Maria Ask

Ocean-continent boundary in the Iberia Abyssal Plain from multichannel seismic data

Abstract The Ocean-Continent Boundary of the West Iberia margin is marked by a basement ridge trending N-S. Four segments of this ridge are recognized, each of them being progressively offset westward from 40°N to 43°N. Because the setting and seismic character of the ridge in the Iberia Abyssal Plain are similar to those of the Galicia margin ridge, which is made of serpentinized peridotite, we think that the southern segments of the ridge are also made of the same mantle material. The segmentation of the ridge suggests that the northward propagation of the continental break-up during the North Atlantic opening in Early Cretaceous times was discontinuous, each segment possibly corresponding to a propagation step. East of the ridge, the basement of the whole Iberia Abyssal Plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the S reflector previously recognized off Galicia. By analogy with the Galicia margin, we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle, the S reflector corresponding to the contact between crustal rocks and serpentinite.

Postglacial faults in Fennoscandia : targets for scientific drilling

During the last stages of the Weichselian glaciation (ca. 9,000–15,000 years B.P.), reduced ice loads and glacially affected stress fields resulted in active faulting in Fennoscandia with fault scarps up to 160 km long and up to 30 m high. These postglacial (PG) faults are usually SE dipping, SW–NE oriented thrusts, and represent reactivated, pre-existing crustal discontinuities. Postglacial faulting indicates that the glacio-isostatic compensation is not only a gradual viscoelastic phenomenon, but also includes unexpected violent earthquakes, suggestively larger than other known earthquakes in stable continental regions. We explore here possibilities and benefits for investigating, via scientific drilling, the characteristics of postglacial faults in northern Fennoscandia, including their structure and rock properties, present and past seismic activity and state of stress, as well as hydrogeology and associated deep biosphere. The research is anticipated to advance science in neotectonics, hydrogeology and deep biosphere studies, and provide important information for nuclear waste disposal, petroleum exploration on the Norwegian continental shelf and studies of mineral resources in PG fault areas.

In-situ stress from breakouts in the Danish sector of the North Sea

Abstract Borehole breakouts from 26 wells situated in the Danish Central Graben and one well situated in the northwestern section of the Ringkobing-Fyn High have been studied. The data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4.7 km. Statistics were used to obtain a mean azimuth of breakouts for each well, and each individual breakout was weighted by its length. Data quality of breakouts is assigned according to the World Stress Map quality ranking scheme. The breakout data reveal that S H orientations in the Danish Central Graben are extremely scattered; no regional trend in mean S H orientation can be identified as a result of the large standard deviation (±77°). Similar scatterings of S H orientations have been reported from adjacent regions of the North Sea. The results indicate that the state of stress in the sedimentary cover of the Danish Central Graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms. Three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the Post Chalk Group, Chalk Group and Pre Chalk Group.

LiDAR-derived inventory of post-glacial fault scarps in Sweden

Abstract Pre-existing bedrock structures that reactivated following deglaciation through a combination of tectonic and isostatic stresses are well documented in northern Fennoscandia. Due to their possible implications for seismic hazards, there is a need to document the locations and geometries of these features. The recent availability of a high-resolution, LiDAR-derived, digital elevation model covering most of Sweden provides an ideal base upon which to map post-glacial fault scarps that appear to crosscut glacial sediments and landforms. The current mapping project has identified new post-glacial fault scarps in central Sweden, and both refined and rejected scarps previously mapped by aerial photographic interpretation in northern Sweden. No post-glacial fault scarps, however, were identified in southern Sweden. The current inventory of post-glacial fault scarps is available for download and will be updated as more data become available.

Carbonatite ring-complexes explained by caldera-style volcanism.

Carbonatites are rare, carbonate-rich magmatic rocks that make up a minute portion of the crust only, yet they are of great relevance for our understanding of crustal and mantle processes. Although they occur in all continents and from Archaean to present, the deeper plumbing system of carbonatite ring-complexes is usually poorly constrained. Here, we show that carbonatite ring-complexes can be explained by caldera-style volcanism. Our geophysical investigation of the Alnö carbonatite ring-complex in central Sweden identifies a solidified saucer-shaped magma chamber at ~3 km depth that links to surface exposures through a ring fault system. Caldera subsidence during final stages of activity caused carbonatite eruptions north of the main complex, providing the crucial element to connect plutonic and eruptive features of carbonatite magmatism. The way carbonatite magmas are stored, transported and erupt at the surface is thus comparable to known emplacement styles from silicic calderas.

8. Deformation and Mechanical Strength of Sediments at the Nankai Subduction Zone Implications for Prism Evolution and Décollement Initiation and Propagation

As in many depositional settings, marine sediments at convergent margins undergo diagenetic changes before, during, and after mechanical consolidation and deformation. These changes influence mechanical behavior within and beneath the prism and along the décollement. To illustrate the interrelations between sediment diagenesis and deformation, we review physical properties and the types and distributions of deformation structures at several ODP and DSDP drill sites from the frontal regions of the Nankai accretionary prism. Both compactive and dilative deformation structures and fabrics are documented, denoting complicated stress paths during consolidation and tectonic deformation. Laboratory deformation experiments conducted on selected samples from these sites also demonstrate enhanced sediment strengths relative to their preconsolidation stress, both above and below the décollement horizon. This mechanical response indicates the presence of intergranular bonding or cementation that allows sediments to resist consolidation and deformation to relatively high stresses. Once their shear strengths are exceeded, however, cemented sediments can undergo rapid failure, leading to transient increases in pore pressures followed by consolidation. This deformation history may account for the localized compactive deformation bands within the prism. An analogous sequence may develop at depth within the underthrust sediments. Stress perturbations, e.g., near the up-dip limit of the seismogenic zone, may locally exceed the enhanced shear strengths of the underthrust sediments, leading to compactive failure and release of trapped pore fluids. Associated increases in pore fluid pressures may enable décollement downcutting and tectonic underplating. The resulting changes in structural and physical properties of the sediments may favor the onset of seismogenic slip along the décollement .

Image log analysis of in situ stress orientation, breakout growth, and natural geologic structures to 2.5 km depth in central Scandinavian Caledonides: Results from the COSC‐1 borehole

Stress-induced borehole deformation analysis in the Collisional Orogeny in the Scandinavian Caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized regi ...

Rock Mass Behavior Under Hydropower Embankment Dams: A Two-Dimensional Numerical Study

Sweden has more than 190 large hydropower dams, of which about 50 are pure embankment dams and over 100 are concrete/embankment dams. This paper presents results from conceptual analyses of the response of typical Swedish rock mass to the construction of a hydropower embankment dam and its first stages of operation. The aim is to identify locations and magnitudes of displacements that are occurring in the rock foundation and grout curtain after construction of the dam, the first filling of its water reservoir, and after one seasonal variation of the water table. Coupled hydro-mechanical analysis was conducted using the two-dimensional distinct element program UDEC. Series of the simulations have been performed and the results show that the first filling of the reservoir and variation of water table induce largest magnitudes of displacement, with the greatest values obtained from the two models with high differential horizontal stresses and smallest spacing of sub-vertical fractures. These results may help identifying the condition of the dam foundation and contribute to the development of proper maintenance measures, which guarantee the safety and functionality of the dam. Additionally, newly developed dams may use these results for the estimation of the possible response of the rock foundation to the construction.

Numerical Analyses of Earthquake Induced Liquefaction and Deformation Behaviour of an Upstream Tailings Dam

Much of the seismic activity of northern Sweden consists of micro-earthquakes occurring near postglacial faults. However, larger magnitude earthquakes do occur in Sweden, and earthquake statistics indicate that a magnitude 5 event is likely to occur once every century. This paper presents dynamic analyses of the effects of larger earthquakes on an upstream tailings dam at the Aitik copper mine in northern Sweden. The analyses were performed to evaluate the potential for liquefaction and to assess stability of the dam under two specific earthquakes: a commonly occurring magnitude 3.6 event and a more extreme earthquake of magnitude 5.8. The dynamic analyses were carried out with the finite element program PLAXIS using a recently implemented constitutive model called UBCSAND. The results indicate that the magnitude 5.8 earthquake would likely induce liquefaction in a limited zone located below the ground surface near the embankment dikes. It is interpreted that stability of the dam may not be affected due to the limited extent of the liquefied zone. Both types of earthquakes are predicted to induce tolerable magnitudes of displacements. The results of the postseismic slope stability analysis, performed for a state after a seismic event, suggest that the dam is stable during both the earthquakes.

Exploration for deep enhanced geothermal systems with Riksriggen and the LTU downhole stress measurement system

Recent developments in drilling technology have resulted in about five to ten times faster penetration rates compared to penetration rates of conventional diamond drilling. The increased penetration rates open up for affordable drilling of deep boreholes in regions with cold crust, such as in Scandinavia, allowing for geothermal energy generation in enhanced geothermal systems. We present two infrastructures for downhole sampling and testing, Riksriggen and the LTU downhole stress measurement system. Riksriggen produces a borehole that allows in-situ sampling and testing to 2.5 km depth. Furthermore, highly relevant data for geothermal energy production is produced, for example fracture geometries and frequencies, as well as depth of zones with elevated transmissivity zones and their in-situ hydraulic conductivities. The LTU downhole stress measurement system allows determination of the three-dimensional stress tensor and its spatial variation with depth in a scientific unambiguously way. Thus, Riksriggen and the LTU downhole stress measurement system provide data needed for planning most stable borehole trajectory with depth and to determine optimal pattern of production and injection wells (e.g. hydraulic stimulation by fraccing), as well as for assessing the risk of induced seismicity. (Less)