Zhang, Li¹, Judith S. Chester¹ 
(1) Texas A&M University, College Station, TX

ABSTRACT: Mechanical Constraints on Trishear Kinematic Models of the Backlimbs of Fault-Bend and Fault-Propagation Folds

Trishear kinematic models of fault-related folding generate kink-like and rounded fold forms providing an analytical tool for predicting relations between fold-form, fault-shape and fault-displacement in deformed terrains. Nevertheless, trishear kinematic models suffer from a non-unique velocity field formulation that allows an infinite number of possible velocity solutions. We use mechanical and analog models of the backlimb region of fault-bend and fault-propagation folds to determine the similarity of the backlimb trishear model to mechanics-based models and the range of physically reasonable trishear parameters for backlimb regions, and to constrain trishear parameters as a function of orientation and magnitude of anisotropy of sedimentary cover and frictional strength of the underlying fault. We find that the linear velocity field of the trishear kinematic model is an acceptable approximation when layering is approximately parallel to the fault surface or the magnitude of anisotropy is relatively small. The roundness of the backlimb fold decreases with magnitude of anisotropy, and hinterland vergence of the trishear zone increases as fault friction increases promoting thickening of layers as they move through the trishear zone. Trishear asymmetry and apical angles of approximately 0º and 75º, respectively, are appropriate for cases of nearly isotropic rock packages above low strength faults, whereas angles of approximately –20º and 30º, respectively, are appropriate for anisotropic sedimentary cover and frictional faults. The existing backlimb trishear model does not provide a reasonable approximation when sedimentary layering is inclined towards the foreland or steeply inclined towards the hinterland.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.