Marta Marcucci

Patterns of faulting in the Mont Blanc granite

Abstract Faults at the meso scale in the Mont Blanc granite have been analyzed considering their strike, dip and pitch of the striae. The resulting pattern is characterized by mainly SW-NE striking, strongly dipping faults with a pronounced dip-slip character, associated with oblique or transversal strike-slip faults. The dominant dip-slip, NE-trending faults constitute a downward converging fan more or less concordant with the wedge shape of the granite mass. Reverse faults largely prevail over normal ones. In some sections of the massif (especially the NW-section) the fault pattern can be analyzed in terms of a plane strain model under compression with the NW azimuth. Towards the southeast, two-dimensional models of deformation become increasingly inadequate, and the pattern needs being analyzed in three dimensions: it can be simulated by a half-cylindrical envelope of fault planes whose axis strongly dips towards NW. Dip-slip movement characterizes the fault planes lying at the bottom of this half-cylindrical “gutter”, while the movement becomes strike-slip in the faults whose planes lie at the gutters flanks. Although this is only a geometrical simulation, it may again be consistent with the hypothesis of a NW-directed overall compression. In all the examined sections of the massif the angle of dip of fault planes is much higher than expected by the Mohr-Coulomb theory with horizontal maximum compression. We suggest that this may be due to an association of faulting with bulk deformation, which may have resulted in both a pattern of shearing stresses along very inclined planes, different from that of perfectly elastic materials, and some rotation of already formed fault planes. The different patterns of NW- and SE-dipping faults, the bimodal distribution of the dip angles around the dominant NE-strike, and other features suggest that faulting occurred in more than one phase.

LONGITUDINAL STRIKE-SLIP FAULTS IN OCEANIC RIFTING : A MESOSTRUCTURAL STUDY FROM WESTERN TO SOUTHEASTERN ICELAND

Abstract Mesostructural analysis carried out in several localities of western, southern-central and southeastern Iceland shows an unexpectedly frequent occurrence of strike-slip faults parallel to, or lying at relatively small angles with, the axis of rifting. Some faults can be interpreted in terms of shear parallel to the rifts, whereas others form conjugate systems referable to rift-parallel compression. Axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral (rift-parallel) displacements between the major lithospheric plates. Axial compression may be attributable to rift-oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling. The data regard 1974 fault surfaces, mainly small-scale.

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

In the Argolis, the Basal Sequence, constituting the eastern Pelagonian margin which bordered the Maliac-Vardar oceanic domain, includes shallow-water carbonates of Late Triassic-Early Jurassic, condensed pelagic limestones of Early-Middle Jurassic, radiolarian cherts of late Middle-Late Jurassic age and siliceous mudstones and sandstones rich in ophiolite fragments. Up-section, coarse breccias, including clasts of boninites derived from the ophiolite obducted onto the Pelagonian margin in Late Jurassic times crop out. Near Angelokastron a small quarry exposes pervasively sheared dark reddish-brown, radiolarian-bearing cherty shales with disrupted fragments of chert and chert nodules impregnated by ferro-manganese oxides. These shales occur in the footwall of a thrust bringing them into contact with the Pantokrator Limestone of the Basal Sequence. We collected more than 30 samples of the chert fragments and the shaly matrix. Thirteen nodules and one matrix sample yielded determinable radiolarians. Low to non-detectable concentrations of trace metals such as Co, Cr, Cu, Ni, Zn, and Pb indicate a hydrothermal origin of the ferro-manganese mineralization. The radiolarian taxa found indicate four age groups for the nodules that are embedded in the siliceous shale matrix that yielded a Middle Jurassic age (middle Bathonian). The first group includes a nodule of Late Triassic age (late Norian to Rhaetian); the second group nodules of Early Jurassic age (late early to late Pliensbachian and probably middle-late Toarcian); the third group nodules of early Middle Jurassic age (Aalenian–Bajocian); the last group finally includes nodules of late Middle Jurassic age (Bajocian–Bathonian). The presence of Upper Triassic to Middle Jurassic Mn-impregnated chert nodules in a Middle Jurassic matrix indicates a deep oceanic environment of deposition outside the Pelagonian realm (easternmost Adria Plate), which at that time was a shallow-water carbonate platform with a thin pelagic limestone cover. The chert nodules are with all certainty derived from the oceanic Maliac-Vardar domain and were, together with their host formation, tectonically emplaced onto the Pelagonian margin. We speculate that these nodules, more lithified than their matrix, were exhumed on the slope of an intra-oceanic accretionary wedge and were redeposited in the Middle Jurassic siliceous mudstones on the floor of the subducting Maliac-Vardar Ocean.

Mesoscopic faults in the Bregaglia (Bergell) massif, Central Alps☆

Abstract The strike, direction of dip and pitch of the striae along mesoscopic faults in the Oligocene granodiorite-tonalite of Val Masino-Val Bregaglia (Bergell) are analysed. Most fault planes are steeply dipping, and show strike-slip or oblique-slip motion. Dominant strikes are NNW or NNE. A relative chronology of fault sets is suggested based on the presence of different minerals (chlorite and epidote) on fault planes. The pattern of mesoscopic faults in the Val Masino-Val Bregaglia massif does not follow the earlier tectonic trends of the Pennidic nappe edifice, nor even the trend of the nearby section of the Insubric Line considered at both regional and mesoscopic scales. The mesoscopic analysis of the Val Masino-Val Bregaglia massif thus reveals a fault system largely oblique to the major Alpine lineaments. The observed fault pattern does not reveal traces of thrusting referable to late Alpine orogenic phases, and can be related to subsequent deformation, dominated by strike-slip movements; this pattern does not match the traditional schemes of extensional dip-slip faulting following orogenesis. It records a stage of tectonic evolution which follows nappe emplacement, yet it precedes vertical or extensional post-orogenic tectonics.

Mesoscopic faults in the granite of Isola del Giglio (Tuscan Archipelago)

Abstract Faults at the mesoscopic scale in the granite at Isola del Giglio are examined by considering the strike, dip, pitch of the striae and minerals on the fault planes. The pattern of faulting is areally variable. Dykes and veins are often parallel to faults, suggesting a genetic connection. A major system of faults, mainly reverse and steeply dipping, strikes north-northwest and is characterized by the frequent occurrence of crushed tourmaline, these faults are often developed along pegmatite veinlets or dykes, and were probably produced by a combination of hydraulic and shear fracturing in the final stage of cooling of the granite. Another prominent phase of faulting is testified to by ENE-striking fault planes, strike-slip to oblique-slip, devoid of tourmaline and mostly coated with chlorite. p ]The prevalence of reverse and strike-slip faults over normal faults contradicts the traditional views about dominantly extensional tectonics in this area during the Pliocene and Quaternary. Extensional deformation at Isola del Giglio ended with the granite intrusion; crustal melting and extension ended at about 5 Ma, after which subvertical and strike-slip movements became dominant.

From obduction to continental collision: New data from Central Greece

The aim of this paper is to contribute to deciphering the evolutionary history of the Hellenides by the study of a large sector of the chain located between the front of the ophiolitic units and the external zones classically attributed to the continental margin of Adria. In particular, the tectonic units located in Boeotia – a key area located in Central Greece at the boundary between the Internal and External Hellenides – were studied from structural, stratigraphic and biostratigraphic points of view. Addressing the main debated aspects concerning the origin of the ophiolite nappe(s), the tectonic evolution of the Hellenic orogen was revised with a particular emphasis on the period between obduction and continental collision. New findings were compared with consolidated data concerning the main metamorphic events recorded in the more Internal Hellenides, geochemistry and age of the ophiolites and main stratigraphic constraints obtained in other sectors of the belt. Finally, a new reconstruction of the tectonic evolution of this area was introduced and, in the context of the dispute concerning the origin of the ‘ophiolitic belts’ as a possible record of multiple oceanic basins, we put forward for consideration a ‘single ocean’ tectonic model spanning from Triassic up to Tertiary times, and valid for the whole Hellenic–Albanian sector.

MIDDLE AND LATE HETTANGIAN RADIOLARIANS FROM THE MT. CAMICIA SUCCESSION (GRAN SASSO, CENTRAL APENNINES - ITALY)

Well-preserved middle and late Hettangian radiolarians have been discovered S-E of Mt. Camicia (Central Apennines) in limestone beds that contain also Hettangian ammonites. These beds are part of an Upper Triassic to Lower Jurassic carbonate succession, which crops out in the eastern part of the Gran Sasso range. This succession includes euxinic deposits (bituminous dolostone) and other pelagic sediments (mudstone and calcarenites) which developed in a basin rimmed by a carbonate platform. In the same time interval, the well-known formations of Dolomia Principale and Calcare Massiccio were forming on this platform. In this paper we examine the radiolarian assemblages collected in three levels of the ammonite bearing beds, and we define two new genera ( Squinabolia and Turritus ) and eight new radiolarian species ( Anaticapitula parvireticulata , Anaticapitula triangularis, Farcus aquilensis, Farcus leonseveroi , Parahsuum vraddense , Squinabolia multispinata, Turritus venturii, Zhamoidellum sphaericus ), and we define twenty four other species, for a total of thirty eight identified species. The ammonites in the lowest level indicate a middle Hettangian age, those from the two upper levels a late Hettangian. In the Western Tethys successions bearing Hettangian radiolarians are rare and have been found together with ammonites only in the Mt. Camicia section described in this paper. This section permits to correlate the radiolarian assemblages with the ammonite zonation.