Evidence of late oligocene/early miocene backthrusting in the central alpine “root zone”

Two groups of stretching lineations can be distinguished in the Central Alpine " root zone " between Ticino and Mera :1) Steeply plunging lineations formed during retrograde metamor-Phism under amphibolite/greenschist facies conditions indicate an uplift movement of the Central Alps. The lineations can be related to an important back-thrusting event of late Oligocene/early Miocene age.2) Gently plunging lineations formed under lower greenschist facies conditions display a pattern typical of a dextral strike-slip system. These lineations are of early Miocene age.This cpmbined movement, achieved by ductile deformation along the lnsubric line was followed by a stage of brittle deformation in a dextral strike-slip system (= Tonale line).The signification of this interpretation is shown in a new crustal cross section through the Central Alpine/Southern Alpine border zone in the Iicino area.