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Séminaires "Calédonides scandinaves"

Lundi 27 janvier, 10H-12H, Salle - 107M

Understand the subduction history of the Baltoscandian margin in the Scandinavian Caledonides, deciphering P-T-t-D evolution of (U) HP deeply subducted rocks

Pauline JEANNERET, Laboratoire Chrono-environnement, Université de Bourgogne-Franche-Comté, France

The recent discovery of evidence for ultra-high pressure (UHP) metamorphism in the Seve Nappe Complex (SNC) in the Swedish Caledonides is providing the basis for new investigations of the Caledonian subduction history of the Baltoscandian margin of continent Baltica. The Caledonian bedrock in the mountains of western Scandinavia provides a comprehensive geological cross-section of a Himalayan-type orogen that formed as a result of the collision of the continents Laurentia and Baltica in the Mid-Paleozoic. Many of the mountain building processes involved still remain enigmatic, including Ordovician subduction-collision events which can be traced in the allochthonous units thrust onto the Baltoscandian margin ; the mechanisms governing exhumation of these allochthons also are obscured. Of particular interest for this project is the Middle Allochthon (MA), which represents the Baltoscandian rifted margin and continent-ocean transition zone. The SNC is the highest grade unit of the MA and bears evidence of deep subduction of the continental crust already during the Ordovician. Thus, it serves as a key for understanding Iapetus closure and formation of the Caledonides. Notably, these traces of high and ultra-high pressure (HP-UHP) metamorphism within the SNC are chronologically much earlier (at least c. 30Myr) than the well-known Western Gneiss Region of southwestern Norway, an archetypical UHP area within this mountain belt. However, the WGR, mainly represented by parautochthonous Baltoscandian basement, was formed during the final stages of the collisional event between Baltica and Laurentia in the Early Devonian and provides little evidence of the earlier subduction history. Therefore, there is a need for detailed study of the MA (especially the SNC) to explore the Ordovician-Silurian subduction history and derive a comprehensive model of the Caledonian orogeny in Scandinavia and the North Atlantic region, in general.

Exhumation of the high-pressure Tsäkkok Lens, Swedish Caledonides resolved by structural and in-situ 40Ar/39Ar geochronological investigations

Christopher. J. Barnes, Department of Earth Sciences, Uppsala University, Uppsala, Sweden

The tectonostratigraphy of the Caledonian Orogeny preserves a record of Iapetus Ocean closure in the middle Cambrian-early Silurian, culminating in continental collision between Baltica and Laurentia in early Silurian-Early Devonian time. Evidence of late Cambrian-Early Ordovician subduction to high pressure conditions is preserved in the Tsäkkok Lens of the Seve Nappe Complex in northern Sweden. A structural investigation combined with in-situ white mica 40Ar/39Ar geochronology to resolve the exhumation history of the Tsäkkok Lens from eclogite-facies conditions. Three deformation events are defined for the metasedimentary rocks that host eclogites. D1 is likely related to the prograde to peak-metamorphic stages, represented by a locally preserved S1 foliation. D2 resulted in vertical shortening and is defined by a pervasive S2 foliation and cm-/m-scale F2 folds. D2 deformation terminated with Scandian thrusting due to continental collision. D3 records NE-SW shortening and constitutes m-/km-scale F3 open folds that folded the Tsäkkok Lens after Scandian thrusting. For the metasedimentary rocks, 40Ar/39Ar geochronology collectively targeted S1, F2 and S2 structures. Results from S1 and coaxial D2 structures provide yield a record of post-decompression cooling at 477 ± 3 Ma. During the later stages of D2, F2 folding and non-coaxial deformation overprinted the pre-existing structures and partially reset the 40Ar/39Ar cooling record. The youngest dates coincide with the timing of Scandian thrusting i.e as young as 428 ± 8 Ma. Altogether, the Tsäkkok Lens records fast exhumation from eclogite-facies conditions, followed by protracted residence likely at mid-crustal depths until the Scandian thrusting phase. This supports an exhumation model governed by a reduction in buoyancy throughout exhumation as the Tsäkkok Lens exhumed from mantle to crustal conditions.

Brittle failure of eclogites at high-pressures as a trigger of metamorphic fluids discharge in a co-seismic regime

Michał Bukała, AGH-University of Science and Technology, Kraków, Poland

Intermediate-depth earthquakes within subduction zones have been correlated with regions where metamorphic dehydration reactions are expected to occur. Of particular importance is the blueschist to eclogite transition. However, due to the scarcity of recognized exhumed seismometamorphic rocks, the relationships between dehydration reactions and intermediate-depth earthquakes remains an open question.
The Lower Ordovician eclogites from the Tsäkkok Lens (Scandinavian Caledonides) preserve petrological and microstructural features suggesting a seismometamorphic evolution. The Tsäkkok tectonic lens consists of sediments and pillow basalts metamorphosed at eclogite-facies conditions. This volcano-sedimentary succession represents distal part of the Baltica passive margin, which was buried at a cold subduction regime and subsequently emplaced onto the Baltica platform at the late stage of the Caledonian orogeny The brittle structures of eclogites comprise single-grain micro-fractures, which likely formed as a result of pore-fluid overpressure on the crystal-scale as a result of fluid production from dehydration of lawsonite and glaucophane. In contrast, the meso-fractures indicate variations of differential stress on the decameter scale.
These metamorphic dehydration reactions were directly responsible for brittle fracturing of the eclogites in HP conditions via increase in pore-fluid pressure on the micro-scale, and associated dehydration-driven stress transfer on at least the meso-scale. The fracturing on both scales, and perhaps pseudotachylyte production, constitute the oldest documented natural example of the effect of lawsonite and glaucophane dehydration reactions associated with HP brittle failure during eclogitization. Brittle-failure in turn allowed the expulsion of the metamorphic fluids from the eclogite in HP conditions up to 21.6 ± 3.2 kbar and 590 ± 60ºC.

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