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Tectonic Evolution of the Internal and Southern Zones of the Alps |
Department of Geology, University College of Wales, Aberystwyth SY23 3DB, Wales, UK
Department of Geology, University College, Swansea SA2 8PP, Wales, UK
A series of thrust and shear zone structures in the upper Pennine Alps dissect the area into a number of horsts and duplexes. The diversity of the mineral stretching lineation directions when combined with shear-sense indicators reveals substantial variation in the movement direction both between and within different units, and in some cases at different times within the same unit. The present-day geometry, illustrated in a new block diagram, is controlled largely by backthrusts. These backthrusts have overprinted, and in some cases reactivated, earlier, more ductile forethrusts and cut out all but one of the regional megascopic folds. While this backthrusting renders structural analysis of the early history of the area intractable, study of the petrogenesis of early, high-pressure assemblages yields some data. Deformation during subduction, leading to the subcretion of rock packets onto the over-riding plate is recorded by a ‘hairpin’ in the P-T paths of several units, and by a few relic structures within ophiolite units. The forms of P-T paths for units which have undergone high pressure metamorphism indicates that they underwent similar histories. Geochronological data for this area are as yet of insufficient resolution to decide if the tectonometamorphic events were continuous or episodic.
