Mountain Building Paradigms and New Tools for Tectonic Analysis In the Continental Crust of the European Alps

Overthrust nappe theory germinated in the foreland belt of the alpine chain, but the structure and kinematics of the axial collision zone soon became the intriguing tectonic problem. About a century ago separation of old continental from Mesozoic oceanic rocks made clear the overall architecture of the suture zone between African and European margins; ductile low angle upthrusting was envisaged to have generated the thick pile of pre-Alpine and Mesozoic continental and oceanic (former geosynclinalic) sequences named Pennine nappe belt (ARGAND, 1911; STAUB, 1917). Later, addition of environmental details to fragments of Mesozoic sediments, stuck solution of the thickening kinematics upon its starting point of pre-orogenic paleogeograpy and this view generated virtual problems of lower crustal mass excess within the collisional mechanics, that lasted till the sixties (LAUBSCHER, 1971). Since the seventies, tectonothermal evidences from metamorphic rocks, displayed by estimates of petrogenetic conditions of mineral assemblages made subduction signatures evident (ERNST, 1971; DAL PIAZ et al., 1972) and imposed lithosphere-scale thickening processes. Crustal location of pre-Alpine protoliths was revealed in many of the stacked continental and oceanic slices from polyphased pre-orogenic tectono-thermal imprints, together with signs of contrasting P-T regimes, related to Alpine collision, to pre-Alpine rifting or Variscan tectono-metamorphic episodes. Origin of the recumbent-like overthrust nappe finite architecture was made manifest as a long polyphase process generated at various crustal depths (HALL, 1972; HIGGINS, 1964; MILNES, 1978) and fossil mineral equilibria preserved in protected strain domains became the effective key to unravel repeated episodes of tectonic coupling and decoupling in the subduction-exhumation-collision- cycle (SPALLA et al., 1996). Multi-scale structural analysis and P-T estimates are the investigation tool (tectono-metamorphic units = TMU; SPALLA et al., 2005) of the relative translational viscous flow of coupling and decoupling tectonic units, that opened promising perspectives in the Alpine subduction-exhumation cycle.