David E. James

Bio: David E. James is an academic researcher from Carnegie Institution for Science. The author has contributed to research in topics: Mantle (geology) & Craton. The author has an hindex of 41, co-authored 94 publications receiving 5960 citations. Previous affiliations of David E. James include Stanford University & University of Washington.

Physical, chemical, and chronological characteristics of continental mantle

Abstract: [1] Unlike in the ocean basins where the shallow mantle eventually contributes to the destruction of the overlying crust, the shallow mantle beneath continents serves as a stiff, buoyant “root” whose presence may be essential to the long-term survival of continental crust at Earth's surface. These distinct roles for subcrustal mantle come about because the subcontinental mantle consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate. Melt removal causes the continental mantle to be cold and strong but also buoyant compared to oceanic mantle. These characteristics allow thick sections of cold mantle to persist beneath continental crust in some cases for over 3 billion years. If the continental mantle becomes gravitationally unstable, however, its detachment from the overlying crust can cause major episodes of intracontinental deformation and volcanism.

Plate Tectonic Model for the Evolution of the Central Andes

Abstract: Data on the geophysics and geology of the central Andes are interpreted in terms of plate theory and a model for Andean evolution is presented. Analysis of upper mantle structure and seismicity shows that the underthrusting Pacific plate is now about 50 km thick and the overriding South American plate 200 to 300 km thick. Underthrusting of the Pacific plate probably began in Triassic time and has continued without substantial change to the present. Prior to underthrusting, the west coast of South America was quiescent, and great thicknesses of Paleozoic continental shelf deposits were laid down in an area east of the present volcanic arc. In Late Triassic or Early Jurassic time, an incipient arc began to form at or west of the present coast of South America. Igneous activity has since migrated eastward, culminating in the Pliocene-Pleistocene volcanic episode. The crust beneath the volcanic cordillera is more than 70 km thick and probably consists largely of rocks compositionally equivalent to those of the volcano-plutonic suites observed at the surface. Increase in crustal volume of the volcanic arc between Cretaceous time and the present implies either that the mantle above the under-thrust plate has undergone 18 to 36 percent partial melting or that 1 to 2 km of rock has been melted from the underthrusting plate. The intrusion of melt into the crust beneath the volcanic cordillera and the resultant crustal dilatation produced continentward compression of the Paleozoic sedimentary rocks which form an easterly belt of thrust and fold mountains. Here crustal shortening has produced crustal thicknesses of 50 to 55 km. Few deposits of the type normally termed eugeosynclinal, and no ophiolites, are observed between trench and volcanic arc; only in the intermontane foredeep behind the arc has a clastic wedge of geosynclinal proportions formed.

Tectospheric structure beneath southern Africa

Abstract: P-wave and S-wave delay times from the broadband data of the southern Africa seismic experiment have been inverted to obtain three-dimensional images of velocity perturbations in the mantle beneath southern Africa. High velocity mantle roots appear to extend to depths of at least 250 km, and locally to depths of 300 km beneath the Kaapvaal and Zimbabwe cratons. Thick roots are confined to the Archean cratons, with no evidence for similar structures beneath the adjacent Proterozoic mobile belts. The Kaapvaal craton was modified ca. 2.05 Ga by the Bushveld magmatic event, which affected a broad swath of cratonic mantle beneath and to the west of the exposed Bushveld Complex. The mantle beneath the extended Bushveld province is characterized by seismic velocities lower than those observed in regions of undisturbed cratonic mantle. The mantle beneath the Limpopo Belt, an Archean collisional zone sandwiched between the Kaapvaal and Zimbabwe cratons, exhibits a cratonic signature.

Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces

Abstract: A teleseismic travel-time study reveals the presence of a fossil plume in the deep upper mantle beneath Brazil, which has apparently remained geographically fixed with respect to the overly-ing continent despite thousands of kilometres of plate motion. This result implies that the upper mantle and lithosphere beneath South America have remained coupled since the breakup of Gondwanaland and may provide an answer to the long-standing question of what forces drive continental plates.

Andean crustal and upper mantle structure

Abstract: Phase and group velocities of Rayleigh and Love waves have been used to derive models of the structure of the crust and upper mantle beneath southern Peru, Bolivia, and northern Chile. A three-dimensional model of crustal structure has been obtained that shows crustal thickness varying from about 11 km (including water layer) in the ocean basin to 30 km along the coast to more than 70 km beneath the western cordillera and western part of the altiplano. The crust thins eastward and beneath the eastern cordillera is only 50–55 km thick. The crust beneath the crest of the Andes appears to thin to the north and south of the altiplano region, and the maximum crustal thickness in those parts of the Andes not associated with the altiplano may be on the order of 55–60 km. Mean crustal velocities within this region of the Andes are characteristically low; typical values of mean P- and S-wave velocities are ∼6.2 km/sec and 3.45 km/sec, respectively. No significant low-velocity zone for shear waves has been found in the upper 150 km of the mantle beneath the continental area studied, although subcrustal velocities for the entire region are somewhat low. The lowest uppermost mantle velocities measured are for the region between La Paz, Bolivia, and Huancayo, Peru. Here the upper mantle shear velocity corresponds to the axis of the high electrical conductivity anomaly that has been found in the Andean region. Beneath the oceanic areas, a major shear-wave low-velocity zone is required to satisfy phase- and group-velocity observational data. The top of this zone is at a depth of 50–60 km, and the low velocities extend to a depth probably in excess of 200 km. There is a slight suggestion that the upper boundary of the low-velocity zone may be dipping toward the continent.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Abstract: Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Composition of the Continental Crust

Abstract: This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.