Shankar Mitra¹

(1) University of Oklahoma, Norman, OK

ABSTRACT: Characteristic Geometry And Evolution Of Faulted Detachment Folds

Faulted detachment folds represent a common structural style in a number of hydrocarbon-producing fold-thrust belts and in the deep-water Mississippi Fan and Perdido fold belts. These structures form in units characterized by high competency contrasts, and display a transition in deformational behavior from detachment folding to progressive fault propagation with increasing shortening. The major forelimb and back limb thrusts are secondary to the fold, and accommodate strain resulting from variations in structural position and lithology. The units first deform by asymmetric detachment folding, with the fold wavelength controlled by the thickness of the dominant unit. Volumetric constraints require the movement of material from the synclinal to the anticlinal hinges. Continued shortening results in a tighter fold and the propagation of a thrust fault through competent units on the steep front limb. The fault initially terminates both up and down section within deformation zones in the incompetent units. With continuing deformation, it connects with the basal detachment to form a throughgoing thrust fault.

Because faulted detachment folds superficially mimic fault-propagation folds, their geometry can be misinterpreted in areas of poor data quality. This can result in incorrect assessments of the size and location of hanging-wall traps, inaccurate prediction of the sealing capacity of the major fault, and bypassing of potential footwall traps. Characteristic features for distinguishing these structures include (1) smaller variations in restored wavelength with shortening, (2) complex variations in fault slip, (3) considerable ductile deformation within the basal units, and (4) occurrence in sequences with significant competency contrasts.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado