Hot Versus Cold Orogenic Behavior: Comparing the Aracuai-West Congo and the Caledonian Orogens

摘要

Observations and modeling show that temperature controls crustal rheology and therefore also the orogenic evolution of continent-continent collision zones and the associated tectonic style. In order to explore the effect of temperature in a natural environment, we compare eroded sections through the unusually cold lower Paleozoic North Atlantic Caledonian (Scandian) collision zone and the very hot Brasiliano/Pan-African Aracuai-West Congo orogen. A cold and stiff subducting Caledonian continental margin was able to subduct as a rather coherent unit to ultrahigh-pressure conditions, twice as deep as the Pan-African/Brasiliano crust that was quickly heated and softened and got involved in partial melting. Furthermore, the Caledonian collision developed large coherent thrust sheets that were transported hundreds of kilometers toward the foreland. This was never achieved in the hot Aracuai-West Congo orogen, where much of the tectonic stress was absorbed by the partially molten central part of the orogen through magmatic state deformation. Major mylonite zones (thrusts) such as those seen in the Caledonides are therefore less common in the Aracuai-West Congo orogen. Further, the deep continental subduction in the Caledonides developed a strongly asymmetric collision zone, with rapid variations in pressure and temperature. In contrast, the Aracuai-West Congo orogen soon developed into a more symmetric geometry due to its easily flowing hot crust, with a relatively flat base and a corresponding plateau in its upper part. Deformation of the cold Caledonian crust was controlled by plate-tectonic stress, while gravitational forces more strongly influenced the hot Brasiliano/Pan-African example.