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} 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.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} 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.
Highlights
Collisional mountain belts form in response to convergent movements between tectonic plates and result from the closure and subduction of oceanic domains, followed by continent-continent collision
We examine the Alpine (∼ 34 Ma to present) thermo-kinematic evolution of the eastern Aar Massif by combining detailed field analyses with the quantitative assessment of peak temperature (T p ) based on Raman spectroscopy on carbonaceous material (RSCM)
6 Conclusions This study demonstrates that RSCM peak temperature data, coupled with detailed structural analyses and independent age constraints, can be used to quantify vertical and horizontal components of collisional deformation and to study the mechanical behavior of basement and cover units during the late-stage orogenic evolution
Summary
Collisional mountain belts form in response to convergent movements between tectonic plates and result from the closure and subduction of oceanic domains, followed by continent-continent collision Such mountain belts typically involve passive continental margins, including basins and normal faults inherited from preorogenic extension (e.g., Beaumont et al 2000; Bellahsen et al 2012; Butler et al 2006; Jackson 1980; Lacombe and Mouthereau 2002; Lafosse et al 2016; Lemoine et al 1989; Manatschal 2004; Marshak et al 2000; Mohn et al 2012). Mouthereau et al (2013) demonstrated that the strength of the lithosphere and the distribution of strain depends to a first order on the thermotectonic age (i.e., the time elapsed since the last pre-orogenic rifting event) Another significant factor is the inherited extensional passive margin structure (e.g., Lafosse et al 2016). This has important implications for the estimation of shortening vs. uplift ratios during compressional deformation
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