Open Session - SWISS GEOSCIENCE MEETINGs

The Bernardo-Gneiss is an element of the cataclastic Gneiss chiari, which Reinhard (1953) considered to form a thin (500 -
1000 m), but large (100 km, Borgosesia – Pizzo del Diavolo) Variscian nappe on top of the crystalline basement and his metasediments,
what remained difficult to explain with modern tectonic models (Schumacher et al. 1997). With its widespread
occurrence of shocked quartz, the Gneiss chiari can be interpreted now as cataclastic target rock representing the
(par)autothochthonuous floor of an impact crater. Because of the extensiveness of the “nappe”, the effects of the impact
must have been significant. Further hints of a huge structure give additional occurrences of shocked quartz beyond the
Southern Alps, as the “Maranerbreccie” of Arosa (Cadisch 1953) or the “Saluverbreccie” near St. Moritz (Finger 1978). Like the
Breccia of Arzo, these formations seem to coincide with processes at the Triassic-Jurassic Boundary.
Due to the Alpine orogenesis and the accompanying alteration and sedimentation, no circular or elliptical impact crater
structure has survived. But probabely, the several kilometres of siliceous limestones in the Monte Generoso and Monte Nudo
basins (Bernoulli 1964) represent a relic of impact-related debris filling depressions of the TrJ-boundary impact structure.
Figure 1. Bernardo-Gneiss from Orlino; Quartz with sub-
parallel planar fractures and with small, high-relief
crystals of ?coesite, plane-polarized light, width of picture
is 0.3 mm
Figure 2. Porfido Luganese from Carona, quartz grain
with PDFs in a granophyric matrix,
cross-polarized light, width of picture is 0.6 mm
REFERENCES
Bernoulli, D. 1964: Zur Geologie des Monte Generoso, Beitr. Geol. Karte Schweiz, N. F. 118.
Buletti, M. 1985: Petrographisch-geochemische Untersuchungen im Luganer Porphyrgebiet, Diss. Univ. Bern, unpublished.
Cadisch, J. 1953: Geologie der Schweizer Alpen, Verlag Wepf & CO., Basel.
Finger, W. 1978: Die Zone von Samaden und ihre jurassischen Breccien. Diss. ETH Zürich.
French B. M. 1998: Traces of Catastrophe, LPI Contribution No. 954, Lunar and Planetary Institute, Houston.
Masaitis V.L. 2005: Redistribution of Lithologies in Impact-induced Dikes of Impact Structures, in Koeberl, C. & Henkel, H.
(Eds.): Impact Tectonics, Springer-Verlag Berlin Heidelberg.
Niggli, E. 1953: Die magmatischen Bildungen, in Cadisch, J.: Geologie der Schweizer Alpen, Verlag Wepf & CO., Basel.
Reinhard, M. 1953: Über das Grundgebirge des Sottoceneri im südlichen Tessin. Eclogae geol. Helv. 46, 214-222.
Schumacher, M. E., Schönborn, G., Bernoulli, D. & Laubscher, H. P. 1997: Rifting and Collision in the Southern Alps, in
Pfiffner. A. O. et al. (Eds): Deep Structures of the Swiss Alps, Birkhäuser Verlag, Basel.
Wiedenmayer, F. (1963): Obere Trias bis mittlerer Lias zwischen Saltrio und Tremona (Lombardische Alpen). Eclogae geol.
Helv. 56, 529–640.
Therriault A.M., Fowler, A.D & Grieve, R.A.F. 2002: The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet,
Economic Geology; November 2002, v. 97, no. 7,1521-1540.
101
Symposium 2: Mineralogy-Petrology-Geochemistry