نتایج جستجو برای: continental crust
تعداد نتایج: 29224 فیلتر نتایج به سال:
The age distribution of the crust is a fundamental parameter in modeling continental evolution and the rate of crustal accretion through Earth’s history, but this is usually estimated from surface exposures. The exposed Yangtze craton in eastern China consists mainly of Proterozoic rocks with rare Archean outcrops. However, the U-Pb ages and Hf isotope systematics of xenocrystic zircons brought...
Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such p...
The results of a laser ablation microprobe–inductively coupled by the more widely used samarium–neodymium system (e.g. Dickin, 1995). As a tracer of petrogenetic processes, plasma mass spectrometry Lu–Hf isotope study of zircons in however, Lu–Hf has one important advantage over Sm– 0·93–1·67 Ga rocks from south Norway indicate that early Nd: zircons retain a robust memory of their initial Hf P...
[1] In this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back-arc basin combining refraction and wide-angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)-2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented inclu...
Sm-Nd isotopic data on Late Archean age metasedimentary rocks from around the Wyoming province yield ENd values at 2.6 Ga from -12.9 to +3.3, which correspond to depleted mantle model ages of 2.7 to 3.8 Ga. These results indicate that crust of a wide variety of ages was exposed and eroding in Late Archean time. The predominance of Middle Archean model ages suggests that continental crust was ex...
Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998^99) and four from Legs 119 and 120 (1987^88), show that the dominant rocks are basalts with geochemical characteristics distinct from ...
Oceanic iron inputs must be traced and quantified to learn how they affect primary productivity and climate. Chemical reduction of iron in continental margin sediments provides a substantial dissolved flux to the oceans, which is isotopically lighter than the crust, and so may be distinguished in seawater from other sources, such as wind-blown dust. However, heavy iron isotopes measured in seaw...
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