ABSTRACT: Volumetric Constriction and Concentric Deformation

WILLIS, JAMES J., Baylor University, Waco, TX

Concentric folding is distinguished from similar folding on the basis that thickness is maintained during deformation. Consequently, anticlines exhibit tightening-down geometries, whereas synclines tighten upward. A major aspect of concentric deformation involves volumetric constriction in these upward-tightening "synclines," which range from outcrop-scale and smaller to basinal proportions. This volumetric constriction can be accommodated by several methods, including loss of volume (e.g., porosity reduction) and transfer of excess volume by bedding plane slippage and "out-of-the-syncline" faulting. Because of important implications toward hydrocarbon exploration and prospect evaluation, we offer an expanded classification system of various resultant crowd structures, which not only rovides both descriptive and identification aspects, but also relates their morphology and location to deformational kinematics.

Brown (1988) previously classified three detachment-originated, "out-of-the-syncline" structures. Rabbit-ear structures develop on the steep flank of the primary structure, whereas back-limb thrusts form on the gentle limb. Cross-crestal structures offset the fold axis of the primary anticlinal structure and may be of back-limb or rabbit-ear type, with back-limb predominant. To this, we add synclinal-hinge faulting, where breakage occurs parallel to the synclinal hinge. The fault originates in the upper section, but propagates downward along the hinge as volumetric constraints progress downward in response to continued synclinal tightening. Synclinal-hinge faults are present in numerous situations but are especially prevalent in fold-thrust structures. In the dual-fault system of fold thrusts, the lower fault appears to originate as a synclinal-hinge fault. Many times, however, synclinal-hinge faults are misinterpreted to be the primary basement-originated faults of fold-thrusts. The synclinal-hinge fault is entirely

secondary in origin, but may contribute to overall uplift of the fold-thrust, especially following shearing of the entire stratigraphic section by downward propagation of the fault. We also add to the classification system pop-out anticlines, an anticlinal detachment feature that literally appears to have "popped out" of the center of the syncline (the anticlinal axis is directly above and parallels the deeper synclinal axis). Minor associated structures are also described and classified.

Numerous examples have been observed throughout the Rocky Mountain foreland and Cordilleran thrust belt, as well as other compressional regimes. Many localities display multiple "out-of-the-syncline" structures. Recognition of these volumetric crowd structures is critical to understanding both the structural geometry and deformational history of the primary structure. In many cases, shallow geometries are controlled by these secondary structures, in effect obscuring deeper subsurface structures of higher order and potential hydrocarbon traps. In other situations, these secondary structures may themselves be productive.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)