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THE EUROPEAN CONTINENT: SURFACE EXPRESSION OF UPPER MANTLE DYNAMICS
Rosaria Tondi, Renata Schivardi, Irene Molinari and Andrea Morelli ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA (INGV) Sezione di Bologna, email: [email protected]
T33G2730
The surface topography of Europe shows important variations, most of which are relatively well explained by isostatic compensation of density constrasts within the crust and litosphere. However, not all of the densitycontrasts leading to topography reside within the litosphere. The crucial problem is how to detect the extra topography signal, in addition to that associated with both crustal and lithospheric anomalies. Among others, Forte and Perry (2000), estimate the amplitude of the dynamic topography by removal of the crustal isostatic topography signal from the surface of the Earth. Faccenna and Becker (2010) infer theequivalent dynamic topography from the normal stress generated at the surface by mantle viscous flow driven by thrmal anomalies.Here, we consider the correlation between residual topography and mantle residual gravity anomalies. As shown by Pekeris (1935) and Hager et al., (1985), the viscous mantle flow that is driven by thethermal density contrasts is responsible for the longwavelength gravity anomalies observed at the surface. They have demonstrated that the gravitational effects of surface deformation caused by the flow is opposite insign and comparable in magnitude to that of the driving density contrast. Thus, the dynamic contribution can be considered less important when mantle gravity anomalies are inversely correlated with the residual topography.Following these ideas, the correlation matrix between the residual topography and the mantle residual gravity anomalies allows us to define the regions where the sublithospheric mantle densitycontributes to surface topography (correlation coefficient equal to 1).
3D images of the European upper mantle isotropic shearwave speeds and mass densities, recently recovered by combined inversion of surfacewave information and GRACE satellite gravity data(Tondi et al., 2012) are used to select the regions where the residual topography and the residual mantle gravity anomalies are strongly correlated.
As residual topography is calculated by removing the isostatic adjustment from the topography, in this poster we compare results obtained when isostatic compensation is achieved within the crustor within the lithosphere. The 1° x 1° , recently assembled European crustal model EPCrust (Molinari and Morelli, 2011) is used to estimate the effects of the isostatic crust. The thickness of mobile lithospheric plates (LAB)is still an important question in debate. So far, it is problematic to detect and a number of different techniques to determine it have been developed. They do not necessarily lead to identical results (see e.g. Anderson, 2007,Jones et alt., 2010). Here, we consider a collection of measurements recovered from receiver function techniques (Geissler et al., 2008, Kind et al., 2012). The data set does not cover the whole region of study; however it gives important information about the Central Europe and the Mediterranean regions. To recover the residual topography, the effects of the isostatic crust/lithosphere are estimated with the Panasyuk and Hager, 2000) algorithm and subtracted from the observed elevation (ETOPO1). The mantle residualgravity anomalies are estimated as the differences between the produced gravity field of the 3D crustal/lithospheric model and the measurements observed. Eventually, we select the region of positive correlaton of the twoquantities on the 3D density model at sublithospheric (210 km) depth. Following Pekeris (1935), we assume surface uplift processes with negative density anomalies and downward pull with positive anomalies.
SURFACE TOPOGRAPHY OF EUROPE
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3D DENSITY MODEL OF UPPER MANTLEBELOW EUROPE (after Tondi et al., 2012)
MOHO DEPTH FROM EPCRUST
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MANTLE GRAVITY RESIDUALS
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CONTRIBUTION OF SUBLITOSPHERIC MANTLE TO SURFACE TOPOGRAPHY
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ISOSTATIC COMPENSATION ACHIEVED WITHIN THE CRUST
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CONTRIBUTION OF SUBLITOSPHERIC MANTLE TO SURFACE TOPOGRAPHY
EEC
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ISOSTATIC COMPENSATION ACHIEVED WITHIN THE LITHOSPHERE
Assuming that isostatic compensation is achieved within the crust, theEast European Craton (EEC) appears mostly overcompensated, withregions of uplift at the northwestern margin, where the central Norwegian Caledonides are located, and at the southern margins thatare associated with the huge DnieprDonets rift system and the AfghanTajik Basin. As observed also by Artemieva (2007), althoughcompositional variations have been incorporated into the model,downpull of the lithosphere below the East European Craton is requiredto explain the strong correlation between mantle residual anomalies andresidual topography. For the regions of uplift at the margins of the craton,the negative density anomaly in the North Atlantic mantle, which isassociated with the Iceland plume, can provide dynamic support for theNorwegian Caledonides (Japsen and Chalmers, 2000). The other twoadjacent regions indicate sites of deep adjustments of the lithosphericplates associated with Mesozoic and Cenozoic collisional events of theEurasian continent (Otto, 1997). The origin of the largescale upwellingbetween the east African and Arabian plates continues to be debated, andit is associated with the Read Sea Riftflank uplift (Wernicke, 1985), orconnected to the seismically slow and thermally buoyant megaplumestructure under South Africa (Daradich et al., 2003). The Mediterraneanregion is composed of a mosaic of blocks within the convergence regionof the Eurasian and African Plate. Downward flow (e.g. below the Hellenicslab) is associated with upwelling return flow (Adria) between the slabsand at their edges (Faccenna and Becker, 2010). For the region of mantleupwellings, close to the western Mediterranean, which extends from theCanary Islands, to Northern Africa, to the Central European VolcanicProvince, they may ultimately be derived from a common sublithosphericmantle source (Hoernle et al., 1995).Assuming that isostatic compensation is achieved within the lithosphere,other interesting regions, where upper mantle dynamics has surfaceexpression are highlighted: the uplift related to the tectonically activeregion of the Southern Balkans, the uplift of the Balearic Promontory, which can be important in the kinematics of the Western Mediterraneanand the downpull of the lithosphere along the TESZ (Trans European Suture Zone) and in particular below the northern part, the TeisseyreTornquist (TTZ) Suture, generally interpreted as a thrust of Caledoniannappes on Baltica (EEC) (Geissler et al., 2010).
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Depth sections of the density percentage perturbations in the 3D Tondiet al. (2012) density model. Model perturbations are expressed aspercentage deviations with respect to the mean velocity of each layer.AN, Anatolian plateau; AP, Arabian Platform; AS, Arabian Sea; EEC, EastEuropean Platform; Hel, Hellenic arc; HK, Hindu Kush; IP, Iranian Plateau;Med, Mediterranean Sea; Mg, Maghrebides; RS, Red Sea; SB, Sirt Basin;WAC, West African Craton
From our analysis it emerges that isostatic compensaton is most likelyachieved within the litosphere and not, within the crust. This misconceptionprobably dates back the first models of isostasy, which where proposedlong before the concept of lithosphere was formalized. Isostaticcompensation can be achieved because the lithosphere essentially floats ona relatively inviscid substrate: the weak peridotite of the asthenosphere.Changes in the buoyancy or elevation of the lithosphere are accommodatedby displacement of asthenospheric mantle (Zhou and Sandifords, 1992). However, as asthenospheric mantle is not completely inviscid (that is, itsviscosity is not negligible), dynamic topography is the topographysupported by the normal stresses generated by the viscous sublithosphericmantle.
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