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- Rasmussen, Thorkild Maack
(författare)
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Magnetotellurics in southwestern Sweden: Evidence for electrical anisotropy in the lower crust?
- 1988
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 93:B7, s. 7897-7907
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Tidskriftsartikel (refereegranskat)abstract
- Magnetotelluric data from measurements conducted on the SW part of the Baltic Shield are analyzed with respect to crustal and upper mantle structures. The interpretations have been done using ordinary 2-D and transverse anisotropic layered models in which distortions of the impedance tensors due to near-surface electrical structures are included in the interpretation procedures. The anisotropic model is able to explain the gross features of the data, whereas the 2-D model only displays features observed at long periods. The 2 main results from the study with anisotropic models are the interpretation of a possible transverse anisotropic lower crust and a depth to the uppermost conductive layer within the mantle of more than 200 km. The minimum depth to the conductive mantle layer is found from a most squares analysis.
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- Rona, P.A., et al.
(författare)
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Serpentinized ultramafics and hydrothermal activity at the mid-atlantic ridge crest near 15°N
- 1987
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 92:B2, s. 1417-1427
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Tidskriftsartikel (refereegranskat)abstract
- The initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the Fifteen Twenty Fracture Zone. The ultramafics crop out in block-faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m. They comprise cumulate harzburgites, pyroxenites, Iherzolites, and wehrlites, as well as gabbronorites, olivine gabbronorites, gabbropegmatites, and alteration products including serpentinites, bastite serpentinites, and asbestos. The Ti-Zr-Y relations and relatively constant Zr/Ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust. Ongoing hydrothermal activity is indicated by chemical anomalies (δ3He, Mn) in the near-bottom water at the ultramafic outcrop. The upwelling hydrothermal circulation apparently follows crust-penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and/or mantle. The observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long-offset (>100 km) ridge-ridge transform faults, where they are serpentinized by hydrothermal processes within the initial 1 × 106 years of generation of lithosphère at a slow spreading axis. The corners formed by the intersections of the walls of a rift valley with both the transform (RT corner) and the nontransform (RN corner) portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics.
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- Svensson, Urban, et al.
(författare)
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Formulae for Pressure Gradients in One-Dimensional Lake Models
- 1989
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 94:C4, s. 4939-4946
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Tidskriftsartikel (refereegranskat)abstract
- Formulae for barotropic and baroclinic pressure gradients, suitable for one-dimensional lake models, are derived and explored. The derivation is based on the equation for free standing oscillations and a continuity relation. The formulae, which are easy to derive and use, give the correct external and internal seiche frequencies for nonrotating, rectangular one- or two-layered basins of constant depth. When applied to situations with continuous stratification, further assumptions need to be introduced. These are discussed in the paper. Applications to a laboratory experiment, dealing with wind-induced entrainment, show that pressure gradients for the entrainment process are significant and that the pressure formulae derived capture this effect. The formulae are also used in combination with a one-dimensional lake model. Comparisons with field measurements of the seasonal stratification show that the inclusion of pressure gradients improves the predictive capabilities of the lake model.
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- Zhang, Ping, et al.
(författare)
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Electric resistivity structure of the Siljan impact region ( Sweden)
- 1988
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 93:B6, s. 6485-6501
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Tidskriftsartikel (refereegranskat)abstract
- Based on 65 magnetotelluric (MT) measuring stations, a cross section of the electric conductivity structure in the Siljan impact area, located in the central part of Sweden, is presented. Static shift effects due to local near-surface inhomogeneities are present in nearly all of the MT soundings.In an attempt to correct for near-surface distortions, average impedance data and distortion parameters were interpreted. Derived 2-D models show some coincidence with seismic reflection results and support an impact origin of the Siljan area. Fluid migration through the fractures generated by the impact is one possible explanation for the anomalously high conductivity within the upper crust
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