| Popis: |
We discuss the important role of tangential stretching in the Central Alps highlighting that the Alps are a truly three-dimensional orogen. A review of pressure-temperature (P-T) conditions allows us to define three major breaks in metamorphic pressure (P-breaks) across the nappe pile. We constrain the kinematics of mylonites associated with these P-breaks and report eight new Rbsingle bondSr multi-mineral isochrons that define the age of the mylonites. Kinematic data and Rbsingle bondSr geochronology indicate that the timing of top-SE normal shearing in the Avers-Turba mylonite zone (≥45–34 Ma; P-break1) and in a zone of distributed top-E normal shearing (>35–30 Ma) above the (ultra)high-P Adula nappe of the distal, thinned European margin (P-breaks 2 and 3). The ages from both sets of mylonite are grading into each other at about 35–34 Ma. All ages appear to postdate high-P metamorphism in the Pennine nappes but are, in part, coeval with and predate (ultra)high-P metamorphism in the underlying Adula nappe of the distal European (Helvetic) margin. The oldest Rbsingle bondSr age of 42.3 ± 2.5 Ma (2σ uncertainties) dates the waning stage of mylonitization and indicates that the Avers-Turba mylonite zone started to operate ≥45 Ma, and predated the exhumation of the Adula nappe by ~10 Ma. We suggest that the motion in the Avers-Turba mylonite zone overlapped with backfolding of the Schams nappe in an extrusion wedge. When this process started, the Adula nappe was still being underthrust/underplated. The ages for distributed top-E normal shearing are contemporaneous with thrusting of the Adula nappe on top of distinctly lower-P Helvetic nappes. When the Adula nappe was being thrust onto non-high-P nappes the necessary reduction of its overburden was largely accomplished by distributed top-E normal faulting above the Adula nappe, and possibly still ongoing top-SE normal shearing. We discuss that major normal shearing occurred during lithospheric shortening. Our data indicate considerable tangential (out-of-plane) movements during overall top-N/NW thrust propagation in the Central Alps and have implications for current tectonic models of the Alps. |
