John A. Korstgård

A major shear zone within the Nagssugtoqidian of West Greenland

Abstract A regional lineament of highly parallelized planar and linear structures can be followed over a distance of 150 km from the coast to the Inland Ice at Nordre Stromfjord in central West Greenland. This shear zone is situated within the Nagssugtoqidian mobile belt and transects an area of intricate interference structures, from which it has been formed by high shear strain. By using data for the orientation of planar structures outside and within the zone, the shear strain is calculated to be approximately 6. This value applies to the shear zone close to the coast where its width is approximately 15 km. The zone is cut by the granulite/amphibolite facies boundary, and towards lower metamorphic grade the width of the zone decreases. At the same time the shear strain increases, so that the offset across the zone could be constant irrespective of width. At the coast, where the zone has been mapped, the fanning of planar structures shows the zone to be wedge-shaped, thinning upwards. It is therefore suggested that the zone represents a deep-seated ductile part of a major, transcurrent fault with a sinistral displacement of at least 100 km.

The mechanics of clay smearing along faults

A clay- or shale-rich fault gouge can significantly reduce fault permeability. Therefore, predictions of the volume of clay or shale that may be smeared along a fault trace are important for estimating the fluid connectivity of groundwater and hydrocarbon reservoir systems. Here, we show how fault smears develop spontaneously in layered soil systems with varying friction coefficients, and we present a quantitative dynamic model for such behavior. The model is based on Mohr-Coulomb failure theory, and using discrete element computations, we demonstrate how the model framework can predict the fault smear potential from soil friction angles and layer thicknesses.

Planar detaching faults in the southern Horn Graben, Danish North Sea

The Danish Horn Graben is located in the south-eastern North Sea and suffered fault controlled differential subsidence during the Triassic. Within the Horn Graben, several faults cut the Triassic succession but detach at or close to the Top pre-Zechstein surface. An E-W striking trend of planar detaching faults is mapped using 2-D seismic sections and the displacement on the faults is analysed with respect to timing and spatial distribution of the displacement. The spatial distribution of the displacement is analysed using the Fault Analysis Projection System (Badley Earth Sciences), whereas the timing is analysed by reconstructing the seismic sections and thus obtaining the spatial distribution of displacement at different times. The occurrence of the detaching planar faults appears to be controlled by the difference in basal friction across the pinch-out line of the mobile Zechstein sediments. A dipping detachment surface is necessary to create a driving mechanism but is of minor importance in initiating the faulting. The timing of the faulting is controlled by a general change in tectonic pattern enabling a N-S extension of the sediments covering the Top pre-Zechstein surface.

A new UHP metamorphic complex in the ~1.8 Ga Nagssugtoqidian Orogen of West Greenland

Abstract The Nagssugtoqidian Orogen is a ca. 1.8 Ga belt of east-west trending, highly deformed rocks that bisects central Greenland. Although a variety of data have suggested this belt marks the location of a continent-continent collision zone, evidence of subduction has been lacking. We report here mineralogical evidence from four samples within a well-defined lithologic unit of metabasic and metasedimentary ocean floor rocks of a previously unrecognized UHP metamorphic episode. The UHP episode is recorded by remnants of orthopyroxene exsolved from majoritic garnet, graphitized diamond, exsolution of rutile from garnet and pyroxenes, exsolution of magnetite from olivine, and complex exsolution textures in ortho- and clinopyroxenes (including omphacite). Associated with these mineralogical features is an unusual occurrence of quartz needles in Mn-rich fayalite. From textural characteristics, we infer that the quartz needles exsolved from the fayalite. To our knowledge, olivine with exsolved silica has not been reported. We note, however, that experimental studies have shown that b-spinel can incorporate excess silica. We therefore speculate these quartz needles may be silica that exsolved from Mn-rich ahrensite, the Fe analog of ringwoodite, upon decompression and inversion to fayalite. If correct, this occurrence would be the first reported sample of naturally occurring olivine (fayalite) that inverted from ahrensite. Corroborating an early UHP history are reaction relationships that delineate a path through high-pressure and high-temperature conditions during decompression. P-T conditions inferred for the UHP episode are ~7 GPa at ~975 °C. The unusually low T for this UHP system at ~1.8 Ga may reflect either very rapid subduction rates at that time, or unexpectedly cool mantle conditions. Preservation of the UHP assemblages probably is due, in large part, to the exceptionally low aH2O during decompression and cooling. These UHP rocks establish that the location of the subduction and suture zones that must have existed prior to and during the collision of continents was along what is now the northern edge of the Nordre Strømfjord shear zone.

Evolution of a Salt-Related Tertiary Growth Fault in the Danish North Sea

A salt-related growth fault in the Danish North Sea shows the combined effects of salt tectonics and compaction on the development of the fault, its geometry and its rollover structure through Tertiary time. Contour displacement diagrams on a vertical plane parallel to the fault strike and vertical displacement maps of the lateral displacement distribution have been produced for different times in the Tertiary. The distortion of the regular semi-elliptical patterns is suggested to be due to salt flow along the fault strike. Backstripped and decompacted seismic sections have enabled the reconstruction of the movement of the salt surface and its influence on the fault geometry through time.

Further observations related to a possible occurrence of terrestrial ahrensite

Abstract Clusters of aligned, highly elongate, prismatic quartz (Qtz) rods occur in a few fayalite (Fa) crystals in an eulysite from a recently identified ~1.8 Gy UHP site in central West Greenland (Glassley et al. 2014). Additional detailed analyses of the crystallography and phase compositions of these olivines were conducted to evaluate the postulate that the Qtz rods formed during inversion of super-silicic ahrensite to Fa+Qtz during decompression. These new observations show the Qtz rods consistently occur in crystallographically coherent clusters with the Qtz grains aligned parallel to [100] of Fa. The contrasting compositions of coexisting primary UHP Fa and Fa postulated to have formed by inversion of ahrensite are consistent with the inversion scenario. We thus conclude that all available data are consistent with the postulate that ahrensite was part of the equilibrium phase assemblage formed during UHP metamorphism and that it inverted to Fa+Qtz upon decompression. If true, this would represent the first occurrence of terrestrial ahrensite formed through natural tectonic processes.

Flexural plate representation of Danish Central Graben evolution

Abstract An inverse flexural plate method is used on five dip seismic lines across the northeastern Danish Central Graben to recover original unloaded plate parameters. The original plate showed remarkable consistency of parameters determined independently from each of the seismic lines with average rigidity D = 1020.6±0.2 Nm; bending moment M = 14.4±0.8 N, and dip angle θ = −7.5±1.5°. By sequentially adding the sedimentary units to the average “bare” basement, it is shown that the 3-D geometry of the basin had major downward warp, and high sedimentary fill, to the SE at Triassic time, leaving the northern section open to possible erosion during Early and Middle Jurassic time. The deep southern basin was then filled by sediments during late Jurassic time, and a new depocenter created across the central and SW parts of the basin. This newer depocenter was then filled during Cretaceous time. The combined 3-D flexure of the basement plate and corresponding sediment fill behaviors with time are indicative of corresponding lithologic variations across the basin and with suspected organic rich sediments. The integrated evolution of basement, sediments and depocenter behavior points to the utility of the inverse procedure for improving our understanding of basinal evolution. The relatively low values of rigidity compared to estimated oceanic crust values point to a thin crust below the sediments and to a higher heat flux through the basement relative to that through oceanic crustal material.