Thickness of the lithosphere

The details of lithospheric composition are best considered for individual locations. Here it is only worth emphasizing that the chemical composition of xenoliths reflect both major element depletion events and subsequent enrichment processes, and so have had complex open-system histories. The sources and characteristics of noble gases must consider this environment, as discussed further below. 

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Lithosphere The outermost, rigid layer of the Earth, which includes the crust and upper mantle. The thickness of the lithosphere usually ranges from about 50 km in ocean basins to 100 km in the high mountainous areas of the continents. 

The thickness of the lithosphere along the Pontine-Campania-Vulture transect increases from about 50 km along the Tyrrhenian Sea border to more than 110 km in the Apulia foreland (Calcagnile and Panza 1981). The depth of the Moho increases from about 20-25 km offshore the Tyrrhenian Sea coast to 40 km beneath the internal zones of the Apennines, to decrease to about 30 km beneath the Apulia foreland (e.g. Locardi and Nicolich 1988 Piromallo and Morelli 2003). In contrast with other zones of the Apennine chain, the sector running from the Campania Province to... 

Jaupart C., Mareschal J.-C., Guillou-Frottier L., and DavaiUe A. (1998) Heat flow and thickness of the lithosphere in the Canadian Shield. J. Geophys. Res. 103, 15269-15286. 

Changes in ocean-floor depth, sediment thickness, and hydrothermai circuiation with increasing age of the lithosphere, (a) Heat loss is by advection, (b) heat loss is by advection and conduction, and (c) heat loss is by conduction alone. Source From Sclater, J. G. (2003). Nature 421, 590-591. 

The separations between the layers as indicated in figure 7.1 are not as clear as shown, but are gradual, which results in a gradual change in density and mineral composition. The thickness of the continental earth s crust or lithosphere (Greek lithos = stone) varies between appr. 20 and appr. 60 km, with an average thickness of appr. 35 km. The elementary composition of the lithosphere is well-known,... 

As discussed in the seminal paper by England and Molnar (1990) paleoelevation reflects the combined chemical and physical state of the lithosphere, including thicknesses, thermal structure and bulk chemistry. Tectonic processes such as lithospheric delamination and growth of mountain ranges through either collisional orogenesis or arc evolution may gradually or abruptly... 

The above discussion illustrates that the interplay between the thickness and composition of the lithosphere and the composition and potential temperature of the underlying upper mantle control the composition, source, and volumes of Phanerozoic intraplate continental magmatism. Exactly how these factors are related to the production of continental magmas is best illustrated by considering the formation of specific continental igneous rocks types. 

Rowland A. D. and Davies H. J. (1999) Buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones. Geophys. Res. Lett. 26, 3037-3040. 

These tests show that the average thickness of the seismic lithosphere (crust plus upper-mantle lid) can be as much as c. 160 km and the minimum S-wave velocity beneath the lid can be as high as c. 4.45kms and still produce synthetic waveforms that match the observed waveforms. A thicker lid or higher S-wave velocities at depth below the lid are not consistent with the regional seismic waveforms. It is the high S-wave velocity lid that is unique to the upper mantle of the shield below that, the S-wave velocity is not significantly different from PREM (Preliminary Reference Earth Model) (Fig. 2). 

Our models include the movement of the lithosphere over a stationary hotspot, thereby allowing us to evaluate the effects of plume proximity on the distribution of plume material susceptible to melting. We have considered models with 150 km (Ebinger Sleep 1998) and 220 km thick cratonic keels, and varied the loca-... 

Fig. 5. Thermal evolution of the lithosphere along a cross-section of Africa through 34°E. Arrow shows the location of the plume relative to the northward-moving African plate since 45 Ma. It should be noted that lithosphere cools as y/t south of plume, as we have placed no restriction on maximum thickness of the continental lithosphere. The long-term effect of the plume heating can be crudely estimated, if we assume that a 40 km equivalent thickness of material that is 200 K hotter than normal mantle, and with specific heat 4 x 106 JK m", ponds beneath a craton every 300 Ma. The mantle heat flow is increased by 3.4 mWm, or 20-25% of typical mantle heat flow from cratonal areas (e.g. Jaupart et al. 1998). (b) Thickness of plume material ponded beneath lithosphere 45 Ma after plume onset.

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