Testing the Existence of a Mantle Diapir Below the Southern Scandes

Christophe Pascal & Odleiv Olesen

NGU, Geological Survey of Norway, N-7491 Trondheim, Norway; Christophe.pascal@ngu.no ; Odleiv.Olesen@NGU.NO

This webpage is a synopsis of the paper: Pascal, C. & O. Olesen, Are the Norwegian mountains compensated by a mantle thermal anomaly at depth?, Tectonophysics, 475, 160-168, 2009.

Click here for Discussion of this webpage

Introduction

The Scandes are a long mountain range that stretches for more than 1400 km through most of Norway and parts of central and northern Sweden. They are traditionally divided into two dome-like areas (the southern and northern Scandes), which are separated by a central area with less pronounced topography (Figure 1a).

Figure 1: a) topography (Dehls et al., 2000) and b) Bouguer gravity anomalies of Fennoscandia (Skilbrei et al., 2000; Korhonen et al., 2002). A, B, C represent the three lines used in the gravity modelling. Note the spatial correlation between the most pronounced gravity lows and the location of the Scandes. Click here or on figure for enlargement.

The origin of the Scandes mountain chain far away from any plate boundary remains a matter of debate in the geoscientific community and various models have been advanced. A non-exhaustive list of invoked causes includes:

1. Opening of the NE Atlantic (Torske, 1972);
2. Isostatic response to glacial erosion (Doré, 1992; Riis & Fjeldskaar, 1992);
3. Pre-subduction instability (Sales, 1992);
4. intraplate stresses (Cloetingh et al., 1990);
5. Mantle convection (Bannister et al., 1991);
6. Climate deterioration and sea-level changes (Eyles, 1996);
7. Small-scale convection (Stuevold & Eldholm, 1996);
8. Rift-shoulder uplift (Doré, 1992, Redfield et al., 2005);
9. Asthenospheric diapirsm (Rohrman & van der Beek, 1996; Rohrman et al., 2002);
10. Migrating phase boundaries (Riis & Fjeldskaar, 1992);
11. Serpentinisation (Skelton & Jakobsson, 2007), and more recently;
12. Modification of the Caledonian topography (Nielsen et al., 2009).

The most accepted model, and probably the most satisfactory one in terms of accounting for most of the observations, is the asthenospheric diapir model advanced by Rohrman & van der Beek (1996).

The aim of the present contribution is to test the asthenospheric diapir model by means of integrated modelling of high-resolution gravity data with modern heat flow data. We first summarise the theoretical background and the implications of the asthenospheric diapir model. We then model the long-wavelength field (i.e., associated with compensating masses below the mountains) of the Bouguer gravity anomalies in southern Norway, in order to constrain the depth of the assumed asthenospheric body and its density deficit or, conversely, its temperature excess. The predictions are then tested against recently acquired heat flow data.

The asthenospheric diapir model

The asthenospheric diapir model (Figure 2) involves impingement of anomalously hot asthenosphere at the base of cold cratonic lithosphere (Rohrman & van der Beek, 1996; Rohrman et al., 2002). According to the model, the hot asthenosphere originates in the Iceland hotspot emplaced in the Norwegian-Greenland Sea at ~30 Ma (Lawver & Müller, 1994). Hot material travels through a thin asthenosphere layer before meeting cold cratonic lithosphere. The contrast in temperature (i.e., viscosity) between the two produces a Rayleigh-Taylor instability. This process is similar to the one that leads to the formation of thunderclouds. Finally the rise of the asthenospheric diapir creates isostatic uplift of the surface. Rohrman & van der Beek (1996) invoke two cases. In the first case, the diapir has reached relatively shallow levels in the lithosphere and subsequent decompression melting results in volcanism at the surface (e.g., at Spitsbergen). In the second case, which would represent the Scandes, the asthenospheric diapir is still located at great depth at the present day and therefore no melting has occurred yet but topography is compensated by the hot diapir at depth.

Figure 2: The asthenospheric diapir model: interaction between anomalously hot asthenosphere and cold cratonic lithosphere produces a Rayleigh-Taylor instability, penetration of hot asthenosphere into mantle lithosphere, and subsequent uplift of the Earth's surface (redrawn from Rohrmann and van der Beek, 1996).

The asthenospheric diapir model is probably the most elegant one while reconciling different pieces of the puzzle. It accounts for both the amount (i.e., 1-2 km) and the apparent timing (i.e., mostly Neogene) of uplift and integrates various geological and geophysical observations into a coherent scheme (Rohrman et al., 2002). The main conclusions derived by Rohrman & van der Beek (1996) are that the top of the diapir is located at ~100 km depth, its vertical extent is ~100 km, its radius 100-150 km and the temperature contrast between the hot asthenosphere and the cold lithosphere is ~400°C (Figure 2). Bearing in mind these numerical values, we evaluate the asthenospheric diapir model.

Integrated gravity-thermal modelling of the southern Scandes

Testing the gravity response of the asthenospheric diapir

We focused our study on the southern Scandes and used the high-resolution (i.e., one measurement every ~3 km) gravity database at NGU (Skilbrei et al., 2000). In order to model Bouguer anomalies we selected data points along three NW-SE profiles across the middle of the southern Scandes (Figure 1). The aim of our modelling was to find the nature and depth of the sources that reproduce a gravity signal consistent with the one suggested by the gravity data.

First, we placed at 100 km depth a 100-km-wide body and tested different density contrast values with respect to the neighbouring rocks (Figure 3). Our analysis shows that a density reduction between the lithosphere and the hot asthenosphere of -50 kg/m³, which would correspond to a reasonable value of ~3250 kg/m³ for the density of the asthenosphere, produces a very smooth gravity signal unlikely to be detected. Increasing the density contrast to -100 kg/m³ increases the amplitude of the signal and its gradient but is far from satisfying the shape of the gravity low as it is suggested by the data points. Apparently a density contrast higher than this latter value and close or equal to -200 kg/m³ would meet our requirements (Figure 3). However, such a density contrast value would result in unrealistically temperatures much higher than 1750°C for the assumed diapir. This latter temperature value already exceeds by ~150°C maximum temperatures estimated for thermal anomalies in the upper mantle (e.g., Goes et al., 2004). In brief, this first modelling exercise demonstrates that, in order to match the observed gravity low, the asthenospheric diapir cannot be located at great depth below the surface.

Figure 3: Gravity modelling assuming an asthenospheric diapir at 100 km depth and as a function of density contrast, Δρ, between the diapir and surrounding lithosphere (or alternatively diapir temperature, T_d, assuming average lithosphere temperatures of ~1000°C). Dots represent gravity data, extracted along profiles A, B and C (Figure 1), and solid lines modelled gravity responses (see details in Pascal & Olesen 2009). Note that it is impossible to reproduce the key characteristics of the observed gravity signal unless diapir temperatures reach unreasonably high values.

Alternatively, the asthenospheric diapir could be at shallower depths than anticipated. We calculated the gravity effect placing the diapir at different depths. A reasonable fit between observed and modelled gravity is obtained only if we assume that the diapir has reached relatively shallow depths (i.e., ~40 km,), close to the Moho (Stratford et al., 2009). It appears that a density contrast of between -50 kg/m³ and -100 kg/m³ would reproduce reasonably well the observed gravity low, implying diapir temperatures higher than 1400°C and lower than or equal to 1750°C (Pascal & Olesen, 2009).

Testing the heat flow response of the asthenospheric diapir

We explored the consequences for surface heat flow assuming that the top of the diapir is located at 40 km depth and its temperature is between 1400°C and 1750°C. In addition, we followed Rohrman et al. (1995, 2002) and assumed that uplift of the Scandes (i.e., emplacement of the diapir) started at 30 Ma. We used the most recent heat flow database for southern Norway, suggesting typical heat flow values of 58 ± 12 mW/m² (Slagstad et al., 2009), and assumed typical pre-uplift heat flows of ~40 mW/m² (i.e., lowermost continental heat flow; Nyblade & Pollack, 1993), the difference between measured present-day heat flow and this latter value (i.e., ~20 mW/m²) being sourced from the asthenospheric diapir.

First-order analytical models of surface heat flow evolution, after emplacement of the asthenospheric diapir, suggested that present heat flow values should be at least 10 mW/m² higher than observed (Pascal & Olesen, 2009). In order to confirm these preliminary findings we conducted more advanced thermal modelling by means of finite-element techniques (Figure 4). We modelled the 2D transient response of the lithosphere after emplacement of a hot diapir. We assumed a sudden rise in temperature at the location of the diapir at t = 0 and let the system evolve. First, we set a temperature of T_d = 1500°C for the diapir and maintained it constant through time. This latter value represents a mantle thermal anomaly with average temperatures (e.g., Goes et al., 2004) but already results in an increase in surface heat flow of more than 55 mW/m², 20 Ma after its application (Figure 4b). We thus decreased the temperature of the diapir down to its lowermost permissible value of T_d = 1300°C (i.e., corresponding to "normal" asthenosphere). The increase in surface heat flow after 20 Ma still reached more than 40 mW/m² (Figure 4b) or, conversely, the model suggested that present-day surface heat flow in southern Norway should be equal to 80 mW/m², exceeding by 20 mW/m² the measured average value. The obvious conclusion from these modelling tests was that the emplacement of a mantle diapir at 40 km depth at 30 Ma is unlikely, because it would have produced much higher present-day surface heat flow values than actually measured.

Figure 4: a) 2D finite-element model: thermal evolution after impingement of an asthenospheric diapir at 40 km depth at t = 0. The temperature of the diapir is initially T_d = 1600°C and it is allowed to evolve. Note the fast decay in diapir temperatures. b) Evolution of surface heat flow vs. time after emplacement of the asthenospheric diapir at 40 km depth. Red curves represent 2D numerical solutions where T_d is diapir temperature and is kept constant. The blue curve corresponds to the transient numerical solution depicted in a). c) Evolution of the density contrast between the diapir and the surrounding lithosphere for the transient numerical solution depicted in a). The results are given for the centre and the lateral edges of the diapir. The evolution of the average density contrast is also shown. Note that previous gravity modelling shows that the gravity signal cannot be reproduced for Δρ > –50 kg/m³. Click here or on figure for enlargement.

From a physical point of view it is doubtful that an intruded diapir could maintain a constant temperature with time. In order to be complete, our last numerical modelling test addressed the effect of cooling of the asthenospheric diapir on the evolution of surface heat flow and mantle densities. We used similar modelling parameters as previously but allowed diapir temperatures to evolve following 2D heat diffusion. We report here only one simulation where T_d was set to its maximum allowed value of 1600°C (i.e., mantle thermal anomaly of + 300°C, Goes et al., 2004) at t = 0. The cooling of the diapir, following its assumed fast intrusion into the lithosphere, is rapid in particular at shallow depths (Figure 4a). Compared to previous modelling results the surface heat flow signal is drastically reduced and decays after having reached ~23 mW/m² at t ~15 Ma (Figure 4b). Therefore, assuming that no magma has reached relatively shallow levels in the crust, the heat flow anomaly would not be detectable despite the high temperature assumed for the mantle diapir.

Based solely on thermal considerations the asthenospheric diapir hypothesis could apparently pass this last modelling test. However, as temperature declines within the diapir, density increases. Using the modelled temperatures we computed the evolution of the density contrast between asthenospheric rocks within the diapir and the surrounding mantle lithosphere (Figure 4c; see details in Pascal & Olesen, 2009). Following the cooling of the diapir its density increased quickly, in particular at its edges. Figure 4c shows that average values of Δρ (the density contrast between the lithosphere and the diapir) are –50 kg/m³ shortly after ~4 Ma and reach –25 kg/m³ at t = 30 Ma, whereas our gravity modelling demonstrates that Δρ values between –100 and -50 kg/m³ are needed in order to explain the observed gravity low. The evident conclusion from this modelling exercise is that the southern Scandes cannot be isostatically balanced by an asthenospheric diapir at depth.

Discussion and conclusions

Our integrated gravity/thermal approach demonstrates that the topography of the southern Scandes is not compensated by a mantle thermal anomaly at depth. A simple analysis of the Bouguer gravity field shows that the mass deficit needs to be located close to the Moho as already demonstrated by Olesen et al. (2002). New heat flow data show that whatever the nature of this mass deficit it cannot, in any case, be related to a thermal anomaly. This statement would find even more support if we had considered thermal advection in our computations, which is a far more efficient, and perhaps more natural, way to transport excess heat to the surface.

Our analysis does not demonstrate the cause of Scandes uplift. Indeed the only criticism that can be addressed to the asthenospheric diapir model seems to be the assumption that the Bouguer gravity low is caused by a hot diapir or, conversely, that the topography is compensated by a mantle thermal anomaly. The other arguments in favour of the model of Rohrman & van der Beek (1996) (to our mind a brilliant one) appear to remain valid. In particular, the existence of a low-velocity anomaly at 75-150 km depth below southern Norway has been confirmed by a recent tomographic study (Weidle & Maupin, 2008). This study resolved the mantle structure of the NE Atlantic with better resolution than previous seismic studies (Husebye et al., 1986; Bannister et al., 1991; Rohrman et al., 2002) and imaged a continuous low-velocity anomaly stretching from Iceland to southern Norway as proposed in the asthenospheric diapir model. All this calls for a modification, but not a firm rejection, of the diapir model.

References

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Discussion

Hermann G W Burchard, 24th November, 2009

Do the authors prefer model #9 while dismissing that model #2 might be a causative agent?

Model 2. Isostatic response to glacial erosion (Doré, 1992; Riis & Fjeldskaar, 1992);
Model 9. Asthenospheric diapirsm (Rohrman & van der Beek, 1996; Rohrman et al., 2002);

Not having read Rohrman et al, perhaps I should not comment. However, this webpage does not mention WHY the diapir should have started to rise.

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