Giulio Viola

Late Oligocene-Neogene evolution of Europe-Adria collision: New structural and geochronological evidence from the Giudicarie fault system (Italian Eastern Alps)

The Giudicarie fault system (Giudicarie sensu stricto, Meran-Mauls, Passeier, Thurnstein, and Jaufen faults) represents a sharp break in the generally E-W strike direction of the orogen-scale Periadriatic fault system and is a key element for understanding the late Oligocene-Neogene evolution of the Alpine chain. The kinematics, timing and magnitude of movements on the Giudicarie fault system are presented here, on the basis of a detailed structural study of the component fault segments combined with zircon and apatite fission track analysis from closely spaced samples. Two main tectonic phases are established: (1) back thrusting of the Austroalpine units over the Southern Alps around 32 Ma, recorded by basement and limestone mylonites along the Giudicarie and Meran-Mauls faults with transport directions toward 100°–110°, and (2) later sinistral transpressive displacement, characterized by structures at the ductile-brittle transition, which overprinted the top to E/ESE thrust-related mylonites but also partitioned into a major system of transcurrent faults in the Southalpine domain. It was during this later event that the Periadriatic fault attained its present-day geometry. However, the amount of sinistral displacement along the Giudicarie system was only ∼15–20 km. The magnitude is established here from the ∼15 km sinistral offset of the Jaufen mylonites across the Passeier brittle fault and the ∼20 km long gap in the otherwise continuous distribution of Oligocene tonalitic lamellae along the Giudicarie fault. A direct structural connection is also established between the Brenner and the Jaufen faults. This constrains the timing of the second phase, since it must postdate the main exhumation phase of the Tauern Window at 20–18 Ma. The results of this study argue against an originally straight Periadriatic fault. The Giudicarie fault formed a restraining bend in this part of the Periadriatic fault system since at least the late Miocene and probably since the late Oligocene.

Preferential distribution along transcontinental corridors of kimberlites and related rocks of Southern Africa

Regional and local structural controls on the emplacement of 1326 Southern African kimberlites and related rocks (kimberlites sensu lato, 11% of which are dated) are analysed using a framework of lineaments defined by combining geology, aeromagnetics, gravity and geomorphological data. Spatial analysis of occurrences within clusters of kimberlites less than 100km across resolves variable trends, depending on the age and position of the cluster; but on a regional scale the distribution of these clusters is statistically controlled by four lineament trends: 040°, 096°, 134° and 165°. Similar regional trends are observed as aspect lineaments that can be followed over large distances from modelling the variation in dip direction of the Southern African topography. These observations suggest that different geological parameters exert a control on the distribution of kimberlites. Local structures may include en-echelon fault arrays, Riedel, R’-, P- or T-structures within trans-continental lithosphere structures (cryptic continental corridors). Many cryptic continental corridors are collinear with fracture zones along the Atlantic and Indian continental margins of Southern Africa, and may have found their origin in events resulting from plate reorganization during the break-up of the supercontinent Gondwana. Fault resistance may have rapidly changed the stress state of the African continent causing the deep lithospheric faults to be the loci of episodic extension, allowing kimberlite fluids to ascend through the faults and cluster within near-surface structures. A progressive age variation of kimberlite magmatism in Southern Africa may be attributed to stress propagation along deep lithospheric fractures.

The analysis of quartz c-axis fabrics using a modified optical microscope

A new fully automated microfabric analyzer (MiFA) is described that can be used for the fast collection of high‐resolution spatial c‐axis orientation data from a set of digital polarized light images. At the onset of an analysis the user is presented with an axial‐distribution diagram (AVA –‘Achsenverteilungsanalyse’) of a thin section. It is then a simple matter to build‐up c‐axis pole figures from selected areas of interest. The c‐axis inclination and colatitudes at any pixel site is immediately available to create bulk fabric diagrams or to select measurements in individual areas. The system supports both the interactive selection of c‐axis measurement sites and grid array selection. A verification process allows the operator to exclude dubious measurements due to impurities, grain boundaries or bubbles. We present a comparison of bulk and individual c‐axis MiFA measurements to pole figures measured with an X‐ray texture goniometer and to data collected from a scanning electron microscope furnished with electron backscatter diffraction (EBSD) facility. A second sample, an experimentally deformed quartzite, illustrates that crystal orientations can be precisely linked to any location within an individual grain.

Growth and collapse of a deeply eroded orogen: Insights from structural, geophysical, and geochronological constraints on the Pan‐African evolution of NE Mozambique

This paper presents results of a large multidiciplinary geological mapping project in NE Mozambique, with a focus on the structural evolution of this part of the East African Orogen (EAO). It integrates field structural studies with geophysical interpretations and presents new geochronological data. The tectonic architecture of NE Mozambique can be subdivided into five megatectonic units on the basis of lithology, structure and geochronology: unit 1, Paleoproterozoic Ponta Messuli Complex in the extreme NW corner of NE Mozambique, which represents the local NW foreland to the EAO; unit 2, a collage of Mesoproterozoic metamorphic complexes, which forms the basement to unit 3, a stack of Neoproterozoic, NW directed imbricate thrust nappes named here the ‘‘Cabo Delgado Nappe Complex’’ (CDNC); unit 4, restricted Neoproterozoic metasedimentary basins; and unit 5, two exotic Neoproterozoic granulite me´ lange complexes. The units were assembled during a long and complex history of NWdirected shortening, which commenced with nappe stacking and emplacement of the CDNC over the Mesoproterozoic basement terranes toward the NW foreland. It is proposed that the CDNC and the Eastern Granulites farther north in Tanzania are remnants of Neoproterozoic volcanic arcs and microcontinents formed ‘‘outboard’’ of the Mesoproterozoic continent after 596 ± 11 Ma. Field and potential field geophysical data show that the nappes were folded by regional-scale NE–SW trending folds that formed in response to a later stage of the same shortening episode and this episode gave rise to the Lurio Belt, a prominent structural feature of northern Mozambique and a key element (often as suture zone) in many Gondwana reconstructions. The Lurio Belt is here interpreted as a structure generated during folding of the CDNC during later stages of the progressive shortening event. It is, however, a repeatedly reactivated shear zone, probably at the site of an older (Mesoproterozoic?) discontinuity, with an intense pure shear deformation history. It is cored by strongly attenuated lenses of a granulitic tectonic me´lange, the Ocua Complex (megatectonic unit 5) and is intruded by Late Pan-African granitoids of the Malema Suite. The compressional phase of the orogen was postdated by NW–SE directed extension. New U-Pb zircon and monazite dates show that extension was initiated at circa 540 Ma in the eastern Lurio Belt. It is argued that extension was the result of a major episode of orogenic collapse of the EAO, initiated by gravitational instabilities resulting from crustal thickening during the shortening phase.

Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on the western coast of South Africa

Abstract Apatite fission-track (AFT) data from two traverses across the Great Escarpment of the western coast of South Africa are used to reconstruct the tectonic evolution and denudation history of this sector of the Atlantic passive margin. Fission-track ages range between 180 and 86 Ma. Modelling of this data identifies two distinct cooling events. The first event, between 160 and 138 Ma, is recorded only by the rocks above the escarpment in the Karoo area, and is tentatively linked to post-Karoo magmatism (c. 180 Ma) thermal relaxation. The second, between 115 and 90 Ma, results instead from a tectonically induced denudation episode responsible for the removal of up to 2.5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau. Based on these results, it is suggested that the Cretaceous is the time when most of the elevated topography of Southern Africa was generated, with only a minor Cenozoic contribution.

The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps

The Pejo fault in the Italian Eastern Alps is a major sinistral transtensional fault. It marks the boundary between basement units displaying contrasting thermal histories, with Alpine (i.e., Mesozoic–Cenozoic) cooling ages preserved in the footwall juxtaposed against Variscan (i.e., Carboniferous– Permian) age in the hanging wall. Structural investigations, together with fission-track analysis, confirm a Late Cretaceous age for the Pejo fault, which excludes any direct kinematic contribution of the Pejo fault to the late Oligocene–Neogene evolution of the central-eastern segment of the Periadriatic fault. However, our results establish the importance of a major early Oligocene north-south to north-northwest–south-southeast shortening phase in the Central-Eastern Alps, which resulted in the development of new reverse shear zones, in the reactivation of the Pejo fault with a reverse motion, and in regionally important folding. The Pejo mylonites are folded on a kilometer scale around an east-northeast–trending axis. Field observations and fission-track analysis suggest a post-Oligocene age for the folding phase. Apatite fission-track data in the Pejo valley area reveal the base of a fossil apatite partial annealing zone exhumed to the surface. This finding argues for >4 km of exhumation since the Miocene, which was related to a major pulse of exhumation that began at ca. 15 Ma. This study suggests that the simple distinction between largely pre-Alpine fabrics of Variscan age in the hanging wall of the Pejo fault (Tonale nappe) and Alpine fabrics (Cretaceous) in the footwall (Campo-Ortler nappe) is not universally valid. Alpine overprinting is confined to the mylonitic shear zone itself. Deeper into the footwall, pre-Alpine structures are still well preserved. Earlier maps and interpretations based on a clear distinction between Tonale and Campo should be viewed with caution.

Structural overview of selected Group II kimberlite dyke arrays in South Africa: implications for kimberlite emplacement mechanisms

Group II kimberlite dykes occur in small, dominantly en-echelon dyke-fracture arrays, with individual dyke-fractures showing small angular variations from their array trends (5° to 15°). The analysed dyke systems are characterized by closely matching opposing dyke contacts, “ in-situ ” breccia, multiple kimberlite stringers within a dilated dyke-parallel fracture cleavage, wedge-shaped apophyses in bent bridges at dyke-fracture offsets/overlaps, kimberlite-free offset/overlap areas and calcite vein fibres orthogonal to dyke contacts. Commonly found microscopic structures include synemplacement/syncrystallization calcite veinlets, containing high aspect ratio stretched fibrous calcite, and elongate phlogopite phenocrysts and serpentinized olivine phenocrysts growing across the width of these veins. Both macro- and microscopic structures support a model of orthogonal host rock dilation during kimberlite emplacement. Terminations of dyke-fracture segments show minimal curvature or overlap, suggesting that remote horizontal stresses dominated during their emplacement (“passive” intrusion), as opposed to magma overpressured systems wherein dyke or dyke-fracture overlaps curve strongly towards each other (“active” intrusion). The application of Mohr diagrams suggests that low differential stresses, with no or only a very minor shear component, prevailed at the time of emplacement. The dominance of remote horizontal forces, imparting small differential stresses to the brittle portions of the crust, a closely-spaced, dilating dyke-parallel fracture cleavage ahead of the dyke tip (imparting a local suction) and the low-volume, low-viscosity, highly volatile nature of kimberlitic magmas may explain their empirically-constrained high emplacement velocities. This, in turn, explains the means by which such magmas may entrain significant volumes of high specific-gravity mantle material. Mobile hydrofracturing in the fringe zones around dilated craton-scale jointing is proposed to be a viable mechanism for kimberlite emplacement.

Deconvoluting complex structural histories archived in brittle fault zones

Brittle deformation can saturate the Earths crust with faults and fractures in an apparently chaotic fashion. The details of brittle deformational histories and implications on, for example, seismotectonics and landscape, can thus be difficult to untangle. Fortunately, brittle faults archive subtle details of the stress and physical/chemical conditions at the time of initial strain localization and eventual subsequent slip(s). Hence, reading those archives offers the possibility to deconvolute protracted brittle deformation. Here we report K-Ar isotopic dating of synkinematic/authigenic illite coupled with structural analysis to illustrate an innovative approach to the high-resolution deconvolution of brittle faulting and fluid-driven alteration of a reactivated fault in western Norway. Permian extension preceded coaxial reactivation in the Jurassic and Early Cretaceous fluid-related alteration with pervasive clay authigenesis. This approach represents important progress towards time-constrained structural models, where illite characterization and K-Ar analysis are a fundamental tool to date faulting and alteration in crystalline rocks.

Manual extraction of bedrock lineaments from high-resolution LiDAR data: methodological bias and human perception

Abstract Manual extraction of topographic features from Light Detection and Ranging (LiDAR) images is a quick, cost effective and powerful tool to produce lineament maps of fractured basement areas. This commonly used technique, however, suffers from several biases. In this contribution, we present the influence of (1) scale, (2) illumination azimuth and (3) operator, which significantly affect results of remote sensing expressed as number, orientation and length of the mapped lineaments. Six operators (N1–N6) with differing experience in remote sensing and different Earth sciences backgrounds mapped the same LiDAR DEM of a fractured bedrock terrain located in western Norway at three different scales (1:20.000, 1:10.000, 1:5.000) and illuminated from three different azimuths (045°, 180°, 315°). The 54 lineament maps show considerable output variability depending on the three factors: (1) at larger scales, both the number and the orientation variability of picked lineaments increase, whereas the line lengths generally decrease. (2) Linear features oriented perpendicular to the source of illumination are preferentially enhanced. (3) Inter-operator result reproducibility is generally poor. Operators have different perceptions of what is a lineament. Ironically, this is particularly obvious for the results of the “most experienced” operators, seemingly reflecting a stronger conceptual bias of what lineaments are and an operational bias on how they should be mapped. Based on these results, we suggest guidelines aimed to improve the reliability of remote sensing lineament interpretations.

Inclined transpression at the toe of an arcuate thrust: an example from the Precambrian ‘Mylonite Zone’ of the Sveconorwegian orogen

Abstract The ‘Mylonite Zone’ (MZ) forms a major, arcuate terrane boundary in the Precambrian Sveconorwegian orogen of SW Scandinavia. SE-directed thrusting along this curvilinear shear zone emplaced the higher-grade Idefjorden Terrane to the west onto the lower-grade Eastern Segment terrane to the east. Detailed structural characterization of the MZ mylonites in two different localities (Värmlandsnäs and Bua peninsulas) reveals a complex three-dimensional strain pattern. Inclined transpression is inferred on the basis of coexisting (and broadly coeval) foliation-parallel oblique shearing (resolvable in a strike-slip and dip-slip component) and across-foliation shortening. The former accommodated the transpressive component of the MZ, and its kinematics is either sinistral or dextral depending on the local strike of the MZ with respect to the regional thrust shortening vector. The latter led to pure-shear shortening perpendicular to the thrust sheet and subsequent lateral extrusion parallel to the mylonitic foliation via the development of antithetic displacements. No significant strain partitioning is observed at the meso-scale and strain is thus truly triclinic. The example described is an outstanding case of triclinic deformation, confirms theoretical analyses of complex strain models and adds valuable natural field constraints to our knowledge of deformation in the crust.