Thick Cratonic Roots Research Articles

Mantle dynamics of the stagnating slab: A case study of Shandong Peninsula and adjacent areas

The Shandong Peninsula is located in the eastern part of the North China Craton (NCC). Influenced by the collision between the North and South China blocks and the subduction of the Pacific Plate, the tectonic evolution process is highly complex. Previous studies have proposed that the thick cratonic root beneath the eastern NCC has been destructed. To better understand the relationship between the craton reactivation and the deep subduction, more constraints on the topography variations of the mantle transition zone (MTZ) discontinuities are required. In this study, we stacked 9073 P‐to‐S receiver functions calculated from 129 broadband seismic stations to image the depth variations of the MTZ discontinuities beneath the Shandong Peninsula and its adjacent area. To decrease the influence of velocity anomaly above the MTZ discontinuities, we converted the apparent depths into corrected depth using regional and global 3D velocity models. The apparent depth of the 410‐km discontinuity exhibits a depression of approximately 6 km, which normalizes after velocity correction. This implies the existence of the low‐velocity anomaly in the upper mantle, possibly caused by slab dehydration and water infiltration into the upper mantle. The significant depression of the 660‐km discontinuity indicated the presence of the cold Pacific slab stagnating at the base of the MTZ. The gradual westward depression of the 660‐km discontinuity by 50 km could provide evidence for the sinking of the western margin of the flat‐lying subducting Pacific slab in the MTZ beneath the Shandong Peninsula.

Cenozoic India-Asia collision driven by mantle dragging the cratonic root

The driving force behind the Cenozoic India-Asia collision remains elusive. Using global-scale geodynamic modeling, we find that the continuous motion of the Indian plate is driven by a prominent upper-mantle flow pushing the thick Indian lithospheric root, originated from the northward rollover of the detached Neo-Tethyan slab and sinking slabs below East Asia. The maximum mantle drag occurs within the strong Indian lithosphere and is comparable in magnitude to that of slab pull (1013 N m−1). The thick cratonic root enhances both lithosphere-asthenosphere coupling and upper-plate compressional stress, thereby sustaining the topography of Tibetan Plateau. We show that the calculated resistant force from the India-Asia plate boundary is also close to that due to the gravitational potential energy of Tibetan Plateau. Here, we demonstrate that this mantle flow is key for the formation of the Tibetan Plateau and represents part of a hemispheric convergent flow pattern centered on central Asia.

Crust and upper mantle structure beneath Southwest China and its implications for Mesozoic multistage gold deposits

The Youjiang Basin (YB) on the southwestern margin of the Yangtze Craton (YC) contains >200 Triassic or Cretaceous gold deposits, but deep dynamic processes contributing to large-scale gold accumulation remain unknown. Based on a south–north trending linear broadband seismic array, we obtained the crust and mantle structure beneath the YB and southwestern YC by P-wave and S-wave receiver functions and ambient noise tomography. Our results show a remarkable lithospheric boundary beneath the northern margin of the YB, which separates the YB with felsic thin crust and thin mantle lithosphere from the YC with both a thick crust and thick mantle lithosphere. A new structural image shows that the southern end of the YC Moho dips southward and overlaps with the YB Moho. A 5-km-thick high-velocity layer is also found at a depth of 10 km beneath the northern YB. Combining this information with previous tectonic and geological results, we reveal the process of Mesozoic lithospheric evolution in the southwestern South China Block. We infer that both the pre-Mesozoic weak crust and mantle lithosphere and the intense intracontinental orogenesis in the Early-Middle Triassic contributed to the formation of first-stage Au deposits in the YB. Lithospheric delamination in the Early Cretaceous thinned the lithosphere of YB rather than YC, forming a steep lithosphere–asthenosphere boundary beneath the transition zone between YB and YC, which favored upwelling of Au-rich asthenosphere below the thick cratonic root to the bottom of the crust in the northern YB and contributed to the second-stage Au deposits in the YB.

Seismic Tomography of the Trans-North China Orogen and Its Dynamic Implications

We present a high-resolution S wave velocity model of the Trans-North China Orogen (TNCO) and its adjacent areas derived from S wave traveltime residuals of teleseismic waveforms recorded by the dense ChinArray seismic network. Our model reveals S wave velocity anomalies beneath the study area, suggesting the strong lateral heterogeneities of the upper mantle structures. The upper mantle of the Datong volcano is dominated by the significant low velocity anomalies. In addition to the low-velocity zone rooted in the mantle transition zone beneath the Datong volcano region, two low velocity layers extend northwestward to the Hetao graben and southwestward to the southern TNCO within depths of 100–300 km, respectively. Based on these low-velocity anomalies, we provide new insights into the origin of the Datong volcano, which may be related to the asthenospheric upwelling originated from the water-rich mantle transition zone and horizontal mantle flows driven by the extrusion northeastward of the Tibetan plateau. A clear velocity contrast in the upper mantle above 300 km depth beneath the Ordos block and the TNCO implies that the Ordos block still preserves thick cratonic root while the lithosphere of the TNCO is modified. We propose that the asthenospheric upwelling and northeastward push of the Tibetan plateau account for the lithospheric deformation of the TNCO and its adjacent areas.

Continental Magmatism: The Surface Manifestation of Dynamic Interactions Between Cratonic Lithosphere, Mantle Plumes and Edge‐Driven Convection

AbstractSeveral of Earth's intra‐plate volcanic provinces lie on or adjacent to continental lithosphere. Although many are believed to mark the surface expression of mantle plumes, our limited understanding of how buoyant plumes interact with heterogeneous continental lithosphere prevents further progress in identifying mechanisms at the root of continental volcanism. In this study, using a suite of 3‐D geodynamical models, we demonstrate that the magmatic expression of plumes in continental settings is complex and strongly sensitive to the location of plume impingement, differing substantially from that expected beneath more homogeneous oceanic lithosphere. Within Earth's continents, thick cratonic roots locally limit decompression melting. However, they deflect plume conduits during their ascent, with plume material channeled along gradients in lithospheric thickness, activating magmatism away from the plume conduit, sometimes simultaneously at locations more than a thousand kilometres apart. This magmatism regularly concentrates at lithospheric steps, where it may be difficult to distinguish from that arising through edge‐driven convection. At times, the flow field associated with the plume enhances melting at these steps long before plume material enters the melting zone, implying that differentiating geochemical signatures will be absent. Beneath regions of thinner lithosphere, plume‐related flow can force material downwards at lithospheric steps, shutting off pre‐existing edge‐related magmatism. Additionally, variations in lithospheric structure can induce internal destabilization of ponding plume material, driving intricate magmatic patterns at the surface. Our analysis highlights the challenges associated with linking continental magmatism to underlying mantle dynamics, motivating an inter‐disciplinary approach in future studies.

Mantle dynamics of the North China Craton: new insights from mantle transition zone imaging constrained by P-to-S receiver functions

SUMMARY Cratons are generally defined as stable continental blocks with a strong cratonic root that typically is at least ∼200 km thick. Many cratons have undergone little internal tectonism and destruction since their formation, but some of them, such as the eastern part of the North China Craton (NCC), the Dharwar Craton and the Wyoming Craton, have lost their thick cratonic root and become reactivated in recent geological history, leading to widespread Meso-Cenozoic volcanisms. The mechanisms responsible for such decratonization remain debated. To provide new constraints on models leading to decratonization, in this study we stack 612 854 source-normalized P-to-S conversions (receiver functions or RFs) from the 410 and 660 km discontinuities (d410 and d660, respectively) bordering the mantle transition zone (MTZ) recorded at 1986 stations in the NCC. Both the number of RFs and the number of stations are unprecedented in the study area. The average apparent depths of the d410 and d660 and the thickness of the MTZ are 413 ± 6, 669 ± 8 and 255 ± 6 km, respectively. A depression of up to 37 km and mean 11 km of the d660 are clearly observed beneath the eastern NCC, mainly caused by the possible existence of a relatively large amount of water in the MTZ. Our study provides strong observational evidence for geodynamic modelling that suggests water in the MTZ can be driven out into the upper mantle by poloidal mantle flow induced by the subduction and retreat of subducted oceanic slabs. The results are consistent with the weakening of the lithosphere beneath the eastern NCC by the release of water (in the form of structurally bound H/OH) brought down to the MTZ by subduction of the Pacific slab. Continuous slab dehydration and the ascent of fluids would have triggered intraplate volcanism and mantle upwelling in the eastern NCC, as evidenced by the spatial correspondence among the lower-than-normal upper-mantle seismic velocities, unusually large depressions of the d660, Cenozoic basaltic volcanism and thinning of the cratonic lithosphere.

Rapid mantle convection drove massive crustal thickening in the late Archean

The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere (Jordan, 1988; Boyd, 1989; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb– and MnO/FeOT–SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60–70 km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10−13–10−12 s−1, requiring mantle flow rates associated with late Archean crust formation to be 10–100 times faster than today.

An oxygen isotope test for the origin of Archean mantle roots

The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform δ18O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform δ18O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.

Radiogenic heating and craton‐margin plate stresses as drivers for intraplate orogeny

ABSTRACTThe Proterozoic belts that occur along the margins of the West Australian Craton, as well as those in intraplate settings, generally share similar geological histories that suggest a common plate‐margin driver for orogeny. However, the thermal drivers for intraplate orogenesis are more poorly understood. The Mutherbukin Tectonic Event records a protracted period of Mesoproterozoic reworking of the Capricorn Orogen and offers significant insight into both the tectonic drivers and heat sources of long‐lived intraplate orogens. Mineral assemblages and tectonic fabrics related to this event occur within a 50 km‐wide fault‐bound corridor in the central part of the Gascoyne Province in Western Australia. This zone preserves a crustal profile, with greenschist facies rocks in the north grading to upper amphibolite facies rocks in the south. The P–T–t evolution of 13 samples from 10 localities across the Mutherbukin Zone is investigated using phase equilibria modelling integrated with in situ U–Pb monazite and zircon geochronology. Garnet chemistry from selected samples is used to further refine the P–T history and shows that the dominant events recorded in this zone are prolonged D1 transpression between c. 1,320 and 1,270 Ma, followed by D2 transtension from c. 1,210 to 1,170 Ma. Peak metamorphic conditions in the mid‐crust reached >650°C and 4.4–7 kbar at c. 1,210–1,200 Ma. Most samples record a single clockwise P–T evolution during this event, although some samples might have experienced multiple perturbations. The heat source for metamorphism was primarily conductive heating of radiogenic mid‐ and upper crust, derived from earlier crustal differentiation events. This crust was thickened during D1 transpression, although the thermal effects persisted longer than the deformation event. Peak metamorphism was terminated by D2 transtension at c. 1,210 Ma, with subsequent cooling driven by thinning of the radiogenic crust. The coincidence of a sedimentary basin acting as a thermal lid and a highly radiogenic mid‐crustal batholith restricted to the Mutherbukin Zone accounts for reworking being confined to a discrete crustal corridor. Our results show that radiogenic regions in the shallow to mid crust can elevate the thermal gradient and localize deformation, causing the crust to be more responsive to far‐field stresses. The Mutherbukin Tectonic Event in the Capricorn Orogen was synchronous with numerous Mesoproterozoic events around the West Australian Craton, suggesting that thick cratonic roots play an important role in propagating stresses generated at distant plate boundaries.

Making Archean cratonic roots by lateral compression: A two-stage thickening and stabilization model

Archean tectonics was capable of producing virtually indestructible cratonic mantle lithosphere, but the dominant mechanism of this process remains a topic of considerable discussion. Recent geophysical and petrological studies have refuelled the debate by suggesting that thickening and associated vertical movement of the cratonic mantle lithosphere after its formation are essential ingredients of the cratonization process. Here we present a geodynamical study that focuses on how the thick stable cratonic lithospheric roots can be made in a thermally evolving mantle. Our numerical experiments explore the viability of a cratonization process in which depleted mantle lithosphere grows via lateral compression into a >200-km thick, stable cratonic root and on what timescales this may happen. Successful scenarios for craton formation, within the bounds of our models, are found to be composed of two stages: an initial phase of tectonic shortening and a later phase of gravitational self-thickening. The initial tectonic shortening of previously depleted mantle material is essential to initiate the cratonization process, while the subsequent gravitational self-thickening contributes to a second thickening phase that is comparable in magnitude to the initial tectonic phase. Our results show that a combination of intrinsic compositional buoyancy of the cratonic root, rapid cooling of the root after shortening, and the long-term secular cooling of the mantle prevents a Rayleigh-Taylor type collapse, and will stabilize the thick cratonic root for future preservation. This two-stage thickening model provides a geodynamically viable cratonization scenario that is consistent with petrological and geophysical constraints.

Large variations in lithospheric thickness of western Laurentia: Tectonic inheritance or collisional reworking?

The ca. 2.0–1.8Ga tectonic assembly of Laurentia provides a record of complex processes resulting in amalgamation of distinct lithospheric domains. In global and continental-scale teleseismic tomographic models, however, the subcontinental lithosphere beneath western Laurentia appears to have a deceptively simple structure that lacks a clear correlation with mapped crustal domains. Here we present a new shear-velocity model of the upper mantle beneath western Laurentia through Rayleigh-wave tomography, using data from several newly deployed broadband seismic arrays. Our models show prominent heterogeneities that appear to correlate well with crustal domains and other geophysical observations. The tomographic results delineate high-velocity keel-shaped anomalies beneath the Archean Hearne Province and the Paleoproterozoic Buffalo Head Terrane; these features are inferred to extend to depths of up to 260km and likely represent ancient thick cratonic roots, whereas relatively thin lithosphere characterizes the adjacent Wabamun domain and Medicine Hat Block. A regional isostatic residual gravity anomaly in the foreland of the Cretaceous–Paleocene southern Canadian Rockies coincides with an area of inferred thick lithosphere in the Hearne Province, suggesting that along-strike variations in flexural rigidity correlate with lithospheric thickness. Taken together, our results suggest that high-amplitude basal topography of the lithosphere–asthenosphere boundary beneath cratons reflects a complex lithospheric evolution that combines effects of both tectonic inheritance and collisional reworking.

Craton stability and longevity: The roles of composition-dependent rheology and buoyancy

Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. High strength of the cratonic root is generally considered to be the most important factor, but the role of lithospheric mantle depletion and dehydration in this strengthening is still debated. Geodynamical models often argue for a significant strength or buoyancy contrast between cratonic and non-cratonic mantle lithosphere, induced by mantle depletion and dehydration. But recent laboratory experiments argue for only a modest effect of dehydration strengthening. Can we reconcile laboratory experiments and geodynamical models?We perform and discuss new numerical models to investigate craton stability and longevity with different composition-dependent rheology and buoyancy. Our results show that highly viscous and possibly buoyant cratonic root is essential to stabilise a geometry in which thick cratonic lithosphere and thinner non-cratonic lithosphere coexist for billions of years. Using non-Newtonian rheology, a modest strengthening factor of Δη=3 can protect compositionally buoyant cratonic roots from erosion by mantle convection for over billions of years. A larger strengthening factor (Δη=10) can maintain long term craton stability even with little or no intrinsic buoyancy. Such composition-dependent rheology is comparable to the laboratory experiments. This implies that a strict isopycnic state of cratonic lithosphere may not be necessary for the preservation of a cratonic root, provided a sufficient level of compositional strengthening is present.

Craton nucleation and formation of thick lithospheric roots

Cratonisation is regarded as coincident with amalgamation and lithosphere thickening during collisional tectonics, leading to the birth of the cratonic entities we know today. These are, however, themselves rifted remnants of earlier plate configurations involving cratonic nuclei whose mode of formation conditioned their properties, hence stability and further evolution. Dating of inclusions in diamonds reveals that some thick cratonic roots existed well in advance of accretionary processes leading to craton amalgamation or reflected in the ages of eclogites residing in cratonic mantle. Major uncertainties relate to (1) whether these nuclei formed at ambient or excess mantle potential temperatures TP, i.e. in mid-ocean ridge or oceanic plateau settings and (2) whether the deep, garnet-bearing cratonic mantle, a portion of which extends into the diamond stability field (>150km), formed by plume subcretion or stacking of oceanic lithosphere. In order to extract constraints from published data sets that include samples modified by mantle metasomatism, olivine inclusions in diamonds are used as a guide to the pre-metasomatic composition of the cratonic subcontinental lithospheric mantle (SCLM). The FeO–MgO relationships of the filtered data set show that many depleted cratonic SCLM sections nucleated by onset of partial melting at pressures>5GPa, possibly pointing to formation at excess TP typical of plumes, rather than at ambient TP in Archaean mid-ocean ridge settings. Cr2O3–Al2O3 relationships constrain pressures of equilibrium partial melt extraction. All cratonic depleted garnet peridotite suites have compositions consistent with formation at pressures≥3GPa and hence formed in situ and not by subduction of oceanic plates. Because most cratonic SCLM sections formed as high-pressure residues consisting of olivine and aluminous orthopyroxene, the absence of a garnet signature in residual peridotites that last equilibrated at pressures >3GPa is not proof for original partial melting in the spinel stability field. Cratonic crust may have formed by shallow plate interactions leading to generation of TTGs and greenstone belts. These crustal nuclei may have been subsequently penetrated and subcreted by plume-derived melts, and, after delamination of dense eclogite and komatiites, were trapped by buoyant plume residues. This ensured craton longevity and implies that cratonic crust and mantle are not entirely cogenetic.

Mantle dynamics of the reactivating North China Craton: Constraints from the topographies of the 410-km and 660-km discontinuities

The topographies of the 410-km and 660-km discontinuities have important implications for mantle dynamics. Here we present high-resolution seismic imaging of the 410-km and 660-km discontinuities beneath the North China Craton (NCC) employing the receiver function method. Depth anomalies (deeper or shallower than the global average depths) at both discontinuities were detected by introducing a three-dimensional regional velocity model. The depressions of the 410-km discontinuity are mostly located in the eastern NCC. A local elevation of the 660-km discontinuity appeared in the northwest of the NCC and a significant depression of the 660-km discontinuity is located in the southeast of the NCC. Two dynamic mantle regimes are speculated to explain the formation of the anomalous depth zones in the NCC. One possibility is a complex mantle upwelling linked to edge-derived convection between the stagnant slab and the thick cratonic root. The other potential dynamic regime is slab stagnating, sinking, and induced upwelling at the neighboring slab front. These regimes hint that the mantle flow was possibly dominated by dynamic interactions among the subducting slab, cratonic root, and ambient mantle beneath the NCC.

Asthenospheric flow and origin of volcanism in the Baikal Rift area

The origin of low-volume, hotspot-like volcanism often observed in continental rift areas is debated, as is the nature of the flow in the mantle beneath. In this paper we assemble seismic constraints on the mantle flow below the Baikal Rift Zone. We combine new evidence from upper-mantle tomography and from a radially anisotropic shear-velocity profile measured beneath and SE of the rift with published shear-wave-splitting constraints on azimuthal anisotropy. Fast directions of azimuthal anisotropy are perpendicular to the rift on both of its sides. Stable Siberian-Platform lithosphere on the northern side extends down to 200 km; on the southern side, the lithosphere is only 60–70 km thick. This asymmetry, with the thick cratonic root forming a barrier to convective flow, inhibits a mid-ocean-ridge-like flow pattern. The radially anisotropic profile yields evidence for horizontal flow in the upper asthenosphere (60–100 km) SE of the rift but no evidence for vertical flow. These results are consistent with horizontal asthenospheric flow NW to SE perpendicular to the rift, from beneath the thick Siberian towards the thin Baikal–Mongolian lithosphere. When asthenosphere ascends from the 200-km depth beneath the craton to shallower depths beneath the rift, decompression melting of embedded veins of enriched rock can produce the hotspot-like basaltic volcanism observed. The occurrence of such veins of enriched mantle in sub-cratonic asthenosphere has been predicted by geochemical mantle models; we propose that sub-horizontal asthenospheric flow from beneath cratons is a common cause for hotspot-like volcanism in continental rift zones.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

Seismic tomographic images of the cratonic upper mantle beneath the Western Superior Province of the Canadian Shield—a remnant Archean slab?

Abstract Knowledge of the velocity structure of the upper mantle beneath the Western Superior Province (WSP) is key to understanding better the accretionary processes active during the Archean. To that end, teleseismic P- and S-wave travel times recorded as part of the Teleseismic Western Superior Transect (TWST) were inverted for their respective seismic velocities. This experiment involved 17 portable broadband stations arrayed in northern Ontario, Canada, so as to cross-cut the strike of many subprovinces as well as the boundary with the Proterozoic Trans-Hudson Orogen to the north. The 5-month deployment yielded 1423 P-wave and 651 S-wave high-quality residuals for inversion. The resulting tomographic images reveal three apparently-robust velocity anomalies represented by: (i) a dipping tabular high-velocity anomaly; (ii) a relatively shallow low-velocity anomaly directly above the positive anomaly; and (iii) a deep low-velocity body. The first anomaly may be interpreted in the Western Superior context as a 30–50 km thick eclogite/dunite layer representing remnant subducted oceanic lithosphere. The presence of such a body within the cratonic root would suggest its origin at around 2.7 Ga and the apparent SE–NW strike is noticeably oblique to the main EW trend of the subprovince boundaries. The low-velocity anomalies may be related to processes that occurred at the edges of the descending slab or they may be expressions of later upwelling material. The presence of a thick cratonic root (≈300 km) may also be revealed by the tomographic images. Overall, these travel time results are considered compatible with late Archean structures at depth resembling those of modern subduction tectonics, though other possibilities (e.g. small-scale convection cells) exist.