Timing of exhumation of the eastern Central Alps from zircon and apatite (U-Th)/He thermochronology (Graubünden, Switzerland)

Abstract

The structurally complex eastern Central Alps, located in eastern Switzerland and adjacent areas, have helped constrain the timing and kinematics of deformation during the Alpine orogeny. Several regional and orogenic scale studies have established a well-defined structural evolution of the eastern Central Alps; however, the low-temperature cooling history of the region remains relatively unconstrained. An understanding of this cooling history may elucidate mechanisms for exhumation of the region and evaluate orogenic scale models for the timing and nature of exhumation within the mountain belt. This study presents new low-temperature zircon and apatite (U-Th)/He age data from the Austroalpine and Penninic nappes of the eastern Central Alps that reveals a three stage cooling history for the region. These data, combined with inverse time-temperature modeling, provide cooling rates for the three phase history. Slow cooling at rates of ~1-10 °C/m.y. through the zircon HePRZ (~200-140 °C) are recorded in the Austroalpine Silvretta, Campo-Grosina, and Err nappes during the Eocene from ~52 to 36 Ma. This period of cooling is related to exhumation resulting from north directed thrusting associated with continental collision. A second stage of rapid cooling at rates of 10-50 °C/m.y. from ~36 to 21 Ma, are recorded by zircon and apatite (U-Th)/He age data from the Bernina and Corvatsch Austroalpine nappes, and zircon (U-Th)/He age data from Penninic units exposed in the Engadine Window. This period of relatively rapid cooling and exhumation is likely a response to a combination of backthrusting along the Insubric Line and crustal duplexing at depth. A final period of cooling and exhumation is recorded only in the Silvretta Austroalpine nappe and Penninic units exposed in the Engadine Window at ~16 Ma. This period of cooling and exhumation is likely a result of uplift along the oblique left-lateral Engadine Line. The overall cooling history of the eastern Central Alps shows that the western exposure of the orogenic lid behaved as a rigid to semi-rigid block during continental collision and continued convergence until the Engadine Line affected the region in the Miocene. The three phase cooling history constrained by this low-temperature thermochronometric data contributes to the growing body of evidence for episodic cooling and apparent exhumation of the Alpine orogen.