3D CRUSTAL ARCHITECTURE OF THE ALPS-APENNINES JOIN - A NEW VIEW ON SEISMIC DATA

Seismic data from the Alps-Apennines join have usually been interprete d in the form of 2D cross-sections, passing either through the Western Alps or the Ligurian Alps-Monferrato Apennines. However, the oblique SE-NW convergence of Adria and Europa and superimposed rotations imply a distinct 3D kinematic development around the Adriatic Indenter (AI) , the westernmost spur of Adria. In order to develop kinematic models, data on motion at the different margins of AI must be coordinated. Al ong the northern margin, the dextrally transpressive Insubric line (IL ) was active between 25 and 16 Ma (Insubric-Helvetic phase of Alpine o rogeny). Contemporaneously, along the southern margin (Paleo-Apenninic phase), a complementary sinistral motion took place along the Villalv ernia-Varzi line (VVL). It emplaced the Monferrato Apennines westward to the north of the Ligurian Alps by carrying them westward on top of AI. Between 14 and 6 Ma (Jura-Lombardic phase of Alpine orogeny) the L ombardic thrust belt developed on the northern margin of AI, now large ly hidden under the Po plain. Its continuation to the southwest is imp eded by older thrust masses along the Western Alps that consist largel y of basement, their sediments having been eroded, as noted on the dee p reflection line CROP ALPI-1 by earlier investigators. This line, mor eover, contains a deep reflection band originating in the autochthonou s Mesozoic of the Apenninic foredeep. In order to better visualize thi s origin and the relation of further elements identified on reflection lines around the northwestern end of the Monferrato Apennines, a 3D f ence diagram was constructed. It helps in establishing a 3D structural -kinematic model of the Alps-Apennines join based on the kinematics of AI. This model features an underthrust of AI under the western Alps i n the Paleo-Apenninic phase. In the course of this underthrust, the Pa leo-Apenninic elements of the Monferrato moved under the marginal thru sts of the western Alps. Subsequent Neo-Apenninic thrusting brought bo th elements together to the surface where they now form the Monferrato and Turin hills. A derivation of the Alpine Collina di Torino from th e south instead of from the west, as recently proposed, meets with ser ious kinematic difficulties.