Volcanism that occurs far from plate margins is di cult to explain with the current paradigm of plate tectonics. The Changbaishan volcanic complex, located on the border between China and North Korea, lies approximately 1,300 km away from the Japan Trench subduction zone and is unlikely to result from a mantle plume rising from a thermal boundary layer at the base of the mantle. Here we use seismic images and three-dimensional waveform modelling results obtained from the NECESSArray experiment to identify a slow, continuous seismic anomaly in the mantle beneath Changbaishan. The anomaly extends from just below 660 km depth to the surface beneath Changbaishan and occurs within a gap in the stagnant subducted Pacific Plate. We propose that the anomaly represents hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific Plate and is now escaping through a gap in the subducting slab. We suggest that this subduction-induced upwelling process produces decompression melting that feeds the Changbaishan volcanoes. Subductioninduced upwelling may also explain back-arc volcanism observed at other subduction zones.

This paper describes a mechanical model of the mid‐ocean ridge axis based on a nonlinear rheology. The oceanic lithosphere is modeled as a strongly temperature‐dependent “power law” material with the Byerlee's friction law describing the brittle behavior of the upper oceanic lithosphere. We examined two simple models. A mantle‐only model predicted an axial rift valley 1–2 km deep and 10–15 km wide at slow half‐rates of 5–25 mm/yr which is in fair agreement with observations. However, the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller. Thus, the mantle‐only model failed to explain the observed transition in axial topography with spreading rate. By including a uniform thickness oceanic crust with theological properties different from the mantle, our second model produced the first order variation of axial topography with spreading rate: a pronounced rift valley with 1–2 km relief at slow rates, a smaller relief (<500 m) rift valley at intermediate rates, and no rift valley at fast rates. There are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models. First, horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis, i.e., the stresses are concentrated in a very narrow zone for the nonlinear rheology. This conclusion is remarkably independent of the boundary conditions at the seafloor. Second, the computed width of the axial plastic failure zone decreases with spreading rate, a result very different from that of a uniform viscosity model. This is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate. Beyond some distance from the ridge axis, the brittle plate (roughly the region above the 750°C isotherm) deforms very little; both the horizontal strain rate ė11 and the maximum shear strain rate ėmax are less than 10−17 s−1 which is in good agreement with the 10−18 s−1 average for oceanic brittle plate deformation inferred from seismic moment data. Within the ductile region (area beneath the 750°C isotherm) the strain rates are in the range of 10−14–10−16 s−1, except within a narrow band near the ridge axis where higher strain rates are located. We found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern (or the rheology) but is very sensitive to the cooling by hydrothermal circulation near the ridge axis. Axial structure is also influenced by factors other than the spreading rate. Within the range of slow and intermediate rates, our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature; in other words, a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley. In the vicinity of hotspots, the absence of an axial rift valley e...

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A compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness, away from plateaus, is 6 km; no systematic increase of crustal thickness with spreading rate is observed. Instead, the data show large variations in crustal thickness at slow spreading ridges (3 – 8 km for half rates < 20 mm/yr) and small variations in thickness at fast rates (5 – 7 km for half rates > 30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid‐ocean ridges. Both data sets support the speculation of a transition from a 3‐D structure of crustal accretion at slow ridges to a 2‐D accretion pattern at fast ridges.

We perform a finite-frequency tomographic inversion to image 3D velocity structures beneath southern and central Tibet using teleseismic body-wave data recorded by the TIBET-31N passive seismic array as well as waveforms from previous temporary seismic arrays. High-velocity bodies dip ~40° northward beneath the Himalaya and the Lhasa Terrane. We interpret these high-velocity anomalies as subducting Indian Continental Lithosphere (ICL). The ICL appears to extend further north in central Tibet than in eastern Tibet, reaching 350 km depth at ~31°N along 85°E but at ~30°N along 91°E. Low P-and S-wave velocity anomalies extend from the lower crust to ≥180 km depth beneath the Tangra Yum Co Rift, Yadong-Gulu Rift, and the Cona Rift, suggesting that rifting in southern Tibet may involve the entire lithosphere. The anomaly beneath Tangra Yum Co Rift extends down to about 180 km, whereas the anomalies west of the Yadong-Gulu Rift and east of the Cona Rift extend to more than 300 km depth. The low-velocity upper mantle west of the Yadong-Gulu Rift extends furthest north and appears to connect with the extensive upper-mantle low-velocity region beneath central Tibet. Thus the northward-subducting Indian Plate is fragmented along north-south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern Tibet.

A simple mechanical model for the axial topographic and gravity variations with spreading rate is developed. The model emphasizes the rapid transition from one mechanism to another with changing spreading rate. Our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis. There exists a potential failure zone of width Lf where the stresses within the brittle plate exceeds its yield strength. If the yield strength of the brittle plate is nearly constant, this width is largely determined by the strain rate and the viscosity of the “strong” mantle. The high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region, a critical feature in this model. The size of the decoupling region Ld is determined by the thickness of the oceanic crust and temperatures within the crust. If the decoupling region is small (Ld < Lf), as shown by numerical results to be the case for slow spreading ridges, strong coupling of the brittle plate to the ductile flow of the mantle would cause the brittle layer to deform plastically and a rift valley to be produced as a result of a steady state necking process. However, if the decoupling region is large (Ld > Lf), as shown to be the case for fast spreading ridges, then each of the two layers responds separately to the horizontal stretching. The brittle plate will be broken only at the axis, and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy. The changing size of the decoupling region gives rise to the transition from Atlantic type axial topography to Pacific type, as well as the transition from a high amplitude, highly variable gravity signature to a low amplitude, very uniform gravity signature. Parameters can be chosen so that such a transition occurs at a half‐rate of 35 mm/yr, which is the critical half‐rate suggested by both topographic and gravity data.

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[1] Gravity-derived crustal thickness models were calculated for the North Atlantic Ocean between 76°N and the Chain Fracture Zone and calibrated using seismically determined crustal thickness. About 7% of the ocean crust is <4 km thick (designated as thin crust), and 58% is 4-7 km thick (normal crust); the remaining 35% is >7 km thick and is interpreted to have been affected by excess magmatism. Thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis. By far the most prominent thick crust anomaly is associated with Iceland and adjacent areas, which accounts for 57% of total crustal volume in excess of 7 km. Much smaller anomalies include the Azores (8%), Cape Verde Islands (6%), Canary Islands (5%), Madeira (<4%), and New England-Great Meteor Seamount chain (2%), all of which appear to be associated with hot spots. Hot spot-related crustal thickening is largely intermittent, suggesting that melt production is episodic on time scales of tens of millions of years. Thickened crust shows both symmetrical and asymmetrical patterns about the Mid-Atlantic Ridge (MAR) axis, reflecting whether melt anomalies were or were not centered on the MAR axis, respectively. Thickened crust at the Bermuda and Cape Verde rises appears to have been formed by isolated melt anomalies over periods of only ∼20-25 Myr. Crustal thickness anomalies on the African plate generally are larger than those on the North American plate; this most likely results from slower absolute plate speed of the African plate over relatively fixed hot spots.