ALBERT

Description: The rapid expansion of broad-band seismic networks over the last decade has paved the way for a new generation of global tomographic models. Significantly improved resolution of global upper-mantle and crustal structure can now be achieved, provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized. Here, we present a new global, vertically polarized shear speed model that yields considerable improvements in resolution, compared to previous ones, for a variety of features in the upper mantle and crust. The model, SL2013sv, is constrained by an unprecedentedly large set of waveform fits (~3/4 of a million broad-band seismograms), computed in seismogram-dependent frequency bands, up to a maximum period range of 11–450 s. Automated multimode inversion of surface and S -wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram. The equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers. Going beyond ray theory, we calculated the phase of every mode at every frequency and its derivative with respect to S - and P -velocity perturbations by integration over a sensitivity area in a 3-D reference model; the (normally small) perturbations of the 3-D model required to fit the waveforms were then linearized using these accurate derivatives. The equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3-D model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle. Elaborate outlier analysis was used to control the propagation of errors in the data (source parameters, timing at the stations, etc.). The selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors, increasing the resolution of the imaging. Our new shear speed model is parametrized on a triangular grid with a ~280 km spacing. In well-sampled continental domains, lateral resolution approaches or exceeds that of regional-scale studies. The close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark. In oceanic regions, spreading ridges are very well resolved, with narrow anomalies in the shallow mantle closely confined near the ridge axis, and those deeper, down to 100–120 km, showing variability in their width and location with respect to the ridge. Major subduction zones worldwide are well captured, extending from shallow depths down to the transition zone. The large size of our waveform fit data set also provides a strong statistical foundation to re-examine the validity field of the JWKB approximation and surface wave ray theory. Our analysis shows that the approximations are likely to be valid within certain time–frequency portions of most seismograms with high signal-to-noise ratios, and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting.

Description: This work presents a teleseismic P -wave receiver function study on 34 stations deployed across Ireland in order to determine the first-order crustal properties, thickness ( H ) and mean crustal V p / V s , over the entire island. We apply the H – V p / V s stacking method, which exploits the information contained in both the Ps and the multiple phases from the free-surface. In this way, we obtain the first Moho depth and V p / V s maps of Ireland based on a uniform distribution of measurements. The results are used to examine in detail the lateral variation of crustal thickness and V p / V s ratio across the major terrane boundaries in Ireland. Our results show a good agreement with the available previous estimates from onshore wide-angle/refraction experiments and add new information in poorly constrained areas such as Northern Ireland and the NW coast of Ireland. The mean V p / V s ratio is 1.73 ± 0.05 with a consistently low (1.70) value in the Leinster domain and in central Ireland. The mean crustal thickness is 30.9 ± 2.3 km. The southern portion of the island shows a nearly flat Moho at a depth of 32–33 km, while north of the Southern Uplands Fault, a relatively higher spatial frequency variation in Moho topography exists with values ranging from 28 to 32 km. This reflects the complex history of multiphase terranes accretion during the Caledonian orogeny, although locally, the superposition of more recent geological processes is not excluded. Crossing the Iapetus Suture Zone, our results support the presence of a ‘transitional’ Moho, that is, a 3–4 km smooth seismic transition between crust and mantle, while Moho depth remains constant. Anomalous values in Northern Ireland are interpreted as evidence of a 5- to 6-km-thick high S -wave velocity layer just above the Moho.

Description: Azimuthal seismic anisotropy, the dependence of seismic wave speeds on propagation azimuth, is largely due to fabrics within the Earth's crust and mantle, produced by deformation. It thus provides constraints on the distribution and evolution of deformation within the upper mantle. Here, we present a new global, azimuthally anisotropic model of the crust, upper mantle and transition zone. Two versions of this new model are computed: the rough SL2016svAr and the smooth SL2016svA. Both are constrained by a very large data set of waveform fits (~750 000 vertical component seismogram fits). Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms in broad period ranges (dominantly from 11 to 450 s, with the best global sampling in the 20–350 s range), yielding resolving power from the crust down to the transition zone. In our global tomographic inversion, regularization of anisotropy is implemented to more uniformly recover the amplitude and orientation of anisotropy, including near the poles. Our massive waveform data set, with complementary large global networks and high-density regional array data, produces improved resolution of global azimuthal anisotropy patterns. We show that regional scale variations, related to regional lithospheric deformation and mantle flow, can now be resolved by the global models, in particular in densely sampled regions. For oceanic regions, we compare quantitatively the directions of past and present plate motions and the fast-propagation orientations of anisotropy. By doing so, we infer the depth of the boundary between the rigid, high-viscosity lithosphere (preserving ancient, frozen fabric) and the rheologically weak asthenosphere (characterized by fabric developed recently). The average depth of thus inferred rheological lithosphere-asthenosphere boundary (LAB) beneath the world's oceans is ~115 km. The LAB depth displays a clear dependence on the age of the oceanic lithosphere, closely matching the 1200 °C half-space cooling isotherm for all oceanic ages. In continental regions, azimuthal anisotropy is characterized by smaller-scale 3-D variations. Quantitative comparisons of the tomographic models with global SKS splitting measurements confirm the basic agreement of the two types of anisotropy analysis; they also offer a new insight into the average rheological thickness of continental lithosphere. In spite of significant recent improvements in the resolution of upper-mantle anisotropic structure, correlations between the anisotropic components of current global tomographic models remain much lower than between the isotropic ones. Our comparisons of the current models show which features are resolved consistently by different models, and therefore provide a means to estimate the robustness of anisotropic patterns and amplitudes. Significantly lower correlations are observed at depths greater than ~300 km, compared to those shallower, which suggests that global azimuthal anisotropy models are yet to reach consensus on the nature of anisotropy in the transition zone.

Description: Lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal; however, its relation to neotectonic activity remains unclear. In northwestern Canada, abundant seismicity throughout the northern Canadian Cordillera (NCC) is geographically confined by several crustal-scale boundaries, yet its southern extent terminates abruptly along the inferred westward extension of a Late Cretaceous rifted margin boundary called the Liard transfer zone (LTZ). We use seismic data to show that the uppermost mantle beneath the Cordillera exhibits a sharp north-south contrast in fabric across the LTZ. South of the LTZ, fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent Canadian shield. North of the LTZ, fast axes are reoriented subparallel to the motion of the Pacific plate and follow the strike of the large dextral strike-slip Tintina and Denali faults. We attribute changes in anisotropic delay times across the Tintina and Denali faults to localized shear within the lithosphere; this implies that the crust and lithospheric mantle remained mechanically coupled during shearing. We propose that the contrast in uppermost mantle structure across the LTZ reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures, which affects the stability of the lithosphere and limits the extent of seismic activity within the NCC. These results indicate that neotectonic activity in modern Cordilleras is controlled in part by inherited upper mantle structures.

Description: We use new models of crustal structure and the depth of the lithosphere–asthenosphere boundary to calculate the geopotential energy and its corresponding geopotential stress field for the High Arctic. Palaeostress indicators such as dykes and rifts of known age are used to compare the present day and palaeostress fields. When both stress fields coincide, a minimum age for the configuration of the lithospheric stress field may be defined. We identify three regions in which this is observed. In north Greenland and the eastern Amerasia Basin, the stress field is probably the same as that present during the Late Cretaceous. In western Siberia, the stress field is similar to that in the Triassic. The stress directions on the eastern Russian Arctic Shelf and the Amerasia Basin are similar to that in the Cretaceous. The persistent misfit of the present stress field and Early Cretaceous dyke swarms associated with the High Arctic Large Igneous Province indicates a short-lived transient change in the stress field at the time of dyke emplacement. Most Early Cretaceous rifts in the Amerasia Basin coincide with the stress field, suggesting that dyking and rifting were unrelated. We present new evidence for dykes and a graben structure of Early Cretaceous age on Bennett Island.

Description: At subduction zones, the deep seismogenic transition from a frictionally locked to steady sliding interface is thought to primarily reflect changes in rheology and fluid pressure and is generally located offshore. The development of fluid pressures within a seismic low-velocity layer (LVL) remains poorly constrained due to the scarcity of dense, continuous onshore-offshore broadband seismic arrays. We image the subducting Juan de Fuca oceanic plate in northern Cascadia using onshore-offshore teleseismic data and find that the signature of the LVL does not extend into the locked zone. Thickening of the LVL down dip where viscous creep dominates suggests that it represents the development of an increasingly thick and fluid-rich shear zone, enabled by fluid production in subducting oceanic crust. Further down dip, episodic tremor, and slip events occur in a region inferred to have locally increased fluid pressures, in agreement with electrical resistivity structure and numerical models of fault slip.

Description: Waveform tomography with very large datasets reveals the upper-mantle structure of the Arctic in unprecedented detail. Using tomography jointly with computational petrology, we estimate temperature in the lithosphere–asthenosphere depth range and infer lithospheric structure and evolution. Most of the boundaries of the mantle roots of cratons in the Arctic are coincident with their geological boundaries at the surface. The thick lithospheres of the Greenland and North American cratons are separated by a corridor of thin lithosphere beneath Baffin Bay and through the middle of the Canadian Arctic Archipelago; the southern archipelago is part of the North American Craton. The mantle root of the cratonic block beneath northern Greenland may extend westwards as far as central Ellesmere Island. The Barents and Kara seas show high velocities indicative of thick lithosphere, similar to cratons. The locations of intraplate basaltic volcanism attributed to the High Arctic Large Igneous Province are all on thin, non-cratonic lithosphere. The lithosphere beneath the central part of the Siberian Traps is warmer than elsewhere beneath the Siberian Craton. This observation is consistent with lithospheric erosion associated with the large igneous province volcanism. A corridor of relatively low seismic velocities cuts east–west across central Greenland. This indicates lithospheric thinning, which appears to delineate the track of the Iceland hotspot. Supplementary material: Figures with comparisons of different tomographic models at 50 and 200 km depths are available at https://doi.org/10.6084/m9.figshare.c.3817810

Description: Lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal; however, its relation to neotectonic activity remains unclear. In northwestern Canada, abundant seismicity throughout the northern Canadian Cordillera (NCC) is geographically confined by several crustal-scale boundaries, yet its southern extent terminates abruptly along the inferred westward extension of a Late Cretaceous rifted margin boundary called the Liard transfer zone (LTZ). We use seismic data to show that the uppermost mantle beneath the Cordillera exhibits a sharp north-south contrast in fabric across the LTZ. South of the LTZ, fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent Canadian shield. North of the LTZ, fast axes are reoriented subparallel to the motion of the Pacific plate and follow the strike of the large dextral strike-slip Tintina and Denali faults. We attribute changes in anisotropic delay times across the Tintina and Denali faults to localized shear within the lithosphere; this implies that the crust and lithospheric mantle remained mechanically coupled during shearing. We propose that the contrast in uppermost mantle structure across the LTZ reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures, which affects the stability of the lithosphere and limits the extent of seismic activity within the NCC. These results indicate that neotectonic activity in modern Cordilleras is controlled in part by inherited upper mantle structures.