Post-collisional mantle delamination in the Dinarides implied from staircases of Oligo-Miocene uplifted marine terraces

The Dinarides fold-thrust belt on the Balkan Peninsula resulted from convergence between the Adriatic and Eurasian plates since Mid-Jurassic times. Under the Dinarides, S-wave receiver functions, P-wave tomographic models, and shear-wave splitting data show anomalously thin lithosphere overlying a short down-flexed slab geometry. This geometry suggests a delamination of Adriatic lithosphere. Here, we link the evolution of this continental convergence system to hitherto unreported sets of extensively uplifted Oligocene–Miocene (28–17 Ma) marine terraces preserved at elevations of up to 600 m along the Dinaric coastal range. River incision on either side of the Mediterranean-Black Sea drainage divide is comparable to the amounts of terrace uplift. The preservation of the uplifted terraces implies that the most External Dinarides did not experience substantial deformation other than surface uplift in the Neogene. These observations and the contemporaneous emplacement of igneous rocks (33–22 Ma) in the internal Dinarides suggest that the Oligo-Miocene orogen-wide uplift was driven by post-break-off delamination of the Adriatic lithospheric mantle, this was followed by isostatic readjustment of the remaining crust. Our study details how lithospheric delamination exerts an important control on crustal deformation and that its crustal signature and geomorphic imprint can be preserved for millions of years.

www.nature.com/scientificreports/ formed on top of previously deformed bedrock 32,33 . So far, no geodynamic scenario has explained the occurrence of these conspicuous surfaces in the External Dinarides.

Methods
In order to map the horizontal surfaces across the External Dinarides, we used the EU-DEM v1.1 34 to produce slope and roughness maps. Based on these maps and additional topographic profiles, we manually mapped flat surfaces within the range of 0°-8° slope and a roughness of 0-22 m. In a second step only flat surfaces with a lower median slope of not more than 6° and a lower median roughness of not more than 7 m were considered for the final selection. Field mapping was performed to ground-proof these results and to look for major Neogene-Quaternary faults. We then analyzed 1 km-wide swath profiles of four pairs of rivers on both sides of the Dinaric drainage divide to quantify river incision as a proxy for regional uplift using the TopoToolbox2 35 . The 3D P-wave velocity model UU-P07 24,36 (Supplementary Videos A2, A3) served to map the top and the leading edge of the subducted Adriatic lithosphere.

Results
Marine terraces and river incision in the external Dinarides. All mapped horizontal surfaces follow the Adriatic shoreline for ca. 600 km from Istria to Albania (Fig. 1, Supplementary KML File, A4). They form a staircase morphology in the Velebit Mountains (Fig. 3), affect different bedrock lithologies, and encompass present-day elevations between 10 and 920 m (Fig. 4). Their morphology is neither influenced by bedding dip nor by faults. The largest surface was identified around the Krka River in the central part of the External Dinarides  www.nature.com/scientificreports/ (Fig. 3). Unfortunately, neither terrestrial nor marine deposits were found on top of these flat surfaces that could irrefutably prove their erosive marine origin. Due to the lack of other denudation processes that would result in large surfaces in close proximity to the present-day shoreline, and which cut folds, faults, and tilted strata on various elevations, we interpret these flat surfaces as degradational, marine wave-cut terraces (Figs. 1, 3). Such marine terraces and the associated staircase morphology are documented worldwide as a result of rather constant uplift and oscillating sea-level variations: thus, they are widely used to reconstruct surface uplift [37][38][39] (Fig. 1). The mean present-day elevations of the mapped marine terraces are between 200 and 300 m (Fig. 4A). The histogram of all terrace elevations may be biased by the largest terraces, because multiple measurements for larger terraces were included. Therefore, all terraces with areas smaller than 0.01 km 2 and larger than 1 km 2 were excluded from the filtered histogram, as well as those with standard deviations of more than 3° for slope and more than 10 m for elevation. The histogram of the filtered dataset shows four statistically significant peaks for smaller terraces at elevation of 100, 200-300, 450, and 600 m (Fig. 4b). The uneven preservation of marine terraces along the Adriatic coast is related to the different bedrock lithologies and coastal-near high elevations. In the Velebit Mountains, most of the terraces are located within the Velebit Breccia 27 (Figs. 3, 4). This well cemented and massive carbonate breccia is mainly exposed at the southwestern slope of the Velebit Mountains and covers the mountain tops. This shows that the Velebit Breccia has a higher erosional resistance compared to the surrounding carbonate bedrock. Consequently, this favors the pristine terrace preservation within the Velebit Beccia all across the frontal Velebit Mt. region.   (Figs. 2, 4). These mid-Eocene (locally early Eocene 30 ) to early Oligocene flexural foreland basin sediments were deposited in deep to shallow marine environments that changed to deltaic, lacustrine, and alluvial environments during the Oligocene 29 . Since then, the global sea level was at maximum between 50 m 40 and 100 m 41 above the present-day level (Fig. 2). Therefore, the formation of at least all terraces presently preserved at elevations higher than 100 m above sea level must be due to a regional surface uplift of the Dinarides since the Oligocene.
All terraces formed within carbonates. Due to the lack of suitable exposure dating techniques for carbonate rocks, the absolute age dating of terrace formation is nearly impossible. However, not a single terrace was found within the lacustrine Miocene sediments of the Dinaride Lake System. These sediments record the minimum age of the change from marine to terrestrial (lacustrine) conditions 32 (Fig. 2). Outcrops of Miocene lake sediments on Pag Island (Fig. 1b) are situated at present-day sea level, that is, topographically below all mapped terraces. There is no structural offset between the Miocene strata on Pag Island and the nearby marine terraces of the frontal Velebit Mt. region. Consequently, the marine terraces must be older than the Miocene sediments on Pag Island. The start of Miocene sedimentation on Pag Island is dated by magnetostratigraphy to 17.1 Ma 33 (Fig. 2). The timing of terrace formation is thus bracketed between the deposition of the youngest early Oligocene Promina Beds   The depth of the up to 150 km deep high velocity P-wave tomography body underneath the central Dinarides 21,23,36 is in agreement with independent surface S-wave tomography 42 and with SKS splitting data, which show northward mantle flow at depths of > 150 km 26 . This uniform northward mantle flow, captured by the average fast SKS across the central External Dinarides, shows no switch of the asthenosphere mantle flow direction even in close proximity to the top of the high velocity body (Fig. 1). Within the margin of error, the comparison of the SKS data, the S-wave and P-wave tomography show no evidence of a slab deep enough to disturb the mantle flow pattern in the central Dinarides as observed in the Alps and Apennines 43 . However, the top of the leading edge of this high P-wave velocity body underneath the entire Dinarides (Fig. 1b, supplementary videos) correlates with the pronounced change in crustal 44 and in the northern to central Dinarides also the lithospheric thickness 45 . All three of these abrupt lithosphere-scale changes show a first order correlation with the Mediterranean-Black Sea drainage divide across the Dinarides (Figs. 1b, 6).
The four river pairs across the drainage divide reveal up to 380 m of symmetric incision in the catchments on either side (Fig, 1c). Disruption on one side of the drainage divide within a catchment would have led to an asymmetrical incision, which we do not observe. We therefore interpret river incision amounting to 150-380 m as a response to an Oligo-Miocene uplift, responsible also for the mean uplift of 200-300 m of the marine terraces (Fig. 4). Consequently, the area of uplift was not only restricted to the Adriatic coastline, but rather affected the entire Dinarides.

Discussion
Neogene (post uplift) shortening in the Dinarides. Although the lack of suitable marker beds younger than the Miocene Dinaric Lake Sediments make it difficult to quantify the exact amount of Neogene shortening in the Dinarides, several studies show a substantial amount of post-uplift Neogene shortening 17,46,47 . For instance, palinspastic reconstructions show a 5° + /− 2° counter clockwise (CCW) rotation of Adria, which is associated with about 100 km of post-20 Ma shortening in the Dinarides 47 . This is supported by the detailed structural analysis from the Internal and southern External Dinarides, which show evidence for post-18 Ma N-S shortening 17,46 . Bennett, et al. 48 proposed an Eocene to present northeastward subduction of continental lithosphere underneath the Dinarides to explain the substantial decrease of the present-day GPS velocities from the External to the Internal Dinarides.
In contrast, AMS (anisotropy of magnetic susceptibility) data from the Neogene Dinaric lake sediments show evidence for mild NE-SW shortening, but do not confirm a CCW rotation of the Dinarides since their deposition   32 . Negligible Neogene shortening in the central External Dinarides is also supported by the absence of a Miocene flexural foreland basin (Fig. 1a) and the lack of Miocene low-temperature cooling ages in the central External Dinarides 49 (Fig. 2). In addition, the pristine preservation of the horizontal 28-17.1 Ma old marine terraces presented here provides evidence for the absence of substantial Neogene shortening in the central External Dinarides. Consequently, the Neogene shortening was most likely accommodated either in the more internal part of the Dinarides or within the even more internal Carpathian-Pannonian system. This is documented in the Dinarides-Alpine-Pannonian basin transition zone in Croatia 50 and in northerly adjacent Slovenia 51 . In contrast, the frequency of the mapped terraces decreases towards the southern Dinarides (Figs. 1, 4). South of the city of Dubrovnik the terraces are scattered and only locally a staircase morphology is preserved (Fig. 5). This might be related to a Neogene structural overprint due to pronounced Neogene shortening in the southern External Dinarides 17,52,53 , which is in agreement with the presence of the Neogene South Adriatic Basin 54 (Fig. 1a).
Interestingly, the seismic zoning, derived from recent earthquakes, shows the highest probability of strong ground motion in the northern and southern Dinarides, whereas the central External Dinarides are categorized as areas of lower expected ground motion 55 . This is in line with the higher preservation of the mapped terraces in the central External Dinarides (Fig. 4). However, GPS velocities 48 and fault plane solution data 56 prove that the present-day Adria-Europe convergence 48,57 has to be accommodated partly by shortening within the entire Dinarides fold and thrust belt 48,58 and in the Dinarides-Pannonian basin transition area 59 .
Deep-seated processes governing the Oligo-Miocene surface uplift. The extent of the uplifted area and the spatial correlation between the drainage divide and the thinnest part of the Adriatic lithosphere in the central Dinarides 45 (60 km, Fig. 6) favor a deep-seated process that governs surface uplift 60 . The lithospherescale cross-section across the central Dinarides (Fig. 6) Fig. 6). This contradicts an expected thickened lithosphere underneath the Dinarides due to Late-Cretaceous to Paleogene nappe stacking and thus pleads for post-collisional removal of the orogenic root. P-wave tomography model UU-P07 reveals an NEdipping positive anomaly down to 180 km (supplementary videos). Such an orogen-wide lithosphere structure is independently substantiated by the inversion of Rayleigh-wave dispersion data, which shows a positive S-wave velocity anomaly underneath the entire Dinarides 42 . Based on its position and shape we interpret this body as delaminated lithospheric mantle of the Adriatic plate (Fig. 6). We suggest that here, delamination caused inflow of less dense asthenosphere, which led to the elevated Oligo-Miocene Moho and induced surface uplift in the entire External Dinarides. The good correlation of the dipping lithosphere structure with the northeastern margin of thick lithosphere in the northern and north-central part of the External Dinarides further supports delamination in this region (Fig. 7). This correlation becomes less clear in the south-central and southern External Dinarides, where large parts of the Adriatic lithosphere thickness 45 is undetermined due to the lack of seismic stations (Fig. 7). In this region the lack of geophysical data is supplemented by geochemical data of the Eo-Oligocene magmatism, which shows mantle affinity (Fig. 7). This affinity is interpreted to reflect mantlecrust-interaction 18 and additionally supports orogen-wide delamination in the Dinarides. Prelević, et al. 61 showed that ultra-potassic volcanics from around the Sava Suture zone (Figs. 1a, 4, 7) exhibit a metasomatized mantle affinity that resulted from mantle-crust-interaction. The radiometric ages of these and other igneous rocks in the Internal Dinarides of 37-22 Ma (Figs. 2, 6, 7) clearly post-date Late Cretaceous collision and were previously explained by either late Eocene slab break-off (slab detachment) of the Adriatic slab 62 or by slab break-off followed by mantle delamination 18 .
Van de Zedde and Wortel 63 showed that the shallow detachment or break-off, i.e. at depths of 30-50 km, leads to a lateral contrast in buoyancy distribution which may trigger a Bird-type 9 delamination, leading to a peeled-off Adriatic lithospheric mantle. Andrić,et al. 62 proved that this model is applicable in the Dinarides, by relating the slab detachment (break-off) to a relamination of the lower crust to explain the foreland propagation of the syn-and post-collisional magmatism in the Internal Dinarides. The results of their best-fit 2D magmaticthermochemical numerical model show that a progressive lithosphere slab retreat could control the change from www.nature.com/scientificreports/ Eocene relamination and contraction to Oligocene-Miocene eduction and extension, as observed in the Internal Dinarides and the Sava Zone 62 (Fig. 2). Other studies, in turn, relate the prominent Oligo-Miocene extension to the contemporaneous subsidence of the northerly adjacent Pannonian Basin 16,64 (Fig. 2). Under the assumption that the present-day Adria-Europe convergence was active since late Eocene, Bennett, et al. 48 calculated a slab length of 138 km underneath the Dinarides to propose an active uninterrupted subduction since late Eocene times and to match the observed up to 180 km deep high velocity P-wave anomaly. This assumption requires a substantial amount of Neogene shortening in the entire Dinarides. However, the preservation of the horizontal staircase morphology presented here rather implies the absence of Neogene crustal shortening during and since the Oligo-Miocene surface uplift for the central External Dinarides.
Comparison with other studies. Although the preservation of Oligo-Miocene flat erosional surfaces within the Alpine-Himalayan-Belt seems to be exceptional, a Late Miocene (10 Ma) exposure age of the Dachstein Paleosurface was reported from the Northern Calcareous Alps 65 . The formation of the Dachstein paleosurface was dated to Late Eocene to Early Oligocene times, when this surface was buried by the 1.3 km thick Augenstein Formation and experienced a Late Miocene uplift pulse, which led to the exposure of the paleosurface 65 . As with the set of Oligo-Miocene marine terraces reported here, the Dachstein paleosurface is also preserved within a thick carbonate sequence. According to Frisch, et al. 65 a karstic environment might favor subsurface erosion and formation of caves over surface erosion, which led to the preservation of the Late Miocene exposed geomorphic features in the Northern Calcareous Alps. A similar process of mainly subsurface erosion might have taken place in the Dinarides, where the pristine preservation of the marine terraces in the External Dinarides recorded an Oligo-Miocene delamination event.
Mantle geometries that were explained by delamination patterns were also reported for the east Carpathians, and the Colorado Plateau. In both scenarios delamination occurred without crustal shortening. The uplift of the Colorado Plateau was ascribed to slab removal, which caused thermo-chemical convection leading to presently active delamination, surface uplift, and concomitant incision of the Grand Canyon 11 . Underneath the Colorado Plateau only P-wave tomography confirms the existence of a partly attached foundering continental slab, while it remains undetected by S-wave receiver functions 11 . This is comparable to our case in the Dinarides, where the continental slab is continuously detected by P-wave tomography (Fig. 1), whereas S-wave receiver functions provide evidence for a much deeper reflector in one location only 45 (Fig. 7).
In the Eastern Carpathians delamination led to surface uplift and foundering of the seismically active, nearvertical lithospheric slab underneath the Vrancea region, which shows eastwards-retreat since collision as a consequence to slab break-off 10 . Numerical models 8 showed that denser, partly attached oceanic lithosphere may have triggered delamination, which resulted in the observed uplift pattern and subsidence geometries within the Carpathians. The numerical model of Göğüş, et al. 66 , in particular the model run (EXP-6) based on  Fig. 1b). Main detachments of mainly Paleogene nappe stacking (red) and younger partly reactivated extensional detachments in black 16 . The LAB depth is projected into the section trace (location: Fig. 1b). The thinnest lithospheric part corresponds to the drainage divide. The fast P-wave tomographic anomaly is interpreted as a remnant of the "peeled-off " Adriatic lithosphere.  66 . The slab depth, the amount of associated uplift, and the duration of lithosphere removal fit our observations and proposed time window of lithosphere removal underneath the Dinarides (Fig. 2). The radiometric ages of igneous and mafic extrusive rocks in the Internal Dinarides, interpreted as proxies for mantle delamination, gradually follow Late Eocene slab detachment and range from 37 to 22 Ma 18,67 . This coincides with the estimated time of surface uplift between 28 and 17.1 Ma (Fig. 2), as inferred from relative age dating of the marine terraces discussed above. Since both are related to mantle delamination, the timing of the uplift can be constrained to  Ma. The potential causes of such post-collisional continental delamination can be related to either (i) a rather shallow Late Eocene slab break-off, which weakened the lithosphere between the Dinarides and Pannonian-Carpathian system and led to a lateral instable buoyancy distribution; or (ii) by eclogitization due to lithospheric over-thickening. The preservation and the relative dating of the marine terraces in the External Dinarides prove that delamination occurred after an early Oligocene slab break off and was not associated with crustal shortening. Our results indicate that delamination, which may have culminated from a slow incipient to a rapid process, is responsible for the orogen-wide surface uplift of the Dinarides in Oligo-Miocene times.

Conclusions
One of the most striking conclusions of our study is that marine terraces, formed and uplifted during Oligocene to Miocene (28-22/17.1 Ma) times, are still discernible at present despite a karstic environment, emphasizing the regional importance of this uplift event. The regional surface uplift signal of up to 600 m is restricted to the along-strike extent of the NE edge of a delaminated continental slab. The position of the leading edge of the slab shows a first order correlation with the Mediterranean-Black Sea drainage divide. Symmetric incision across the drainage divide pleads for uniform uplift, with its crest underneath the drainage divide. Relative ages of the terraces coincide with the age of post-collisional magmatism in the Internal Dinarides. The combination of deep geophysical and surface geomorphic data suggests an Oligo-Miocene delamination event following early Oligocene slab detachment leading to Moho-and surface uplift. This delamination is confirmed by a shallow, sub

Data availability
Tomography model UU-P07 is available at https ://www.atlas -of-the-under world .org.