Thermal regime of fold and thrust belts—an application to the Bolivian sub Andean zone

Abstract

A quantitative analysis of the various parameters influencing the thermal regime in orogenic belts and related foredeeps shows that (i) the increasing heat flow in internal zones is mainly due to the thickening of radiogenic layers, although there is no simple proportionality between crustal thickness and heat flow signal at large scale; (ii) in external zones, where the horizontal strain rate is large (such as in the Bolivian Andes), surface processes can be of first order within the first kilometers of the crust. Hence, they induce a large scatter in the thermal data which are acquired at shallow depths. The deep thermal regime can be restored only by a quantitative assessment of these parameters. Active erosion (respectively sedimentation) can increase (resp. reduce) the heat flow by a factor of 2 in the uppermost kilometers. The effects of fluid circulation percolating at depth can also generate significant local disturbances. Other processes such as heat advection during thrusting, surface morphology and climate change have a minor influence in most settings, compared to the aforesaid effects.In the Bolivian Sub Andean Zone, between 18°S and 22°S, the very active deformation enhances the surface thermal perturbations (particularly erosion and sedimentation) and disturb the thermal field. The analysis of these data accounting for the kinematics of the belt allows the lateral variations of the thermal regime at various scales to be assessed. A slight eastward increase in the thermal regime towards the Chaco plain is evidenced as well as towards the Boomerang area, as the Mesozoic and Cenozoic sedimentary cover gets thinner.