Geometry and lateral slip distribution along large thrust fault systems

Quantitative relationships between fold and fault shapes allow detailed analyses of thrust fault geometries and slip histories. Fold limbs (kink bands) that grow by axial surface migration above bends in thrust faults record dip-slip motion. Folds imaged in high-resolution seismic reflection profiles record this total fault dip-slip and reflect causative thrust fault geometry. Growth (syntectonic) strata deposited during the active history of underlying thrusts develop limb widths equal to the amount of fault dip-slip since their deposition. Therefore, narrowing upward kink bands (growth triangles) form as sediments deposited earlier in the slip history record wider limb widths than do sediments deposited later. Ages of selected syntectonic strata (determined independently) in growth triangles allow estimates of long term fault-slip rates. Maps of axial surfaces that bound kink bands highlight changes in thrust fault geometry along strike and record lateral fault-slip distribution. In addition, end and offsets of kink bands in map view highlight fault terminations and lateral changes in thrust fault geometry. Detailed analyses of fold and fault geometries constrain balanced, three-dimensional structural models that show how large thrust faults develop and slip through time. These models and cross sections integrate G.P.S. measurements and seismicity from active thrust fault systems. Lateralmore » variations in fold shape caused by changes in thrust fault geometry may form lateral closure along fold trends that trap hydrocarbons. Combined, fault slip rates and fault geometry yield estimates of the size and recurrence of potentially damaging earthquakes on blind thrust fault systems.« less