sentences:  The arc stabilized until about 30 Ma, when it began to rift to form the Parece Vela Basin.Spreading also began in the northernmost part of the IBM arc about 25 Ma and propagated south to form the Shikoku Basin.Parece Vela and Shikoku basin spreading systems met about 20 Ma and the combined Parece Vela Basin-Shikioku Basin continued widening until about 15 Ma, ultimately producing Earth's largest back-arc basin.