The geology of the Vicentinian Alps (NE-Italy) : with special reference to their paleomagnetic history

Abstract

This geological study on the Vicentinian Alps is mainly an analysis of the tectonic and paleomagnetic data, collected by the author in the years 1959, 1960, and 1961. The stratigraphy is based for the greater part on data published in the first decenniums of this century. In the westtern part of the region the exposed basement consists of quartz -phyllites. The age of the strata overlying the basement ranges from Permian, to Miocene. During the Permian and the Mesozoic, sedimentation was mainly the result of a slow subsidence and a supply of clastic material from a land in the south (northern Adria and Po-block). Facieschanges are due to WNW-ESE trending uplifts, accompanied by magmatic (volcanic) activity (the Schio-Recoaro horst). In the late Cretaceous the southern land area subsided and the Alpine geanticline began to rise. The sedimentation in the Tertiary was mainly controlled by the presence of this source area in the north. In this period changes in facies took place also,which were mainly controlled by NNW-SSE trending uplifts (e. g. EuganeiBerici- Lessini horst). Volcanic activity is known to have taken place in the Vicentinian Alps in various periods. Next to basaltic dikes of either devonian or early permian age, two major volcanic periods can be distinguished; the Permo-Triassic and the Tertiary. During the Permo-Triassic mostly acid magmas intruded and effusive rocks were formed in either marine or terrestrial environment. During the Tertiary mostly basic magmas ascended, forming thick tuff- and lava deposits. It is noteworthy that the centres of maximal volcanic activity moved from southwest to northeast in the early Tertiary The tectonics are rather complex and may be ass igned to three tectogenetic periods. The first comprises the Hercynic orogenesis, in which two phases can be distinguished, one of subvertical movements (primary tectonics) and another of subhorizontal movements (secondary tectonics). During the first the region was elevated and heavily eroded; the second caused WNW-dipping overthrust and NNE-SSW striking folds (originally, Le. prior to the permotriassic rotation as determined by paleomagnetic data, the overthrust planes were dipping WSW and the fold axes were E-W. The second tectogenetic period is of permo-triassic age. At this time, a horst-like doming-up was caused in het Valli-Becoaro area by ascending basic magmas in an ENE-WSW striking zone of weakness. Originally, this zone had a WNW-ESE direction (see paleo-mag-. netism). Normal faults were formed,striking ENE-WSW (dipping north or south) and NNW-SSE (dipping east or west). The thi I'd tectogenetic period coincided with the alpine orogenesis. The Vicentinian region was located on the intersection of two hinge zones. One is ENEWSW and lies between the rising Alpine geanticline and the subsiding Adria mass; the other is NW-SE and forms the northern flank of the subsiding Po area. In both hinge belts the basement and the sedimentary cover reacted differently to differential vertical movements. In the basement, the older, permo - triassic zones of weakness were reactivated;ENEWSW striking step faults, dipping south, were formed as reaction to the subsidence of the Adria mass and NNW-SSE faults dipping east originated on the NE-side of the Po geosyncline. This second set of faults is closely related to the doming-up along the northeastern margin of the subsiding Po region, which can be interpreted as a correction of the isostatic equilibrium. The updoming was accompanied by volcank acti vity. The centra of volcanic acti viiy shifted northeastward (each time over about 30 km) between upper Cretaceous and Oligocene. The sedimentary cover slided off the south flank of the alpine geanticline to the south ("decollement "); this caused approximately E-W (Tauern-decollement) and NNE -SSW trending fold axes (Oetzdecollement). Only the former have been of importance in the Vicentinian Alps. These southward movements of the sedimentary cover were locally obstructed by the permo-triassic horst ("bufferzone"), causing the fold axes there to be striking NE-SW. In the younger Tertiary, NNE -SSW sinistral wrench faults caused anti -clockwise rotations of minor blocks. From paleomagnetic data of permotriassic and tertiary rocks may be concluded that NE-Italy has been subjected to majol' translations and rotations since the Permian. Three relatively fixed points are known. Firstly, the present position of the region (450 45' N. L., 110 30' E. L. ). Secondly, the position in the Eocene (NE-Italy must have been located on the +500 isocline of the Eocene, which runs across the present position of southern Italy). Thirdly, the position during the Permian (NE-Italy was probably on the intersection of the permian -300 isocline and the Tethys zone; this point is at the present location of the western Himalayas). Two major movements may be distinguished: awestdrift of the region during the Permian and Mesozoic (over maxima14800km) and a northdrift during the Tertiary (over 800 km). During the westdrift, the Vicentinian region rotated about 600 anti-clockwise in the late Triass ic. This rotation corresponds with right lateral shear movements along the north side of the Tethys zone. The northward movement of the region during the Tertiary may have been caused by a NW drift of Africa.