UPPER-MANTLE STRUCTURE OF THE BALTIC SHIELD BELOW THE SWEDISH NATIONAL SEISMOLOGICAL NETWORK (SNSN) RESOLVED BY TELESEISMIC TOMOGRAPHY

Abstract

Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P-phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array (±30 •). Results indicate P-wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60 • –64 • N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250– 300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north. For a better understanding of the evolution of Baltic Shield, robust imaging of the seismic velocities in the upper mantle is required to help constrain geodynamical models. The Baltic Shield is the exposed part of the Fennoscandian Shield which constitutes the northwestern part of the East European Craton and is composed of Precambrian crystalline rocks (Fig. 1). The East European Cra-ton includes a range of lithospheric provinces from Archaean to late Proterozoic (Artemieva 2003). The evolution of the Fennoscandian Shield can be described in a few stages (Gorbatschev & Bogdanova 1993): First the formation of the Karelian in the Archaean province over 3.5 Ga. Then in the northeast part of the shield area the for-mation of the late Archaean–Paleoproterozoic Lapland Kola mo-bile belt. The high topography of such Archaean–early Proterozoic cratons is interpreted as implying depleted, low density underlying lithospheric mantle (Abbott et al. 1997). Semi-simultaneously, there was accretion to the southwest of the Proterozoic Svecofennian do-main. To the west, the Svecofennian Orogenic zone is delineated by the ∼1.8–1.65 Ga Transcandinavian Igneous Belt. The final stage of crustal shield formation occurred by accretion to western Scan-dinavia between ∼1.75 and 1.5 Ga ago. A major part of Sweden lies in the Svecofennian part of the shield. Nironen et al. (2002) explain the evolution of the Svecofennian domain by five subsequent oro-genies alternating with extensional activity between 1.92 and 1.79 Ga. Crustal structures of the Baltic Shield have been extensively in-vestigated in several controlled source seismic studies including Guggisberg et al. (1991); Kinck et al. (1993); Balling (2000); Lund & Smirnov (2001); Abramovitz et al. (2002); Juhlin et al. (2002) and Korja & Heikkinen (2005). Two large-scale projects in the region, namely FENNOLORA (a seismic refraction experiment, 1979) and BABEL (a wide-angle seismic experiment, 1989) are especially im-portant in earlier studies of the large-scale structures. The structure of upper mantle in the Baltic Shield is an interesting target for deep seismic studies (i.e. Calcagnile 1982; Gee & Zeyen 1996; Perchuc & Thybo 1996; Thybo & Perchuc 1997; Plomerova et al. 2002a; Shomali et al. 2002; Bruneton et al. 2004; Sandoval et al. 2004; Shomali et al. 2006) partly due to the presence of at least two po-tentially different lithospheres in the shield (the Archaean Karelia and Proterozoic Svecofennian), which has been tectonically sta-ble since 1.2 Ga. Another noteworthy geodynamic phenomenon in the area is the glaciation and deglaciation related to the Late Pleis-tocene glacial cycle that is responsible for large scale redistributions of ice and water on the Earth's surface (Kaufmann et al. 2000). Current crustal deformation in Fennoscandia is dominated by post-glacial isostatic adjustment (Milne et al. 2001).