A unified framework for earthquake sequences and the growth of
geological structure in fold-thrust belts
Abstract
Observations of fold growth in fold-thrust belt settings show that
brittle deformation can be localized or distributed. Localized shear is
associated with frictional slip on primary faults, while distributed
brittle deformation is recognized in the folding of the bulk medium. The
interplay of these processes is clearly seen in fault-bend folds, which
are folds cored by a fault with an abrupt change in dip (e.g., a
ramp-décollement system). While the kinematics of fault-bend folding
were described decades ago, the dynamics of these structures remain
poorly understood, especially the evolution of fault slip and off-fault
deformation over different periods of the earthquake cycle. In order to
investigate the dynamics of fault-bend folding, we develop a numerical
modeling framework that combines a long-term elasto-plastic model of
folding in a layered medium with a rate-state frictional model of fault
strength evolution in order to simulate geologically and mechanically
consistent earthquake sequences. In our simulations, slip on the
ramp-décollement fault and inelastic fold deformation are mechanically
coupled processes that build geologic structure. As a result, we observe
that folding of the crust does not occur steadily in time but is
modulated by earthquake cycle stresses. We suggest combining
seismological and geodetic observations with geological fault models to
uncover how elastic and inelastic crustal deformation generate
fault-bend folds. We find that distinguishing between the elastic and
inelastic response of the crust to fault slip is possible only in the
postseismic period following large earthquakes, indicating that for most
fault systems this information currently remains inaccessible.