OROGENIC BELTS THROUGH TIME
Orogenic belts comprise present and past great mountain belts of the Earth. Most belts reflect continent-continent or continent-island arc collisions. Most structures in orogenic belts are subhorizontal, and most belts are two-sided, with thrusts over forelands on both sides. Orogenic belts form during assembly and breakup of supercontinents, but most continental fragments do not converge and collide with pieces away from which they rifted. Modern tectonic activity in the Western Pacific illustrates intraoceanic complexity that may be assembled between two collided continents. Circumpacific deformed belts represent partially developed orogenic belts, pending the ultimate closure of the Pacific Ocean. In convergence and collision, continents converging along subduction zones sweep before them and collect intervening island arcs, other oceanic islands, and microcontinents. Orogenic belts exhibit ophiolites (oceanic crust preserved by early continent-"reverse dip" subduction zone collisions), remnants of oceanic collisions, rifts, subduction zone flips, multiple sutures, intraoceanic strike-slip faults, "pre" and "syn-orogenic" granitic intrusions and arc deposits. Collided pieces adjust to continental margin shapes by local strike-slip faults and thrusts of possible opposite sense. Major subhorizontal thrust complexes cut all rocks. Mountain roots and major topographic welts form and are worn down by chemical changes and mantle-crust adjustment, and exhumation and erosion, respectively. Canadian Shield observations (Hammer et al., 2011) indicate that similar processes have operated since the Neoarchean (2.7 Ga). 2.7-1 Ga magmatic oceanic crust was thicker; ophiolites are present in orogenic belts as thrust slices of mostly magmatic rocks ("greenstone belts") with only rare mantle tectonites.