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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 3. (March)

First Page: 336

Last Page: 344

Title: Geometric Constraints Derived from the Law of Conservation of Volume and Applied to Evolutionary Models for Detachment Folding: Geologic Note: (1)

Author(s): CLINTON D. A. DAHLSTROM (2)

Abstract:

This paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones. The common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel. Limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane. The basic depth to detachment calculation, which derives from the Law of Conservation of Volume, requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment. This imposes a geometric constraint with whi h a constant limb length anticline, growing by increasing limb dip above a fixed detachment plane, cannot comply unless, in the early stages, mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then, in the later stages, the flow direction reverses. Relatively few detachment zones contain a significant amount of mobile salt or shale. Therefore, in most concentric detachment fold trains, a basic geometric incompatibility exists between the conventional geometric model and the Law of Conservation of Volume. The conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows. In this model, the anticlinal axes generally are fixed and the synclinal axes active, which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons.

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