Reconstructing the Alps–Carpathians–Dinarides As a Key to Understanding Switches in Subduction Polarity, Slab Gaps and Surface Motion

Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-014-1060-3 REVIEW ARTICLE Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion Mark R. Handy · Kamil Ustaszewski · Eduard Kissling Received: 16 September 2013 / Accepted: 13 July 2014 © The Author(s) 2014. This article is published with open access at Springerlink.com Abstract Palinspastic map reconstructions and plate part of the Alpine orogen and induced widespread crust– motion studies reveal that switches in subduction polarity and mantle decoupling, thus facilitating Pannonian extension and the opening of slab gaps beneath the Alps and Dinarides were roll-back subduction of the Carpathian oceanic embayment. triggered by slab tearing and involved widespread intracrustal The second slab-tearing event triggered uplift and peneplaini- and crust–mantle decoupling during Adria–Europe collision. zation in the Eastern Alps while opening a second slab gap, In particular, the switch from south-directed European sub- still present between the Eastern and Central Alps, that was duction to north-directed “wrong-way” Adriatic subduction partly filled by northward counterclockwise subduction of beneath the Eastern Alps was preconditioned by two slab- previously unsubducted Adriatic continental lithosphere. In tearing events that were continuous in Cenozoic time: (1) late Miocene time, Adriatic subduction thus jumped westward Eocene to early Oligocene rupturing of the oppositely dipping from the Dinarides into the heart of the Alpine orogen, where European and Adriatic slabs; these ruptures nucleated along a northward indentation and wedging of Adriatic crust led to trench–trench transfer fault connecting the Alps and Dinar- rapid exhumation and orogen-parallel escape of decoupled ides; (2) Oligocene to Miocene steepening and tearing of the Eastern Alpine crust toward the Pannonian Basin. The plate remaining European slab under the Eastern Alps and western reconstructions presented here suggest that Miocene subduc- Carpathians, while subduction of European lithosphere con- tion and indentation of Adriatic lithosphere in the Eastern tinued beneath the Western and Central Alps. Following the Alps were driven primarily by the northward push of the Afri- first event, post-late Eocene NW motion of the Adriatic Plate can Plate and possibly enhanced by neutral buoyancy of the with respect to Europe opened a gap along the Alps–Dinar- slab itself, which included dense lower crust of the Adriatic ides transfer fault which was filled with upwelling astheno- continental margin. sphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This Keywords Alps · Carpathians · Dinarides · Adria · upwelling also weakened the upper plate of the easternmost Subduction polarity switch · Slab tearing · Slab gaps · Crust–mantle decoupling · Surface uplift Electronic supplementary material The online version of this article (doi:10.1007/s00531-014-1060-3) contains a link to supplementary material (Appendix, Parts A, B and C, including Introduction figures) which is available to authorized users. Switches in subduction polarity—either stationary or * M. R. Handy ( ) migrating along convergent boundaries—exist in both sub- Freie Universität Berlin, Berlin, Germany e-mail: [email protected] duction orogens (Taiwan, e.g., Tsai et al. 1977; Ustaszewski et al. 2012) and collisional mountain belts (Pamir-Hindu- K. Ustaszewski kush, e.g., Burtman and Molnar 1993; Sippl et al. 2013), Friedrich-Schiller Universität Jena, Jena, Germany including the circum-Mediterranean Alpine mountain belt E. Kissling (e.g., Faccenna et al. 2004). The central part of this highly Eidgenössische Technische Hochschule, Zürich, Switzerland arcuate belt (Fig. 1) comprises the Alps proper, a classical 1 3 Int J Earth Sci (Geol Rundsch) 5˚ 25˚ 10˚ 15˚ 20˚ 1993) and collision (Moretti and Royden 1988; Faccenna 50˚ 50˚ et al. 2004). Likewise in the Dinarides, Adria was the lower plate during the entire Cretaceous to Cenozoic orogenic evolution (Schmid et al. 2008). These opposing subduction polarities persist today; the rigid northern promontory of 45˚ 45˚ Adria (the so-called Adriatic indenter) is still rotating counterclockwise and indenting the Eastern Alps and Dinarides (e.g., Nocquet and Calais 2004; Vrabec and Fodor 2006 and references therein), while the southern part is subduct- ing beneath the advancing Hellenides (Grenerczy et al. 40˚ 40˚ 2005; Burchfiel et al. 2008; Bennett et al. 2008). Debate on changing subduction polarity in the Alps has been galvanized in the past decade by teleseismic tomographic images of two positive compressive-wave velocity 35˚ 35˚ ( V ) slab anomalies with contrasting orientations beneath + p 5˚ 25˚ the Alps (Fig. 4a–c): one beneath the Central and Western 10˚ 15˚ 20˚ Alps that dips to the southeast to a depth of about 200 km and is consistent with the classical view of south- to south- Fig. 1 Central segment of the Alpine–Mediterranean chain with orogenic fronts, basins and main localities mentioned in the text. The east-directed subduction of European lithosphere (e.g., thrust vergence reflects the polarity of subduction, including polarity Schmid et al. 1996), and another beneath the Eastern Alps switches at the Alps–Apennines and Alps–Dinarides junctions dis- that is oriented the wrong-way for European subduction; cussed in this paper instead, it dips northward to a depth of at least 210 km (Lip- pitsch et al. 2003) or more (Dando et al. 2011; Mitterbauer et al. 2011). Controversy surrounds the proposed northward collisional orogen which joins at either end with the Apen- dip of the latter anomaly, partly due to its incompatibility nines, Carpathians and Dinarides (Fig. 2). Despite along- with conventional notions of southward European subduc- strike changes in thrust vergence, these mountain belts tion beneath the Alps, and partly because of the ambiguity contain nearly continuous exposures of ophiolite and accre- of tomographic images in this region (Mitterbauer et al. tionary prisms that mark the remains of two oceanic basins, 2011). Nevertheless, the generally northward dip of the V + p the Triassic–Jurassic Neotethys (Ricou 1994) and Jurassic– Eastern Alps anomaly in Lippitsch et al. (2003) is regarded Early Cretaceous Alpine Tethys (e.g., Stampfli and Borel as a robust feature because their model is unique in incor- 2002; Schmid et al. 2008), which are overprinted by Late porating a high-resolution 3D model of crustal velocities Cretaceous-to-Cenozoic calc-alkaline magmatism (Fig. 3). specific to the Alps. An important feature of all tomographic The Alps themselves, long considered an archetypal models so far is a narrow but distinct gap between the two orogen for cylindrism and uniform-sense subduction V anomalies at the junction of the Western-Central and + p (Argand 1924), are actually characterized by two along- Eastern Alps as marked in Fig. 4a. We will return to this strike reversals in subduction polarity: one at their junc- feature below, but emphasize that the existence of this gap tion with the Apennines (Laubscher 1988; Molli 2008; is difficult to reconcile with the classical notion of a single, Vignaroli et al. 2008) and another at their transition to the continuous slab beneath the entire length of the Alps. Dinarides (Laubscher 1971; Lippitsch et al. 2003; Kissling Taken at face value, the tomographic model of Lippitsch et al. 2006; Ustaszewski et al. 2008). These reversals coin- et al. (2003) indicates a fragment of Adriatic lithosphere cide with striking differences in deformational style along entrained beneath the Eastern Alps (Schmid et al. 2004; the boundaries of the Adriatic Plate (here termed Adria) Horváth et al. 2006; Kissling et al. 2006). This is consist- that formed during its Cenozoic convergence with Europe ent with the southwestward vergence of thrusts and folds (Royden and Burchfiel 1989; Handy et al. 2010): In the in the Dinarides, as well as with images of a northward- Alps, the northern margin of Adria, together with the Adria- dipping slab along the still-active Hellenic arc-trench sys- derived Austroalpine nappe pile, formed the upper plate to tem (Fig. 4d, Bijwaard and Spakman 2000; Piromallo and the Paleogene subduction of Alpine Tethys. The ensuing Morelli 2003; Spakman and Wortel 2004; Zhu et al. 2012). collision involved substantial accretion and exhumation Alternatively, Mitterbauer et al. (2011) proposed that the of deeply subducted units from the downgoing European V anomaly beneath the Eastern Alps is vertical to steeply + p plate (Schmid et al. 1996, 2004). In the Apeninnes, how- northeast-dipping and represents European lithosphere that ever, Adria was the compliant lower plate during Neogene originally subducted to the south, but was subsequently roll-back subduction (Malinverno and Ryan 1986; Royden steepened and overturned. 1 3 Int J Earth Sci (Geol Rundsch) 5˚ 25˚ 10˚ 15˚ 20˚ 50˚ 50˚ Fig. 11b 240 150 45˚ 45˚ 40˚ 40˚ 35˚ 35˚ 5˚ 25˚ 10˚ 15˚ 20˚ accreted active leading edge of Western Alps thrust slab at 210 km depth undeformed inactive Europe suture Eastern Alps slab contours (top) I at 150 & 240 km depths unaccreted I normal fault Ivrea (rotation axis for accreted active strike-slip fault + Adria from 20 - 0 Ma) Adria Apenninic/ E. Ligurian inactive transfer fault Fig. 2 Tectonic map of the Alps, Apennines, Carpathians and Dinar- Milan Thrust Belt, NCA—Northern Calcareous Alps, PF—Peri- ides showing main faults, tectonic units, and surface traces of slabs adriatic Fault System, including Balaton Fault (BA), SA—Southern beneath the Alps. Red and blue lines indicate depth contours (km) of Alps Front, SK—Split-Karlovac Fault, SV—Sava Suture Zone, SP— positive P-wave velocity ( Vp) anomalies projected to the surface, Scutari-Peç Fault, TD—Tisza–Dacia boundary fault, TK—Timok respectively, for the Eastern+ Alps slab and the leading edge of Euro- Fault, TW—Tauern Window, WV—West Vardar ophiolite front. Cir- pean slab. Localities: D Dubrovnik, G Genoa, L Lyon, M Munich, W cles with crosses represent the Bükk Mtns.

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