The China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins. Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in...

Tibet’s ancient topography and its role in climatic and biotic evolution remain speculative due to a paucity of quantitative surface-height measurements through time and space, and sparse fossil records. However, newly discovered fossils from a present elevation of ∼4,850 m in central Tibet improve substantially our knowledge of the ancient Tibetan environment. The 70 plant fossil taxa so far recovered include the first occurrences of several modern Asian lineages and represent a Middle Eocene (∼47 Mya) humid subtropical ecosystem. The fossils not only record the diverse composition of the ancient Tibetan biota, but also allow us to constrain the Middle Eocene land surface height in central Tibet to ∼1,500 ± 900 m, and quantify the prevailing thermal and hydrological regime. This “Shangri-La”–like ecosystem experienced monsoon seasonality with a mean annual temperature of ∼19 °C, and frosts were rare. It contained few Gondwanan taxa, yet was compositionally similar to contemporaneous floras in both North America and Europe. Our discovery quantifies a key part of Tibetan Paleogene topography and climate, and highlights the importance of Tibet in regard to the origin of modern Asian plant species and the evolution of global biodiversity.

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Monsoonal climates at low latitudes (<32 °N)

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The complex paleobotanical investigations carried out in the Maoming sedimentary basin (Guangdong Province, South China) yielded first data on the taxonomic composition and ecological prop erties of two large paleofloras from the Youganwo and Huangniuling formations. The palynomorph assem blages from these formations indicate their middle-late Eocene age (Lutetian-Bartonian and Priabonian for the former and latter, respectively). It is shown that sediments of the Youganwo Formation were deposited in an intermittently swamped lacustrine-fluvial plain, which gave way to a freshwater lake. Vegetation of this period was represented by wet subtropical forests with evergreen Fagaceae, Lauraceae, and Palmae. The Hua ngniuling flora reflects the biome of seasonal tropical forests developed in a broad fluvial plain and its margins. The Eocene floras of the Maoming Basin are marked by the appearance of several recent plant genera, which is also evident from finds of remains of their reproductive structures. The Eocene flora from low latitudes of South China exhibits a notable share of floral elements from middle and high latitudes of East Asia.

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