MODELING FEEDBACKS BETWEEN TRANSIENT THERMAL EVOLUTION, RHEOLOGICAL BEHAVIOR, AND STRAIN LOCALIZATION IN COLLISIONAL THRUST BELTS

In orogens, thermal processes govern rheology, which controls material strength, strain localization, and ultimately, the structural architecture of collisional systems. Although the feedback between mechanical and thermal processes is most evident in numerical models of orogens that show a progressive evolution from small cold accretionary wedges to large hot orogenic plateaus, many such models involve a necessarily simplified viscous or plastic deformation continuum that cannot account for the effects of large fault and shear zones. In natural systems, however, two key observations indicate that these major strain discontinuities play a fundamental role in orogen thermal evolution: (1) Large faults and shear zones often separate components of the composite orogen that have experienced broadly different thermal and deformational histories, and (2) quantitative metamorphic and diffusional studies indicate that heating rates are much faster and the duration of peak conditions much shorter in natural collisional systems than those predicted by numerical continuum deformation models. Because heat transfer processes such as conduction usually operate at much slower time scales than rates of other tectonic processes, thermal evolution is often transient and thus can be strongly influenced by tectonic disturbances (faults, magmatic intrustions, etc.) that occur at rates much faster than thermal relaxation. Here, we use coupled thermal-mechanical finite element models of thrust belts to explore how fault kinematics (geometry, slip rate, and spacing between major thrusts) fundamentally influence the thermal evolution of individual footwall and hanging wall thrust slices. We further examine the implications that these models may have for the non-steady interplay between rheological behavior and strain localization as a function of a transiently evolving thermal structure.