Integrated modelling investigating the link between topography and lithosphere of the Scandes Sofie Gradmann1, Jörg Ebbing1, Christophe Pascal1, Javier Fullea2 1Geological Survey of Norway (NGU), Trondheim, Norway 2Dublin Institute for Advanced Studies, Dublin, Ireland SUMMARY: The debate about the high topography yet old age of the Norwegian mountains (the Scandes) is addressed in the framework of the TopoEurope projects of the European Science Foundation. Multiple of the mechanisms that have been proposed for causing the currently high topography are linked to lower lithospheric processes. We incorporate several sets of recent geophysical data as well as their interpretations into an integrated numerical modelling approach to characterize the current lithospheric conditions of the Scandes and the adjacent Fennoscandian shield. This study is the first step of numerical investigations to determining the processes that affected the lithosphere and topography since the Cenozoic break-up. INTRODUCTION: The Caledonides of Norway formed in the Silurian (440-410 Ma ago), followed by post-orogenic extensional collapse and a long period of alternating phases of rifting (i.e. Permian, Triassic and Late Jurassic) and tectonic quiescence and erosion. The last major tectonic phase occurred with the opening of the North Atlantic in the Early Cenozoic (ca. 53 Ma ago). Albeit the lack of onshore Cenozoic shortening processes, widespread evidence from various geological and geophysical studies depict a scenario of increased rock uplift and erosion in post-rift Cenozoic times (Dore et al., 2002). This evidence comprises subcropping offshore Mesozoic strata, enhanced sedimentary influx, presence of overcompacted sediments, and the plateau-like topography of the Scandes (Japsen and Chalmers, 2000; Lidmar-Bergström et al., 2000). Numerous processes have been suggested for Paleogene and Neogene uplift in Norway, including influence of the Iceland plume, mantle upwelling, intraplate stresses or dynamic topography (Dore et al., 2002). An alternative explanation assigns the profound changes in erosion and exhumation history to climatic conditions and isostatic response, thereby negating any tectonic uplift component (Nielsen et al., 2009). Many of the above mechanisms are related to processes in the lithospheric mantle and are likely to be reflected in its current state, structure and composition. Bouguer gravity anomalies show a prominent low along the Scandes, which indicates mass deficit below the mountains and a form of isostatic compensation (Figure 1a). But seismic studies reveal that the Moho underneath the Scandes does not form a pronounced crustal root structure as it would be expected for isostatic compensation of the high topography (Kinck et al., 1993; Grad et al., 2009; Stratford et al., 2009). This discrepancy between a missing crustal root yet apparently isostatically compensated topography cannot be explained by the relatively well known upper crustal structures. But it can be resolved by a dense lower crustal body underneath the eastern extension of the Scandes (Figure 1b, Ebbing 2007). Additionally, a mantle component is still likely, especially below the Southern Scandes, in order to explain the differences between the Northern and Southern Scandes (Olesen et al. 2002; Ebbing 2007). Similarly, recent results from seismological experiments point to major lateral variations in the lower lithosphere. Weidle and Maupin (2008) employ seismic tomography to map a narrow low velocity zone in the mantle lithosphere extending from Iceland towards southern Norway. Travel time residuals from southern Scandinavia reveal a relatively narrow transition zone along eastern Norway which separates two domain of fast arrival of seismic waves to the west and south and delayed arrivals to the east (Bondo Medhus et al. 2010). A growing heat-

flow data base of Norway allows us to constrain the thermal state. We include these new data and interpretational results to perform combined geophysical-petrological modelling of the lithosphere and sublithospheric upper mantle with the LitMod3D (Fullea et al., 2009), building on first-order studies by Pascal and Cloetingh (2009). It is here important to combine onshore and offshore data sets since the onshore topography can be closely coupled to the offshore tectonic history. METHODS: We use results of seismic studies (Stratford et al., 2009) and recent Moho depth compilations (Grad et al., 2009) to constrain the geometry of the subsurface of the Scandes. Crustal densities are obtained from converted seismic velocities and surface petrophysics. Heat flow studies provide thermal properties of the crust and constrain those of the lithospheric mantle (Pascal and Olesen, 2009). Whereas the Southern Scandes have been intensively studied, the data coverage in the northern Scandes is sparse, and a careful choice of parameters is needed. Densities and thermal properties of the lithospheric mantle are calculated throughout the model run from the chosen mantle composition and thermal field. Two critical factors in the model are (1) assumption of isostatic compensation at base lithosphere, and (2) presence of a high-density lower crustal body below the central Fennoscandian shield. Large-scale variations of mantle composition and temperature as well as the extent and nature of the proposed lower crustal body are tested for, and the aim is to find a model solution which shows seismic mantle velocities variations that are similar to those of Weidle and Maupin (2008) and Bondo Medhus et al. (2009). The analysis should allow us to differentiate between the effect of composition and of temperature on seismic velocities and consequently on mantle densities. Important is also the link to postglacial rebound models. Such models provide information about the recent uplift of Fennoscandia and the rheology of the mantle. Although postglacial rebound is a dynamic effect, our analysis aims to identify similarities in the mantle rheologies derived from our modelling approach, glacial rebound analysis and spectral methods. OUTLOOK: We employ LitMod3D to investigate the effects of the crustal composition and lateral variations of the lithospheric mantle underneath the Fennoscandian shield. Matching the calculated values of, e.g., geoid, gravity signal or seismic travel times, to the observed data is directly tied to conditions on the thermal regime and the mantle composition. Determining and quantifying these conditions allows us to constrain the rheology of the lithospheric mantle and to evaluate which of the mechanisms proposed for the Cenozoic uplift of the Norwegian mountains can be validated with the available geophysical data base. In particular, the rheology and thermal structure of the lithosphere will strongly control the duration and wavelengths of uplift processes. In a second step we will employ dynamic modelling to validate the various uplift mechanisms.

Figure1: a) Bouguer gravity anomaly of Norway and western Fennoscandia. White lines mark the regions of high topography. b) Thickness map of a high density lower crustal body as determined from isostasy and gravity models by Ebbing, (2007).

Figure2: Diagram of LitMod3D, adapted to the Southern Scandes. a) Topography, top of lower crustal body and Moho. b) spatial transformation and subdivision of lithospheric columns into vertical prisms used in the computation of geoid and gravity anomalies. Not to scale. (modified from Fullea et al., 2009) REFERENCES: Bondo Melhus et al., 2009. Deep-structural differences in southwestern Scandinavia revealed by P-

wave travel time residuals. Norwegian Journal of Geology 89, 203-214. Dore et al., 2002. Exhumation of the North Atlantic margin: introduction and background. In Doré

et al., (eds.): Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geol. Soc. London Special Publication 196, 1-12.

Ebbing, 2007. Isostatic density modelling explains the missing root of the Scandes. Norwegian Journal of Geology 87, 13-20.

Fullea et al., 2009. LitMod3D: an interactive 3-D software to model the thermal, compositional, density, seismological, and rheological structure of the lithosphere and sublithospheric mantle. G-cubed 10, doi:10.1029/2009/GC002391.

Grad et al., 2009. The Moho depth map of the European plate. Geophys. J. Int. (2009) 176, 279-292. Japsen and Chalmers, 2000. Neogene uplift and tectonics around the North Atlantic: Overview.

Global and Planetary Change 24, 165-173. Kinck et al., 1993. The Moho depth distribution in Fennoscandia and the regional tectonic evolution

from Archean to Permian times, Precambrian Res., 64, 23-51. Lidmar-Bergström et al., 2000. Landforms and uplift history of southern Norway. Global planet.

Change, 24, 211–231. Nielsen et al., 2009. The evolution of western Scandinavian topography: a review of Neogene uplift

versus the ICE (isostacy-climate-erosion) hypothesis. Journal of Geodynamics 47, 72–95. Olesen et al., 2002. Bridging the gap between the onshore and offshore geology in Nordland,

northern Norway. Norwegian Journal of Geology 82, 243-262. Pascal and Cloetingh, 2009. Gravitational potential stresses and stress field of passive continental

margins: insights from the south-Norway shelf, Earth and Planetary Science Letters, 277, 464-473.

Pascal and Olesen, 2009. Are the Norwegian mountains compensated by a mantle thermal anomaly at depth?, Tectonophysics, 475, 160-168.

Rohrman et al., 1995. Meso-Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology. Tectonics 14, 704 - 718.

Stratford et al., 2009. New Moho map for onshore southern Norway. Geophys. J. Int. 178, 1755-1765.

Weidle and Maupin, 2008. An upper-mantle S-wave velocity model for Northern Europe from Love and Rayleigh group velocities. Geophys. J. Int. 175, 1154-1168.