The impact of the Bohemian Spur on the cooling and exhumation pattern of the Eastern Alpine wedge

Fold and thrust belt dynamics and architecture may largely be impacted by the geometry of the overridden basement. The Bohemian Spur, the subcrop extension of the Bohemian massif, guided thrust propagation leading to the arcuate shape of the orogen and a narrowing of the Molasse Basin at the transition to the between the W-E trending Eastern Alps and the SW-NE trending Western Carpathians. Thermochronological studies in the Eastern Alps were mainly focused on the core of the collisional orogen, where deformation has been most prominent. Further to the east, some FT work is concentrated along fault zones but thermochronometers with lower closure temperatures have hardly been applied to higher elements of the nappe pile. Due to the scarcity of the dataset and preferential application of fission track dating uppermost crustal cooling below ca. 80 °C remains undetected. In this study we present new apatite (U-Th)/He and apatite fission track data from clastic units of the Rhenodanubian Flysch zone and the Northern Calcareous Alps. We find reset ages, that monitor a so far un(der)appreciated phase of prominent Late Oligocene to Miocene cooling. Thermal modeling of age data from the flysch samples reveals rapid Early Miocene cooling at rates of up to 40 °C/Ma between ca. 20 and 15 Ma. We propose a buttressing effect of the underlying tectonically structured eastern rim of the Bohemian Spur to be the driving mechanism for this phase of intensified exhumation. Our tectonic model (Fig. 1a) invokes contractional reactivation of pre-existing normal faults inherited from Penninic continental rifting. This positive inversion led to the shortening of the Jurassic half-graben infill and its extrusion as a major fold. Thermochronological data and thermal modeling of data from samples in the Lunz nappe of the Northern Calcareous Alps nappe pile indicate less punctuated cooling and exhumation. Modeling defines an increase of cooling rates at the latest at ca. 27 to 25 Ma, i.e., earlier than in the Flysch samples. Cooling occurred at a much lower rate of 3 to 6 °C/Ma and was synchronous with northward movement of the deformation front. In our tectonic model (Fig. 1b), we propose a staircase pattern that influences wedge dynamics: The topographically segmented downgoing plate leads to less localized and more distributed deformation invoking a broader area of uplift than the spatially focused uplift of the Flysch samples. Wedge propagation is initially inhibited or retarded by the relief of the basement. The ongoing northward movement of the propagating wedge is compensated through deep duplexing of the autochthonous foreland sequence. When calling upon deep-seated processes to explain the exhumation pattern the buttressing effect needs to be taken into account. Early Miocene drainage pattern reorganization in the Molasse Basin is proposed to be a consequence of uplift induced by the subcrop promontory

The Oligocene Reifnitz tonalite (Austria) and its host rocks : implications for Cretaceous and OligoceneNeogene tectonics of the south-eastern Eastern Alps

In the south-eastern Eastern Alps, the Reifnitz tonalite intruded into the Austroalpine metamorphic basement of the Wörthersee half-window exposed north of the SarmatianPliocene flexural Klagenfurt basin. The Reifnitz tonalite is dated for the first time, and yields a laser ICP-MS UPb zircon age of 30.720.30 Ma. The (UThSm)/He apatite age of the tonalite is 27.6 1.8 Ma implying rapid Late Oligocene cooling of the tonalite to ca. 60 C. The Reifnitz tonalite intruded into a retrogressed amphibolite-grade metamorphic basement with a metamorphic overprint of Cretaceous age (40Ar/39Ar white mica plateau age of 90.7 1.6 Ma). This fact indicates that pervasive Alpine metamorphism of Cretaceous age extends southwards almost up to the Periadriatic fault. Based on the exhumation and erosion history of the Reifnitz tonalite and the hosting Wörthersee half window formed by the Wörthersee anticline, the age of gentle folding of Austroalpine units in the south-eastern part of the Eastern Alps is likely of Oligocene age. North of the Wörthersee antiform, Upper CretaceousEocene, Oligocene and Miocene sedimentary rocks of the Krappfeld basin are preserved in a gentle synform, suggesting that the top of the Krappfeld basin has always been near the Earths surface since the Late Cretaceous. The new data imply, therefore, that the Reifnitz tonalite is part of a post-30 Ma antiform, which was likely exhumed, uplifted and eroded in two steps. In the first step, which is dated to ca. 3127 Ma, rapid cooling to ca. 60 C and exhumation occurred in an EW trending antiform, which formed as a result of a regional NS compression. In the second step of the SarmatianPliocene age a final exhumation occurred in the peripheral bulge in response to the lithospheric flexure in front of the overriding North Karawanken thrust sheet. The Klagenfurt basin developed as a flexural basin at the northern front of the North Karawanken, which represent a transpressive thrust sheet of a positive flower structure related to the final activity along the Periadriatic fault. In the Eastern Alps, on a large scale, the distribution of Periadriatic plutons and volcanics seems to monitor a northward or eastward shift of magmatic activity, with the main phase of intrusions ca. 30 Ma at the fault itself.(VLID)268606

Polyphase exhumation in the western Qinling Mountains, China: Rapid Early Cretaceous cooling along a lithospheric-scale tear fault and pulsed Cenozoic uplift

The western sector of the Qinling–Dabie orogenic belt plays a key role in both Late Jurassic to Early Cretaceous “Yanshanian” intracontinental tectonics and Cenozoic lateral escape triggered by India–Asia collision. The Taibai granite in the northern Qinling Mountains is located at the westernmost tip of a Yanshanian granite belt. It consists of multiple intrusions, constrained by new Late Jurassic and Early Cretaceous U–Pb zircon ages (156 ± 3 Ma and 124 ± 1 Ma). Applying various geochronometers (40Ar/39Ar on hornblende, biotite and K-feldspar, apatite fission-track, apatite [U–Th–Sm]/He) along a vertical profile of the Taibai Mountain refines the cooling and exhumation history. The new age constraints record the prolonged pre-Cenozoic intracontinental deformation as well as the cooling history mostly related to India–Asia collision. We detected rapid cooling for the Taibai granite from ca. 800 to 100 °C during Early Cretaceous (ca. 123 to 100 Ma) followed by a period of slow cooling from ca. 100 Ma to ca. 25 Ma, and pulsed exhumation of the low-relief Cretaceous peneplain during Cenozoic times. We interpret the Early Cretaceous rapid cooling and exhumation as a result from activity along the southern sinistral lithospheric scale tear fault of the recently postulated intracontinental subduction of the Archean/Palaeoproterozoic North China Block beneath the Alashan Block. A Late Oligocene to Early Miocene cooling phase might be triggered either by the lateral motion during India–Asia collision and/or the Pacific subduction zone. Late Miocene intensified cooling is ascribed to uplift of the Tibetan Plateau

The Oligocene Reifnitz tonalite (Austria) and its host rocks: implications for Cretaceous and Oligocene–Neogene tectonics of the south-eastern Eastern Alps

In the south-eastern Eastern Alps, the Reifnitz tonalite intruded into the Austroalpine metamorphic basement of the Wörthersee half-window exposed north of the Sarmatian–Pliocene flexural Klagenfurt basin. The Reifnitz tonalite is dated for the first time, and yields a laser ICP-MS U–Pb zircon age of 30.72±0.30 Ma. The (U–Th–Sm)/He apatite age of the tonalite is 27.6 ± 1.8 Ma implying rapid Late Oligocene cooling of the tonalite to ca. 60 °C. The Reifnitz tonalite intruded into a retrogressed amphibolite-grade metamorphic basement with a metamorphic overprint of Cretaceous age (40Ar/39Ar white mica plateau age of 90.7 ± 1.6 Ma). This fact indicates that pervasive Alpine metamorphism of Cretaceous age extends southwards almost up to the Periadriatic fault. Based on the exhumation and erosion history of the Reifnitz tonalite and the hosting Wörthersee half window formed by the Wörthersee anticline, the age of gentle folding of Austroalpine units in the south-eastern part of the Eastern Alps is likely of Oligocene age. North of the Wörthersee antiform, Upper Cretaceous–Eocene, Oligocene and Miocene sedimentary rocks of the Krappfeld basin are preserved in a gentle synform, suggesting that the top of the Krappfeld basin has always been near the Earth’s surface since the Late Cretaceous. The new data imply, therefore, that the Reifnitz tonalite is part of a post-30 Ma antiform, which was likely exhumed, uplifted and eroded in two steps. In the first step, which is dated to ca. 31–27 Ma, rapid cooling to ca. 60 °C and exhumation occurred in an E–W trending antiform, which formed as a result of a regional N–S compression. In the second step of the Sarmatian–Pliocene age a final exhumation occurred in the peripheral bulge in response to the lithospheric flexure in front of the overriding North Karawanken thrust sheet. The Klagenfurt basin developed as a flexural basin at the northern front of the North Karawanken, which represent a transpressive thrust sheet of a positive flower structure related to the final activity along the Periadriatic fault. In the Eastern Alps, on a large scale, the distribution of Periadriatic plutons and volcanics seems to monitor a northward or eastward shift of magmatic activity, with the main phase of intrusions ca. 30 Ma at the fault itself

Holocene sediments from the Southern Chile Trench: a record of active margin magmatism, tectonics and palaeoseismicity

Sedimentology, petrography and the provenance of Holocene sediments from the Southern Chile Trench (36–47°S) were investigated in an integrated approach combining description of a collection of gravity cores, measurements of physical properties, quantitative X-ray petrography and modal analysis. The sediments studied were trench hemipelagic sediments, fan deposits, and more distal hemipelagic sediments from the Nazca Plate. The trench is mostly fed by multiple point sources via submarine canyons. Sandy turbidites show a southward increase in sediment maturity. Whereas volcanic lithic fragments and plagioclase represent the dominant fraction in the north, quartz content strongly increases in the southern part of the study area, in line with source lithologies. Further north, active volcanoes in the Main Cordillera represent almost the entire provenance signal as a result of a strong contribution of highly erodible volcanic rocks. Recurrence rates of sandy and silty turbidites in the trench fan sediments indicate a link to the palaeoseismic record on land. Our study documents the potential usefulness of proximal turbidites to reconstruct palaeoseismicity, even at a scale of single segments of the plate boundary

Postcollisional cooling history of the Eastern and Southern Alps and its linkage to Adria indentation

Indentation of rigid blocks into rheologically weak orogens is generally associated with spatiotemporally variable vertical and lateral block extrusion. The European Eastern and Southern Alps are a prime example of microplate indentation, where most of the deformation was accommodated north of the crustal indenter within the Tauern Window. However, outside of this window only the broad late-stage exhumation pattern of the indented units as well as of the indenter itself is known. In this study we refine the exhumational pattern with new (U–Th–Sm)/He and fission-track thermochronology data on apatite from the Karawanken Mountains adjacent to the eastern Periadriatic fault and from the central-eastern Southern Alps. Apatite (U–Th–Sm)/He ages from the Karawanken Mountains range between 12 and 5 Ma and indicate an episode of fault-related exhumation leading to the formation of a positive flower structure and an associated peripheral foreland basin. In the Southern Alps, apatite (U–Th–Sm)/He and fission-track data combined with previous data also indicate a pulse of mainly Late Miocene exhumation, which was maximized along thrust systems, with highly differential amounts of displacement along individual structures. Our data contribute to mounting evidence for widespread Late Miocene tectonic activity, which followed a phase of major exhumation during strain localization in the Tauern Window. We attribute this exhumational phase and more distributed deformation during Adriatic indentation to a major change in boundary conditions operating on the orogen, likely due to a shift from a decoupled to a coupled system, possibly enhanced by a shift in convergence direction.ISSN:1437-3254ISSN:1437-326