Geology and permian paleomagnetism of the Merano region, province of Bolzano, N. Italy - PhDData

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Geology and permian paleomagnetism of the Merano region, province of Bolzano, N. Italy

The thesis was published by , in September 2022, Utrecht University.


Geology Stratigraphically, the facies of the permo-triassic deposits of the Merano area is closely connected with that of the Southern Dolomites. Structurally, the Merano region belongs to the southern flank of the east-alpine geanticline. The southern part of the alpine geanticline is cut in the Merano region by a major fault with a southwest-northeast trend, called Judicaria fault (fig. 2, p. 15). In former syntheses of the structure of the Eastern Alps, the Judicaria fault has generally been considered as a sinistral transcurrent or wrench fault. This transcurrent fault was assumed to have displaced the suture line between the northern or alpine branch sensu stricto and the southern or dinaric branch of the Alpine Mountain System (sensu largo). The net slip between the Insubric or Tonale line in the west and the Pusteria-Drau line in the east would then be about 80 km. However, the structure and the analysis of the minor tectonic features in the Merano area do not confirm this concept. On account of the outcrop of the Judicaria fault its present character is that of a steep upthrust with a northwestern dip (fig. 27, sections I,ll, VI, VII and IX, p. 38). However, according to the tectonic features encountered close to the Judicaria fault, this fault was originally a normal fault with a southeastern dip (fig. 28, subphase b, p. 41). The vertical component of the displacement of the fault amounts to 5-9 km near the town of Merano. The formation of the fault facilitated the diapiric ascent of tonalitic magma, causing the emplacement of the massifs of Monte Croce, Ivigna, and Bressanone (fig. 28, subphase c). These plutonic masses belong to the peri-adriatic series of tonalites, which most probably, intruded during the mid-tertiary or insubric phase of the alpine orogenesis. During the intrusion of the tonalitic bodies the upper part of the Judicaria fault assumed a steep northwestern dip, probably because of a magma-tectonic mass-circuit (fig. 28, subphase c). The geological data of the Merano area fit in with van Bemmelen’s concept of the east-alpine structural evolution (van Bemmelen, 1957, 1960-b, 1960-c). Paleomagnetism A study was made of the paleomagnetic properties of the permian quartz-porphyries occurring at the northwestern margin of the Bolzano volcanic province. Progressive partial demagnetization experiments on orientated samples, by means of alternating magnetic fields indicate that the remanent magnetization of the quartz-porphyries is composed of two components, namely a relatively unstable component, and a stable component. The stable component probably represents the original permian magnetization acquired upon cooling of the volcanics. The unstable component probably resulted from the induction of postpermian magnetic fields. These secondary components can be removed by exposing the samples to alternating magnetic fields. This procedure of magnetic “cleaning” greatly reduces the scatter of the primary directions of magnetization. The permian magnetic south pole position deduced from the average direction of magnetization (declination 164°, inclination -7,5°) is 1460 Wand 45° N. Though the latitude of this pole position is in agreement with that of the poles inferred from other permian rocks from the european continent, its longitude deviates appreciably from that of the average european permian pole position, namely about 40° (fig. 42, p. 53). The following two possibilities for the cause of the above mentioned deviation may be suggested: The divergence of the north italian permian magnetic pole might be the result of secular variation; but also a geotectonic cause might be possible, for instance a counter-clockwise rotation of the Merano region around a vertical axis

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