Ausserberg–Avat-Zone

Representation and status

Color CMYK

N/A

Color RGB

R: 241 G: 239 B: 237

Rank

tectonic zone

Validity

Unit is in Use

Status

valid

Status discussion

Nomenclature

Deutsch

Ausserberg–Avat-Zone

Français

Zone d'Ausserberg–Avat

Italiano

Zona di Ausserberg–Avat

English

Ausserberg–Avat zone

Origin of the Name

Ausserberg (VS) und Piz Avat (GR)

Historical Variants

Südliche Gneise auct., Zone IV = Südliche Gneisse (Alb. Heim), Südliche Gneiszone = Southern Gneiss Zone (Abrecht 1994), Ausserberg-Avat Zone (Berger et al. 2017)

Nomenclatorial Remarks

Entspricht die «Südliche Gneiszone».

Description

Thickness

Die Breite der Zone variiert zwischen 3 km im Tavetschgebiet, max. 7 km bei Naters und min. 1 km im Urserental oder sogar 500 m zwischen Gütsch und Tiefenbacg.

Hierarchy and sequence

Subordinate units

• Engi-Granit
• Gärsthorn-Gneiskomplex
• Sogn-Placi-Gneiskomplex
• Massa-Gneiskomplex
• Pulanera-Gneiskomplex

Geography

Geographical extent

Schmale Band am Südrand des Aar-Massivs zwischen Rhônetal (Ausserberg, Naters), Urserental, Tavetschgebiet und Graubünden (Piz Avat).

References

Definition

Berger Alfons, Mercolli Ivan, Herwegh Marco, Gnos Edwin (2017) : Geological Map of the Aar Massif, Tavetsch and Gotthard Nappes. Geological Special Map 1:100'000, Explanatory Notes 129

p.24: A new name, Ausserberg-Avat Zone, replaces the older name “Southern Gneiss Zone” (ABRECHT 1994 and references therein). Early to post-Variscan plutonic rocks define the northern boundary of this zone. Its shape is characterised by pronounced changes in thickness. At Naters, the apparent thickness of the Ausserberg-Avat Zone reaches its maximum of some 7 km. The thickness diminishes regularly towards the east and reaches a minimum of approximately 1 km in the Urserental. Further east, in the Tavetsch area, it increases again up to 3 km. The internal lithological composition of the Ausserberg-Avat Zone consists of three subunits: (1) the Massa Gneiss Complex (GMM), (2) the Sogn Placi Gneiss Complex (GS) and (3) the Gärsthorn Gneiss Complex (GOG).

In general, all rock types of the Ausserberg -Avat Zone show petrographic and structural characteristics that are similar to the other polycyclic metamorphic basement units (ABRECHT 1994). One exception is the widespread occurrence of large metagranitoid bodies that are rare in the other units. The three distinguished gneiss complexes can be found throughout the entire zone, but the relative proportions vary considerably. The high-grade metamorphism that affects all the gneiss complexes must be of pre -Visean age as demonstrated by the intrusive relationships with the Baltschieder Granodiorite (Rötifirn Group, intrusive age ~336 Ma, see below). The Alpine overprint of the basement units is variable. It is intense along the southern boundary of the Ausserberg-Avat Zone, where biotite-chlorite-sericite gneiss and schist are frequent (e. g., NIGGLI 1965).

Besides the Central Aar Granite and the Southwestern Aar Granite of the Haslital Group, which intrude the Ausserberg-Avat Zone along ca. 70 % of its northern boundary, the zone is intruded by numerous other intrusives belonging to the Fruttstock Group and the Rötifirn Group. The Ausserberg-Avat Zone is therefore the only basement zone that hosts members of all three generations of Variscan plutonic rocks. Furthermore, the Late Carboniferous Val Lumpegna Formation is wedged into this zone, forming a thin layer at its northeastern end.

HUBER (1948), ZBINDEN (1949), LABHART (1965), NIGGLI (1965) and STECK (1966) mapped in detail the rocks belonging to the Ausserberg-Avat Zone. All these authors describe the complex relationships between metasedimentary, migmatitic and granitic gneisses. In fact, the high-grade metasedimentary gneisses frequently show transitions into migmatites, whereas the latter can grade into “Augengneiss”.

The fact that this primary banding is overprinted by migmatitic and additionally by post-anatectic deformation does not facilitate mapping in the field. Therefore, the present subdivision based on the three main rock types (gneiss complexes) must be regarded as a compromise in order to translate the mapped units available in the GeoCover vector datasets into the classification scheme of the map sheet.