A review of the NE Atlantic conjugate margins based on seismic refraction data

The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s−1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

Seismic refraction data and results from receiver functions were used to compile the depth to the basement and Moho in the NE Atlantic Ocean. For interpolation between the unevenly spaced data points, the kriging technique was used. Free-air gravity data were used as constraints in the kriging process for the basement. That way, structures with little or no seismic coverage are still presented on the basement map, in particular the basins off East Greenland. The rift basins off NW Europe are mapped as a continuous zone with basement depths of between 5 and 15 km. Maximum basement depths off NE Greenland are 8 km, but these are probably underestimated. Plate reconstructions for Chron C24 (c. 54 Ma) suggest that the poorly known Ammassalik Basin off SE Greenland may correlate with the northern termination of the Hatton Basin at the conjugate margin. The most prominent feature on the Moho map is the Greenland–Iceland–Faroe Ridge, with Moho depths >28 km. Crustal thickness is compiled from the Moho and basement depths. The oceanic crust displays an increased thickness close to the volcanic margins affected by the Iceland plume

An extinct, Late Mesoproterozoic, Sveconorwegian mantle wedge beneath SW Fennoscandia, reflected in seismic tomography and assessed by thermal modelling

A channel‐like, low‐velocity zone in the lithospheric mantle beneath W Norway coincides spatially with the extension of a recently discovered 200 × 50 km granite batholith, which formed as a result of oceanic subduction beneath the SW Fennoscandian margin between 1.07 and 1.01 Ga. Based on results from numerical modelling, we argue that the low‐velocity zone, at least in part, reflects the thermal (radioactive) effects of the refertilized mantle wedge of this magmatic arc. The geological record in SW Fennoscandia suggests that active‐margin magmatism terminated as a result of rapid slab rollback and trench retreat starting at ca. 1 Ga. The rapid shift from active‐ to passive‐margin processes was probably critical in preserving the mantle wedge, and its identification can therefore shed light on how active‐margin processes terminated in ancient orogens. © 2017 John Wiley & Sons, Inc

Crustal structure and intraplate seismicity in Nordland, Northern Norway: insight from seismic tomography

The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. However, the exact cause of seismicity in this region is still debated. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. Here, we address this problem with the aid of a new high-resolution 3-D VP and VP/VS ratio images of the crust in Nordland using seismic traveltime tomography. These images show the existence of a localized, 10–15 km Moho step that runs parallel to the coast. The north–south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho step. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP/VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture.publishedVersio

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

Seismic refraction data and results from receiver functions were used to compile the depth to the basement and Moho in the NE Atlantic Ocean. For interpolation between the unevenly spaced data points, the kriging technique was used. Free-air gravity data were used as constraints in the kriging process for the basement. That way, structures with little or no seismic coverage are still presented on the basement map, in particular the basins off East Greenland. The rift basins off NW Europe are mapped as a continuous zone with basement depths of between 5 and 15 km. Maximum basement depths off NE Greenland are 8 km, but these are probably underestimated. Plate reconstructions for Chron C24 (c. 54 Ma) suggest that the poorly known Ammassalik Basin off SE Greenland may correlate with the northern termination of the Hatton Basin at the conjugate margin. The most prominent feature on the Moho map is the Greenland–Iceland–Faroe Ridge, with Moho depths >28 km. Crustal thickness is compiled from the Moho and basement depths. The oceanic crust displays an increased thickness close to the volcanic margins affected by the Iceland plume

A review of the NE Atlantic conjugate margins based on seismic refraction data

The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s−1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed

Glacial isostatic adjustment model with composite 3-D Earth rheology for Fennoscandia

Models for glacial isostatic adjustment (GIA) can provide constraints on rheology of the mantle if past ice thickness variations are assumed to be known. The Pleistocene ice loading histories that are used to obtain such constraints are based on an a priori 1-D mantle viscosity profile that assumes a single deformation mechanism for mantle rocks. Such a simplified viscosity profile makes it hard to compare the inferred mantle rheology to inferences from seismology and laboratory experiments. It is unknown what constraints GIA observations can provide on more realistic mantle rheology with an ice history that is not based on an a priori mantle viscosity profile. This paper investigates a model for GIA with a new ice history for Fennoscandia that is constrained by palaeoclimate proxies and glacial sediments. Diffusion and dislocation creep flow law data are taken from a compilation of laboratory measurements on olivine. Upper-mantle temperature data sets down to 400 km depth are derived from surface heatflow measurements, a petrochemical model for Fennoscandia and seismic velocity anomalies. Creep parameters below 400 km are taken from an earlier study and are only varying with depth. The olivine grain size and water content (a wet state, or a dry state) are used as free parameters. The solid Earth response is computed with a global spherical 3-D finite-element model for an incompressible, self-gravitating Earth. We compare predictions to sea level data and GPS uplift rates in Fennoscandia. The objective is to see if the mantle rheology and the ice model is consistent with GIA observations. We also test if the inclusion of dislocation creep gives any improvements over predictions with diffusion creep only, and whether the laterally varying temperatures result in an improved fit compared to a widely used 1-D viscosity profile (VM2). We find that sea level data can be explained with our ice model and with information on mantle rheology from laboratory experiments, heatflow and seismology and a pure olivine rheology above 400 km. Moreover, laterally heterogeneous models provide a significantly better fit to relative sea level data than the VM2 viscosity, for our ice model as well as for the ICE-5G model that is based on the VM2 profile. The new ice model gives different constraints on mantle rheology than the ICE-5G model, indicating a possible bias towards mantle viscosity in the latter or shortcomings in our ice model. Present-day uplift rates for a dry rheology are close to GPS observed uplift rate for certain combinations of grain size and temperature fields. Sea level data show a preference for a wet olivine rheology, but in that case uplift rates are too low for all grain sizes and temperature fields. The difficulty to fit sea level data and uplift rate data simultaneously can not be resolved by varying creep parameters below 400 km. Uncertainties in the flow law and the neglect of other materials in the upper mantle, as well as the neglect of flow in the crust could affect our conclusions.Space EngineeringAerospace Engineerin