GEOSITE

The Err detachment in SE Switzerland: a witnessof how continents break apart

The Err detachment exposed in the Lower Austroalpine Err nappe in

SE Switzerland, north of the Engiadina Valley, is one of the world’s

few exposed, most spectacular, and preserved rift-related low-angle

detachment faults. This structure can be mapped over 200 km2

exhibiting primary relationships within Mesozoic pre-, syn-, and post-

rift sediments, documenting the final stage of the rift history leading

to the formation of the Alpine Tethys ocean about 160 Ma ago.

Structures similar to the Err detachment are assumed to lie beneath

thick sedimentary sequences at present-day deepwater, magma-poor

rifted margins. The access to such exposed structures offers therefore

a unique opportunity to study the processes of how continents break

apart and new oceans form.

Parts of the Err detachment were first mapped and described by

Cornelius (1935) as an Alpine structure (Fig. 1a). The occurrence of

characteristic black indurated fault gouges along this structure,

interpreted as ‘‘Reibungs-Konglomerate’’ by von Rath (1857), and the

finding of clasts of these gouges reworked in Mid-Jurassic sedimen-

tary breccias led Froitzheim and Eberli (1990) to interpret this fault as

a Jurassic low-angle detachment fault. This interpretation has been

supported by numerous studies using structural, geochemical, petro-

logical, and sedimentological methods. The most characteristic fea-

tures of the fault zone are green, silica-impregnated cataclasites, and

black fault gouges (Fig. 1b). They form as the result of fluid- and

reaction-assisted weakening processes, controlling the strain locali-

zation along this long-offset fault (Manatschal 1999).

The Err detachment corresponds to a well-defined surface, either

overlain by extensional allochthons (Fig. 1a, c), or, where the fault

was directly exhumed at the seafloor, by syn-rift sediments (Fig. 1d,

Manatschal and Nievergelt 1997). The Err detachment formed during

the final stage of rifting, by sequential faulting and exhumation of a

lower plate (e.g., future distal Adriatic margin) underneath an upper

plate (e.g., future European margin). The extensional allochthons

overlying the Err detachment can therefore be considered as remnants

of the European margin left behind on the exhumed distal Adriatic

margin (see explanations in Fig. 3 of Masini et al. 2013). These

extensional allochthons, tens to thousands of meters long and less

than 1 km thick, are characteristic features of detachment faults and

testify the allochthonous nature of extension during final rifting. The

preservation of the supra-detachment syn-rift sediments along the Err

detachment system provides a precise stratigraphic and sedimento-

logical record of allochthon formation and detachment footwall

exhumation (Fig. 1a, Masini et al. 2012).

Such a geological record is usually hidden below deepwater and/or

thick sedimentary sequences in present-day oceans. The Err detachment

represents therefore a unique witness of how continents break apart. It

allows to study the interaction between deformation (Fig. 1a, b), fluids

(Fig. 1c), magma, and sedimentation (Fig. 1d), during the final stages

of rifting eventually leading to lithospheric break up and seafloor

spreading (Manatschal and Muntener 2009).

References

Cornelius HP (1935) Geologie der Err-Julier Gruppe: Das Baumate-

rial (Strastigraphie und Petrographie, excl. Quartar). Betr geol

Karte Schweiz NF 70/1:321

Froitzheim N, Eberli GP (1990) Extensional detechment faulting in

the evolution of a Tethys passive continental margin, Eastern

Alps, Switzerland. Geol Soc Am Bull 102:1308

Manatschal G (1999) Fluid- and reaction-assisted low-angle normal

faulting: evidence from rift-related brittle fault rocks in the Alps

(Err Nappe, eastern Switzerland). J Struct Geol 21:777–793

Manatschal G, Muntener O (2009) A type sequence across an ancient

magma-poor ocean–continent transition: the example of the

western Alpine Tethys ophiolites. Tectonophysics 473:4–19. doi:

10.1016/j.tecto.2008.07.021

Manatschal G, Nievergelt P (1997) A continent-ocean transition

recorded in the Err and Platta nappes (Eastern Switzerland).

Eclogae Geol Helv 90:3–27

Masini E, Manatschal G, Mohn G, Unternehr P (2012) Anatomy and

tectono-sedimentary evolution of a rift-related detachment

system: the example of the Err detachment (central Alps, SE

Switzerland). Geol Soc Am Bull 124:1535–1551. doi:10.1130/

B30557.1

Masini E, Manatschal G, Mohn G (2013) The Alpine Tethys rifted

margins: reconciling old and new ideas to understand the

stratigraphic architecture of magma-poor rifted margins. Sedi-

mentology 60:174–196. doi:10.1111/sed.12017

Rath GV (1857) Geognostische Bemerkungen uber das Berni-

nagebirge in Graubunden. Z Deutsch Geol Ges

E. Masini (&) � G. Manatschal

Ecole et Observatoire des Sciences de la Terre (EOST), Universite de Strasbourg, 1, Rue Blessig, 67084 Strasbourg Cedex, France

e-mail: masini@unistra.fr

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Int J Earth Sci (Geol Rundsch)

DOI 10.1007/s00531-013-0924-2

Received: 22 February 2013 / Accepted: 8 June 2013� Springer-Verlag Berlin Heidelberg 2013

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