Isostatic state and density structure from surface to depth of the lithosphere below the Norwegian continental margin

Transcription

1 Isostatic state and density structure from surface to depth of the lithosphere below the Norwegian continental margin Jörg Ebbing (1,2), Odleiv Olesen (1), Torleif Lauritsen (1), R. Fjalar Reynisson (3,2) (1) Geological Survey of Norway, Trondheim (2) Department for Petroleum Technology and Applied Geophysics, NTNU, Trondheim (3) Statoil Research Centre, Trondheim imagine Arctic Days Tromsø June, 1st 2010

2 Bouguer anomaly Topography & reg. seismics Ebbing et al. 2010

3 Moho Depth Topography & reg. seismics Ebbing et al. 2010

4 Moho Depth Topography & reg. seismics...the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations. An appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data. P.J. Barton (1986) Ebbing et al. 2010

5 Isostatic concept ρ Topo ρ Water ρ Upper Crust 1. Normal lithospheric column 2. Loading by Topography ρ LowerCrust 3. Isostatic Moho 4. Observed seismic Moho ρ Mantle density lower crust 5. Compensate by High- ( underplating ) ρ Water < ρ Topo < ρ Upper Crust < ρ LowerCrust <ρ Mantle ρ Water < ρ Topo < ρ Upper Crust < ρ LowerCrust < ρ HighDensityLowerCrust <ρ Mantle

6 Isostasy vs. Seismic Isostatic highdensity lower crust Seismic high-velocity lower crust (Perez-Gussinye et al. 2004)

7 Isostasy and gravity residuals Isostatic highdensity lower crust Gravity residual of isostatic model => Correlation with surface geology?

8 Petrophysical database: Density ~ 28,000 rock samples have been measured with respect to density, susceptibility and remanence maps of susceptibility and density are produced by calculating the average values within each geological g unit represented on the Norwegian part of the northern Europe bedrock map Non-representative samples (e.g., sulphide mineralisations, hydrothermal alterations, mylonites, diabase, eclogite) were removed from the dataset before map production Olesen et al. 2010

9 Upper crustal gravity effect and isostatic gravity residuals Gravity residual of refined isostatic model

10 Onshore-offshore structures Olesen et al. 2010

11 Hammerfest Trom sø Basin Sørvest snagdbas in SenjaR idge LoppaHigh Basin alvøya gerh Varan ja Sen S FZ gra Via h Hig Møre Basin Ba sin n fote Lo Ve stf jord en ge Rid Bodø Bo rd er F.C. h Hig in ard as Utg ab ge æn Tr.C. drid F.C. lan etf gard ell Nord - Ut vfj Fl es e -R Basin sin en dba olm lan erh lge Ytt He ru st Fr ont Bo rder F.C. 66º ian Th Tr øndelagplatform Caled on Klakk F.C. Helland-Hanse n Vøring n te fo Lo igh kh Ny Bo rde rf.c. ll Gja e idg arr HaltenT errace n igh lh ina arg gm Fr øy aving leiaf.c. one re Z cu act Fra en May Jan ri Vø Rø st Hig Utr h Ma øst R idg rm e æ Rib ban le Sp Bas ur in Tromsø sin Ba an Fro Tertiary volcanic esca rpment Oceanic fracture zone BoundaryofTertiary lavas(inner flows) Subcropof top Basement below Qua ternary Onshore geology Permian OldRed Sandstone Östersund Trondheim UppermostAllochthon UpperAllochthon MiddleAllochthon Sedimentarycover Precambrian crystallinerocks Lower Allochthon Sedimentarycover Precambrian crystallinerocks Parautochthonand LowerAllochthon NormalFaults W- dipping E - dipping Bergen Oslo Stavanger Norwegian-Danish basin Ka tteg at Ebbing et al Reynisson et al. 2010

12 Origin and extend of LCB? Mjelde et al. 2009

13 Crustal structure from 3D Model Modelling software: IGMAS Interactive Gravity and Magnetic Application l System Modelling constrained by Seismic i information i from OBS lines (e.g. Mjelde et al. 1992, 2005, unpublished) Structural interpretations (e.g. Blystad et al. 1995, Brekke & Riis 1987, Eldholm et al. 2002, Osmundsen et al. 2006) Densities from petrophysical studies, wells and velocitydensity conversions Interpreted seismic horizons

14 Modelled and isostatic Moho Reynisson et al. 2010

15 Difference between isostatic and modelled Moho Reynisson et al. 2010

16 LCB and Moho Reynisson et al. 2010

17 Reynisson 2010

18 Origin of LCB? Reynisson et al. 2010

19 Summary Top basement and Moho geometry reflect the tectonic evolution of the Northeast Atlantic margin Gravity and magnetic are especially helpful to study their geometry Isostasy is important to consider to validate the crustal geometry Onshore-offshore late-caledonian l detachments representing the collapse of the orogene can be linked by their magnetic signature below the margin high-density lower crust is a feature to the east of the Scandes and at the outer margin: LCB LCB: multiple origin suggested Magmatic origin at outermost margin Serpentinisation in central part of margin Eclogite below central Fennoscandia