Laurent Gernigon

A recent aeromagnetic survey was carried out to refine our understanding of the structure and nature of the basement rocks beneath the Finnmark Platform (Norwegian Barents Sea). Magnetic depth source estimation and potential field models were used to explore the platform's basement architecture and evaluate the existence of Paleozoic basins. In the western part, magnetic fabrics suggest a continuation of Precambrian rocks into the adjacent platform. Further east, magnetic data reveal NW–SE-trending basement lineaments related to the Sørøya–Ingøya Shear Zone, formed during the Caledonian orogeny. Additionally, lower-frequency and discrete NNW–SSE lineaments may reflect the influence of older Precambrian structures. The Gjesvær Low, previously considered a post-orogenic basin, is now seen as a Caledonian backthrust system tied to the Sørøya–Ingøya Shear Zone. Both Precambrian and Caledonian inherited structures controlled and accommodated the development and segmentation of subsequent rift basins during the Paleozoic and Mesozoic. The NW–SE to WNW–ESE Precambrian trends guided early rift segmentation from the Devonian to Early Carboniferous, while Mesozoic rift border faults align with NE–SW to ENE–WSW-trending magnetic lineaments, largely interpreted as Caledonian in origin.

The tectonic status, the mode of Caledonian deformation and the transport distance of the Precambrian units on the Fennoscandian Shield that constitute the so-called tectonic windows within the realm of the Scandinavian Caledonian nappe pile have long been a subject of dispute. It has been argued for several hundred kilometres of eastward transport for some of these tectonic units, whereas parautochthonous to autochthonous positions have also been suggested. In this contribution, geophysical data have been studied to shed light on this controversy. A dominant regional NNW–SSE to north–south structural grain of Precambrian origin extends beneath the Caledonian nappes. Indeed, similar structural patterns appear in several of the tectonic windows within the orogen. However, magnetic anomalies in southern Norway and the Nordland area show an apparent c. 90° counterclockwise rotation into the Caledonian trend along a line from Nordfjord in western Norway to Senja in northern Norway, whereas the anomalies in Finnmark (northernmost Norway), seem to be rotated clockwise along a line from Kvænangen to Porsangerfjorden. We suggest that this pattern extending through the Caledonides likely represents a boundary between two crustal domains with different Caledonian reworking. The eastern domain demonstrates less extensive modification during the Caledonian continent–continent collision. Some thrusts within basement windows in the eastern domain accommodated transport of a few tens of kilometres, revealing a parautochthonous status for most of these windows. Interpretations of aeromagnetic and gravity data have shown that the Caledonian nappes occurring along detachments extend to a maximum depth of c. 9 km below sea-level. The thickest nappe piles occur along the late Caledonian detachments. The eastern basement domain with minor Caledonian deformation has a thinner nappe stack (up to 4–5 km below). Our combined interpretations also show the need to modify the tectonostratigraphy of the northern Scandinavian Caledonides. The Lower Laksefjord Nappe in Finnmark, for instance, is interpreted as a parautochthonous window into the Precambrian basement and is not part of the Laksefjord Nappe Complex that has been transported hundreds of kilometres.

The disintegration of Pangea north of the Charlie Gibbs fracture zone led to the formation of the NE Atlantic and Arctic Oceans. Both these oceans are exceptionally complex in terms of diversity of the structures they contain and the sequence of events leading to their formation. Recent, extensive work by cross-disciplinary international groups has cast a great deal of new light on the structure and evolution of both oceans. Both have experienced fan-shaped oceanic-type spreading and ridge growth by linear propagation. Both contain shallow, linear bathymetric highs which comprise substantially or almost wholly, continental crust. There are also regions of continental crust, some hyper-extended, capped with lavas. Much of the NE Atlantic Ocean is floored by oceanic crust produced by classical, albeit piecemeal, oceanic spreading. The spreading rate is low and dwindles to ultra-low on the Gakkel Ridge in the Eurasia Basin of the Arctic Ocean. The Gakkel Ridge is flanked by linear, oceanic-like magnetic anomalies although it is not entirely clear whether these represent fully oceanic crust formation or whether some residual stretched continental crust remains beneath this region. The same may be true of the extinct Canada Basin spreading axis in the Amerasia Basin. Likewise, the nature and location of the continent-ocean transition in the NE Atlantic is currently under discussion and it has recently been proposed that the oldest linear magnetic anomalies, closest to the continental edges, characterize some form of magma-injected continental crust. A similar structure has been recently proposed for the Greenland-Iceland-Faroe Ridge  and the Alpha-Mendeleev Rise. What is currently unclear is the extents, in both oceans, of the three kinds of crust – true continental crust including microcontinents, magma-injected continental crust, and fully oceanic crust. There is furthermore likely a structural and geological continuum between these types. Classical linear magnetic anomalies are discontinuous between sections of the spreading ridge, raising the question of whether continuous fully oceanic crust connects these sections. In our presentation we will summarize what is known geologically and tectonically about both oceans, compare and contrast them, and outline their evolution. We will discuss the extents of the three types of crust and explore the implications for the history and mechanisms of ocean formation and the origins and extents of flood basalts. Of particular interest also is the control of pre-existing structure on the style of breakup.

We use high-resolution and regional geophysical data to study a bathymetric high near the Mohns/Knipovich ridges junction, in the Norwegian-Greenland Sea. Near-seafloor magnetic data over hydrothermal site Loki’s Castle first support the basaltic nature of the seafloor. We then combine this result with regional magnetic and bathymetric considerations to investigate the crustal architecture in the vicinity of the junction. We show that the spreading asymmetry is insufficient to allow the development of Oceanic Core Complexes. Instead, this atypical off-axis hill is dominantly basaltic and should be interpreted as the first inside corner hogback structure identified along an active mid-ocean ridge system. Our conclusion tempers the definition of Oceanic Core Complex and underlines that bathymetric highs located off axis from slow-spreading centers cannot always be interpreted as such. This intermediate type of spreading paves the way to the introduction of a new class of oceanic structure referred to as Proto-Core Complexes.