Geophysical and geodynamic studies of the North Atlantic Realm

• Main Author: Schiffer, Christian
• Format: Book
• Language: English
• Published: 2015
• Subjects: North Atlantic; East Greenland; Caledonides; Geophysics; Geodynamics; Geology; Arctic; Greenland; Jan Mayen; Iceland
• Online Access: https://pure.au.dk/portal/da/publications/geophysical-and-geodynamic-studies-of-the-north-atlantic-realm(642649a1-8b72-485e-9f53-f49a4f395443).html

The geology of the North Atlantic Realm (NAR), including the North Atlantic, Greenland, the Arctic, Iceland, Scandinavia, Northern Europe and Northeast America has been studied for more than a century and inspired some of the most fundamental theories in geoscience, such as plate tectonics, the supercontinent-cycle and the plume theory. In general, the major elements of the geological evolution in this region during the past 450 Ma are understood. However, the crucial details of this evolution are the subject of much discussion and include the following items: a) The formation of the Caledonian mountain range (approx. 425 Ma), the exact series and number of collision and subduction events as well as subduction polarity. b) The formation of the North Atlantic (approx. 60 Ma) and accompanied high magmatic activity which formed distinct and conspicuous structures and features, such as the Iceland Melt Anomaly magma-rich passive margins, hyperextension, exhumed serpentinised mantle and vertical movements. c) The present-day state of dynamic support of the mantle, and the isostatic compensation of the crust and lithosphere and topography. The aim of this PhD project was to collect information on specific areas of the NAR and thereby contribute to the understanding of the geodynamic processes that shaped the region. The central part is the geophysical and geodynamic study of Central East Greenland, based on seismological data acquired by 11 broadband seismometers, deployed and maintained by the Department of Geoscience, Aarhus University for period of 2 years. The results are summarised in two manuscripts, describing receiver function analysis as well as gravity and petrological modelling (P1 and P2). These studies reveal an east-dipping high velocity structure in the upper mantle of the Central Fjord (CF) region, overlain by a zone of considerably low mantle velocities. This distinct signature is interpreted as a fossil subduction zone of Caledonian age, with a slab of eclogitised mafic crust as well as a hydrated and partly serpentinised mantle wedge. Manuscripts P3 and P4 are inspired by the results from East Greenland and focus on continuing tectonic considerations on how a fossil subduction zone might have affected the formation of the NAR. Assuming fossil subduction zones are the common rule along suture zones (which share the same genetic origin) these might have a considerable impact on the rift evolution and passive margin formation, especially because eclogite and serpentinite are “weaker” materials, and therefore could localise extension of the lithosphere. Numerical modelling shows that a fossil subduction complex, together with variations in crustal thickness and extension rate have a major impact on rifting and the shape and style of the formed passive margins. These simple parameters also seem to control whether a margin becomes magma-poor or magma-rich (P3). Correlation of the presumed fossil subduction zone in the CF region with the very similar Flannan structure offshore northern Scotland leads to the hypothesis that these two structures share a genetic origin and once formed a coherent lineament along the western Caledonian Front. The geophysical signature and strong correlation to the Caledonian geology suggests that this lineament represents an east-dipping subduction zone along the western edge of the Caledonian Orogen and is consistent with tectonic scenarios of the Caledonian orogeny, which include early eastward subduction events (Taconian and Grampian). Further, this structure might have had a crucial impact on major post-Caledonian tectonic events in the North Atlantic, such as the occurrence of the Iceland Melt Anomaly and the North Atlantic Igneous Province, as well as the separation from the Jan Mayen microcontinent (P4). Finally a novel modelling approach was developed to iteratively improve predictions of the lithospheric geopotential stress field with regard to the World Stress Map, by testing different combinations of sub-lithospheric pressure and LAB depth changes. This approach provides independent constraints on dynamic topography in the North Atlantic mantle and thereby contributes to the understanding of the geodynamic state and development in the NAR. The results of these case studies conducted have implications for several major and obviously interconnected tectonic events in the NAR, such as the Palaeozoic Caledonian orogeny, the subsequent Cenozoic continental-break up and opening of the North Atlantic and the present-day state of the North Atlantic topography, crust and mantle.