Eo-Oligocene normal faulting in the Alps: Orogen-perpendicular subduction channel reactivation or orogen-parallel collapse following slab breakoff?
Mineralogy, Petrology and Geochemistry
Final Report Abstract
In subduction zones one plates dives beneath another plate. Commonly, oceanic crust subducts beneath another plate. Once oceanic crust situated between two continents is consumed, the continents collides. The subducting oceanic crust pulls the attached continental plate down towards the earth mantle. The low density of the continental crust opposes the downward drag. The two opposing forces commonly result in the break off of the oceanic slab, termed slab breakoff. From these findings, the hypothesis was formulated, that before a slab breakoff, the continental crust is dragged a certain distance into the mantle along the subduction channel – a reverse fault – but slides back after the break off reactivating the subduction channel as a normal fault. This research project aimed to test this hypothesis in pre-Miocene structures of the Alps. Previous studies showed the presence of presence of extensional structures at or near the former plate interface, however especially their timing has not been clearly determined. Therefore no temporal relation with the slab breakoff of the former Penninic ocean at ~35 Ma could be established and therefore processes not related. To investigate temporal relations, the project aimed to use three rarely used and innovative geochronologic methods to directly date deformation structures: U-Pb dating of carbonate mineralizaions, 40Ar/39Ar dating of fluid inclusions in quartz mineralization and 40Ar/39Ar in situ dating of K-bearing minerals. Unfortunately, it has to be concluded that none of the three methods lead to reliable geological dates. To diversify the sample set and potentially gain methodological insights, the sample set was expanded with samples from northern Helvetic units, the Glarus thrust, faults of the lower Rhine Graben and the foreland of the Rhodopes. All these analyses were unsuccessful in defining meaningful geological ages (76 samples U-Pb carbonate; 15 samples 40Ar/39Ar fluid inclusions in quartz). Especially problematic for future applications of the methods is that not even hints could be identified pointing to more or less promising samples and structures to date for future applications. In all cases the entire, time consuming analytics had to be worked through before retrieving no results. Dating of deformation structures, especially those formed at low temperatures remains a challenge.
Publications
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Boutique neutrons advance 40 Ar/ 39 Ar geochronology. Science Advances, 5(9).
Rutte, Daniel; Renne, Paul R.; Morrell, Jonathan; Qi, Liqiang; Ayllon, Mauricio; van Bibber, Karl; Wilson, Jonathan; Becker, Tim A.; Batchelder, Jon; Bernstein, Lee A.; Lebois, Mathieu; James, Jay; Chong, Su-Ann; Heriot, Will L.; Wallace, Max; Marcial, Angel; Johnson, Charles; Woolley, Graham & Adams, Parker A.
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An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA). Tectonics, 39(10).
Rutte, Daniel; Garber, Joshua; Kylander‐Clark, Andrew & Renne, Paul R.
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The Alichur Dome, South Pamir, Western India–Asia Collisional Zone: Detailing the Neogene Shakhdara–Alichur Syn‐collisional Gneiss‐Dome Complex and Connection to Lithospheric Processes. Tectonics, 39(1).
Worthington, James R.; Ratschbacher, Lothar; Stübner, Konstanze; Khan, Jahanzeb; Malz, Nicole; Schneider, Susanne; Kapp, Paul; Chapman, James B.; Stevens, Goddard Andrea; Brooks, Hanna L.; Lamadrid, Hector M.; Steele‐MacInnis, Matthew; Rutte, Daniel; Jonckheere, Raymond; Pfänder, Jörg; Hacker, Bradley R.; Oimahmadov, Ilhomjon & Gadoev, Mustafo
