Abstract

The discovery and preliminary mapping of large recumbent fold structures in the Pennine zone of the Central Alps (Switzerland and Italy) justifies a review of the overall structure of this zone. For descriptive purposes, the zone is subdivided into an Upper Pennine and a Lower Pennine complex, defined roughly as in earlier literature. Below these, a “Subpennine” complex, which includes the Gotthard massif, is recognized. The complexes consist of a “nappe structure” — interleaved or intermixed pre-Triassic basement and Mesozoic cover units (nappes), already showing a complicated history of thrusting, intense folding, and imbrication or even formation of tectonic mélanges — affected by later (“postnappe”) recumbent folding on a regional scale. In an even later phase of deformation, the “root zones” developed. These are referred to as steep belts, because the implied genetic connection of “roots” to nappe formation can no longer be upheld. Paleogeographic reconstruction of the Pennine realm depends on a correct “unfolding” of these fold phases. This is not yet possible for the postnappe recumbent folds, first because they have not yet been completely mapped, and second because reliable criteria for deciding how the nappe structure lay before folding are not yet available. Plate tectonic interpretation of the Pennine zone, which contains the only ophiolite complexes in the Alps, will depend to some extent on a clarification of regional relations on the basis of this newly emerging structural picture.

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