LITHOSPHERIC THINNING IN EASTERN ASIA; CONSTRAINTS, EVOLUTION, AND TESTS OF MODELS

Abstract

The North China Craton (NCC) is the only place currently recognized where an Archaean craton developed a continental root in the Archaean, and subsequently lost half of that root in younger tectonism. In this volume, various authors have advanced different models of root loss, and provided geological, geophysical and geochemical data that help constrain the geometry and timing of root loss. Understanding why and how roots are lost may help us under-stand how often this process may have occurred in the geological past, and how much lithospheric material has been recycled to the convecting mantle through this mechanism, potentially drasti-cally changing our current understanding of crustal growth rates and processes. With current data, there are several equally plausible possibilities that require further data collection for testing. There are several possible tectonic triggers that may have caused half the root to be lost, acting either separately or together. These include collisional, extensional, plume-related, fluid-weakening, spontaneous, and more complex hybrid mechanisms. We also do not know why only the eastern half of the root was lost, and not the root from beneath the whole craton. One tantalizing idea is that the root grew independently, by tectonic underplating of subducted buoyant oceanic lithosphere, beneath the previously separate eastern and western halves of the craton by 2.5 Ga, with modification at 1.8 Ga. If so, perhaps only the eastern half of the root was lost in younger tectonism because there was some physical or geometric difference between the two halves. Alternatively, collisional or subduction-related tectonic processes acting only on the Eastern Block may have caused the disruption of the tectosphere there in the Mesozoic. The timing of and mechanism for loss of the root is not uniquely resolvable with current data, but a solution to the problem is in reach. Possible triggering mechanisms include, but are not limited to, collision of the South China (Yangtze) and North China Cratons in the Trias-sic, the India– Asia collision, closure of the Solonker and Mongol – Okhotsk oceans, Mesozoic subduction of the Pacific plate beneath Eastern China, impingement of mantle plumes, mantle hydration from long-term subduction, and several rifting events. In this concluding review, we link studies of crustal tectonics with investigations aimed at determining the nature and timing of the formation and loss of the root, to better understand mechanisms of continental root for-mation, evolution and recycling –removal. Cratons are structurally complex regions that attained prolonged stability (1 Ga) within the con-tinents and thus, by definition, are Precambrian in age (Fig. 1). Indeed, most formed in the Archaean (Rudnick 1995; Windley 1995; Kusky & Polat 1999). Some first-order observations about Archaean cratons include the following. (1) Many are preserved in the centre of conti-nental masses, surrounded by younger fold belts (Fig. 1). (2) Surface heat flow is typically low within cratons, averaging 41 + 11 mW m 22 (Morgan 1984; Nyblade & Pollack 1993; Artemieva & Mooney 2001). (3) Cratons are underlain by regions of fast P and S velocities, extending to depths of at least 200–250 km, and perhaps deeper (Jordan 1975; James et al. 2001; Gung et al. 2003). (4) Kimberlites erupted within Archaean cratons carry diamonds interpreted to be derived from the lithospheric mantle, consistent with the presence of a cool and thick keel (Boyd & Gurney 1986). (5) Mantle xenoliths from craton-penetrating kimberlites are highly refractory peridotites, which experienced large degrees of melt extraction (Boyd 1989), followed by variable degrees of meta-somatic overprinting (Erlank et al. 1987).