Cimmerian block detachment from Gondwana: A slab pull origin?

The Cimmerian blocks are ribbons-like micro-continents that detached from the northern margin of Gondwana during the closure of the Paleotethys ocean at the end of the Permian. While it has been hypothesized that slab pull forces induced by subduction of the Paleotethys beneath Eurasia could have driven extension in the lower plate after the Paleotethys mid-oceanic ridge subduction, so far, numerical modeling studies have concluded that these forces alone are not sufficient to drive continental breakup. However, the previous modeling studies neglect olivine phase transitions in the mantle and likely underestimated the slab pull. Here, we present numerical simulations that consider the evolution of the density of the slab associated with metamorphic reactions and analyze carefully when, where, and how the slab pull can lead to the formation of micro-continents. Our results show that the marked increase in slab density related to olivine phase transitions at 410 km discontinuity, in addition to gravitational potential energy, can cause continental rifting of the lower plate. Nonetheless, continental breakup is only achieved in absence of horizontal shear flow at the base of the continental lithosphere. In these successful experiments, the deformation localizes 100-200 km on shore due to bending stress pointing out that this process probably always leads to the formation of small continental ribbons. Our results confirm that the tensional force induced by the Paleotethys subduction dynamics may have caused the detachment of the Cimmerian blocks. Since continental breakup of the lower plate is promoted in the absence of basal shear below Gondwana's lithosphere (with respect to the underlying mantle), we argue that the formation of the Cimmerian blocks provides information on the relative motion between the tectonic plates and the underlying mantle. Our simulations demonstrate that there is a significant time-lag between ridge subduction and continental breakup (i.e. the time needed for the slab to reach the 410 km discontinuity), which is rarely accounted by paleogeographic reconstructions.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the detachment of Cimmerian blocks from the northern margin of Gondwana during the closure of the Paleotethys ocean. It suggests that the subduction of Paleotethys beneath Eurasia induced tensional forces that led to the formation of micro-continents. The study also emphasizes the importance of considering olivine phase transitions in numerical simulations of plate tectonics.