April 27th 2021 Geophysical signature of the alpine slab: Field analogues and direct models. 1.ISTeP, Sorbonne University, Paris. 2.LG TPE, Jean Monnet University, Saint Etienne 1 1 2 3 4 4 4 Sonnet M ., Labrousse L . ,Bascou J ., Plunder A ., Nouibat A ., Paul A ., Stehly L . 3.BRGM, Orléans. 4.ISTerre, Grenoble-Alpes University. 1 • The alpine dipping panel is in • Bulk rock chemistries & thermo- 4 continuity with the European dynamic modeling (Holland & Powell • The alpine lower crust I. Validation of the method out of the mountain range Lower Crust (LC) (Zhao et al. 1998). forms the top of the 2015). • P-T velocity diagrams from seismic lithospheric slab. • Interpretation of tomography properties modeling (Abers & Hacker • Chemical composition of Zoom 1: i m a g e s l i m i t e d b y o u r 2016). rocks is constant except Mantle-type lithologies knowledge of rocks seismic Testing the impact of different • for water (OH). fit into wraps of the Why? properties. How? processes on the effective velocities of • T h e E u r o p e a n a n d • Direct method: predict the rocks along the slab. Adriatic lithospheres are Adriatic and European possible seismic properties • Comparison with effective seismic thermally equilibrated mantle. o f t h e L C f r o m fi e l d velocities along the Cifalps profile Assuming that: away from the Alps. It would thus seem that analogues. (Nouibat et al. in prep). they are close to a 2 stable continental Tested lithologies have been sampled geotherm. in the External Crystalline Massifs. Other relevant lithologies are also Modified from Agard, 2021 borrowed from Boundy et al. 1992 (Bergen), Galli et al. 2011 (Gruf), and Martin et al. 2011 (Corsica) (see part 4). II. One first parameter tested : the OH content of the A: paragenesis Zoom 2: A single chemistry does not explain the AB increase in Vs drawn b y t h e f o r e l a n d Sample from felsic group Sample from envelope (European plate). B: For this, in a crustal model consisting of felsic and mafic rocks intercalations, the share of mafic ones must increase with Zoom 3: C depth. Rocks may have undergone hydration or dehydration during strain localization and metamorphism during burial and since. Other parameter such as fabric orientation, layering and 3 metamorphic disequilibrium are also in the scope of the present study parameters. Sample from intermediate group Sample from References: • Abers, G.A., Hacker, B.R., 2016. C: 5 Geochem. Geophys. Geosyst. 17, 616– • The European LC must involve with depth from felsic to mafic lithologies with 624.https://doi.org/ Gruf charnockite, Corsica kinzigite and felsic group being the best analogues 10.1002/2015GC006171 • Agard 2021, Earth-Science Reviews for the middle crust and intercalated mafic restites for the lowermost levels. 214, 103517. https://doi.org/10.1016/ The alpin slab shows velocities below the ones predicted for fully reacted j.earscirev.2021.103517. • • B o u n d y, T. M . , e t a l . 1 9 9 2 , J . analogues lithologies, hence processus lowering the apparente velocity must metamorphic Geol. 10, 127-146. act at depth : • Galli A., et al. 2011. Lithos 124, 17-45. doi:10.1016/j.lithos.2010.08.003. Reaction overstepping and delay. • Martin A.J, L., 2011. Lithos 125, 620-640. doi:10.1016/ Discussion Metamorphic texture and layering. j.lithos.2011.03.015. • N o u i b a t e t a l . , s u b m i t t e d t o Post subduction thermal re-equilibration and subsequence re-equilibration. Geophysical Journal International. • Zhao, L., 2015. Geology 43, 815–818. From Nouibat et al., submitted Presence of free fluids not taken into account in our calculations. Sample from mafic group Sample from https://doi.org/10.1130/G36833.1.