Previous Next Tübinger Geowissenschaftliche Arbeiten, Series A, Vol. 52, pp. 19 - 23.
Abstracts of the 4th Workshop on Alpine Geological Studies, Tübingen 21-24 Sept. 1999

BLOCK PRESENTATION B017

Oligocene-Present brittle tectonics and differential exhumation of the western Alps between the Canavese line and the Mont Blanc massif. Integrated remote sensing, structural analysis, paleomagnetism, fission track and seismotectonic data

Andrea Bistacchi 1, Matteo Massironi 1, Massimiliano Zattin 2, Paolo Baggio 3, Maria L. Balestrieri 4, Giorgio V. Dal Piaz* 1, Antonio Guermani 1, Silvana Martin 1, Giorgio Pennacchioni 1


 1 

Dipartimento di Geologia e Geofisica, University of Padova, Italy

 2 

Dipartimento di Scienze della Terra e Geologia Ambientale, University of Bologna, Italy

 3 

Laboratorio Analisi Territoriali, University of Padova, Italy

 4 

Dipartimento di Scienze della Terra, University of Firenze, Italy

 * 

Correspondence:  via Giotto 1, 35137 Padova, Italy (dalpiaz@geol.unipd.it)

 

This block presentation deals with the Oligocene to Present brittle deformation and differential exhumation of the western Austroalpine-Penninic nappe stack, between the Canavese line, the Penninic frontal thrust, the Simplon normal fault and the Gran Paradiso massif, focusing on the Aosta valley and surroundings (figures and more details are reported in Bistacchi et al, poster). As shown by the CROP-ECORS deep seismic experiment, the Austroalpine-Penninic wedge is a floating belt devoid of welded lithospheric mantle, sandwiched between the Southalpine indenter and the European lower plate (Nicolas et al., 1990; Polino et al., 1990). From top to bottom, it consists of the Austroalpine tectonic system (Sesia-Lanzo zone, Dent Blanche nappe s.l.), the structurally composite ophiolitic Piedmont zone, the Monte Rosa-Gran Paradiso and Grand St. Bernard continental nappes and the Sion-Courmayeur (Valais, North-Penninic) zone, including the Versoyen ophiolite. The Mesozoic starting configuration and the Cretaceous-Eocene contraction of this sector of the western Alps are illustrated in the poster session (Cortiana et al.; Dal Piaz). Briefly, the Austroalpine-Penninic nappe stack was accomplished before and during the continental collision. The contraction was characterised by a subduction-related low thermal regime, which, in the Late Eocene, changed to relatively high-T conditions, as it can be inferred from the eclogitic (locally UHP) and blueschist facies imprint of Late Cretaceous-Middle Eocene age, followed by the Late Eocene-Lower Oligocene greenschist facies regional overprint. Since the Austroalpine-inner Penninic nappe stack had been already uplifted, cooled and hardened during the Oligocene, its brittle evolution begun coeval with the intrusion of post-metamorphic calc-alkaline to ultrapotassic dikes and plutons at about 31-30 Ma. In the NW Alps, brittle tectonics has been often underestimated, mainly because of the lack of reliable reference horizons and distribution of brittle deformation along wide bands. An integrated approach, by the combined use of different field and laboratory techniques, was attempted to tackle these problems.

 

 

Bistacchi, A. ,   Eva, E. ,   Massironi, M. ,   Solarino, S. , in press.,  Miocene to Present kinematics of the NW-Alps: evidences from remote sensing, structural analysis, seismotectonics and thermochronology. J. Geodynamics, in press.

Dal Piaz, G. V. ,   Venturelli, G. ,   Scolari, A. , 1979,  Calc-alkaline to ultrapotassic postcollisional volcanic activity in the internal northwestern Alps. Mem. Sc. Geol. Padova, 32: 16pp.

Diamond, L.W. , 1990,  Fluid inclusion evidence for P-V-T-X evolution of hydrothermal solutions in Late-Alpine gold-quartz veins at Brusson, Val d'Ayas, northwest Italian Alps. Am. J. Sci., 290: 912-958.

Freeman, S.R. ,   Inger, S. ,   Butler, W.H. ,   Cliff, R. A. , 1997,  Dating deformation using Rb-Sr in white mica: Greenschist facies deformation ages from the Entrelor shear zone, Italian Alps. Tectonics, 16: 57-76.

Gallagher, K. , 1995,  Evolving temperature histories from apatite fission track data. Earth. Planet. Sci. Letters, 136: 421-435

Hubbard, M. ,   Mancktelow, N.S. , 1992,  Lateral displacement during Neogene convergence in the western and central Alps. Geology, 20: 943-946.

Hunziker, J.C. ,   Desmons, J. ,   Hurford, A.J. , 1992,  Thirty-two years of geochronological work in the Central and Western Alps: a review on seven maps. Mém. Géol., Lausanne, 13: 59pp.

Hurford, A.J. ,   Hunziker, J.C. ,   Stöckhert, B. , 1991,  Constraints on the late thermotectonic evolution of the western Alps: evidence for episodic rapid uplift. Tectonics, 10: 758-769.

Mancktelow, N. S. , 1985,  The Simplon Line: a major displacement zone in the western Lepontine Alps. Eclogae geol. Helv., 78: 73-96.

Mancktelow, N. S. , 1990,  The Simplon Fault Zone. Beitr. Geol. Karte Schweiz, 163: 74pp.

Mancktelow, N.S. , 1992,  Neogene lateral extension during convergence in the Central Alps: Evidence from interrelated faulting and backfolding around the Simplon pass (Switzerland).. Tectonophysics, 215: 295-317.

Maurer, H.R. ,   Buckhard M. ,   Deichmann, N. ,   Green, A.G. , 1997,  Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley. Terra Nova, 9: 91-94.

Nicolas, M. ,   Hirn, A. ,   Nicolich, R. ,   Polino, R. ,   ECORS-CROP Working Group , 1990,  Lithospheric wedging in the western Alps inferred from the ECORS-CROP traverse. Geology, 18: 587-590.

Polino, R. ,   Dal Piaz, G. V. ,   Gosso, G. , 1990,  Tectonic erosion at the Adria margin and accretionary processes for the Cretaceous orogeny of the Alps. in Roure, F. ,   Heitzmann, P. ,   Polino, R., Deep Structure of the Alps, Mém. Soc. géol. France

Seward, D. ,   Mancktelow, N. S. , 1994,  Neogene kinematics of central and western Alps: evidence from fission-track dating. Geology, 22: 803-806.

 

SUB-BLOCKS155

 

Fault network reconstruction by satellite remote-sensing investigation

Matteo Massironi


  

Dipartimento di Geologia e Geofisica, University of Padova, Italy
   Correspondence:  via Giotto 1, I-35137 Padova, Italy (matteo@dmp.unipd.it)

 

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).

 

 

 

SUB-BLOCKS156

 

Thermochronological constraints to brittle tectonics

Massimiliano Zattin


 

Dipartimento di Scienze della Terra e Geologia Ambientale, University of Bologna, Italy
   Correspondence:  via Zamboni 67, I-40127 Bologna, Italy (zattin@geomin.unibo.it)

 

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 m and 13.5 m) 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 230C - during the Oligocene. A fast cooling can be envisaged for both units down to 125C, when a differential exhumation begun. The Sesia-Lanzo zone continued to cool fast down to at least 60C, 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. 60-120C): 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 95C for samples with oldest ages and more than 120C 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.

 

 

 

SUB-BLOCK S157

 

Paleomagnetic constraints to fault-bounded block rotations

Andrea Bistacchi


  

Dipartimento di Geologia e Geofisica, University of Padova, Italy
   Correspondence:  via Giotto 1, I-35137 Padova, Italy (bistek@dmp.unipd.it)

 

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. 150-300C. 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 250-150C 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 100C) 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.

 

 

 

 

SUB-BLOCK S158

 

Kinematic analysis and the Oligocene to Present evolution

Andrea Bistacchi


Dipartimento di Geologia e Geofisica, University of Padova, Italy
  Correspondence:  via Giotto 1, I-35137 Padova, Italy (bistek@dmp.unipd.it)

 

In the Oligocene, a general cooling (due to exhumation) lead to the onset of brittle conditions in the Austroalpine and Penninic units of the Aosta Valley. Kinematic analysis of brittle features always indicates direct to transcurrent displacements. Two brittle tectonic phases, developed from the Oligocene to the Present, have been recognised by consistent crosscutting relationships. In this progressively cooling environment, all brittle structures obviously crosscut the ductile ones. Hence a pre-Oligocene age may be envisaged for the ductile backthrusting-backfolding processes across the NW Alps, previously referred to the Oligo-Miocene by a comparison with the Central Alps. This is confirmed for the Entrelor back-thrust, which yields about 35 Ma (Freeman et al., 1997). The first brittle tectonic phase developed in the Oligocene, through combined displacements along three principal fault sets, giving an overall NW-SE extension: a) the system of low-angle, NW- and SE-dipping brittle-ductile detachment horizons, mainly developed within the relatively weak Piemont calcschists; b) the high-angle, NW- and SE-dipping set of brittle normal faults (always overprinting the former); c) the Aosta-Ranzola fault system. During this phase, a T-related rheological transition took place in the Piemont calcschists underlying the Austroalpine system. In fact, the diffuse low-angle detachments (a) evolved into sharp high-angle fault planes (b), always showing the same NW-SE extension direction. Conversely, in harder rocks like Austroalpine and Penninic gneisses, only high-angle brittle faults, and no low-angle detachments, have been found. This extensional phase lead to the differential exhumation of large fault-bounded blocks of the nappe stack, well represented by cooling rate contour maps, based on published and new cooling ages (Ap FT, Zr FT, Rb/Sr Bt ages; Hunziker et al. 1992; Seward and Mancktelow 1994). In addition, the Oligocene age of this extensional phase is supported by the syn-tectonic emplacement of calc-alkaline dikes (30-32 Ma, Dal Piaz et al. 1979), gold-bearing veins and hydrothermal alteration/metasomathism preferentially developed along large normal fault zones (31-33 Ma, Diamond, 1990) The second brittle tectonic phase is related to the well known orogen-parallel escape, which is coeval with the tectonic unroofing of the Lepontine dome along the SW-dipping Simplon fault (Hubbard and Mancktelow, 1992). During this phase, the Pennine-Graian nappe stack constitutes a continuous block (PGA block), characterised by high-strain border-zones and a rather homogeneous internal deformation. The border-zones of the PGA block are: 1) the normal SW-dipping Simplon fault; 2) the broad dextral strike-slip system, constituted by the Rhone and Chamonix lines an by the Penninic and Briançonnais reactivated frontal thrusts; 3) the recently discovered sinistral strike-slip Ospizio Sottile fault. A complex network of faults and fractures dissects the PGA block. Even if these faults did not generally experience large displacements during the Miocene, they constitute an important strain marker, since they have a regular frequency and indicate an overall NE-SW extension of the block. Seismicity is mainly concentrated along the strike-slip border-zones and focal plane solutions generally agree with surface data. Thermochronology indicates that, from the Late Miocene onwards, no major changes took place in the exhumation pattern, and hence in the overall kinematics. This evidence supports the general agreement between present-day deformation (seismotectonic data) and kinematics in the recent-past (structural geology data).