Modes and geometry of crustal-scale detachment folds in hot orogens – insights from physical modeling

The concept of detachment folding was developed between the '60s and '80s and generally describes displacement and buckling deformation of a competent layer above a weak, usually low viscosity horizon during tectonic shortening. From this definition, and based on the Biot-Ramberg theory, it is clear that geometrical parameters of such folds depend on contrasting rheology in both layers or on the rheological gradient in a complex multilayer. These systems were originally studied in association with the thin-skinned deformation and salt tectonics, and recently with regards to large-scale lithospheric deformation. In the latter, the rapidly heated lower crust is partially melted and a thin melt layer at the MOHO depth serves as the detachment horizon during collision and shortening. 

Our experimental work contributes to the understanding of the geometrical, kinematic and dynamic behaviour of such types of detachment folds as this deformation process strongly depends on a thermally dependent rheological gradient and nonlinear shortening velocity. The natural prototype for our models is for example the Chandmann or Bugat metamorphic domes in central Asia (CAOB). Our aim is to parametrize the style of such crustal-scale detachment folds depending on the rheological properties of the layered crust and the thermal gradient.

For this purpose, we developed an apparatus for thermal analogue models capable of producing thermal gradients and programmable shortening. Paraffin wax is used as the analogue for the partially molten lower crust. The advantage of this material is, that it reproduces the temperature-controlled rheological stratification of the crust in hot orogens with a melt layer at the bottom (at Moho) superposed by partially molten crust. The upper crust is represented by a granular mixture of low-density cenosphere particles and silica sand, respectively. To keep the models properly dynamically scaled, we take into account the relationship for progressively decelerated plate convergence in the orogens. 

With increasing of both, basal heating and shortening rate, the folds' finite geometry converges to a system of pseudo-symmetric folds, cored by various amounts of the melt with respect to their position in the fold sequence. The dynamics of the fold amplification also depends on the position in the array of folds and is described by four evolutionary steps; initial perturbation, amplification with the melt inflow into axial zones of the folds, locking and simple vertical extrusion. 

With decreasing intensity of basal heating, the total melt amount is lower, deformation is more localized and converges to brittle-ductile coupling. Typical products are thrust systems on a local scale or pop-up structures on a large scale. Melt is localized in form of small fingers underneath the pop-up structures. Relatively colder and slower models display homogeneous thickening.

A higher degree of heating results in melt redistributed along the axial planes of folds. Analysis of the layer interfaces curvature, paths and tortuosity of the material particles in these high-temperature experiments (based on resultant displacements calculated by the PIV method) also revealed asymmetrical evolution of the P-T-t paths for associated limbs of the pseudo-symmetrical folds.