Nature of the crust-mantle transition zone and the thermal state of the upper mantle beneath Iceland from gravity modelling

Abstract

Summary Gravity data from Iceland and its surroundings are analysed and modelled with respect to seismic data. A Bouguer gravity map of Iceland is recomputed based on admittance between the topography and the gravity and with corrections for glacial ice sheets. From seismic data and with the help of relations between the residual topography and the depth to seismic boundaries we construct maps of the main seismic boundaries, including the Moho. By inversion calculations we recomputed these maps, assuming different density values for Seismic Layer 4 to fit the observed gravity field. We found that the average density of Layer 4 has to be in the range 3050–3150 kg m−3 in order to fit both seismic and gravity data. Thus we conclude that Layer 4 is a transition zone between the mantle and the oceanic crust in Iceland. Furthermore by assuming that the upper‐mantle density variations necessary to compensate for the gravity effect of crustal layers, are caused by thermal variations in the upper mantle, we calculate the depth to the 1200 °C isotherm to be at 30–50 km depth below Iceland but rising up to less than 20 km below parts of the volcanic zone in Northern Iceland. We conclude that the temperature within the Seismic Layer 4 is close to 600 °C at its top, increasing to approximately 950 °C at its bottom (Moho), which makes a widespread layer of partially molten material within Layer 4 unlikely. By use of cross spectral analysis of the gravity field and the external topographic load at short wavelengths, we conclude that the elastic plate thickness in Iceland can hardly exceed 6 km. In addition we point out that the residual isostatic anomalies have circular forms east of the eastern volcanic zone but are near parallel to the ridge axis on the western side. This form of the anomalies may be caused by pressure from the eastward moving mantle plume below the eastern volcanic zone.

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

“…Of particular concern were the measurements on Iceland, as Schlindwein (2006) pointed out that the P-S converted phases there are only weak, which is why the use of the receiver function technique may be limited. For this reason, the 53 available measurements by Kumar et al (2007) were cross-checked with the seismic refraction data and the seismically controlled gravity inversion of Kaban et al (2002). In this process, 14 receiver functions were rejected, as they were not consistent with the other methods.…”

“…To allow for the best possible quality control and internal consistency, existing local compilations were not incorporated into our database. Examples of such compilations are the seismically constrained gravity inversion for Iceland (Kaban et al 2002) or the model for the Barents Sea (Ritzmann et al 2007) that is based on both seismic reflection and refraction data, as well as on gravity modelling.…”

Temporal Gravity Variations near Shrinking Vatnajökull Ice Cap, Iceland

“…The plume viscosity, g, is varied: g = (3, 7, 10, 30) 9 10 17 PaÁs. On top of the plume is a 6-km-thick elastic lithosphere (e.g., KABAN et al, 2002). The small lateral variations of the shear modulus, l, and the density, q, are neglected.…”

Crustal accretion at high temperature spreading centres: Rheological control of crustal thickness

“…As discussed by Menke (1999) this low density contrast seems too low to be easily explained by high mantle temperatures, retained melt or depletion. Based on similar reasoning, the isostatic models of Kaban et al (2002) require anomalously high densities of a layer usually associated with the lowermost crust and attribute this layer to a mantle -crust transition zone.…”

“…White,1993, Ito et al, 2003Ruedas et al 2004). The most prominent example is Iceland, where high melt production rates in combination with low spreading rates lead to crustal thicknesses estimated to range up to 40 km (Darbyshire et al, 2000;Kaban et al, 2002;Fedorova et al, 2005) (Fig. 1).…”