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ABSTRACT: Style and Balance of Extensional Structures

Richard H. Groshong

Based on structural style and rules of balance, quantitative models of extensional structures provide a powerful and direct approach to the goal of finding the correct interpretation of a cross section despite incomplete, misleading, or erroneous data. A balanced section has the same area before and after deformation and can be restored to its original geometry. These conditions, together with a quantitative statement of the style as a relationship between bed dip, fault geometry, and internal rotation, are sufficient to define a model.

Most oil-field scale extensional structures are half-grabens, formed either by domino-block rotation or by slip on a curved or ramp-and-flat master fault; equal-displacement conjugate fault systems appear to be common only in association with salt domes. The two fundamental style elements may be modified by deposition during deformation, compaction, and footwall uplift. In the domino model, beds and faults rotate together producing a quantitative relationship between bed dip, fault dip, block width, and displacement. Along curved or ramp-and-flat faults, including low-angle ramps, hanging-wall fold geometry can be related to displacement by a general oblique simple-shear model or an antithetic simple-shear model. Simple shear oblique to bedding causes changes in bed length of the roll ver adjacent to the master fault. These changes can be accomplished either by conjugate fault systems or by dominoes acting as the deformation mechanism within the rollover.

The style elements described previously are illustrated by experimental models, field examples, and recently published quantitative interpretations. The development and testing of an interpretation, based on geologic and seismic data from the Schell Creek fault, Nevada, show by using the model relationships how a cross section is constrained, and underline the critical importance of the restoration datum.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990