Franz Neubauer

Evolution of late Neoproterozoic to early Paleozoic tectonic elements in Central and Southeast European Alpine mountain belts: review and synthesis

Abstract Late Neoproterozoic to early Paleozoic, Cadomian tectonic elements are widespread in the southeastern Alpine–Mediterranean mountain belts, with discontinuous exposure extending from the Alps to the Menderes Massif in Turkey. The sequences include voluminous plutonic island and continental arc successions, some ophiolites of variable, late Neoproterozoic and Cambrian to early Ordovician ages, high- to medium-grade metamorphic sequences and subordinate metasediments. Geological and geochronological data suggest that these Cadomian elements partly experienced tectonothermal activity including metamorphism and magmatism between ca. 650 and 600 Ma, followed by further I- and S-type plutonism in an Andean-type continental margin setting ca. between 570 and 520 Ma. Subsequently, a major rift zone developed which resulted in continental stretching and subsequent formation of a back arc basin which is dated as late Cambrian. Rifting is followed by oceanic spreading which commenced ca. at the Cambrian/Ordovician boundary, possibly in a back-arc setting. This tectonic scenario suggests that Alpine–Mediterranean Cadomian tectonic elements were accreted to Gondwana during early Cadomian events (ca. 600–650 Ma). Subsequently, they were likely part of a long-lasting ‘outer’ subduction zone of Gondwana at the margin of a Panthalassa-type ocean during late Neoproterozoic III and Cambrian. Due to back-arc spreading, continental pieces started to split off from Gondwana ca. at the Cambrian–Ordovician boundary.

Late Cretaceous exhumation of the metamorphic Gleinalm dome, Eastern Alps: kinematics, cooling history and sedimentary response in a sinistral wrench corridor

Abstract The metamorphic Gleinalm dome, Eastern Alps, was uplifted and exhumed within a releasing structure in a sinistral wrench corridor during the Late Cretaceous. The dome is confined by a system of ductile shear zones including low-angle normal faults and steep sinistral tear faults which define a large releasing structure with the metamorphic dome in its center. The fabrics developed within all ductile shear zones record processes which were operating during decreasing temperatures from initial epidote-amphibolite/upper greenschist facies conditions (with crystal plastic fabrics in quartz) to temperatures below ca. 300°C (with predominantly cataclastic fabrics). A cooling path based on 40Ar/39Ar amphibole (95.4 ± 1.2 Ma) and muscovite ages (87.6 ± 0.6; 84.3 ± 0.7 Ma) together with sphene, zircon and apatite fission track data indicate cooling through ca. 500°C at ca. 94 Ma to below ca. 250-200°C at 65 Ma. Subsidence of the adjacent Late Cretaceous Kainach Gosau basin occurred synchronously with cooling and uplift of the Gleinalm dome. Internal depositional patterns record rapid subsidence at the time of cooling with internal synsedimentary block rotation above an intra-crustal ductile normal fault. The sinistral wrench corridor of the Eastern Alps developed by sinistral displacement of the Austroalpine units against a relatively stable Europe during the Late Cretaceous.

Cretaceous to Cenozoic thermal evolution of the southwestern South Carpathians: evidence from fission-track thermochronology

Abstract The southwestern South Carpathian orogen is composed of various nappe complexes which were assembled during the Cretaceous–Cenozoic orogeny. These are from footwall to hangingwall: (1) the Danubian nappe complex including a Cadomian/Variscan basement; (2) the Arjana and Severin units with Jurassic to Early Cretaceous rift and oceanic sequences; and (3) the Getic nappe complex with Variscan continental basement. Fission track (FT) thermochronology on apatite, zircon and sphene from samples collected from various units of the South Carpathians, in conjunction with field constraints and previous geochronology is used to characterise the Alpine tectonic events and to restore the pattern and amount of exhumation since the Cretaceous. Zircon from the flysch unit and the Danubian Liassic cover sequence yields FT ages around 200 Ma suggesting cooling of the rift flanks prior to the opening of the Severin rift. Zircon and sphene from the Getic and Danubian basement units yield FT ages averaging 110 Ma and indicating cooling under 240°C of the basement contemporaneous with, or postdating thrusting. Apatite FT ages display a decreasing age trend from the hangingwall (65 Ma) to the footwall units (30 Ma). The age data and corresponding horizontal confined track length distributions suggest that exhumation of the nappe pile occurred in two stages: the first is related to the Late Cretaceous nappe stacking and the second one to the final thrusting of the South Carpathians onto the top of the Moesian platform. Apatite FT ages along major brittle wrench faults indicate reheating above ca. 120°C during fluid flow associated with fault (re)activation during Oligocene and Neogene times. Thus, shear zone rocks experienced a higher temperature overprint during Cenozoic time than rocks of the unaffected nappe pile. Temperatures of hydrothermal flow along these zones decreased below 100°C progressively starting with the Late Oligocene–Early Miocene when the area began to override the Moesian platform.

Chronology of late Paleozoic tectonothermal activity in the southeastern Bohemian Massif, Austria (Moldanubian and Moravo-Silesian zones): 40Ar/39Ar mineral age controls

Abstract New 40Ar/39Ar hornblende and muscovite ages from the Moldanubian and Moravian zones of the southeastern Bohemian Massif indicate rapid cooling associated with continental underplating during Early Carboniferous plate collision and closure of the Moravo-Silesian foredeep. 40Ar/39Ar ages have been determined for hornblende concentrates from: (1) amphibolite from the Moldanubian Nappe Complex (including the Dobra gneiss, Rehberg amphibolite and Loosdorf complex: all part of the “Variegated unit”); (2) micaschist structurally overlying the Bittesch gneiss; and (3) amphibolite within the Bittesch gneiss (Moravian Zone). The five hornblende concentrates display variably discordant 40Ar/39Ar age spectra because of contamination with extraneous argon components. However, 36Ar/40Ar vs. 39Ar/40Ar isotope correlations are well defined for the five hornblende concentrates (MSWD The hornblende and muscovite ages are interpreted to date post-Variscan metamorphic cooling through ca. 500°C and ca. 400°C, respectively. No record of pre-Variscan thermal events is preserved in either the hornblende or muscovite argon systems; however, the widespread occurrence of extraneous argon components suggest a pre-Variscan stage of tectonothermal development. The near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures, and indicate that relatively high crustal levels had likely been attained during the Early Carboniferous.

The significance of Gosau-type basins for the Late Cretaceous tectonic history of the Alpine-Carpathian Belt.

Abstract A key feature of Late Cretaceous tectonics throughout the Alpine-Carpathian-Pannonian (ALCAPA) region is the synchronous formation of sedimentary basins (Gosau basins) and exhumation of metamorphic domes. Initial subsidence, spatially varying in time (Cenomanian-Santonian), within Gosau-type basins is associated with the development of a fluvial-lacustrine to shallow marine environment and the deposition of conglomerates, sandstones, coal-bearing marls and rudist limestones were deposited. The progressive deepening of the marine basins is documented by a second subsidence pulse during the Campanian-Early Maastrichtian leading to the establishment of an open marine environment. Gosau basins on top of the Northern Calcareous Alps and equivalent nappes of the Western Carpathians which were located at or close to the northern to northwestern active margin of the Austroalpine realm (external basins) locally subsided below the CCD. In contrast a distinctly shallower water environment prevailed in Gosau basins in central areas of the actively evolving Alpine-Carpathian mountain chain (internal basins). Deposition of flysch-type deposits is common for the deep-water facies. Subsidence analysis of internal Gosau basins were performed and their mutual relationship to coevally exhuming metamorphic domes, documented by a number of geochronologic data, is emphasised. A compilation of these data revealed a diachronic evolution of the ALCAPA region. Major vertical movements post-dating nappe imbrication and metamorphism started first in the Tisza-Dacia related orogenic compartments (East-, South Carpathians and Apuseni Mts.) as early as Late Aptian, whereas exhumation and subsequent cooling of metamorphic crust in the East Alpine-West Carpathian domain occurs from the Cenomanian onward. This characteristic basement-basin relationship suggests a strong coupling between lithospheric and surface processes, largely controlled by the rheology of the orogenic system. Formation of internal Gosau basins is seen in context with collapse of thickened and gravitationally unstable continental crust.

Deformation-induced resetting of Rb/Sr and 40Ar/39Ar mineral systems in a low-grade, polymetamorphic terrane (Eastern Alps, Austria)

A paradox in dating metamorphic events in low-grade, polymetamorphic terranes is exemplified by the eastern Lower Austroalpine nappes of the Eastern Alps. Here, the last metamorphic event, best recorded in post-Variscan cover rocks, is dated as Late Cretaceous in age (c. 80–85 Ma) by white mica Rb/Sr and 40Ar/39Ar systems. Within the underlying polymetamorphic basement, 40Ar/39Ar and Rb/Sr ages of phengitic white mica record only Early and/or Late Variscan ages (375–270 Ma), indicating that the Alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of Variscan mineral ages. Based on these results, the timing of a penetrative, ductile, top–to-WNW simple shear deformation recorded within both basement and cover rocks was contradictory. Deformation within the post-Variscan cover rocks had to be Alpine in age, whereas phengite 40Ar/39Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63–30 and 30–10 μm) from highly strained shearbands within the Wechsel basement nappe. A resulting Rb/Sr errorchron pointed to an Upper Cretaceous age (c. 85 Ma) for the deformation, consistent with the timing of Alpine metamorphism in the cover rocks. Coarser-grained white mica (150–300 μim) from similar basement mylonites do not reflect any Alpine overprint of either K/Ar and/or Rb/Sr systems. It follows that dynamic re-and/or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the Rb/Sr system locally virtually re-equilibrated. This is valid even for overprinting metamorphic conditions below the temperatures required for Ar diffusional loss in phengitic white mica (c. <350°C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica 40Ar/39Ar) should be considered with caution.

Orogen-parallel strike-slip faults bordering metamorphic core complexes: the Salzach–Enns fault zone in the Eastern Alps, Austria

Abstract The approximately ENE-trending Salzach–Enns fault (Eastern Alps) contributed to late-stage exhumation of the Tauern metamorphic core complex in response to oblique convergence of European and Adriatic plates. Six stages of kinematic evolution with early ductile and later brittle deformational structures were recognized as follows: (1) Initial ductile deformation is represented by a combination of sinistral shear-dominated noncoaxial strain and coaxial strain. (2) Early brittle sinistral strike-slip is combined with a pure shear deformation and represents roughly N–S contraction. It is followed by (3) sinistral strike-slip and (4). ca. N–S extension due to normal-slip striations. (5) Subsequent N–S compression resulted in counterclockwise rotation of the maximum principal stress from NE to NW and led to final dextral strike-slip displacements along the Salzach–Enns fault. The exhumation of the Tauern Window was accommodated by dip-slip on the fault plane at shallow levels and coaxial flattening at deeper structural levels. Brittle sinistral faults at shallow structural levels produced a larger amount of strike-slip displacement than ductile shear zones at the deeper structural levels, which is explained as a scissors-like movement and which resulted in the eastward tilting of the Tauern Window.

Thick-skinned versus thin-skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic - Austria)

The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low-grade metamorphic conditions, and (3) frontal, thin-skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick-skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin-skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal-scale ramps during foreland-directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal-scale rheological con-trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (>610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I-type affinities (∼ 590 Ma).

Contrasting Late Cretaceous with Neogene ore provinces in the Alpine-Balkan-Carpathian-Dinaride collision belt

Abstract Internal sectors of the Alpine-Balkan-Carpathian-Dinaride (ABCD) orogen comprise fundamentally different ore deposits along strike in three temporally and spatially distinct belts. These were formed by several short-lived, late-stage collisional processes (including slab break-off) during the Late Cretaceous and Oligocene to Neogene times. Reconstruction of Late Cretaceous (c. 92–65 Ma) collisional structures, magmatic features and mineralization reveals contrasting variations along strike, including the following. (1a) The Late Cretaceous ‘banatite’ magmatic belt, which extends from the Apuseni mountains to the Balkans, associated mainly with porphyry Cu-Au, massive sulphide and Fe-Cu skarn mineral deposits. In respect to their country rocks and geodynamic setting, the magmatism is interpreted to represent either post-collisional or Andean-type calc-alkaline due to continuous subduction or break-off of the subducted lithosphere. (1b) The Alpine-West Carpathian sector, which is characterized by strong Late Cretaceous metamorphic/deformational overprint, lack of magmatism and both syn- and late-orogenic formation of metasomatic and metamorphogenic talc, magnesite, siderite and vein- and shear zone-type Cu and As-Au due to the exhumation of metamorphic core complexes. (2a) The Oligocene-Miocene Serbomacedonian-Rhodope metallogenic zone extends across several structural units from the Bosnian Dinarides to the Rhodopes and to Thrace. It includes both a belt with volcanic-hosted and vein-type Pb-Zn deposits and a belt of porphyry Cu-Au-Mo and epithermal Au mineralization, which is more common in the south. Both belts appear to relate to microcontinent collision and associated subsequent magmatism, again possibly due to slab break-off. (2b) Different types of mineralization were also formed along the internal Inner Carpathian and Alpine sectors during Late Oligocene to Miocene collision. In the Alps, mineralization formed due to eastward extrusion of fault-bounded blocks into the Carpathian arc. Associated mineral deposits are always related to exhumation of metamorphic core complexes and include: sub-vertical mesothermal Au-quartz veins and replacement As-Ag-Cu ore bodies within the metamorphic core complex, fault-bounded mineralization (Pb-W-Au) along low-angle ductile normal faults along the upper margins of the metamorphic core complex, mineralized (Sb-Au) strike-slip faults and sub-vertical Au-Ag-Sb-bearing tension veins. (2c) In contrast, nearly all Miocene ore deposits within the Carpathians are related to volcanic activity contemporaneous with the invasion of fault-bounded blocks into the Carpathian arc. These have been related to slab break-off and cessation of subduction. Mineral deposits include structurally controlled Au-Sb-Cu-Pb-Zn ore bodies within shallow volcanic edifices, with a preference for steep tension veins parallel to the motion direction of laterally escaping crustal blocks.

40Ar/39Ar muscovite ages from the Penninic‐Austroalpine plate boundary, Eastern Alps

The Radstadt Mountains, Eastern Alps, expose the tectonic boundary between the base of the Austroalpine continental plate (hanging wall) and the Penninic oceanic units (foot wall). Parts of the Austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the Permian (290–250 Ma). Austroalpine Permo-Mesozoic cover rocks were deformed during the Cretaceous (∼80 Ma) and the Paleogene (55–50 Ma). These ages are interpreted as representing two distinct events of nappe stacking. The younger ages represent the collision of Austroalpine and Penninic tectonic units. The Penninic nappe complex displays a successive decrease of ages from ∼37 to 25 Ma from high to deep tectonic levels. A second age group of ∼22 Ma was found both in low-temperature release steps and as plateau ages close to the Penninic-Austroalpine boundary. It is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge.