The aim of the modelling presented in this example was to examine the gravity effect of the detaching lithospheric slab beneath the Vrancea region in Romania (Figure 1). The geometry of the slab is constrained by recent seismic tomography results derived at the Universität Karlsruhe as part of the SFB461 Project.

Figure 1 Map of Romania showing topography and earthquakes in various depth ranges from the catalogue of Romanian earthquakes (Oncescu et al., 1999).  The concentration of deeper seismicity (>70 km) in the south-east of the Carpathians reflects the break-off of the last piece of the subducting lithospheric slab beneath the Vrancea region. Symbol size is linearly proportional to earthquake magnitude (in the range 3--7.9 Mw).  Topography data are from the GTOPO30 global digital elevation model.
 

2. Summary of Geological Setting

Geological and geophysical studies of the south-eastern Carpathians are driven by the existence of intense earthquake activity in a remarkably confined volume (Wenzel et al., 1998; Wenzel et al., 1999). In this region, the Vrancea region, earthquakes within the depth range 70--180 km are confined to an almost vertical volume that is restricted to a surface area of about 30 by 80 km (Oncescu et al., 1999;Wenzel et al., 1999; Martin et al., 2001) (Figure 1). Earthquake locations also coincide with regions of fast seismic velocity in seismic tomography models of the Vrancea region (Wenzel et al., 1999; Martin et al., 2001).

The confined seismic activity is generally explained by models in which roll-back of the Carpathian subduction zone during the Miocene resulted in steepening and segmentation of the slab into small pieces (see Wenzel et al., 1999). As convergence ceased, post-collisional break-off of the slab was triggered. This break-off migrated from north to south, and each segment was progressively severed and sank into the lower mantle. Slab break-off models suggest that the last segment of the Carpathian lithospheric slab lies beneath the Vrancea region. This segment is believed to either be still attached or in the process of detaching. This scenario explains the spatially limited seismic zone in the south-eastern Carpathians (Girbacea & Frisch, 1998; Chalot-Prat & Girbacea, 2000). Aspects of the slab break-off model have been disputed (Pana & Morris, 1999), but at present, the break-off model appears to be favoured by most researchers (e.g. Matenco et al., 1997; Nemcok et al., 1998; Sperner et al., 2001; Sperner et al., in press). More recently, discussion has turned to the mechanism of slab break-off (e.g. Sperner et al., 2001): is it abrupt break or a gradual stretching of the slab into the mantle?

3. Modelling Procedure

The gravity model of the Vrancea slab is based on a density model derived by Dr. Alik Ismail-Zadeh. This density model is derived from the tomographic model using pressure-dependant (but not temperature-dependant) velocity--density relationships derived by Krasovsky (1981). Various sections through the resulting density model are shown in Figure 2. These figures show density contrast (in g/cm³) relative to a background density model. Red colours represent high density contrasts that coincide with fast seismic velocities in the tomography model. Therefore, the red colours are representative of the geometry of the Vrancea slab.

Figure 2 (Left) Horizontal slices through the Vrancea density model for the depths indicated. The sections are centred on 26°15'E/45°30'N and the sides of the boxes represent 1000 km. (Right) Vertical east--west sections through the Vrancea density model for the Y (latitude) coordinate shown. The lower left image shows the background reference density model. Depth axis represents 350 km (the depth extent of the tomographic model). [Click on the images for a more detailed view].

For modelling in IGMAS, the "edge" of the slab was assumed to coincide with a density constrast of 0.025 g/cm³. On each of the vertical cross-sections shown in Figure 2 (right), the coordinates of this contour line were extracted and inserted into an IGMAS input file. The series of cross-sections were then triangulated by IGMAS to produce a 3D body representative of the slab (Figures 3 and 4). In the 3D views of Figure 4, the top of the slab appears to be deeper in the north-east. This is the region where the slab is currently detaching, but the deeper slab-top is misleading --- the "gap" reflects poor resolution of the tomographic model in this region. In fact, above about 70 km, the tomographic model is poorly resolved and does not give a good representation of the slab geometry.

Figure 4 Perspective views of the 3D Vrancea slab density model. (left) view toward the north-west; (right) view toward the south-east. In both images, the depth axis (green) represents 350 km depth. These views of the slab are generated directly from IGMAS as IVIS3D files.

4. Results

The computed gravity and geoid effect of this model slab were then compared to observed values (Figure 5). The gravity effect predicted for the Vrancea slab is about 20 mGal, and the geoid effect about 5 m. Figure 5 shows that the gravity effect of the slab is not obvious in the Romanian gravity field, but when the slab gravity effect is removed from the observed Bouguer anomalies, the signature associated with the Carpathian foredeep (most negative Bouguer anomalies) is modified. It is possible that this modified anomaly pattern better reflects the geometry of the foredeep basin.

Figure 5 (top left) EGM96 geoid for Romania (degree 2--360); (top right) Bouguer anomalies from Romania (after Ioane & Atanasiu, 1998); (bottom left) Predicted geoid heights from the Vrancea slab; (bottom right) Predicted Bouguer anomalies from the Vrancea slab.

References:

Chalot-Prat, F. & Gîrbacea, R. (2000). Partial delamination of continental mantle lithosphere, uplift-related crust--mantle decoupling, volcanism and basin formation: a new model for the Pliocene--Quaternary evolution of the southern East-Carpathians, Romania, Tectonophysics, 327, 83--107.

Gîrbacea R. & Frisch, W. (1998). Slab in the wrong place: Lower lithospheric mantle delamination in the last stage of the Eastern Carpathian subduction retreat, Geology, 26 (7), 611--614.

Ioane, D. and Atanasiu, L. (1998). Gravimetric geoids and geophysical significances in Romania, in Ioane, D. (ed.), Monograph of Southern Carpathians, Reports on Geodesy, 7, pp. 157--175, Institute of Geodesy and Geodetic Astronomy, Warsaw University of Technology, Warsaw, Poland.

Krasovsky, S. S. (1981). Reflection of continental-type crustal dynamics in the gravity field, Naukova Dumka, Kiev, 268p (in Russian).

Martin, M., Wenzel, F., Achauer, U., Kissling, E., Mocanu, V., Musacchio, G. & Radulian, M. (2001), High-resolution images of a slab detachment process, submitted to Seismological Research Letters.

Matenco, L., Zoetemeijer, R., Cloetingh, S. & Dinu, C. (1997).  Lateral variations in mechanical properties of the Romanian external Carpathians: inferences of flexure and gravity modelling, Tectonophysics, 282, 147--166.

Nemcok, M., Pospisil, L., Lexa, J. & Donelick, R. A. (1998). Tertiary subduction and slab break-off model of the Carpathian--Pannonian region, Tectonophysics, 295, 307--340.

Oncescu, M. C., Marza, V. I., Rizescu, M. & Popa, M. (1999). The Romanian earthquake catalogue between 1984--1997, in Wenzel, F., Lungu, D. & Novak, O. (eds.), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation, Kluwer Academic Publishers, Dordrecht, Netherlands, pp. 43--47.

Pana, D. & Morris, G. A. (1999). Slab in the wrong place: Lower lithospheric mantle delamination in the last stage of the Eastern Carpathian subduction retreat: Comment, Geology, 27, 665--666.

Sperner, B., Lorenz, F. P. , Bonjer, K.-P. , Hettel, S., Müller, B. & Wenzel, F. (2001), Slab break-off --- abrupt cut or gradual detachment? New insights from the Vrancea region (SE Carpathians, Romania), Terra Nova, 13, 172--179.

Sperner, B. , Ioane, D. & Lillie, R. J. (in press). Slab dynamics and its surface expression: new insights from kinematic gravity modelling in the Eastern Carpathians, Tectonics.

Wenzel, F., Achauer, U., Enescu, D., Kissling, E., Russo, R., Mocanu, V. & Musacchio, G. (1998). Detailed look at final stage of plate break-off is target of study in Romania, EOS Transactions American Geophysical Union, 79 (48), 589--594.

Wenzel, F., Lorenz, F. P., Sperner, B. & Oncescu, M. C. (1999). Seismotectonics of the Romanian Vrancea area, in Wenzel, F. and Lungu, D. and Novak, O. (eds.), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation, pp. 15--25, Kluwer Academic Publishers, Dordrecht, Netherlands.
 

This modelling was conducted with support from the Deutscher Akademischer Austauschdienst (DAAD).