Alasdair Skelton

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program’s Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Hydrogeochemical changes before and after a major earthquake

Hydrogeochemical changes were detected by monitoring ice age meteoric waters before and after a magnitude (M) 5.8 earthquake on 16 September 2002 in the Tjornes Fracture Zone, northern Iceland. Significant Cu, Zn, Mn, and Cr anomalies reached our sampling station 1, 2, 5, and ≥10 weeks before the earthquake, respectively. By comparison with published experimental, geophysical, and geochemical studies, we suggest stress-induced source mixing and leakage of fluid from an external (hotter) basalt-hosted source reservoir, where fluid-rock interaction was more rapid. Rapid 12%–19% increases in the concentrations of B, Ca, K, Li, Mo, Na, Rb, S, Si, Sr, Cl, and SO 4 , and decreases in Na/Ca, δ 18 O, and δD, occurred 2–9 days after the earthquake. The rapidity of these changes is consistent with time scales of fault sealing due to coupled deformation and fluid flow. We interpret fluid-source switching in response to fault sealing and unsealing, with the newly tapped aquifer containing chemically and isotopically distinct ice age meteoric water. Variation in Na/Ca ratio appears to be sensitive to the changing stress state associated with M > 4 earthquakes. This study highlights the potential of hydrogeochemical change in earthquake-prediction studies.

The relative timing of serpentinisation and mantle exhumation at the ocean–continent transition, Iberia: constraints from oxygen isotopes

Legs 173 and 149 of the Ocean Drilling Program profiled a zone of exhumed mantle peridotite at the ocean-continent transition (OCT) beneath the Iberia Abyssal Plain. The zone of exhumed peridotite appears to be tens of kilometers wide and is situated betw

Centrifuge modelling of the evolution of low-angle detachment faults from high-angle normal faults

Abstract Centrifuge models composed of a ductile layer overlain by a semi-brittle layer are used to study how deformation localised by a high-angle normal fault promotes detachment faulting. During lateral extension driven by centrifugal force, localised extension along a pre-existing fault initiated localised isostatic upwelling of the denser lower layer. Where the lower ductile layer was significantly less dense than the semi-brittle upper layer, localised extension along the prescribed cut initiated upwelling of the ductile lower layer. Based on model results, we argue that the transition from high-angle normal faulting to low-angle ‘detachment’ faulting is an inevitable consequence of localising extension, provided that there is viscous coupling between the extending upper layer and the upwelling lower layer. In models with a lower layer of equal density or a denser lower layer, this rotation takes place at the later stages of localised thinning in the upper semi-brittle layer, whereas in models with a less dense lower layer, the rotation takes place earlier due to the buoyant rise of the ductile lower layer. In areas of distributed crustal stretching (e.g. rift basins), where extension of the upper layer is accommodated by numerous steep faults distributed over a wide area and upwelling of lower ductile materials is ‘distributed’ across the area, normal faults remain more planar despite a large amount of extension. Models show that distributed extension along several closely spaced normal faults encourages rotation of blocks rather than their distortion to form listric faults. We further conclude that the only configuration whereby localisation of extension would not result in detachment-style faulting is when the upper and lower layers were completely decoupled.

Magmatic origin of giant ‘Kiruna-type’ apatite-iron-oxide ores in Central Sweden

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Swedens main iron-source, the ‘Kiruna-type’ apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish ‘Kiruna-type’ deposit at Grängesberg have δ18O values between −0.4 and +3.7‰, while the 1.90−1.88 Ga meta-volcanic host rocks have δ18O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ18Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ18Omgt ≤ +0.9‰), in turn, equilibrated with high-δ18O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish ‘Kiruna-type’ ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

Flux rates for water and carbon during greenschist facies metamorphism

The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by ...

Fluid-flux and reaction rate from advective-diffusive carbonation of mafic sill margins in the Dalradian, southwest Scottish Highlands

Abstract Greenschist facies mafic sills in the Dalradian of the southwest Highlands, Scotland, have been carbonated by infiltration of a CO 2 -bearing hydrous fluid from adjacent calc-phyllites. The primary amphibole-epidote bearing assemblage is preserved in the cores of many sills in which the margins were altered to calcite, chlorite and quartz. The asymmetric widths of the carbonated margins allow determination of flow direction and magnitude, and indicate that sill margins were buffered to nearly constant fluid composition by copious flow in the more permeable phyllites. Partially carbonated amphibole-epi-dote assemblages within the reaction fronts preserve evidence for sluggish reaction kinetics. Downstream margins developed by diffusion against the flow direction and this allows calculation of reaction front broadening due to both diffusion and reaction kinetics. An advective-diffusive transport model with linear reaction kinetics has been fitted to the reaction progress profile for a sill at Port Cill Maluaig, Knapdale. This implies a cross-layer time-integrated fluid flux of 62.1 ± 1.3 m 3 /m 2 , a Damkohler Number of 22.4 ± 4.2 and a Peclet Number of 66.2 ± 20.3 (1 σ errors). The Peclet Number and time-integrated fluid flux imply that the flow event lasted between 0.02 and 20 Ma for plausible porosities in the range 10 −3 to 10 −6 . The inferred rate constant for the reaction kinetics is two or more orders of magnitude slower than that calculated by extrapolation of experimentally determined surface reaction rates. Either the kinetic dispersion arose from factors additional to reaction kinetics or fluid-solid reaction was controlled by a slower mechamism such as diffusion away from flow channels. The latter conclusion implicates deformation as an important control on the rates of fluid infiltration and fluid-rock reaction.

Major earthquake at the Pleistocene-Holocene transition in Lake Vättern, southern Sweden

Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.

Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake

Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15–20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8–10 years after the earthquake.

Controls of tor formation, Cairngorm Mountains, Scotland

Tors occur in many granitic landscapes and provide opportunities to better understand differential weathering. We assess tor formation in the Cairngorm Mountains, Scotland, by examining correlation ...