Crustal structure and composition of the Oslo Graben, Norway W. Stratford ⁎, H. Thybo Institute of Geology and Geography, University of Copenhagen, Copenhagen, Denmark abstract article info Article history: Received 24 August 2010 Received in revised form 8 February 2011 Accepted 10 February 2011 Available online 10 March 2011 Editor: P. Shearer Keywords: Oslo Graben refraction seismics continental rift Poisson's ratio Although more than 250 my old, the Oslo Graben has retained a distinctive distribution of P-wave velocities associated with rifting, intrusion and underplating of the thinned crustal section. We calculate Poisson's ratio values from crustal P and S-wave velocities along an ~400 km long profile across the southern Scandinavian Peninsula, with a focus on the Oslo Graben. Plutonic rocks are now exposed at the surface in the graben due to post rifting erosion and the corresponding low (5.5 km/s) P-wave velocities extend to depths of ~ 3 km. The P- wave velocity and Poisson's ratio between depths of 6 and 34 km are 5% higher inside than outside the rift boundaries which, together with thickening of the N 7 km/s lower crust beneath the rift, indicate magmatic intrusions. Poisson's ratio of 0.27 and high 7.1–7.4 km/s velocities indicate a heterogeneous composition lower crust. The thickened high velocity lower crust is 1.5 times wider than the surface expression of the rift and extends to both the east and west of the main rift axis. Depth dependant extension may have occurred within the rift and may account for the significant volumes of volcanic output despite the small surface extension observed. The added volume of intrusive material may explain why the Moho up warp is small (as little as 2 km) at the rift. Based on the new P-wave velocities, Poisson's ratio measurements and the thickness of the magmatic underplate, apparent β values for the rift are estimated to be 1.5 but, when intrusion within the crust is included, could be as high at 2.3. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Oslo Rift has been the subject of extensive exploration since its discovery in the late 1800s. The rift was labelled in early studies as a “type locality” for continental rifting and a plume origin was inferred (Ramberg, 1976). A high density “rift pillow” in the lower crust was interpreted by early gravity studies (Ramberg and Smithson, 1971). More recent studies favour a less typical, low temperature, wet mantle rift regime (Neumann et al., 1992; Pedersen and Van der Beek, 1994; Pedersen et al., 1998), and formation of the rift by impingement of a hot mantle plume has largely been discredited (Pedersen and Van der Beek, 1994). The key observations against the plume model are the insufficient volume of volcanics, and the lack of surface uplift prior to rifting (Heeremans et al., 1996). Despite the favoured non-plume model, alteration of the crust by magmatic activity is inferred (Ebbing et al., 2005; Ramberg, 1976; Ramberg and Smithson, 1971). Quantifying the volumes of new material added to the crust by rifting is essential for elucidating the mode of rifting within the graben; in particular in the lower crust where underplated magmatic rocks may form the source zone for the differentiation of more sialic composition rocks. Crustal thickness variations within the Fennoscandian Shield are largely accommodated within the lower crust (Kukkonen et al., 2008). Post accretion alteration mechanisms affecting the Fennoscandian Shield may have included delamination (Dewey et al., 1993), or convective removal (Houseman and Molnar, 1997) of the lower crust in the Archean and Svecofennian Domains after metamorphism in eclogite facies (Kukkonen et al., 2008). Within the South Scandinavian Domain in southern Norway, the youngest part of the Fennoscandian Shield, the post accretion mechanisms for alteration of crustal structure may be more complex. The high P-wave velocity (Vp N 7 km/s) lower crust beneath the southern Scandes Mountains is thin (Stratford and Thybo, 2011), whereas within the Oslo Graben, a 60 million year history of rifting and magmatism has likely increased the amount of high velocity magmatic material above the Moho (Neumann et al., 1986, 1992; Ramberg, 1976). Measurements of Poisson's ratio can provide insight on the variations in crustal alteration throughout the Fennoscandian Shield. Combining available information from deep crustal xenoliths, Vp and Poisson's ratio can add constraint to the composition and volume of new material added to the crust, and the general compositional variations between regions. Measurements of Poisson's ratio are, however, rarely carried out because strong S-wave arrivals are seldom recorded in active source projects. S-wave data are required to resolve variation in Poisson's ratio within crustal layers. Unusually strong S-waves were recorded from chemical shots in this study. These new Poisson's ratio estimates are used to infer bulk Earth and Planetary Science Letters 304 (2011) 431–442 ⁎ Corresponding author. Now at the Department of Earth Sciences, Durham University, United Kingdom. E-mail addresses: ws@geo.ku.dk, wandastratford@gmail.com (W. Stratford). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.02.021 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl