Receiver function analysis of the crust and upper mantle in Fennoscandia – isostatic implications

Earth and Planetary Science Letters 381 (2013) 234–246

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Receiver function analysis of the crust and upper mantle in Fennoscandia – isostatic implications Andrew Frassetto ∗,1 , Hans Thybo Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark

a r t i c l e

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Article history: Received 16 January 2013 Received in revised form 1 July 2013 Accepted 1 July 2013 Available online 2 September 2013 Editor: P. Shearer Keywords: receiver functions continental margins lithospheric structure Fennoscandia isostasy Vp/Vs ratio

a b s t r a c t The mountains across southern Norway and other margins of the North Atlantic Ocean appear conspicuously high in the absence of recent convergent tectonics. We investigate this phenomenon with receiver functions calculated for seismometers deployed across southern Fennoscandia. These are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust. Such parameters are key to understanding crustal isostasy and assessing its role, or lack thereof, in supporting the observed elevations. Our study focuses on the southern Scandes mountain range that has an average elevation >1.0 km above mean sea level. The crust–mantle boundary (Moho) is ubiquitously imaged, and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal, chemical, or viscous boundaries in the upper mantle. The Moho resides at ∼25–30 km depth below mean sea level in southeastern coastal Norway and parts of Denmark, ∼35–45 km across the southern Scandes, and ∼50–60 km near the Norwegian–Swedish border. That section of thickest crust coincides with much of the Transscandinavian Igneous Belt and often exhibits a diffuse conversion at the Moho, which probably results from the presence of a high wave speed, mafic lower crust across inner Fennoscandia. A zone of thinned crust (<35 km) underlies the Oslo Graben. Crustal Vp/Vs ratio measurements show trends that generally correlate with Moho depth; relatively high Vp/Vs occurs near the coast and areas affected by postCaledonide rifting and lower Vp/Vs appears in older, unrifted crust across the southern Scandes. Our results indicate that most of the observed surface elevation in the southern Scandes is supported by an Airy-like crustal root and potentially thin mantle lithosphere. To the east, where thicker crust and mantle lithosphere underlie low elevations, the presence of dense mafic lower crust fits a Pratt-like model for isostatic compensation. Because the Scandes mountains occupy the location of the ancient Caledonian orogeny, which created presumably much thicker crust and lithosphere by ca. 400 Ma, much of the dense lower crust or mantle lithosphere that is expected to form beneath large mountain belts must have been removed sometime afterwards to instill the current lithospheric architecture that underlies the region. © 2013 Elsevier B.V. All rights reserved.

1. Motivation The existence of high, rugged topography far from plate boundaries is a prominent feature of the passive margins around the North Atlantic Ocean (Japsen and Chalmers, 2000; Doré et al., 2002; Anell et al., 2009). These orogens show little evidence for recent, substantial crustal tectonic activity, and their ubiquity across these latitudes coincides with both past and present continental ice sheets and proximity to the Atlantic Ocean basin and Icelandic hotspot, providing an opportunity to investigate hypotheses ranging from the effect of glaciers on landform evolution

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Corresponding author. E-mail address: [email protected] (A. Frassetto). 1 Now at Incorporated Research Institutions for Seismology, 1200 New York Avenue, NW, Suite 400, Washington, DC, 20005, USA. 0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.07.001

to the surface manifestation of dynamic, mantle-scale processes (Nielsen et al., 2009; Anell et al., 2010; Gabrielsen et al., 2010). The Scandes mountains of southern Norway (Fig. 1) form a notable link in this chain of circum-Atlantic high topography. The original models for the formation of the Scandes are primarily derived from geomorphic observations, which indicate tectonically or dynamically driven recent uplift (e.g. Lidmar-Bergström et al., 2000; Gabrielsen et al., 2010) in conjunction with increased offshore sedimentation rates (e.g. Japsen and Chalmers, 2000; Anell et al., 2009, 2010). Recent studies advocate that the Scandes result from icedriven erosion (the “glacial buzzsaw”) of long-lived (>400 My) high elevations (e.g. Nielsen et al., 2009); this model utilizes constraints from apatite fission track data and observations of a small crustal root (Svenningsen et al., 2007). Vigorous debate continues over the merits of these opposing ideas (Gabrielsen et al., 2010; Chalmers et al., 2010; Nielsen et al., 2010; Steer et al., 2012).

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Fig. 1. One-half by one-half degree smoothed topography and bathymetry (in meters) for southern Fennoscandia and northern Europe. The seismic stations analyzed include temporary (circle) and permanent (triangle) deployments. The topography shows a broad dome of relatively high elevation in southern Norway. Generalized geologic boundaries after Gorbatschev (2004) are drawn and labeled, with abbreviations: SvnD—Sveconorwegian Domain, SvfD—Svecofennian Domain, OG—Oslo Graben (blue), TIB—Transscandinavian Igneous Belt (grey), WGR—Western Gneiss Region, and CdN—Caledonian Nappe Sequences. The subset of elevations highlighted in Fig. 11 is derived from measurements obtained from within the white box. The black squares from south to north represent the cities of Oslo, Bergen, and Trondheim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Buoyancy forces related to the thickness and density of the crust support most of the observed elevation on continents (e.g. Watts, 2001). These forces are usually a combination of the Airy (varying thickness of crustal columns of similar density) and Pratt (varying density of crustal columns of similar height) end-member mechanisms of isostatic compensation. The Caledonian orogeny in present-day southern Norway had waned by ca. 390 Ma (e.g. Gee et al., 2008). Because of the recent tectonic quiescence of the region, the current mode of isostatic compensation for the southern Scandes may deviate from Airy isostasy, which is typically invoked for young orogens via recent tectonic crustal thickening, or involves an additional isostatic or dynamic component of support from the upper mantle. Unfortunately, the absence of an onshore sedimentary record corresponding to the post-orogenic history of southern Fennoscandia inhibits unique geodynamic interpretations of the evolution of the region (e.g. Anell et al., 2009). Characterizing the structure and properties of the crust and upper mantle is one of the few avenues to advance this discussion. Seismological constraints, including depth to the Mohoroviˇcic´ Discontinuity ´ 1910; signifying the boundary between seismic wave (Mohoroviˇcic, speed in the crust and uppermost mantle and hereafter called the Moho), are vital to investigating how subterranean factors influ-

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ence the topography in this part of Fennoscandia, which may serve as an analog for other parts of the Atlantic margins. The number of relevant seismic studies in Fennoscandia has grown recently. Regional tomography across Norway and surrounding regions (Bannister et al., 1991) detected a zone of relatively slow upper mantle beneath portions of the southern Scandes and adjacent Norwegian margin. Similarly resolved crustal thickness measurements (Kinck et al., 1993) demonstrated clear differences between the southern Scandes (35–40 km), west-central Sweden (thicker, >45 km), and the Oslo Graben (thinner, <35 km). Recent tomographic studies (Köhler et al., 2012; Medhus et al., 2012; Wawerzinek et al., 2013) all confirm the presence of a relatively confined zone of low wave speed upper mantle beneath southwest Norway. Maupin et al. (2013) presents a detailed summary of these findings. Recent studies have focused on the southern Scandes (Svenningsen et al., 2007; Stratford et al., 2009; Stratford and Thybo, 2011a, 2011b; England and Ebbing, 2012). Two transects of broadband/short-period seismometers analyzed by Svenningsen et al. (2007) were the first temporary seismic stations deployed across the mountains and imaged a small crustal root to ∼43 km depth. Their teleseismic migration procedure assumed a one-dimensional crustal wave speed model. The controlled source seismic survey MAGNUS-REX (MAntle investiGations of Norwegian Uplift Structure-Reflection EXperiment) (e.g. Stratford et al., 2009) produced much greater detail in its crustal thickness and wave speed measurements, but two of its three transects were confined to the same areas. Stratford et al. (2009) found the maximum Moho depth beneath the southern Scandes to be comparatively shallower at 38–40 km, decreasing towards the coast and Oslo Graben. Analysis of P- and S-wave refractions located a relatively fast, mafic lower crust beneath easternmost Norway that is absent under the southern Scandes. The crust throughout the Scandes is predominantly felsic-to-intermediate composition (Stratford and Thybo, 2011a, 2011b). Most recently, England and Ebbing (2012) analyzed the SCANLIPS deployment, a transect of broadband seismometers at the northern extent of the southern Scandes near Trondheim. Their single station forward models and receiver function transect show that the Moho increases in depth from ∼34 km along the coast to ∼42 km under the highest elevations extending into Sweden. Modeled receiver functions also show a high wave speed layer of lower crust that generally matches regional gravity measurements when converted to density. Repeatedly imaged, the observed locally thicker crust led investigators to postulate that the southern Scandes are in Airy isostatic balance (e.g. Svenningsen et al., 2007). Stratford et al. (2009) found slightly thinner crust overall and confirmed that the root is offset from the highest average elevations and surrounded by thinner crust. Additionally, the crustal wave speed structure and calculated bulk density is substantially different between the Scandes, the Oslo Graben, and westernmost Sweden (Stratford and Thybo, 2011a, 2011b). Conclusions drawn from these studies retain caveats, having been limited in their geographic sampling, confined primarily to the crust, or lacking complementary resolution in adjoining parts of Sweden. Initial observations to the east (BABEL Working Group, 1993b; Kinck et al., 1993; Abramovitz et al., 1997; Olsson et al., 2008) hint that the Moho is deeper in this region, possibly due to the added presence of a high wave speed lower crust. Well-constrained measurements of crustal wave speed are limited to controlled source transects and previous receiver function studies use non-unique or overly simple assumptions for the crustal wave speed structure. Although the studies outlined above produced valuable new constraints on first-order structure and composition of the crust

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of southern Norway, these observations have not been integrated into a combined study. Our goal is to combine receiver functions calculated for the broadband seismic array experiment MAGNUS (Weidle et al., 2010) with those recalculated from previous seismometer deployments and crustal wave speed values provided by MAGNUS-REX to produce a unified study of the southern Scandes and adjacent regions. Receiver functions produced for seismometer arrays ideally elucidate the three-dimensional geologic structure and properties of the crust and upper mantle. Generating a uniform image of the Fennoscandian lithosphere aids the effort to understand the origin and evolution of the southern Scandes and potentially distinguish the geologic signature of older tectonic events in the current lithospheric structure. Characterizing variations in the bulk composition of the crust, depth to the Moho, its seismic sharpness, and relation to topography, and underlying mantle structure places key constraints on factors that influence the character of the Earth’s surface. In this study we use this broad set of new observations to discuss geographic variations in the character of the litosphere and to examine potential explanations for high topography in southern Fennoscandia. 2. Geologic history The Fennoscandinavian basement records over three billion years of geologic history (see Gaál and Gorbatschev, 1987 for a useful review). The 3.00–2.50 Ga core of the Baltic craton in northern Finland and northwest Russia preserves a section of Archean continental crust and represents the oldest rocks in the region. The 2.00–1.75 Ga Svecofennian Domain extends to the south and west of this cratonic core and comprises predominantly plutonic crust metamorphosed up to amphibolite and granulite facies. The 1.85–1.65 Ga Transscandinavian Igneous Belt, a series of alkaline and calcic batholiths formed in a continental arc setting (Gorbatschev, 2004), bounds this area to the southwest. Further westward, the Southwest Scandinavian Domain comprises plutonic and metamorphic basement formed 1.75–1.50 Ga, and accreted to the edge of Baltica around 900 Ma (Pesonen et al., 1989) and subsequently affected by the Sveconorwegian–Grenvillian and Caledonide orogenies (Starmer, 1996). The relation of observable seismic structures to these continental lithosphere-forming events and orogens is unclear across this region, but similarly ancient structures have been clearly imaged in other, older cratonic areas (e.g. BABEL Working Group, 1990; Abramovitz et al., 1997). The collision of Laurentia and Baltica culminated in the Himalayan-style, end-stage Caledonide orogeny (440–410 Ma) throughout Scandinavia (e.g. Torsvik et al., 1996). Allochthonous nappe sequences containing material from both the closed Iapetus Ocean and Baltica (Stephens, 1988) overthrust the deformed Fennoscandian basement and today the lower parts of the orogen remain exposed across parts of southern Norway. The Western Gneiss Region, known for numerous outcrops of ultra-high-pressure rocks, was metamorphosed up to eclogite facies at depths exceeding 100 km within the subduction zone (Walsh et al., 2007). These deeper rocks were subsequently exhumed during the collapse of the Caledonides (Andersen, 1998). The Oslo Graben, formed by rifting associated with the Variscan orogeny in greater Europe (Thybo, 2000), penetrated the Southwestern Scandinavian Domain by ca. 300 Ma (Heeremans et al., 1996). Intrusive igneous rocks associated with the rift span ∼200 km and show that significant magmatism persisted there until ca. 245 Ma (Sundvoll et al., 1990). The magmatism evolved temporally from initial basalts derived from an enriched mantle to a progressively depleted source, and melting was eventually dominated by anatexis of previously emplaced material. Extension persisted and culminated with the opening of the North Atlantic Ocean ca. 55 Ma (e.g. Mosar et al., 2002), transitioning the continental edge to a passive margin. Icelandic plume-

related magmatism after ca. 40 Ma (Vink, 1984) and the central Alpine orogeny (e.g. Schmid et al., 1996) at ca. 30 Ma encompasses the only subsequent tectonic activity in the general vicinity of southern Fennoscandia. It is unknown what bearing, if any, these most recent events have had on the topography in southern Norway (Anell et al., 2009). 3. Methods 3.1. Station and event selection Digital seismographs have been temporarily and permanently deployed across Fennoscandia since the early 1990s. Station coverage in southern Norway grew considerably with MAGNUS, lasting from October 2006 to June 2008. We combine these stations with previous experiments, complementary deployments, and a growing framework of permanent stations (see supplementary Table 1). Together these form a relatively uniform grid (Fig. 1). Most stations are spaced around 50 km apart but are considerably closer in some cases. The MAGNUS data are broadband, but other temporary and permanent stations operate short- and intermediate-period seismometers. We consider earthquakes recorded between January 1990 and March 2011, occurring at great-circle-path distances of 25–95◦ and M > 5.6 for P-arrivals and 95–180◦ and M > 6.4 for PP-arrivals. These parameters yield good coverage across a wide range of distances but are azimuthally biased towards northern Pacific subduction zone sources. We keep only events with a signal-to-noise ratio >5 for the P-arrival and its coda on the vertical component, retaining 18 218 P and PP seismograms representing unique eventstation pairs. 3.2. Receiver function processing and analyses Receiver functions (e.g. Langston, 1979) measure the seismic impedance structure of the crust and upper mantle using the waveforms of distant earthquakes. We band-pass seismograms from 0.05 to 5 Hz to reduce interference from the microseism and cultural noise, rotate into the R–T–Z coordinate system, and calculate low-, medium-, and high-band receiver functions using iterative, time-domain deconvolution (Ligorría and Ammon, 1999). Deconvolution removes source and instrument effects to preserve P-SV conversions (hereafter called Ps) generated by an up-going teleseismic P-wave propagating through the structure around a seismometer. Iterative deconvolution avoids the drawbacks of spectral division and has proven particularly robust in noisy environments. For each event, we iterate 400 times or until the misfit of the calculated receiver function that is reconvolved with the source signal decreases by <0.001. We choose Gaussian width factors (termed “a”-values) of 1, 2.5, and 4 for the deconvolution, corresponding to low-pass filters with corner frequencies of 0.5, 1.2 Hz, and 1.9 Hz, which are capable of resolving discrete structures that are separated vertically by about 3.7, 1.5, and 1 km for S-wave speed values typical of continental crust in our region (∼3.7 km/s) (Stratford and Thybo, 2011b). The included short-period stations produce typical receiver functions for the low-band filter, so most are retained in the corresponding CCP stacks. We apply H–K and common-conversion-point stacking for receiver functions with variance reductions 70% (Ligorría and Ammon, 1999) which exhibit the expected response for a normalized radial receiver function: the maximum amplitude equals 1 near 0 s lag-time, negative amplitudes do not exceed −1, the P-arrival is not preceded by a negative trough or significant delay, and the coda of the receiver function contains a root-mean-square amplitude of 0.01 to 0.4 from 1.5 to 10 s for H–K stacking and from 40

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to 95 s for CCP stacking. This last step removes traces with excessive harmonic reverberations or otherwise anomalous waveforms. In order to further refine the dataset, we discard lower variance reduction events at stations that yield hundreds of passable receiver functions, e.g. the long running NORSAR array of borehole seismometers. H–K stacks (Zhu and Kanamori, 2000) for stations with at least 10 middle-band (a = 2.5) receiver functions are calculated to determine the thickness (H) and average Vp/Vs ratio (K) of the crust. The stacking procedure is similar to the EarthScope Automated Receiver Survey (EARS) (Crotwell and Owens, 2005). P-wave speed for each station is calculated from a model combining CRUST2.0 (Bassin et al., 2000) and the results of MAGNUS-REX (Stratford and Thybo, 2011a, 2011b), with most stations utilizing the MAGNUSREX values. The traces are not normalized during stacking and like EARS we apply phase-weighting (Schimmel and Paulssen, 1997) to improve coherence of the H–K maxima. The final H–K stack is the mean of 50 separate stacks using bootstrapping (Efron and Tibshirani, 1986). To constrain error due to spurious noise, we discard one-third of the picked maxima occurring furthest from the median picked depth for each bootstrapped stack and calculate the standard deviation (σ ) for the remaining measurements. We ignore the station if σ is >0.05 for K or >3 for H. We subtract station elevation from the H value to yield Moho depth measurements. Additionally, we also eliminate stations with measurements that represent geologically implausible scenarios (e.g. K > 2). We utilize common-conversion-point (CCP) stacking (Dueker and Sheehan, 1997; Frassetto et al., 2011) to create a threedimensional structural image of the crust and upper mantle. Receiver functions are normalized so that the P-arrival has unit amplitude equal to 1 near 0 s lag-time, increasing the amplitudes of subsequent arrivals, an effect which is addressed in stacking. We align our bins in a north-south, east-west grid. To account for the +2 km of relief across the region we correct the elevations to sea level. We use a two-step depth migration scheme with an earth-flattening transform (Gurrola et al., 1994), which incorporates a customized model of the wave speed structure across this region. For the crust, we combine measurements of P- and Swave speed from MAGNUS-REX, CRUST2.0, and the H–K stacking results from this study. For the upper mantle, we obtain S-wave speed parameters from a global surface wave model (Shapiro and Ritzwoller, 2002). We calculate corresponding P-wave speed terms using the Vp/Vs relations in AK135 (Kennett et al., 1995). These models are interpolated into a grid with coverage of 0.2◦ longitude by 0.1◦ latitude by 0.5 km, encompassing the entire survey area and smoothly averaged between 50 and 60 km depth. Each receiver function is migrated through this model-space using the mean P- and S-wave speed models to obtain values specific to each back-projected ray-path. All receiver functions are subsequently remigrated using the discretized models. Through this process, receiver functions are corrected for moveout and propagation through a sphere, converted to depth, and back-projected into gridded bins spaced at 20 km intervals with each bin having a horizontal radius of 30 km and vertical thickness of 1/2 km for high-band seismograms and 1 km for low-band. Sharing data across bins closes gaps in ray coverage caused by non-uniform spacing and enhances the lateral coherency of conversions. Bins containing less than five receiver functions are discarded. Each trace is multiplied by the ratio i r /i, where i r is the incidence angle calculated from the S-wave speed and P-slowness and i is the incidence angle calculated from P-wave speed and Pslowness. This term rescales normalized amplitudes as a function of distance and localized wave speed relationships. The stacks are bootstrapped 100 times and summed using phase-weights with a 3 km wide smoothing window to suppress various forms of

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signal-generated noise typically present in lithosphere-scale images (Frassetto et al., 2010). 3.3. Uncertainties Identifying sources of uncertainty in the CCP stacks adds vital constraints to subsequent interpretations. At 40 km depth 699 of 1356 bins stacking high-band data contain on average 49 receiver functions with a mean variance reduction of 87.9%, demonstrating robust deconvolution and ray coverage. Bootstrapping the stack 100 times allows us to sufficiently quantify the effects of spurious noise. We visually constrain a range of depths for picking the Moho across the CCP bins, reject the one-third of picks farthest from the median picked depth at each bin, compute the standard deviation of the remaining values, and omit estimates with σ > 3 km. Despite low-band data having the poorest practical depth resolution, the mean error for these measurements is only 0.2 km (vs. 0.12 for high-band), with the largest at any bin being 0.56 km (0.46 for high-band). Additionally, Moho depth estimates from low- and high-band data are extremely close, 40.89 km (from 756 out of 1365 bins) vs. 41.06 km (from 747 out of 1356 bins). Understanding the error in migrated depth introduced by the Vp and Vs assumptions is also important (e.g. Frassetto et al., 2011). Mean errors from refraction modeling are about 0.7% for P-waves and 1% for S-waves (Stratford and Thybo, 2011a, 2011b) for the entire crust, with corresponding Vp/Vs errors of <2%. Errors from accepted H–K stacking measurements are around 1%. For a Ps conversion arriving at 5 s, perturbing an average crustal Vp = 6.6 km/s by its uncertainty changes the depth of the arrival by < ±0.3 km. Similarly varying Vp/Vs = 1.77 by 2% alters the migrated depth by < ±2.1 km. Therefore, the most extreme combination of these perturbations results in an uncertainty of < ±2.4 km. Errors will be proportionally smaller for migrated conversions arriving earlier. Considering these dependencies we can expect a combined potential uncertainty of < ±2.6 km for the average depth of the Moho throughout southern Fennoscandia, with deeper and shallower interfaces having proportionally larger and smaller errors. 4. Observations 4.1. Single station observations and H–K stacks Middle-band receiver functions clearly resolve both the direct conversion and reverberations from the Moho and thus are ideal for H–K stacking. Many of our stations provide robust crustal thickness and Vp/Vs measurements (Fig. 2; supplementary Table 2). Typically stations with many receiver functions yield the best measurements, but stations with as few as 15 receiver functions produce robust results. The direct conversion from the Moho and its reverberations are clearly observed across southwestern Sweden, southernmost coastal Norway, and some parts of the Scandes. The S-wave conversion usually arrives 4–5 s after the P-wave arrival. The southernmost stations deployed as part of MAGNUS show remarkably pristine reverberations, implying a sharp impedance contrast at the Moho coupled with homogeneous overlying crust. The sharpness and recognizable moveout pattern of the direct conversion and its reverberations degrades to the north and west from the center of the Scandes; this likely stems from scattering by heterogeneous, allochthonous crust and smaller amplitude conversions at the Moho. The MAGNUS-REX controlled source seismic data shows unusually strong S-waves in the same areas, which has been interpreted to result from crustal heterogeneity introduced by Caledonide nappe structures (Stratford and Thybo, 2011b).

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Fig. 2. Example results from the H–K analysis. For each row of stations the center columns show the absolute amplitude of medium-band receiver function arrivals stacked in the H (crustal thickness)–K (Vp/Vs) domain. The number of receiver functions used, assumed P-wave speed, and calculated mean H and K values are listed. Circles (red and blue) represent the H–K maxima from 50 bootstrapped stacks with X marking the peak maxima. Only the two-thirds of the measurements (blue) closest to the median maxima are used. These plots are flanked by corresponding depth converted stacks, showing the mean stacked receiver function (black) and error bar from bootstrapping (blue), and move out stacks that superimpose predicted arrivals from the Moho conversion and its 2P and 2S reverberations over the raw receiver functions. The locations of these stations are marked in Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Mapped results from H–K [crustal thickness (left)–Vp/Vs (right)] stacks of medium-band receiver functions. Crustal thickness measurements have been corrected for station elevation and thus represent Moho depth. The locations of the stations presented in Fig. 2 are marked and labeled. Generalized geologic boundaries are included. Four measurements (HEF, SGF, SJU, and TRO) reside outside the map boundaries.

Moho depth measurements vary by ∼30 km within the study area (Fig. 3). The crust is thinnest across southwestern coastal Norway, northern Germany, and Denmark, where the Moho is as shallow as 25 km depth. The crust is <30 km thick in most of the Danish basin and >45 km thick in southern Sweden, with a narrow transition at the Sorgenfrei–Tornquist Zone (Thybo, 2001). Thin crust is clearly resolved beneath the NORSAR array at the north-

ernmost extent of the Oslo Graben. Elsewhere the Moho deepens westward from the Oslo Graben to ∼45 km beneath the southern Scandes mountains and eastward beneath southern Sweden to a maximum depth of ∼54 km at station VIK. The crust is considerably thicker beneath low elevation, interior Sweden than under comparatively high southern Norway. We make additional measurements at permanent stations situated to the north of the central study area which show crustal thicknesses of ∼40 km along the coast near Tromsø and ∼46 km at three stations distributed across low-elevation Archean-age basement in northernmost Sweden and northwestern Finland. Crustal Vp/Vs measurements (Fig. 3) show wide regional variation that is much larger than the mean error from bootstrapping and correlates with Moho depth in several areas. Throughout the interior of the southern Scandes values of 1.65–1.75 are typical of quartz-rich continental crust (Zandt and Ammon, 1995). Similar values are observed within the northernmost Oslo Graben, and through parts of southern Sweden. Crustal Vp/Vs is high (>1.8) near the Sorgenfrei–Tornquist Zone, along the southern coast of Norway near the Oslo Graben, within the Western Gneiss Region between Bergen and Trondheim, and across the northernmost southern Scandes. The highest Vp/Vs measurements are generally observed along low elevation portions of the Norwegian coast, the Danish basin, and interior Sweden. However, observations are scattered in Sweden with coarser station coverage and fewer highquality measurements to the east and north of the Oslo Graben. Stations in northern Fennoscandia show consistent Vp/Vs measurements of 1.73–1.75.

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Fig. 4. Stacking bins and piercing point locations for high-band receiver functions. Piercing points (red) at 100 km depth are shown in relation to the grid of 20 km by 20 km common-conversion-point (CCP) bins (blue). Seismometers are shown by triangles (yellow). Letters identify CCP transects presented in this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2. Common-conversion-point transects Picked depths illustrate the large variation in Moho depth within the area stacked with CCP gathers (Figs. 4 and 5). High- and low-band receiver functions show conversions from the Moho that are generally consistent across different frequency ranges. These results are also very close to the results from H–K stacking: a zone of thicker crust across the Scandes, a locally shallower Moho within the Oslo Graben, and substantially thicker crust in western Sweden and eastern Norway beneath the Transscandinavian Igneous Belt (TIB). The crust is 25–30 km thick along the coast of southern Norway, consistent with low average elevations (Fig. 1). However the coastal Western Gneiss Region around latitude 62◦ N, despite having the same general topography, has a Moho that is 5–10 km deeper than regions to the south especially at the highband. The low-band receiver functions also resolve an additional, cryptic interface at ∼55 km depth near Trondheim. It is coher-

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ent enough to be picked and mapped as a Moho conversion for the low-band but not on the high-band (Fig. 5). The northernmost Moho mapped by both sets of receiver functions shows that the crust thins from the TIB towards the Gulf of Bothnia but remains thicker to the south. The conversions from the Moho in the high-band stacks have strong signal-to-noise ratio and are easily distinguishable across southern Norway and Sweden (Fig. 6, transects A–H). The Moho generally deepens and its amplitude diminishes from west to east. High-band data indicate a shallow notch of Moho in the northern Oslo Graben (E–E ) that is less pronounced to the south. Farther north, CCP stacks show a mostly uniform Moho depth with a gradual deepening towards the TIB (C–C ). Intracrustal conversions are mainly observed in the northwestern part of the survey area and almost entirely absent in the south. These signals and their occasionally high amplitude reverberations are likely due to the Caledonian nappe sequences that are found across this portion of Norway (B–B ). We use low-band stacks to focus on the sub-Moho structure to a depth of 200 km. These stacks also resolve the Moho (Figs. 7 and 8) and improve upon its resolution in the eastern section of the CCP volume. In the upper mantle the biggest features are the 2P and 2S multiples from the Moho conversion. These are a common artifact in P-wave receiver function studies and stack from 120–180 km depth as a positive arrival and a trailing negative arrival. Despite these spurious features, several diffuse negative conversions are observed between these reverberations and the Moho arrival (G–G , I–I ). In the southwestern portion of the study area a negative conversion appears laterally coherent at depths of 60–80 km. Several discrete conversions also appear at greater depths within inferred Precambrian-age lithosphere across southern Sweden. A localized negative conversion is observed only 10–15 km under the Moho within the Oslo Graben. In contrast, no well-defined negative conversions are observed in the northern part of the survey area. However, a localized positive conversion underlies the northernmost segment of the TIB sampled by our stations (A–A , J–J ). 5. Discussion 5.1. Comparison to relevant recent seismic results This work combines modern, comprehensive models of crust and upper mantle wave speed (Shapiro and Ritzwoller, 2002; Stratford and Thybo, 2011a, 2011b) and an integrated dataset of receiver functions to most accurately determine lithospheric structure and crustal properties. Comparison to previous studies reveals

Fig. 5. Moho depth maps generated from CCP stacking. Interpolated depth to picked conversion from the Moho for low-band (left) and high-band (right) receiver functions. Black circles mark CCP bins providing depth picks. Generalized geologic boundaries are included.

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Fig. 6. Structural transects from common-conversion-point stacks of high-band receiver functions. Transects are ordered vertically from north to south and labeled alphabetically here and in Fig. 4. Scale is ±10% of the P-arrival amplitude, saturating at blue (negative) and red (positive). Black tick marks are the picked depth of the conversion from the Moho. Number of receiver functions contributing to each transect at 50 km is marked on the right. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

greater heterogeneity in crustal composition from new Vp/Vs measurements, as well as discrepancies in Moho depth that result both from differences in methodology and previous use of simpler wave speed models. For MAGNUS-REX, models of crustal Vp/Vs are horizontally averaged and confined near transects. In contrast, our receiver function estimates are vertically averaged in a narrowly sampled cone around the station but also are more evenly distributed across the region. In both studies Vp/Vs values generally are higher near the Western Gneiss Region and lower across the Caledonide nappe sequences. Results from Stratford and Thybo (2011a) yield a bulk crustal Vp/Vs of 1.7–1.8. Our H–K stacking results demonstrate a wider range (1.65–1.85) across the same area and in surrounding regions. Only the Oslo Graben, which the receiver function study does not sample well enough for a detailed comparison, showed locally higher crustal Vp/Vs in the MAGNUS-REX study. Svenningsen et al. (2007) assumed a two-layer model of constant Vp/Vs values of 1.76 and 1.78 for the upper and lower crust. Although this is still a reasonable mean value for the area, applying a model with regionally varying Vp/Vs ensures more accurate depth migration of the Moho. Our new Moho depths are shallower along the coast due to generally higher Vp/Vs and deeper across a broad region under the Scandes, exceeding the previous maximum observed depth of 43 km. For the northern stations including SCANLIPS the results are also similar to England and Ebbing (2012),

which uses station specific conversions to depth. Overall the results diverge from the somewhat shallower depths seen in Stratford et al. (2009) despite the use of the same crustal wave speeds during the CCP migration. These differences appear to result from inherent differences in seismic technique and are discussed in Section 5.4. 5.2. Character of the crust and upper mantle beneath southern Fennoscandia The regional scale of the change in the seismic signature of the Moho between southern Norway and Sweden is a key new observation. The crust is thicker and the amplitude of the converted phase from the Moho is weaker beneath Sweden. The lessened amplitude indicates a reduced or gradual transition in seismic wave speed across the Moho. This suggests that high wave speed lower crust occupies a large portion of southern Fennoscandia to the east and north of the Oslo Graben, including the TIB. This interpretation is supported by complementary observations from a regionally focused receiver function study (Olsson et al., 2008) and controlled source surveys (Lund, 1979; BABEL Working Group, 1993a, 1993b) in Sweden and the Baltic Sea. Such high wave speed lower crust is absent or very thin (<4 km thick) in southern Norway (Stratford and Thybo, 2011a, 2011b). Analyses from the MAGNUS-REX study demonstrate that this layer may be mafic in composition and allow

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Fig. 7. Structural transects from low-band receiver function stacks. Ordering, scale, and annotations are the same as in Fig. 6.

Fig. 8. Structural transects from low-band receiver function stacks. Transects are ordered vertically from west to east. Scale and annotations are the same as in Fig. 6.

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Fig. 9. The modeled low- and high-band receiver function responses for an incident wave (p = 0.06 s/km) for three proposed transitions (A – sharp, B – two step, and C – 20 km wide) between (6.2/3.65 km/s) crust and (8.0/4.44 km/s) mantle.

us to infer that it could have been previously removed or perhaps never formed beneath southern Norway. Receiver functions are routinely calculated for simple structural models, and this practice can be used to best inform this interpretation of the Moho. We use the ray-tracing code ray3d (Owens et al., 1984) to reproduce the potential range of seismic responses from the Moho. The amplitude of the Moho conversion varies with impedance contrast across the Moho and the Gaussian filter used for the deconvolution. We use a = 1 and a = 4 values to remain consistent with the observations from our CCP stacks and compare the response of three models: (A) a layer over half-space at 40 km depth, (B) two steps in wave speed at 35 and 45 km depth, and (C) a 20-km thick gradient with one km steps in wave speed centered at 40 km (Fig. 9). Synthetic receiver functions for Model A produce a negligible difference in the normalized amplitude between low- and highband deconvolution results. A 10-km thick lower crustal layer (Model B), dividing the Moho into two steps, shows a small reduction in low-band amplitude compared to Model A. On its highband stack, the Moho appears as two separate peaks of similar, still prominent amplitude. In contrast to both, Model C shows a 20% difference in amplitude depending on Gaussian used. The high-band conversion is effectively eliminated when the potential effects of noise are considered. Overall the observed difference in seismic response of the Moho between southern Norway and Sweden is likely explained by a gradient of transitional lower crust beneath Sweden. It is also similar to observations related to absent or present mafic to ultramafic lower crust in western North America (Frassetto et al., 2011). This example demonstrates the continued usefulness of varying the low-pass filter used in receiver function calculation for the purposes of more effectively interpreting the Moho. Only in the vicinity of Trondheim do Moho depth measurements from H–K (Fig. 3) and CCP (Fig. 5) stacking deviate significantly. The complicated nature of this local Moho may stem from transitional lower crust close to the coast, complex reverberations generated by the nearby fjord, structural complexities introduced by active faulting (e.g. Olsen et al., 2007), or merely is an artifact of incomplete azimuthal coverage from the smaller event sampling for most nearby stations. The initial work of England and Ebbing (2012) focused on the structure down to the modeled shallow Moho arrival, and the deep arrival seen on low-band receiver functions is a new result from the SCANLIPS stations combined with MAGNUS (NWG01) and Norwegian National Seismic Network (TBLU) stations. Acceptable H–K results are only produced at two SCANLIPS stations and TBLU, and these measurements are borderline in event count and quality of the observed reverberations. A local study that incorporates TBLU, NWG01, the SCANLIPS

dataset, and additional seismometers may provide better insight into this complicated area. We detect a complex series of conversions that originate from the uppermost mantle within the study area. These conversions appear at depths common to both the LithosphereAsthenosphere Boundary (LAB) and the mid-lithospheric discontinuity (MLD)/8-degree discontinuity (8-deg.) and may represent one or both (e.g. Thybo and Perchuc, 1997; Fischer et al., 2010). The CCP stacks show a locally continuous and coherent negative converter at depths of 60–80 km under the southeastern quadrant of the CCP volume (G–G , I–I ) that deepens eastward from the continental margin. This depth is at the shallow limit for global observations of the LAB in young tectonically reworked regions (Rychert and Shearer, 2009) and coincides with a feature identified as the LAB using S-receiver functions (Wawerzinek and Ritter, 2011). Hence, the base of the lithosphere also appears to track with the depth of the Moho, increasing beneath the Scandes from 60 km in the west and south to 110 km in the east and north (I–I ). If this arrival signifies the true base of the continent and if we assume that the lithosphere was probably comparatively thick beneath the Himalayan-type Caledonian orogeny, then it demonstrates that the collapse of the orogen and/or the rifting and break-up of the North Atlantic Ocean and subsequent extensional episodes imparted markedly thin continental lithosphere. Body wave tomographic and thermal models indicate 100–150 km thick lithosphere beneath southern Norway and >200 km thick lithosphere under the Baltic craton (Grand, 2002; Artemieva, 2007; Artemieva and Thybo, 2008), findings that are also supported by recent high-resolution P-wave tomography in the region (Medhus et al., 2012). Our new results therefore indicate that the LAB or similarly significant mantle structure may be particularly shallow and dipping eastward from the North Sea. Most of the profiles show a negative conversion beneath Precambrian-age Sweden (e.g. G–G ). It is ambiguous whether this represents a continuation of the diffuse LAB arrivals observed to the west. This depth corresponds to the depth of a general, heterogeneous low wave speed zone in the Fennoscandian Shield as observed from very-wide angle controlled source seismic data (Perchuc and Thybo, 1996). There has been no relevant data collected for making similar observations in southern Norway, and it is unknown if the zone is present there. Perchuc and Thybo (1996) attribute the low wave speed zone to rocks with temperature close to the solidus. Similar observations have been made in other continents, and the low wave speed zone may be a global feature (Thybo and Perchuc, 1997; Thybo, 2006). These types of structures are not observed at similar depth below the Transscandinavian Igneous Belt or elsewhere in the north-

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ern half of this study, possibly because they are situated deeper or possess longer wavelength. Reverberations from the Moho may also obscure these arrivals. However, a positive conversion is observed at 100–110 km depth beneath the TIB and clearly arrives before diffuse reverberations from the Moho. Furthermore, the lack of energetic conversions within the crust discounts the possibility of this arrival being another shallowly sourced reverberation. Therefore, it likely represents a true converter in the upper mantle that may distinguish the top of a zone of depleted mantle related to the melt extraction (e.g. Jordan, 1979) that developed as the Transscandinavian Igneous Belt formed. In this case the stratified upper mantle, or TIB “mantle root” (A–A , J–J ), may indicate part of a stepwise increase in wave speed within the lithosphere. Interpreting stratified and chemically and/or thermally distinct mantle layers fits with the recent conclusion of Yuan and Romanowicz (2010), who examined depth variations in azimuthal anisotropy when investigating the transition between the cratonic core of North America and its active margins. However, the limited coverage of this observation and lack of complementary data on anisotropy precludes a more detailed interpretation here. This structure seems best resolved by the close station spacing of the SCANLIPS deployment east of Trondheim. Its presence southward under the TIB is poorly resolved, perhaps due to sparse station coverage. 5.3. Relation to surface geology This study expands the seismic characterization of major geologic structures in southern Norway and broadly extends into adjacent regions. Regional differences in crustal Vp/Vs closely link to prominent features in the surface geology including the Danish basin, Tornquist zone, Transcandinavian Igneous Belt, Precambrian basement outcrops, Oslo Graben, and Western Gneiss Region. For example, Vp/Vs results are notably high near the Western Gneiss Region, where eclogite facies rock has been exhumed to the surface. Lower Vp/Vs observations at stations on Precambrianage basement are comparable to previous H–K analyses of nearby Swedish National Seismic Network stations (Olsson et al., 2008). The crust below southern Norway is substantially thinner by 10–20 km than elsewhere in the study area, despite the southern Scandes exhibiting Moho depths up to 45 km. In contrast, the thinnest crust is found near the southern coast and below the Oslo Graben and generally coincides with the area with the thinnest lithosphere. There is an apparent difference between the southern Scandes with Vp/Vs < 1.73 and the coastal areas and southern Sweden with Vp/Vs > 1.77. These higher Vp/Vs values may demonstrate basaltic input into the crust during nearby continental rifting. The difference from areas in Sweden with thick crust may be ascribed to the higher mafic content in the Svecofennian crust. This high Vp/Vs conflicts with some results by Kuusisto et al. (2006) that found an average Vp/Vs of 1.73 throughout the Fennoscandian Shield. This discrepancy may originate from their inclusion of observations throughout Finland and our measurements in the TIB, where the thick crust may be more mafic than elsewhere in the Fennoscandian Shield. We refer to Stratford and Thybo (2011b) for a comprehensive discussion of these features. The thickest crust (>50 km thick) within the study area is observed in a narrow NW-SE trending belt that at least partly coincides with the TIB. This presumably relates to magmatic thickening of the crust in a continental arc setting at 1.85–1.65 Ga when the Transscandinavian Igneous Belt formed via intrusion of a series of alkaline and calcic batholiths (Gorbatschev, 2004). If so, the TIB crust is an original feature that has existed for at least 1.65 Gy. Vp/Vs is mainly high in this zone, although there are exceptions and overall the zone is not as well sampled by H–K analyses. This

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Fig. 10. Difference between interpolated depth of the Moho from high-band receiver functions and depth obtained by interpolation between stations from the MAGNUS-REX refraction/wide-angle reflection experiment (grey symbols) and other controlled-source studies (Stratford et al., 2009). The residual depth difference shows regions where the Moho on receiver functions is deeper (red) and shallower (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

is due partly to the weak Moho conversion relating to mafic lower crust. 5.4. Comparison to MAGNUS-REX results The multiple recent seismic imaging studies conducted in southern Norway present a rare opportunity to directly compare different methods of mapping the Moho. The MAGNUS-REX and receiver function measurements agree well south of 61.5◦ N (Fig. 10). The interpolated Moho depths based on refraction/reflection modeling and teleseismic converted waves are within a few kilometers, inside their calculated ranges of uncertainty. However, these measurements deviate significantly to the north of 61.5◦ N. Receiver functions consistently show thicker crust than the controlled source results, >5 km in the Western Gneiss Region and >10 km further eastward even where measurements directly coincide. These largest differences may be explained by the comparative scarceness of controlled source measurements, which leads to interpolation between older coarsely spaced data points not sampling the thickest crust below TIB. These differences not only demonstrate the improved coverage from 3D sampling, but also reveal inherent differences in estimates of Moho depth between lower frequency, bottom-up receiver function analyses and higher frequency, azimuthally limited top-down controlled source seismic profiling. Both methods have well-understood limitations, and this combination allows us to infer the relative simplicity of the Moho in southernmost Norway compared to the complexity across more geologically reworked neighboring regions. As seen from single-station analyses (Fig. 2) the crust generates a sole, pristine conversion from the Moho in southern Norway. To the north and east, the Moho is less distinct due to more ambiguous arrivals, weaker wave speed contrast across the boundary, and crustal scattering. The Moho likely has a complex structure and thicker transition to the mantle in the north and east. This may lead to refracted waves bottoming at the shal-

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Fig. 11. Examining crustal isostasy across southern Norway and Sweden. The graph shows one-half by one-half degree smoothed elevation (Fig. 1) plotted against Moho depths measured in CCP stacks of high-band receiver functions (Fig. 5). Linear regression fits are applied to all measurements (black), only those west of 11.5◦ E and south of 62.5◦ N (green), and the remainder to the north and east (red). Correlation coefficients are R 2 = 0.35 for all measurements (black line), R 2 = 0.44 outside of the Southern Scandes (red line), and R 2 = 0.75 within the southern Scandes (green line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

lowest portion of the wave speed increase while receiver functions detect a depth closer to its midpoint or the base of the transition zone. The disagreement in Moho depth near Trondheim is also quite significant and may relate to the factors discussed earlier. 5.5. Isostasy and topography Comparison of surface elevation to Moho depth provides first order evidence for the importance of crustal isostatic compensation of topography. As a whole, there is no clearly demonstrated relationship between elevation (Fig. 1) and Moho depth (Fig. 5) across most of Fennoscandia (Fig. 11). However, we find a correlation coefficient of R = 0.75 between isolated elevation values and corresponding Moho depth measurements in the southern Scandes. The positive correlation indicates that Airy isostasy due to thickened crust is a key component of compensating high elevations of the southern Scandes in Norway, although the data points are scattered around the best fitting regression line in a band with a width of about 10–12 km for Moho depth and 500–800 m for topography. This scatter may reflect the ∼60 km lateral offset between the highest topography and the deepest Moho in the area and that not all the elevation can be explained by Airy type of crustal isostatic compensation (Stratford et al., 2009; Anell et al., 2010). Substantial flexural strength of the lithosphere is required to maintain the lateral offset between maximum topography and the compensating vertical forces. The elastic thickness of the lithosphere has been estimated to 10–20 km in southern Norway based on admittance-coherence techniques (Djomani et al., 1999; Pérez-Gussinyé et al., 2004) and mechanical modeling (Fjeldskaar, 1997; Pascal and Cloetingh, 2009). Because these estimates are low, better knowledge of the variation in lithospheric strength in the shield, southern Norway, and the continental shelf is required before final inferences can be made regarding the mechanisms that maintain the high elevations. A potentially missing component of compensation may reside in the uppermost mantle, as concluded from previous studies examining the short and long wavelength gravity field across the region (Ebbing and Olesen, 2005; Ebbing et al., 2012).

Conversely, the remaining elevations in easternmost Norway and Sweden show a poor correlation with crustal thickness. The trend and large scatter suggests that Pratt isostasy is the dominant mode of crustal compensation in this region. This conclusion is further supported by the presence of seismically fast, presumably dense, mafic lower crust in the same geographic area. However, tomographic seismic models also include a substantial difference in upper mantle wave speed between the east and west portions of the study area, separated by a sharp transition over short distance (Medhus et al., 2012). This indicates a large variation of density within the mantle, thus confirming the results of integrated crustal and gravity studies (Artemieva et al., 2006) across this portion of Fennoscandia and reinforcing the inference that mantle density plays a role in maintaining the low topography in the shield proper. The Scandes are located within the former Caledonides, which were conceivably underlain by crust of up to 70 km thickness, analogous to the contemporary Himalayan mountain belts (e.g. Galvé et al., 2006). Removal of this dense, over-thickened lower crust would have led to a density structure permitting overall lower elevations in the Scandes, because the void would have become filled with mantle. Likewise, erosion at the top of the crust would have increased the average density of the crustal column, which again would have led to lowering of the topography. However, if the lower crust already had become metamorphosed into eclogite facies during the late part of the Caledonian orogeny, delamination of this part could have led to uplift of the surface (cf. Abramovitz and Thybo, 2000; Artemieva and Meissner, 2012). Our new crustal thickness measurements across southern Fennoscandia allow us to reflect on the ongoing debate regarding Norwegian uplift. The lithospheric structure appears to have changed significantly since the Caledonian orogeny with direct implications for the observed change in surface elevation. These detailed observations of seismic structure beneath southern Fennoscandia will allow future interdisciplinary studies and feed geodynamic models that will place important constraints on the evolution of the mantle beneath southern Norway and its relationship to the present topographical variation across the region. 6. Conclusions These results demonstrate the benefit of combining past data with newly made observations in a comprehensive study. The images of lithospheric structure are the most highly resolved and broadly distributed yet available for this region. The coherent, relatively shallow, and seismically sharp Moho beneath southernmost Norway, in contrast to southern Sweden, is central to the interpretation that thinned crust and lithosphere relative to inboard, older continental lithosphere of the Fennoscandian Shield may provide the primary support of high elevation beneath southern Norway. Several mechanisms (e.g. metamorphic reactions and/or delamination) may have led to the removal of the dense lower crust beneath southwestern Fennoscandia. The reduced amplitude of the Moho conversion, along with previous observations of fast lower crust beneath Sweden, suggests that mafic lower crust is still present below the shield proper, where the relatively dense crust and thick mantle lithosphere may provide an anchor to elevation. The sharp Moho below the area of the Scandes, inferred from receiver function amplitudes and refraction seismic waveforms (Stratford and Thybo, 2011b), may indicate that previous high-density lower crust may have been removed or that the rocks have been metamorphosed into denser rock types, which today appear as mantle material. The present evidence indicates that most, but not all, of the present topography of southern Fennoscandia may be explained by a combination of Airy and Pratt isostasy.

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Acknowledgements The authors thank editor Peter Shearer, Ian Bastow, and an anonymous reviewer for helpful reviews of the manuscript. The lead author wishes to recognize the collegiality of the Nordic seismological community, particularly Bo Holm Jacobsen, Neils Balling, and Anna Bondo (Aarhus University); Lars Ottemøller and Jens Havskov (Norwegian National Seismological Network); and Ulf Baadshaug and Jan Fyen (NORSAR), all who enabled access to a wider set of data. Their good example improved this study. Special thanks go to Wanda Stratford, Valerie Maupin, and the entire TopoScandiaDeep team for useful discussions on the region. The lead author reserves the highest possible gratitude to Megan Fogarty for endorsing their Danish adventure together and noted-Texan and non-seismologist Barrington L. Reno who did a barry nice review of the manuscript. This study used the m_map package maintained by Rich Pawlowicz. MAGNUS waveforms were recorded with the mobile KArlsruhe BroadBand Array (KABBA) of the Karlsruhe Institute of Technology, Germany. Financial support for the MAGNUS experiment was provided by the Universities of Aarhus, Copenhagen, Karlsruhe and Oslo as well as NORSAR. Other event waveforms analyzed in this study were accessed from the ORFEUS, SZGRF, and GFZ data centers. The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used for access to waveform and metadata required in this study. Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF). The IRIS DMS is funded through the National Science Foundation and specifically the GEO Directorate through the Instrumentation and Facilities Program of the National Science Foundation under Cooperative Agreement EAR-0552316. This work has been done in the framework of the ESF EUROCORES TOPO-EUROPE Programme 07-TOPOEUROPE-FP-014: The Scandinavian mountain chain – deep processes (TopoScandiaDeep). We acknowledge financial support from the Danish Research Council for Natural Sciences. Appendix A. Supplementary material Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2013.07.001. References Abramovitz, T., Thybo, H., 2000. Seismic images of Caledonian, lithosphere-scale collision structures in the southeastern North Sea along MONA LISA Profile 2. Tectonophysics 317, 27–54. Abramovitz, T., Berthelsen, A., Thybo, H., 1997. Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data. Tectonophysics 270, 259–277, http://dx.doi.org/10.1016/S0040-1951(96)00213-2. Andersen, T.B., 1998. Extensional tectonics in the Caledonides of southern Norway, an overview. Tectonophysics 285, 333–351. Anell, I., Thybo, H., Artemieva, I.M., 2009. Cenozoic uplift and subsidence in the North Atlantic region: Geological evidence revisited. Tectonophysics 474, 78–105, http://dx.doi.org/10.1016/j.tecto.2009.04.006. Anell, I., Thybo, H., Stratford, W., 2010. Relating Cenozoic North Sea sediments to topography in southern Norway: The interplay between tectonics and climate. Earth Planet. Sci. Lett. 300, 19–32, http://dx.doi.org/10.1016/j.epsl.2010.09.009. Artemieva, I.M., 2007. Dynamic topography of the East European craton: Shedding light upon lithospheric structure, composition and mantle dynamics. Glob. Planet. Change 58, 411–434. Artemieva, I.M., Meissner, R., 2012. Crustal thickness controlled by plate tectonics: A review of crust–mantle interaction processes illustrated by European examples. Tectonophysics 530, 18–49, http://dx.doi.org/10.1016/j.tecto.2011.1012.1037. Artemieva, I., Thybo, H., 2008. Deep Norden: Highlights of the lithospheric structure of Northern Europe, Iceland, and Greenland. Episodes 31, 98–106. Artemieva, I.M., Thybo, H., Kaban, M.K., 2006. Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga. In: Gee, D., Stephenson, R. (Eds.), European Lithosphere Dynamics. Geological Society Sp. Publ., London, pp. 11–41.

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