Giulio Viola - Academia.edu (original) (raw)

Papers by Giulio Viola

Bingen, B.; Viola, G.; Boyd, R.; Thomas, RJ; Jacobs, J.; Henderson, IHC; Nordgulen, Ø.; Sandstad,... more Bingen, B.; Viola, G.; Boyd, R.; Thomas, RJ; Jacobs, J.; Henderson, IHC; Nordgulen, Ø.; Sandstad, JS; Engvik, AK; Smethurst, M.; Skar, Ø.; Solli, A.; Key, RM; Daudi, EX. 2011 Geological evolution of Northeastern Mozambique, in the context of the Pan-African ...

Geological Society, London, Special Publications, 2010

The &#x27;Mylonite Zone&#x27; (MZ) forms a major, arcuate terrane boundary in the... more The &#x27;Mylonite Zone&#x27; (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 ...

Geological Society, London, Special Publications, 2009

Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on... more Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on the ... Africa and South America) from east Gondwana (Australia, Antarctica, India and New Zealand), post ... Modelling of the AFT data along this traverse has to take into account the ...

Precambrian Research, 2011

New structural and geochronological investigations of the “Mylonite Zone” (MZ), an arcuate terran... more New structural and geochronological investigations of the “Mylonite Zone” (MZ), an arcuate terrane boundary in southwest Scandinavia, contribute to a refined conceptual model for the Grenvillian–Sveconorwegian tectonic evolution of the Mesoproterozoic Sveconorwegian orogenic belt. During late convergence, around 970 Ma, the MZ acted as a top-to-the-SE thrust that accommodated crustal shortening in the eastern part of the orogen by juxtaposing the Idefjorden Terrane in the hanging wall in the west against the “Eastern Segment” footwall in the east. The eastward vergence of the MZ and of similarly oriented second-order nearby shear zones is interpreted as reflecting late back-thrusting within the overall W-vergent orogeny. Back-thrusting was possibly promoted by the backstop role played by the rigid block formed by the 1810–1650 Ma Transscandinavian Igneous Belt. During subsequent E–W crustal extension, the MZ thrust-related fabrics were reactivated in an extensional fashion with bulk top-to-the-W kinematics. This was triggered by gravitational instabilities resulting from crustal overthickening during the shortening phase. 40Ar-39Ar biotite and white mica ages from a greenschist-facies and extension-related mylonite range between 922 and 860 Ma. This long-lived episode is expressed by extensional structures that evolved continuously from purely ductile to brittle during progressive exhumation of the footwall. The “Eastern Segment” is interpreted as an immature asymmetric core complex, exhumed in the footwall of the extensional MZ, through the antithetic normal displacement of the MZ itself and of the top-to-the-E Sveconorwegian Frontal Deformation Zone farther to the east. The core complex is bound to the north by the transtensional Hammarö Shear Zone, characterized by penetrative constrictional fabrics, interpreted as indicative of an overall transtensional regime.► Structural analysis in the Mesoproterozoic Grenville-Sveconorwegian orogenic belt. ► Build up and collapse of the orogen in southwestern Scandinavia. ► E-vergent back-thrusting in overall west-vergent orogen along the Mylonite Zone. ► Asymmetric core complex development in a Precambrian transtensional regime. ► 40Ar-39Ar biotite and white mica ages record extension between 922 - 860 Ma.

Journal of Structural Geology, 2011

Sandbox experiments with sand and sand-silicone models were performed above a circular strike-sli... more Sandbox experiments with sand and sand-silicone models were performed above a circular strike-slip fault to understand the influence of curvature on the development of second-order faults. Riedel R faults appear already at low displacements in the concave side of the circular plate where they always form more numerous. They are followed by R faults in the convex side and eventually by throughgoing D faults that join the R faults and that develop parallel to the underlying circular fault. The angle between the Riedel faults and the trace of the main circular fault at the surface of the models is c. 26° on the concave side and 15° on the convex side. We infer a larger obliquity of σ1 in the concave side of the circular plate, which corresponds to a larger transpressional component than in the convex side of the main wrench fault due to the more confined volume of deforming sand. In sand experiments, most of the faults root into the underlying strike-slip fault. In sand-silicone experiments instead, the faults form close to the displacement discontinuity in case of a high displacement rate only. Uplifted areas are located all along the main fault in sand experiments. In sand-silicone experiments, they are mostly located in wedges defined by the Riedel faults and the main wrench fault and the width of these uplifted areas appear to be related to the length and activity of the Riedel faults. Our results differ significantly from those of experiments with straight strike-slip faults where strain and second-order faults are symmetrically arranged on both sides of the main fault.► The faults propagate on both sides of circular strike-slip fault although they remain more numerous on the concave side. ► The angle between the R and D fault is always more opened on the concave side of the plate than on the convex side. ► The asymmetry of second-order faults can constitute relevant indications of a circular component along a wrench fault.

Geochimica Et Cosmochimica Acta, 2006

The Allalin Gabbro, which forms part of the Zermatt-Saas ophiolite, contains both relicts of the ... more The Allalin Gabbro, which forms part of the Zermatt-Saas ophiolite, contains both relicts of the original igneous mineralogy and high-pressure metamorphic assemblages. Incomplete recrystallisation of mineral assemblages has therefore allowed the preservation of different metamorphic stages and reaction paths associated with the gabbro to eclogite transition that have been well documented by numerous previous studies. This knowledge base provides an ideal framework within which to investigate the redistribution of trace elements during the gabbro to eclogite transition.

The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies ... more The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted M max is ~5.8, and that deformation is governed by a NNW-SSE oriented horizontal σ 1 , typical of an Andersonian strike-slip regime (σ 1 > σ v > σ 3 ). The history and dynamics of this large scale (≥ 2x 10 6 Km 2 ) stress field, known as the Wegener stress anomaly, appears to be complex. The palaeostress record suggests that a stress field comparable to the current one became established after the opening of the Atlantic, perhaps at ~102 Ma and waned at about ~72 Ma, when it was replaced by a markedly different Andersonian thrust regime (σ 1 > σ 2 > σ v ) oriented NNE-SSW. It is uncertain when the current Wegener stress field was re-established, but some evidence points to a pre-Quaternary event.

Tectonophysics

The brittle structural history of western South Africa has been investigated by remote sensing an... more The brittle structural history of western South Africa has been investigated by remote sensing and field studies to build a conceptual scheme for its > 500 Ma long evolution. Paleostress tensors were computed from a significant fault-slip dataset and a relative geochronological succession of brittle deformation events was established. This was aided by separating in time faulting events through the usage of Cretaceous weathering horizons, silicified fluvial deposits, paleosols and 77–54 Ma olivine melilitite plugs as time markers. The oldest features recognized formed during four compressional episodes assigned to the Neoproterozoic Pan African evolution. This history is expressed by sub-vertical conjugate fracture sets and fits well the inferences derived from remote sensing. The greatest compressive direction rotated from NW-SE to NNE-SSW and finally to almost E–W. A subsequent ENE-WSW-oriented extensional episode is associated with the local effects of the opening of the Atlantic Ocean and was followed by a second, ca. E–W extensional episode, linked to the well-acknowledged Mid-Cretaceous (115–90 Ma) event of margin uplift. A late Santonian (85–83 Ma) NW-SE compressive paleostress deformed the Late Cretaceous sequences and was in turn followed firstly by a renewed episode of NE-SW extension and later by ca. NNE-SSW Late Maastrichtian (69–65 Ma) shortening. The latter is broadly coeval with the emplacement of the Gamoep magmatic suite. A phase of WNW-ESE Cenozoic extension is assigned to the extensional phase recorded in the Okawango delta, interpreted as reflecting propagation of the East African Rift System into southern Africa. No stress tensor was computed for the present day “Wegener anomaly” stress field, oriented NW-SE. However, in situ stress measurements were used to perform slip tendency analysis, which indicates that, under the currently existing stress conditions, WNW-ESE- and NNW-SSE-striking faults are critically stressed and are the most likely reactivated, in agreement with the present seismicity.►Stress tensor inversion and remote sensing document 500 Myr of brittle evolution. ►Pan African and younger brittle phases due to Mesozoic and Cenozoic evolution. ►Continued fault reactivation controlled accommodation of younger strain increments.

South African Journal of Geology, 2010

Journal of The Geological Society, 2008

Abstract The Naukluft Thrust forms the floor thrust to the Naukluft Nappe Complex, a far-travelle... more Abstract The Naukluft Thrust forms the floor thrust to the Naukluft Nappe Complex, a far-travelled, nappe stack of the Pan-African Damara belt in Namibia. The thrust tectonostratigraphy comprises three dolomitic components, a calc-mylonite horizon, and a discrete brittle fault. Stable isotope data indicate that the leading edge is characterized by positive δ 13 C values, whereas the trailing edge is characterized by negative δ 13 C values. There is a significant range in the δ 18 O values, over 15‰ in different sections, with the ...

Tectonics, 2008

[2] The term &amp;amp;amp;#x27;&amp;amp;amp;#x27;Pan-African&amp;amp;amp;... more [2] The term &amp;amp;amp;#x27;&amp;amp;amp;#x27;Pan-African&amp;amp;amp;#x27;&amp;amp;amp;#x27;is widely used to refer to the central part (largely in Africa) of the global orogeny that resulted in the formation of the supercontinent Gondwana between Late Neoproterozoic and Cambrian times [eg, Unrug, 1997]. Although there is general ...

Journal of Metamorphic Geology, 2007

Pan-African high-pressure granulites occur as boudins and layers in the Lurio Belt in north-easte... more Pan-African high-pressure granulites occur as boudins and layers in the Lurio Belt in north-eastern Mozambique, eastern Africa. Mafic granulites contain the mineral assemblage garnet + clinopyroxene + plagioclase + quartz ± magnesiohastingsite. Garnet porphyroblasts are zoned with increasing almandine and spessartine contents and decreasing grossular and pyrope contents from core (Alm46Prp32Grs21Sps2) to rim (Alm52Prp26Grs19Sps3). This pattern is interpreted as a retrograde diffusion zoning with the preserved core chemistry representing the peak metamorphic composition. Mineral reaction textures occur in the form of monomineralic and composite plagioclase ± orthopyroxene ± amphibole ± biotite ± magnetite coronas around garnet porphyroblasts. Thermobarometry indicates peak metamorphic conditions of up to 1.57 ± 0.14 GPa and 949 ± 92 °C (stage I), corresponding to crustal depths of ∼55 km. Zircon yielded an U–Pb age of 557 ± 16 Ma, inferred to date crystallization of zircon during peak or immediately post-peak metamorphism. Formation of plagioclase + orthopyroxene-bearing coronas surrounding garnet indicates a near-isothermal decompression of the high-pressure granulites to lower pressure granulite facies conditions (stage II). Development of plagioclase + amphibole-coronas enclosing the same garnet porphyroblasts shows subsequent cooling into amphibolite facies conditions (stage III). Symplectitic textures of the corona assemblages indicate rapid decompression. The high-pressure granulite facies metamorphism of the Lurio Belt, followed by near-isothermal decompression and subsequent cooling, is in accordance with a long-lived tectonic history accompanied by high magmatic activity in the Lurio Belt during the late Neoproterozoic–early Palaeozoic East-African–Antarctic orogeny.

$Research paper thumbnail of Passive kimberlite intrusion into actively dilating dyke–fracture arrays: evidence from fibrous calcite veins and extensional fracture cleavage$

Lithos, 2004

Calcite veins are invariably associated with en-echelon kimberlite dyke–fracture arrays. A detail... more Calcite veins are invariably associated with en-echelon kimberlite dyke–fracture arrays. A detailed microstructural study of veining indicates four vein types. Type I stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock, by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins. Type I vein mineralogy indicates syn-dilational crystallization of vein minerals in local tensional areas within the kimberlite. Vein Types II (stretched to syntaxial elongate-blocky) and III (antitaxial) indicate late crystallization vein mineral growth during subsequent or repeated dilation. Calcite fibers in Type I to Type III veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins. Type IV calcite veins, with blocky or mosaic/polycrystalline textures, are attributed to minor post-intrusion extension, which was potentially accompanied by repeated kimberlite intrusion within a given dyke array. Syn-crystallization/syn-intrusion Type I veins and an ubiquitous dyke-parallel fracture cleavage, in a zone up to 4 m on either side of dyke contacts, suggest that en-echelon kimberlite dyke–fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust. Type II and III veins indicate that extension or dilation continued, independently of an occupying kimberlite fluid phase, after initial intrusion. Arrested mobile hydrofracturing, under low differential stress within the upper brittle or seismic carapace of the continental crust, followed by repeated dilation of the dyke–fracture system, is proposed as a mechanism for producing the features observed in this study. The conditions constrained in this study indicate passive dyke intrusion into dilating fracture arrays during crustal extension.

South African Journal of Geology, 2008

Precambrian Research, 2010

The Nampula Block covers over 100,000 km2, making it the largest Mesoproterozoic crustal segment ... more The Nampula Block covers over 100,000 km2, making it the largest Mesoproterozoic crustal segment in northern Mozambique and an important component of the Neoproterozoic to Cambrian (Pan-African) East African Orogen. It is bounded in the north by the WSW–ENE trending Lúrio Belt. The oldest rocks (Mocuba Suite) are a polydeformed sequence of upper amphibolite-grade layered grey gneisses and migmatites associated with intrusive trondhjemite-tonalite-granodiorite and granitic orthogneisses. A banded gneiss, interpreted as a meta-volcanic rock, yielded a U-Pb SIMS zircon date of 1127 ± 9 Ma. Metamorphic rims, dated at ca. 1090 Ma, probably grew during a later magmatic phase, represented by the tonalitic Rapale Gneiss, two samples of which were dated at 1095 ± 19 and 1091 ± 14 Ma, respectively. The earliest (D1) deformation that took place at approximately this time, was associated with high grade metamorphism and migmatisation of the Mocuba Suite. The geochemistry of these rocks suggests that they were generated in a juvenile, island-arc setting. The Mocuba Suite is interlayered with extensive belts of meta-pelitic/psammitic, calc-silicate and felsic to mafic meta-volcanic paragneisses termed the Molócuè Group. U-Pb data from detrital zircons from a calc-silicate paragneiss gave a bimodal age distribution at ca. 1100 and 1800 Ma, showing derivation from rocks of the same age as the Mocuba Suite and a Palaeoproterozoic source region. The age of the Molócuè Group has been directly determined by dates of 1092 ± 13 and 1090 ± 22 Ma, obtained from two samples of the leucocratic Mamala Gneiss (meta-felsic volcanics?), one of its major constituent components. The final phase of Mesoproterozoic activity is represented by voluminous plutons and sheet-like bodies of foliated megacrystic granite, augen gneiss and granitic orthogneiss of the Culicui Suite, which have A-type granite geochemical characteristics and are interpreted to have been generated in a late tectonic, extensional setting. Three samples from the suite gave identical ages of ca. 1075 Ma. The Nampula Block was extensively re-worked during the major (D2: Pan-African) collision orogen in Late Neoproterozoic to Cambrian times, when the major regional fabrics were imposed upon the Mesoproterozoic rocks under amphibolite-facies metamorphic conditions. In the dated samples, this orogenic event is represented by metamorphic zircon rim ages of ca. 550 to 500 Ma. The new data indicate that the Mesoproterozoic rocks of the Nampula Block were originally accreted to a Palaeoproterozic crustal Block and the Nampula Block only reached its current position, separated from the other Mesoproterozoic blocks of NE Mozambique by the Lúrio Belt, during Neoproterozoic collision and plate movements. The geological history of the Nampula Block is comparable with that described from other parts of the Mesoproterozoic orogenic belts of the Kalahari craton and helps to constrain an integrated model of their evolution.

Earth and Planetary Science Letters, 2005

Recently discovered mud volcanoes in the Orange Basin, offshore southwestern Africa, denote the e... more Recently discovered mud volcanoes in the Orange Basin, offshore southwestern Africa, denote the existence of neotectonic faults in the submerged continental shelf. Interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures, diagnostic of strike-slip faulting along a N/NNW direction. Analysis at the regional scale of onland neotectonic features in southwestern Africa shows that recent faulting occurred both in central Namibia and Namaqualand, South Africa and that it created both N/NNW- and NW-trending lineaments. It is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field. These structures form a set of conjugate transtensive faults, which constrain the regional horizontal greatest compressive stress in a NW/NNW direction. Such stress orientation, also supported by in situ stress measurements, defines the so-called Wegener stress anomaly, the predominant present-day stress field of southwest Africa. The Wegener anomaly is incompatible with the stress orientation required by plate-scale tectonic constraints, mainly in the form of recently published GPS motion values for the African plate.

Journal of Structural Geology, 2005

Folding of axial plane cleavage can occur during progressive deformation without a change in the ... more Folding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow. Two field examples of upright (Lachlan Fold Belt, SE Australia) and recumbent (Naukluft Nappe Complex, central Namibia) folds are presented, in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification. Finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded. Cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment. The differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric. For folding in simple shear flow, as specifically modelled here, refraction is only strong on the fold limb that rotates against the imposed sense of shear. In known shear environments, this provides a potential kinematic indicator in folded units at relatively low strain (e.g. in simple shear, γ of around one), where other higher-strain indicators, typical of mylonites, are not yet sufficiently developed or are equivocal.