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

95 Staring out across the lush and unforgiving landscape of China’s Yunnan and southwestern Sichuan Provinces with a geologic map in hand, one is immediately humbled by the skill of the Chinese geologists who were tasked with a seemingly impossible job. The mapping was, by and large, accurate; however, the Cenozoic chronology was based entirely on biostratigrapic and lithostratigrapic correlation (e.g., Ge and Li, 1999) with little, if any, absolute age control. With limited exposure and elusive datable horizons except for a few dikes cross-cutting stratigraphy and interbedded lava flows (e.g., Studnicki-Gizbert et al., 2008), there were few constraints on absolute time in the Cenozoic. Because the area is important for understanding the evolution of the Tibetan Plateau as a whole, the community working in the area pressed on, leaning heavily on the regional stratigraphy in building a Miocene history of how rivers re-organized and incised the Tibetan Plateau’s southeast margin (e.g., Clark et al., 2004; Schoenbohm et al., 2006) in response to passive surface uplift resulting from flow of the lower crust (Clark and Royden, 2000) (Fig. 1A). This narrative was reinforced by subsequent low-temperature thermochronology that yielded a middle Miocene timing of river incision (Clark et al., 2005; Ouimet et al., 2010). Using largely the same geologic age constraints from the sedimentary basins, paleo-altimetry studies determined that the Plateau margin was already high in the Eocene (Hoke et al., 2014; Li et al., 2015; Tang et al., 2017), consistent with high elevations in southern and central Tibet (Rowley and Currie, 2006). 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. 1A). This reassignment of time, at a minimum, should force us to step back and assess how our interpretations of the southeast margin might change. Given their revised age for the Lühe Basin, Linnemann et al. point out that the much of the modern biodiversity was established in the early Oligocene, 20 m.y. earlier than previously thought. The past, apparently, looked very much like the present, and likely included the establishment of high elevation prior to the late Paleogene. Perhaps as important, time is much more compressed than initially envisioned. Taking the results from Gourbet et al. (2017) with those of Linnemann et al., it would appear that deposition of the Shuanghe and Xiaolongtan Formations occurred over a relatively brief window of time between 37 Ma and 32.5 Ma. These shake-ups in time will likely continue: the youngest zircon U-Pb ages (n = 12) for a tuffaceous sandstone reported by Wissink et al. (2016), in what is mapped as the Paleocene Yunlong Formation, average 36.3 Ma, making them equivalent in time with the Shuanghe Formation of the Jianchuan Basin. Keeping in mind that lithostratigraphy and biostratigraphy were the glue that bound correlations in the area, what other surprises await? It is certainly within the realm of possibility that what we once thought was Miocene throughout the southeast margin of the Tibetan Plateau now resides in the later part of the Paleogene. This opens up the interesting possibility that these units record late Eocene to earliest Oligocene terrestrial temperatures from the region. There are a large number of plant fossil-derived temperature estimates data throughout the region from basins formerly thought to be Miocene (Xu et al., 2008; Xia et al., 2009; Sun et al., 2011; Yao et al., 2010; Jacques et al., 2011; Jacques et al., 2014), yet with a mean temperature of 17.6 ± 2.3°C over the 10° of latitude there appears to be no meaningful trend from basins that share fossil assemblages with the flora of Lühe and Jianchuan Basins (Fig. 1B). Branched glycerol dialkyl glycerol tetraether (GDGT) temperatures derived from a survey of modern soils of southeast Tibet and the southeast margin of the Tibetan Plateau yield coherent lapse rates that match weather station data (Wang et al., 2017), suggesting a potential horizon for understanding Paleogene temperatures and potentially elevation histories.