Improved Resolution of Kinematic and Metamorphic Paths in (Ultra)High-Pressure Rocks Using Multiple Porphyroblast Growth Phases

A comparative investigation is proposed of oriented eclogite and blueschist facies samples from the Eastern Alps (Tauern, Koraple, Saualpe), Sifnos (Cyclades), Western Alps (Dora Maira) and Dabie Shan (China), representing rocks subducted to depths between 50 and 150 km, and subsequently exhumed partially during the same orogenic cycle. The project intends to provide now insights in crustal strain geometries associated with the deep burial of these rocks, and their subsequent exhumation within relatively short geological time periods, Current burial models envisage different combinations of two competing processes, namely, underthrusting of relatively rigid crustal units, versus more distributed crustal thickening (folding) implying more viscous crustal behaviour. Proposed exhumation mechanisms include extensional collapse, high-angle tectonic extrusion, buoyancy driven uplift, hydrodynamic return flow. The project will attempt to provide constraints on these models through an original approach of 3D micro-structural analysis of relictual tectonic foliations in (mainly garnet) porphyroblasts of the samples. During the past decade, quantitative data have systematically demonstrated strong regional consistencies in the orientation of internal porphyroblast fabrics in different areas, including in Variscan HP units previously studied by the applicant. The orientation data were fruitfully applied to the unraveling of superposed crustal strain geometries. We would now like to apply several complementary techniques of 3D micro-structural analysis to determine spatial preferred orientations of relative rnatrix-porphyroblast rotation axis, and internal foliations included by successive porphyroblast growth phases in our (ultra)high-pressure rocks samples. Regional consistencies in the data, as we expect will be disclosed considering the above mentioned earlier work, would provide a completely new avenue for correlating porphyroblastic phases, between isolated locations, and establishing their relative timing from consistent orientation changes between core (old) and rim (younger) growth zones of individual porphyroblasts. Combined with detailed microprobe work on the composition of inclusions and of porphyroblast zoning, such correlation will allow integration of metamorphic and deformation histories at a much finer resolution. as previously possible, and new levels of precision in. Deciphering motion of rock through the earth`s crust. This should generate new insights into tectonic processes during orogenesis.