Gernold Zulauf

Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)

Abstract The Tepla–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Tepla–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Tepla–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit. The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch (Stěchovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dobřis conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (Těsovice granite), 524±3 Ma (Vsepadly granodiorite), 523±3 Ma (Smržovice tonalite), 523±1 Ma (Smržovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.

Late Carboniferous plutonism within the pre-Alpine basement of the External Hellenides (Kithira, Greece): evidence from U–Pb zircon dating

In memory of Theodor Doutsos, for his invaluable contribution to the understanding of geology of Greece. U–Pb zircon dating of three granitic orthogneisses, sampled from Kithira island, SW Greece, provide the first reliable evidence for Carboniferous plutonism within the pre-Alpine basement of the External Hellenides. Concordant zircons from two samples yielded ages at 324 ± 2 Ma and 323 ±3 Ma, whereas the third sample yielded a lower intercept age at 320 ± 1.2 Ma. The concordant ages are interpreted to date the emplacement of the igneous protolith. Ages of inherited zircons of two orthogneisses range from 2.5 to 2.4 Ga, indicating Late Archaean–Early Proterozoic crustal components. In combination with published ages for other Aegean metagranitoids, the new U–Pb ages provide additional evidence of a temporally restricted period of plutonism in the pre-Alpine Aegean region from 325 to 300 Ma. Comparing the post-Neoproterozoic evolution of the investigated basement with that of the Cycladic and Pelagonian basements and of the continental massifs of NE Greece and Turkey, we argue that all these crustal blocks were part of the Gondwana-derived Cimmerian terrane. Based on the spatial and temporal distribution of Late Carboniferous Aegean plutonism, we suggest that this period of magmatic events results from the southward subduction of Palaeotethys beneath the marginal fragments of northern Gondwana.

Analogue modeling of lithospheric-scale orocline buckling: Constraints on the evolution of the Iberian-Armorican Arc

We report on a series of analogue modeling experiments that study the oroclinal buckling process as a thick-skinned process involving the entire lithosphere. The results obtained in the experiments suggest that, during oroclinal buckling, extension in the outer arc and significant shortening in the inner arc are produced by tangential longitudinal strain as the main mechanism of deformation. The models also reveal that the mantle lithosphere thickens in different noncylindrical ways depending on the initial lithospheric mantle thickness: from almost recumbent to folding with subvertical axial planes for the thinnest to the thickest mantle lithosphere, respectively. The results provide useful insights into thick-skinned orocline buckling as it is interpreted to have happened in the Iberian-Armorican Arc.

Experimental folding and boudinage under pure constrictional conditions

Abstract Constrictional folds are characterized by true fold-axis parallel extension if the rock-volume does not vary during deformation. Studies of such folds in experiments, using plasticine layers of different apparent viscosity and power-law exponent, clearly indicate that fold-axis parallel stretch may be accompanied by plastic elongation as well as boudinage of the competent layer. Characteristic aspects of the experimentally folded competent layers are: (1) coeval development of folds and boudins; (2) layer thickness not changing during deformation; (3) layer-parallel shortening in sections perpendicular to the fold (stretching) axis; (4) enlargement of the initial thickness of the competent layer results in increasing fold wavelength and decreasing number of boudins. The ratio of dominant wavelength to layer thickness of the constrictional folds can be described mathematically approximately by the equation developed for plane strain folding of power-law materials

Late to post-Variscan deformation phases and palaeostresses in the KTB pilot research well (Bohemian Massif, Germany)

Abstract Brittle faults and veins are very widespread in the crystalline rocks of the northern Oberpfalz, particularly in the 4000 m deep Kontinentale Tiefbohrung (KTB) pilot well. In the late Variscan (Late Carboniferous), subvertical tension gashes were formed under NE-SW extension. A genetic relationship between the vein development and late-phase magmatic activity of the late Variscan granites is obvious. Subsequently, but still during the Late Carboniferous, graphite-enriched reverse faults developed under E-W and NE-SW compression. In the deeper part of the KTB pilot hole these faults were formed within the brittle-ductile transition regime of the paragneisses. Since, during the late Variscan, the geothermal gradient was remarkably elevated, the brittle-ductile regime was situated at a relatively high crustal level where cataclasis, crystal plasticity and diffusion-controlled deformation mechanisms were active simultaneously. In post-Variscan time, probably during the Cretaceous, a further generation of reverse faults developed in addition to subhorizontal tension gashes and minor strike-slip faults. During these events the temperature was much lower compared with the conditions during the late Variscan deformations. The reverse faults evolved primarily under N-S compression, which is probably related to the Cretaceous subduction and collision events of the Alpine orogeny further to the south. The youngest, normal, faults are probably connected to the subsidence of the nearby Eger Graben which evolved during the Neogene uplift of the investigated area. The stress data, derived by using faults and striations from surface outcrops and from several oriented core intervals from the KTB pilot well, display weak depth-dependent variations. Although the deformation was polyphase, the “inversion method” has proved to be a suitable mode for determining reliable palaeostress data. However, if the brittle-ductile transition regime or broad shear zones are reached, the data become more and more invalid.

Strain-dependent rheology and the memory of plasticine

Abstract Plasticine and plasticine-like materials have been widely used as analogue materials for experimental deformation, but not many workers have conducted detailed investigations on their rheology. The physical properties of Becks green and black plasticine , a modelling material made in Gomaringen, Germany, and plasticine/oil mixtures were investigated by means of uniaxial compression and relaxation tests. Becks plasticine is a non-Newtonian fluid characterised by strain rate-dependent plastic yielding and strain hardening. Strain hardening is more pronounced at low strain rates leading to an increase of both stress exponent and viscosity. The addition of oil leads to an increase of the stress exponent and a decrease in viscosity. The strain dependence of viscosity decreases with increasing oil content. Compression tests on preflattened plasticine were also conducted in order to study possible ‘strain memory’ of the materials. Preflattened plasticine is characterised by a later onset of yielding and an increase in both stress exponent and viscosity. Our results suggest that Becks green and black plasticine is a suitable analogue material for modelling rocks that deform by dislocation creep and exhibit pronounced strain hardening. Nevertheless, plane strain modelling of boudinage has verified analytical solutions for the dominant wavelength at viscosity contrasts of approximately 1.5 and 2.5.

Quartz dislocation microstructure between 7000 m and 9100 m depth from the Continental Deep Drilling Program KTB

We investigated the quartz microstructures from gneiss samples recovered from the German Continental Deep Drilling Program (KTB) main hole between 7000 m and the final depth of 9100 m. Optical microscopy and transmission electron microscopy (TEM) show similar microstructures for most of the studied profile. At the final depth, enhanced recovery is indicated by fewer dislocation tangles, fewer submicroscopic fluid inclusions, and well-developed low-angle grain boundaries. Between 7000 and 9100 m depth, the mean dislocation density is reduced from 4×109 cm−2 to 1×109 cm−2. Using dislocation density as a piezometer, the differential stress recorded in samples from 9100 m is estimated as approximately 140 MPa. Microstructures indicate that the drill hole reached the semibrittle transition zone and that strain is partitioned between brittle deformation, solution precipitation creep, and plastic flow. Differential stress estimates from in situ measurements extrapolated down to 9.1 km range from 170 to 220 MPa. Fluid injection induced microearthquakes do not seem to occur at a depth greater than 9 km, possibly indicating the absence of critically stressed brittle faults. Microstructural observations and differential stress estimates from entirely different techniques suggest that in situ differential stresses are not likely to increase with further depth. Stresses predicted from extrapolated quartz flow laws are mostly smaller for the low strain rates assumed for the KTB tectonic environment.

Emplacement depths and radiometric ages of Paleozoic plutons of the Neukirchen–Kdyně massif: differential uplift and exhumation of Cadomian basement due to Carboniferous orogenic collapse (Bohemian Massif)

Abstract The igneous complex of Neukirchen–Kdyně is located in the southwestern part of the Tepla–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the Vsepadly granodiorite and the Smržovice diorite intruded at shallow crustal levels ( 20 km). The Teufelsberg (Certův kamen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.

Strain and strain rate in a synkinematic trondhjemitic dike: evidence for melt-induced strain softening during shearing (Bohemian Massif, Czech Republic)

Abstract Emplacement-related deformation of a Cambrian trondhjemitic dike, located at the western border of the Tepla-Barrandian unit (central European Variscides), is studied in detail. The trondhjemitic melt was emplaced at T = 750 ° C into an active low-temperature (ca 350 °C) east-northeast trending dextral transcurrent shear zone. Thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling, respectively. The major part of the shearing occurred within 8 days after the melt emplaced. The zonation of the dike, in strain magnitude, mineralogy and geochemistry, can be explained by the laterally varying rheology (from margin to centre) during cooling and shearing. The deformation was volume-constant, and the strain data plot in the apparent constrictional field. The unusually high value of the calculated longitudinal strain rate (> 2.8 × 10 −6 s −1 ; equivalent to a displacement rate of > 2 cm h −1 ) is probably related to the intruding melt that ‘lubricated’ the shear zone and thus enhanced its displacement velocity. Consequently, melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust.

Element mobility and volumetric strain in brittle and brittle–viscous shear zones of the superdeep well KTB (Germany)

Abstract Mass-balance studies of brittle and brittle–viscous shear zones of the superdeep well KTB (Germany) show that element mobility and associated volumetric strain is markedly different in metabasites and paragneisses. Shear zones in metabasites show thickening and volume increase due to mineralization of prehnite (±epidote±calcite) within open fractures and pore space. Gains in Al 2 O 3 , CaO and SiO 2 are compatible with these observations. Shear zones in paragneisses show either constant volume or volume loss. Volume loss of the paragneiss shear zones can be explained by pressure solution of quartz and by retrograde mica-forming reactions associated with significant gains in potassium-group elements (K, Rb, Ba) and losses in SiO 2 . The differences in volumetric strain between paragneiss and metabasite shear zones can be explained by the different deformation mechanisms and rheology of paragneiss and metabasite at T =ca. 250–350°C. The rigid metabasites supported long-lived open fractures and pores where new minerals could precipitate from a fluid phase. The paragneisses, on the other hand, were weak because of increasing amounts of crystal plasticity and pressure solution of quartz which is typical for the brittle–viscous transition of quartz-bearing rocks. Open fractures and pores were rapidly closed hampering the deposition of new minerals. Graphite enrichment has been found in both paragneiss and metabasite shear zones. There is clear evidence that graphite enrichment results from fluid phases which carried C into the shear zones from external sources suggesting that C was largely mobile during the shearing processes.