A Reappraisal of Pacific-type Orogeny and Subduction Erosion

The accepted concept of orogeny is based on the Theory of Plate Tectonics and implies two types of orogeny: collision-type (C-type) and Pacific-type (P-type). Pacific-type oro- genic belts are directly linked with Pacific-type convergent margins (PCM) existing over subduction zones, where oceanic lithosphere is submerged under intra-oceanic arcs or active continental margins. PCMs are major sites of new crustal growth through juvenile arc magmatism and accretion of upper strata of the subducting oceanic lithosphere and destruction by subduction erosion. The subduction erosion destroys formerly formed mag- matic arcs and accretionary prisms. Related Pacific-type orogenic belts thus keep records of both juvenile crustal growth and subduction erosion. There have been found many sites of ongoing subduction erosion in the Circum-Pacific, but it is harder to find evidence for that of fossil Pacific-type orogenic belts. Recent studies of several accretionary complexes in the Central Asian Orogenic Belt (CAOB), the largest in the world Paleozoic to Mesozoic accretionary foldbelt, have found several probable cases of subduction erosion once hap- pened at active margins of the Paleo-Asian Ocean, an old analogue of the modern Pacific Ocean. Evidence for that comes from magmatic lulls as recorded in U–Pb detrital zircon age distributions of arc-derived greywacke sandstone. The modern and fossil cases of sub- duction erosion are still waiting for their quantitative estimations in order to contribute to our knowledge about the global balance of crust gains and losses.