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Palaeogeographic perturbations in the key-area between the Alpine Tethys and the Neotethys Realms during the time of tectonic overturns: Jurassic of the Alpine-Dinaric transition zone

Rožič Boštjan

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

60--61

The major Mesozoic palaeogeographic disintegration of the present-day transitional area between the Alps and the Dinarides (Slovenia) occurred due to the Middle Triassic rifting event related with the opening of the Neotethys Ocean. By the Norian, three major palaeogeographic units were formed: the Dinaric (Adriatic, Friuli) Carbonate Platform (DCP) in the south, intermediate, E-W extending Slovenian Basin (SB) and the Julian Carbonate Platform (JCP) in the north. The platforms were characterized by a Dachstein type platform, while the basin was filled with hemiplegic and resedimented limestones, most of which are now dolomitized. To the west, there was a shallow water “bridge” between the two platforms. After the Triassic-Jurassic Boundary crisis, the palaeogeographic setting was preserved, but the margins of the platforms turned into ooidal factories. During the Early Jurassic, SB was almost exclusively filled with ooid calciturbidites from the north, which can be explained by the wind/leeward position of the basin with respect to the particular platform. The first rifting phase of the opening Alpine Tethys, generally dated to the earliest Jurassic, is poorly expressed in this area. The main products are limestone breccias that occur in the western part of the SB. In contrast, the second rifting phase (dated to the Pliensbachian in Slovenia) completely disintegrated JCP. The margins subsided first and were characterized by open shelf conditions with crinoid meadows, while the inner parts of the JCP remained shallow-marine. In the SB, the initial subsidence can be seen in the altered composition of the calciturbidites. Namely, the ooid/peloid dominated resediments changed to crinoid/ lithoclast dominated. In the Toarcian, sedimentation ended on most of the JCP, with only sporadic marls occurring at the margins. At the same time, the sedimentary environment of the DCP also deepened and nodular or crinoid limestone was deposited. The SB is characterized by uniform clay-rich sediments that vary greatly in thickness, indicative of differential subsidence caused by the second rifting phase. In the Middle Jurassic, shallow-water sedimentation re-established on the DCP, the margin being characterized again by ooidal shoals, the sedimentation of the SB gradually changed to siliceous limestone, while the JCP and the “bridge” between the JCP and DCP are characterized by non-sedimentation. The last important Jurassic change occurred during the Bajocian-Bathonian stages. Condensed Ammonitico Rosso-type limestone began to be deposited on the “bridge” and the JCP, while sedimentation in the SB changed to pure radiolarite. In the past, this was interpreted as a result of thermal subsidence associated with oceanization of the Alpine Tethys. However, studies in the last decade suggest a more complex tectonic evolution. Because the area in question lies between the opening Alpine Tethys to the west and the concurrent onset of subduction of the Neotethys to the east, it has been subject to strong differential subsidence between the largescale DCP and all units north of it. The exact nature of the tectonic deformation is not yet clear, but a transtensional regime is most probable. These events resulted in the disintegration and collapse of the northern DCP margin, as evidenced by the sedimentation of limestone breccia megabeds along the entire SB southern margin. These megabeds not only indicate enhanced tectonics, but also provide important information about the pre-Middle Jurassic architecture of the DCP margin, which is no longer preserved. They consist of very diverse limestone lithoclasts and an ooid packstone matrix. Analysis of the clasts revealed that the Late Triassic DCP margin was characterized by Dachstein-type reefs and the Early Jurassic by ooid shoals. In the interior of SB, these strata merge into ooid calciturbidites interlayered between radiolarite and become completely wedged in the northern part of the basin. Corresponding gravity-flow deposits also sedimented on the subsided “bridge” between the DCP and the JCP, and even on the northern margin of the DCP itself. An important difference is the simpler composition of the resediments in this area. Namely, they consist entirely of Middle Jurassic platform margin and slope lithoclasts. This is explained by the less pronounced palaeotopography between the active platform and submerged “bridge”, which did not allow erosion of the older platform limestone (as observed in SB). The described collapse of the DCP margin caused it to retreat, and marginal reefs formed over the underlying inner platform limestones in the Late Jurassic. The emersion phase in the Kimmeridgian ended reef growth and the margin turned back into ooid rich shoals. At the same time, the SB was characterized by continuous radiolarite sedimentation and drowned JCP together with the “bridge” with the Ammonitico rosso facies, characterized by several stratigraphic gaps. Rare calciturbidites are interbedded in areas near the DCP (southern SB and a drowned “bridge”). At the end of Jurassic, all areas north of the DCP show uniform sedimentation of the Biancone Limestone Formation.

Detrital rutile U-Pb geochronology of the Alpine convergence in the External Western Carpathians

Doliwa Zieliński Ludwik de, Potočný Tomáš, Bazarnik Jakub, Kooijman Ellen, Kośmińska Karolina, Mazur Stanisław, Majka Jarosław

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

84--84

The Carpathian Flysch Belt represents a Paleogene accretionary wedge (External Western Carpathians – EWC) located in front of the narrow Pieniny Klippen Belt zone and the Cretaceous Central Western Carpathian nappe stack. The Flysch Belt is formed of several nappes thrust over the slope of the European Platform in the Miocene. This study is focused on the uppermost Magura Nappe, which consists of the Rača, Bystrica and Krynica subunits. As there are no relics of pre-Miocene oceanic crust in the EWC, the sedimentary rocks of the Flysch Belt are the only source of information available about the Alpine collisional events. U-Pb geochronology was applied to detrital rutile from sandstones of the Magura Nappe in order to better understand the closure of the Alpine Tethys in the Western Carpathians. Ten medium-sized sandstone samples were collected across the Bystrica and Krynica subunits in the Nowy Targ region in southern Poland. The samples represent synorogenic clastic sediments with inferred deposition ages between the Late Cretaceous and Oligocene. Approximately 200 rutile grains were separated from each sandstone sample and around half of them were selected for further analyses. The age and appearance (shape, inclusions, zoning etc.) of the dated rutile show significant variations, suggesting derivation from various sources. The most prominent age peaks represent the Variscan (c. 400–280 Ma) and Alpine (c. 160–90 Ma) tectonic events which are well-pronounced in all but the oldest dated sample. It is also noteworthy that four distinct Alpine signals were detected in our rutile data set. The two most prominent peaks with ages of 137–126 Ma and 115–105 Ma are found in majority of the samples. In two sandstone samples, deposited between the Eocene –Oligocene and the Late Cretaceous–Paleocene, the youngest peak of 94–90 Ma appears. Another peak of 193–184 Ma is also present in these two samples, as well as in another sandstone deposited between the Paleocene and the Eocene. In addition, most samples show few Proterozoic ages (approx. 1770 Ma, 1200 Ma, 680 Ma and 600 Ma). Since metamorphic rutile requires relatively high pressure to crystallize, its formation in the course of an orogeny is possible in a subduction setting. Hence, our new age data may reflect tectonic events related to subduction of oceanic crust and overlying sediments. Tentatively, we propose that recognizable events include the Jurassic subduction of the Meliata Ocean (~180–155 Ma), the Early Cretaceous thrust stacking of the Veporic and Gemeric domains (140–105 Ma) and possibly the Late Cretaceous subduction of the Váh Ocean (c. 90 Ma). In addition to dating, the Zr content of the rutile formed during the Alpine orogeny was measured by electron microprobe at the AGH University in Krakow. The amount of Zr varies between 37–420 ppm in almost all grains, with the exception of 4 rutile grains where ~1100 ppm was reached. The Zr in rutile thermometer, based on the approach of Kohn (2020) was used to calculate the possible metamorphic conditions at 450–650°C and >7.5 kbar. This data set corroborates formation of the Alpine rutile under relatively high pressure and rather low to moderate pressure/temperature gradient, i.e. typical of subduction-related tectonic environments.