CONTROLS ON HINGE-PARALLEL EXTENSION FRACTURING IN SINGLE-LAYER TANGENTIAL-LONGITUDINAL STRAIN FOLDS

The stress history results from a published viscous layer folding solu tion are used as the basis for a fracture mechanics analysis of the fa ctors that control hinge-parallel extension fracturing in tangential-l ongitudinal strain folds. The analysis incorporates published results for the change in sedimentary rock mode I fracture toughness at increa sing confining stress to examine the relationship between regional str ain rate, depth of burial, pore fluid pressure, initial crack size, la yer viscosity, and the amount of fold shortening required for the prop agation of a bed-perpendicular, hinge-parallel extension fracture. Tan gential-longitudinal strain folding of layers can occur at all scales in a foreland thrust system and is the result of the buckling and bend ing of stratigraphic units during the development of decollement, faul t bend, and fault propagation folds. Hinge-parallel extension fracture s oriented perpendicular to bedding are a common fracture set observed in tangential-longitudinal strain folds. The fractures propagate as a result of local tensile stresses that develop by the stretching of la yers in the outer are. of fold hinges during bending. We considered a range of geologically reasonable boundary conditions to show that at o ne extreme, fracturing can occur as a result of only minor shortening by folding to the other extreme where a tight fold can form with no as sociated extension fracturing. For folds formed at shallow depths, whe re the confining stress on the system is less than the bending stresse s in the layer and where the confining stress has not greatly increase d the fracture toughness of the rock, hinge-parallel extension fractur es can grow under hydrostatic fluid pressure conditions. As depth incr eases, however, much higher pore fluid pressures are required to cause fracturing under similar strain rates. The observed controls are used to hypothesize how hinge-parallel extension fracturing in fault bend folds can vary spatially and temporally across a thrust belt as a func tion of strain (thrusting) rate, the amount of bending at thrust ramps , and the depth of folding.