Complex sentence: Magmatism migrated away from the trench, moving northeast towards the Gulf of Mexico--giving the arc its characteristic E-W orientation, the inland push of the arc showed progressively drier melting, and eventually slab melting began to occur--suggesting flattening of the subducted slab.
Split into simpler sentences: Magmatism migrated away from the trench, moving northeast towards the Gulf of Mexico--giving the arc its characteristic E-W orientation. The inland push of the arc showed progressively drier melting, and eventually slab melting began to occur--suggesting flattening of the subducted slab.