Evidenze di tettonica recente e di glaciotettonica nel Mendrisiotto (Ticino meridionale, Svizzera)

Abstract

The detailed study of deformational structures present in the Messinian to Quaternary successions of Mendrisiotto (Southern Ticino, Switzerland) gives important new data on the local recent evolution of the Southern Alps. The Messinian Conglomerato di Pontegana is affected by two main sets of extensional joints and tension gashes, indicating E-W and NNW-SSE extension. The lower Pliocene Argille di Castel di Sotto and the Pleistocene tillites forming the lower part of the Spinee Member (Conglomerato di Mendrisio) are generally disrupted by E-W trending normal and normal-oblique faults, indicating an important extensional event. The stress tensor obtained using fault populations measured in these formations shows a subvertical σ1 axis and a roughly N-S trending subhorizontal σ3 axis. These fault systems are developed especially along the tectonic contact between the Gonfolite Group (Oligo-Miocene) and the Pliocene clays. The boundary between the two formations consists in an E-W trending, north-dipping normal fault which bounds the southern part of the Mendrisiotto region. These structures may represent the eastern prosecution of the active ENE-WSW dextral strike-slip faults recognized west of Lago Maggiore (Linea della Cremosina). In another view, these structures might be related to superficial accommodation of deep folds and thrusts possibly active in the Po Plain along the southern margin of the Southern Alps. Timing of motions along these fault systems can be constrained between Upper Pliocene and the first glaciations (Late Pliocene-Lower Pleistocene). In the upper part of the Spinee Member and in the Novazzano Member (Congiomerato di Mendrisio - Pleistocene) compressive structures as large folds developed in still soft sediments. Water escape structures and plastic deformation of unconsolidated layers indicate folding before final diagenesis. Glaciotectonic phenomena are believed responsible for the formation of such structures. Fold systems are generally conformable to the pushing direction of a glacier coming from the Como Lake region.