Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

Area of Science:

• Structural Geology
• Tectonics
• Metamorphic Petrology

Background:

• The eastern Aar Massif in the Swiss Alps provides a key window into the complex thermo-kinematic evolution of mountain belts.
• Understanding the interplay between deformation, metamorphism, and exhumation is crucial for deciphering the Alpine orogeny.

Purpose of the Study:

• To investigate the thermo-kinematic evolution of the eastern Aar Massif.
• To establish the timing of metamorphism and deformation.
• To track long-term orogenic movements and assess the influence of temperature and structural inheritance on kinematic evolution.

Main Methods:

• Peak temperature data estimation using Raman spectroscopy of carbonaceous material.
• Detailed field analyses, including new shear zone mapping and structural cross sections.
• Step-wise retrodeformation and compilation of temperature-time constraints.

Main Results:

• Basement-involved thrusting initiated before or during peak metamorphism ($$).
• Subsequent deformation (22–14 Ma) was dominated by steep, NNW-vergent reverse faults, leading to significant structural relief and vertical displacements (7 km shortening, 11 km exhumation).
• Post-14 Ma exhumation occurred "en bloc" with a transition to strike-slip dominated deformation, resulting in 13 km shortening and 9 km exhumation.

Conclusions:

• Evolving temperature conditions controlled both the timing of basement-involved deformation and the structural style (shear zone dip).
• Decreasing rheological contrasts between basement and cover units at higher temperatures influenced deformation.
• Inherited normal faults were not significantly reactivated; new thrusts developed in the basement below syn-rift basins.