Large scale detachment folding of thermally softened crust within a closing orocline in the Chinese Altai - insights from analog modeling

The ribbon-like Altai accretionary sedimentary wedge, representing the SW exteriors of the the Tuva-Mongol Orocline, suffered important Devonian and Permian deformation, metamorphism and melting. The last Permian deformation was associated with massive lower crustal melting, granulitization and lateral lower crustal flow of anatectic material. This lateral transfer was controlled by upwelling of the mantle below the extended parts of the crust. The subsequent Permian shortening led to development of a series of crustal scale detachment folds cored by migmatite-magmatite complexes and surrounded by weakly metamorphosed rocks in marginal synforms.

 

The current study aims to understand the geometry, kinematics and dynamics of such large scale folding in the Chinese Altai during compression of thermally softened crust confined in the Tuva-Mongol Orocline. In such a setting, the angle of convergence is progressively increasing during collision, as the curvature of the orocline increases. To visualize and quantify this process, we employed analog modeling by using paraffin wax for ductile lower crust and sand-cenosphere mixture for brittle upper crust. The model domains (60cm×70cm×3cm) are preheated for 15 hours to attain a stable initial thermal and rheological gradient. The base of the models sustains the temperature at 51 °C (the melting point for the paraffin wax) while the top part of the model is heated to 48 °C by convective air. Strain in the models is quantified from the top view using the stereoscopic digital image correlation system from Lavision GmbH. The models are shortened by movement of indenter wall driven by a step-motor. Three series of experiments were designed to simulate the above detachment folds. In the first series of models, the indenter wall is perpendicular to the shortening direction. In the second scenario, the indenter wall is initially obliquely oriented to the shortening direction. As for last scenario, the angle of convergence α (defined as the angle between the plate motion vector and the plate boundary) is continuously increased from initial 60° to 90°. This last mode mimics the effect of the closing orocline confining the thermally softened crust. All models display progressive development of an array of folds with crestal grabens that are cored by molten and partially molten wax. We describe how the style of folding, degree of strain partitioning and distribution of transcurrent movements differ between the modes of convergence.