What force caused the crust to subside in the left hand figer?

The crust in the left-hand figure likely subsided due to tectonic forces, specifically the downward movement associated with extensional tectonics or subsidence caused by sediment loading. This can occur when geological processes, such as the weight of accumulated sediments or volcanic activity, cause the Earth's crust to sink. Additionally, tectonic plate interactions, such as divergent boundaries, can contribute to this subsidence.