Abstract
Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.
Highlights
Switches in subduction polarity—either stationary or migrating along convergent boundaries—exist in both subduction orogens (Taiwan, e.g., Tsai et al 1977; Ustaszewski et al 2012) and collisional mountain belts (Pamir-Hindukush, e.g., Burtman and Molnar 1993; Sippl et al 2013), including the circum-Mediterranean Alpine mountain belt (e.g., Faccenna et al 2004)
The trend of this rotation axis is parallel to the Periadriatic Fault which forms the southern boundary of the Eastern Alps (Fig. B3); (2) backrotation of the slab edge about a vertical axis at Ivrea corresponding to the aforementioned Miocene-to-Recent rotational pole for the counterclockwise rotation of the Adria (Fig. B4)
We link the Miocene thrust front in the Southern Alps to a northern continuation of the dextrally transpressive Split-Karlovac Fault (Chorowicz 1970, 1975) which we propose transferred most of the Miocene shortening in the Southern and Eastern Alps to the Miocene orogenic front in the southern External Dinarides (Fig. 10)
Summary
Switches in subduction polarity—either stationary or migrating along convergent boundaries—exist in both subduction orogens (Taiwan, e.g., Tsai et al 1977; Ustaszewski et al 2012) and collisional mountain belts (Pamir-Hindukush, e.g., Burtman and Molnar 1993; Sippl et al 2013), including the circum-Mediterranean Alpine mountain belt (e.g., Faccenna et al 2004). The Alps themselves, long considered an archetypal orogen for cylindrism and uniform-sense subduction (Argand 1924), are characterized by two alongstrike reversals in subduction polarity: one at their junction with the Apennines (Laubscher 1988; Molli 2008; Vignaroli et al 2008) and another at their transition to the Dinarides (Laubscher 1971; Lippitsch et al 2003; Kissling et al 2006; Ustaszewski et al 2008) These reversals coincide with striking differences in deformational style along the boundaries of the Adriatic Plate (here termed Adria) that formed during its Cenozoic convergence with Europe (Royden and Burchfiel 1989; Handy et al 2010): In the Alps, the northern margin of Adria, together with the Adriaderived Austroalpine nappe pile, formed the upper plate to the Paleogene subduction of Alpine Tethys. Mitterbauer et al (2011) proposed that the +Vp anomaly beneath the Eastern Alps is vertical to steeply northeast-dipping and represents European lithosphere that originally subducted to the south, but was subsequently steepened and overturned
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