David C. Tanner

Orogenic processes: quantification and modelling in the Variscan belt

Research into the orogenic processes that shaped the continental crust of Europe has a long-standing tradition. Why the need to quantify and model? It is not just satisfactory to identify ancient examples of subduction zones, accretionary prisms, island arcs, extensional collapse and other standard items of the geodynamic menu. Such interpretations need to be quantified: what was the extent and composition of subducted crust, angle and speed of subduction, amount and composition of melts produced, heat sources for metamorphism? All such interpretations have to conform to first principles, and also to stand the test of quantitative balancing—a concept first developed for the conservation of length or volume in tectonic cross sections. Also in other fields, the correlation of causes and effects and the internal consistency of dynamic models requires a numerical approach. Modelling a Palaeozoic orogen, for some people, may look like a hopeless task from the very beginning. The fossil record of ancient orogenic crust is limited by erosion from above, and re-equilibration of the Moho from below. The Variscan basement had already been eroded and buried in late Permian and Mesozoic time, and was later exhumed again by Alpine foreland compression, uplifted on the shoulders of grabens or above a mantle plume. Good outcrops are scarce. Living orogens are more attractive in many respects, but it was not advisable to have our group dispersed over the planet: the project needed regional focus and interdisciplinary collaboration. The European Variscides were the obvious target, since a wealth of basic information

Anatomy and kinematics of oblique continental rifting revealed: A three-dimensional case study of the southeast Upper Rhine graben (Germany)

The Upper Rhine graben is a north-northeast-trending, small-displacement, crustal-scale rift of Tertiary age. Retrodeformation of its southeastern part demonstrates that it is a product of sinistral oblique rifting. Early extension was toward 80°. Later, the major stretching axis changed to a 60° direction. The modeling results suggest that the eastern Main Border fault developed first, and that faulting later propagated into the evolving graben interior. Considerable along-strike variations in heave, throw, and displacement are evident. Displacement partitioning causes warping of the rift floor with a 30-35-km wavelength. We consider this to be a characteristic of oblique rifting. Contact deformation of the wall rocks to the major faults may have caused widespread smaller scale faulting and brecciation and may be the location of later movements. Close spatial coincidence of the depth projections of some of the faults studied and the hypocenters of recent small earthquakes indicates continuing activity of the fault system. Apparently, three fault segments in the Freiburg area are currently active and may be an increased earthquake risk.

Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength

Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

The Slaufrudalur pluton, southeast Iceland—An example of shallow magma emplacement by coupled cauldron subsidence and magmatic stoping

The Tertiary Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland. Five glacial valleys cut through the uppermost 900 m of the pluton, exposing spectacular sections through its roof, walls, and interior. The wall contacts are subvertical and sharp. Only in the northeast and southwest is the wall contact characterized by brittle faulting. The pluton roof is smooth at map scale, so that the overall cross-sectional shape of the pluton and its internal layering indicate emplacement by incremental floor sinking through cauldron subsidence. A pronounced elongation of the pluton, parallel to the trend of regional fissure swarms, and its angular shape in map view indicate strong tectonic control on horizontal ring-fault propagation, whereas faulted wall contacts represent step-over structures between the earlier-formed ring faults. On outcrop scale, the roof contact exhibits numerous steps, faults, and apophyses associated with conjugate fracture sets that are parallel and perpendicular to the strike of the length of the pluton. These structures were presumably formed by sequential inflation and deflation of the pluton during episodic magma intrusion and therefore are closely coupled to cauldron subsidence. As a result of roof fracturing and magma injection along the fractures, roof material is found partly or completely detached within the granite. The Slaufrudalur pluton therefore provides new insight into the coupling of the emplacement mechanisms of cauldron subsidence and magmatic stoping in the upper crust.

Three-dimensional geometry of concentric intrusive sheet swarms in the Geitafell and the Dyrfjoll volcanoes, eastern Iceland

Sheet intrusions (inclined sheets and dykes) in the deeply eroded volcanoes of Geitafell and Dyrfjoll, eastern Iceland, were studied at the surface to identify the location, depth, and size of their magmatic source(s). For this purpose, the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries. For the Geitafell Volcano, the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km, the location of which coincides with several gabbro bodies exposed at the surface. Assuming that these gabbros represent part of the magma chamber feeding the inclined sheets, a source depth of 2 to 4 km below the paleoland surface is derived. A second, younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm. The source of this second swarm is estimated to be located to the SE of the source of Swarm 1, below the present-day level of exposure and deeper than the source of the first swarm. For the Dyrfjoll Volcano, we show that the sheets can be divided into seven different subsets, three of which can be interpreted as swarms. The most prominent swarm, the Njardvik Sheet Swarm, converges toward a several kilometers wide area in the Njardvik Valley at a depth of 1.5 to 4 km below the paleoland surface. Two additional magmatic sources are postulated to be located to the northeast and southwest of the main source. Crosscutting relationships indicate contemporaneous, as well as successive activity of different magma chambers, but without a resolvable spatial trend. The Dyrfjoll Volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as Askja, whereas in Geitafell, the sheet swarms seem to have originated from a single focus at one time, thus defining a single central volcanic complex, such as Krafla Volcano.

Prediction of subseismic faults and fractures: Integration of three-dimensional seismic data, three-dimensional retrodeformation, and well data on an example of deformation around an inverted fault

In addition to seismically mapped fault structures, a large number of faults below the limit of seismic resolution contribute to subsurface deformation. However, a correlation between large- and small-scale faults is difficult because of their strong variation in orientation. A workflow to analyze deformation over different scales is described here. Based on the combination of seismic interpretation, coherency analysis, geostatistical analysis, kinematic modeling, and well data analysis, we constrained the density and orientation of subseismic faults and made predictions about reactivation and opening of fractures. We interpreted faults in seismic and coherency volumes at scales between several kilometers and a few tens of meters. Three-dimensional (3-D) retrodeformation was performed on a detailed interpreted 3-D structural model to simulate strain in the hanging wall at the time of faulting, at a scale below seismic resolution. The modeling results show that (1) considerable strain is observed more than 1 km (0.62 mi) away from the fault trace and (2) deformation around the fault causes strain variations, depending on the fault morphology. This strain variation is responsible for the heterogeneous subseismic fracture distribution observed in wells. We linked the fracture density from the well data with the modeled strain magnitude and used the strain magnitude as a proxy for fracture density. With this method, we can predict the relative density of small-scale fractures in areas without well data. Furthermore, knowing the orientation of the local strain axis, we predict a fault strike and opening or reactivation of fractures during a particular deformation event.

Quantitative fracture prediction from seismic data

ABSTRACT This paper presents results obtained from an area located east of Bremen, Germany, where gas is produced from a deep Rotliegend sandstone reservoir. Faults, fractures and associated deformation bands at reservoir depth have an important influence on the productivity of the gas field as fractures are cemented and tight and may act as permeability barriers. This contribution comprises the development of new coherency tools to better image sub-seismic faults and lineaments from seismic data, and the development of fracture attributes in order to quantify fracturation and its areal distribution. The fractal behaviour of faults was used to establish a relationship between coherency processed seismic data and borehole images at log scale. The ‘fractal dimension’ (FD) of the length of a fault population can be interpreted as a characteristic parameter describing local geology in terms of fracturation. Calculating FD for each point of a seismic grid yields an areal distribution of this value. Correlating seismic-derived FD values and fracture populations derived from borehole images defined a linear relationship which can be used to forecast the distribution of sub-seismic fractures and deformation from seismic data.

Kinematic retro-modelling of a cross-section through a thrust-and-fold belt: the Western Irish Namurian Basin

Abstract The Western Irish Namurian Basin (WINB) developed into a fold-and-thrust belt at the front of the northward-propagating Variscan orogenic wedge. Part of this basin is well exposed along the coast of County Clare, Ireland. From a detailed study that used an integrated GPS mapping approach, we produced a c. 50 km long, balanced cross-section, parallel to the tectonic transport vector. We sequentially decompacted and retro-deformed the Namurian strata in 7 stages to evaluate the palinspastic situation of the basin and the amount of shortening. By using passive markers in the model and a highly-detailed timescale, we were able to determine that shortening of the WINB, from the onset of Central Clare Group sedimentation was 7.44% (or 4.07 km) of which shortening due to folding accounts for c. 2.64% (c. 1.37 km), and therefore c. 4.80% (c. 2.69 km) was solely because of thrusting. The rate of horizontal shortening ranges from 1–34 mm a−1; this is within typical orogenic values.

Three-dimensional retro-modelling of transpression on a linked fault system: the Upper Cretaceous deformation on the western border of the Bohemian Massif, Germany

Abstract The Zone of Erbendorf-Vohenstrauß (ZEV) on the western margin of the Bohemian Massif was deformed by an Upper Cretaceous intra-plate deformation event. Dextral transpression was caused by the reactivation of pre-existing structures. Using the extensive geological database available, we have constructed a three-dimensional virtual model of the ZEV. The model was deformed in reverse, to remove the effects of the Upper Cretaceous event. This involved moving the hanging wall (the ZEV) in a sinistral transtensive sense northwards above a composite active fault surface composed of two steep faults, perpendicular to each other in strike, and a detachment intersecting both faults at 9.5 km depth. Hanging-wall deformation was accommodated by antithetic inclined shear. Seven kilometres heave of the hanging wall fulfilled the geological constraints. Calculated uplifts range from 2 to 6 km. Deformation is mostly only contained within the ZEV. The hanging-wall deformation above a linked fault system was highly complex, causing rollover above one fault and drag-folding above the other. The most important control on the vertical movement and deformation of the hanging wall was a 30° change in the strike of one of the coupled faults.

Structural Architecture and Deformation Styles Derived from 3D Reflection Seismic Data in the North German Basin

The Upper Cretaceous Ilhabela sandstones, which represent the main reservoir unit in the Santos Basin, were deposited in a fluvial to shallow marine environment during a period of active basaltic volcanism. The volcanics have been partly eroded, providing the reservoir with volcanic rock fragments and causing a complex diagenetic history. This has resulted in the local occurrence of sandstones with complex lithology and elastic properties differing from the basin trend (Klarner et al, 2005). For better prediction and interpretation of the amplitudes and AVO behaviour of the Ilhabela reservoir it is essential to map the occurrence of the volcanics as well as the transport direction of their erosional products. In this paper, typical seismic features are presented which may help to identify volcanics in the Upper Cretaceous sequence.