Tomographic inversion of Pn waves beneath southern Scandinavia : a study to reveal the upper mantle velocity structure to explain anomalous topography of Scandinavian Mountains

• Main Author: Latif, Adnan
• Other Authors: Valerie Maupin
• Format: Master Thesis
• Language: English
• Published: 2012
• Subjects: VDP::450; Myers; Norway; Iceland
• Online Access: http://hdl.handle.net/10852/12575; http://urn.nb.no/URN:NBN:no-32741

Scandinavian Mountain Range is located at the continental margin of the Eurasian plate. This area is part of the Baltic shield so topography is expected to be generally low. But this is not the case here. In South western corner of the Baltic shield (Southern Norway) topography is rough and ranges up to 1500 m above sea level forming a NE-SW mountain range. Absence of active tectonics in this area and results from various geophysical studies show that the cause of this topography is merely not a pure crustal phenomenon. There could be some mantle process that caused and sustained this topography. The objective of this study is to analyze the upper mantle composition and heterogeneities by using the concept of Pn tomography (by imaging P wave velocities in the upper mantle). This could be helpful to explain anomalous topography in terms of velocity anomalies in mantle, if any in this region. The tomographic formulation of Myers et al. (2010) and preconditioned conjugate inversion is employed to invert travel times for model parameters. In addition to the mantle velocity perturbations this tomographic formulation also allows to image mantle vertical velocity gradient and crustal modification in the subsurface. Results of mantle velocity imaging indicates a general trend of low velocity in the thinner and younger part of Baltic shield (Norway and offshore) whereas high velocity in the thicker and older parts (Sweden and Baltic). Further, as expected, low velocities are observed in the younger part of central Europe. North Sea is observed to have high velocity surrounded by low velocities of southern Scandinavia and central Europe. Velocity structure beneath southern and central Norway is anomalous (having low velocity) with respect to the rest of Baltic shield. These low velocities are probably the connected to Iceland plume in the west by a narrow channel where low velocities are related to hot and shallow mantle beneath the plume. These results are in good agreement with previous studies (Bannister et al. 1991; Bondo et al. 2009; Husebye et al. 1986). Another important observation is the crustal modifier after inversion. In southern Norway crustal modifier suggests that either the crust in this area has low velocity compared to the rest or that a low velocity anomaly in the mantle is leaked into the crustal part during inversion.