ABSTRACT Under fast-moving oceanic plates, the asthenosphere seismic B″ region becomes isolated from the convecting mantle by plate drag and acts as an advecting layer, which can serve as a long-lived source for intraplate volcanism. Geochemical enrichment of B″ can occur via infiltration by melts generated from the breakdown of serpentinite at ~200 km depth in subducting slabs. Ocean-island chains arise when melts generated within metasomatized B″ by shear melting and localized convection are released along lithospheric fractures controlled by the stress field of the plate. Intersection of metasomatized B″ with ocean-ridge systems produces oceanic plateaus. A strong anisotropy anomaly (V SH /V SV >1) at depths of ~150 km in the Pacific asthenosphere marks a metasomatized B″ domain that originated in the western paleo-Pacific basin in the Carboniferous, and that is now associated with Hawaiian volcanism. Metasomatized B″ can be trapped beneath orogenic belts during continental aggregation and tapped by edge-driven convection upon rifting to produce the correlation between intraplate volcanism and the fabric of sutures in opening ocean basins such as the Atlantic Ocean basin.

ABSTRACT Widespread episodes of major contractional orogenesis correlate commonly with ages of high-pressure eclogitic rocks formed during bottom-driven, induced subduction of crustal terranes. Rapid exhumation of the deeply emplaced crust has led to the development of the concept of a “tectonic dunk.” The dunk process is a hallmark component of a suite of linked tectonic, magmatic, metamorphic, and sedimentologic processes that systematically follow plate interactions, including collision, coupling, and capture resulting in plate reconfiguration and changes of movement. Plate capture, which takes place during mechanical connection of plates within a “clutch” zone, is followed generally by an abrupt transition to plate stretching in response to drag or plate spin. Plate stretch, which is accommodated during drag by a network of complementary strike-slip and normal faults or during spin by regional domains of transtension, is recorded by “postorogenic,” back-arc extension, basin formation, and magmatism, extensive domains of which comprise large igneous provinces. As a captured continental plate is dragged or rotates, ductile mantle is disrupted and displaced by protuberances, such as a slab coupled against the base of an overriding plate and/or orogenic roots extending down from a cratonic core. The mantle turbulence resembles a wave-like ship’s wake with tsunami-like movement, albeit below crust. The arrival of a moving mantle bulge or wave is inferred to be focused along continental plate margins where subduction is induced, as recorded by magmatism and eclogitic rocks that form during deep emplacement of crustal terranes. Concurrent shortening of crust in the vicinity of the plate margin is inferred from inversion and uplift of marginal rift basins, obduction, and development of fold-and-thrust belts. As the mantle wave passes beneath plate interiors, tens to hundreds of meters of uplift, recorded by oceanic atolls, continental stream incision, regional unconformities, and local transitions to evaporite within shelf settings, record epeirogeny. After passage of the wave, common development of sheet-like bodies of quartzose sandstone, especially during the early Paleozoic, suggest postwave, regional subsidence. Resumption and re-invigoration of extension are recorded by eduction of dunked crust and conspicuous, widespread, volcanic eruptions recorded by tuffaceous layers intercalated with carbonaceous black shale within broad basins developed above thickened crust.

The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km 3 /yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km 3 /yr) than in the opposite east- or northeast-directed subduction zones (~74 km 3 /yr). This difference is consistent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlying mantle and that there is an “easterly” directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantle flow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.