt).

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990