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.