Fold interference pattern and crustal decoupling in northern Tanzania

Abstract

High-temperature granulite facies metamorphism and deformation evolved through Neoproterozoic tectonic processes in large parts of southeastern Africa. The Eastern Granulites of Tanzania, one example for these extreme conditions, show progressive decoupling between lower and middle crust together with progressive cooling and rheology stratification. Coupled deformation during earliest stages of deformation arises from extremely small viscosity contrast of all minerals and rocks at granulite facies metamorphic conditions around 850 °C. With cooling of the orogen rheology stratification increases and deformation decouples into horizontal stretching through gravity driven channel flow in the lower crust and thickening and horizontal west-east shortening in the middle/upper crust. As soon as a stiff and cold indenter, represented by external parts of the Archean Tanzania Craton (Western Granulites) enters the system, the indenter acts as lower crustal ramp along which deep crustal units are exhumed and displaced towards the foreland. During this orogenic period the crust was fully stratified and a second phase of decoupling, detachment over crustal scale ramp, occurs. Superposition with late orogenic north-south shortening structures resulted in type 1 fold interference pattern and formation a of dome-and-basin geometry in northeastern Tanzania. Structural modelling of fold superposition helps to explain the complex structural pattern and to predict sites of decollement horizons. By comparison with fold interference pattern exposed in southern Madagascar we define different types of dome-and-basin formation. Those with neglectable rheology contrast are viscosity determined where mixing of upper and lower crustal material is enabled by vertical flow folding. Those with strong viscosity contrast are mechanically determined where domes and basins are formed by changes in regional principal stress directions.