New U-Pb dates show a Paleogene origin for the modern Asian biodiversity hot spots

Abstract

Yunnan, in southwestern China, straddles two of the world's most important biodiversity hot spots (i.e., a biogeographic region that is both a reservoir of biodiversity and threatened with destruction) and hosts more than 200 fossiliferous sedimentary basins documenting the evolutionary history of that biodiversity, monsoon development, and regional elevation changes. The fossil biotas appear modern and have been assumed to be mostly Miocene in age. Dating has been by cross-correlation using palynology, magnetostratigraphy, and lithostratigraphy because numerical radiometric ages are lacking. Here we report the first unequivocal early Oligocene age (33-32 Ma) of a section in the Lühe Basin (25.141627°N, 101.373840°E, 1890 m above mean sea level), central Yunnan, based on U-Pb zircon dates of unreworked volcanic ash layers in a predominantly lacustrine succession hosting abundant plant and animal fossils. This section, located in Lühe town, is correlated with an adjacent section in the Lühe coal mine previously assigned to the upper Miocene based on regional lithostratigraphic comparison. Our substantially older age for the Lühe town section calls into question previous estimates for the surface uplift and climate history of the area, and the age of all other correlative basins. The modernization of the biota ~20 m.y. earlier than previously thought overturns existing concepts of vegetation history in southwestern China, and points to Paleogene modernization of the biota in Yunnan and associated Asian biodiversity hot spots.

Oligocene Deformation of the Chuandian Terrane in the SE Margin of the Tibetan Plateau Related to the Extrusion of Indochina

“…We assume that the feldspar age most closely reflects the date of ash deposition and matches that of the youngest biotite. Step heating of a feldspar from ash Layer 11 gave a similar age of 33.32 ± 0.36 Ma, which is indistinguishable from that of ash Layer 3 when uncertainties are taken into account and suggests that the bottom part of the Lühe mine section was deposited at ~33 Ma, which is the same as U‐Pb ages (33 ± 1 Ma) from zircons in primary ashes exposed ~2.6 km to the southeast in Lühe town (Linnemann et al, 2018; see Figure 3 for location).…”

“…These paleoclimate proxy data were also used to reconstruct the paleoelevation of the Lühe area, which suggested an elevation of ~1 ± 0.8 km in the late Miocene (Hoke et al, 2014). However, Linnemann et al (2018) reported a 33–32 Ma U‐Pb zircon age of a primary volcanic ash within the plant‐bearing sediments from the southern tip of the Lühe Basin (Lühe town section; see Figure 3 for location). This new age, together with updated ages from the Jianchuan Basin (Gourbet et al, 2017), is significantly transforming our view about the tectonics, paleoclimate, and biodiversity of the SE margin of the Tibetan Plateau (Hoke, 2018; Linnemann et al, 2018).…”

“…Note that the fold axes are deflected when close to the Chuxiong fault, modified from Tapponnier et al (1990). Green star and Lühe town denote the locations of our magnetostratigraphic section and Linnemann et al (2018), respectively.…”

Geochronology transforms our view of how Tibet’s southeast margin evolved

“…Over the past few years, sedimentary basins of the southeastern margin of the Tibetan Plateau have finally begun to yield absolute age constraints, including two new ages from volcanic ashes in the Lühe Basin reported in this issue of Geology (p. 3-6) by Linnemann et al (2017). This spate of geochronology (Hoke et al 2014;Gourbet et al 2017;Linnemann et al, 2017) squarely reassigns the age of at least two basins from middle and late Miocene to the Paleogene (Fig.…”

Evolution of the Yangtze River network, southeastern Tibet: Insights from thermochronology and sedimentology

“…Because Cenozoic continental sediments are difficult to date, the difference between the assumed and revised depositional age in SE Tibet can reach more than 20 m.y. (Gourbet et al, 2017;Linnemann et al, 2018). As a consequence, we cannot estimate an incision rate based on the age of the red beds, and so we simply used the observed range of 0.5-1.2 km as an estimate for the total incision since deposition.…”