[1] Uniaxial reconsolidation experiments conducted on Ocean Drilling Program drill cores along the Muroto Transect of the Nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the décollement zone. Tests were conducted on samples collected from similar stratigraphic levels below the décollement and its seaward projection. Consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples. The sediments, however, exhibited substantial postyield strength: up to 2.8 times the predicted in situ effective vertical stress beneath the protothrust zone. This enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section. The close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state, the sediment matrix remains sensitive to in situ stress conditions. The low yield stress of one sample, collected within $40 m of the décollement fault at Site 808, may reflect diagenesis under reduced effective stress conditions, due to postconsolidation increases in pore pressure along the décollement. Due to their cemented state, the strong underthrust sediments resist décollement downcutting beneath the toe of the prism, leading to preferential incorporation of weaker, continuously deforming accreted sediments during shear. Seismogenic slip along the décollement at depth may create stresses in excess of sediment strength, causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the décollement zone.

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[1] Experimental consolidation of uncemented clay-rich marine sediments provides information concerning their stress history. A main finding is that some of the well-known behavior of soft sediment deformation in geotechnical applications cannot validly be extrapolated to sediments that have been subjected to higher stresses and longer times of geologic conditions. This study confirms that the yield stress of the uncemented sediment accurately reflects its previous maximum consolidation state. Furthermore, we have identified a new phase of post-yield strain that is associated with higher values of the modified compression index (the slope of the porosity versus the logarithm of effective vertical stress) than that of elastic deformation, but with much lower values than that for primary consolidation. This post-yield behavior is a linear, non-elastic deformation, and is termed tertiary consolidation. Yield stress appears independent of creep time or strain rate, whereas the tertiary-primary consolidation transition is sensitive to these parameters. During post-yield creep (secondary consolidation) the slope of the porosity versus the logarithm of time curve, or the secondary consolidation index, is generally assumed constant. However, this is not valid for claystones at effective vertical stresses above about 1 MPa, where the secondary consolidation index increases with stress. At a given effective vertical stress, the secondary consolidation index also increases with creep times greater than about 10 5 s (%28 h).

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 .

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.

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 region in central Sweden. Two acoustic televiewer logging campaigns, with more than 1 year between campaigns, provide detailed images along the full length of the 2.5 km deep borehole for breakout, drilling‐induced tensile fracture (DITF), and natural occurring structural analysis. Borehole breakouts occur in 13 distinct zones along total length of 22 m, indicating an average maximum horizontal stress, SHmax, orientation of 127° ± 12°. Infrequent DITFs are constrained within one zone from 786 to 787 m depth (SHmax orientation: 121° ± 07°). These SHmax orientations are in agreement with the general trend in Scandinavia and are in accordance with many mechanisms that generate crustal stress (e.g., ridge push, topographic loading, and mantel driven stresses). The unique acquisition of image logs in two successions allows for analysis of time‐dependent borehole deformation, indicating that six breakout zones have crept, both along the borehole axis and radially around the borehole. Strong dynamic moduli measured on core samples and an inferred weak in situ stress anisotropy inhibit the formation of breakouts and DITFs. Natural fracture orientation below 800 m is congruent to extensional or hybrid brittle shear failure along the same trend as the current SHmax. Analysis of foliation in the image logs reinforces the interpretation that the discontinuous seismic reflectors with fluctuating dip observed in seismic profiles are due to recumbent folding and boudinage.

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.

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.