PATTERNS OF GROWTH STRATA ASSOCIATED WITH DETACHMENT FOLDS

Detachment folds represent a common structural style in a number of fold-thrust belts and deepwater fold belts. The evolution of these structures involves a number of distinct stages, which are recorded by synkinematic growth strata. Early structural evolution involves the formation of broad, symmetric or asymmetric folds, with the fold wavelength controlled by the thickness of the dominant units. Volumetric constraints require sinking of units in the synclines, and movement of the ductile unit from the synclines to the anticlines. Continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and synclinal hinge migration. Initially, limb rotation occurs primarily by hinge migration and flexural-slip folding; however, late-stage fold tightening may involve rotation between fixed hinges, resulting in the development of deformation zones and the propagation of faults through the fold limbs.

Patterns of growth strata associated with detachment folding depend on the mechanisms of fold evolution. Synclinal sinking in the early stages of folding results in thickened growth strata, which are subsequently rotated onto the fold limbs. Flexural-slip folding involving hinge migration results in parallel growth strata and growth triangles, whereas limb rotation between fixed hinges results in fanned growth strata. The simultaneous occurrence of limb rotation and anticlinal or synclinal hinge migration results in progressive onlap and folded growth strata. These growth patterns can therefore be used to understand the kinematic evolution of detachment folds, and the relative importance of different mechanisms during fold evolution.