Alexander Minakov | University of Oslo (original) (raw)

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Papers by Alexander Minakov

In this poster we show results from 2D acoustic pre-stack depth migration and full waveform inver... more In this poster we show results from 2D acoustic pre-stack depth migration and full waveform inversion using multichannel seismic data, complemented by coincident travel-time tomography of wide-angle ocean bottom seismometer data. The study area is located within the deep ocean basin in the northeastern parts of the Norwegian-Greenland Sea. This area was affected by intense Quaternary glacial sedimentation in the Storfjorden and Bjørnøya Fans and formation of submarine mega-slides. The seismic source used for the data acquisition consisted of an array of six airguns, and the wavefield was recorded by a 3-km-long 240-channel streamer. After some initial processing, pre-stack depth migration and waveform inversion was performed in order to obtain an image the glacial sedimentary package. The background velocity model was obtained from travel time tomography on the coincident ocean bottom seismometer data. We first show inversion results for a test model which is based on the our knowle...

Proceedings 76th EAGE Conference and Exhibition 2014, 2014

ABSTRACT The northern Barents Sea continental margin has remained the least investigated province... more ABSTRACT The northern Barents Sea continental margin has remained the least investigated province of the Barents Sea because of very limited seismic data due to a largely permanent ice cover. An understanding of the structure and the evolution of the continental margin is essential to figure out the history of geological development and the hydrocarbon potential of the northern Barents Sea region. A series of crustal-scale geotransects illustrating the architecture of the continental margin were constructed using seismic reflection profiles and both inverse and forward gravity modeling. The applied method includes solution of the inverse gravimetric problem with respect to the gravity effect of a thermally differentiated mantle. An iterative grid-based gravity inversion for Moho depth and stretching factors was applied. Two-dimensional gravity inversion for Moho depths was carried out along synthetic profiles taking into consideration the distribution of sedimentary cover from sparse seismic profiles and depth to magnetic source estimates. The crustal transects reveal a narrow and steep continent-ocean transition which is characteristic of sheared more than rifted margins. This may reflect a short-lived phase of shear during breakup prior to the opening of the Eurasia Basin which was initiated at the Paleocene-Eocene transition. The free-air gravity field shows large positive anomalies associated with Plio-Pleistocene glacial fans deposited in front of the Franz-Victoria and St. Anna troughs, which are prominent bathymetric features in the northern Barents-Kara Sea. These sediments were derived from uplifted and eroded areas in the Barents-Kara Sea region.

We suggest that thermo-mechanical feedback between shear heating and temperature-dependent viscos... more We suggest that thermo-mechanical feedback between shear heating and temperature-dependent viscosity in continental zones of oblique extension has the potential to significantly weaken the lithosphere and facilitate subsequent rapid breakup. We perform thermo-mechanical experiments of strike-slip and coeval but longer lasting extension using the finite element method. The style of rifting in our model depends mainly on the efficiency of shear heating which, in turn, depends on the magnitude of lithospheric stresses and strain rates. We develop and apply a numerical model to explain the formation of the Lomonosov Ridge microcontinent. Our proposed model provides a physically consistent mechanism for the detachment of the Lomonosov Ridge from the Barents–Kara Sea margin at 68–56 Ma. The narrow rift mode predicted by the numerical model is supported by the crustal structure of conjugate passive margins of the Eurasia Basin.Style of continental rifting depends on the efficiency of shear heating.Narrow rifted margins may develop by oblique extension assisted by shear heating.Strike-slip deformation facilitated formation of the Lomonosov Ridge 68–56 Ma.

The northern Barents–Kara Sea continental margin is a poorly investigated area because of a perma... more The northern Barents–Kara Sea continental margin is a poorly investigated area because of a permanent ice cover hampering seismic exploration. The available geological and geophysical data show that the magma-poor margin developed in response to early Cenozoic break-up and subsequent opening of the Arctic Eurasia Basin. In this study, a series of crustal-scale geotransects illustrating the architecture of the continental margin are constructed using sparse seismic reflection profiles and a gravity inversion method incorporating a thermal model of rifting. The continental side of the northern Barents Sea margin is underlain by Palaeozoic–Early Mesozoic deep sedimentary basins separated from the oceanic side by the marginal uplift. A bathymetry analysis complements low-resolution seismic data to predict the sedimentary depocenters beyond the shelf break. These depocenters are associated with troughs, perpendicular to the shelf edge. The depocenter in front of the St. Anna Trough may contain a sedimentary section more than 4 km thick. The gravity correction for the effect of sedimentary cover was added to the inversion. This correction used an exponential density–depth function. The inversion supports a narrow and steep continent–ocean transition (COT; ca. 100 km). The conjugate Lomonosov Ridge margin is modelled using the same technique. Palaeoreconstructions were made to predict the break-up setting. The northern Barents Sea–Lomonosov Ridge rift system can be described as an initially narrow symmetric rift. A transitional zone of extreme thinning is assumed between the oldest spreading magnetic anomaly and the stretched continental crust. The free-air gravity anomaly in the western part of the margin can be predicted by the upwelling divergent flow model implying the exhumation of the lower crust and the continental upper mantle within the COT. It is suggested that an episode of shear or oblique extension before breakup is required to explain the observed narrow symmetric conjugate margins in the Eurasia Basin.

Geology, Jan 1, 2011

The Cenozoic sedimentary record revealed by the Integrated Ocean Drilling Program's Arctic Coring... more The Cenozoic sedimentary record revealed by the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44–18 Ma. According to conventional thermal kinematic models, the microcontinent should have subsided to >1 km depth owing to rifting and subsequent separation from the Barents–Kara Sea margin at 56 Ma. We propose an alternative model incorporating a simple pressure-temperature (P-T) relation for mantle density. Using this model, we can explain the missing stratigraphic section by post-breakup uplift and erosion. The pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central Eurasia Basin could have been restricted and confined by non-volcanic thinning of the mantle lithosphere at an early stage (ca. 56–40 Ma). In response to a rise in temperature, the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite. The consequence of lithosphere heating and related mineral phase transitions would be post-breakup uplift followed by rapid subsidence to the deep-water environment observed on the Lomonosov Ridge today.

Tectonophysics, Jan 1, 2011

A seismic refraction and reflection tomography experiment was performed across the igneous provin... more A seismic refraction and reflection tomography experiment was performed across the igneous province east of Svalbard which is a part of the Cretaceous High Arctic Large Igneous Province. Seismic travel times from 12 ocean bottom seismometers/hydrophones deployed along a 170 km line are inverted to produce smooth 2D images of the crustal P-wave velocity and geometry of the acoustic basement and Moho. The inversion of travel times was complemented by forward elastic wave propagation modeling. Integration with onshore geology as well as multichannel seismic, magnetic and gravity data have provide additional constraints used in the geological interpretation. The seismic P-wave velocity increases rapidly with depth, starting with 3 km/s at the sea floor and reaching 5.5 km/s at the bottom of the upper sedimentary layer. The thickness of this layer increases eastward from 2 km to 3.5 km. On average the P-wave velocity in the crystalline crust increases with depth from 5.5 km/s to 6.8 km/s. The crustal thickness is typical for continental shelf regions (30–34 km). Finger-shaped high-velocity anomalies, one reaching 12% and two of 4–6% velocity perturbation, are obtained. These velocity anomalies are concomitant with Lower Cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs, traced towards the northern Barents Sea passive continental margin. We interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the Arctic Amerasia Basin.

The Barents Sea region possesses huge economical potential owing to its resources of oil and gas.... more The Barents Sea region possesses huge economical potential owing to its resources of oil and gas. At the same time, its geological structure and fundamental processes responsible for sedimentary basins formation remain largely ambiguous because of a lack of reliable data. In particular, the northern continent-ocean margin of the Barents Sea is almost not explored by seismic data due to permanent ice cover.
As a part of PETROBAR research project an effort has been undertaken to reveal deep structure of the crust and upper mantle below the northern Barents Sea from new seismic tomography experiments with ocean bottom seismometers. These data are also used to constrain generally more uncertain modelling of gravity anomalies and numerical simulations of lithospheric deformation.
The obtained results agree with the hypothesis that the Arctic Ocean started its formation through breakup of the lithosphere and seafloor spreading in the Early Cretaceous (about 130 million years ago). This process was accompanied by extensive basaltic volcanism known as the High Arctic Large Igneous Province and manifested in the northern Barents Sea as numerous subhorizontal and vertical sheet-like bodies of igneous rocks radiating from a magma source in the central Arctic Ocean. After a period of tectonic uplift (100-70 million years ago) the northern shelf edge of the Barents Sea chopped off and drifted away by magma-poor breakup and sparsely magmatic seafloor spreading. Thus, during the Cretaceous the style of lithospheric deformation at the northern Barents Sea continent-ocean margin has changed from a magma-controlled to tectonically controlled regime which was followed by development of slow cooling passive continental margin.

A new combined magnetic database and a magnetic-profile map are developed for the Eurasia Basin a... more A new combined magnetic database and a magnetic-profile map are developed for the Eurasia Basin as a result of adjusting all available historical and recent Russian and American magnetic data sets. The geohistorical analysis of magnetic data includes several steps: identification of linear magnetic anomalies along each trackline, calculation of the Euler rotation pole positions for the relative motion of the North American and Eurasian plates, analysis of temporal and spatial variations in the spreading rate, and plate reconstructions. The pattern of key Cenozoic magnetic isochrons (24, 20, 18, 13, 6, 5, 2a) is constructed for the entire Eurasia Basin. In the western half of the basin, this pattern is consistent with a recently published scheme [16]. In its eastern half, magnetic isochrons are determined in detail for the first time and traced up to the Laptev Sea shelf. The main stages in the seafloor spreading are established for the Eurasia Basin. Each stage is characterized by a specific spreading rate and the degree of asymmetry of the basin opening. The revealed differences are traced along the Gakkel Ridge. Systematic patterns in wandering of the Eurasia Basin opening pole are established for particular stages. The continent-ocean transition zone corresponding to the primary rupture between plates is outlined in the region under consideration on the basis of gravimetric data. The nature of different potential fields and bottom topography on opposite sides of the Gakkel Ridge is discussed. The characteristic features of the basin-bottom formation at main stages of its evolution are specified on the basis of new and recently published data. The results obtained are in good agreement with plate geodynamics of the North Atlantic and the adjacent Arctic basins.

In this poster we show results from 2D acoustic pre-stack depth migration and full waveform inver... more In this poster we show results from 2D acoustic pre-stack depth migration and full waveform inversion using multichannel seismic data, complemented by coincident travel-time tomography of wide-angle ocean bottom seismometer data. The study area is located within the deep ocean basin in the northeastern parts of the Norwegian-Greenland Sea. This area was affected by intense Quaternary glacial sedimentation in the Storfjorden and Bjørnøya Fans and formation of submarine mega-slides. The seismic source used for the data acquisition consisted of an array of six airguns, and the wavefield was recorded by a 3-km-long 240-channel streamer. After some initial processing, pre-stack depth migration and waveform inversion was performed in order to obtain an image the glacial sedimentary package. The background velocity model was obtained from travel time tomography on the coincident ocean bottom seismometer data. We first show inversion results for a test model which is based on the our knowle...

Proceedings 76th EAGE Conference and Exhibition 2014, 2014

ABSTRACT The northern Barents Sea continental margin has remained the least investigated province... more ABSTRACT The northern Barents Sea continental margin has remained the least investigated province of the Barents Sea because of very limited seismic data due to a largely permanent ice cover. An understanding of the structure and the evolution of the continental margin is essential to figure out the history of geological development and the hydrocarbon potential of the northern Barents Sea region. A series of crustal-scale geotransects illustrating the architecture of the continental margin were constructed using seismic reflection profiles and both inverse and forward gravity modeling. The applied method includes solution of the inverse gravimetric problem with respect to the gravity effect of a thermally differentiated mantle. An iterative grid-based gravity inversion for Moho depth and stretching factors was applied. Two-dimensional gravity inversion for Moho depths was carried out along synthetic profiles taking into consideration the distribution of sedimentary cover from sparse seismic profiles and depth to magnetic source estimates. The crustal transects reveal a narrow and steep continent-ocean transition which is characteristic of sheared more than rifted margins. This may reflect a short-lived phase of shear during breakup prior to the opening of the Eurasia Basin which was initiated at the Paleocene-Eocene transition. The free-air gravity field shows large positive anomalies associated with Plio-Pleistocene glacial fans deposited in front of the Franz-Victoria and St. Anna troughs, which are prominent bathymetric features in the northern Barents-Kara Sea. These sediments were derived from uplifted and eroded areas in the Barents-Kara Sea region.

We suggest that thermo-mechanical feedback between shear heating and temperature-dependent viscos... more We suggest that thermo-mechanical feedback between shear heating and temperature-dependent viscosity in continental zones of oblique extension has the potential to significantly weaken the lithosphere and facilitate subsequent rapid breakup. We perform thermo-mechanical experiments of strike-slip and coeval but longer lasting extension using the finite element method. The style of rifting in our model depends mainly on the efficiency of shear heating which, in turn, depends on the magnitude of lithospheric stresses and strain rates. We develop and apply a numerical model to explain the formation of the Lomonosov Ridge microcontinent. Our proposed model provides a physically consistent mechanism for the detachment of the Lomonosov Ridge from the Barents–Kara Sea margin at 68–56 Ma. The narrow rift mode predicted by the numerical model is supported by the crustal structure of conjugate passive margins of the Eurasia Basin.Style of continental rifting depends on the efficiency of shear heating.Narrow rifted margins may develop by oblique extension assisted by shear heating.Strike-slip deformation facilitated formation of the Lomonosov Ridge 68–56 Ma.

The northern Barents–Kara Sea continental margin is a poorly investigated area because of a perma... more The northern Barents–Kara Sea continental margin is a poorly investigated area because of a permanent ice cover hampering seismic exploration. The available geological and geophysical data show that the magma-poor margin developed in response to early Cenozoic break-up and subsequent opening of the Arctic Eurasia Basin. In this study, a series of crustal-scale geotransects illustrating the architecture of the continental margin are constructed using sparse seismic reflection profiles and a gravity inversion method incorporating a thermal model of rifting. The continental side of the northern Barents Sea margin is underlain by Palaeozoic–Early Mesozoic deep sedimentary basins separated from the oceanic side by the marginal uplift. A bathymetry analysis complements low-resolution seismic data to predict the sedimentary depocenters beyond the shelf break. These depocenters are associated with troughs, perpendicular to the shelf edge. The depocenter in front of the St. Anna Trough may contain a sedimentary section more than 4 km thick. The gravity correction for the effect of sedimentary cover was added to the inversion. This correction used an exponential density–depth function. The inversion supports a narrow and steep continent–ocean transition (COT; ca. 100 km). The conjugate Lomonosov Ridge margin is modelled using the same technique. Palaeoreconstructions were made to predict the break-up setting. The northern Barents Sea–Lomonosov Ridge rift system can be described as an initially narrow symmetric rift. A transitional zone of extreme thinning is assumed between the oldest spreading magnetic anomaly and the stretched continental crust. The free-air gravity anomaly in the western part of the margin can be predicted by the upwelling divergent flow model implying the exhumation of the lower crust and the continental upper mantle within the COT. It is suggested that an episode of shear or oblique extension before breakup is required to explain the observed narrow symmetric conjugate margins in the Eurasia Basin.

Geology, Jan 1, 2011

The Cenozoic sedimentary record revealed by the Integrated Ocean Drilling Program's Arctic Coring... more The Cenozoic sedimentary record revealed by the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44–18 Ma. According to conventional thermal kinematic models, the microcontinent should have subsided to >1 km depth owing to rifting and subsequent separation from the Barents–Kara Sea margin at 56 Ma. We propose an alternative model incorporating a simple pressure-temperature (P-T) relation for mantle density. Using this model, we can explain the missing stratigraphic section by post-breakup uplift and erosion. The pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central Eurasia Basin could have been restricted and confined by non-volcanic thinning of the mantle lithosphere at an early stage (ca. 56–40 Ma). In response to a rise in temperature, the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite. The consequence of lithosphere heating and related mineral phase transitions would be post-breakup uplift followed by rapid subsidence to the deep-water environment observed on the Lomonosov Ridge today.

Tectonophysics, Jan 1, 2011

A seismic refraction and reflection tomography experiment was performed across the igneous provin... more A seismic refraction and reflection tomography experiment was performed across the igneous province east of Svalbard which is a part of the Cretaceous High Arctic Large Igneous Province. Seismic travel times from 12 ocean bottom seismometers/hydrophones deployed along a 170 km line are inverted to produce smooth 2D images of the crustal P-wave velocity and geometry of the acoustic basement and Moho. The inversion of travel times was complemented by forward elastic wave propagation modeling. Integration with onshore geology as well as multichannel seismic, magnetic and gravity data have provide additional constraints used in the geological interpretation. The seismic P-wave velocity increases rapidly with depth, starting with 3 km/s at the sea floor and reaching 5.5 km/s at the bottom of the upper sedimentary layer. The thickness of this layer increases eastward from 2 km to 3.5 km. On average the P-wave velocity in the crystalline crust increases with depth from 5.5 km/s to 6.8 km/s. The crustal thickness is typical for continental shelf regions (30–34 km). Finger-shaped high-velocity anomalies, one reaching 12% and two of 4–6% velocity perturbation, are obtained. These velocity anomalies are concomitant with Lower Cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs, traced towards the northern Barents Sea passive continental margin. We interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the Arctic Amerasia Basin.

The Barents Sea region possesses huge economical potential owing to its resources of oil and gas.... more The Barents Sea region possesses huge economical potential owing to its resources of oil and gas. At the same time, its geological structure and fundamental processes responsible for sedimentary basins formation remain largely ambiguous because of a lack of reliable data. In particular, the northern continent-ocean margin of the Barents Sea is almost not explored by seismic data due to permanent ice cover.
As a part of PETROBAR research project an effort has been undertaken to reveal deep structure of the crust and upper mantle below the northern Barents Sea from new seismic tomography experiments with ocean bottom seismometers. These data are also used to constrain generally more uncertain modelling of gravity anomalies and numerical simulations of lithospheric deformation.
The obtained results agree with the hypothesis that the Arctic Ocean started its formation through breakup of the lithosphere and seafloor spreading in the Early Cretaceous (about 130 million years ago). This process was accompanied by extensive basaltic volcanism known as the High Arctic Large Igneous Province and manifested in the northern Barents Sea as numerous subhorizontal and vertical sheet-like bodies of igneous rocks radiating from a magma source in the central Arctic Ocean. After a period of tectonic uplift (100-70 million years ago) the northern shelf edge of the Barents Sea chopped off and drifted away by magma-poor breakup and sparsely magmatic seafloor spreading. Thus, during the Cretaceous the style of lithospheric deformation at the northern Barents Sea continent-ocean margin has changed from a magma-controlled to tectonically controlled regime which was followed by development of slow cooling passive continental margin.

A new combined magnetic database and a magnetic-profile map are developed for the Eurasia Basin a... more A new combined magnetic database and a magnetic-profile map are developed for the Eurasia Basin as a result of adjusting all available historical and recent Russian and American magnetic data sets. The geohistorical analysis of magnetic data includes several steps: identification of linear magnetic anomalies along each trackline, calculation of the Euler rotation pole positions for the relative motion of the North American and Eurasian plates, analysis of temporal and spatial variations in the spreading rate, and plate reconstructions. The pattern of key Cenozoic magnetic isochrons (24, 20, 18, 13, 6, 5, 2a) is constructed for the entire Eurasia Basin. In the western half of the basin, this pattern is consistent with a recently published scheme [16]. In its eastern half, magnetic isochrons are determined in detail for the first time and traced up to the Laptev Sea shelf. The main stages in the seafloor spreading are established for the Eurasia Basin. Each stage is characterized by a specific spreading rate and the degree of asymmetry of the basin opening. The revealed differences are traced along the Gakkel Ridge. Systematic patterns in wandering of the Eurasia Basin opening pole are established for particular stages. The continent-ocean transition zone corresponding to the primary rupture between plates is outlined in the region under consideration on the basis of gravimetric data. The nature of different potential fields and bottom topography on opposite sides of the Gakkel Ridge is discussed. The characteristic features of the basin-bottom formation at main stages of its evolution are specified on the basis of new and recently published data. The results obtained are in good agreement with plate geodynamics of the North Atlantic and the adjacent Arctic basins.