Evidence for the thickening of the lithosphere beneath accretionary continental crust from the subsidence of intercontinental basins

The relationship between the thickness, age and bathymetry of the oceanic lithosphere is well established. Our aim is to investigate whether such a relationship exists for areas of juvenile continental lithosphere. Accretionary crust is formed from the collision of island arcs, accretionary complexes, fragments of reworked older crust and other mobile terranes at subduction zones. It therefore has a thin lithosphere due to subduction related processes such slab driven convection and the formation of accretionary prisms without any initial mantle lithosphere. Once the accretion has finished we predict that the lithosphere cools and thickens over time producing widespread subsidence. To investigate this process we backstripped large basins situated on accretionary crust in North Africa to give the water loaded tectonic subsidence. This was compared to results from a 1D numerical plate cooling model similar to lithospheric cooling models for the oceans. The topography of the 1D column is calculated by isostatically comparing it to a mid ocean ridge. The crustal composition and structure used in the model has been varied around average values of accretionary crust to represent the heterogeneity of accretionary crust. The model fits the backstripped subsidence curves best with a 30 km thick crust and a model thickness of 155 km which causes the 1200 °C isotherm (chosen to represent the base of the lithosphere) to settle at 124 km. Lithospheric growth may also occur where the lithosphere has initially been thinned by a hot spot in the asthenosphere. One area where this may occur is the West Siberian Basin. We used our model to look at lithosphere growth in the situation where the lithosphere is not only initially thin, but also has a temperature anomaly directly beneath it. We found it was possible to fit a backstripped subsidence curve from within the Urengoy rift and one from the wider basin using a 50 km thick initial temperature anomaly of 1500 °C and a final lithospheric thickness of 150 km, but varying the crustal thickness from 28 km in the rift to 34 km outside the rift. The subsidence produced by the model is most sensitive to the thickness of the continental crust and the final thickness of the lithosphere. However, there is not a direct trade off between the two because only varying the final lithospheric thickness alters the timescale subsidence occurs over. The model indicates that the lithosphere reaches a stable thickness after 250 Myrs. Interestingly this is different in our two study areas. The model does not show why, though it could be due to different crustal compositions or dynamics in the upper mantle. Conversely this work also shows that thickening of the lithosphere beneath continental crust is a potential basin forming mechanism.