Modelling Deformation at Plate Boundaries

There is growing understanding that processes at plate boundaries control the long-term deformation of the lithosphere and influence plate motion, and therefore may also significantly affect global convection. The type examples of deformation at convergent (Andean type) and continental transform plate boundaries (Dead Sea Transform and San Andreas Fault system) were analysed within several interdisciplinary projects (SFB 267, DESERT/DESIRE, ICDP-SAFOD) using advanced thermomechanical modelling techniques dealing with highly nonlinear rheology and involving high-performance parallel computing. The modelling was constrained by the comprehensive interdisciplinary field observations collected in those projects.

Deformation at Convergent Plate Boundary: Modeling Subduction Orogeny Mountain building at convergent plate boundaries is among key orogenic processes. The type example of the orogeny at an active continental margin is the late Cenozoic mountain building in the central Andes at the central part of the Pacific margin of South American plate (Fig.1). The South American plate is drifting westwards, at a rate that has increased from ~2 to 3 cm/yr over the last 30 million years, over the Nazca plate, which is subducting eastwards at about 5 cm/yr. The Andean mountain belt stretches along the entire western margin of the South American plate where it overlies the subducting Nazca plate. The Altiplano-Puna plateau of the central Andes, which is the second greatest plateau in the world, has resulted from up to 300 km of late Cenozoic, crustal shortening at the western edge of the South American plate (Hindle et al., 2005). This shortening generated unusually thick, hot and felsic, continental crust. In contrast to the central Andes, no high plateau exists in the northern and southern Andes (Fig.1), where only minor (less than 50 km) tectonic shortening has been reported.
The key question about Andean orogeny is why the high plateau has developed only in the central Andes and only in Cenozoic times (mostly during the last 30 million years), even though the Nazca plate has been subducting along the entire western margin of South America for more than the last 200 million years. In order to answer this question we used coupled, thermo-mechanical, numerical modelling of the dynamic interaction between subducting and overriding plates (Sobolev and Babeyko, 2005; Sobolev et al., 2006) constrained by numerous geological and geophysical observations collected within the SFB 267 project.

The general conclusion from this modelling is that the tectonic shortening of the overriding plate at the convergent plate boundary is mostly controlled by the overriding rate but not by the subduction rate, as was previously thought. The higher the overriding rate is, the larger is the motivation for the shortening of the overriding plate. However, significant shortening occurs only if in addition to the high overriding rate, there is relatively high coupling at plates interface and the crust has critical thickness of about 45 km allowing for eclogitization of the lower crust. All these conditions were met in the Central Andes in the late Cenozoic time, but not in the Southern Andes, which explains the dramatic difference in the evolution of the Central and Southern Andes. The two GFZ papers presenting thermomechanical model of the orogeny at an active continental margin, published in Geology (Sobolev and Babeyko, 2005; Babeyko and Sobolev, 2005) have been highlighted by Nature (vol. 436, p.756, 2005).

Deformation at Convergent Plate Boundary: Modeling Deformation in Andean Back Arc To answer the question why are the Altiplano and Puna segments of the central Andes different, we have run several numerical experiments focused on the deformation in the Andean back-arc (Babeyko and Sobolev, 2005). Fig. 2 shows the finite strain distribution of two models, replicating well the deformation patterns in the Altiplano (Fig. 2a) and Puna (Fig. 2b) segments of the central Andes along with corresponding geological cross sections. The significantly different deformation styles seen in the models of Fig. 2 result from the two segments differing in upper crustal strength (weak sediment in the Altiplano foreland and no such sediment in the Puna foreland) and lithospheric temperature (cold lithosphere in the Altiplano foreland and warm in the Puna foreland) in agreement with previous expectations based on geological arguments (Allmendinger and Gubbels, 1996).

Deformation at continental transform plate boundary Another fundamental type of plate boundary is transform boundary. Of particular interest is the dynamics of transform boundaries crossing continental lithosphere, as their activity generates powerful earthquakes strongly influencing human activities. The type example of such a boundary is the Dead Sea Transform which accommodates about 105 km of left lateral displacement between the Arabian plate and the Sinai sub-plate during the last 15-20 Myr (Fig.3). Recently the DST was much studied in the framework of the DESERT multi-disciplinary project and its continuation DESIRE, focussed on the Dead Sea Basin.

The thermomechanical modelling of the DST (Sobolev et al., 2005) demonstrated that the typical narrow rift-like structure of the DST results from localization of strike-slip deformation in the initially cold lithosphere with variable crustal thickness when there is an additional small (few km) extension. In this case the shear strain is localized in a sub-vertical shear zone, which crosses the entire lithosphere. In the upper crust the strike-slip deformation localizes at one or two major vertical faults located at the top of this zone. The width of this zone in the lower crust and upper mantle (20–40 km) is controlled by shear heating and temperature- and stress-dependence of the viscosity of the rocks (right-side inset in Fig.3). If the left-lateral transform fault deviates (or jumps) to the left, this creates a pull-apart basin, whose thickness is controlled by the strength of the lithosphere. The unusually large length and sediment thickness of the Dead Sea basin, the classical pull-apart basin associated with the Dead Sea Transform, can be explained by 100 km of strike-slip motion and a thick (20–22 km, up to 27 km locally) brittle part of the cold lithosphere beneath the basin (Petrunin and Sobolev, 2006).

References  

• Babeyko, A. Y. and Sobolev, S.V. (2005) Quantifying different modes of the Late Cenozoic shortening in the Central Andes, Geology 33, 621–624.
• Babeyko, A.Y., Sobolev, S.V., Vietor, T., Trumbull, R.B. and Oncken, O. (2006) Weakening of the Upper Plate during Tectonic Shortening: Thermo-Mechanical Causes and Consequences. In: Oncken O et al. (eds) The Andes – Active Subduction Orogeny. Frontiers in Earth Sciences, 1, Springer, 495-512.
• Hindle, D., Oncken, O., Kley, J., Sobolev, S. (2005) Crustal balance and crustal flux from shortening estimates in the Central Andes, Earth Planet. Sci. Lett., 230, (1-2), 113 – 124.
• Petrunin, A and Sobolev, S.V. (2006) What controls thickness of sediments and lithospheric deformation at a pull-apart basin? Geology, 34 (5) 389-392.
• Sobolev, S.V. and Babeyko, A.Y. (2005) What drives orogeny in the Andes? Geology 33, 617–620.
• Sobolev, S.V., Petrunin, A., Garfunkel, Z., Babeyko, A.Yu., and DESERT Group (2005) Thermo-mechanical model of the Dead Sea transform: Earth Planet. Sci. Lett., 238, 78-95.
• Sobolev, S.V., Babeyko, A.Y., Koulakov, I. and O. Oncken (2006), Mechanism of the Andean orogeny: insight from the numerical modeling, In: Oncken O. et al. (eds) The Andes – Active Subduction Orogeny. Frontiers in Earth Sciences, 1, Springer, 513-535.

Surface topography and major structural features of the Andes. The trench adjacent to the high central Andes has no sedimentary fill, which may increase friction in the subduction channel. Insets show results of thermomechanical modelling for cross-sections through the high Central Andes (upper inset) and Southern Andes (lower inset).


Models and observations for two cross sections in the central Andes: (a) the Altiplano and (b) the Puna. Geological sections are from Kley et al. (1999). Grey colours in the model cross sections indicate the magnitude of the cumulative, finite-strain norm. Note the significant difference in the deformation styles between the Altiplano and Puna segments, and the good correspondence between the models and the geological data.


Surface topography and major structural features of the Middle East. Insets show results of thermomechanical modelling (recent finite stain) for the cross-section between the Red Sea and the Dead Sea (right-side inset) and a model of the strain rate distribution in the pull-apart basin like the Dead Sea Basin (left-side inset).