Icelandic Lithosphere & Upper Mantle Structure. Natalia Solomatova

Transcription

1 Icelandic Lithosphere & Upper Mantle Structure Natalia Solomatova

2 Location of Iceland maps.google.com

3 Lithosphere Thickness Anomalously thick lithosphere beneath Greenland and Baltic shield may be due to Precambrian collisional tectonic events. Artemieva and Thybo (2008) model from Grand (2002)

4 Interaction between a mantle plume and the MidAtlantic Ridge 9 r /y m m plume? 9 r /y m m

5 Structure of the Lithosphere Anomalously thick lithosphere beneath Greenland and Baltic shield may be due to Precambrian collisional tectonic events. Artemieva and Thybo (2008)

6 The Ponded Plume Model Poore et al. (2011) orange body = plume flowing beneath lithosphere black line = MOR ridge straddling the plume red patches = blobs of hot plume material which expand radially outwards by Poiseuille flow red circle = intersection of youngest V-shaped ridge (VSR) with Reykjanes Ridge (RR); blue circle=position beyond youngest VSR blue and grey block = lithosphere cut-away yellow prism = melting region below MOR beneath

7 Velocity Variations Along 2 Sections absolute arrival times relative arrival times (from surface waves) (from body waves) Vertical cross sections through the mantle S velocity model ICEMAN-S. lateral velocity variations relatively high velocities in plume head Allen et al. (2002)

8 Velocity Anomaly Conclusions (Allen et al., 2002): The anomaly has a radius of km. Velocity anomaly of 3.8% for VS and 2.1% for VP are consistent with a K temperature anomaly. Above 200 km the nature of the velocity anomaly changes due to the presence of a horizontal low-velocity plume head with relatively high lateral velocities. The degree of melt being extracted from the plume core is higher than before.

9 Velocity Anomalies within the Plume Head Allen et al. (2002)

10 Density Anomalies Large lateral variations in density between 25 and 80 km. *from joint inversion of teleseismic P-wave delay times and GRACE gravity N W E S O Donnell et al. (2011)

11 Density Anomaly Conclusions (O Donnell et al., 2011): Lateral variations can be due to a ~400oC thermal anomaly. However, they argue that the anomalies must reflect compositional, rather than plume-driven thermal contrasts. The low velocities are not necessarily slow compared to normal mantle. Possible reasons for compositional variations are: terrane accretion during the Iapetus Ocean closure frozen melt generated during the opening of the Atlantic Ocean frozen Iceland plume related magmatic intrusions Argue for small-scale convection at the base of lithosphere. The ponded plume hypothesis is also at odds with heat flow measurements.

12 Assuming there is a plume, does the plume form at the base of the upper mantle or at the core-mantle boundary?

13 Shallow Plume Model Lithosphere extension over a pre-existing steady-state mantle plume. lateral T gradients between thin oceanic lithosphere and thick Greenland craton lithosphere induces small-scale convection stretching & thinning of lithosphere causes magmatism restricted to the upper km of the mantle Problems: convection is not strong enough wrt to spreading rate does not explain geoid anomalies (positive gravity anomaly of ~60 mgal) (King and Anderson, 1995)

14 CMB Plume Model A transient broad plume head originating at the core-mantle boundary impacts the base of the lithosphere. The heavy line segments indicate the diffracted paths along the CMB. The SKPdS segments associated with KEV and KRK appear to be sampling a localized ULVZ located beneath Iceland. (Helmberger et al., 1998)

15 Challenges to the Plume Model seismic tomography is debated weak visibility of plume evidence of eclogite source rather than depleted mantle geochemical signatures are highly varied no volcanic track as seen with Hawaii

16 Alternative to the Plume Model Melting of fossil slab: Not dynamically or chemically stable. Uncertainty about the thermal effect of such massive melting. Not imaged in tomography. modified from Sichel et al. (2008)

17 References King, S. D., & Anderson, D. L. (1995). An alternative mechanism of flood basalt formation. Earth and Planetary Science Letters, 136(3), Allen, R. M., Nolet, G., Morgan, W. J., Vogfjörd, K., Bergsson, B. H., Erlendsson, P.,... & Stefánsson, R. (2002). Imaging the mantle beneath Iceland using integrated seismological techniques. Journal of Geophysical Research: Solid Earth ( ), 107(B12), ESE-3. Artemieva, I. M., & Thybo, H. (2008). Deep Norden: highlights of the lithospheric structure of Northern Europe, Iceland, and Greenland. Episodes,31(1), 98. Helmberger, D. V., Wen, L., & Ding, X. (1998). Seismic evidence that the source of the Iceland hotspot lies at the core mantle boundary. Nature,396(6708), Poore, H., White, N., & Maclennan, J. (2011). Ocean circulation and mantle melting controlled by radial flow of hot pulses in the Iceland plume. Nature Geoscience, 4(8), Sichel, S. E., Esperança, S., Motoki, A., Maia, M., Horan, M. F., Szatmari, P.,... & Mello, S. L. (2008). Geophysical and geochemical evidence for cold upper mantle beneath the Equatorial Atlantic Ocean. Revista Brasileira de Geofísica, 26(1), Grand, S. P. (2002). Mantle shear wave tomography and the fate of subducted slabs. Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, 360(1800),