GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 23, PAGES 3389-3392,NOVEMBER 15, 1996 Shear-wave velocity structure of the crust and upper mantle beneath the Kola peninsula I. G. Dricker and S. W. Roecker Department of EarthandEnvironmental Sciences, Rensselaer Polytechnic Institute, Troy, NY G. L. Kosarev and L. P. Vinnik Institute of Physics of the Earth,Russian Academy of Sciences, B. Gruzinskaya 10, Moscow, Russia Abstract. We determined the shear-wavevelocity structure of the crust and upper mantle beneaththe central part of the Kola peninsula from the analysis of P-wave receiver functions and mantle P-SV converted phases recordedat stations Apatity (APA) and Lovozero (LVZ). The times of P-SV converted phasesfrom the 410 and 660 km discontinuities are close to thosepredicted by the IASP91 model. Phase conversions at the crust-mantle boundary beneath the Baltic shield northeast of LVZ and southwest of APA are consistent with a sharptransi- tion from crust to mantle at a depth of 40 km, while conver- sions from the intervening Khibina plutonic region are consis- tent with a gradual transition between depths of 20 and 40 km. We infer that short (-50 km) wavelength lateral variations in the crust-mantle transition persist in this region, despite the inactivity of the Kola peninsula since Devonian times. Introduction The Kola peninsula is locatedin the northeastern part of the Baltic Shield (Figure 1). For the mostpart, tectonic activity in plutons, although LVZ is surrounded almost entirelyby smaller magmatic intrusions of theKAP. Because of this geography, analysis of seismograms recorded from different azimuths can provide informationabout any structural signature that remains from this magmatic intrusion. Data Station APA is an intermediate band(0.05 - 5 Hz) analog sta- tion equipped with Kirnosseismometers that hasbeenin opera- tion since the early 1960s. Analog seismograms of 26 tele- seismic events in the distancerange of 30ø to 1000 and with magnitudes greater than 5.8 recordedat APA since 1977 were scanned and then digitized using a versionof the DGS software developed at the IRIS/SYNAPSE data center in Moscow. The large amountof data archivedat APA allowed us to review more than five hundredseismograms before selecting a subset with an acceptablesignal to noise ratio. Station LVZ is an IRIS/IDA station installed in 1992 and equipped with a broadband STS-1 sensor. Digital seismograms the BalticShield ceased afterits main units consolidated at from about 200 earthquakes recorded atLVZ were obtained from about 1.8Ga [e.g., Lobach-Zhuchenko et al., 1972; Berthelsen the IRIS Data Management Center (DMC), using the same dis- and Marker, 1986; Park, 1991]. However, during Paleozoic times the Kola peninsulaexperienced a rifting event that re- sulted in the formation of the Kontozero graben and the em- placement of a sequence of ultrabasic to alkaline eruptive cen- ters that make up the Kola Alkaline Province (KAP) [e.g., Kramm and Kogarko, 1994]. The entire KAP was emplaced be- tween 380 and 360 Ma [Kramm et al, 1993], implying that the conditions for magma formationand rifting existedhere for only a brief time. The two major complexes of the KAP are the Khibina and Lovozero agpaitic nepheline-syenite massifs, both of which are 20-40 km in diameter (Figure 1). Previous seismic studies have traced these intrusionsto depths of 2-7 km, beneath which their dimensions decrease to 10-15 km di- ameter. These studies also show that the Khibina and Lovozero complexes extendat leastto 15 and 7 km depth,respectively. To determine the structure of the crust and upper mantle in this part of the Baltic shield,we analyzed data from two seismic stations in the Kola peninsula: the analogRussian stationAPA (67.6øN, 33.3øE) near the town of Apatity, and an IRIS/IDA broadband digital station at LVZ (67.9øN, 34.6øE) near the town of Lovozero. APA is located southwest, and LVZ north- 69 •PENINSULA 68, .LV,•,•' 67 ß i ! i , i , i • i , i , i ß 31 33 35 37 east, of the mountains formed bythe Khibina and Lovozero Figure 1. Simplified tectonic map of the Kola peninsula, af- ter Berthelsen andMarker [1986]. Numbers on mapcorrespond to principal tectonic units: (1) Murmansk unit, (2) Sorvaranger Copyright 1996 by the American Geophysical Union. unit, (3) Kola suture belt, (4) Tanalev unit; (5) South Lapland- Karelia craton (6) Khibina pluton; (7) Lovozero pluton. Paper number 96GL03262. Dashed line indicates the trendof the Kontozero graben (KGB). 0094-8534/96/96GL-03262505.00 Triangles show the location of stations APA and LVZ. 3389