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

Girault, Jean-Baptiste; Bellahsen, Nicolas; Boutoux, Alexandre; Rosenberg, Claudio; Nanni, Ugo; Verlaguet, Anne; Beyssac, Olivier

doi:10.1029/2018tc005334

Cited by 21 publications

(21 citation statements)

References 73 publications

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“…Similar results were obtained in Europe through the western external Alps (e.g., Bellahsen et al, 2014), although with different kinematics, where Oligocene times are characterized by distributed shortening before the deformation localized along the frontal crustal ramps (Boutoux et al, 2016;Girault et al, 2020). The main difference with the Pyrenees is that, in the western external Alps, the shortening is accommodated by west-verging shear zones.…”

Section: Insights Into the Early Growth Of Orogenic Wedgessupporting

confidence: 77%

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

et al. 2021

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Bellahsen et al., 2019). Moreover, rifting postdated the Variscan orogeny which strongly deformed rocks currently outcropping in the AZ. Alpine shortening started in the NPZ with the closure of the Pyrenean rift and subsequently propagated to the AZ and the external zones. In the Meso-Cenozoic basins of the South Pyrenean Zone (SPZ; Figures 1a and 1b), the timing and amount of shortening of each major structure are well defined and thus provide constraints for the AZ structural evolution: the classical interpretation of the ECORS (Etude Continentale et Océanique par Réflexion et Réfraction Sismique) deep reflection profile suggests the development of an antiformal stack in the AZ (e.g.

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Section: Insights Into the Early Growth Of Orogenic Wedgessupporting

confidence: 77%

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

et al. 2021

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“…Girault et al. ( 2020 ) found significant peak metamorphic and retrograde deformation in the Lower Helvetic Doldenhorn (western Aar Massif) and Morcles (eastern Mont Blanc - Aiguilles Rouges Massifs) nappes. In the Mont Blanc and Aiguilles Rouges Massifs, massif-internal deformation appears to mostly coincide with the thermal peak, which persisted for 5–10 Myrs (Boutoux et al.…”

Section: Rscm Results and Interpretationmentioning

confidence: 99%

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

et al. 2021

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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.

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“…In contrast, peak‐ T contours folded in shallower parts of an orogen are typically highly oblique to the lithological and nappe boundaries (e.g. Girault et al., 2020; Wiederkehr et al., 2011).…”

Section: Discussionmentioning

confidence: 99%

“…Bousquet et al., 2008; Frey et al., 1999). For instance, the pattern of peak‐metamorphic temperatures in the Helvetic nappes was passively folded when the Aiguilles Rouges and Aar massifs were uplifted along basement thrusts (Girault et al., 2020).…”

Section: Introductionmentioning

confidence: 99%

Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

Groß

Pleuger

Handy

et al. 2020

Journal Metamorphic Geology

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We investigate the evolution of the three‐dimensional thermal structure of a palaeo‐subduction channel exposed in the Penninic units of the central Tauern Window (Eastern Alps). Structural and petrological observations reveal a sheath fold with an amplitude of some 20 km that formed under high‐P conditions (~2 GPa). The fold is a composite structure that isoclinally folded the thrust of an ophiolitic nappe derived from Alpine Tethys Ocean onto a unit of the distal European continental margin, also affected by the high‐P conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of the T‐dominated Barrovian metamorphism that itself overprints the high‐P metamorphism partly preserved in the sheath fold. Using Raman spectroscopy on carbonaceous material (RSCM), we are able to distinguish peak‐temperature domains related to the original subduction metamorphism from domains associated with the later temperature‐dominated (Barrovian) metamorphism. The distribution of RSCM temperatures in the Barrovian domain indicates a lateral and vertical decrease of peak temperature with increasing distance from the centres of the thermal domes. This represents a downward increase of palaeo‐temperature, in line with previous studies. However, we observe the opposite palaeo‐temperature trend in the lower limb of the sheath fold, namely an upward increase. We interpret this inverted palaeo‐temperature domain as the relic of a subduction‐related temperature field. Towards the central part of the sheath fold's upper limb, RSCM temperatures increase to a maximum of ~520°C. Further upsection in the hangingwall of the sheath fold, palaeo‐peak temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Peak‐temperature contours of the subduction‐related metamorphism are oriented roughly parallel to the folded nappe contacts and lithological layering. The contours close towards the northern, western and eastern parts of the fold, resulting in an eye‐shaped, concentric pattern in cross‐section. The temperature contour geometry therefore mimics the fold geometry itself, indicating that these contours were also folded in a sheath‐like manner. We propose that this sheath‐like pattern is the result of a two‐stage process that reflects a change of the mode of nappe formation in the subduction zone from thrusting to fold nappe formation. First, thrusting of a hot oceanic nappe onto a colder continental nappe created an inverted peak‐thermal gradient. Second, sheath folding of this composite nappe structure together with the previously established peak‐temperature pattern during exhumation. This pattern was preserved because temperatures decreased during retrograde exhumation metamorphism and remained less than the subduction‐related peak temperatures during the later Barrovian overprint. The fold ascended with diapir‐like kinematics in the subduction channel.

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

Cited by 21 publications

References 73 publications

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

Central Pyrenees Mountain Building: Constraints From New LT Thermochronological Data From the Axial Zone

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

Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

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