Effects of the lower crust on slab detachment – a case study in the Hindu Kush

Earthquake nests are defined as volumes of intense intermediate-depth seismicity which are isolated from any surrounding seismic activity. The high seismic activity within these earthquake nests occurs continuously and thus sets them apart from other seismic sequences such as earthquake swarms or aftershocks. These intermediate-depth earthquakes cannot be explained by the same causes as shallow earthquakes. Instead, they are often linked to slab detachment (e.g. in the Hindu Kush).

To constrain the conditions at which these large intermediate-depth earthquakes occur, numerical models are required to better understand their tectonic environment. Here, we use two-dimensional thermomechanical models with a nonlinear visco-elasto-plastic rheology were to determine the deformation state and the controlling mechanisms of the detachment process.

In this study, we focus on the question how the viscosity ratio (η_lith /η_lc) between the lithosphere and the lower crust and the depth d_lc to which lower crust may have been subducted influence the subduction process. Both is poorly constrained for the Hindu Kush. To this end, we varied the viscosity ratio η_lith /η_lc between 0.01 and 1000 and the subduction depth of the lower crust d_lc between 160 km and 240 km. We obtained detachment depths ranging from 110 km to 470 km, which fall within the range of the Hindu Kush earthquake nest, extending up to 280 km. The deformation behaviour from the 264 models can be classified into five different regimes based on stress, strain rate, detachment depth, and coupling between subducting and overriding plate. The five regimes represent the dependency of the detachment depth (d_det) to its viscosity ratio (η_lith /η_lc). Detachment in regime two is enhanced via shear heating and detachment in the other regimes occurs via necking. The relationship between lower crustal depth and detachment depth varies by model category. This variability reflects the complex influence of the “lubrication effect” of a weak lower crust and the limitation of subduction depth governed by its rheological properties.