Mesozoic metamorphism has affected all the formations prior to the Upper Cretaceous, i.e. the initially metamorphic pre- Perrnian basement and its Permo-Jurassic cover consisting of shelf sediments and not "geosynclinal" trough deposits. The mineral associations encountered (pumpellyite + prehnite, lawsonite + albite, and lawsonite + glaucophane) define a very low grade metamorphism which has occurred under the effect of only a slight geothermal gradient (low temperatures ranging from 200 to 400°C and high pressures of 3 000 to 8 000 bars). The metamorpqic zonation points out to an increase in intensity from northeast to south- west. This metamorphic phase coincides with the major Eocretaceous orogeny (= Rangitata orogeny), the folds of which were initially overturned or slided towards the north-east. An evolutionary model for the Rangitata orogeny is proposed, involving the subduction towards the south-west of a fragment of the continental crust (including the "Central Chain"), which became detached from the Australian continent during the Jurassic, underneath another fragment of continental crust (including the "West Coast" ), which was subsequently separated from Australia as a result of oceanization of the Tasman Sea during the Paleocene.