Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs

The different segments of the tectonic boundary between external (European) and internal (Penninic) units in the Western Alps, the so-called Penninic Front (PF), formed at different times and according to different kinematic scenarios. During a first episode (Eocene), the PF corresponds to a transpressive suture zone between Penninic and European units. North- to NNW-trending stretching lineations, found along internal nappe contacts within the Penninic units, are related to this episode. This subduction zone was sealed by the Priabonian flysch of the Aiguilles d'Arves, a detrital trench formation that formed during the final stages of subduction. During a second episode, starting in mid-Oligocene times, the PF, imaged along the ECORS-CROP profile, acted as a WNW-directed thrust. This thrust, the Roselend Thrust (RT), only partially coincides with the PF. South of Moûtiers, the RT propagates into the Dauphinois units, carrying the former Eocene PF (including the Priabonian flysch) passively in its hangingwall. South of the Pelvoux massif the RT finds its continuation along the "Briançonnais Front", an out-of-sequence thrust behind the Embrunais-Ubaye nappes. On a larger scale, our findings indicate oblique (sinistral) collision within the future Western Alps during the Eocene, followed by westward indentation of the Adriatic bloc

Reply to Comment by W. Kurz on "Tectonic map and overall architecture of the Alpine orogen”

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Structural and metamorphic evolution of the Camughera - Moncucco, Antrona and Monte Rosa units southwest of the Simplon line,Western Alps

Abstract.: This structural and petrological study examines relationships between Alpine deformation and metamorphism in the Camughera-Moncucco, Antrona and northeastern Monte Rosa units, and it correlates major late stage deformation structures, such as the Vanzone antiform and Simplon normal fault. D1/D2 deformation and related top-N or top-NW thrusting started under high-pressure conditions (12.5-16kbar) at relatively high temperatures (c. 620-700°C). Petrological and structural data, together with published radiometric data, suggest that top-SE shearing within the structural top of the high-pressure units in the upper Penninic Alps is coeval with top-N or top-NW thrusting at their structural base. This suggests differential ascent of high-pressure units relative to the surrounding units during nappe stacking and associated crustal shortening. Barrovian metamorphism in the upper Penninic Alps is related to a first phase of backfolding, active between c. 35Ma and c. 29-26Ma ago (D3). D3 involves dextral shearing, combined with top-WSW shearing and orogen-parallel extension. Unroofing by orogen-parallel extension delays cooling during decompression and leads to isothermal decompression after the high-pressure stage. Towards deeper structural levels in the east, D3 deformation becomes progressively younger and prevailed at increasingly higher temperatures. Hence, compared to the higher structural levels further west, the brittle-ductile transition occurs later in the Ossola valley. Masera synform and Brevettola antiform form an open fold pair and represent the eastern continuation of the major Vanzone antiform (D4). Normal faulting across the Simplon line overlaps in time with the formation of these major D4 backfolds. Displacement along the Simplon normal fault decreases to insignificant values towards the southeast. Contemporaneous dextral shearing, however, occurs within the southern limbs of the Vanzone and Brevettola antiform

The thermal history of the Miocene Ibar Basin (Southern Serbia): new constraints from apatite and zircon fission track and vitrinite reflectance data

The Ibar Basin was formed during Miocene large scale extension in the NE Dinaride segment of the AlpineCarpathian-Dinaride system. The Miocene extension led to exhumation of deep seated core-complexes (e.g. Studenica and Kopaonik core-complex) as well as to the formation of extensional basins in the hanging wall (Ibar Basin). Sediments of the Ibar Basin were studied by apatite and zircon fission track and vitrinite reflectance in order to define thermal events during basin evolution. Vitrinite reflectance (VR) data (0.63—0.90 %Rr) indicate a bituminous stage for the organic matter that experienced maximal temperatures of around 120—130 °C. Zircon fission track (ZFT) ages indicate provenance ages. The apatite fission track (AFT) single grain ages (45—6.7 Ma) and bimodal track lengths distribution indicate partial annealing of the detrital apatites. Both vitrinite reflectance and apatite fission track data of the studied sediments imply post-depositional thermal overprint in the Ibar Basin. Thermal history models of the detritial apatites reveal a heating episode prior to cooling that began at around 10 Ma. The heating episode started around 17 Ma and lasted 10—8 Ma reaching the maximum temperatures between 100—130 °C. We correlate this event with the domal uplift of the Studenica and Kopaonik cores where heat was transferred from the rising warm footwall to the adjacent colder hanging wall. The cooling episode is related to basin inversion and erosion. The apatite fission track data indicate local thermal perturbations, detected in the SE part of the Ibar basin (Piskanja deposit) with the time frame ~7.1 Ma, which may correspond to the youngest volcanic phase in the region

Low-temperature thermochronology of the flanks of the southern Upper Rhine Graben

The Upper Rhine Graben (URG) is the most perceptible part of the European Cenozoic Rift System. Uplifted Variscan basement of the Black Forest and the Vosges forms the flanks of the southern part of the graben. Apatite and zircon fission-track (FT) analyses indicate a complex low-temperature thermal history of the basement that was deciphered by inverse modelling of FT parameters. The models were tested against the observed data and independent geological constraints. The zircon FT ages of 28 outcrop samples taken along an E-W trending transect across the Black Forest and the Vosges range from 136 to 312Ma, the apatite FT ages from 20 to 83Ma. The frequency distributions of confined track lengths are broad and often bimodal in shape indicating a complex thermal history. Cooling below 120°C in the Early Cretaceous to Palaeogene was followed by a discrete heating episode during the late Eocene and subsequent cooling to surface temperature. The modelled time-temperature (t-T) paths point to a total denudation of the flanks of URG in the range of 1.0-1.7km for a paleogeothermal gradient of 60°C/km, and 1.3-2.2km for a paleogeothermal gradient of 45°C/km since the late Eocen

The low-temperature thermal history of northern Switzerland as revealed by fission track analysis and inverse thermal modelling

Abstract.: New zircon and apatite fission track (FT) data from four boreholes, which penetrate the Mesozoic and pre-Mesozoic sediments and crystalline basement of northern Switzerland, are presented. Inverse thermal modelling of the measured apatite FT parameters unravels the low-temperature (below ~120°C) thermal history of the crystalline basement of northern Switzerland. Zircon FT central and single-grain ages cluster around 250Ma, thus maximum palaeotemperatures did not exceed ∼ 330°C after late-Variscan consolidation of the crystalline basement. Apatite FT central ages vary between 25 and 87Ma. Confined mean track lengths range between 9.3μm and 11.6μm, suggesting substantial track annealing within all apatite samples. Modelled time-temperature paths offer a clear picture about the low-temperature thermal history of the crystalline basement of northern Switzerland: Cretaceous cooling is followed by an Eocene heating event and subsequent cooling to present-day temperatures. The Eocene heating episode is contemporaneous with the initial rifting stage of the nearby Upper Rhine Graben and the associated increasing volcanic activity. Crustal-scale faults of the Permo-Carboniferous Trough of northern Switzerland could have acted as major pathways for circulating hydrothermal fluids giving rise to the observed Middle to Late Eocene thermal even

The contact zone between the ALCAPA and Tisza-Dacia mega-tectonic units of Northern Romania in the light of new paleomagnetic data

Abstract.: Paleomagnetic analyses were carried out on samples from 19 localities within two different mega-tectonic units in Northern Romania: Tisza-Dacia (11 localities) and ALCAPA (8 localities). The samples cover a range of different lithologies: (1) Late Cretaceous red-coloured marl to marly limestone, (2) Eo-Oligocene flysch sediments, and (3) mid-Miocene (Langhian) tuffite (Dej tuff and related sediments). The Late Cretaceous and mid-Miocene specimens carry secondary paleomagnetic signals exhibiting a counter clockwise deflection of the paleo-declinations by some 30°, while the Eo-Oligocene localities indicate an overall clockwise deflected (between some 45° and >90°) paleodeclination with respect to present-day north. Clockwise rotation postdates the age of sedimentation (Lower Oligocene), as well as (at least partially) thrusting of the Pienides onto the Tisza-Dacia mega-tectonic unit, which occurred between 20.5 and 18.5 Ma. Clockwise rotation predates post-12 Ma counter clockwise rotations inferred for the mid-Miocene localities. Surprisingly, the clockwise rotations of the first rotational stage not only affected the (par-) autochthonous sedimentary cover of the Tisza-Dacia megatectonic unit, but also the allochthonous flysch nappes of the Pienides, i.e. the eastern tip of the ALCAPA mega-tectonic unit. Well-documented opposed rotation of the remainder of ALCAPA necessitates a detachment of this eastern tip of ALCAPA after 18.5 Ma. The most likely location for this detachment zone is along the margins of the Transcarpathian depression. During a second (post-12 Ma) stage, counter clockwise rotations of up to 30° affected the entire working area. Regarding timing and magnitude, these second stage rotations are similar to rotations documented for the East Slovak basin, but different from those reported from the South Apuseni Mountains and the Central and Inner West Carpathians located west of the East Slovak basi

The timing of polyphase Miocene tectonics in Northern Romania

This study addresses the polyphase Miocene tectonic evolution in the Maramures area (northern Romania) by combining field observations, stratigraphic arguments and fission-track analysis (Tischler et al. in press). Fission-track analysis has been carried out on basement samples from the Rodna horst, situated in the East Carpathians (Bucovinvian nappes). This area was affected by Cretaceous medium- to low-grade metamorphism, followed by post-collisional exhumation and renewed moderate thermal overprint due to the deposition of Eocene to Early Miocene sediments. Based on paleostress analyses of mesoscale structures, three main tectonic phases can be disdinguished in the study area, all of which are postdate the earliest Miocene (Aquitanian, 20.5 Ma). In late Early Miocene (Burdigalian) the Pienide nappes, nonmetamorphic flysch series, were emplaced onto the Paleogene to Early Miocene sedimentary cover of the Bucovinian nappes...conferenc