Low-temperature thermochronometric ages in fold-and-thrust belts

Abstract

We use a 2-dimensional thermal-kinematic thrust fault model to generate low-temperature thermochronometric ages in a fold-and-thrust belt to understand better the rates and timing of thrust-deformation recorded by cooling ages. Our model links the kinematic structural model 2DMove to a finite-element advection–diffusion thermal model to track the cooling history of material exhumed by erosion during thrust faulting and fault bend folding. We use the cooling histories to model fission track annealing and He retention and predict zircon and apatite fission track and apatite (U–Th)/He thermochronometric ages. We model thermochronometric age patterns generated during motion along individual and multiple forward-propagating thrust faults, and explore the effects of fault slip rate and fault geometry. All our models generate a characteristic U-shaped thermochronometric age pattern along a transect perpendicular to the fault. This pattern is formed by the distribution of reset, unreset, and partially-reset thermochronometric ages. Above the thrust ramp, reset material forms a zone of young thermochronometric ages. Partially reset and unreset material lies on either side of this zone of young ages. Although some thermochronometric ages predicted by our fault-bend-fold model are similar to those predicted using a 1-d thermal model, the patterns of ages are markedly different. The smooth U-shaped pattern is generated in both the individual thrust fault and multiple thrust-fault models. In the foreland-propagating, multiple-fault models, there are no distinct changes in age across individual thrust faults. In these multiple fault models the thermochronometric age pattern changes with thrust-fault spacing. Depending on the fault spacing and amount of slip along each fault, early faulting can be recorded either in the hinterland or foreland of the thrust-belt.