The 3‐D Thermal Structure of the Helvetic Nappes of the European Alps: Implications for Collisional Processes

Understanding the rheology of orogenic wedges requires the knowledge of the structural and thermal evolution of collisional units. In this study, we document the maximum temperature reached by the sedimentary cover nappes of the External Crystalline Massif (Western and central Alps) by Raman spectroscopy of carbonaceous material, between the Belledonne (France) and the Aar (Switzerland) Massifs. These cover units form the Helvetic/Dauphinois nappe complex. Maximum temperatures reached by the Upper Helvetic nappes lie in a range spanning from below 220 and 350 °C ± 50 °C. For the Lower Helvetic nappes, the temperatures spread between 226 and 358 °C ± 50 °C. These temperatures were projected on two structural cross sections in order to constrain the 3‐D thermal structure. From these data, we propose that the Helvetic nappes were deformed and emplaced before and/or during the thermal peak, which supports recent findings that shortening in the External Crystalline Massif was mainly accommodated during a 5‐ to 10‐Myr‐long thermal peak before deformation localized along crustal thrusts, which exhumed and cooled down the wedge. During this late exhumation, the isotherms corresponding to the thermal peak were passively folded. Key Points Maximum temperatures attained in the Helvetic nappes increase from their core to their front, from 220 to 358 °C The 3‐D thermal structure is constrained by combining the maximum temperature distribution in map view and in cross sections Helvetic nappes were deformed and emplaced before and/or during the thermal peak of Alpine collision