On the structure of the Ivrea‐Verbano Zone (northern Italy) and its implications for present‐day lower continental crust geometry

The Ivrea‐Verbano Zone in northern Italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the Alpine orogeny. Recent and on‐going structurally‐oriented geological mapping in this region is providing new information about the geometry of the complex. The central part of the zone is dominated by a large basic complex (the ‘mafic formation') which is intrusive into the surrounding gneisses. The foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning, but in the region south‐west of Monte Capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c. 10 km. This fold locally inverts the stratigraphy of the layered basic group of the complex, and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust. Several high‐temperature shear zones have now been traced within the country rock for distances up to 20 km. The geometry of these, and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body. Close parallels can be drawn between the observed structure in the Ivrea‐Verbano Zone (after removing the effects of late, low‐temperature faulting and folding related to emplacement of the rocks into their present position), and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the US Basin and Range province

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 1. The Miagliano Pluton

The Canavese Line in the Western Alps represents the position in the Alpine chain, where alkaline and calc-alkaline magmatism occur in close spatial and temporal association. In addition to available data on the alkaline Valle del Cervo Pluton, we present petrological and geochemical data on the Miagliano tonalite. The latter is of special interest, because it is located in the south-eastern side of the Canavese Line, in contrast to most Periadriatic Plutons. The dioritic to tonalitic rocks of the Miagliano Pluton represent an intermediate stage of a calc-alkaline differentiation, demonstrated by relics of two different pyroxenes as well as the texture of allanite. Hornblende barometry indicates pressures of similar to 0.46 GPa consistent with the presence of magmatic epidote. Field relationships between the two Plutons, the volcanic and volcaniclastic rocks of the Biella Volcanic Suite and numerous dykes cross-cutting the different units, allow reconstruction of a more refined chronology of the calc-alkaline and alkaline magmatic series. High precision zircon geochronology yields an age of 33.00 +/- A 0.04 Ma for the central tonalitic part of the Miagliano Pluton and 30.39 +/- A 0.50 Ma for the granitic core of the Valle del Cervo Pluton. The difference in age combined with cooling data and intrusion depth indicates dissimilar tectonic transport east and west of the Canavese Line. The earlier emplaced Miagliano Pluton has to be exhumed from an intrusion depth of similar to 12-15 km, whereas the neighbouring and younger Valle del Cervo Pluton is exhumed from a depth of 5-7 km. This tectonic scenario is related to upper crustal rigid block rotation responsible for the burial of the lowermost Rupelian paleosurface of the Sesia-Lanzo Zone. Thus, the new ages constrain the paroxysm of the orogenic magmatism in the internal Western Alps to an extremely short lapse of time in the first half of the Rupelian.</p