The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rocks that were metamorphosed in the Late Eocene; and the Metchosin Formation, a Lower Eocene basalt and gabbro unit. The Leech River fault, which was clearly imaged on both profiles, dips 35–45 °northeast and extends to about 10 km depth. The Survey Mountain fault lies parallel to and above the Leech River fault and extends to similar depths. The San Juan fault, the western continuation of the Survey Mountain fault, was not imaged, although indirect evidence suggests that it also is a thrust fault. These faults accommodated the Late Eocene amalgamation of the Leech River and Metchosin terranes along the southern perimeter of Wrangellia. Thereafter, these terranes acted as a relatively coherent lid for a younger subduction complex that has formed during the modem (40 Ma to present) convergent regime.Within this subduction complex, the LITHOPROBE profiles show three prominent bands of differing reflectivity that dip gently northeast. These bands represent regionally extensive layers lying beneath the lid of older accreted terranes. We interpret them as having formed by underplating of oceanic materials beneath the leading edge of an overriding continental place. The upper reflective layer can be projected updip to the south, where it is exposed in the Olympic Mountains as the Core rocks, an uplifted Cenozoic subduction complex composed dominantly of accreted marine sedimentary rocks. A middle zone of low reflectivity is not exposed at the surface, but results from an adjacent refraction survey indicate it is probably composed of relatively high velocity materials (~ 7.7 km/s). We consider two possibilities for the origin of this zone: (1) a detached slab of oceanic lithosphere accreted during an episodic tectonic event or (2) an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust. The lower reflective layer is similar in reflection character to the upper layer and, therefore, is also interpreted as consisting dominantly of accreted marine sedimentary rocks. It represents the active zone of decoupling between the overriding and underthrusting plates and, thus, delimits present accretionary processes occurring directly above the descending Juan de Fuca plate. These results provide the first direct evidence for the process of subduction underplating or subcretion and illustrate a process that is probably important in the evolution and growth of continents.

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The role of sediment melting in Earth's mantle remains controversial, as direct observation of melt generation in the mantle is not possible. Geochemical fingerprints provide indirect evidence for subduction-delivery of sediment to the mantle, however sediment abundance in mantle-derived melt is generally low (0-2%), and difficult to detect. Here we 1 provide evidence for bulk melting of subducted sediment in the mantle through isotopic analysis of granite sampled from an exhumed mantle section. Peraluminous granite dikes that intrude peridotite in the Oman-United Arab Emirates ophiolite have U-Pb ages of 99.8±3.3 Ma that predate obduction at ca. 85 to 90 Ma. The dikes have unusually high oxygen isotope (δ 18 O) values for whole rock (14-23‰) and quartz (20-22‰), and yield the highest δ 18 O zircon values known (14-28‰; values relative to Vienna standard mean ocean water). The extremely high oxygen isotope ratios uniquely identify the melt source as high δ 18 O marine sediment (pelitic and/or siliciceous mud), as no other source could produce granite with such anomalously high δ 18 O. Formation of high δ 18 O sediment-derived (S-type) granite within peridotite requires delivery of sediment to the mantle by subduction, where it melted and intruded the overlying mantle wedge. The granite suite described here contains the most evolved oxygen isotope ratios reported for igneous rocks, yet intruded mantle peridotite below the Mohorovičić seismic discontinuity, the most primitive oxygen isotope reservoir in the silicate Earth. Identifying the presence and quantifying the extent of sediment melting within the mantle has important implications for understanding subduction recycling of crust and mantle heterogeneity over time.

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resent-day Earth's mantle structure is dominated by a degree-2 spherical harmonic featuring two equatorial and antipodal mantle domains bisected by a subduction girdle surrounding the Pacific Ocean. Each of the two antipodal African and Pacific mantle domains features a large low shear-wave velocity province (LLSVP) in the lower mantle. Although most mantle plumes are believed to have originated from the edges of the LLSVPs in the two mantle domains since at least ca. 200 million years ago (Ma) 1 , we still know little about the nature and evolutionary histories of the two LLSVPs.Opposing models exist regarding how and when the two LLSVPs formed. According to one model, they are quasi-stationary, long-lived through Earth history (4.0-2.0 billion years ago) and uninfluenced by plate tectonics 1,2 . By contrast, another model claims the LLSVPs are dynamic in their formation, evolution (including demise) and geographic locations and are linked to the assembly and breakup of supercontinents 3,4 . Palaeomagnetic data, mantle plume records during the last two supercontinent cycles and dynamic modelling results have been used to suggest that the antipodal LLSVPs are related to whole-mantle convection driven by plate motion 5,6 , particularly by circum-supercontinent subduction 7 . As such, the shape and location of LLSVPs both in time and space are dynamically linked to the formation of supercontinents. It has been further speculated 3,7,8 that when the antipodal LLSVPs, whose locations are controlled by the subduction girdle surrounding the supercontinent, are positioned off the Equator, the centrifugal force of Earth's spin would bring them to the Equator through true polar wander, that is, wholesale rotation of the entire silicate Earth relative to the spin axis 9 . Such a strong coupling between the outer layer of the planet (tectonic plates) and deeper mantle domains is consistent with both continental and oceanic plume records over much of Earth history 10,11 . However, to test contrasting geodynamic models, it is fundamental to characterize and compare the geochemical compositions and evolutionary paths of the two mantle domains.Radiogenic isotopes (lead (Pb), strontium (Sr) and neodymium (Nd)) and noble gas isotopes (for example, helium (He)) of basaltic lava flows originating from hotspots in the present-day oceans appear to be the best tools for evaluating any systematic compositional difference between the two mantle domains 12 . A first-order isotopic distinction can be made between mid-ocean-ridge basalts (MORBs) and hotspot basalts (or ocean island basalts (OIBs)), which are thought to tap reservoirs in the depleted upper mantle 13 and the more primitive lower mantle 14 , respectively. While most MORBs are isotopically relatively uniform, OIBs show substantial diversity due to deep recycling of various subducted lithospheric components such as oceanic and continental lithospheric materials, including sediments 13 .Geographically, isotopic signatures from oceanic basalts have a purported hemispheric distinc...

Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

The Georgetown Inlier of northeast Australia provides evidence of critical links between Australia and Laurentia during the late Paleoproterozoic and the early Mesoproterozoic. Detrital zircon age spectra from sedimentary strata within the inlier show two distinct changes in sedimentary provenance: (1) the lowermost units (depositional age ca. 1700–1650 Ma) have detrital zircon age spectra that strongly resemble Laurentian magmatic ages and detrital zircon age spectra of the similar-aged Wernecke Supergroup of northwest Laurentia; (2) sediments deposited from ca. 1650 to 1610 Ma show a unimodal proximal signature; and (3) postorogenic sediments deposited after 1550 Ma have detrital zircon age spectra like the Mount Isa Inlier of the North Australia craton. Along with new paleocurrent measurements, the detrital age data challenge current models that suggest that the Georgetown Inlier was part of Australia before ca. 1700 Ma. Rather, we argue it was a continental ribbon rifted from west Laurentia during slab rollback ca. 1680 Ma; by 1650 Ma, the Georgetown Inlier had completely separated from Laurentia, and ca. 1600 Ma collided with Australia during supercontinent Nuna amalgamation.