The thermal and density structure of passive margins

Transcription

1 The thermal and density structure of passive margins Magdalena Scheck-Wenderoth, Yuriy Maystrenko, Julia Autin, Hans Jürgen Götze, Sabine Schmidt, Christian Reichert

2 basins in their plate-tectonic setting intracontinental rift sediments Wilson cycle continental breakup passive margin crust mantle ocean basin MOR passive margin passive margin trench foreland basin subduction Wilson, J. T., 1966, Did the Atlantic close and then re-open?, Nature, 211(5050),

3 basins in their plate-tectonic setting intracontinental rift sediments Do all margins look the same? Wilson cycle continental breakup passive margin crust mantle ocean basin MOR passive margin Heat Gravity passive margin trench foreland basin subduction Wilson, J. T., 1966, Did the Atlantic close and then re-open?, Nature, 211(5050),

4 Heat at passive margins continent margin ocean influenced by crust syn-rift deposits post-rift deposits Sources Lithospheric mantle LAB Transport asthenosphere

5 continent margin ocean heat sources? post-rift deposits radiogenic heat crust syn-rift deposits Lithospheric mantle asthenosphere 1300 C LAB amount of heat from the asthenospheric mantle depth to the thermal boundary layer (~1300 C isotherm) local heat anomaly in the asthenosphere (plume, small-scale convection)

6 transport? continent margin ocean crust syn-rift deposits Lithospheric mantle asthenosphere post-rift deposits LAB Conduction dominant on the scale of the lithosphere 0 = div (λ*gradt) + S & boundary conditions 3D conductive thermal model steady-state, FEM Bayer et al., 1997 S radiogenic heat production [mw/m³] T temperature [ C] λ- conductivity [W/mK]

7 Thermal effects continent margin ocean Sediments: mostly insulating, moderate radiogenic heat production => heat transport, thermal blanketing crust syn-rift deposits post-rift deposits crust: conductive, high radiogenic heat production => heat budget, heat transport Lithospheric mantle asthenosphere 1300 C LAB mantle: conductive, low radiogenic heat production => heat transport depth to LAB isotherm => heat budget

8 gravity anomaly (free-air offshore, Bouguer onshore) Observed modelled Residual Data from Earth Gravitational Model - EGM2008 (International Gravimetric Bureau - BGI) and from the DNSC08GRA global marine gravity field (Andersen et al., 2010) Cumulative effects of all masses, determined by subsurface density distribution

9 main density contrasts & characteristic wavelength in gravity response: continent margin ocean post-rift deposits sediments/crystalline crust ( km) lithology, geometry degree of compaction crust syn-rift deposits crystalline crust/mantle ( km) Lithospheric mantle asthenosphere LAB mantle lithosphere/asthenosphere (>100 km) composition thermal state crustal thickness composition of the crystalline crust high-density bodies! continental versus oceanic crust

10 3D models from differently old margins Million years before present Mueller et al., 2008

11 data-based regional models sediments LAB observations Field wells Stratigraphy Lithology: ρ,λ,k,θ, S Temperature Seismic

12 data-based regional models crust sediments observations Field wells Stratigraphy Lithology: ρ,λ,k,θ, S Temperature Seismic Potential fields

13 data-based regional models sediments observations Field Lithosphere crust Moho wells Stratigraphy Lithology: ρ,λ,k,θ, S Temperature Asthenosphere Seismic Seismology MT Potential fields

14 data-based regional models tools GMS Constrained by 3D thermal & gravity modelling Götze (1978); Schmidt et al. (2011) FEM Bayer et al., 1997

15 South Atlantic: breakup ~132 ma Million years before present Mueller et al., 2008

16 Data base African margin Bathymetry/Topography Gravity Several shallow seismic surveys A few wells Deep seismic experiments Published results on LAB Bauer et al., 2000; Hirsch et al., 2008, 2009, 2010, Fishwick, 2010, Stewart et al., 2000 ; Light et al., 1993; Pawlowski, 2008; Elliott et al., 2009; Andersen et al., Ma +100 Ma +200 Ma -100 Ma 0 Ma INKABA ye AFRICA SPP SAMPLE

17 lithosphere-scale 3D model of the West African continental margin 5 C 1300 C 4 times vertically exaggerated

18 thickness water pre-aptian Cenozoic Precambr-Paleoz. postturonian Aptian- Turonian Sediment thickness largest parallel to coast Densities increase in response to compaction (logs) Impact on thermal field : insulating Maystrenko et al., submitted

19 thickness of crustal layers oceanic crust (density 2900 kg/m3) thermally conductive, low heat production High-velocityhigh-density body (density 3050 kg/m3) thermally conductive, low heat production continental lower crystalline crust (density 2950 kg/m3) thermally conductive, low heat production continental upper crystalline crust (density 2810 kg/m3) thermally conductive, high heat production

20 Depth 20 Top cryst. crust Moho LAB Fishwick, 2010 slightly modified (3D gravity model Density contrast of 3222/3160 kg/m3) 1300 C isotherm

21 3D density model cross sections 3 SDR: seaward-dipping reflections COB: continent-ocean boundary

22 Section Walvis Basin: ocean continent thickness of crystalline crust (heat production!) increases toward continent LAB (1300 C) ~ flat, mantle lithosphere uniform Maystrenko et al., submitted

23 Predicted Temperature 2km depth 10km depth 20 km depth Sediments thick ->thermal blanketing Crust thick ->heat generation Maystrenko et al., submitted

24 predicted temperature depth LAB 70km depth 90km depth Lateral variations in temperature decrease with depth in the lithosperic mantle Maystrenko et al., submitted

25 heat flow trend inversion with depth Surface Moho Heat production controlled shallow thermal field of old (~130 mio a) passive margin

26 volcanic passive margin Norway: breakup at 55 mio a breakup at 55 mio a Oslo Bergen Amsterdam Universities Million years before present Mueller et al., 2008

27 3D model N ocean margin continent Resolution: horizontal 2.5 km vertical 14 layers Scheck-Wenderoth et al., 2007

28 The lithospheric mantle: Gravity misfit for uniform lithospheric mantle (continent +ocean) Mantle 3330 Maystrenko & Scheck-Wenderoth, 2009

29 The lithospheric mantle: oceanic mantle less dense than continental mantle! thermal? Maystrenko & Scheck-Wenderoth, 2009

30 best-fit model consistent with gravity & heat flow 1300 C temperature -dependent densities in lithospheric mantle Thermal field at young margin controlled by cooling of oceanic lithosphere! Maystrenko & Scheck-Wenderoth, 2009

31 temperature variation with depth 2000 m 3000 m 5000 m laterally variable margin hotter than continent & ocean At 5km~ C =>potential source rocks in the oil window Scheck-Wenderoth & Maystrenko, Geology, 2008

32 temperature variation with depth 5 km 10 km 50 km margin hotter than continent & ocean (thermal blanketing of sediments) margin & ocean hotter than continent lithospheric mantle hotter ( 400 K) below the ocean than below the continent thermal expansivity density reduction

33 increasing heat flow from continent to ocean Surface Moho

34 conclusions old margin controlled by radiogenic heat of the cryst.crust young margin controlled by cooling of oceanic lithosphere Thermal & density heterogeneities originate mainly in the crust lithospheric mantle is +/- homogenous smooth transition between continental and oceanic lithosphere Thermal and density heterogeneities orginate in the crust and in the lithospheric mantle lithosph. mantle less dense & hotter below ocean sharp transition between continental and oceanic lithosphere