Andreas Henk

Post-collisional granite generation and HT-LP metamorphism by radiogenic heating : the Variscan South Bohemian Batholith

The Palaeozoic Variscan Orogen of Europe is a well‐documented example of a collision zone characterized by widespread late‐orogenic high‐temperature metamorphism and associated crustal magmatism. However, the heat source is still under debate. Based on the Bohemian Massif in the internal zone of the Variscides as case study, we present geological, geochemical, petrological and geochronological data arguing against a substantial mantle involvement in metamorphism and magma genesis in the area of the South Bohemian Batholith. In order to provide an alternative explanation consistent with heat transfer mechanism, we apply a two‐dimensional thermal–kinematic modelling approach. The model calculates the transient lithospheric temperature field during crustal thickening and subsequent thinning by erosiorf from material parameters and boundary conditions specific to the study area. Model results show that the increased contribution of radiogenic heat in the thickened crust can indeed cause a substantial temperature increase in the middle and lower crust. Model predictions are in good agreement with observations, e.g. the P–T–t evolution of the country rocks, the formation of syn‐kinematic migmatites, the large volumes of peraluminous granites derived from dehydration melting of metasediments and the small volumes of lamprophyric melts from the mantle lithosphere. The results of this study emphasize the importance of radiogenic heat as the source for high‐temperature metamorphism and granite petrogenesis in the Bohemian Massif and potentially in other areas of the Variscan Orogen.

Magmatic Underplating, Extension, and Crustal Reequilibration: Insights From A Cross‐Section Through the Ivrea Zone and Strona‐Ceneri Zone, Northern Italy

The thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the Ivrea‐Verbano Zone and Strona‐Ceneri Zone in northern Italy. Geochronological and petrologic data are compared to a two‐dimensional thermal‐kinematic model. Field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite‐ to amphibolite‐facies metamorphism as well as reequilibration of mineral chemical and isotopic systems. Magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution, as information on the earlier history is preserved only at upper crustal levels.

Syn-convergent high-temperature metamorphism and magmatism in the Variscides: a discussion of potential heat sources

Abstract A period of pervasive high-temperature metamorphism and igneous activity from 340 to 325 Ma is a well-established characteristic of the Variscan Orogen of Central Europe. During this stage, the internal zone of the orogen was virtually soaked by granitic to granodioritic magmas. Petrological data point to temperatures of 600–850 °C at upper- to mid-crustal levels. These elevated temperatures occurred during the final convergence stage and may be comparable with similar processes inferred from geophysical evidence for the present-day Tibetan Plateau, in both regional extent and significance for the orogen’s evolution. We review various geodynamic scenarios that may have provided the heat for melting and metamorphism, and compare model predictions with field data from the Variscides. All lines of evidence point to a geodynamic scenario that led to thickening of the continental crust with increased internal radiogenic heating, but without simultaneous thickening of the mantle lithosphere. Possible mechanisms include convective removal of the thermal boundary layer, delamination of part of the lithospheric mantle, and subduction of the mantle lithosphere of the downgoing plate. However, with the present stage of knowledge it is virtually impossible to single out one of these three mechanisms, as their geological consequences are so similar.

Late orogenic Basin evolution in the Variscan internides: the Saar-Nahe Basin, southwest Germany

Abstract The late Variscan evolution of the Saar-Nahe Basin in southwest Germany is closely controlled by the kinematics of the Hunsruck Boundary Fault, which also separates two main tectonostratigraphic units of the Variscides. The Saar-Nahe Basin comprises a half-graben structure, which formed due to extensional reactivation of a Variscan thrust. Between the early Westphalian and late Rotliegend, about 8.5 km of alluvial fan, fluvial and lacustrine sediments accumulated. During the Permo-Carboniferous, W-E oriented extension was accommodated by a system of NW-SE trending transfer faults and orthogonal normal faults. Balanced cross-section construction and subsidence analyses suggest a 35% extension of the previously thickened crust in the late stage of the orogeny. The subsidence analyses show discontinuous depth-dependent extension, with laterally varying extension factors in the crust and mantle. The offset between syn- and post-rift depocentres is explained by a mantle stretching zone, shifted laterally with respect to the area of maximum crustal extension. Finally, a geodynamic model for the evolution of the Saar-Nahe Basin, with special reference to the earlier Variscan development and the general evolution of Permo-Carboniferous basins in Central Europe is presented.

Pre-drilling prediction of the tectonic stress field with geomechanical models

Knowledge of the tectonic stress field in a reservoir is essential to optimize drilling and production. Borehole stability, orientations of natural and hydraulically induced fractures, fluid flow anisotropies, among others, all depend critically on the present-day stress distribution. Several techniques ranging from dipmeter analysis of borehole breakouts to anelastic strain recovery and shear acoustic anisotropy analysis of core samples (e.g., Yale, 2003; Sperner et al., 2003) can be used to determine the in-situ stress orientations and relative magnitudes, but obviously this valuable information will only become available after the well has already been drilled. However, there are also numerous cases where the stress orientation should be known prior to drilling. For example, if multiple fracs in a horizontal well are planned, the stress field orientation needs to be known beforehand because for optimal frac design the horizontal well path must be aligned parallel to the orientation of the least principal stress axis σ3. Similarly, the planning of well trajectories with respect to borehole stability as well as the design of secondary and tertiary recovery measures (e.g., water injection, hydraulic fracture treatments) are significantly improved by a pre-drilling knowledge of the subsurface stress field. Information on the regional stress orientations can be derived from large-scale data collections like, for example, the world stress map project (Zoback, 1992; Sperner et al., 2003). The orientation and magnitude of the stress field in sedimentary basins, however, can be highly variable and, particularly near faults, the local stress orientations can differ by up to 90° from the regional trend (e.g., Yale, 2003). In such cases, inference of reservoir-scale in-situ stress orientations from regional scale maps would inevitably lead to an incorrect pre-drilling prediction. This paper uses a numerical modelling approach to determine the magnitude and orientation of the tectonic stresses in a reservoir and, particularly, the local stress perturbations near faults. The model is based on reservoir and fault geometries taken from seismic data and boundary conditions representing the regional stress field. Thus, this tool is also applicable to cases where well data are absent. Following a brief outline of the modelling approach, a case study is presented to assess the practical value of such geomechanical models for the pre-drilling prediction of the tectonic stress field in fault-controlled reservoirs.

Subsidenz und tektonik des Saar-Nahe-Beckens (SW-Deutschland)

The Saar-Nahe-Basin in SW-Germany is one of the largest Permo-Carboniferous basins in the internal zone of the Variscides. Its evolution is closely related to movements along the Hunsrück Boundary Fault, which separates the Rhenohercynian and the Saxothuringian zones. Recent deep seismic surveys indicate that the Saar-Nahe-Basin formed in the hanging wall of a major detachment which soles out at lower crustal levels at about 16 km depth. Oblique extension along an inverted Variscan thrust resulted in the formation of a half-graben, within more than 8 km of entirely continental strata accumulated. The structural style within the basin is characterized by normal faults parallel to the basin axis and orthogonal transfer fault zones. Balanced cross-section construction and subsidence analysis indicate extension of the orogenically thickened lithosphere by 35%. Subsidence modeling shows discontinuous depth-dependent extension with laterally varying extension factors for crust and mantle lithosphere. Thus, the offset between maximum rift and thermal subsidence can be explained by a zone of mantle extension shifted laterally with respect to the zone of maximum crustal extension.

Strike-slip fault bridge fluid pumping mechanism: insights from field-based palaeostress analysis and numerical modelling

Abstract We present a finite-element study of stress perturbation in evolving compressive and extensional strike-slip fault bridges. The results are compared with a fracture study of a compressive bridge at St Donats, UK. Horizontally interbedded calcareous mudstone and bioclastic calcilutite at St Donats have a distinct vertical permeability anisotropy. This sedimentary sequence behaves as a set of horizontal aquifers. The fluid flow in these aquifers is sensitive to mean stress gradients. Paleostress analysis of field fracture data, verified by finite-element modelling, indicates a rotation of σ 1 towards parallelism with boundary faults inside the growing compressive bridge. Boundary faults and bridge faults recorded numerous fluid flow events. The modelled mean stress pattern shows a regional maximum within the bridge and local maxima/minima pairs at boundary fault tips. Finite-element modelling of an extensional bridge indicates that σ 3 rotates towards parallelism with boundary faults. The mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged. The stress patterns are reestablished by each stress build-up preceding the rupturation of the boundary faults throughout the development stages of strike-slip fault bridges. Mean stress gradients developed pre-failure control the fluid flow in fractures of the strike-slip fault system at and after the end of each stress build-up and the fluid flow in boundary faults post-failure. Fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge. Rupture and sealing of the main bounding-faults is a step-wise process that opens and closes fluid conduits between areas with different pressures.

Continental break-up along strike-slip fault zones; observations from the Equatorial Atlantic

Abstract The study focuses on Equatorial Atlantic margins, and draws from seismic, well, gravimetric and magnetic data combined with thermo-mechanical numerical modelling. Our data and numerical modelling indicates that early drift along strike-slip-originated margins is frequently characterized by up to 10°–20° spreading vector adjustments. In combination with the warm, thinned crust of the continental margin, these adjustments control localized transpression. Our observations indicate that early-drift margin slopes are too steep to hold sedimentary cover, which results in their inability to develop a moderately steep slope undergoing cycles of gravitational instability resulting in cyclic gravity gliding. These slopes either never develop such conditions or gain them at later development stages. Our modelling suggests that the continental margin undergoing strike-slip-controlled break-up experiences warming due to thinning along pull-apart basin systems. Pull-apart basins eventually develop sea-floor spreading ridges. Margins bounded by strike-slip faults located among pull-apart basins with these ridges first undergo cooling. However, spreading ridges leaving the break-up trace along its strike eventually pass by these cooling margins, warming them again before the final cooling proceeds. As a result, the structural highs surrounded by several source rock kitchens witness a sequential expulsion onset in different kitchens along the trajectory of spreading ridges. Supplementary material: Discussion of the methods used, chronostratigraphic results and strike-slip margin characteristics are available at http://www.geolsoc.org.uk/SUP18518

Late Variscan exhumation histories of the southern Rhenohercynian Zone and western Mid-German Crystalline Rise: results from thermal modeling

Thermal modeling techniques constrained by published petrological and thermo-chronometric data were applied to examine late orogenic burial and exhumation at a Variscan suture zone in Central Europe. The suture separates the southern Rhenohercynian zone from the Mid-German Crystalline Rise and traces the former site of a small oceanic basin. Closure of this basin during Variscan subduction and subsequent collision of continental units were responsible for different tectono-metamorphic evolutions in the sutures footwall and hanging wall. Relative convergence rates between the southern Rhenohercynian zone and western Mid-German Crystalline Rise can be inferred from the pressure-temperature-time evolution of the Northern Phyllite Zone. During Late Viséan-Early Namurian times, horizontal thrusting velocities were at least 20 mm/a. Thermal modeling suggests that exhumation of the Mid-German Crystalline Rise occurred temporarily at rates of more than 3 mm/a. Such rapid exhumation cannot be produced by erosion only, but requires a substantial contribution of extensional strain. Exhumation by upper crustal extension occurred contemporaneously with convergence and is explained by continuous underplating of crustal slices and thrusting along faults with ramp-flat geometry. Finally, implications for the tectono-metamorphic history of the study area and the thermal state of the crust during late Variscan exhumation are discussed.

Foreland-directed lower-crustal flow and its implications for the exhumation of high-pressure-high-temperature rocks

Abstract Thermo-mechanical finite element models are used to study foreland-directed lower-crustal flow as a potential process to transport high-pressure-high-temperature (HP-HT) rocks from a continental collision zone to areas that have never experienced crustal thickening or deep burial. The numerical simulations show that lower-crustal rocks can indeed flow over substantial horizontal and vertical distances, provided a thermal anomaly reducing lithospheric strength exists in the foreland. As lower-crustal flow is a fast process compared with conductive heat transport and occurs subparallel to the isotherms, the resulting PT path of the rocks exhumed will be characterized by near-isothermal decompression. Modelling results are applied to the Saxonian Granulite Massif in Eastern Germany, where HP-HT granulites have been exhumed beneath a marine sedimentary basin during the Variscan Orogeny.