events. l This model can therefore be used to explain lithospheric thinning in different periods of time. Asthosphere Folding Model This model is specifically proposed for how the Yang Tze Craton and the North China Craton collided and thinned the lithosphere. The collision of the 2 cratons first thickened the crust by folding. Eclogite formed in the lower crust, which made the lower crust denser. New shear zones also developed in the lower crust. The asthenosphere convected and seeped into weak points developed in the lower crust shear zones. The heavy lower crust was then fragmented and sunk into the lithosphere. The lithosphere of the North China Craton was then thinned. Biostratigraphy The North China Craton is very important in terms of understanding biostratigraphy and evolution. In Cambrian and Ordovician time, the units of limestone and carbonate kept a good record of biostratigraphy and therefore they are important for studying evolution and mass extinction. The North China platform was formed in early Palaeozoic. It had been relatively stable during Cambrian and the limestone units are therefore deposited with relatively few interruptions. The limestone units were deposited in underwater environment in Cambrian. It was bounded by faults and belts for example Tanlu fault. The Cambrian and Ordovician carbonate sedimentary units can be defined by six formations: Liguan, Zhushadong, Mantou, Zhangxia, Gushan, Chaomidian. Different trilobite samples can be retrieved in different strata, forming biozones. For example, lackwelderia tenuilimbata (a type of trilobite) zone in Gushan formation. The trilobite biozones can be useful to correlate and identify events in different places, like identifying unconformity sequences from a missing biozones or correlates events happening in a neighbouring block (like Tarim block). The carbonate sequence can also be of evolutionary significance because it indicates extinction events like the biomeres in the Cambrian. Biomeres are small extinction events defined by the migration of a group of trilobite, family Olenidae, which had lived in deep sea environment. Olenidae trilobites migrated to shallow sea regions while the other trilobite groups and families died out in certain time periods. This is speculated to be due to a change in ocean conditions, either a drop in ocean temperature, or a drop in oxygen concentration. They affected the circulation and living environment for marine species. The shallow marine environment would change dramatically, resembling a deep sea environment. The deep sea species would thrive, while the other species died out. The trilobite