Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

Abstract

The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time.

Magmatic Forcing of Cenozoic Climate?

“…Topography in the Tibetan Plateau was built up in several steps, likely starting well before the collision between India and Asia (Murphy et al, 1997;Wang et al, 2017). Paleoelevations may have been comparable to the present since at least the late Oligocene (DeCelles et al, 2007;Dupont-Nivet et al, 2008;Van Der Beek et al, 2009), but estimated plateau elevations during the Eocene vary from near sea level to~5 km (e.g., Botsyun et al, 2019;Ding et al, 2014;Wei et al, 2016). Normal faulting in Tibet at about 13 Ma suggests an increase in Plateau elevation at that time (Molnar et al, 1993;Murphy et al, 2009) and may have had climatic effects such as initiation of aeolian accumulation in the main part of the Chinese Loess Plateau and the onset of intense monsoonal circulation (e.g., Sun et al, 2008Sun et al, , 2014.…”

Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene

“…The elevation of the TP since the Eocene as reconstructed in stable isotope paleoaltimetry studies is close to modern values (>4000 m) (8-10). However, other lines of evidence from paleobotany (11,12) and paleontology (13,14) suggest much lower elevations of the TP in the past. Improved constraints on the early topographic history of the TP are critical to better understand the geodynamic evolution of India-Eurasian collision and its consequences on the climate system-namely, the Asian monsoon onset.…”

“…Corresponding sample sites, geographic locations are provided in fig. S6(8,10,13,24,(57)(58)(59)(60)(61)(62)(63)(64)(65)(66). d18 O c are reported relative to V-PDB.…”

Aridification signatures from fossil pollen indicate a drying climate in east-central Tibet during the late Eocene

“…In addition to the proportions of the ancestral vs. derived type of Ephedripites, a significant spike in tropical forest pollen in one sample at this time, combined with a large decrease in steppe-desert pollen, suggests that Zone II of (Wang et al, 1990a, b); 2, Hoh Xil Basin (Miao et al, 2016); 3 and 4, Nangqian Basin (this study; Yuan et al, 2017); 5, Qaidam Basin (Zhu et al, 1985;Zhang, 2006;Miao et al, 2016); 6, Xining Basin (Wang et al, 1990a, b;Hoorn et al, 2012); 7, Jianchuan Basin (Wu et al, 2018); and 8, the Xigaze region (Li et al, 2008). The dominant ancient vegetation reconstructed from palynological assemblages is shown to the right of each section.…”

“…4). The Jianchuan Basin to the south was dominated by mixed tropical-subtropical coniferous and broad-leaved forest (Wu et al, 2018), and it is also forested today (but with species of a less thermophilic nature). Similarly, the Markam and Gonjo basins host alpine meadow and forest today; although detailed palynological records have not yet been recovered, macrobotanical fossils suggest these areas were dominated by mixed broad-leaved and coniferous forest in the late Eocene-early Oligocene (Su et al, 2018;Studnicki-Gizbert et al, 2008).…”