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.

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Composition of the Continental Crust

A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

The chemical composition of subducting sediment and its consequences for the crust and mantle

There is now a large published literature on the strengths and weaknesses of downscaling methods for different climatic variables, in different regions and seasons. However, little attention is given to the choice of downscaling method when examining the impacts of climate change on hydrological systems. This review paper assesses the current downscaling literature, examining new developments in the downscaling field specifically for hydrological impacts. Sections focus on the downscaling concept; new methods; comparative methodological studies; the modelling of extremes; and the application to hydrological impacts.

Consideration is then given to new developments in climate scenario construction which may offer the most potential for advancement within the ‘downscaling for hydrological impacts’ community, such as probabilistic modelling, pattern scaling and downscaling of multiple variables and suggests ways that they can be merged with downscaling techniques in a probabilistic climate change scenario framework to assess the uncertainties associated with future projections. Within hydrological impact studies there is still little consideration given to applied research; how the results can be best used to enable stakeholders and managers to make informed, robust decisions on adaptation and mitigation strategies in the face of many uncertainties about the future. It is suggested that there is a need for a move away from comparison studies into the provision of decision-making tools for planning and management that are robust to future uncertainties; with examination and understanding of uncertainties within the modelling system. Copyright © 2007 Royal Meteorological Society

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Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological modelling

Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating

Calibration of the interlaboratory 40Ar-39Ar dating standard, MMhb-1

Differential zircon fertility of source terranes and natural bias in the detrital zircon record: Implications for sedimentary provenance analysis

KAr data has been compiled from over 18 laboratories on the 40Ar39Ar dating standard, MMhb-1. The new recommended age for the monitor is 520.4 ± 1.7 Ma. The interlaboratory precision for K is 1.06% and for Ar is 1.89%. 40Ar-39Ar analyses of MMhb-1 have shown that the 40Ar★40K ratio is uniform at better than the 0.1% level, thus variations in the conventional determinations are a result of systematic variations and interlaboratory biases. We recommend a set of 40Ar39Ar standards be prepared and adopted by the KAr community to reduce uncertainty in intersample age comparison. Further, an organization like the U.S. National Bureau of Standards should be encouraged to “unconventionally” calibrate these monitors if the precision of which the 40Ar39Ar technique is capable is to be approached.

Calibration of the interlaboratory 40Ar39Ar dating standard, MMhb-1

The Yadong cross structure (YCS), occurring at ∼89°30′ east longitude in the Himalaya, is the largest across-strike discontinuity in the geologic structure and topography of the High Himalaya between the Himalayan syntaxes. It is manifest by a plan view left offset of the topographic crest of the range and a coincident, apparent, left strike offset of the South Tibetan Detachment System (STDS) of about 70 km. New field mapping indicates that the STDS intersects the west side of the YCS at Zherger La and further suggests that the YCS is the surface expression of a large north-northeast striking west facing lateral ramp in the Himalayan thrust system. The Greater Himalayan allochthon is apparently draped across this lateral ramp, resulting in a north-northeast striking monoclinal flexure of the allochthon manifest in the Chomolhari range. Superimposed steep west-northwest directed normal shear appears to have obliterated earlier northerly directed STDS shear fabric along along the YCS segment of the Tethyan belt/Greater Himalayan belt contact. Recent seismicity suggests that the Greater Himalayan allochthon is still moving southward along the lateral ramp. U-Pb monazite dates on leucogranites in the footwall of the STDS on opposite sides of the YCS are suggestive of a south-to-north decrease in the crystallization age of these granites. The age pattern is consistent with southward “extrusion” of the Greater Himalayan allochthon and suggests the possibility of determining an average slip rate for the STDS by dating granites in the immediate footwall of the STDS that are substantially separated in north-south section. An average slip rate of about 7 mm/yr between 23 and 12 Ma is suggested by the data presented here.

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Yadong cross structure and South Tibetan Detachment in the east central Himalaya (89°-90°E)

Sm–Nd and Rb–Sr isotopic studies of several Phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked, pre-existing continental crust. In contrast, isotopic data collected from two of the largest terranes in the Canadian Cordilleran orogenic belt, the Alexander and Stikine terranes, show that these tectonic fragments are composed of mantle-derived continental crust. In many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some Proterozoic crustal regions

Scott D. Samson

Papers

Calibration of the interlaboratory 40Ar-39Ar dating standard, MMhb-1

Differential zircon fertility of source terranes and natural bias in the detrital zircon record: Implications for sedimentary provenance analysis

Calibration of the interlaboratory 40Ar39Ar dating standard, MMhb-1

Yadong cross structure and South Tibetan Detachment in the east central Himalaya (89°-90°E)

Evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the Canadian cordillera