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If one takes into account active as well as remnant or inactive trenches and island arcs, the southwest Pacific island arc-trench system (New Zealand, Fiji plateau, New Guinea area, Banda arc, Philippines) is a complex of bilateral symmetrical structures. As such it is generally comparable with mountain belts; hence foredeeps and deep-sea trenches, crystalline belts and volcanic arcs, and intermediate furrows and interarc basins, respectively, are equivalent structures. The southwest Pacific island arcs, back-deeps, and interarc basins represent a postorogenic (subsequent) stage and are characterized by subsidence and inversion of former intermediate massifs. The active deep-sea trenches form narrow foredeeps in a late geosynclinal stage.
The origin of the southwest Pacific island arc-trench systems specifically and of mountain belts in general is explained in terms of the hypothesis of global vertical tectonics. The key areas for understanding the mountain belts and island arc-trench systems are the intermediate massifs, intermediate furrows, and interarc basins which represent the top of diapir-like upwelling material from the asthenosphere. These subcrustal asthenoliths are characterized by crustal thinning, extension, inversion structures, high mean heat flow, deep earthquakes, positive gravity anomalies, extrusion of mantle-derived tholeiite basalts, and intrusion of ultramafic massifs.
The forces caused by the rising of asthenoliths are primarily vertical; horizontal stresses of secondary origin are gravity controlled. Accretion ("cratonization") and destruction ("oceanization") of continental crust, extension in the interarc basins and intermediate furrows and local shortening in the deep-sea trenches and foredeeps are contemporaneous in island arc-trench systems and mountain belts. Consequently the proposed model of global vertical tectonics is incompatible with the hypothesis of plate tectonics. Global vertical tectonics explains the evolution of island arc-trench systems and mountain belts in a much simpler way than the hypothesis of plate tectonics, which needs some auxiliary assumptions to interpret the bilaterally symmetrical mountain belts and the magmatic a d geophysical phenomena behind the island arcs.
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