Manuel Sintubin

The Cambrian to mid Devonian basin development and deformation history of Eastern Avalonia, east of the Midlands Microcraton: new data and a review

Abstract A review is given of recently published and new data on Avalonia east of the Midlands Microcraton. The three megasequences from Cambrian to mid Devonian described in Wales and Welsh Borderland are also present east of the Midlands Microcraton (Brabant Massif, Condroz, Ardennes, Remscheid and Ebbe inliers, Krefeld high). The three megasequences are caused by a tectonic driving mechanism and are explained by three different geodynamic contexts: an earlier phase with extensional basins or rifting and rather thick sequences, when Avalonia was still attached to Gondwana; a second phase with a shelf basin with moderately thin sequences when Avalonia was a separate continent and a later phase with a shelf or foreland basin development and thick sequences. Deformation of the megasequences 1 and 2 or 1 to 3 varies between areas. In Wales and the Lake District the Acadian phase is long-lived and active from early to mid Devonian. In the Ardennes inliers a deformation is active between the late Ordovician and the Silurian (Ardennian Phase), with a similar intensity as the core of the Brabant Massif, when present erosion levels are compared. The Brabant Massif is partly deformed by the long-lived Brabantian Phase from late Silurian till early mid Devonian. Both the Ardennes inliers and the Brabant Massif are not classic orogenic belts, only slate belts where no more than the epizone is reached at present erosion levels. Areas supposedly close to the microcraton or basement are nearly undeformed (SW Brabant Massif and central Condroz). A model of anticlockwise rotation of Avalonia of about 55° from Caradoc to Emsian is proposed to explain the deposition setting of megasequence 3 and the subsequent Acadian and Brabantian deformation. Immediately after the Avalonian microcontinent touched Baltica in Caradoc times it created a short-lived subduction magmatic event from The Wash to the Brabant Massif and soon after the magmatism ended a foreland basin developed. Possibly during and after that development a long-lived and slow compressional event occurred, leading to the deformation of the Anglo-Brabant Deformation Belt. In the early Devonian, contemporaneous with the shortening of the Anglo-Brabant Deformation Belt, extension occurred in the Rheno-Hercynian Zone, possibly caused by the same slow rotation of Avalonia. More evidence emerges that Avalonia east of the Midlands Microcraton comprises not one but probably two terranes: the remnant of the palaeocontinent Avalonia, and what is called the palaeocontinent Far Eastern Avalonia; the latter is only occasionally observed in the few deep boreholes into the Heligoland-Pomerania Deformation Belt, in southern Denmark, NE Germany and NW Poland, with scant available indirect data in between indicating only Proterozoic basement and no Caledonian deformation. For Far Eastern Avalonia a similar palaeogeographical history is postulated as Avalonia, with rifting from Gondwana in Arenig or earlier times, collision with Baltica before the mid-Ashgill and deformation between the late Ordovician and latest Silurian. The Avalonia concept might need to be expanded to an ‘Avalonian Terrane Assemblage’ with cratonic cores and small short-lived oceans as in the Armorican Terrane Assemblage.

The use of a combined structural, stable isotope and fluid inclusion study to constrain the kinematic history at the northern Variscan front zone (Bettrechies, northern France)

Abstract A detailed structural, stable isotope and fluid inclusion study of distinct vein generations has been performed in a kilometre-scale syncline to correlate the temperature–pressure conditions of vein formation with specific deformation episodes. The structural relationship of the veins with cleavage, folds, faults and with one another, allows the identification of pre-, syn- and post-Variscan carbonate vein generations. The stable isotope composition of the vein calcites and dolomites is very similar to that of the surrounding limestones and dolostones, respectively. This indicates that the cements were precipitated from fluids buffered by the host-rock and implies that the temperature–pressure characteristics of the fluids reflect the thermal history of the deformation history within the syncline. Microthermometric data of primary fluid inclusions were used to estimate the temperature–pressure conditions at precipitation time, and thus of the deformation. Pressure-corrected trapping temperatures for the veins show a temperature evolution from pre-Variscan (≤310°C) to Variscan (260–200°C) and post-Variscan (75°C). This evolution indicates that the syncline developed not at a specific depth, but rather as an active, progressive deformation process during transport along the northern Variscan thrust front.

Arcuate fold and cleavage patterns in the southeastern part of the Anglo–Brabant Fold Belt (Belgium): tectonic implications

Abstract The Brabant Massif (Belgium) is situated at the southeastern extremity of the Anglo–Brabant Fold Belt, the predominantly concealed eastern branch of the British–Belgian Caledonides. In contrast to the other parts of this Caledonian slate belt, the deformation history in the Brabant Massif has to date never been constrained properly. In this paper a synthesis is presented of the structural observations in the limited outcrop area of the Brabant Massif, in particular with respect to cleavage and fold trajectories and mutual cleavage–fold relationships. Arcuate fold and cleavage patterns, as well as the presence of cleavage-transected folds are demonstrated, similar to the other parts of the belt. Moreover, the arcuate trajectories are considered to reflect the trace of the Caledonian deformation front, once obscured by the Variscan Ardenne Allochthon. Combined with interpretations of the aeromagnetic and Bouguer anomaly maps, an overall kinematic model is proposed for the Acadian deformation event in the Brabant Massif. With the proposed model, calling upon processes of indentation by promontories and lateral escape of lithostructural domains, as well as on the control exerted by rigid granitoid basement blocks, the strong similarities with the other parts of the British–Belgian Caledonides become apparent. By defining an overall shortening direction and proposing a trace for the Caledonian deformation front in this southeastern extremity of the Anglo–Brabant Fold Belt, an integrated tectonic image is obtained for the entire British–Belgian Caledonides.

The anisotropy of magnetic susceptibility (AMS) in low-grade, cleaved pelitic rocks: influence of cleavage/bedding angle and type and relative orientation of magnetic carriers

Abstract Cambrian and Silurian, low-grade, pelitic rocks of the single-phase deformed Brabant Massif consistently have a maximum magnetic susceptibility axis (K1) parallel to the cleavage/bedding intersection. In contrast, the minimum susceptibility axis (K3) either coincides with the bedding pole, with the cleavage pole or occupies an intermediate position. Anisotropy of anhysteretic remanence (AARM) and X-ray pole figure goniometry allow the distinguishing of the orientation distributions of the ferromagnetic and paramagnetic (white mica and chlorite) carriers, respectively. Mismatches between K3 and the poles to the macroscopic fabric elements (i.e. bedding and cleavage) are attributed to different orientations of the different magnetic (s.l.) carriers. A strong relationship exists between the cleavage/bedding angle and the shape parameter: low, respectively high angles leading to oblate, respectively prolate susceptibility ellipsoids. However, differences are observed between the Cambrian and Silurian samples in terms of the shape parameter and the behaviour of the degree of anisotropy with changing cleavage/bedding angle. This is tentatively attributed to differences in relative orientation and mineralogy of the magnetic (s.l.) carriers. These results demonstrate the influence of the relative orientation of the different carriers on AMS and suggest that, although being a petrofabric tool, AMS cannot be used as a strain gauge in the case of composite magnetic fabrics.

Stress-state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types (High-Ardenne slate belt, Germany)

Abstract: In the frontal part of the Rhenohercynian fold-and-thrust belt (High-Ardenne slate belt, Germany), two successive types of quartz veins, oriented normal and parallel to bedding respectively, are interpreted to reflect the early Variscan compressional tectonic inversion of the Ardenne–Eifel sedimentary basin. Fracturing and sealing occurred in Lower Devonian siliciclastic multilayers under very low-grade metamorphic conditions in a brittle upper crust. A geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure–temperature conditions of both vein types, allowing the reconstruction of the stress-state evolution in a basin during tectonic inversion. It is demonstrated that bedding-normal extension veins, which developed under low differential stresses and repeatedly opened and sealed (crack-seal) under near-lithostatic fluid pressures, reflect the latest stage of an extensional stress regime. Bedding-parallel veins, which developed at differential stresses that were still low enough to allow the formation of extension veins, cross-cut the bedding-normal veins and preceded the regional fold and cleavage development. These veins show a pronounced bedding-parallel fabric, reflecting bedding-normal uplift and bedding-parallel shearing under lithostatic to supra-lithostatic fluid pressures during the early stages of a compressional stress regime. This kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny.

A Logical Methodology for Archaeoseismology: A Proof of Concept at the Archaeological Site of Sagalassos, Southwest Turkey

Archaeoseismology is currently controversial, with criticisms over the extent to which this research field can contribute to seismic-hazard analysis and, in- deed, as to whether man-made structures can be used as earthquake indicators at all. Addressing these concerns—in other words, refining the utility of archaeologi- cally derived earthquake information to permit its inclusion in earthquake-hazard assessments—requires first a systematic, quantitative, and interdisciplinary approach to the seismocultural record. Here, we modify a semiquantitative logic-tree formalism developed for palaeoseismology to explore a methodological scheme that can track uncertainties in successive stages of archaeoseismological investigation. The end result of our logic-tree formalism is a value that expresses the level of certainty to which an archaeological site has recorded a palaeoearthquake and thus reflects its relative significance with respect to a seismic-hazard analysis; we call this measure the archaeoseismic quality factor (AQF). We illustrate how the methodological scheme might work by applying it to Saga- lassos in southwest Turkey, an archaeological site for which earthquake effects are extensively reported. The resulting AQF that we derive from our critical review of the Sagalassos dataset is currently meaningless, because comparable values from equiva- lent reappraisals of earthquake evidence at other archaeological sites are needed for its relative significance to be appreciated. Nevertheless, the varying levels of uncertainty that we assign for different stages in the logic tree allow us to identify key weaknesses in the earthquake hypothesis at Sagalassos, deficiencies that might be redressed through future investigations onsite. The logic-tree formalism offers the potential of a standardized procedure to compile, categorize, and evaluate archaeoseismological information in a form that might, with refinement from wider earthquake archaeology studies, be appropriate for seismic-hazard analysis.

Large-scale slumping deduced from structural and sedimentary features in the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult, especially when the scale of the slump folds exceeds outcrop scale. In the southeastern part of the single-phase deformed, Lower Palaeozoic Anglo-Brabant fold belt a comparison of cleavage/fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle Caradoc fine-grained turbidites in the core of a large synform was overturned prior to tectonic deformation. This overturning is attributed to large-scale slumping, which was most likely a result of middle Caradoc seismic activity. The exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias, making up less than 5% of the exposed thickness. If the beds were not overturned, large-scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall ‘stable’ sedimentary pile. This may explain why so few ancient large-scale slides and slumps have been reported: the small amount of internal deformation makes them very difficult to recognize, especially when dealing with poorly exposed areas. As such, large ancient slides and slumps may be more common than suggested by the geological literature.

Origin and migration pattern of palaeofluids during orogeny: discussion on the Variscides of Belgium and northern France

Abstract The origin, geochemistry and migration patterns of palaeofluids during an orogeny have been investigated at the Variscan front zone in Belgium and northern France. A review has been made of four structural, mineralogical and geochemical studies of Variscan vein systems. The comparison of these case studies shows that open versus closed fluid flow systems active during compressional deformation can be distinguished based on the data acquired. However, a detailed knowledge of the basin and of its diagenetic and tectonic evolution is necessary to determine the origin and migration pattern of the ancient fluids. This prerequisite partly restricts the predictability of the characteristics of the palaeofluids and the migration systems.

The "boudinage" question in the High-Ardenne Slate Belt (Belgium): a combined structural and fluid-inclusion approach

Abstract The terms “boudin” and “boudinage” 1 were first introduced, as descriptive terms, for particular geometric features associated with quartz veins in competent metaquartzitic layers in the High-Ardenne Slate Belt (Belgium). The occurrence of the “boudins” has subsequently been used to draw kinematic conclusions on a pre-Variscan extensional event in the Rhenohercynian belt. In this paper, a combined structural and fluid-inclusion approach allows proposing a new and more representative kinematic model for the development of the veins and associated structures. Moreover, the kinematic significance of “interboudin” veins is emphasized. In this regard, microthermometry of primary CO 2 –N 2 fluid inclusions in the quartz of the “interboudin” veins suggests that the origin of vein development is hydraulic fracturing in an already compressional setting. This suggestion implies the presence of fluid and near-lithostatic fluid pressures to permit fracturing at great depths where the rocks otherwise would be either stable or in the realm of ductile behaviour. The vein filling fluid had a local origin and was most likely related to the formation of bedding-parallel dissolution seams within the host rock. Veining is followed by layer-parallel shortening, resulting in the development of “double-sided” mullions at both interfaces of metaquartzitic layers, pinned by pre-existing veins. Subsequently, these polyphase structures behave passively in the Variscan deformation.

Archaeological, geomorphological and geological evidence for a major earthquake at Sagalassos (SW Turkey) around the middle of the seventh century AD

Abstract The ancient city of Sagalassos, located 7 km north of the ilçe (centre of an administrative unit) of Ağlasun in the Turkish province of Burdur, was an important centre during the Roman period. The collapse and subsequent repairs of buildings may suggest that at least four distinct earthquakes struck this city. These could have occurred in the second half of the first century AD, the middle of the third century AD, the first quarter of the sixth century AD and the middle of he seventh century AD. The youngest of these four earthquakes could have been so destructive that the city was abandoned. The age of this earthquake is based on coins and pottery dated to the middle of the seventh century AD, overlain by the collapsed monuments. Additional evidence is provided by the fractures in a baked mosaic floor belonging to a library, set on fire in the third quarter of the fourth century AD and filled in with waste material and earth during the fire. A directional analysis of the fractures in the mosaic floor of the Library, on the pavement of the Upper Agora and on the Theatre steps gives some insight into the local stress field associated with this earthquake. The palaeostress field, inferred from this analysis, is similar to that created by a transtensional strike-slip activity on an E-W-trending tear fault, situated just north of the city. Although it cannot be proven that the earthquake was related to activity along this fault, the magnitude of the destruction suggests an epicentre in the proximity of the city.