Conference Agenda
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3.7-2 The Alpine Orogen: Reconciling Recent Images of its Deep Structure and Crustal Tectonics
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by Thomas Meier1, Jan Behrmann2, Eline LeBreton3, Wim Spakman4 1: Christian-Albrechts Universität Kiel, Germany; 2: GEOMAR Kiel, Germany; 3: Freie Universität Berlin, Germany; 4: Utrecht University, The Netherlands The Alps provide the fascinating opportunity to study the complexity of mountain building processes because of the availability of extensive and detailed geological and geophysical data sets. The highly arcuate geometry of the Alpine chain originated in the Neogene, when previously southeast-directed subduction gave way to hard collision and indentation in the western, central, eastern Alps, and to roll-back subduction in the Carpathians and Apennines, respectively. The Alps Mountains are composed of deformed and amalgamated crustal slices derived from Eurasia, Adria, the intervening Brianconnais microcontinent and the Piemont-Ligurian and Valais oceanic basins. The driving forces of collision are however still less understood because segments of mantle lithosphere subducting beneath the Alps remain poorly imaged. Examples for open questions are: Down to which depth is Eurasian mantle lithosphere subducting eastwards beneath the western Alps? Is there evidence for Eurasian slab break-offs? Is Adria subducting northwards beneath the eastern Alps? Furthermore, the deformation of crustal units in the core of the orogen as well as the tectonic uplift in the western Alps and the eastward lateral escape in the eastern Alps north of the Periadriatic Fault remain to be understood quantitatively. New images of the Alpine deep structure using data of AlpArray and complementary projects are becoming available. We invite contributions on the interpretation of geophysical images of the Alpine crustal and upper mantle structure as well as on the analysis of surface deformation and tectonics aimed to cross-fertilise discussions on quantitative tectonic models able to explain mountain building in the Alps. Specifically, we want to focus on coupling models of the Alpine deep structure with multi-scale (near-)surface observations. | ||||||||
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3:20pm - 3:35pm
ID: 345 Invited Virtual Presentation | Keynote Lithospheric architecture from amphibious AlpArray data across the Ligurian Basin 1GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Germany; 2CAU, Christian Albrechts Universität zu Kiel, Germany; 3IDPA-CNR, Istituto per la dinamica dei processi ambientali, Sezione di Milano, Milano, Italy; 4IPGP, Institut de Physique du Globe de Paris, Laboratoire de Géosciences Marines, Paris, France; 5ISTerre, Institut des Sciences de la Terre, Université Grenoble, Grenoble, France; 6AlpArray Working Group: http://www.alparray.ethz.ch The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, the seismic events indicate the closure of this back-arc basin. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. Local seismicity was recorded on broad band ocean bottom seismometers over a period of 8 month. The majority of the refraction seismic data show mantle phases at offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The mantle shows rather high velocities >7.8 km/s. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly does not initiated oceanic spreading before the opening of the Ligurian Basin stopped. In the centre of the Ligurian Basin, a cluster of 15 seismic events with magnitudes lower than 2.5 occurred in the upper mantle in a depth of 10-15 km below the seismic Moho. The fault plane solutions indicate reverse faulting and a convergence in NW-SE direction.
3:35pm - 3:50pm
ID: 346 Invited Virtual Presentation | Keynote Crust and upper mantle structure of the Ligurian Sea revealed by ambient noise tomography and receiver function analysis 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany; 2Kiel University; 3Institut de physique du globe de Paris, Paris, France; 4ISTerre - Institut des Sciences de la Terre, Grenoble, France; 5AlpArray Working Group: http://www.alparray.ethz.ch The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea using 29 broadband Ocean Bottom Seismometer (OBS). The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018, as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data, including corrections for instrument tilt and seafloor compliance, to improve the signal-to-noise ratio. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong earthquake events that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. Our group velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). In general, the velocities increase with depth and the velocity anomalies can be related to varying sediment thickness and magmatism. The longer periods (20-90 s) show a smoother velocity structure that reveals mantle velocities in the vicinity of the Ligurian margin, north of the basin centre. However, resolution at greater depth is limited along the Corsican margin due to less station coverage. Our results do not indicate an oceanic spreading centre, however, may hint to an asymmetric opening of the Ligurian Basin.
3:50pm - 4:05pm
ID: 343 Invited Virtual Presentation | Keynote Neogene kinematics and structural evolution of the eastern Southern Alps (Giudicarie Belt, Northern Italy) 1Freie Universität Berlin, Germany; 2Department of Earth Sciences, ETH Zürich, Switzerland; 3GFZ German Research Centre for Geosciences, Potsdam, Germany The eastern Southern Alps formed as a retro-wedge of the Alpine orogen and shortening is partitioned into three kinematically linked fold-and-fault systems: (1) the Giudicarie Bel; (2) the Valsugana Thrust System; and (3) the external orogenic front, including the strike-slip Schio-Vicenza Fault. Here, we present closely spaced geological cross-sections from the Northern Giudicarie Fault to the South Alpine orogenic front to assess the amount of Neogene shortening and better understand deformation of the Adriatic indenter in Mio-Pliocene time. The amount of Neogene, NNW-SSE shortening varies from 8 km in the vicinity of the Adige embayment to 25 km further to the northeast, with most shortening (18-22 km) accommodated within the Giudicarie Belt and Valsugana Thrust System. Shortening estimates vary on either side of the Trento-Cles, Schio-Vicenza and Ballino-Garda strike-slip faults. These faults coincide with major Permian-Mesozoic paleogeographic boundaries across which the stratigraphic thickness and rheological properties of rock units significantly change. We interpret these to be responsible for strain partitioning in the eastern Southern Alps. Moreover, balancing and forward modelling of the cross-sections with MOVE Suite Software indicate that the depth of the detachments within the Pre-Permian basement is no greater than 20km. The Northern Giudicarie Fault has recorded 75 km of Neogene sinistral offset, of which 40 km was accommodated by shortening along the Val Trompia fold-and-thrust belt and 35 km by shortening along the Giudicarie/Valsugana domain east of the Trento-Cles Fault. Additional Neogene shortening may have occurred within a decoupled lower crustal Adriatic wedge beneath the Tauern Window.
4:05pm - 4:20pm
ID: 350 Invited Virtual Presentation | Keynote First interpretation of mantle structure in the Alps-Apennines-Carpathian-Pannonian region from teleseismic Vp studies 1Geowissenschaften, Ruhr-Universität Bochum, Germany; 2Geowissenschaften, Freie Universität Berlin, Germany; 3Institut für Geophysik, ETH-Zürich, Switzerland A pattern of teleseismic Vp anomalies in the greater Alpine area reflect asymmetrical mantle structure in the Alps as well as pronounced orogen-parallel variations. The foreland of the Western and Central Alps shows layered +Vp and -Vpanomalies that extend from the Variscan belts to beneath the Alpine orogenic front. The base of the lower -Vp layer lies at a depth of c. 180 km. However, below the Tauern Window and to the east, this layering is less pronounced and subhorizontal, with an undefined base. In the Eastern Alps, the +Vp anomalies define an ultrathin (≤ 50 km) lithospheric lid underlain by shallow -Vp anomalies trending E-W, parallel to the direction of eastward-lateral motion of the AlCaPa unit into the Pannonian Basin. Isolated +Vp anomalies underlie the Eastern Alps at c. 200 km depth. South of the Alps the Adriatic lithosphere is some 80-100 km thick and underlain by variably strong -Vp anomalies. No slab anomaly is detected beneath the northern Dinarides. We interpret the Vp anomaly pattern in the Western and Central Alps to indicate SSE-directed subduction and partial detachment of a thick, compositionally heterogeneous European tectosphere comprising inherited (Variscan?) Vp anomalies. +Vp anomalies beneath the Alps represent subducted lithosphere in various stages of tearing and delamination, with complete delamination east of the western Tauern Window. Shallow -Vp anomalies in this domain document asthenospheric upwelling which facilitated crust-mantle decoupling during Miocene escape of AlCaPa. The slab anomaly beneath the northern Apennines is detached from its foreland and hangs subvertically.
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