THE PACIFIC ISLAND ARCS: PRODUCED BY POST-OROGENIC VERTICAL TECTONICS?

Abstract

Geotectonics is simpler than we thought, the gross structures stare naked from the world physical chart and things really are what they look like. S. Warren Carey, 1958, p.179 Abstract A short review of some basic results of the IPOD drilling of the Pacific active margins is presented. From these it is deduced that the present conceptual models of tectonic processes operating at these margins do not seem to reflect the complex geological reality of these active regions. The direct data contradict the key concepts of active compression, progressive accretion and uplift of the fore-arc region. Instead, they have evidenced a generalized extensional regime since the Neogene and a rapid subsidence with submergence of Late Paleogene subaerial erosion surfaces in the fore-arc. The Neogene “molassic” slope deposits transgress regionally over block-faulted and eroded pre-Neogene orogenic belts. Particularly noteworthy are the post-orogenic collapse and extension of the back-arc area, which is generally centered over the median portions of the circum-Pacific bivergent orogens (i.e., the rigid zones of older chains, strongly folded and partly metamorphosed). These Tertiary collapse tectonics assume varying sizes and forms according to the relative evolutionary stage reached by the Pacific margin. The initial stages are represented by the central valleys of the Andes up to the great continental depressions of Basin and Range type. The successive stages lead to the marginal basins of the western Pacific, here classified by families, according to the age of the beginning of the oldest basin area opening. The great spatial extension and depth of the transcurrent faulting subparallel to the margins of the Pacific Basin are also highlighted. It is thought possible that such a strike-slip motion may take place also in correspondence with the Benioff seismic zones. The entity of the transcurrence phenomenon has led the author to postulate that the post-orogenic collapse of the marginal basins is accompanied by tectonic rotation and great torsions which have disrupted the Pacific perimeter. The Benioff zone is considered to become steeper and deeper in relation to the increase in surface subsidence and lateral extension of the foundering reached by the marginal basins. The depth and pattern of the “oceanization” of the margin seem to indicate a broad control by events in the mantle (e.g., existence of transient “tectonospheric” waves).