two principal contrasting tectonic theories were published in greater detail between 1972 and 1992 as related below. one theory involved closure of a back-arc basin between the western continental margin and a volcanic arc over an east-dipping subduction zone. a second theory involved collision of the continent with an island arc above a west-dipping subduction zone. both were based on the basic understanding that the western facies assemblage is composed of oceanic deposits and that it is underlain by an extensive thrust fault.