Search

We investigate the structure of the continental lithosphere by combining two approaches: a systematic survey of abrupt changes in seismic properties detected by P‐to‐S converted body waves and an integrated geophysical‐petrological inversion for temperature and density in the upper mantle. We refine the global thermo‐chemical model WINTERC‐G in eastern North America by including detailed regional information on the crust into petrological inversions and combine it with the upper mantle layering beneath eastern North America yielded by anisotropy‐aware receiver‐function analysis. Eastern North America's Archean, Proterozoic and Paleozoic lithospheres show an excellent agreement between the depth to the 1,300°C isotherm that bounds the lithosphere and the depth range where converted waves detect abrupt changes in seismic properties. Boundaries with these abrupt changes reside within the rigid mechanical lithosphere and are uncommon in the convecting mantle beneath it. The boundaries include both impedance increases and decreases with depth, as well as anisotropy changes, and must have developed over the course of the assembly and evolution of the lithosphere. In the asthenosphere below, such heterogeneities appear to have been largely mixed out by convection. The existence of abundant interfaces with diverse origin can account for the commonly observed scattered signals from within the continental lithosphere and presents an alternative to the end‐member concept of the mid‐lithospheric discontinuity as a ubiquitous feature with a uniform origin. Generally, we can define continental lithosphere as a region of conductive heat transport and steep geotherm that is characterized by pervasive internal layering of density, elastic moduli and texture. Plain Language Summary: The outer shell of the Earth called the lithosphere is defined most generally as a layer of rigid colder material overlying a warmer and weaker asthenosphere below. Within the lithosphere heat is conducted from the interior to the outside, resulting in a characteristic increase in temperature with depth. The region below it convects on geological (millions of years) time scale and has a different depth dependence of temperature. In this contribution we use two definitions of the lithosphere to estimate its vertical extent beneath a region of North American continent where lithosphere formed between 3 and 0.3 Ga ago. We detect abrupt changes in physical properties in the upper layers of the Earth using seismic waves from distant earthquakes. We perform a joint inversion of geophysical and petrological constraints to derive temperature profiles with depth. Boundaries scattering seismic waves are detected throughout the depth range where temperature is below 1,300°C. The deepest scattering boundaries are close to this depth in areas where lithosphere formed within the last billion years. In the oldest part of the continent the deepest boundaries are ∼50 km above the temperature of 1,300°C, confirming long‐recognized difference of such ancient lithosphere. Key Points: Beneath eastern North America sharp vertical gradients in seismic properties are pervasive, vary in nature and number, and extend to ∼170 kmDetected gradients reside above the depth to the thermally and mechanically defined lithosphere‐asthenosphere boundaryContinental lithosphere retains pervasive internal layering of density, elastic moduli, and texture, which is mixed out in the asthenosphere [ABSTRACT FROM AUTHOR]

The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P‐to‐s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990–2019. We exploit a recent high‐resolution African continental P‐wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (∼100–150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ∼1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar. Plain Language Summary: The African plate has experienced widespread Cenozoic hotspot and rift‐related activity during the development of the East African Rift, the Cameroon Volcanic Line, and other magmatic provinces, for example, central Madagascar. Warmed by the Earth's core, hot mantle rock rises slowly over millions of years within mantle plumes, and melts upon reaching the near‐surface. Mantle plumes may therefore be partly responsible for the formation of volcanoes and crustal magmatism across Africa. However, the number, size, and physical properties of African mantle plumes are debated because geochemical and seismic imaging techniques often used to investigate them are typically sensitive to temperature or mineral chemistry. Instead, we measure the depth at which seismic waves undergo conversions from compressional‐to‐transverse that reflect where mantle minerals undergo changes in their crystal structure. This informs us of variations in both temperature and mineral chemistry in the mantle because both factors affect the precise crystal rearrangement depths. Mantle plumes rooted below 1,000 km depth may exist below East Africa and perhaps also, Cameroon, and Madagascar. Specifically, warm mantle rising below Kenya and Madagascar may transport material from chemically distinct regions on the core‐mantle boundary. Plumes elsewhere are likely purely thermal in nature, not sampling chemically distinct deep mantle regions. Key Points: We investigate mantle transition zone and mid mantle discontinuity structure below Africa using P to s receiver functionsA thin transition zone underlies East African Cenozoic magmatism implying influence of processes at mantle transition zone depths or belowA thermochemically distinct upwelling underlies Kenya while a purely thermal upwelling underlies Ethiopia [ABSTRACT FROM AUTHOR]

We have calculated the structure and elasticity of low-spin ferromagnetic ε-FeOOH to 140 GPa using density functional theory calculations with a Coulombic self-interaction term ( U). Using these data, the elastic moduli and sound velocities of ε-FeOOH were calculated across the pressure stability of the hydrogen bond symmetrized structure (30 to 140 GPa). The obtained values were compared with previously published values for phase H (MgSiH2O4) and δ-AlOOH, which likely form a solid solution with ε-FeOOH. In contrast to these Mg and Al end-members, ε-FeOOH has smaller diagonal and larger off-diagonal elastic constants, leading to an eventual negative pressure dependence of its shear wave velocity. Because of this behavior, iron-enriched solid solutions from this system have smaller shear wave velocities than surrounding mantle and therefore are a plausible contributor to large low-shear velocity provinces (LLSVPs) which exhibit similar seismic properties. Additionally, ε-FeOOH has substantial shear wave polarization anisotropy. Consequently, if iron-rich solid solutions from the FeOOH-AlOOH-MgSiH2O4 system at the core-mantle boundary exhibit significant lattice-preferred orientation due to the strong shear stresses which occur there, it may help explain the seismically observed S H > S V anisotropy in this region. [ABSTRACT FROM AUTHOR]