Linking Alpine deformation in the Aar Massif basement and its cover units – the case of the Jungfrau–Eiger mountains (Central Alps, Switzerland)

Abstract

Abstract. The northwest (NW) rim of the external Aar Massif was exhumed from ∼ 10 km depth to its present position at 4 km elevation above sea level during several Alpine deformation stages. Different models have been proposed for the timing and nature of these stages. Recently proposed exhumation models for the central, internal Aar Massif differ from the ones established in the covering Helvetic sedimentary units. By updating pre-existing maps and collecting structural data, a structural map and tectonic section were reconstructed. Those were interpreted together with microstructural data and peak metamorphic temperature estimates from collected samples to establish a framework suitable for both basement and cover. Deformation temperatures range between 250 and 330 °C, allowing for semi-brittle deformation in the basement rocks, while the calcite-dominated sedimentary rocks deform in a ductile manner at these conditions. Although field data allow to distinguish multiple deformation stages before and during Aar Massif's exhumation, all related structures formed under similar P, T conditions at the investigated NW rim. In particular, we find that the exhumation occurred during two stages of shearing in Aar Massif's basement, which induced in the sedimentary rocks first a phase of folding and then a period of thrusting, accompanied by the formation of a new foliation.

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

“…This implies that at least 50 • C of the cooling is associated with post-obduction extension, i.e., before doming. A twostage exhumation history of the Jebel Akhdar Dome has also been inferred from structural data (Grobe et al, 2018;Mattern and Scharf, 2018) and the stratigraphic record (Fournier et al, 2006;Mann et al, 1990). Top-to-NNE shearing is associated with tectonic thinning of the ophiolite (Grobe et al, 2018).…”

“…The organic particles lack sufficient size or surface qual-ity for reflectance measurements and are therefore investigated by confocal Raman spectroscopy of carbonaceous material. The technique measures vibrational energies of chemical bonds, which change during temperature-induced reorganization of amorphous carbonaceous material (kerogen) to graphite (e.g., Aoya et al, 2010;Beyssac et al, 2002;Kouketsu et al, 2014;Mair et al, 2018). Measurements were conducted at the Geoscience Center, Göttingen, on a Horiba Jobin Yvon HR800 UV spectrometer attached to an Olympus BX-41 microscope and a 100× objective.…”

The effect of rock composition on muon tomography measurements

“…The origin of this peculiar rock type can be traced back to Hayman et al (1963), Miyake et al (1964), Mandò and Ronchi (1952) and George (1952), who gave slightly different definitions of its physical parameters (mass density ρ, atomic weight A and atomic number Z). A comprehensive compilation thereof can be found in Table 1 of Higashi et al (1966).…”

The role of frost cracking in local denudation of steep Alpine headwalls over millennia (Mt. Eiger, Switzerland)

“…The region surrounding the Eiger is located at the geological contact between the crystalline rocks of the Aar massif, its sedimentary cover rocks and the Helvetic thrust nappes (Berger et al, 2017). The Eiger itself is mainly made up of micritic Jurassic and bioclastic Cretaceous limestone, with local chert layers and nodules, all of which were re-crystallized under lower greenschist facies conditions (c. 300°C; Mair et al, 2018; and references therin). During Alpine orogenesis, the bedrock was heavily deformed through multiple phases of folding and thrusting, which are recorded by a complex fabric in the exposed rock (Herwegh et al, 2017;Wehrens et al, 2017).…”