THE TRANSITION FROM PASSIVE TO ACTIVE MARGIN TECTONICS - A CASE-STUDYFROM THE ZONE OF SAMEDAN (EASTERN SWITZERLAND)

The Zone of Samedan is part of a fossil, early Mesozoic rift system or iginally situated in the distal, Lower Austro-Alpine domain of the Adr iatic passive continental margin. An early Mesozoic configuration of a symmetrical rift basins bounded by relative structural highs compartme ntalized Late Cretaceous active margin tectonics; Jurassic half-graben s were folded into arcuate synclines, whereas relative structural high s engendered thin, imbricated thrust sheets. West-directed thrusting a nd folding initiated at the surface and continued to depths favoring m ylonitization under lower greenschist-facies conditions. At this time Liguria-Piemontese ophiolites were accreted to Lower Austro-Alpine uni ts directly underlying the Zone of Samedan. Late Cretaceous orogenic c ollapse of the Adriatic active margin involved the reactivation of wes t-directed thrusts as low-angle, top-to-the-east, normal faults. These faults accommodated extensional uplift of Liguria-Piemontese ophiolit es and Lower Austro-Alpine units beneath and within the Zone of Sameda n. During Paleogene collision, some Late Cretaceous faults in the Zone of Samedan were reactivated under lower anchi-zonal conditions as nor th-directed thrusts. The latter stages of this early Tertiary thickeni ng were transitional to brittle, high-angle normal faulting associated with top-to-the-east extension and spreading above the warm, upliftin g Lepontine dome.