this study presents a review of published geological data, combined with
original observations on the tectonics of the simplon massif and the
Lepontine gneiss dome in the Western Alps. New observations concern the
geometry of the Oligocene Vanzone back fold, formed under amphibolite
facies conditions, and of its root between Domodossola and Locarno,
which is cut at an acute angle by the Miocene, epi- to anchizonal,
dextral centovalli strike-slip fault. the structures of the simplon
massif result from collision over 50 Ma between two plate boundaries
with a different geometry: the underthrusted European plate and the
Adriatic indenter. Detailed mapping and analysis of a complex structural
interference pattern, combined with observations on the metamorphic
grade of the superimposed structures and radiometric data, allow a
kinematic model to be developed for this zone of oblique continental
collision. the following main Alpine tectonic phases and structures may
be distinguished:
1. NW-directed nappe emplacement, starting in the Early Eocene (similar
to 50 Ma);
2. W, SW and S- verging transverse folds;
3. transpressional movements on the dextral simplon ductile shear zone
since similar to 32 Ma;
4. formation of the Bergell - Vanzone backfolds and of the southern
steep belt during the Oligocene, emplacement of the mantle derived 31 -
29 Ma Bergell and Biella granodiorites and porphyritic andesites as well
as intrusions of 29-25 Ma crustal aplites and pegmatites;
5. formation of the dextral discrete Rhone-Simplon line and the
centovalli line during the Miocene, accompanied by the pull-apart
development of the Lepontine gneiss dome - Dent blanche (Valpelline)
depression.
It is suggested that movements of shortening in fan shaped NW, W and sW
directions accompanied the more regular NW- to WNW-directed displacement
of the Adriatic indenter during continental collision.