The stacking of thrust slices in collision zones and its thermal consequences

Abstract

The thermal consequences of stacking‐up thrust slices in collision zones are investigated using simple one dimensional thermal models. The metamorphic evolution (geotherms and (P,T, time) paths) of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects: cooling and pressure decrease associated with erosion, cooling by the lowest units (screen effect), heating by the upper units (cover effect). It is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone. The metamorphic evolutions are very sensitive to the following tectonic parameters: the number of thrusted units involved in thickening, the time delay between each thrust and finally the mode of stacking of the different units (over and understacking). It appears that, for a given depth of burial, the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units. The screen effect during understacking is more efficient for a short time delay (∼ 10 Ma) between each thrust.Overstacking leads to higher temperatures before uplift when compared to understacking. It is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small (∼ 10 km). It is shown that, for an erosion‐controlled uplift, the shape of the (P,T,t) path depends on the position of the rocks within a given unit and on the unit position within the pile. Finally, the general metamorphic evolutions during crustal thickening (HP‐LT metamorphism and subsequent overprint) are discussed in terms of the previously mentioned parameters. In the case of the Western Alps, it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events.