Complex sentence: Prior to the collision of Patagonia, the southwestern margin of Gondwana (which contained the modern day South American landmass) consisted of the autochthonous Rio de la Plata craton and a number of accreted terranes, as indicated by paleomagnetic studies.
Split into simpler sentences: Prior to the collision of Patagonia, the nucleus of modern day South America was contained within a portion of the southwest margin of Gondwana. This margin consisted of the ancient Rio de la Plata craton and a number of accreted terranes, whose boundaries have been discovered using paleomagnetic studies.