The Shatsky Rise is one of the largest oceanic plateaus, a class of volcanic features whose formation is poorly understood. It is also a plateau that was formed near spreading ridges, but the connection between the two features is unclear. The geologic structure of the Shatsky Rise can help us understand its formation. Deeply penetrating two-dimensional (2-D) multichannel seismic (MCS) reflection profiles were acquired over the southern half of the Shatsky Rise, and these data allow us to image its upper crustal structure with unprecedented detail. Synthetic seismograms constructed from core and log data from scientific drilling sites crossed by the MCS lines establish the seismic response to the geology. High-amplitude basement reflections result from the transition between sediment and underlying igneous rock. Intrabasement reflections are caused by alternations of lava flow packages with differing properties and by thick interflow sediment layers. MCS profiles show that two of the volcanic massifs within the Shatsky Rise are immense central volcanoes. The Tamu Massif, the largest (~450 km × 650 km) and oldest (ca. 145 Ma) volcano, is a single central volcano with a rounded shape and shallow flank slopes (<0.5°–1.5°), characterized by lava flows emanating from the volcano center and extending hundreds of kilometers down smooth, shallow flanks to the surrounding seafloor. The Ori Massif is a large volcano that is similar to, but smaller than, the Tamu Massif. The morphology of the massifs implies formation by extensive and far-ranging lava flows emplaced at small slope angles. The relatively smooth flanks of the massifs imply that the volcanoes were not greatly affected by rifting due to spreading ridge tectonics. Deep intrabasement reflectors parallel to the upper basement surface imply long-term isostasy with the balanced addition of material to the surface and subsurface. No evidence of subaerial erosion is found at the summits of the massifs, suggesting that they were never highly emergent.

Abstract Based on present knowledge of mantle peridotites from the Ligurian Tethys ophiolites, this paper presents new ideas and a new model for passive rifting to ocean spreading in the slow–ultraslow rifting Europe–Adria realm. Relevant points include: (i) the positive feedback between deformation and melt percolation during passive magmatic rifting; (ii) the positive feedback between natural evidence and experimental data on the behaviour of the mantle lithosphere during passive rifting; (iii) the significance of hidden magmatism and the associated melt thermal advection; (iv) the role of the wedge-shaped weakened and softened axial zone; and (v) the evidence of a transition from passive to active rifting in the Ligurian Tethys. Passive rifting induced passive asthenospheric upwelling and the onset of partial melting. Fractional melts migrated through the mantle lithosphere and stagnated at shallow levels (the hidden magmatism). Melt thermal advection heated the mantle lithosphere to temperatures ( T ) of ≥1200°C and formed a wedge-shaped axial zone of rheological softened/weakened mantle peridotites that served as the future locus of continental break-up. The hotter/deeper asthenosphere ascended within this axial zone, underwent partial melting and formed aggregated mid-ocean ridge basalts (MORBs) that migrated within dunite channels to form olivine gabbro intrusions and basaltic lava flows. Rifting evolved from passive to active, and the actively upwelling asthenosphere established a ridge-type system and thermal regime.

The eastern Laurentian margin in northeastern North America is marked by promontories and embayments that are defined by northeast-striking rift zones offset by northwest-striking transform faults. The complete history of the northeastern margin, from the initiation of continental rifting to the onset of passive-margin thermal subsidence, is preserved in a dynamic stratigraphic succession and in anorogenic magmatic suites. Late Neoproterozoic–Early Cambrian clastic and volcanic deposits overlie ca. 1.0 Ga and older Laurentian basement and define multiphase continental extension that rifted Laurentia out of Rodinia, opening the Iapetus Ocean as well as the more marginal Humber Seaway. Continental extension is also expressed in a set of basement fault systems that extend into the craton perpendicular to the northeastern Laurentian margin. Lower Cambrian sandstones at the base of a transgressive passive-margin succession overlie synrift rocks and basement, defining the time of transition for the eastern Laurentian margin from an active rift to a passive-margin environment. The passive margin is expressed as a broad late Early Cambrian through early Middle Ordovician carbonate bank and associated offshelf facies. Synthesis of the available data reveals significant along-strike variations in the thickness, composition, age, and facies of important synrift and postrift stratigraphic successions between the northern Appalachian rift zones. These variations are consistent with models for low-angle detachment rift systems and allow for the resolution of the underlying basement architecture of the eastern Laurentian margin specific to low-angle detachments, including upper-plate margins, lower-plate margins, and transform faults that bound zones of oppositely dipping low-angle detachments.

The Elder Creek ophiolite, which crops out along the South, Middle, and North Forks of Elder Creek, is the largest exposure of mid-Jurassic Coast Range ophiolite in the northern Coast Ranges of California. The Elder Creek ophiolite contains almost all of the components of a classic ophiolite (mantle tectonites, cumulate ultramafics and gabbro, dike complex, volcanics), although most of the volcanic section has been removed by erosion and redeposited in the overlying Crowfoot Point breccia. It differs from classic ophiolite stratigraphy in that it has substantial volumes (25%–30% of the complex) of felsic plutonic rocks intimately associated with the other lithologies. The felsic lithologies include hornblende diorite, quartz-diorite, tonalite, and trondhjemite, which crop out in four distinct associations: (1) as rare, small pods within the sheeted dike complex, (2) as the felsic matrix of igneous breccias (agmatites), (3) 1–25-m-thick dikes that crosscut cumulate or isotropic gabbro, and (4) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of the plutonic section. Typical phase assemblages include quartz, plagioclase, hornblende, and pyroxene, in a hypidiomorphic texture. The Elder Creek tonalite-trondhjemite-diorite (TTD) suite spans a wide range in composition: 54%–75% SiO 2 , 3.3%–14.3% FeO*, and 2.7%–6.4% MgO; all are low in K 2 O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO 2 ) than the dikes, pods, and agmatite matrix (average 60% SiO 2 ). Mg#’s range from 65 to 17, with Cr up to 227 ppm at 58% silica. High-Mg diorites with 4%–7% MgO at 53%–58% SiO 2 are common in the dike suite, but relatively high MgO, Mg#, and Cr values are found in the large plutons as well. The major- and trace-element characteristics are consistent with partial melting of subducted, amphibolite-facies oceanic crust at relatively low pressures (5–10 kbar) outside the garnet stability field. Melting of subducted oceanic crust at these pressures can only occur during the collision and subduction of an active spreading center. Subsequent reaction of these melts with the overlying mantle wedge has increased their refractory element concentrations. The occurrence of zircons with inherited Pb isotope characteristics implies the involvement of subducted sediments containing an ancient zircon component. Formation of the Elder Creek TTD suite was a transient event associated with ridge collision-subduction. This is consistent with previous models for the Coast Range ophiolite and other suprasubduction-zone ophiolites; it is not consistent with an ocean-ridge spreading-center origin.

The polygenetic Ingalls ophiolite complex in the central Cascades, Washington, is one of several Middle to Late Jurassic ophiolites of the North American Cordillera. It consists primarily of mantle tectonites. High-temperature mylonitic peridotite, overprinted by serpentinite mélange (Navaho Divide fault zone), separates harzburgite and dunite in the south from lherzolite in the north. Crustal units of the ophiolite occur as steeply dipping, kilometer-scale fault blocks within the Navaho Divide fault zone. These units are the Iron Mountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanic rocks of the Iron Mountain unit have transitional within-plate–enriched mid-ocean-ridge basalt affinities, and a rhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentary rocks include local oolitic limestones and cherts that contain Lower Jurassic (Pliensbachian) Radiolaria. This unit probably formed as a seamount within close proximity to a spreading ridge. The Esmeralda Peaks unit forms the crustal section of the ophiolite, and it consists of gabbro, diabase, basalt, lesser felsic volcanics, and minor sedimentary rocks. U-Pb zircon indicates that the age of this unit is ca. 161 Ma. The Esmeralda Peaks unit has transitional island-arc–mid-ocean ridge basalt and minor boninitic affinities. A preferred interpretation for this unit is that it formed initially by forearc rifting that evolved into back-arc spreading, and it was subsequently deformed by a fracture zone. The Iron Mountain unit is the rifted basement of the Esmeralda Peaks unit, indicating that the Ingalls ophiolite complex is polygenetic. Ingalls sedimentary rocks consist primarily of argillite with minor graywacke, conglomerate, chert, and ophiolite-derived breccias and olistoliths. Radiolaria from chert give lower Oxfordian ages. The Ingalls ophiolite complex is similar in age and geochemistry to the Josephine ophiolite and its related rift-edge facies and to the Coast Range ophiolite of California and Oregon. The Ingalls and Josephine ophiolites are polygenetic, while the Coast Range ophiolite is not, and sedimentary rocks (Galice Formation) that sit on the Josephine and its rift-edge facies have the same Radiolaria fauna as Ingalls sedimentary rocks. Therefore, we correlate the Ingalls ophiolite complex with the Josephine ophiolite of the Klamath Mountains. Taking known Cretaceous and younger strike-slip faulting into account, this correlation implies that the Josephine ophiolite either continued northward ~440 km—thus increasing the known length of the Josephine basin—or that the Ingalls ophiolite was translated northward ~440 km along the continental margin.

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfigured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafloor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is flawed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-influenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low field strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduction recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in contrast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a subduction zone. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be preserved where it is scraped off of the subducting slab.

The Steep Rock Group, Little Falls assemblage, Finlayson greenstone belt, and Lumby Lake greenstone belt form a once-continuous Mesoarchean terrane at the southern margin of Wabigoon subprovince, Canadian Shield. Synchronous with eruptive activity, oceanic plateau basalts in this region were intruded by tonalitic batholiths, which fed intermediate to felsic volcanic cones and associated mass-flow sediment aprons. A sedimentary succession capping the volcanic pile records stream-induced channel incision into upraised basalt and tonalite, feeding sediment to a delta complex. Base-level rise caused the incised channels to backfill, stromatolitic carbonates to develop in shallow-marine areas, and finally, deposition of iron formation to dominate. A Mesoarchean terrane was also examined in the Wallace Lake area of Manitoba. Here a kilometer-thick, transgressive succession records a transition from fluvial to delta-front to prodelta turbiditic environments, with eventual drowning and siliciclastic sediment starvation leading to dominance by carbonate and iron formation deposition. Sedimentary rocks that cap the oceanic Cretaceous to Holocene Kerguelen Plateau are a direct analogue for the depositional environments, sediment source rocks, and drowning events that define the stratigraphy of the Mesoarchean successions studied here. The Steep Rock–Lumby oceanic plateau formed and actively grew during an interval of 230 m.y. This sequence produces serious problems for Mesoarchean tectonic models in which increased heat dissipation produces fast spreading or numerous spreading ridges. A Mesoarchean, mantle plume–dominated heat dissipation process is in much better agreement with the data presented here.

A conceptual shift is overdue in geodynamics. Popular models that present plate tectonics as being driven by bottom-heated whole-mantle convection, with or without plumes, are based on obsolete assumptions, are contradicted by much evidence, and fail to account for observed plate interactions. Subduction-hinge rollback is the key to viable mechanisms. The Pacific spreads rapidly yet shrinks by rollback, whereas the subduction-free Atlantic widens by slow mid-ocean spreading. These and other first-order features of global tectonics cannot be explained by conventional models. The behavior of arcs and the common presence of forearc basins on the uncrumpled thin leading edges of advancing arcs and continents are among features indicating that subduction provides the primary drive for both upper and lower plates. Subduction rights the density inversion that is produced when asthenosphere is cooled to oceanic lithosphere: plate tectonics is driven by top-down cooling but is enabled by heat. Slabs sink more steeply than they dip and, if old and dense, are plated down on the 660 km discontinuity. Broadside-sinking slabs push all sublithosphere oceanic upper mantle inward, forcing rapid spreading in shrinking oceans. Down-plated slabs are overpassed by advancing arcs and plates, and thus transferred to enlarging oceans and backarc basins. Plate motions make sense in terms of this subduction drive in a global framework in which the ridge-bounded Antarctic plate is fixed: most subduction hinges roll back in that frame, plates move toward subduction zones, and ridges migrate to tap fresh asthenosphere. This self-organizing kinematic system is driven from the top. Slabs probably do not subduct into, nor do plumes rise to the upper mantle from, the sluggish deep mantle.