Alpine and Pacific styles of Phanerozoic mountain building: subduction-zone petrogenesis of continental crust

The geological complexities of contractional orogenic belts have been studied for two centuries, yet our understanding of them is still rapidly changing. Most mountain belts form at or near the active edges of continents and/or fringing island arcs. Virtually all result from consumption of lithospheric plates, involving the underflow of oceanic crust, and the consequent transport and descent of spreading centres, oceanic plateaus, exotic island arcs, far-travelled terranes, microcontinents and salients of continental crust. Long-continued subduction allows the construction of a calcalkaline volcanic–plutonic arc on the stable, hangingwall plate. No two orogenic belts are identical. Indeed, most mountain chains are unique, and exhibit important structural and petrological contrasts along their lengths. A few contain mineralogical relics reflecting ultrahigh pressure (UHP) stages of recrystallization, but reflecting thorough and complete retrograde reaction, many other compressional mountain belts which may have been subjected to similar P–T conditions of metamorphism fail to ABSTRACT