Magnus-Rex and the new refraction Moho Map for southern Norway W. Stratford and H. Thybo Department of Geography and Geology, University of Copenhagen
When did the present mountains form? Topography has been present since the Caledonian orogeny -> requires crustal root for long term support of topography. Topography is younger, last significant uplift during the Neogene -> crustal root not essential and other forms of support for the topography can be considered.
Previous models of Moho depth Gravity modeling has been used to infer both a crustal root, and the lack of a crustal root beneath the mountains. Bouguer gravity signature indicative of support from low densities..
Previous interpretation Crustal thickness review: Kinck et al, 1993 mapped a Moho thickness under the southern Scandes of ~ 36-38 km. Early Receiver Functions studies produced similar results Recent Receiver function profile across southern Scandes interpreted a thicker crust and a root. Oslo Graben has been inferred as having both, thick underplating and thin underplating from gravity interpretations. Inferred Moho upwarp beneath the graben range from a few km to ~ 10 km.
Overview Magnus-Rex - Mantle investigations of Norwegian uplift structure- Refraction experiment Project goals and results from Magnus-Rex Vp, Vs and Poisson s Ratio Models for the crust Construction of the new refraction Moho map for southern Norway. Are the Mountains supported by their crustal thickness?
Magnus-Rex: Mantle investigations of Norwegian uplift structures Refraction seismic Experiment Refraction profiling of the lithosphere; Best method for constraining the velocity structure and thickness of the crust. 22 teams, ~750 texan instruments, 26 shots. Top down approach to forward modelling Constraint on crustal thickness -> ± 1 km where Pn and PmP are available. ± 2 km where only PmP is available.
~360 km array 9 shots (100, 200 and 400 kg charge sizes) 174 instruments at 2 km spacing. 400 kg charge size Pg Vp = 6.0-6.3 km/s Pn Vp = 8.05 ± 0.1 km/s at xcross > 160 km Weak PmP reflection, Mantle reflection Shot 1 - line 1
Shot 4 - line1 200 kg charge size Pg Vp = 5.8-6.6 km/s PmP -> ~ flat moveout
~450 km array, 8 shots (100, 200 and 400 kg charge sizes) 330 instruments at 2 km spacing, including a 120 km section at 0.75 km spacing across the Oslo Graben 400 kg charge size Pg Vp = 5.8-6.3 km/s, PmP Pn Vp = 8.05 ± 0.1 km/s, Mantle reflection behind Pn Shot 26 - line 3
Velocity structure - Line 1 Thick lines -> refraction velocity and depth constraint Thick dashed lines -> reflection structural constraint
Velocity structure - Line 2 Thick lines -> refraction velocity and depth constraint Thick dashed lines -> reflection structural constraint
Velocity structure- Line 3 Thick lines -> refraction velocity and depth constraint Thick dashed lines -> reflection structural constraint
We also recorded S-waves Shot 17 - line 2 400 kg charge size Sg Vs = 3.35 3.65 km/s Sn Vs = 4.65 ± 0.1 km/s SmS reflection
Line 1 - S-waves Joint inversion of P and S-wave arrivals. Lower crust and Moho depth remains fixed from the P-wave model due to higher pick uncertainty for the lower crust/mantle arrivals
Line 2 S-waves Anomaly at ~350 km distance in Upper allochthon rocks (Køli nappes). Moho depth increases to the north.
Line 3 S-waves Upwarp in high velocities beneath the Oslo Graben.
Caledonides: Lower allochthon Granites, granodiorites. Western Gneiss Region. Fennoscandian shield: granite, granodiorite Caledonides: Middle Allochthon, quatzite, greywacke, siltstone, limestone 0.26 0.25 0.27 0.25
Middle and lower Allochthon: granite, gabbro, anorthosite, quartzite, greywacke Fennoscandian shield: granite, granodiorite Upper Allochthon: granite, diorite, gabbro, ultramafics, greywacke, conglomerate, felsic, mafic, metavolcanics 0.27 0.255 0.255 0.275 0.25 0.265
Middle and lower allochthon: granite, quartzite, greywacke, limestone Fennoscandian shield: granite, granodiorite, mica shist Oslo Graben: granite, diorite, gabbro, rhyolite, basalt. g granodiorite, metagreywacke, quartzite 0.25 0.265 0.235 0.245 σ 0.25 0.27 Caledonides Basement Oslo Graben Basement
Red lines = previous refraction experiments. Green line = Reflection profile. Blue dashed lines = receiver function profiles. Green = Calenonides, Blue = Oslo Graben. Average Vp for the upper crust 6.25 ± 0.1 km/s, Average Vp for the lower crust 6.75 ± 0.05 km/s. A. Fedje-Grimstad, (Sellevoll and Warrick, 1971). B. Cannobe, South Norway, (Cassell et al., 1983). C. Olso-Trondheim, (Kanestrøm, 1971). D. Otta-Årsund, (Mykkeltveit, 1980). E. Flora-Åsnes, (Sellevoll and Warrick, 1971). F. Oslo Graben, (Tryti and Sellevoll, 1977). G. Trondheim-Sundsvall (Vogel and Lund, 1971). I and J (Svenningsen et al., 2007).
Contour map of crustal thickness (Moho depth) Before Refraction Moho depth beneath southern Norway. Incorporating data from: Sellevoll and Warrick, 1971, Cassell et al., 1983), Kanestrøm, 1971, Mykkeltveit, 1980, Sellevoll and Warrick, 1971, Tryti and Sellevoll, 1977 and Vogel and Lund, 1971.
Contour map of crustal thickness After Original constraints from earlier refraction seismic studies still stand. Changes inferred are in gaps between previous surveys. Black is constraint from Pn = ± 1 km. Grey is constraint from PmP = ± 2 km.
Receiver function Moho interpretation on line 1. Interpretation of Svenningsen et al, 2007. Svenningsen, L., Balling, N., Jacobsen, B.H, Kind, R., Wylegalla, K., and Schweitzer, J. 2007. Crustal root beneath the highlands of southern Norway resolved by teleseismic receiver functions, Geophys. J. Int.,170,1129-1138.
Crustal contribution to topography Crustal contribution to topography estimated from 1D isostatic calculations h t = h i +h c (1 + ρ c /(ρ m - ρ c )) ρ c = average crustal density (2830 kg/m 3 ) ρ m = mantle density (3300 kg/m 3 ) h i = initial crustal thickness (30 km) h t = total crustal thickness h c = Topograhic relief
Line 1: Airy-isostasy Two models: Average crustal density = 2830 kg/m 3 High density lower crust (LC) = 2950 kg/m 3
Line 2: Airy-isostasy Two models: Average crustal density = 2830 kg/m 3 High density lower crust (LC) = 2950 kg/m 3
Line 3: Airy-isostasy Two models: Average crustal density = 2830 kg/m 3 High density lower crust (LC) = 2950 kg/m 3
Summary Crustal thickness beneath the southern Scandes mountains reach ~40 km. Crust thickens to the northeast Thins from the central mountains towards the coast and Oslo Graben, and to the south Airy-isostasy calculations indicate undercompensation for the high mountains in the west, and over compenstation to the east. Increasing lithospheric thickness and the flexural strength of the crust lithosphere need to be considered.