T8

The Sava suture zone of the internal Dinarides contains Maastrichtian trench-fill sediments, termed “Sava flysch” that record the closure of the northern branch of the Neotethys. Subsequent collision between Adria-derived thrust sheets and blocks of European affinity in Latest Cretaceous to Paleogene times culminated in the formation of the Dinarides fold-and-thrust belt. The suture zone hosts numerous Oligocene plutons of I-type granitic composition. Many of these intrusions are located in the center of metamorphic core complexes (MCCs) that were exhumed in early Miocene times. This phase of post-collisional extension was concomitant with the opening of the northerly adjacent Pannonian Basin and associated with granitic S-type magmatism. Both the processes responsible for extensional deformation and magmatic activity in the internal Dinarides are still a matter of debate.

Our Study contributes spatio-temporal constraints to better understand the tectono-magmatic processes of this area. We present field-kinematic, geochronological, and thermobarometric data from two MCCs at the transition between the internal Dinarides and the Pannonian Basin. Both MCCs are characterized by plutonic rocks in the center, surrounded by up to amphibolite-grade mylonites of exhuming shear zones. Heterogeneous extensional reactivation of formerly contractional structures that gave rise to these core complexes as low-angle detachments in the early Miocene is indicated by a variation in deformation ages of 3 Ma, obtained by Ar-Ar in-situ dating of white mica from deformed rocks of the respective shear zones. While Motajica MCC was exhumed from within the Sava zone during E-W extension at approximately 20 Ma, Cer MCC was exhumed as part of the underlying Adriatic basement during N-S extension between 17-16 Ma. For the Cer MCC, a concordia age of 17.6±0.1 Ma (2σ) obtained by U-Pb LA-ICP-MS on zircons from an S-type granite in combination with an Ar-Ar inverse isochron age of 16.6±0.2 Ma (2σ) obtained on white mica from the same sample, indicate a cooling rate of approximately 400°C/Ma.

Our results contribute to the idea of rapid exhumation of mid-crustal material in the form of MCCs in response to the opening of the Pannonian Basin. This is further corroborated by results of Raman spectroscopy on carbonaceous material, as the temperature profile across the shear zone implies extremely condensed isotherms of 250°C/km. Additionally, U-Pb analyses show that zircons of the I-type intrusion contain inherited cores with age maxima at 270 Ma and 516 Ma and newly formed rims with an age maximum at 31.7 Ma, indicating the timing of intrusion. The S-type granite of Cer in parts reworks the I-type intrusion, as inherited cores include ages of 31-32 Ma, while the rims show an age of 17-18 Ma, suggesting a syn-extensional emplacement. Our data further shows that zircons of the I-type intrusion contain a significant amount of inherited cores with an age spectrum that resembles the detrital age spectrum from sediments of the Sava zone. This challenges the idea that these I-type melts were solely generated from igneous protoliths, and rather suggests a formation from melting of Paleozoic to Mesozoic successions constituting tectonically buried nappes of the internal Dinarides.

How to cite: Löwe, G., Prelević, D., Sperner, B., Schneider, S., Pfänder, J. A., Balling, P., Nabhan, S., von Quadt Wykradt-Hüchtenbruck, A., and Ustaszewski, K.: Exhumation of metamorphic core complexes of the internal Dinarides was triggered by the opening of the Pannonian Basin, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-34, https://doi.org/10.5194/egusphere-alpshop2022-34, 2022.

The Dinarides fold and thrust belt resulted from the collision of the Adriatic Microplate with Eurasia and shows an overall SW-vergent and in-sequence structural architecture. In the Paleocene the ophiolite-bearing internal Dinarides were exclusively affected by crustal shortening. The outward SW propagation of the deformation front reached the eastern Adriatic passive continental margin mainly composed of Mesozoic carbonate platform rocks in Mid-Eocene times. This led to high crustal Mid Eocene to Oligocene shortening and the formation of the external Dinarides. Two balanced cross-sections across the external Dinarides show an along-strike contrasting deformation styles observed in two orogenic segments separated by  the 250 km long dextrally transpressive Split-Karlovac Fault:  the southern segment dominated by SW-vergent forethrusts, and the northern segment dominated by NE-vergent backthrusts, located to the SE and NW from the Split-Karlovac Fault, respectively. So far, it is not known why the regionally rather uniform Mesozoic Adriatic carbonate platform sequence had undergone such contrasting along-strike deformation.

To improve the understanding of the initiation of the NE-vergent backthrusts and to assess the amount of crustal shortening in the NW segment, a 2D kinematic forward model across the central Velebit Mt. was set up. The Velebit Mt. extends for about 130 km along the eastern Adriatic coast and form a SW-dipping monocline with topographic elevations reaching close to 1800 m. This fault-related monocline is formed in the hanging wall of a NE-vergent backthrust system. The 2D kinematic forward model approach applied to a pre-deformed lithostratigraphic template scaled to reported stratigraphic thicknesses enabled us to test various geometries and temporal successions of fault activity not only for the Mid Eocene – Oligocene contraction, but also for the Mesozoic passive margin extension. Through an iterative trial-and-error method, we were able to reproduce the present-day deformed reference section across the Velebit Mt. and the Lika Plateau in its northeastern hinterland.

Our best-fit balanced kinematic model suggests that the reactivation of Middle Triassic and Upper Jurassic basement-rooted half grabens played a key role in the initiation of the backthrusts. These half grabens were mainly reactivated by hanging wall shortcuts. This inversion of normal faults led to predetermination of the thin-skinned NE-vergent back thrusts, forming the upper part of a complex 68 km wide triangle structure. The structurally lower part comprised of a SW-vergent antiformal stack involving Paleozoic basement. We assessed a crustal shortening for the triangle structure of 47 km and a shortening of 98 km for the entire cross-section. Our results show that the differences in both the lithostratigraphic and Mesozoic half grabens along the eastern Adriatic passive margin played a crucial role in the Mid Eocene – Oligocene deformation of the external part of the Dinarides fold and thrust belt, which led to the contrasting along strike deformation styles to the NW and SE of the Split-Karlovac Fault.

How to cite: Balling, P., Tomljenović, B., and Ustaszewski, K.: The inversion of a passive continental margin portrayed by a 2D balanced kinematic forward model across the Velebit Mt.  in the northern external Dinarides fold and thrust belt, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-22, https://doi.org/10.5194/egusphere-alpshop2022-22, 2022.

Fluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450–550 °C and 0.3–0.6 GPa and below. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th–Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe- Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90–70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36–30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25–12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12–5 Ma).

How to cite: Gnos, E., Mullis, J., Ricchi, E., Bergemann, C., Janots, E., and Berger, A.: Episodes of open fissure formation in the Alps, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-2, https://doi.org/10.5194/egusphere-alpshop2022-2, 2022.

The Ivrea-Verbano Zone (IVZ, Southern Alps) is a fossil exhumed passive margin section of the pre-Alpine middle to lower continental crust that escaped Alpine subduction. Following the Variscan orogeny, the IVZ was affected by Permian post-orogenic extension and Triassic-Jurassic polyphasic rifting stages. Rift-related deformation was accommodated by several km-scale shear zones active at different crustal levels (e.g., Beltrando et al., 2015). Due to the intrinsic importance of these tectonic structures, a detailed characterization of their compositional, metamorphic and structural patterns, as well as the timing of activity may provide key information for the models of shear development in relation to the evolution of the regional tectonics.

In this contribution, we investigate one of these major extensional structures - the Anzola shear zone - with the aim to assess the conditions that promoted the strain localisation. We also provide U-Pb dating on titanite as attempt to constrain the timing of the high-temperature crystal-plastic deformation occurred within the shear zone. Recent field and meso-structural investigations revealed that the Anzola shear zone overprinted basement rocks characterized by inherited lithological and structural heterogeneities (Corvò et al., 2022). Gabbroic rocks and migmatites define the hanging wall and footwall, respectively. According to the petrography and geochemistry (ultra-)mylonitic rocks developed at the expense of a multi-lithological sequence showing amphibolite to granulite facies metamorphic conditions and deformation features related to pre-shearing event. Estimated P-T conditions indicate that mylonitic deformation started at high temperature (~820°C) with presence of melt and continued as solid-state deformation down to amphibolite facies (~650°C). As regard the timing, we show preliminary petrochronological results from titanite of the mylonitic amphibolites that recorded recrystallization event under amphibolite facies at about 185 Ma, which is coeval to deformation occurred at different crustal levels in the IVZ (Simonetti et al. 2021).

On the base of our findings, we argue that the shear zone development was promoted by the rheological contrasts derived from the inherited compositional and structural patterns. Moreover, we emphasizes evidence of syn-deformational partial melting and small amounts of free fluids localized in certain layers that enhanced the viscosity contrasts within the multi-lithological complex. Melts/fluids played a key role in both weakening mechanisms controlling the strain localization, as well as the syn-tectonic growth-recrystallization processes of the titanite, resulting in a strong influence of the U-Pb petrochronology results. Finally, our results are discussed in the framework of the geodynamic evolution of IVZ.

How to cite: Corvò, S., Maino, M., Piazolo, S., Kylander-Clark, A., Seno, S., and Langone, A.: Multiscale heterogeneity control on the nucleation of a crustal shear zone: petro-structural investigation and U-Pb titanite dating from the Anzola shear zone (Ivrea-Verbano Zone, Southern Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-21, https://doi.org/10.5194/egusphere-alpshop2022-21, 2022.