Determining relative age of out-of-sequence faults: integration of foreland basin deposits to a flexural thrust belt evolution and a case study in Western Nepal
Creating forward modeled, balanced cross sections, while accounting for both flexural loading and erosional unloading have the potential to verify and refine the kinematic sequence of deformation in fold and thrust belts. Insight into the relative order of events is particularly apparent for out-of-sequence faults that have the ability to either vertically exhume deeper strata in the case of thrusts, or normal faults which create basins that may or may not be preserved in the mapped geology. Strata exhumed by thrusts (either in- or out-of-sequence) may leave a distinct provenance or detrital age signature in the foreland basin. A predicted foreland basin and foreland basin stratigraphy develops as a result of the isostatic model, allowing correlation between modeled and measured stratigraphic sections in the vicinity of the cross section. Incremental modeling in short displacement steps (~10 km) creates “pseudostratigraphy” in the foreland which predicts the location and magnitude of preserved sediment, and identifies the location and amount of material eroded in the incremental thrust event, i.e. the provenance of the sediments within the pseudostratigraphy. We present a case study for this method using two cross sections through the Himalayas of Far-Western Nepal and assess the validity of the resulting dated kinematic histories, displacement rates, flexural wave response, section geometry, and matches to provenance for both sections. In addition, we compare and contrast flexural forward models which take an additional sediment loading step into account and those that do not.