ABSTRACT A new and simple integrated approach is proposed for qualitatively unravelling the crustal thickness of fossil magmatic systems based on the chemical and thermal records in amphibole-bearing magmatic rocks. Statistical analyses applied to a large multidimensional amphibole database show that Ti-rich and Si-poor magmatic amphiboles, which formed at high-temperature (T) conditions (>950 °C), were dominantly developed in basaltic to basaltic-andesitic (SiO 2 -poor, i.e., <55 wt%) magma within relatively thin crust (5–10 km). We find that for crustal thicknesses larger than 10 km, the occurrence of high-T amphiboles and basaltic magma decreases with increasing crustal thickness. This is because of mineral filtering in “mature” deep crustal hot zones that occur at the crust-mantle boundary (Moho). Given that subducting plates exert a direct control on the structural evolution (shortening or extension) of the overriding plates, probing crustal thickness in the past provides first-order information on the geodynamic processes that took place at plate margins. Using this approach, we document the progressive buildup of a thick (>40 km) Jurassic to Cretaceous accretionary belt along the circum-Pacific orogenic belts that bounded the Panthalassa Ocean. The destruction of this thick belt started at ca. 125 Ma and was initially recorded by the thinnest magmatic systems hosting amphibole-bearing magma. Thinning of the circum-Pacific orogenic belts became widespread in the northern regions of western America and in the western Pacific after ca. 75 Ma, possibly in response to oceanic plate segmentation, which triggered slab rollback and overriding plate extension.

Four popular tectonic models are discussed that attempt to explain the formation of arc-backarc systems. These systems develop in a convergent setting with shortening in the forearc region, extension in the backarc region, and progressive out-bowing of the arc. The models include the gravitational collapse model, the rollback model, the extrusion tectonics model, and the orogen-parallel compression model. The rollback model can explain the progressive development of most arcs, such as those found in the Western Pacific and the Mediterranean, in combination with backarc extension. Slab rollback ultimately is a consequence of the negative buoyancy of the slab. Collapse models can explain radial thrusting in the foreland and extensional deformation in high-standing mountain belts, since these regions involve large potential energy contrasts between mountain range and foreland. However, these models cannot explain the development of Western Pacific and Mediterranean style arc-backarc systems. In such settings, the extending region has a small potential energy, which cannot drive arc formation, fore-arc shortening, and backarc extension. The extrusion tectonics model can explain strike-slip structures such as those observed in the Eastern Alps, Anatolia, and East Asia, but fails to explain backarc extension. The extrusion of a wedge cannot produce shortening at its leading edge contemporaneously with extension in the middle of the wedge. The orogen-parallel compression model cannot explain arc formation and backarc extension, since an orogen does not behave elastically in the plane of the lithosphere at a length scale of ~≥1000 km.