F. Beekman

Lithospheric folding in Iberia

Integration of stress indicator data, gravity data, crustal kinematics data, and analysis of topography and recent vertical motions demonstrates the occurrence of consistently oriented spatial patterns of large-scale Alpine to recent intraplate deformation in Iberia. The inferred upper crustal and lithospheric deformation patterns and the timing of the associated expressions at or near the surface support the existence of a close coupling with plate boundary processes operating at the margins of Iberia. Patterns of lithosphere and upper crustal folds are oriented perpendicular to the main axis of present-day intraplate compression in Iberia inferred from structural analysis of stress indicator data and focal mechanism solutions. These findings suggest the presence of lithospheric folds, with wavelengths compatible with theoretical predictions of folding wavelengths of Variscan lithosphere. Stress-induced intraplate deformation set up by plate interactions is compatible with indications for the absence of present-day deep mantle-lithosphere interactions inferred from seismic tomography.

Post-rift compressional reactivation potential of passive margins and extensional basins

Abstract Poly-phase deformation of a compressional nature is a common feature in the post-rift evolution of passive margins and rifts. The compressional mode of deformation in these sedimentary basins, originally formed by extension in an intraplate setting, is characterized by a spectrum of spatial wavelengths spanning several tens of kilometres up to several hundreds of kilometres. The actual mode of compressional deformation appears to be strongly affected by the rheological structure of the underlying lithosphere, the level of the regional intraplate stress field, and the geometry of the rifted basin configuration prior to late-stage compressional reactivation. The interplay of plumes and intraplate compressional deformation can lead to temporal transitions from basin inversion to lithospheric folding. These modes of deformation lead to substantial differential vertical motions, late-stage anomalies in subsidence and uplift patterns. The development of innovative combinations of numerical and analogue modelling techniques is the key to differentiating different modes of compressional deformation of passive margins and extensional basins.

Lithosphere folds in the Eurekan orogen, Arctic Canada?

Cornwall and Princess Margaret arches are major regional uplifts in the Tertiary Eurekan orogen of the northeastern Canadian Arctic Islands and have influenced the development of adjacent synorogenic sedimentary basins. The arches are subparallel structures, about 200 km apart, and gravity and seismic-refraction data indicate that they are underlain at depth by crust-mantle upwarps. They may have developed as a result of crustal-scale folding during Eocene compression. Finite-element models for a layered quartz-diorite-olivine lithosphere rheology suggest that the horizontal stresses required are about 75 to 200 MPa. The strength of the continental lithosphere in the Eurekan orogen is affected by the existence of a thick succession of Paleozoic and younger sedimentary rocks in the crust and by the occurrence of a major igneous (thermal) event in the area 20-30 m.y. prior to the main phase of deformation.

Cause of tectonic reactivation and subtle uplifts in the Rocky Mountain region and its effect on the stratigraphic record

Stratigraphic studies show that widespread, albeit subtle, topographic uplifts have occurred throughout the U.S. Rocky Mountain region during middle and late Mesozoic time. Individual paleotopographic highs generally have wavelengths on the order of tens of kilometres and amplitudes on the scale of metres. In places, these features correspond to sites of previous or later deformation and so suggest the possibility of tectonic reactivation along pre-existing zones of structural weakness in the crust. Observations of modern features and modeling studies indicate that uplifts of small magnitude are both a common and expected result of changes in intraplate stress levels. Finite-element modeling suggests that broad-wavelength, low-amplitude topography can develop in a heterogeneous elastic plate under high-stress magnitudes along preexisting faults and under low-stress magnitudes where zones of high and low strength are juxtaposed. Although these features are small, they have sufficient topography to bring about large changes in paleocurrent patterns for low-gradient streams and significantly affect isopach patterns. Although subtle uplifts from this cause are difficult to predict, they are likely a common occurrence and may be the most dramatic features in an area that is otherwise tectonically quiescent. Thus, subtle uplift may be a background noise rather than a harbinger of regional orogenesis.

Geo-Mechanical And Rheological Modelling Of Upper Crustal Faults And Their Near Surface Expression In The Netherlands.

Abstract The pattern of fault reactivation, basin deformation and concentration of seismicity along the main trans-Netherlands fault zone, located NW–SE across the centre of the Netherlands, indicates that this zone is a major zone of weakness. Gravity modelling reveals after back-stripping of the sedimentary succession a distinctive continuous positive anomaly that can be explained by lithospheric sources. This zone of weakness is therefore likely to have a major influence on the tectonic processes currently active in the Netherlands region. We give a review of the tectonic history of the Netherlands and then present the results of a quantitative study of the reactivation of basin boundary faults and the influence on the surrounding basin. Well-data, balanced and back-stripped cross-sections are used to constrain the lithosphere rheology. The lithosphere rheology modelling results show a weak coupling between upper crustal deformation and the subcrustal lithosphere. A finite element modelling approach focussing on the upper crust is carried out in which the basin boundary faults are assigned various dips. The modelling results indicate that, for continuous reactivation of basin boundary faults, the presence of both a pre-existing weakness and a reduced friction angle is required. The latter implies that large displacements accommodated by primary faults cannot be directly attributed to the relative weakness of these faults compared to the secondary faults, which is in close accordance with inferences from trenching. A reduced friction angle has a significant effect on lithospheric strength and appears to be the major controlling factor in the reactivation of basin boundary faults.

Subduction and deformation of the continental lithosphere in response to plate and crust-mantle coupling

Physical analogue experiments are used to investigate the effect of plate and intra-lithospheric coupling on the efficiency of continental lithosphere subduction and the style of collision. Key parameters investigated in this study are: the degree of plate coupling, regulated by the viscosity ratio of the decoupling zone and the surrounding crust and/or mantle lithosphere; and the depth of decoupling. The experimental results show that subduction of the slab is deepest in cases with strong decoupling at the plate interface and at the level of the lower crust of the downgoing plate, with upper-plate deformation restricted to the area close to the plate contact. In these cases, the strongly asymmetric orogenic wedge is widest, consists of a series of imbricated upper-crustal slices derived from the lower plate, and lacks a retro-wedge. In contrast, a reduced strength contrast across the plate interface, at the depth of either the lithospheric mantle or the ductile crust, leads to a combination of subduction and thickening of the mantle lithosphere in both the upper and the lower plates. The degree of plate coupling determines the efficiency of subduction of continental lithosphere under conditions of collision of neutrally buoyant lithospheres, whereas the vertical position of decoupling horizons within the subducting plate controls the amount of subducted lower crust. Transfer of strain to the upper plate depends critically on (1) the degree of plate coupling, with stronger coupling leading to more deformation, and (2) the presence of decoupling horizons within the upper plate, which act as strain guides to propagate deformation into the upper plate. The experimental results explain the geometry and the sequence of deformation in subduction dominated orogens, such as the Carpathians or the Dinarides, and provide a mechanical basis for the transfer of strain to the upper plate.

Faulting, fracturing and in-situ stress prediction in the Ahnet Basin, Algeria - a finite element approach.

Abstract Many low-efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures. Accurate and low-cost information on basic fault and fracture properties, orientation in particular, is critical in reducing well costs and increasing well recoveries. This paper describes how we used an advanced numerical modelling technique, the finite element method (FEM), to compute site-specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strain end-member FEM models of a hydrocarbon-bearing fault-propagation-fold structure. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still allows comparisons with (the little available) geological and geophysical data. In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth. If pre-existing fractures exist, only (sub)horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models that do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hanging wall. Some of these faults cut reservoir and/or seal layers, and thus may influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure (and proximity to failure). These numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.

Tectonic Models for the Evolution of Sedimentary Basins

The tectonic evolution of sedimentary basins is the intrinsic result of the interplay between lithospheric stresses, lithospheric rheology, and thermal perturbations of the lithosphere–upper mantle system. The thermomechanical structure of the lithosphere exerts a prime control on its response to basin-forming mechanisms, in both extension and compression. Tectonic reactivation has strongly affected the structure and fill of many sedimentary basins. The long-lasting rheological memory of the lithosphere appears to play a far more important role in basin reactivation than hitherto assumed. The temporal evolution of the strength of continents and the spatial variations in stress and strength at continental margins, rifts, and orogenic belts govern the mechanics of basin development in time and space. Polyphase deformation is a common feature of many sedimentary basin systems. Compressional reactivation of extensional basins during their postrift phase appears to occur in many intraplate rifts and passive margins, reflecting temporal and spatial changes in the orientation and magnitude of the intraplate stress regime. Similarly, foreland basins are frequently characterized by preorogenic extension. The actual subsidence patterns of these polyphase systems are often more complex than predicted by end-member models that only consider the basin formation mechanism. Folding of the lithosphere, involving positive and negative deflections, appears to be of more importance in the large-scale deformation of intraplate domains than hitherto realized. In the intraplate domain of continental Europe that was thermally perturbed by Cenozoic upper mantle plume activity, lithospheric folding, for instance, plays an important role and strongly affects the pattern of vertical motions, in terms of both the basin subsidence and the uplift of broad arches. Tectonic processes operating during basin formation and during the subsequent deformation of basins can generate significant differential topography in basin systems. In view of the close link between erosion of topographic highs and sedimentation in subsiding areas, constraints are needed on uplift and coeval subsidence to validate quantitative process-oriented models for the evolution of sedimentary basins. Integration of analog and numerical modeling provides a novel approach to assess the feedback mechanisms between deep mantle, lithospheric, and surface processes.

Thermo-mechanical controls on Alpine deformation of NW Europe

Abstract The lithosphere of the Northern Alpine foreland has undergone a polyphase evolution with an intense interplay between upper mantle thermal perturbations and stress-induced intraplate deformation that points to the importance of lithospheric folding of the thermally weakened lithosphere. In this paper we address relationships between deeper lithospheric processes, neotectonics and surface processes in the Northern Alpine foreland with special emphasis on tectonically induced topography. We focus on lithosphere memory and neotectonics with special attention to the thermo-mechanical structure of the lithosphere, mechanisms of large-scale intraplate deformation, Late Neogene anomalies in subsidence and uplift, and links with surface processes and topography evolution.

Thermal perturbation, mineral assemblages and rheology variations induced by dyke emplacement in the crust

We constructed a thermomechanical model to examine the changes in rheology caused by the periodic intrusion of basaltic dykes in a two-layered continental crust. Dyke intrusion can locally change the mineralogical composition of the crust in space and time as a result of temperature-induced metamorphism. In our models we paid particular attention to determine how different mineral assemblages and reaction kinetics during metamorphism impact on the thermomechanical behavior of the crust, in terms of differential stress values. We investigated several lithologies characteristic for intracontinental crust: (1) a quartz-feldspathic crust (QF), (2) a crust with a mineralogical assemblage resembling the average chemical composition occurring in literature (CC), and (3) a micaschist crust (MS). Our model shows that temperature profiles are weakly influenced by metamorphism, with negligible variations in the T-t paths. The results indicate that intrusion-induced changes in the crustal rheology are strongly dependent on mineralogical assemblage variation. The strength of a dyke aureole in the upper crust increases during dyke emplacement, which may cause migration of later dykes and influence the dyke spacing. In contrast, in the lower crust the strength of a dyke aureole decreases during dyke emplacement. Fast kinetics results in a ductile lower crust in proximity of the dykes, whereas slower kinetics leads to the formation of partial melts and subsequent switch from ductile to brittle behavior. Lithology exerts a dominant role on the quantity of melt produced, with higher volume percentages occurring in the MS case study. Produced melts may migrate and support acidic volcanic activity.