--> Abstract: A Test of Geometric and Kinematic Models for Fault-Bend Folds Using a Fully-Exposed Structure in the Dolomitic Alps, Italy, by C. D. Hawkins; #90937 (1998)

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Abstract: A Test of Geometric and Kinematic Models for Fault-Bend Folds Using a Fully-Exposed Structure in the Dolomitic Alps, Italy

HAWKINS, C. DIAHN, The University of Alabama

A fully-exposed fault-bend fold in the Dolomites of northeastern Italy was used to test geometric and kinematic models for fault-bend folds. Two models were tested: a constant bed length, constant bed thickness model (Suppe, 1983) and a constant area model that allows bed thickness and bed length changes (Epard and Groshong, 1993). The Suppe (1983) model requires fault development prior to folding and does not allow for footwall deformation. Both models require layer-parallel slip between competent units for fold evolution. The test indicates that, while the structure does maintain constant area throughout folding, bed lengths and thicknesses do not remain constant. It also suggests that folding may have been initiated prior to fault develoment.

The purpose of this study is to test how well existing geometric and kinematic models for fault-bend folds fit the geometry of fully-exposed natural structures. Fault-bend folds form as the hanging wall of a thrust fault moves up a ramp and onto an upper detachment surface. The existing models were developed primarily for map-scale structures and thus, the geometry of the fold may be largely inferred. The models typically utilize limited information from one or two key beds involved in the structure and the detachment zone or surface, as well as additional beds within the structure, are unseen. In addition, the models use the predicted geometry of the structure in order to infer the structural evolution.

The structure is located at Piz Boe in the eastern Dolomites. The stratigraphy involved in folding is the Dachstein Limestone (Upper Triassic) and consists of 10-50 cm mudstone beds, which are separated by 1-3 cm shale beds. The structure is between 15 and 20 m in height and the main thrust has a flat-ramp-flat geometry. A small thrust fault, which diverges from the main thrust and forms a horse, offsets hanging wall stratigraphy, and there is a small fold in the footwall below the lower detachment. This small fold appears to detach in a shale bed below the main thrust.

The fold was photographed using a digital camera and all length, area and angle measurements were made using the computer program Canvas. Application of the models to the structure indicates that the Suppe (1983) model does not describe the geometry of the fold, and that the fold did not maintain constant bed length and thickness. The Epard and Groshong (1993) model accurately predicts both the lower detachment surface of the main structure and the detachment for the small fold beneath the main thrust. The footwall syncline is truncated by the main thrust ramp, suggesting that it formed before the ramp and that the structure evolved by breaking through an older detachment anticline.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah