SYNCONVERGENT EXTENSION IS DRIVEN BY SUBDUCTION

Subduction mechanics require common intra-arc and backarc extension (e.g., Paleogene western Cordillera). Subduction hinges roll back into incoming oceanic lithosphere, slabs sink more steeply than they dip, and the dominant regime in overriding plates and arcs is extensional. This is shown by geometric evolution of arcs and backarcs, narrowing of the Pacific, scarcity of forearc crumpling, geodesy, and more. Slabs do not slide down slots from fixed hinges.

Subduction hinges are broad curves back under overriding plates, not sharp inflections at trenches. Leading parts of overriding plates are thinly tapered above gently inclined oceanic lithosphere and usually bear forearc basins (e.g., California Cretaceous-Paleogene and Oregon Eocene-Quaternary) that disprove crumpling. Forearcs float along behind retreating hinges and above pancaking slabs, from which they commonly are separated by melange. Forearc shortening, and subduction erosion from fronts and bottoms of overriding forearcs and arcs, do occasionally occur; e.g., early Paleogene SW U.S.

Where old and dense enough to sink that far, slabs lie down on, and seldom or never penetrate, the 660-km discontinuity. Pancaking slabs push all sub-lithospheric mantle back under incoming oceanic lithosphere (the shrinking Pacific spreads rapidly), and suck overriding arcs or lithosphere plates toward them; hence, common extension. Thermal weakening of arcs, and feedback weakness of extending backarcs, maintain extensional regimes. Only downplated slabs are transferred behind overriding plates and recycled upward into gaps (the nonsubducting Atlantic spreads slowly). This self-organizing circulation rights the density inversion produced by top-down cooling that forms oceanic lithosphere above low-density asthenosphere. The mass of that lithosphere forces it toward subduction systems, its only exit from the surface. Whole-mantle convection (including plumes and deep subduction) does not operate, and fixed hotspots and plate boundaries do not exist.

Continental backarc thrusting represents cratonward spreading, much of it above basement, of thickened crust. The causative thickening represents primarily the mantle component of arc magmatism, including underplating, in excess of correlative extension, and not crustal shortening.