Combing geochemical and seismological results constrains the composition of the middle and lower continental crust better than either field can achieve alone. The inaccessible nature of the deep crust (typically >15 km) forces reliance on analogue samples and modeling results to interpret its bulk composition, evolution, and physical properties. A common practice relates major oxide compositions of small- to medium-scale samples (e.g. medium to high metamorphic grade terrains and xenoliths) to large scale measurements of seismic velocities (Vp, Vs, Vp/Vs) to determine the composition of the deep crust. We provide a framework for building crustal models with multidisciplinary constraints on composition. We present a global deep crustal model that documents compositional changes with depth and accounts for uncertainties in Moho depth, temperature, and physical and chemical properties. Our 3D deep crust global compositional model uses the USGS global seismic database (Mooney, 2015) and a compilation of geochemical analyses on amphibolite and granulite facies lithologies (Sammon McDonough, 2021). We find a compositional gradient from 61.2 ± 7.3 to 53.8 ± 3.0 wt.% SiO2 from the middle to the base of the crust, with the equivalent lithological gradient ranging from quartz monzonite to gabbronorite. In addition, we calculate trace element abundances as a function of depth from their relationships to major oxides. From here, other lithospheric properties, such as Moho heat flux, are derived (18.8 ± 8.8 mW/m2). This study provides a global assessment of major element composition in the deep continental crust.

A high-resolution 3-D crustal and upper-mantle shear-wave velocity model of Northeast China is established by joint inversion of receiver functions and Rayleigh wave group velocities. The teleseismic data for obtaining receiver functions are collected from 107 CEA permanent sites and 118 NECESSArray portable stations. Rayleigh wave dispersion measurements are extracted from an independent tomographic study. Our model exhibits unprecedented detail in S-velocity structure. Particularly, we discover a low S-velocity belt at 7.5-12.5 km depth covering entire Northeast China (except the Songliao basin), which is attributed to a combination of anomalous temperature, partial melts and fluid-filled faults related to Cenozoic volcanism. Localized crustal fast S-velocity anomaly under the Songliao basin is imaged and interpreted as late-Mesozoic mafic intrusions. In the upper mantle, our model confirms the presence of low velocity zones below the Changbai mountains and Lesser Xing’an mountain range, which agree with models invoking sub-lithospheric mantle upwellings. We observe a positive S-velocity anomaly at 50-90 km depth under the Songliao basin, which may represent a depleted and more refractory lithosphere inducing the absence of Cenozoic volcanism. Additionally, the average lithosphere-asthenosphere boundary depth increases from 50-70 km under the Changbai mountains to 100 km below the Songliao basin, and exceeds 125 km beneath the Greater Xing’an mountain range in the west. Furthermore, compared with other Precambrian lithospheres, Northeast China likely has a rather warm crust (~480-970 °C) and a slightly warm uppermost mantle (~1200 °C), probably associated with active volcanism. The Songliao basin possesses a moderately warm uppermost mantle (~1080 °C).

Indonesian seismicity provides important insights into the tectonics and hazards of a region that is characterized by a remarkable diversity in faulting, including subduction, extension, thrusting, and strike-slip faulting. We present a synthesis of Indonesian seismotectonics by documenting the distributions of hypocenters (≥ M 4.6) and focal mechanisms (≥ M 5.0) over ~20 years, quantifying seismicity rates, and comparing observed seismicity trends with proposed tectonic models. Of the 20,622 events ≥ M 4.6 observed in the study region, ~77% of seismicity are shallow (≤ 70 km depth) and of magnitudes ≤ M 5.0 (68%). 61 events ≥ M 7.0 occurred, five of which exceeded M 8.0, including the 2004 Mw 9.1 Sumatra-Andaman earthquake. Regionally, about ~320 ≥ M 5.0 earthquakes occur per year, and rates decrease exponentially between 50-300 km with significantly elevated seismicity in the Mantle Transition Zone (MTZ). Intermediate and deep events (≥ 70 km depth) trace the Wadati-Benioff zones of several subducting slabs exhibiting a geometry consistent with recent tomography models. Seismicity extends to a maximum depth of 678 km. Oblique convergence, lithospheric age, ambient mantle temperatures and viscous resistance at the 410, 520, and 660 km phase boundaries likely contribute to the non-uniform depth distribution of intermediate and deep earthquakes. Shallow seismicity provides insight into how complex oblique convergence is accommodated near the surface, with primary sources including megathrusting, crustal faulting, and shallow intraslab faulting. All sources of shallow seismicity constitute significant seismic hazards.

Extension and rifting of the lithosphere is fundamental to the evolution of the continents, but the mechanism by which the lithosphere thins remains enigmatic. Using new dense magnetotelluric array data collected within the rifted margin and adjacent areas of Southeast China, we resolve the three-dimensional electrical structure of the lithosphere to constrain the process of rifting and thinning. Our measurements discover a brittle-ductile transition zone featuring low electrical resistivity and low seismic velocity in the Cathaysia Block. A southeast-directed dip is resolved for the Jiangshan-Shaoxing Fault that documents the Neoproterozoic suturing of the Yangtze and Cathaysia Blocks, and has been reactivated by the Early Paleozoic and Early Mesozoic intracontinental orogenies. It acted as a low-angle detachment fault during the Mesozoic-Cenozoic extension and rifting. Given the asymmetries of topography, electrical resistivity, Bouguer gravity anomaly and Mesozoic volcanism across the Gan-Hang Rift, an asymmetric simple shear extension model is proposed for the South China Mesozoic-Cenozoic rift system. Water content of up to 0.1 wt% and melt fraction of up to 1% are estimated at 70 km depth beneath the central Wuyi Mountains, suggesting hydration of the mantle lithosphere. The hydration weakening of the mantle lithosphere promoted both the gravitational instability and convective removal of the lowermost lithosphere in South China.

Recently, the continually increasing availability of seismic data has allowed high-resolution imaging of lithospheric structure beneath the African cratons. In this study, S-wave seismic tomography are combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure are employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S-wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons.