Flexural modelling of continental lithosphere deformation: a comparison of 2D and 3D techniques

Abstract

The flexural isostatic response of the lithosphere in response to loading caused by continental tectonics has been modelled in 3D. The modelling approach used has been to determine hanging wall deformation following movement over a pre-defined fault surface. In addition, the lower crust is assumed to deform by a pure shear mechanism. The changes in crustal thickness resulting from these structural processes impose loads upon the lithosphere, which responds by isostatic adjustment. Algorithms have been developed to quantify the flexural isostatic response to these loads in 3D. These deflections are then superimposed upon the results from the structural modelling to generate isostatically compensated hanging wall, footwall and fault surfaces. Schematic models are presented for extensional, compressional and strike-slip deformation. Model results are dependent upon the interaction between fault geometry, displacement along the fault, which can be varied along strike, and the methodology used to quantify the flexural response of the lithosphere to loading. Emphasis has been placed upon contrasting models, which include a structural component only with those that incorporate both structural and flexural isostatic processes. Following extension, structural processes generate a relatively deep half-graben with no deformation at the basin edges. Isostatic compensation modifies this structure to produce a relatively shallow, but variable, basin depth with uplift (typically between 1 and 2 km) experienced at the basin edges. Compressional models have been generated which show the formation of large uplift structures, which are modified by isostatic compensation so that they are considerably reduced in magnitude. A regional depression (i.e. foreland basin) is also generated adjacent to the remaining uplift. Both 2D and 3D implementations of flexural isostasy have been investigated to provide insights into the validity of results provided by commonly applied 2D methods. A major advantage arising from 3D tectonic modelling is the ability to investigate the effects of oblique or entirely strike-slip components of fault movement. Strike-slip deformation has been modelled in the context of a single fault surface, which varies along strike, to show the development of pressure ridge and pull-apart basin structures. The isostatic compensation of these structures shows complex patterns of uplift and subsidence due to the interference of negative and positive loading and associated flexural deflections.