Turning the Orogenic Switch: Slab‐Reversal in the Eastern Alps Recorded by Low‐Temperature Thermochronology

• Published in: Geophysical research letters Vol. 48; no. 6
• Main Authors: Eizenhöfer, Paul R., Glotzbach, Christoph, Büttner, Lukas, Kley, Jonas, Ehlers, Todd A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.03.2021
• Subjects: Bridges; continent‐continent collision; Dipping; doubly vergent orogen; Geological time; Lithosphere; Miocene; Mountains; Orogeny; Plate tectonics; Plates (tectonics); Polarity; Rock; Rocks; Seismic activity; Seismicity; slab reversal; Subduction; Subduction (geology); thermochronology; Tomography; TRANSALP; Alps; Southern Alps
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL092121

Many convergent orogens, such as the eastern European Alps, display an asymmetric doubly vergent wedge geometry. In doubly vergent orogens, deepest exhumation occurs above the retro‐wedge. Deep‐seismic interpretations depict the European plate dipping beneath the Adriatic, suggesting the pro‐wedge location on the north side of the orogen. Our new thermochronometer data across the Eastern Alps confirm distinct shifts in the locus of exhumation associated with orogen‐scale structural reorganizations. Most importantly, we find a general Mid‐Miocene shift in exhumation (in the Tauern Window and the Southern Alps) and focus of modern seismicity across the Southern Alps. Taken together, these observations suggest a subduction polarity reversal at least since the Mid‐Miocene such that the present‐day pro‐wedge is located on the south side of the Alps. We propose a transient tectonic state of a slow‐and‐ongoing slab reversal coeval with motion along the Tauern Ramp, consistent with a present‐day northward migration of drainage divides. Plain Language Summary When tectonic plates collide, they bend downwards and form two lithospheric wedges dipping in opposite directions, such as in the Eastern Alps. We present new crustal cooling data along a transect in the Eastern Alps confirming that surface rocks across the central Tauern Window originated from the deepest structural levels along the transect. South of the Tauern Window rocks were exhumed from higher depths compared to those north of it and were exhumed more recently, while seismic activity is also focused across the Southern Alps. These observations suggest a subduction polarity reversal because they are inconsistent with the original southern and northern locations of overriding and subducting plates, respectively, >15 million years ago. This interpretation is contrary to lithosphere‐scale tomography that shows no change in subduction polarity. Therefore, we propose a transient tectonic state, that is, a slow‐and‐ongoing subduction polarity reversal that initiated when Tauern Window rocks began their steep ascent to the surface along a deep‐seated fault known as the Tauern Ramp. This study bridges observations in the mantle, crust and on the surface over geologic time. Key Points Thermochronologic data in the Eastern Alps is consistent with a transient tectonic state toward complete slab reversal The pro‐wedge has switched from north to south of the Periadriatic Fault along TRANSALP Mid‐Miocene motion along the Tauern Ramp is the consequence of slab‐reversal