From subduction to collision: Thermal overprint of HP/LT meta-sediments in the north-eastern Lepontine Dome (Swiss Alps) and consequences regarding the tectono-metamorphic evolution of the Alpine orogenic wedge

Abstract

the cenozoic-age metamorphic structure of the Alps consists of a throughgoing pressure-dominated belt (blueschists and eclogites) that strikes parallel to the orogen and was later truncated by two thermal domes characterised by barrow-type metamorphism (Lepontine dome and tauern window). this study documents for the first time that relics of Fe-Mg carpholite occur also within meta-sedimentary units that are part of the north-eastern Lepontine structural and metamorphic dome, where so far exclusively barrovian assemblages were found. they occur in meta-sediments of both Valais Oceanderived Lower Penninic bündnerschiefer and structurally lower Europe-derived sub-Penninic cover nappes and slices. these high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for barrovian metamorphism.We present evidence for a two-stage metamorphic evolution in the northern part of the Lepontine dome: (1) Early subduction-related syn-D1 (safien phase) HP/Lt metamorphism under blueschist facies conditions (350-400 °c and 1.2-1.4 GPa) was immediately followed by "cold" isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) collisionrelated barrovian overprint (500-570 °c and 0.5-0.8 GPa) postdates the D3 nappe-refolding event (Domleschg phase) and represents a late heating pulse, separated by D2 and D3 from the D1 high-pressure event. It occurred before and/or during the initial stages of D4 (chièra phase) representing a second nappe-refolding event.In discussing possible heat sources for the late barrow-type heating pulse it is argued that heat release from radioactive decay of accreted material may play an important role in contributing much to heat production. based on the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution. Introductionthe zoning of Alpine metamorphism is rather complex, evolving over a very long period of time before, during and after the collision of Europe with Adria, i.e. from Late cretaceous to Late cenozoic times. Mapping of metamorphic facies in the Alps started with early pioneering studies based on the spatial distribution of index minerals and mineral assemblages (Wenk 1962;Niggli & Niggli 1965;trommsdorff 1966;Frey 1969; Fox 1975;Frey et al. 1980). Metamorphic maps at the scale of the Alpine orogen, showing the spatial arrangement of the different metamorphic facies types, were repeatedly synthesised and improved (Ernst 1971;Niggli & Zwart 1973;Oberhänsli et al. 2004). the cenozoic-age metamorphic pattern is characterised by a pressure-dominated belt (blueschists and eclogites) that strikes orogen-parallel but is interrupted by two thermal domes, the Lepontine dome in the central Alps and the tauern window in the Eastern Alps (Oberhänsli et al. 2004).Our area of investigation is located at the NE border of the Lepontine thermal dome. there, along strike of the tectonic units, a remarkable metamorphic field gradient that rang...

Thermochronometry Across the Austroalpine‐Pennine Boundary, Central Alps, Switzerland: Orogen‐Perpendicular Normal Fault Slip on a Major “Overthrust” and Its Implications for Orogenesis

“…In Oligocene and Miocene time, rocks of the Pennine zone were juxtaposed above the Helvetic domain, coeval with the development of the northern Alpine molasse foredeep basin and with the formation of the Helvetic nappe complex (Figure ). The postkinematic Barrovian metamorphism, mainly affecting the Lepontine dome region of the Central Alps (e.g., Vance & O'Nions, ), affected the southern part of the nappe stack, peaking at circa 18 Ma (Janots et al, ; Wiederkehr et al, ). Neogene crustal‐scale dome‐and‐basin structure (i.e., Aar and Lepontine culminations, and Wildstrubel and Rawil depressions) overprints the entire system, accompanied by thrust faulting, normal faulting, and strike‐slip faulting (e.g., Insubric shear zone, Simplon fault, and Engadine fault zone, respectively; Figures , and ).…”

On the causes and modes of exhumation and lateral growth of the Alps

“…In the Western Alps this outward shift of the exhumation front took place in the Miocene, as shown by the progressive younging of zircon fission track ages (ZFT) from the axial zone (Periadriatic Fault) to the ECM (Figure 5a) [ Fügenschuh and Schmid , 2003]. In the central Alps (Lepontine) an outward younging of the zircon fission track ages is not observed, because there the cooling pattern is affected by a young, high‐temperature and late to posttectonic metamorphic event in the Lepontine [ Wiederkehr et al , 2008]. This event is inferred to have lasted until 20–18 Ma [ Janots et al , 2009], and therefore, it overprinted a cooling pattern that may eventually have been more similar to that of the Western Alps.…”

Alpine orogenic evolution from subduction to collisional thermal overprint: The40Ar/39Ar age constraints from the Valaisan Ocean, central Alps

“…Additionally 40 Ar/ 39 Ar stepwise heating experiments on biotite grain separates were performed for comparison with in situ dating and complete the data set. These new data offer the opportunity to test and further quantify the geodynamic evolution proposed by Wiederkehr et al [2008]. Moreover, this work represents a case study that addresses the transition from subduction to collision in general and thereby provides important field and geochronological data for testing numerical models for the geodynamic evolution of orogenic belts [e.g., Bousquet et al , 1997; Jamieson et al , 1998; Roselle et al , 2002; Goffé et al , 2003].…”

“…The area investigated by this study occupies a key location in the central Alps, located at the northeastern edge of the Lepontine dome (Figure 1a). The area offers uninterrupted excellent exposure of metasedimentary units derived from the Valaisan and adjacent distal European domains within which subduction‐related HP/LT metamorphism dominates in the northeast (Engadine window and Grisons area) while collision‐related Barrow‐type amphibolite facies metamorphism dominates in the southwest (Figure 1) [see also Wiederkehr et al , 2008]. This allows for collecting samples from a continuous along‐strike section and for investigating the geodynamical relationships between these two contrasting tectonometamorphic events.…”

“…For the first time this study provides geochronological data regarding both stages of a polymetamorphic evolution and from within the same working area located in the central Alps: HP/LT metamorphism including subsequent retrogression, followed by a Barrow‐type thermal metamorphic event. We analyzed samples that are well described in terms of their metamorphism [see Wiederkehr et al , 2008; Wiederkehr , 2009; Bousquet et al , 1998] in order to date mineral growth and/or reactions by using 40 Ar/ 39 Ar techniques. We will present results of in situ 40 Ar/ 39 Ar dating of successive white mica generations reflecting the subduction‐related metamorphism on the one hand, and dating of biotite grown during collision‐related Barrovian overprint on the other hand.…”