Abstract

A penetrative composite foliation and stretching lineation in the ophiolite of the Zermatt-Saas-Vanzone region developed under eclogite-facies conditions in mid-Cretaceous time. Eclogite-facies mineral parageneses in this foliation were progressively replaced by blueschist-facies and greenschist- to amphibolite-facies mineral assemblages during Late Cretaceous and mid-Tertiary time and were subsequently completely retrogressed. Kinematic indicators associated with the various stretching lineations of the different metamorphic facies reveal that shearing was initially approximately top-to-west (high-P phase) and changed during the greenschist/amphibolite-facies stage into a top-to-west/northwest orientation. Forethrusting was followed by low-angle normal shearing during the final retrograde phase. This sense of shear was upper block-to-southwest/west-southwest. Unambiguous structural relics of the high-pressure event were not recognized in the gneiss of the Monte Rosa nappe, but the close similarity of the metamorphic evolution of the Zermatt-Saas and Monte Rosa units (including the high-P event) makes a common tectonic history probable.

In late Oligocene and Miocene time, the Zermatt-Saas-Vanzone region was affected by an initial phase of backfolding (Mischabel phase, predating top-southwest/west-southwest-directed, low-angle normal shearing) toward the south/southeast followed by a second backfolding event (Vanzone phase). The main geometrical and structural features suggest that the Mischabel phase resulted from vertical extrusion and horizontal intrusion of the ductile basement (Monte Rosa) into shallower units (Antrona and Zermatt-Seas ophiolites) and is in accord with a recently proposed kinematic model for the Central Alps. No clear-cut relations between the second phase of backfolding and the retrograde phase were observed; this suggests that both events took place largely simultaneously. Backfolding northeast of the Simplon line postdated retrograde normal shearing and is of late Miocene age. It is concluded that backfolding in the Zermatt-Saas-Vanzone region cannot be causally linked with backfolding northeast of the Simplon line.

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