Eclogite dating and subduction zones in the Alps

We applied Lu-Hf geochronology on eclogites from different tectonic units in the Penninic nappe stack of the Western Alps. Overall, older ages were found in structurally higher units of the nappe stack and younger ages in lower units, in accordance with a progression of subduction and accretion from originally more internal (SE) to external (NW) domains. The oldest age, 52,96 ± 0,91 Ma, was determined for Monte Emilius, a unit originally at the transition from the Cervinia Microcontinent (Sesia nappe) to the Zermatt-Saas Basin of the Piemont-Ligurian Ocean (Weber et al., 2021). Eclogites from the Zermatt-Saas Basin yielded ages of 49,79 ± 0,52 Ma (Champorcher), 47,98 ± 0,21 (Punta Nera), and 47,39 ± 0,34 Ma (Colle delle Finestre). A sample from the top of the Monte Rosa Nappe, a continental thrust sheet beneath the Zermatt-Saas Ophiolites, yielded 44,24 ± 0,83 Ma (Passo dei Salati). A significantly youger age (36,09 ± 0,59 Ma) was determined for an eclogitic meta-andesite from the ultrahigh-pressure Dora-Maira Nappe (locality Parigi), a continental thrust sheet thought to be a lateral equivalent of the Monte Rosa Nappe. This age accords well with a U-Pb zircon age of 35.4 ± 1.0 Ma from the same area (Gebauer et al., 1997) and confirms that this nappe records the youngest subduction-related metamorphism of the Western Alps. The ca. 36 Ma old Parigi UHP rocks come from the structurally lowermost unit of the Dora-Maira Nappe, whereas the ca. 44 Ma old Passo dei Salati eclogite mentioned above comes from the top of the Monte Rosa nappe. Therefore, the age difference may again reflect progradation of subduction and accretion from SE to NW and from higher to deeper units. In the Central and Eastern Alps, the youngest eclogites, ca. 37 and ca. 33 Ma old, occur in the distal parts of the former European margin (Adula Nappe and Eclogite Zone in the Tauern Window, respectively). This supports the derivation of Dora-Maira from the European margin

Jurassic ophiolites within the Valais domain of the Western and Central Alps: geochronological evidence for re-rifting of oceanic crust

Metabasic rocks from different parts of the Antrona ophiolites, Western Alps, as well as from the Misox zone, Central Alps, were dated using ion microprobe (SHRIMP) U-Pb analyses of zircon, in association with cathodoluminescence (CL) imaging. HP metamorphism must have affected at least the major part of the Antrona ophiolites, although HP relics are rarely preserved, probably due to the Lepontine metamorphic overprint. HP metamorphism has affected also the area of the Misox zone. The origin of the Antrona ophiolites is arguable. They were interpreted as part of both the Piemont-Ligurian (PL) and the Valais ocean, the two main oceans in the area of the Alps before Alpine convergence. SHRIMP-analyses of co-magmatic zircon domains from the Antrona ophiolites (Guggilihorn, Passo del Mottone and Quarata areas) yielded identical (within uncertainty) weighted mean206Pb/238U ages of 155.2±1.6Ma, 158±17Ma (or 163.1±2.4Ma: one analysis; 1σ error) and 155.6±2.1Ma, respectively, interpreted as the time of crystallization of the magmatic protoliths. These Late Jurassic ages fit well to the time span considered for the formation of Piemont-Ligurian oceanic crust. The metagabbro of the Misox zone (Hinterrhein area), for which a Valaisan origin is generally accepted, gave also a Late Jurassic, PL protolith age of 161.0±3.9Ma. The metamorphic zircon domains from the amphibolitized eclogite of Mottone yielded an age of 38.5±0.7Ma, interpreted as the time of HP metamorphism. This age is in good agreement with the time of metamorphism reported from previous zircon SHRIMP-data for eclogites and amphibolites of other parts in the Valais domain. In order to bring in line the PL protolith ages with the Valaisan metamorphic ages, we suggest a scenario involving emplacement of part of the PL oceanic crust to the north of the newly formed Briançonnais peninsula, inside the Valais geotectonic domain. This paleotectonic configuration was probably established when younger Valaisan oceanic crust formed by spreading and re-rifting, partly within PL oceanic crus

Elastic anisotropies of deformed crustal rocks in the Alps

The crust within collisional orogens is very heterogeneous, in composition as well as in type and intensity of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. We characterized the range of elastic anisotropies of deformed crustal rocks in the Alps according to the crystallographic preferred orientation (CPO) of their constituent mineral phases. In the Lago di Cignana area of the Italian Western Alps, we sampled eclogites, blueschists and greenschists of oceanic origin (Zermatt-Saas-Zone). From the lower crustal and mantle rocks of the Ivrea Zone near Finero metagabbros and marbles were collected. The Adula Nappe in the central Alps, which was intensely deformed during the Alpine orogeny, was sampled for rocks from a typical deformed upper continental crust within the Alps. The two major rock types, orthogneisses and paragneisses and small lenses of eclogites, amphibolites and marbles were sampled. CPOs of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. Similarities can be found within the mafic samples. The eclogite and amphibolite lenses within the continental crust, the eclogites and blueschists of oceanic origin, as well as the metagabbros of the lower crust exhibit highest P-wave velocity (VP) in lineation direction. An exception within the mafic samples is the greenschist, which shows a distribution of high VP within the foliation plane. P-wave anisotropy (AVP) of the metagabbros and eclogites is generally low with about 1.5%. The greenschist, blueschist and amphibolite samples show higher AVP of 2-4.5%. Marble also yields highest VP in lineation direction and high AVP of 8%. It can be distinguished from all other samples in the set by its high VP/VS ratio of 1.8. The felsic rocks of the continental crust show high variability. Those with high mica contents also show VP maxima in the foliation plane. They can be distinguished from greenschists, which show an average VP of 7.30 km/s, by their generally lower average VP of 6.18 - 6.81 km/s. To approximate an average for upper crustal rocks units, we picked common CPO types of rock forming minerals within gneiss samples of the Adula Nappe representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps yielding 4 %. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal or smaller scale seismic model

The Schwarzhorn Amphibolite (Eastern Ratikon, Austria): an Early Cambrian intrusion in the Lower Austroalpine basement

The Alpine nappe stack in the Penninic-Austroalpine boundary zone in the Ratikon (Austria) contains a 4x1 km tectonic sliver of meta-diorite, known as the Schwarzhorn Amphibolite. It was deformed and metamorphosed in the amphibolite facies and is unconformably overlain by unmetamorphic Lower Triassic sandstone, indicating pre-Triassic metamorphism. Cataclastic deformation and brecciation of the amphibolite is related to normal faulting and block tilting during Jurassic rifting. Zircon dating of the Schwarzhorn Amphibolite using LA-ICP-MS gave a U-Pb age of 529+9/-8 Ma, interpreted as the crystallization age of the protolith. Geochemical characteristics indicate formation of the magmatic protolith in a supra-subduction zone setting. The Cambrian protolith age identifies the Schwarzhorn Amphibolite as a pre-Variscan element within the Austroalpine basement. Similar calc-alkaline igneous rocks of Late Neoproterozoic to Early Cambrian age are found in the Upper Austroalpine Silvretta Nappe nearby and in several other Variscan basement units of the Alps, interpreted to have formed in a peri-Gondwanan active-margin or island-arc setting

Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

Agentúra na Podporu Výskumu a Vývoja, Grant/Award Number: APVV-18- 0107; Deutsche Forschungsgemeinschaft, Grant/ Award Number: FR700/18-1We thank Christopher Barnes (AGH University of Science and Technology, Kraków) for providing us with some of the studied samples. Kathrin Fassmer thanks Svenja Trapp and Matthias Hauke (University of Bonn) for help during Lu–Hf laboratory work. We would also like to thank M. Smit, F. Corfu and A. Kylander-Clark for their reviews which greatly contributed to improving the manuscript. This research was funded by DFG-Grant FR700/18-1 to N. F. and the Slovak Research and Development Agency project APVV-18- 0107 to M.J, and partially supported by the National Science Centre (Poland) project 2014/14/ST10/00321 to J. Majka. M.Bukała acknowledges The Polish National Agency for Academic Exchange for the scholarship no. PPN/ IWA/2018/1/00046/U/0001. This is contribution no. 64 of the DFG-funded LA-ICP- MS Laboratory at the Institute for Geosciences, University of Bonn, Germany.The collision of Baltica and Laurentia during the Caledonian Orogeny happened at c. 400-420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsakkok Lenses. Eclogites occur in the Vaimok and Tsakkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. By contrast, eclogites of the Tsakkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean-continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages between c. 500 and 480 Ma, in contrast to younger (460-450 Ma) ages of ultra high-P metamorphism in the Seve Nappe Complex further south in Jamtland. Eclogites from the Vaimok (one sample) and Tsakkok (three samples) lenses were dated using Lu-Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well-defined isochrons of the following ages: 480.4 +/- 1.2 Ma (Vaimok); 487.7 +/- 4.6 Ma, 486.2 +/- 3.2, 484.6 +/- 4.6 Ma (Tsakkok). The ages from Tsakkok are interpreted to date the burial of the Iapetus-Baltica ocean-continent transition in a west-dipping subduction zone around c. 485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jamtland (c. 460-450 Ma) and Norrbotten (c. 488-480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.Agentura na Podporu Vyskumu a Vyvoja APVV-18-0107German Research Foundation (DFG) FR700/18-

Kinematik des Deckenkontaktes zwischen der Combinzone und der Zermatt-Saas-Zone (Penninische Decken, Westalpen) und deren Bedeutung für die Exhumierung der Zermatt-Saas-Zone

Die Grenze zwischen zwei ophiolithischen Decken der penninischen Alpen, der Zermatt-Saas-Zone (unten) und der Combinzone (oben), markiert zugleich einen bedeutenden Sprung der bei der tertiären alpinen Metamorphose maximal erreichten Drücke. Während die Zermatt-Saas-Zone Ultrahochdruckmetamorphose (25–30 kbar/550–600°C, Bucher et al. 2005) erfuhr, erreichte die Combinzone lediglich blauschieferfazielle Bedingungen (13–18 kbar/380– 550°C, Bousquet et al. 2004). Vor allem die Polarität des Drucksprunges führte dazu, daß die Deckengrenze zumeist als gewaltige südostvergente Abschiebung interpretiert wurde (z.B. Ballèvre & Merle 1993, Reddy et al. 1999). Strukturgeologische Geländebeobachtungen ergeben jedoch sowohl für das Hangende als auch das Liegende der Combinstörung die folgende kinematische Entwicklung: i) Nordwestvergente, überschiebende Scherung (D1), ii) (Süd)westvergente Scherung (D2),iii) Südostvergente, abschiebende Scherung (D3). Alle drei Deformationsphasen fanden in beiden Einheiten unter grünschieferfaziellen Bedingungen statt... Die Rekonstruktion ergibt, daß die Combinstörung hauptsächlich als Überschiebung aktiv war. Die Exhumierung der Gesteinseinheiten im Liegenden wurde nicht durch Extension, sondern durch vertikale Ausdünnung der Kruste während horizontaler Kontraktion bewirkt.conferenc

Kinematics and Age of Syn-Intrusive Detachment Faulting in the Southern Alps: Evidence for Early Permian Crustal Extension and Implications for the Pangea A Versus B Controversy

Permian basin formation and magmatism in the Southern Alps of Italy have been interpreted as expressions of a WSW‐ENE‐trending, dextral megashear zone transforming Early Permian Pangea B into Late Permian Pangea A between ~285 and 265 Ma. In an alternative model, basin formation and magmatism resulted from N‐S crustal extension. To characterize Permian tectonics, we studied the Grassi Detachment Fault, a low‐angle extensional fault in the central Southern Alps. The footwall forms a metamorphic core complex affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, the synmylonitic Val Biandino Quartz Diorite and the postmylonitic Valle San Biagio Granite. U‐Pb zircon dating yielded crystallization ages of 289.1 ± 4.5 Ma for the former and 286.8 ± 4.9 Ma for the latter. Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S. Even though a dextral continental wrench system has long been regarded as a viable model at regional scale, the local kinematic evidence is inconsistent with this and, rather, supports N‐S extensional tectonics. Based on a compilation of >200 U‐Pb zircon ages, we discuss the evolution and tectonic framework of Late Carboniferous to Permian magmatism in the Alps

Reuniting the Ötztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Rheinische Friedrich-Wilhelms-Universität Bonn (1040