by Staff Writers
Beijing, China (SPX) Nov 28, 2012
Archean cratons are commonly underlain by a cold, thick (>200 km), isotopically enriched, and compositionally refractory lithospheric keel, and represent some of the most stable regions on our planet.
However, the North China Craton (NCC), an old continental geological structure dating back 4.0 Ga, is underlain by a thin lithosphere (<100 km) and shows many signs of thermo-tectonic reactivation. This observation challenges the traditional view that the lithosphere beneath cratons has always been stable and supports the new concept of "craton destruction".
This concept has now been widely accepted and has contributed to our understanding of the processes of intraplate magmatism generation and ore-forming, and of continental evolution in general. In 2006, Chinese earth scientists sponsored by the National Natural Science Foundation of China (NSFC) embarked on a major research program entitled "The Destruction of the North China Craton".
Over the past six years, they have carried out a comprehensive study covering the geology, geophysics, and geochemistry of the NCC with a global perspective using the "natural laboratory research" scientific model. A summary of recent developments and outcomes, taking the same title of the NSFC's major research program, was published in Science China, Earth Sciences, 2012, Vol. 55(10).
The scientists focused on important scientific issues relating to NCC destruction including the spatio-temporal distribution of the NCC, the structure of the deep earth and thermal-tectonic-fluid processes, correlation between the surface geology and cratonic destruction, processes and mechanisms, and the role of cratonic destruction in global geological and continental evolution.
Using mobile seismic stations and in-situ isotope tracer technology, they obtained high-resolution, large-scale and multi-attribute observations, and collected and analyzed a huge amount of data. Interdisciplinary approaches based on the observations and experimental data have resulted in new evidence and insights into the destruction of the NCC and its implications for near-surface resources and global continent evolution.
Mesozoic and Cenozoic mantle-derived rocks and the peridotite xenoliths entrained in them were sampled throughout the NCC for petrological and geochemical investigations that used newly developed tracers of radiogenic isotopes (Hf and Os) and non-traditional stable isotopes (Li, Mg and Fe). These studies led to new insights into the properties of the Phanerozoic lithospheric mantle beneath the craton and the processes that modified these properties.
The sub-continental lithospheric mantle of the late Mezozoic was found to be rich in water while that of the Cenozoic was highly deficient in water.
It was found that the lithosphere of the NCC had thinned substantially since the Mesozoic and had undergone a significant change in composition from a refractory cratonic lithospheric mantle before the thinning to a relatively fertile "oceanic" mantle after its thinning, while during the decratonization the composition of the lithospheric mantle was very heterogeneous.
It has been demonstrated that this compositional change was caused by peridotite-melt interaction. This evidence and the results of Li-Fe-Mg isotope studies infer that the reactive melts are crustal melts originating from deep continental subduction or from recycled oceanic crust and mantle-derived silicate and carbonatite melts from the asthenosphere. The formation of these melts was related to the subduction of circum-craton plates, in particular the continental Yangtze plate and the Pacific plate.
Exploring the role of craton destruction within the framework of global continental evolution, it is necessary to understand not only the nature of the lithosphere and its modification processes, but also the dynamic and tectonic system that caused craton destruction.
The crust and upper mantle structures pose important constraints on the tectonic elements affecting the NCC. Since 2000, a total of 975 stations equipped with portable broadband seismometers for recording temporal data have been deployed in the NCC with an average spacing of about 10?? km. Additionally, three wide-angle reflection/refraction profiles totaling 3400 km were recorded.
A survey using ocean bottom seismometers (OBSs) and portable land seismometers was carried out in the Bohai region where crustal and mantle structures were studied in unprecedented detail. Significant spatial heterogeneity was observed in the NCC lithospheric thickness and crustal structure, placing constraints on the extent of the NCC destruction.
This structural variance in the lithosphere indicates that parts of the NCC, especially to the east of Taihang Mountains, experienced significant destruction of the lithospheric mantle. Data relating to the upper mantle structure and the anisotropy pattern provide evidence of dynamic interactions among the subducting slab, cratonic root, and the ambient mantle beneath the NCC.
These systems suggest that the destruction of the craton was dominated by the interaction between the lithospheric mantle and the asthenosphere mantle controlled by the Pacific subduction.
The surface expression of the NCC destruction includes extensional structures whereas the deep processes associated with the NCC destruction involved lithospheric transformation and thinning, as revealed by mantle- and crust-derived magmatism.
The consistency between the magmatism peak period (130 Ma) and the formation ages of the metamorphic core complexes, which correspond to a period of intense extension, suggests a close linkage between the deep process and the shallow geology.
The correlation between the shallow extensional activities and the Late Jurassic and Paleogene magmatism also demonstrates a close temporal-spatial relationship. The correlation between the magmatism and the surface geological response confirms that the evolution of the shallow geological features was governed by the cratonic destruction processes.
The geodynamic factor that triggered the destruction of the NCC is important to our understanding of the formation and destruction of the cratonic lithosphere in the global plate tectonic system.
The findings of the NCC research project emphasizes the role of the westward subduction of the Pacific plate based on the observations of the upper mantle velocity structures, episodes of late Mesozoic magmatism, Cenozoic basaltic evolutionary trends, and subducted slab components associated with Mesozoic-Cenozoic mafic magmas.
In particular, the dehydration of the subducted paleo-Pacific Plate released significant amounts of water into the overriding lithospheric mantle beneath the eastern NCC. This lowered the viscosity of the lithosphere significantly and the continental lithosphere in this region was severely weakened, which facilitated its convective removal by the underlying asthenosphere and the ultimate destruction of the NCC.
Such a process is reminiscent of North America where the formation of the Cordillera belt and the partial destruction of the North American craton were related to the eastward subduction of the Farallon plate. In this sense, the craton destruction was a result of the tectonic activity of the plate margins.
The main outcomes of this study have important implications for the relationship between craton destruction and plate tectonics. Compared with typical cratons worldwide, although lithospheric thinning occurs in many other cratons, only some of them are accompanied by craton destruction. It appears that a craton that loses its lithospheric keel because of mantle pluming may preserve its inherent cratonic features.
Craton destruction seems to take place only in cratons severely affected by oceanic subduction. Validating this generalization will require further studies and understanding of the physical-chemical processes that occur in the lithosphere-asthenosphere interface during the process of craton destruction.
The implementation of the NSFC's major research program on Cratonic Destruction realized multi-discipline integration of geology, geophysics, and geochemistry, provided opportunities for many young scientists, and provided a platform for promoting the contribution of Chinese earth scientists in the international community of geosciences.
See the article: Zhu R X, Xu Y G, Zhu G, et al. Destruction of the North China Craton. Sci China Earth Sci, 2012, 55: 1565, doi: 10.1007/s11430-012-4516-y
Science China Press
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