Geological map of the Aar massif and Tavetsch and Gotthard nappe, Geol Spec. Map 129 Explanatory Notes 129

Berger, Alfons; Mercolli, Ivan Pietro; Herwegh, Marco; Gnos, Edwin (2017). Geological map of the Aar massif and Tavetsch and Gotthard nappe, Geol Spec. Map 129 Explanatory Notes 129. In: Geologische Spezialkarten, 129. Wabern: Federal Office of Topography Swisstopo

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In these explanatory notes to the geological map of the Aar Massif, Tavetsch and Gotthard Nappes 1:100000, geological observations concerning the different units are summarized. Data from the literature and maps at 1:25000 scale from the GeoCover® vector dataset of Swisstopo have been used to produce the map. It represents therefore the state of the art knowledge of the occurrence and regional distribution of geological units that can be mapped at this scale. The primary goals of the compilation were: (i) to subdivide the polycyclic basement into tectonic units (zones) (ii) to classify plutonic rocks on the base of their radiometric age and geochemical affinities (groups) (iii) to enhance graphically the occurrences of volcanic and volcaniclastic layers in order to stress their importance for the tectonic subdivision of the basement. The explanatory notes offer a short summary of the characteristics of the represented units and some general statements on the pre-Alpine and Alpine tectono-metamorphic evolution. Aar Massif and Gotthard Nappe consist of a polymetamorphic crystalline basement in which early- to post-Variscan mafic to acid magmatic melts intruded resulting in today’s granitoid bodies. Large sections of the polycyclic basement consist of paragneisses (frequently migmatitic) associated with amphibolites, marbles, calcsilicate and ultramafic rocks. In these basement units relics of Ordovizian eclogites and granulites occur in the Tavetsch and Gotthard Nappes, but rare in the Aar Massif. This association has been interpreted as an assemblage indicating subduction of oceanic crust and subsequent continent-continent collision. Large volumes of Ordovician/Silurian granitoids rocks (e.g., “Streifengneis” Complex) intrude the high-grade metamorphic Ordovician basement. The Ordovizian tectonics and metamorphism is overprinted by the Variscan cycle. This includes Variscan low-pressure metamorphism, which is characterized by the occurrence of pinitized cordierite in anatectic granitoid melts and is attributed to extensional tectonics. The main Variscan processes are different magmatic pulses. Three groups of 346-331 Ma, 315-307 Ma and 299-295 Ma can be distinguished inside the Aar Massif. Mafic plutonic rocks are frequent only in the oldest group. In the Tavetsch Nappe Variscan granitoids are absent. While in the Gotthard Nappe only late to post-Variscan granitoids of 299-295 Ma and 295-290 Ma occurs. The Alpine metamorphism is best recorded in mono-metamorphic Mesozoic sediments and in Variscan granitoids and increases from north to south. The transition to greenschist facies conditions is located in the centre of the Aar Massif, the chloritoid-in isograd at the southern end of the Aar Massif and the staurolite-in isograd near the southern limit of the Gotthard nappe. The Alpine tectonic evolution of the units reported in the map can be described as follows: (1) Formation of Helvetic nappes by the detachment of sediments from the future Gotthard- and Tavetsch Nappes and from parts of the Aar Massif. (2) Thrusting of the Tavetsch- and Gotthard Nappes onto the Aar Massif resulting in thickening causing metamorphic overprinting in the Aar Massif with a peak at ~20 Ma. This metamorphism led to formation of stilpnomelane and green biotite in granitoids of the Aar Massif. (3) NW-NNW dominated reverse faulting in the Aar-massif with the related vertical displacements and simultaneous steepening of the Tavetsch and Gotthard Nappes associated NW-SE shortening and the development of the Northern Steep Belt. (4) Localized dextral strike-slip movements in the South of the Aar Massif. (5) North- and south-directed thrusting along moderately south- and north-dipping shear zones and (6) Late brittle deformation continuing until today. The formation and crystallization of minerals in Alpine fissures and clefts is associated with stages (2) to (4).

Item Type:

Other

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Tectonics

UniBE Contributor:

Berger, Alfons; Herwegh, Marco and Gnos, Edwin

Subjects:

500 Science > 550 Earth sciences & geology

Publisher:

Federal Office of Topography Swisstopo

Language:

English

Submitter:

Alfons Berger

Date Deposited:

10 Nov 2017 09:52

Last Modified:

10 Nov 2017 09:52

URI:

https://boris.unibe.ch/id/eprint/105456

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