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The brittle structural pattern of NW Alps was comprehensively investigated using the regional synoptic vision of satellite images. One Landsat TM scene and two ERS-1 Geocoded Terrain Corrected (GTC) images were processed, analysed and compared. The Landsat TM image interpretation was accomplished taking into account six of the seven TM bands, with the exclusion of the thermal infrared band for its low spatial resolution. The two high overlapping GTC scenes, acquired in ascendant and descendent orbits respectively, were integrated to partially avoid the radiometric distortions on sensor-looking slopes. Contrast histogram adjustments (equalisation, linear-piecewise, logarithmic and exponential stretch) were frequently implemented, for the enhancement of differently illuminated areas, during interpretation both on optical and radar data. In addition, some spatial-frequency and directional filters were applied on the Landsat TM product to complete line detection. 429 out of 1093 detected lineaments (Landsat TM) and 201 out of 747 lineaments (ERS-1-GTC) were checked through systematic field survey and bibliography. The reliability ranges from 65,6% to 84,2%, in function of lineament length and of different satellite products. Fields analysis proves that most of the lineaments, detected by image interpretation, are nearly subvertical faults (inclination angle * 60°) pervasively dissecting the NW Alpine nappe-stack into fault-bounded blocks. The remote-sensing and fieldwork approach evidences a pervasive network of brittle faults at the regional scale and gives more details on the already known major faults (Aosta-Ranzola, Canavese, Cremosina, Simplon, Tonale). In the following we will list the main points of this analysis. a) Three principal sets of high angle brittle elements (E-W, NE-SW and NW-SE trending) have been recognised all over the Pennine-Graian Alps (PGA). b) The Aosta-Ranzola Line was traditionally known as a single E-W trending vertical fault extending from the Ranzola Pass (Gressoney valley) to Aosta. Our work shows that: 1) this brittle feature is a 2 km wide half-graben type fault system, developed on both sides of the Aosta Valley, dipping to the North (50-70°) and with a normal displacement ranging from 500 m, at its eastern tip, to 2 km in the Aosta area; 2) the fault extends westwards as far as the Briançonnais and Penninic frontal thrusts in the Piccolo S. Bernardo area, branching into some WNW-ESE conjugate lines which suggest a dextral transcurrence; 3) the eastern portion of the fault, from the Ranzola pass to Aosta, is consistently associated with an intense hydrothermal activity, recorded by a diffuse syn-kinematic carbonatic metasomathism, especially developed in ophiolitic serpentinites (listvenitization), and by the presence of syn-kinematic quartz-gold lode deposits in the Brusson district. c) The presence of many E-W striking lineaments in the Southern Valais is confirmed by field and seismotectonic observations (Maurer et al., 1997; Bistacchi et al., in press). d) The NE-SW Ospizio Sottile fault is a narrow brittle horizon, at least 60 km long, steeply dipping to the SE and bounding at the eastern end the Aosta-Ranzola fault system. This tectonic line crosscuts the Austroalpine units from the Sesia Valley to the Ayas Valley (external sector of the Sesia Lanzo Zone and Verres slice), whilst to the south-west it runs through the ophiolitic units which border the eastern edge of the Gran Paradiso massif. The fault zone is evidenced by wide cataclastic horizons, pseudotachylytic vein networks, polished fault-planes and large volumes of damaged host rock, often associated with hydrothermal alteration (Bistacchi et al., in press). e) The linear configuration of the Simplon fault, recognisable on satellite images, suggested that the brittle overprint described by Mancktelow (1985, 1990, 1992) at the Simplon Pass could extend south-eastwards. This is confirmed along the master fault (Bognanco Valley) and also in the fault footwall (Varzo area). f) The brittle Centovalli line extends westwards beyond its complex intersection with the Simplon fault, to the northern edge of the Dent Blanche nappe. g) A regular set of NW-SE normal faults, spreading from the Simplon line to the Gran Paradiso massif, is related to the Neo-Alpine lateral escape suggested by Hubbard and Mancktelow (1992). h) The Canavese line is somewhere fragmented by intersections with other faults, especially in the Fobello-Rimella area. Mesostructural analysis, carried out on the middle portion of the line (Cervo Valley), evidenced N-W dipping brittle extensional features that overprint the mylonitic belt and dissect the andesitic cover of the inner Sesia Lanzo zone. i) Some E-W and NE-SW oriented lineaments occur in the northern, eastern and western edges of the Gran Paradiso dome. j) Low-angle, NW- and SE-dipping brittle-ductile detachments develop into the relatively weak Piedmont calcschists, which underlie the Sesia Lanzo zone and the northern Austroalpine klippen. These detachments consist of wide horizons with a pervasive extensional crenulation cleavage and of more localised fault planes with a foliated clayey-chloritic gouge. Similar structures, dipping to the north, have been found also to south of the Aosta-Ranzola fault system. All these detachments are overprinted by the previously mentioned high-angle fault sets (point a).

Fission-track analysis is a powerful tool to reconstruct the tectono-thermal evolution following the Eocene collision and related greenschist facies metamorphism. A geochronological survey, mainly based on fission-track analysis of apatite and zircon, was planned in the Aosta Valley to put temporal and spatial constraints to the brittle tectonics along major structures. At the time of writing, only some apatites have been dated. A first set of samples was collected inside the Aosta-Ranzola fault system, close to Aosta. With respect to the "classic" location of the lineament, five samples come from the northern side (Mont Mary klippe) and three from the southern sector (Austroalpine unit of Brissogne). Ages range from 19 to 29 Ma, without any correlation with sample elevation. Track length distributions (mean between 12.7 µm and 13.5 µm) indicate a moderate cooling rate. A second set of samples was collected across the Ospizio Sottile fault. Apatite ages span from 24 to 35 Ma, irrespectively with the altitude. One sample, located further south (Pont Boset), near the frontal thrust of the Sesia-Lanzo zone, yields an age of about 20 Ma. All samples show track length distributions typical of fast cooling. At a first glance, data show neither a significant age difference between the Sesia Lanzo zone and the Dent Blanche system nor a peculiar trend. Taking into account literature data on a couple of samples collected further north (Hurford et al., 1991), a relevant jump in age (14 Ma) seems to be evident at the same latitude in both sectors. A first intuitive explanation requires the existence of a large undetected E-W striking discontinuity and/or large displacements along the Neogene NW-SE striking normal faults, parallel to the Simplon line. Thermal modelling of fission track ages and length distributions by a robust optimisation procedure, based on genetic algorithms (Gallagher, 1995), was used to test this hypothesis and to refine the thermal history of the two sectors. The starting frame is given by the zircon ages, which appear to be uniform in all the Sesia-Lanzo zone and Dent Blanche system (about 30-33 Ma; Hurford et al., 1991), suggesting that rocks were at a similar temperature - about 230°C - during the Oligocene. A fast cooling can be envisaged for both units down to 125°C, when a differential exhumation begun. The Sesia-Lanzo zone continued to cool fast down to at least 60°C, whereas the Dent Blanche nappe started to cool slowly. The age of this variation in cooling rate can be placed between 28 and 30 Ma. The large range of apatite ages finds an explanation in the extreme track sensitiveness to temperature in the partial annealing zone (c.a. 60-120°C): in this range each change in temperature causes a significant response in the fission-track age. Thermal modelling shows that, for these samples, the discriminating factor is the temperature of the transition between fast and slower cooling rate. This temperature is about 95°C for samples with oldest ages and more than 120°C for samples with younger ages. Different temperatures correspond to different depths, hence modelled data suggest a first phase of rapid exhumation to crustal depths getting shallower to the south. Age variation in a restricted area inside the Aosta-Ranzola fault system may be due to relatively small vertical displacement subsequent to the change of cooling rate (28-30 Ma). On the other hand, along the Ospizio Sottile fault, no evidence of along-dip displacements during the Neogene is given by fission-track data. Elsewhere, local clusters of younger ages could be linked to warm fluid circulation through permeable inherited brittle structures.

Paleomagnetic analysis has been carried out on more than 50 sites, in all the main Austroalpine and Penninic units. A similar analysis has never been undertaken in this part of the Alps. Therefore, at the beginning many different rocks have been investigated in order to assess their magnetic mineralogy and the intensity and stability of their natural remnant magnetisation (NRM). A thermal demagnetisation approach has been followed, since magnetisation of metamorphic rocks, cooling down during exhumation, is very likely to be a thermo-remnant magnetisation (TRM), with some components of chemical remnant magnetisation (ChRM). From this preliminary study, it was evidenced that the ophiolitic calcschists are characterised by a low-temperature TRM carried by pyrrhotite, with an un-blocking temperature spectrum of c.a. 150-300°C. Rotations that took place in the low-temperature (brittle) field can be selectively recorded by such a low-temperature remnant magnetisation. Therefore, the following detailed analysis focused on the carbonatic metasediments. At the time of writing, 27 sites have been investigated in calcschists form both the hangingwall (N) and the footwall (S) of the Aosta-Ranzola fault system, and 16 samples gave reliable results. Even if this study is still in progress, it is possible to summarise some preliminary results. a) Cross-correlation between FT cooling ages, and thermal demagnetisation data yields an Oligocene age for the TRM acquired in the 250-150°C range. b) The directions of this component are generally characterised by low inclinations and scattered declination. Tilting of fault-bounded blocks, around horizontal or oblique axes, may explain this scattering. However, in some cases the effect of strain on remanence cannot be ruled out. Anyway, this complex pattern cannot be explained by simple rotations around vertical axes. c) Some very low temperature TRM (acquired at about 100°C) or ChRM components show the same average orientation as the present-day geomagnetic field, possibly indicating that no major rotations took place from the Miocene to the Present.