International audienceEastward subduction of oceanic tectonic plates has shaped the geologic history of western North America over the past 150 million years. The mountain-building and volcanism that brought forth the spectacular landscapes of the West are credited to the vast ancient Farallon plate, which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle. Here, we use finite-frequency travel-time and amplitude measurements of teleseismic P-waves in seven frequency bands to obtain a high-resolution tomographic image to approx1,800 km depth. We discover several large, previously unknown pieces of the plate which show that two distinct stages of whole-mantle subduction are present under North America. The currently active one descends from the Pacific northwest coast to 1,500 km depth beneath the Great Plains, whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent. We argue that the separation between them is linked to the Laramide era 70–50 Myr ago, a time of unusual volcanism and mountain-building far inland generally explained by an episode of extremely flat subduction

The western quarter of North America consists of accreted terranes--crustal blocks added over the past 200 million years--but the reason for this is unclear. The widely accepted explanation posits that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today's western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building.

This is a survey of mantle provinces (large‐scale seismic anomalies) under North America, from the surface down to 1500–1800 km depth. The underlying P velocity model was obtained by multifrequency tomography, a waveform‐based method that systematically measures and models the frequency‐dependence of teleseismic body waves. A novel kind of three‐dimensional rendering technique is used to make the considerable structural complexities under North America accessible. In the transition zone and below, the North American mantle is dominated by seismically fast provinces, which represent distinct subduction episodes of the Farallon plate. I attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes. Differences in vertical sinking velocity have led to large vertical offsets across adjacent, coeval slabs. Some of the mantle provinces have not been discussed much previously, including (1) a seismically slow blanket overlying the oldest Farallon subduction along the eastern continental margin, (2) a transition zone slab coeval with the Laramide orogeny (ca. 80–60 Myr), which I discuss in analogy to the “stagnant slab” subduction style commonly found in the western Pacific today, (3) the lower mantle root of present‐day Cascadia subduction, which may have started out as intraoceanic subduction,(4) a lower mantle slab under Arizona and New Mexico, the last material to subduct before strike‐slip motion developed along the San Andreas boundary, and (5) two narrow plate tears thousands of kilometers long, one of which is the subducted conjugate of the Mendocino Fracture Zone.

SUMMARY In global-scale seismic tomography, teleseismic P and PP waves mainly constrain structures in the upper two thirds of the mantle, whereas core-diffracted waves (Pdiff) constrain the lower third. This study is the first to invert a very large data set of Pdiff waves, up to the highest possible frequencies. This results in tomographic resolution matching and exceeding that of global S-wave tomographies, which have long been the models of choice for interpreting lowermost mantle structure. We present three new global tomography models of 3-D isotropic P-wave velocity in the earth’s mantle. Multifrequency cross-correlation traveltimes are measured on all phases in passbands from 30 s dominant period to the highest frequencies that produce satisfactory fits (≈3 s). Model DETOX-P1 fits ≈2.5 M traveltimes from teleseismic P waves. DETOX-P2 fits the same data, plus novel measurements of ≈1.4 M traveltimes of Pdiff waves. DETOX-P3 fits the same data as DETOX-P2, plus ≈ 1.2 M PP traveltimes. Synthetics up to 1 s dominant period are computed by full wave propagation in a spherically symmetric earth using the spectral-element method AxiSEM. Traveltimes are linked to 3-D velocity perturbations (dVP/VP) by finite-frequency Fréchet kernels, parametrized on an adaptive tetrahedral grid of ≈400 000 vertices spaced by ≈80 km in the best-sampled regions. To complete spatial coverage, the waveform cross-correlation measurements are augmented by ≈5.7 million analyst-picked, teleseismic P arrival times. P, Pdiff and PP traveltimes are jointly inverted for 3-D isotropic P-velocity anomalies in the mantle and for events corrections, by least squares solution of an explicit matrix–vector equation. Inclusion of Pdiff traveltimes (in DETOX-P2, -P3) improves the spatial sampling of the lowermost mantle 100- to 1000-fold compared to teleseismic P waves (DETOX-P1). Below ≈2400 km depth, seismically slow anomalies are clustered at southern and equatorial latitudes, in a dozen or more intensely slow patches of 600–1400 km diameter. These features had long been classed into two large low shear velocity provinces (LLVP), which now appears questionable. Instead, patches of intensely slow anomalies in the lowermost mantle seem to form a nearly continuous, globe-spanning chain beneath the southern hemisphere, according to our increased resolution of LLVP-internal subdivisions and newly imaged patches beneath South America. Our tomography also supports the existence of whole-mantle plumes beneath Iceland, Ascension, Afar, Kerguelen, Canary, Azores, Easter, Galapagos, Hawaii, French Polynesia and the Marquesas. Seismically fast structure in the lowermost mantle is imaged as narrowly elongated belts under Eastern Asia and the Americas, presumably reflecting the palaeo-trench geometries of subduction zones and arcs that assembled Eastern Asia and the American Cordilleras in Palaeozoic and early Mesozoic times. Mid-mantle structure is primarily constrained by teleseismic P waves, but Pdiff data have a stabilizing effect, for example, sharpening the geometries of subducted slabs under the Americas, Eurasia and the Northern Pacific in the upper 2000 km. PP traveltimes contribute complementary constraints in the upper and mid mantle, but they also introduce low-velocity artefacts beneath the oceans, through downward smearing of lithospheric structure. Our three new global P-wave models can be accessed and interactively visualized through the SubMachine web portal (http://submachine.earth.ox.ac.uk/).

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Crustal blocks accreted to North America form two major belts that are separated by a tract of collapsed Jurassic-Cretaceous basins extending from Alaska to Mexico. Evidence of oceanic lithosphere that once underlay these basins is rare at Earth's surface. Most of the lithosphere was subducted, which accounts for the general difficulty of reconstructing oceanic regions from surface evidence. However, this seafloor was not destroyed; it remains in the mantle beneath North America and is visible to seismic tomography, revealing configurations of arc-trench positions back to the breakup of Pangea. The double uncertainty of where trenches ran and how subducting lithosphere deformed while sinking in the mantle is surmountable, owing to the presence of a special-case slab geometry. Wall-like, linear slab belts exceeding 10,000 km in length appear to trace out intra-oceanic subduction zones that were stationary over tens of millions of years, and beneath which lithosphere sank almost vertically. This hypothesis sets up an absolute lower-mantle reference frame. Combined with a complete Atlantic spreading record that positions paleo-North America in this reference frame, the slab geometries permit detailed predictions of where and when ocean basins at the leading edge of westwarddrifting North America were subducted, how intra-oceanic subduction zones were overridden, and how their associated arcs and basement terranes were sutured to the continent. An unconventional paleogeography is predicted in which mid-to late Mesozoic arcs grew in a long-lived archipelago located 2000-4000 km west of Pangean North America (while also consistent with the conventional view of a continental arc in early Mesozoic times). The Farallon Ocean subducted beneath the outboard (western) edge of the archipelago, whereas North America converged on the archipelago by westward subduction of an intervening, major ocean, the Mezcalera-Angayucham Ocean. The most conspicuous geologic prediction is that of an oceanic suture that must run along the entire western margin of North America. It formed diachronously between ca. 155 Ma and ca. 50 Ma, analogous to diachro nous suturing of southwest Pacific arcs to the northward-migrating Australian continent today. We proceed to demonstrate that this suture prediction fits the spatio-temporal evidence for the collapse of at least 11 Middle Jurassic to Late Cretaceous basins wedged between the Intermontane and Insular-Guerrero superterranes, about half of which are known to contain mantle rocks. These relatively late suturing ages run counter to the Middle Jurassic or older timing required and asserted by the prevailing, Andean-analogue model for the North American Cordillera. We show that the arguments against late suturing are controvertible, and we present multiple lines of direct evidence for late suturing, consistent with geophysical observations. We refer to our close integration of surface and subsurface evidence from geology and geophysics as "tomotectonic analysis." This type of analysis provides a str...

Geophysical Journal International, v. 167, n. 1, p. 271-287, 2006. http://dx.doi.org/10.1111/j.1365-246X.2006.03116.xInternational audienceWe have developed a method to measure finite-frequency amplitude and traveltime anomalies of teleseismic P waves. We use a matched filtering approach that models the first 25 s of a seismogram after the P arrival, which includes the depth phases pP and sP. Given a set of broad-band seismograms from a teleseismic event, we compute synthetic Green's functions using published moment tensor solutions. We jointly deconvolve global or regional sets of seismograms with their Green's functions to obtain the broad-band source time function. The matched filter of a seismogram is the convolution of the Green's function with the source time function. Traveltimes are computed by cross-correlating each seismogram with its matched filter. Amplitude anomalies are defined as the multiplicative factors that minimize the RMS misfit between matched filters and data. The procedure is implemented in an iterative fashion, which allows for joint inversion for the source time function, amplitudes, and a correction to the moment tensor. Cluster analysis is used to identify azimuthally distinct groups of seismograms when source effects with azimuthal dependence are prominent. We then invert for one source time function per group. We implement this inversion for a range of source depths to determine the most likely depth, as indicated by the overall RMS misfit, and by the non-negativity and compactness of the source time function. Finite-frequency measurements are obtained by filtering broad-band data and matched filters through a bank of passband filters

Plate reconstructions since the breakup of Pangaea are mostly based on the preserved spreading history of ocean basins, within absolute reference frames that are constrained by a combination of age‐progressive hotspot tracks and paleomagnetic data. The evolution of destructive plate margins is difficult to constrain from surface observations as much of the evidence has been subducted. Seismic tomography can directly constrain paleotrench locations by imaging subducted lithosphere in the mantle. This new evidence, combined with the geological surface record of subduction, suggests that several intraoceanic arcs existed between the Farallon Ocean and North America during late Mesozoic times—in contrast to existing quantitative models that typically show long‐lived subduction of the Farallon plate beneath the continental margin. We present a continuously closing plate model for the eastern Pacific basin from 170 Ma to present, constrained using “tomotectonic analysis”—the integration of surface and subsurface data. During the Middle to Late Jurassic, we show simultaneous eastward and westward subduction of oceanic plates under an archipelago composed of Cordilleran arc terranes. As North America drifts westward, it diachronously overrides the archipelago and its arcs, beginning in the latest Jurassic. During and post‐accretion, Cordilleran terranes are translated thousands of kilometers along the continental margin, as constrained by paleomagnetic evidence. Final accretions to North America occur during the Eocene, ending ~100 Myr of archipelago override. This model provides a detailed, quantitative tectonic history for the eastern Pacific domain, paving the way for tomotectonic analysis to be used in other paleo‐oceanic regions.