Rates of folding and faulting determined from growth strata

Abstract

Many upper crustal folds on the scale of 1–10 km in compressive mountain belts grow by kink-band migration as a result of fault-bend, fault-propagation, or box folding. One or both kink-band boundaries sweep through the rock as the kink bands widen during fold growth. The kink bands typically have a constant width in pregrowth strata, which are strata that existed before deformation, whereas we predict — and observe on seismic lines — an upward decrease in kink-band width within the stratigraphic sequence deposited during fault slip and associated fold growth -here called growth strata. In fact this growth stratigraphic sequence provides a complete, decipherable record of the kinematics of deformation, much in the same way that sea-floor magnetic anomalies provide a decipherable record of plate kinematics. It is the continual addition of material that provides the detailed record of motion in both cases. In the simplest folds the kink band has a constant width within the pregrowth strata, narrows upward through the growth strata, and finally has a zero width at the top of the growth stratigraphy.

The rate of folding can be described geometrically as the rate of kink-band migration divided by the sedimentation rate, which is dimensionless. Furthermore, if key beds can be dated, absolute fault-slip rates can be computed. Examples are presented from the Gulf of Mexico, the Philippines, Venezuela, California, and Oklahoma. Folding rates in many of these examples are observed to be 1–2 mm/y lasting for 1–8 Ma. Translation of folding rates into fault-slip rates requires knowledge of the fault shape and its relationship to fold shape. Nevertheless, even if fault geometry is poorly constrained, fault slip rates generally can be estimated to within narrow limits based on rates of kink-band migration, a fact that makes growth structures valuable for assessing earthquake hazards.