TWO- AND THREE-DIMENSIONAL MODELING OF DETACHMENT-STYLE FOLDS

Geologists have come to recognize the presence and abundance of detachment-style folds in frontal fold-thrust belt zones. Typically, these folds are interpreted to develop by hinge migration, limb rotation, or a combination of the two processes. Poblet & McClay (1996) describe four different 2-D geometric models for such detachment-style folds; three of which are area-balanced and all possessing a constant regional level outside the fold proper. We derived two new 2-D geometric models for detachment-style folds that incorporate hinge migration and limb rotation as their deformation mechanism, respectively. These area-balanced models differ from all previous models, however, in that they allow hinterland uplift to occur, thereby creating a higher regional level in the hinterland relative to the foreland. These models produce different limb dips and lengths from Poblet and McClay’s (1996) counterparts and simulate uplift due to penetrative shortening in the hinterland relative to the fixed and undeformed foreland.

By varying along-strike displacement and/or changing the along-strike detachment depth (effectively creating lateral ramps), we constructed pseudo-three-dimensional models of terminations for all six types of detachment-style folds. We observed considerable variability in the 3-D geometry of the model terminations. Specifically, some model terminations developed along-strike transitions from upright to overturned limbs, curved axial surfaces that were both concave and convex towards the foreland, and folding perpendicular to the main fold axis. Preliminary comparison of the pseudo-three-dimensional model terminations with the Nuncios Fold termination near Monterrey, Mexico suggests that model results can help constrain cross-sectional interpretations for the structure.