Complex sentence: As plate convergence proceeded, the younger and more buoyant lithosphere of the Farallon Plate subducted at a shallow angle, in what is known as a "flat slab".
Split into simpler sentences: As plate convergence proceeded, the younger and more buoyant lithosphere of the Farallon Plate started to become subducted. This caused it to subduct at a much more shallow angle, in what is known as a "flat slab".