The Role of Isostasy in the Evolution and Structural Styles of Fold and Thrust Belts

Ibrahim, Youseph1, Rey, A/ Prof. Patrice1

1University Of Sydney, Sydney, Australia

Fold and thrust belts (FTB) are highly deformed regions that form as the crust accommodates shortening. The evolution of FTB’s records the dynamic interplay between crustal and surface
processes, in conjunction with the rocks’ intrinsic properties. The stacking of thrust sheets and mass transfer of sediment during orogenesis imposes a load on the lower crust and the mantle underneath, inducing isostatic adjustment and a flexural response, which may also contribute to the overall architecture of FTB’s. The tempo at which a fold and thrust belt forms is a consequence of plate kinematics. The tempo of the isostatic response, however, is reliant on the rheology of the mantle and the elastic thickness of the crust. Here, we focus on the role isostasy plays in controlling structural style in FTB’s. We run two-dimensional, coupled thermal and mechanical, numerical experiments using the Underworld framework to explore the interplay between the rate of compression and the rate of isostasy on the structural evolution of FTB’s.

The numerical model runs in a cartesian domain by solving the conservation of energy, mass, and momentum equations. The numerical domain is 42 km wide and 16 km tall, with a grid resolution of 80 m. From top to bottom, the model consists of ‘sticky air’, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a virtual basal layer, which allows us to implement a local ‘psuedo-isostasy’ boundary condition. Models are run with varying compressional velocities and isostatic rates.

Our suite of models demonstrates the relationship between tectonic and isostatic rates. When the tectonic rate is greater than the isostatic rate, subsidence or flexure is post-tectonic mainly, and
therefore isostasy is unlikely to play a role in the development of the FTB, however, it may modify its architecture post-loading. Alternatively, when the tectonic rate is slower than or equal to the isostatic rate, subsidence will keep pace with tectonic loading. In this scenario, isostasy plays an important role in the development of FTB’s, influencing the topographic elevation generated, the outward extent of the FTB, and thrust fault angles.


Youseph is a first year Ph.D. student at the University of Sydney studying the evolution and structural styles of fold and thrust belts in Central Australia and Papua New Guinea.

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