Subduction lithosphere

The step-wise model describes the rise of three successive plateaus along discrete vertical boundaries separated in time by tens of millions of years. Attainment of elevation occurs episodically and progresses away from the collision zone at distinct time intervals that are coeval with periods of upper crustal shortening, magmatism related to mantle lithosphere subduction, and high erosion rates that produce sediment and basin infilling to subdue relief (Fig. 2A). 

As slabs of lithosphere subduct into the mantle, olivine and other minerals are carried out of their stability fields and into the stability fields of denser phases, into which they transform. If these changes occur rapidly enough to radiate seismic waves, they constitute earthquakes, and their mechanisms will have isotropic components. They probably will also have deviatoric components, because the process of phase transformation will release shear strain, much as explosions are often observed to release tectonic shear strain. There seems to be no reason, however, why such a deviatoric component should be a DC rather than a CLVD, and in fact the CLVD force system was first invented as a possible mechanism for deep earthquakes caused by phase transformations (Knopoff and Randall 1970). 

Tatsumi Y, Hamilton DL, Nesbitt RW, (1986) Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas evidence from high-pressure experiments and natural rocks. J Volcanol Geotherm Res 29 293-309... 

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

A central tenet to the step-wise model is that the continental lithosphere deforms as a series of small plates, and that plate boundary stresses are sufficient to reactivate old suture boundaries as sites of crustal thickening and mantle subduction far from the plate boundary. The continental crust thickens homogeneously above internally undeformed mantle lithosphere that is partially subducted at these boundaries. Localized strain at sites of strike-slip and mantle subduction also requires a mechanically competent lower crust and mantle lithosphere such that stresses are efficiently transmitted from the mantle lithosphere through the crust. 

The timing and extent of the Colorado Plateau uplift bears on the possible mechanisms that caused it. Various hypotheses have been suggested including influence of a mantle plume (Parsons et al. 1994), lithospheric thinning and/or delamination (Spencer 1996 Lastowka et al. 2001), and subduction of the Farallon plate margin (East Pacific Rise) (Dickinson and Snyder 1979). With a definitive uplift history, it will be possible to place constraints on the mechanisms of uplift and thereby shed light on mantle processes and lithospheric evolution. 

Spencer JE (1996) Uplift of the Colorado Plateau due to Lithosphere attenuation during Laramide low-angle subduction. J Geophys Res 101 13595-13609... 

The Sicily volcanoes are located in a variety of structural settings and on different types of bedrocks. Mount Etna (about 0.6 Ma to present) occurs on the accretionary prism of the Africa-Europe subducting system, at the contact between the Ionian lithosphere, the northern margin of the African plate (Pelagian Block, a promontory of the African foreland) and the corrugated Sicilian-Maghrebian chain (Fig. 8.1 Behncke 2001). 

The variable isotopic compositions (especially for Pb) reveal geochemi-cally heterogeneous sources. The origin of this heterogeneity is debated and may derive either from shallow mantle processes, such as variable me-tasomatic modifications of the lithosphere by asthenospheric melts, or from mixing between a deep-mantle plume and asthenosphere-lithosphere material. Etna and Ustica show some trace element and isotopic characteristics (e.g. Rb/Nb, Ce/Pb, and boron isotopes), indicative of a contribution from subduction-derived components. 

Serri G, Innocenti F, Manetti P (1993) Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quatemary magmatism of central Italy. Tectonophysics 223 117-147... 

The source of diamond-bearing kimberlites is uncertain but the stability field of diamond implies a deep-seated origin. Kimberlites are generally believed to have resulted from metamorphism or melting at the deep end of a subducted oceanic plate, the source material being the oceanic lithosphere with... 

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