Fully self-consistent modelling of the southern African lithospheric/sublithospheric mantle using elevation, surface heat flow, magnetotelluric, surface wave, and petrological data.
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Magnetotellurics
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Core‐mantle boundary (CMB) topography is estimated from numerical simulations of instantaneous mantle flow in a three‐dimensional spherical shell geometry. Density anomaly models of the Earth's mantle are derived from geodynamic models constrained principally by seismic tomography results. To account for the small (±∼1.5 km) CMB topographic relief inferred from recent seismological results, lateral viscosity variations in the mantle, compositionally dense piles in the deep mantle, and a low‐viscosity D″ layer are all required for the numerical models. Low‐viscosity subducting slabs in the lower mantle serve to reduce the topographic amplitude below high‐velocity regions (i.e., the circum‐Pacific belt). The existence of dense piles may cause the local topographic depressions even below low‐velocity regions (i.e., the South Pacific and Atlantic Africa regions).
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Seismic Tomography
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Magnetotellurics
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Recent advances in computational petrological modeling provide accurate methods for computing seismic velocities and density within the lithospheric and sub‐lithospheric mantle, given the bulk composition, temperature, and pressure within them. Here, we test an integrated geophysical‐petrological inversion of Rayleigh‐ and Love‐wave phase‐velocity curves for fine‐scale lithospheric structure. The main parameters of the grid‐search inversion are the lithospheric and crustal thicknesses, mantle composition, and bulk density and seismic velocities within the crust. Conductive lithospheric geotherms are computed using P‐T‐dependent thermal conductivity. Radial anisotropy and seismic attenuation have a substantial effect on the results and are modeled explicitly. Surface topography provides information on the integrated density of the crust, poorly constrained by surface waves alone. Investigating parameter inter‐dependencies, we show that accurate surface‐wave data and topography can constrain robust lithospheric models. We apply the inversion to central Mongolia, south of the Baikal Rift Zone, a key area of deformation in Asia with debated lithosphere‐asthenosphere structure and rifting mechanism, and detect an 80–90 km thick lithosphere with a dense, mafic lower crust and a relatively fertile mantle composition (Mg# < 90.2). Published measurements on crustal and mantle Miocene and Pleistocene xenoliths are consistent with both the geotherms and the crustal and lithospheric mantle composition derived from our inversion. Topography can be fully accounted for by local isostasy, with no dynamic support required. The mantle structure constrained by the inversion indicates no major thermal anomalies in the shallow sub‐lithospheric mantle, consistent with passive rifting in the Baikal Rift Zone.
Asthenosphere
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Low-velocity zone
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Rift zone
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Seismic tomography can provide unique information on the structure of the subcontinental lithospheric mantle (SCLM), but seismic velocity reflects both temperature and composition. We present a methodology for evaluating and isolating the relative contributions of these effects, which produces maps of regional geotherm and broad compositional constraints on the SCLM from the inversion of shear wave (Vs) seismic tomography. This approach uses model geotherms quantized in steps of 2.5 mW/m 2 and three mantle compositions corresponding to typical Archean, Proterozoic, and Phanerozoic SCLM. Starting from an assumed composition for a volume of SCLM, lithospheric density at surface pressure and temperature is calculated for each geotherm at each point; the optimum geotherm is taken as the one yielding a density closest to the mean value derived from mantle xenoliths (3.31 g/cm 3 ), since density varies with composition. Results requiring densities or geotherms outside the known natural range of these parameters worldwide require the choice of a different mantle composition. This technique, applied iteratively to a 275 km × 275 km Vs model developed by S. Grand (University of Texas, Austin), results in maps of the geotherm and regional density, which allow interpretation of SCLM composition within broad limits. These results can then be compared with local (paleo)geotherms and data for mantle composition, derived from xenolith suites. Application of this technique to the SCLM beneath Africa, Siberia, and North America shows good correlation with regional geological features, xenolith data, and other geophysical data.
Seismic Tomography
Xenolith
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Abstract We apply a novel 3‐D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite‐derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present‐day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere‐asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small‐scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present‐day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%.
Seismic Tomography
Asthenosphere
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A combined geophysical‐petrological methodology to study the thermal, compositional, density, and seismological structure of lithospheric/sublithospheric domains is presented. A new finite‐element code (LitMod) is used to produce 2‐D forward models from the surface to the 410‐km discontinuity. The code combines data from petrology, mineral physics, and geophysical observables within a self‐consistent framework. The final result is a lithospheric/sublithospheric model that simultaneously fits all geophysical observables and consequently reduces the uncertainties associated with the modeling of these observables alone or in pairs, as is commonly done. The method is illustrated by applying it to both oceanic and continental domains. We show that anelastic attenuation and uncertainties in seismic data make it unfeasible to identify compositional variations in the lithospheric mantle from seismic studies only. In the case of oceanic lithosphere, plates with thermal thicknesses of 105 ± 5 km satisfy geophysical and petrological constraints. We find that Vp are more sensitive to phase transitions than Vs, particularly in the case of the spinel‐garnet transition. A low‐velocity zone with absolute velocities and gradients comparable to those observed below ocean basins is an invariable output of our oceanic models, even when no melt effects are included. In the case of the Archean subcontinental lithospheric mantle, we show that “typical” depleted compositions (and their spatial distribution) previously thought to be representative of these mantle sections are compatible neither with geophysical nor with petrological data. A cratonic keel model consisting of (1) strongly depleted material (i.e., dunitic/harzburgitic) in the first 100–160 km depth and (2) less depleted (approximately isopycnic) lower section extending down to 220–300 km depth is necessary to satisfy elevation, geoid, SHF, seismic velocities, and petrological constraints. This highly depleted (viscous) upper layer, and its chemical isolation, may play a key role in the longevity and stability of cratons.
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