Interpretative Synthesis of Metamorphism in the Alps

Compilation maps are presented showing major tectonic features, selected lithologies, and zones of progressive metamorphism in the eastern half of the western Alps (scale 1:400,000) and the eastern Alps (scale 1:800,000). Two principal complexes are distinguished on them: (1) the Caledonian and Hercynian metamorphosed terrane in the Southern Alps + Austroalpine sheets, overlain by deformed but largely unrecrystallized uppermost Paleozoic and younger platform-type sedimentary rocks; and (2), the tectonically lower Sesia-Lanzo + Lepontine-Pennine + Helvetic realms, a sequence of Hercynian and pre-Hercynian plutonic igneous + metamorphic rocks and a younger, chiefly Mesozoic cover sequence consisting of shelf, slope, and deep-sea sediments + ophiolites, incompletely to pervasively recrystallized during Alpine metamorphism. 1. The pre-Mesozoic metamorphism involved several cycles in both complexes, but it has not been possible to distinguish these on the maps. Judging from the mineral para-geneses, recrystallization events seem to have taken place under moderate to very high temperatures at low to moderately high pressures. 2. Three contrasting but intergradational, temporally overlapping episodes of Alpine metamorphism are recognized in the Sesia-Lanzo + Lepontine-Pennine + Helvetic terrane: (a) an early, high-pressure, low-temperature event syntectonic with nappe formation, which produced eclogites + albite amphibolites, glaucophane schists, and allied greenschists, with lower grade, more recently metamorphosed sections lying externally (that is, toward the European foreland) relative to the older, progressively higher grade, more internal, imbricated sections lying to the south and east; (b) a middle syntectonic to post-tectonic stage characterized by more “normal” physical conditions, resulting in the partial or complete conversion of the products of event (a) to greenschist (prasinite) and low-rank amphibolite facies metamorphic rocks; and (c), a late, and in most cases syntectonic to post-tectonic recrystallization involving moderately high temperatures and pressures, which locally obliterated the products of both (a) and (b). Of the recrystallization continuum, event (a) is best preserved in the Franco-Italian Alps, and in Switzerland in the cantons of Wallis and Graubunden, (b) is nearly ubiquitous in the Sesia-Lanzo + Pennine + Helvetic complex, and (c) is confined to the Lepontine gneiss area of the Italian and Swiss Alps, and to the central gneiss domes of the Tauern Fenster, central Austria. The timing of Alpine metamorphism evidently varied laterally along and across strike of the belt. For instance, in Austria, event (a) may have begun in Late Cretaceous (?) time, whereas it probably commenced during Paleocene-Eocene time in the western Alps. Moreover, “early” Alpine, low-grade zeolitization occurred in the external parts of the Helvetic realm probably during Oligocene time— nearly contemporaneously with the more internal “late” Alpine higher grade Lepontine recrystallization of event (c). The contact between (1) the Southern Alps + Austroalpine nappes on the one hand, and the structurally lower (2) Sesia-Lanzo + Lepontine-Pennine + Helvetic realms on the other, juxtaposes rocks of markedly contrasting petrologic and tectonic histories. This zone, here referred to as the Alpine Suture, is postulated to represent the crustal expression of a Late Mesozoic-early Tertiary convergent lithospheric plate junction. The early Alpine high-pressure paragenesis appears to reflect subduction and shuffling of the more northerly terrane beneath the stable lithospheric dab capped by the Southern Alps + Austroalpine sheet. If so, the observed blueschist-type metamorphic zoning probably was generated by progressively greater depths of underflow and a consequent depression of the isotherms. A variable rate or time at which the complex was exhumed locally could account for the later establishment of a more normal thermal regime, and thus, succeeding higher temperature mineral assemblages as displayed in the Lepontine gneiss area and the Tauern gneiss domes.