TECTONOTHERMAL EVOLUTION OF THE CENTRAL-WESTERN CARPATHIANS AND THEIR FORELAND

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1 TECTONOTHERMAL EVOLUTION OF THE CENTRAL-WESTERN CARPATHIANS AND THEIR FORELAND Ph.D. candidate: ADA CASTELLUCCIO, III course Tutor: Prof. MASSIMILIANO ZATTIN Co-tutor: Prof. STEFANO MAZZOLI Cycle: XXVII Abstract The tectonic and thermal evolution of the Carpathians has been studied for many years and it has still some open issues. They have noteworthy implications in the oil and gas exploration. The sequential restoration of balanced cross-sections coupled with low-temperature thermochronometry (apatite fission track and apatite (U-Th-Sm)/He data) well-constrain the burial and exhumation history of this flysch belt and the relative shortening. The interplay between thick and thin-skinned thrusting characterized the Carpathian tectonic evolution, this latter triggering even the exhumation of the Western Outer Carpathians. The Middle-Late Miocene post-thrusting normal faulting and late Miocene regional uplift are the main mechanism exhuming the Eastern Polish Carpathians and the Ukrainian Carpathians, respectively. A reliable modelling of the isotherm through time has been also performed, highlighting that the Mesozoic and Palaeozoic successions of the Central Western Carpathians reached the maximum temperatures (ca. 200 ) during the Early Miocene. Introduction For years, the Carpathian thrust and fold belt draws the attention of many European researchers not only for their scientific complexity leading controversial theories on their evolution, but also for the increasing interest of the oil companies in understanding their tectonic evolution and the variation of the thermal regime. In this area the occurrence of any branches of the Piemont-Ligurian ocean is still debated and there are not works coupling the tectonic and thermal evolution. This work is the first attempt to correlate these two aspects in an unique model. Our sequentially restored balanced cross-sections integrated with new and published low-temperature thermochronometric data (apatite fission track and apatite (U-Th- Sm)/He ages) allows the definition of a thermo-kinematic model in which the isotherm forward modelling and the prediction of the exhumation ages come directly from the kinematic restoration of a chosen profile. In this study the tectonic evolution of the Carpathians has been reconstructed back to the Early Cretaceous. For the last 20 Ma, three different exhumation processes can be recognized in the Outer Carpathians: (i) thrusting, controlling the exhumation of the westernmost sector, (ii) post-thrusting normal faulting in the eastern Polish region and (iii) post-thrusting regional uplift in the Ukrainian and Central Carpathians. In addition, the original width of the depositional basins of the Inner and Outer Carpathians and the relative amount of shortening can be estimated together with the maximum temperature experienced by the different sector of this thrust belt. Geological setting The Central-Western Carpathians are part of a curved orogenic system extending from the Danube Valley in Austria to southern Romania. The origin of this chain is related to the collision between the European Platform and the Alps-Carpathians-Pannonia (ALCAPA) and Tisza-Dacia Mega-Units belonging to the Adriatic palaeogeographic domain. The closure of the southern branch of the Alpine Tethys (sensu Schmid et al., 2008) started in the Jurassic-Early Cretaceous. The movement of the ALCAPA unit, shaping the Western Carpathians, started during the Late Cretaceous and lasted until the Neogene. It led to the emplacement of the Western Carpathian accretionary wedge on top of the southern margin of the European Platform. The Carpathians are subdivided into two different tectonic domains (Książkiewicz, 1977): the Inner Carpathians (IC) and the Outer Carpathians (OC). The former are made up of Variscan crystalline basement, including Paleozoic metamorphic rocks, and its Mesozoic sedimentary cover, incorporated into a series of thick-skinned thrust sheets. These thrust sheets are unconformably overlain by clastic deposits belonging to the Central Carpathian Paleogene Basin (CCPB). The OC consist of a fold and thrust belt formed mainly by siliciclastic turbiditic deposits of Upper Jurassic to Lower Miocene age, which were deformed since Oligocene times. The OC and IC realms are separated by a few hundreds 1

2 of meters to twenty kilometres wide zone called Pieniny Klippen Belt (PKB) (e.g. Birkenmajer, 1986). It consists of Mesozoic blocks of shallow to deep-water facies embedded in a less competent Upper Cretaceous to Paleocene matrix. Methods Six balanced geological sections have been constructed across the Central-Western Carpathians. Four of them have been sequentially restored using Move, a software developed by Midland Valley Exploration Ltd. and dedicated to cross-section building and structural restoration. Our own field data integrated with geological maps allowed us to constrain the surface geology, whereas the seismic profiles and the borehole data has been used to construct the geometry of the deep structures. The thickness of the eroded strata above the present-day topography has been constrained by published low-t thermochronometric data and vitrinite reflectance. Flexural slip restoration coupled with 2D forward kinematic modeling has been performed in order to check the geometries of the tectonic structures and then validate the crosssections and the related tectonic scenario. Vertical simple shear and fault parallel flow algorithms were used to restore/forward model the listric normal faults and the reverse faults, respectively. The flexural slip algorithm has been applied to simulate the flexure of the underplate. After performing the kinematic modeling we chose ten steps of the restoration of a selected section to be processed with FETKIN, software dedicated to thermochronometric ages prediction (Almendral et al., submitted). It solves the advection-diffusion heat flow equation in 2D and calculates the temperature distribution honoring the deformation mechanism at all time steps of the restoration. Applying an iterative workflow we changed the erosional rate and the paleo-topography in order to achieve the best fit between modeled and measured data. The final result is a calibrated kinematic model in which a certain geological scenario has been consistently integrated with an admissible thermal history Results The balanced geological sections cross the Carpathian thrust and fold belt from the foreland to the CCPB. The sequential restoration allows the reconstruction of the Early Cretaceous pre-orogenic setting. In this scenario, both the IC and the OC successions deposited in continental passive margin basins. Shortening started during the Neocomian and involved the inner part of the Central Western Carpathians (CWC). Deformation then propagated northwards, in the IC realm, producing the reverse-slip reactivation of preexisting normal faults. This thick-skinned deformation produced the imbrication of basement-involved thrust sheets and their subaerial exposure during the Paleocene. The subsequent erosion provided the sediment supply filling the Pieniny foredeep basin, north of the IC front. Some previous studies (Birkenmajer, 1956b; Roca et al., 1995) document the southern provenance of the olistolithes and olistotromes included in the Pieniny wildflysch, as well as their sedimentological similarity with the IC successions. The erosional event affecting the IC domain is marked by the regional unconformity of the Eocene nummulitic Fm. directly on top of the Mesozoic IC nappes. A change in tectonic style occurred during the Paleocene, switching from thick-skinned to thin-skinned thrusting. Shortening rates increased during the Oligocene, when thrusting propagated further north into the OC paleogeographic realm. Insequence, thin-skinned thrust propagation in the OC domain continued up to the Early Miocene, when shortening migrated at depth into the basement. During this stage, erosion started to involve the uppermost successions of the inner part of the OC thrust and fold belt. In the Polish region, the gravitational instability of the wedge and its subsequent extensional collapse led to the development of normal faults [Mazzoli et al., 2010], some of them reactivating preexisting tectonic contacts. This tectonic event was followed by a regional uplift localized in the CWC region. No evidences for Middle-Late Miocene normal faults are present in the section across the Ukrainian Carpathians. The last 20 Ma has been constrained by low-temperature thermochronometric data coming from published works (e.g. Andreucci et al., 2013, 2014; Králiková et al., 2014, and references therein) and new AFT and AHe data from the PKB. Cooling ages for this latter are consistent with the published exhumation ages for the IC domain, thus suggesting a common Late Miocene cooling event for these two different tectonic domains, as shown in our structural model. The FETKIN thermo-kinematic model associated with the above- 2

3 described sequential restoration allows us to link the wide range of cooling ages recorded in this area, to different tectonic processes. The thermo-kinematic model provides a good match between the predicted and measured data. The Early Miocene cooling ages recorded for the Outer Polish Carpathians are mainly associated with thrusting. This is not the case of the Eastern Polish and Ukrainian Carpathians where the Middle Early Miocene cooling ages are respectively associated with low-angle normal faulting and regional uplift. The thermo-kinematic model performed for the profile across the Western Polish Carpathians highlights the occurrence of two region experiencing temperatures lower than 60 (the foreland basin together with the neighbor Silesian Unit and the Liptov Basin). Higher temperature (< 120 ) has been recorded in the CCPB. Its last cooling event is almost coeval with the PKB exhumation and the rest of the CWC. In addition, the forward kinematic modeling allows us to calculate the shortening rate for each step of the tectonic evolution. For the first convergent tectonic stage (Cretaceous thick-skinned inversion) it was of 0.8 mm/yr. A lower rate, 0.5 mm/yr dominated the Paleocene to Early Oligocene time interval. A remarkable increase of the shortening rate, reaching a value of 6.5 mm/yr, characterized the Late Oligocene-Early Miocene time interval. This was associated with a major change in the style of thrusting, from thick-skinned to thin-skinned. Thrusting was then followed by Middle Miocene normal faulting characterized by an extension rate of ca. 0.6 mm/yr. Tectonic evolution This work provides a comprehensive picture of the whole Carpathian orogen-foreland basin system, focusing on the relationships among the IC, PKB and OC. A new scenario is proposed for the tectonic significance of the PKB in the Carpathian orogen. Our model involves an Early Cretaceous preshortening tectonic setting consisting of a sedimentary basin made of thinned continental crust, on which all the preserved successions of the IC and the OC domains were deposited, and an unknown but probably limited if not null amount of oceanic lithosphere. Unlike all the paleogeographic reconstruction suggesting the occurrence of several deep-water basins separated by horsts, our model shows a sedimentary basin in which IC and OC successions were deposited. Our reconstruction is based on the occurrence of some continuous markers throughout the whole sedimentary basin. During the Early Cretaceous-Paleocene, the imbrication and stacking of the IC units on top of the thinned European Platform produced the flexure of this latter and the deposition of olistolites and olistostromes coming from the eroded IC Mesozoic nappes in the so-formed foredeep basin. These mega-blocks together with the Upper Cretaceous Paleocene matrix form the Pieniny wildflysch. Subsequent thrust propagation into the foreland basin led to the partial tectonic superposition of Pieniny wildflysch units on top of the OC successions during the Eocene-Early Oligocene. This scenario is in contrast with the interpretation of the PKB as an oceanic basin suture. According to the latter interpretation, the subduction of the Pieniny Ocean during the Cenomanian, the subsequent shortening event and erosion (Birkenmajer, 1986) would have led to the exhumation of the PKB during the Early Miocene. However, our new thermochronometric data indicate that exhumation of the PKB occurred later, in Middle-Late Miocene times. The cooling ages from the PKB are consistent with those of the surrounding thrust belt units, thus confirming that the PKB formed part of the thrust belt and did not underwent a different and peculiar tectonic evolution marking the existence of a suture zone. The deposition of the Pieniny wildflysch on the flexured foreland lithosphere and its subsequent thrusting on top of the OC successions mark the end of the IC thickskinned deformation. The change in tectonic style from thick-skinned to thin-skinned was followed by insequence propagation of thrusting in the OC and a relevant increase in the shortening rate (6.5 mm/yr). Post-thrusting normal faulting affected the western part of the accretionary wedge, probably due to the extensional collapse associated with the deactivation of the sole thrust. Locally (as for the Eastern Polish Carpathians) low-angle normal faults caused the unroofing of some portion of the belt (Andreucci et al., 2013). In the Ukrainian region there are no evidences of post-thrusting normal faults. Here the exhumation is mainly controlled by regional uplift (Andreucci et al., 2014). New AFT and AHe data from the PKB constrain its cooling to the last 15 Ma, pointing out that exhumation of the PKB is part of the same event producing the final uplift of the IC region. 3

4 Conclusion This work provides an new interpretation for the tectonic evolution of the Central-Western Carpathians. In particular, a new scenario has been proposed for the Cretaceous paleogeography and for the origin of the PKB which represents an intensely deformed sedimentary unit (wildflysch) deposited in the foredeep of the IC belt rather than a subdction mélange. One of the main features of this work is the successful combination of sequentially restored sections and low-t thermochronometry, that allows us to provide a valid tectonic scenario supported even by a well-constrained thermal field. According to our sequential restoration, the width of the original sedimentary basin, independently from the occurrence of any oceanic crust between the IC and OC, ranges between ca. 260 km, in the western polish sector and 460 km in the central part of the study area, getting less wide in the Ukrainian region (ca 240 km).the estimated shortening is variable, from ca. 50% to 58 %(the maximum values in the central part of the accretionary wedge). Both the structural and the thermochronometric data suggest the occurrence of different processes triggering the exhumation of the Carpathian thrust and fold belt, acting in different times: thrusting for the Outer Polish Carpathians, post-thrusting normal faulting for the eastern polish sector and Late Miocene regional uplift (Andreucci et al, 2014) for the Ukrainian Carpathians and the CWC region. References ALMENDRAL, A., ROBLES W., PARRA M., MORA A. and KETCHAM R. Fetkin: Coupling kinematic restorations and temperature to predict exhumation histories. AAPG Bulletin submitted. ANDREUCCI, B., CASTELLUCCIO, A., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., and ZATTIN, M Burial and exhumation history of the Polish Outer Carpathians: Discriminating the role of thrusting and post-thrusting extension. Tectonophysics. 608, ANDREUCCI, B., CASTELLUCCIO, A.,CORRADO, S., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., and ZATTIN, M Interplay between the thermal evolution of an orogenic wedge and its retro wedge basin: an example from the Ukrainian Carpathians. GSA Bull.in press. BIRKENMAJER, K b. Sedimentary characteristics of the Jarmuta Beds (Maestrichtian) of the Pieniny Klippen Belt (Central Carpathians). Bull. Acad. Polon. Sci., 3, 4, 10, Varsovie BIRKENMAJER, K Stage of structural evolution of the Pieniny Klippen Belt, Carpathians. Stud. Geol. Pol., 88, KRÁLIKOVÁ, S., VOJTKO, R., ANDRIESSEN, P., KOVÁČ, M., FÜGENSCHUH, B., HÓK, J. and MINÁR, J a. Late Cretaceous-Cenozoic thermal evolution of the northern part of the Central Western Carpathians (Slovakia): revealed by zircon and apatite fission track thermochronology, Tectonophysic , KSIĄŻKIEWICZ, M Tectonics of the Carpathians, in POZARYSKI W. (ed.). Geology of Poland, Tectonics, Wyd. Geol., 4, , Warsaw, Poland. MAZZOLI, S., JANKOWSKI, L., SZANIAWSKI,R., ZATTIN, M Low-T thermochronometric evidence for post-thrusting (< 11 Ma) exhumation in the Western Outer Carpathians, Poland. C.R. Geosci., 342, ROCA, E., BESSEREAU, G., JAWOR E., KOTARBA, M., and ROURE F Pre-Neogene evolution of the Western Carpathians: Constraints from the Bochnia-Tatra Mountains section (Polish Western Carpathians), Tectonics,14(4), SCHMID, S. M., BERNOULLI, D., FÜGENSCHUH, B., MATENCO, L., SCHEFER, S., SCHUSTER, R., TISCHLER, M. and USTASZEWSKI, K The Alpine-Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss J. Geosci., 101,

5 SUMMARY OF ACTIVITY IN THIS YEAR Communications: XX Carpathian-Balcan Geological Association Tirana, Albania, 24 th -26 th September Castelluccio A., Andreucci B., Grigo D., Jankowski L., Ketcham R. A., Mazzoli S., Szaniawski R. and Zattin M., Coupling sequential restoration of balanced cross-sections and low-temperature thermochronometry: the case study of the Polish- Ukrainian Carpathians. SGI-SIMP 2014 Milano-Italy, 10 th - 12 th September Castelluccio A., Andreucci B., Grigo D., Jankowski L., Ketcham R. A., Mazzoli S., Szaniawski R. and Zattin M., Structure and tectonic evolution of the Western Carpathians: new insights from sequentially restored balanced cross-sections integrated with low-temperature thermochronometry. 14th International Conference on Thermochronology- Chamonix, France, 06 th -12 th September 2014 Castelluccio A., Andreucci B., Ketcham R. A., Jankowski L., Mazzoli S., Szaniawski R. and Zattin M., Coupling sequential restoration of balanced cross-sections and low-temperature thermochronometry: the case study of the Polish Carpathians. AAPG Europe Office, the 25 th -26 th March, Naples (Italy). Castelluccio A., Andreucci B., Grigo D., Jankowski L., Ketcham R. A., Mazzoli S., Szaniawski R. and Zattin M., Tectonic and thermal evolution of the Carpathian fold and thrust belt: insights from low-temperature thermochronometry and sequential restoration of balanced cross-sections. Posters: EGU th April - 2 nd May, Vienna, Austria. Castelluccio, A., Andreucci, B., Grigo, D., Jankowski, L., Ketcham, R., A., Mazzoli, S., Szaniawski, R., and Zattin, M., New constraints on the tectonic and thermal evolution of the Central-Western Carpathians Publications: ANDREUCCI, B., CASTELLUCCIO, A., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., and ZATTIN, M Burial and exhumation history of the Polish Outer Carpathians: Discriminating the role of thrusting and post-thrusting extension. Tectonophysics. 608, ANDREUCCI, B., CASTELLUCCIO, A.,CORRADO, S., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., and ZATTIN, M Interplay between the thermal evolution of an orogenic wedge and its retro wedge basin: an example from the Ukrainian Carpathians. GSA Bull.in press. CASTELLUCCIO, A., ANDREUCCI, B., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., and ZATTIN, M. Building and exhumation of the Western Carpathians: new constraints from sequentially restored, balanced cross-sections integrated with low-temperature thermochronometry. Tectonics, submitted. Teaching activities: Teaching assistant: 21 hours Sedimentary Geology at University of Padua (2013/2014) 5

THERMAL HISTORY AND TECTONIC EVOLUTION OF THE WESTERN CARPATHIANS

THERMAL HISTORY AND TECTONIC EVOLUTION OF THE WESTERN CARPATHIANS Ph.D. candidate: BENEDETTA ANDREUCCI, I course Tutor: Prof. MASSIMILIANO ZATTIN Cycle: XXV Abstract The Cenozoic tectonic evolution of