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B. H. Jacobsen

Upper-mantle velocities below the Scandinavian Mountains from P- and S- wave traveltime tomography

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More than 20000 arrival-times of teleseismic P- and S-waves were measured over a period of more than 10 years in five separate temporary and two permanent seismic networks covering the Scandinavian (Scandes) Mountains and adjacent areas of the Baltic Shield. The relative traveltime residuals were inverted to 3D tomograms of P- and S- velocities and the VP/VS ratio. Resolution analysis documents that good 3D resolution is available under the dense network south of 64° latitude (Southern Scandes Mountains), and patchier, but highly useful resolution is available further north, where station coverage is more uneven. A pronounced upper-mantle velocity boundary (UMVB), transecting the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre-Trøndelag Fault Complex), from where it follows the coast and runs between Lofoten and the crest of the Northern Scandes Mountains and stays off the coast further north.
Seismic velocities in the depth interval 100-300 km change across the UMVB from low relative VP and even lower relative VS on the western side to high relative VP and even higher relative VS to the east. Therefore, this main velocity boundary also separates relatively high VP/VS ratio to the west and relatively low VP/VS to the east. Under the Southern Scandes Mountains (most of southern Norway) we find low relative VP, even lower relative VS and hence high VP/VS. These velocity values are indicative of thinner lithosphere, higher temperature and less depletion and/or fluid content, and possible even partial melt in a relatively shallow asthenosphere. At first sight, this might support the idea of a mantle buoyancy source for the high topography. Under the Northern Scandes Mountains we find the opposite condition: High relative VP, even higher relative VS and hence low VP/VS, consistent with thick dry depleted lithosphere, similar to what is found in most of the Baltic Shield. This shows that upper-mantle velocity anomalies are very poor predictors of topography in this region.
An important deviation from these patters is found near the topographic saddle between the Southern and Northern Scandes Mountains. Centred around 64°N, 14°E, a zone of lower S-velocity and hence higher VP/VS ratio is detected in the depth interval between 100 and 300 km. This “Trøndelag-Jämtland Mantle Anomaly” (TJMA) is still interpreted as part of relatively undisturbed lithosphere of shield affinity, because of high relative P-velocity, but the relatively low VP/VS indicates lower depletion, possibly higher fluid content, and most likely lower viscosity relative to the adjacent shield units. We conjecture that this mechanical mantle anomaly may have influenced the collapse of the Caledonian Mountains and in particular guided the location and development of the Møre-Trøndelag Fault Complex. Thus, the TJMA is likely also indirectly to have played an important part in conditioned the “two-dome architecture” of the Scandes Mountains.
Original languageEnglish
JournalGeophysical Journal International
Volume208
Issue1
Pages (from-to)177-192
ISSN0956-540X
DOIs
Publication statusPublished - Jan 2017

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