Tredimensjonal gravimetrisk modellering på Vøring marginen

En tredimensjonal litosfæremodell ble konstruert for Vøring marginen, utenfor kysten av Midt-Norge. Grunnlaget for modellen var 32 tolkede OBS-profiler. 11 regionale grenseflater ble digitalisert fra hvert av de 32 OBS-profilene, og bunnen av modellen ble satt til å være 100 kilometer. Dypet til grenseflatene utenfor de digitaliserte basisprofilene ble beregnet ved å bruke en vektet interpolasjon. 3D-gravitasjonen fra 3D-modellen ble kalkulert ved hjelp av Parkers formel. Den kalkulerte gravitasjonen ble sammenlignet med friluftsanomalier fra satellittdata. Den største forskjellen mellom kalkulert- og observert felt ble redusert ved å minke tettheten i den oseanske mantellitosfæren relativt til tettheten i den kontinentale mantellitosfæren. Denne tetthetsreduksjonen er mest sannsynlig et resultat av at mantellitosfæren under oseanskorpen er varmere enn mantellitosfæren under kontinentalskorpen. De store forskjellene mellom kalkulert- og observert felt i Råsbassenget og Utgardshøyden ble redusert ved å legge Moho grunnere. Store negative differanseverdier i nordlige deler av Vøringbassenget er tolket som sterkt intruderte sedimentære bergarter. Sørlige deler av Vøring transform margin viste for høy kalkulert gravitasjon. Dette kan reduseres ved å legge Moho dypere eller ved å legge en tetthetsreduksjon i mantelen lenger mot øst. 3D-modellen ble brukt til å studere forskjeller på 2D gravimetri og 3D gravimetri. Det ble vist at feilene som gjøres ved å bruke 2D modellering kan være betydelige, spesielt i områder hvor det er store endringer i geologi nært profilet som modelleres. Ved å ta hensyn til forskjellene mellom 2D-feltet og 3D-feltet kan det oppnås en bedre 2D modellering. En tredimensjonal litosfæremodell tilsvarende den som er bygd i denne oppgaven er et nyttig hjelpemiddel innenfor mange geologiske/geofysiske grener. Ved å inkludere mer data kan 3D-modellen forbedres ytterligere. Modellen vil da kunne brukes til mer detaljert gravimetrisk modellering, samt isostatiske beregninger, beregninger av strekningshistorie, termal modellering og bassengrekonstruksjon

An integrated geophysical study of Vestbakken Volcanic Province, western Barents Sea continental margin, and adjacent oceanic crust

This paper describes results from a geophysical study in the Vestbakken Volcanic Province, located on the central parts of the western Barents Sea continental margin, and adjacent oceanic crust in the Norwegian-Greenland Sea. The results are derived mainly from interpretation and modeling of multichannel seismic, ocean bottom seismometer and land station data along a regional seismic profile. The resulting model shows oceanic crust in the western parts of the profile. This crust is buried by a thick Cenozoic sedimentary package. Low velocities in the bottom of this package indicate overpressure. The igneous oceanic crust shows an average thickness of 7.2 km with the thinnest crust (5-6 km) in the southwest and the thickest crust (8-9 km) close to the continent-ocean boundary (COB). The thick oceanic crust is probably related to high mantle temperatures formed by brittle weakening and shear heating prior to continental breakup. The COB is interpreted in the central parts of the profile where the velocity structure and Bouguer anomalies change significantly. East of the COB Moho depths increase while the vertical velocity gradient decreases. Below the assumed center for Early Eocene volcanic activity the model shows increased velocities in the crust. These increased crustal velocities are interpreted to represent Early Eocene mafic feeder dykes. East of the zone of volcanoes velocities in the crust decrease and sedimentary velocities are observed at depths of more than 10 km. The amount of crustal intrusions is much lower in this area than farther west. East of the Knølegga Fault crystalline basement velocities are brought close to the seabed. This fault marks the eastern limit of thick Cenozoic and Mesozoic packages on central parts of the western Barents Sea continental margin

Statistical Analysis of Displacement and Length Relation for Normal Faults in the Barents Sea

This paper is devoted to the statistical analysis of dependence between fault length (L) and displacement (D). The main purpose of this work is to study the scaling relations between fault length and displacement using a database that includes datasets of 21 faults with geometric data extracted from 3D seismic coherence cubes of the Norwegian Barents Sea. Multiple linear regression and Bayesian and Akaike information criterions are applied to obtain optimal regression parameters. Our dataset is unique since it includes segment lengths of individual faults, unlike the previously published datasets. Hence, we studied both the dependence of fault segment length and accumulated fault length on displacement. The latter relation (accumulated fault length versus displacement) shows a general agreement (positive correlation and power-law relation) with the previously published results that are mainly obtained from outcrop studies, although the slopes vary for different lithologies. The differences could be attributed to the unique characteristics of our dataset that includes data of all segment lengths of individual faults

Crustal structure across the Møre margin, mid-Norway, from wide-angle seismic and gravity data

The Møre Margin in the NE Atlantic represents a dominantly passive margin with an unusual abrupt transition from alpine morphology onshore to a deep sedimentary basin offshore. In order to study this transition in detail, three ocean bottom seismometer profiles with deep seismic reflection and refraction data were acquired in 2009; two dip-profiles which were extended by land stations, and one tie-profile parallel to the strike of the Møre–Trøndelag Fault Complex. The modeling of the wide-angle seismic data was performed with a combined inversion and forward modeling approach and validated with a 3D-density model. Modeling of the geophysical data indicates the presence of a 12–15 km thick accumulation of sedimentary rocks in the Møre Basin. The modeling of the strike profile located closer to land shows a decrease in crustal velocity from north to south. Near the coast we observe an intra-crustal reflector under the Trøndelag Platform, but not under the Slørebotn Sub-basin. Furthermore, two lower crustal high-velocity bodies are modeled, one located near the Møre Marginal High and one beneath the Slørebotn Sub-basin. While the outer lower crustal body is modeled with a density allowing an interpretation as magmatic underplating, the inner body has a density close to mantle density which might suggest an origin as an eclogized body, formed by metamorphosis of lower crustal gabbro during the Caledonian orogeny. The difference in velocity and extent of the lower crustal bodies seems to be controlled by the Jan Mayen Lineament, suggesting that the lineament represents a pre-Caledonian structural feature in the basement

Continent–ocean-transition across a trans-tensional margin segment: off Bear Island, Barents Sea

A 410 km long Ocean Bottom Seismometer profile spanning from the Bear Island, Barents Sea to oceanic crust formed along the Mohns Ridge has been modelled by use of ray-tracing with regard to observed P-waves. The northeastern part of the model represents typical continental crust, thinned from ca. 30 km thickness beneath the Bear Island to ca. 13 km within the Continent–Ocean-Transition. Between the Hornsund FZ and the Knølegga Fault, a 3–4 km thick sedimentary basin, dominantly of Permian/Carboniferous age, is modelled beneath the ca. 1.5 km thick layer of volcanics (Vestbakken Volcanic Province). The P-wave velocity in the 3–4 km thick lowermost continental crust is significantly higher than normal (ca. 7.5 km s–1). We interpret this layer as a mixture of mafic intrusions and continental crystalline blocks, dominantly related to the Paleocene-Early Eocene rifting event. The crystalline portion of the crust within the south-western part of the COT consists of a ca. 30 km wide and ca. 6 km thick high-velocity (7.3 km s–1) body. We interpret the body as a ridge of serpentinized peridotites. The magmatic portion of the ocean crust accreted along the Knipovich Ridge from continental break-up at ca. 35 Ma until ca. 20 Ma is 3–5 km thicker than normal. We interpret the increased magmatism as a passive response to the bending of this southernmost part of the Knipovich Ridge. The thickness of the magmatic portion of the crust formed along the Mohns Ridge at ca. 20 Ma decreases to ca. 3 km, which is normal for ultra slow spreading ridges