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

Abstract

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From $$\text{ca.\;26\,Ma}$$ ca.\;26\,Ma onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults ($$\text{ca.}$$ ca. 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than $$50^{\circ}$$ 50 ∘ already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Orogen‐Parallel Migration of Exhumation in the Eastern Aar Massif Revealed by Low‐T Thermochronometry

“…The assumed geothermal gradient is comparable to the average transient geotherm calculated by the inverse model for that time, which is a requirement for the comparison of the two data sets in Figures 7b and 8b. Furthermore, the assumed geothermal gradient is in agreement with the spacing of peak temperature isograd surfaces in the study area (Nibourel et al., 2021) and with independent pressure temperature time constraints (Challandes et al., 2008; Goncalves et al., 2012; Rolland et al., 2009). Peak metamorphism in the Lower Helvetic domain is due to the burial in the footwall to the basal Helvetic thrust, representing the main active tectonic boundary at that time (Pfiffner, 2015).…”

“…Peak metamorphism in the Lower Helvetic domain is due to the burial in the footwall to the basal Helvetic thrust, representing the main active tectonic boundary at that time (Pfiffner, 2015). According to numerical models (Shi & Wang, 1987), temperature perturbations across shallow‐dipping active thrusts with slip rates no higher than 5 mm/yr, such as the basal Helvetic thrust, re‐equilibrate rapidly (Nibourel et al., 2021; Pfiffner, 2015) so that the assumption of a constant geothermal gradient of C/km appears justified for this time step, even if minor temperature perturbations cannot be excluded.…”

“…We correlate this early phase of rapid and differential exhumation with the exposed east‐northeast‐striking and steeply south‐southeast dipping to sub‐vertical reverse faults and with a pervasive greenschist facies foliation (Figure 10, “Reverse faults” on Figure 11, e.g., Herwegh et al., 2017; Rolland et al., 2009; Wehrens et al., 2017). This early phase produced (a) the structural relief as seen in the internal parts of the central Aar Massif (“early differential exhumation” on Figure 8a), and (b) the N‐S metamorphic gradient (Figures 2 and 8a, Nibourel et al., 2018, 2021). In the Vättis area, our data (thermochronometric ages and peak temperatures) do not record significant exhumation during this period. 14–10 Ma: In the southern central Aar Massif, cross‐cutting relationships and isotopic ages suggest a transition to transpressional tectonics with a dominant strike‐slip component (Figure 10, “strike‐slip faults” on Figure 11).…”

“…This may be achieved (a) by displacement or dip variations along the strike of thrusts and reverse faults, (b) by laterally varying spacing of thrusts and reverse faults, or (c) by decoupling individual fault segments by transverse faults. There is some field evidence for large displacement variations, for example along the strike of the Windgällen-Färnigen zone (Figure 10, Nibourel et al, 2018Nibourel et al, , 2021 and the Frisal line (Figure 10, Pfiffner, 1978). Both reverse faults appear to die out laterally, but in opposite directions (Nibourel et al, 2021).…”

“…There is some field evidence for large displacement variations, for example along the strike of the Windgällen-Färnigen zone (Figure 10, Nibourel et al, 2018Nibourel et al, , 2021 and the Frisal line (Figure 10, Pfiffner, 1978). Both reverse faults appear to die out laterally, but in opposite directions (Nibourel et al, 2021). An overall change in the spacing of faults from the central to the eastern Aar Massif is not observed (Baumberger, 2015;Nibourel et al, 2021).…”