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Feasibility of double slab breakoff (Cretaceous and Tertiary) during the Alpine convergence

von Blanckenburg, F. and Davies, John Huw ORCID: 1996. Feasibility of double slab breakoff (Cretaceous and Tertiary) during the Alpine convergence. Eclogae Geologicae Helvetiae 89 (1) , pp. 111-127.

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It has been proposed in newer models of the Alpine convergence that closure of oceanic basins and following continental collision was separated into two discrete events: Eoalpine (Cretaceous) closure of a presumed west ward extension of a South-Tethys ocean (Vardar-Meliata-Hallstatt), followed by Mesoalpine (Tertiary) closure of the Penninic basins (Piemont, Valais). Further, according to the slab breakoff model, closure of an oceanic basin and partial subduction of continental lithosphere can lead to detachment and falling away of the oceanic lithosphere. Therefore the geological imprints of slab breakoff could be found twice in the Alps. In this paper we test the feasibility of this hypothesis by compiling literature data relevant to the surface expressions predicted from the slab breakoff model. a) Ages from high-pressure continental rocks fall into two groups (after taking into account the likely effect of excess argon in phengites): 110-95 in Austroalpine units, 50-30 Ma in Penninic units. b) Greenschist facies metamorphism affected parts of the Austroalpine from 100 to 85 Ma, Penninic units from 40-30 Ma. c) High-temperature regional metamorphism peaked at ca. 90 Ma in the Austroalpine, at ca. 29 Ma in the Penninics. d) Most importantly, some minor basaltic magmatism occurred at 100 Ma in the Austroapline, more pronounced bimodal magmatism is prominent along the Periadriatic Lineament from 43 to 25 Ma. e) Rapid erosion is witnessed by basins formed from 90 to 60 Ma in the Austroalpine (Gosau), Molasse and Po-Basin from 35 to 10 Ma. f) East-west directed extensional exhumation is documented by low-angle normal faulting at 90-70 Ma in the Austroalpine and from 30-10 Ma along the Tauern Window and Central Alps, respectively. All these features are predicted by the slab breakoff model. The broad similarities and good separation in time between both orogenic cycles may suggest that they were affected by the same process in different periods. However, none of the above expressions is exclusive to slab breakoff, nor is the timing in every case well-constrained. Therefore, rather than using these observables as definite proof of the double-slab breakoff hypothesis, the aim of this paper is to demonstrate how these criteria can be used to constrain the model. This should allow its confirmation or rejection in the future.

Item Type: Article
Status: Published
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Publisher: Birkhäuser
ISSN: 0012-9402
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Last Modified: 18 Oct 2022 12:38

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