High and dry in central Tibet during the Late Oligocene
Introduction
The timing of attainment of regional high elevation (4 to 5 km) in the Tibetan Plateau (Fig. 1) is a significant problem in continental tectonics because elevation strongly influences the force distribution in and adjacent to orogenic plateaux [1], [2], and the timing and mechanisms of elevation gain are important in competing general models of plateau formation (compare [1], [2], [3], [4], [5], [6], [7], [8]) and global and regional climate change [9], [10], [11]. Our knowledge of the uplift history of the Tibetan Plateau remains poor, however, because of the scarcity of well-dated ancient sediments containing unambiguous indicators of paleoelevation. The pursuit of paleoaltimetry data from Tibet is motivated in part by the need to better constrain the initial development of high elevation in this region as a means of testing tectonic models for Tibet. For example, models that infer uplift of the Plateau in response to removal of a convective instability in the upper mantle call for rapid, regional uplift during Late Miocene time [1], whereas models that build the Plateau by addition of Indian crustal material from the south predict a progressive northward increase of elevation from Late Eocene time onward [3], [7], [8]. Other models call for gravitationally driven north- and northeastward expansion of the Plateau by ductile flow of lower to middle crustal material [2], [5], a process that demands an explanation for the origin of thick crust in central Tibet at the onset of crustal flow.
Oxygen isotopic values from carbonates (expressed as δ18Occ in ‰) and the waters from which they precipitate (δ18Omw) decrease with increasing elevation, making them potentially useful paleoaltimeters. Ancient carbonates in the geologic record have been analyzed as paleoelevation indicators in a variety of orogenic belts [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Paleobotanic [27] and oxygen isotopic data from southern Tibet suggest that high (> 4 km) paleoelevations were attained at Thakkhola [14] and in the Oiyug Basin [22] (Fig. 1) by the Middle to Late Miocene (roughly 15–10 Ma). The picture is less clear further back in the geologic record. Oxygen isotopic analysis of carbonates from Hoh Xil in northern Tibet (Fig. 1) suggests that paleoelevation during the Late Eocene was ∼ 2000 m, significantly lower than the modern elevation of 4700–5300 m in this region [21]. This reconstruction of paleoelevation depends critically on the choice of isotopic lapse rate, which differs dramatically between northern and southern Tibet. In the Hoh Xil study, modern δ18Omw versus elevation relationships established in the distant Nepalese Himalaya [13], [18], [30] – where monsoon-dominated isotopic lapse rates are very steep – were used to reconstruct paleoelevation [21], instead of the more modest lapse rates characteristic of northern Tibet (encompassing Xoh Xil) today. In fact, reconstructed δ18Omw values from Hoh Xil are very similar to δ18Omw values of modern precipitation in the northern Tibetan Plateau, which is derived from recycled continental moisture with δ18O (SMOW) values of − 9‰ to − 10‰ [28], [29]. The use of the gentler lapse rate currently in place for northern Tibet implies that elevation has not changed substantially in this region since the Late Eocene.
At Lunpola basin in central Tibet (Fig. 1), isotopic evidence points to Late Eocene paleoelevations comparable to the present ∼ 4500 m elevation [23]. However, the age of the Lunpola basin deposits is based on palynological biozonation without geochronological control (see [23]). In general, age control is lacking for putative Cenozoic deposits in the Plateau interior, and adjacent successions of lithologically similar nonmarine deposits can range in age from Early Cretaceous to Miocene. Previous age assignments based largely on biostratigraphy and lithostratigraphic correlation are highly suspect (e.g., [30], [31]). Coupled with the potential underestimates of Eocene paleoelevation at Hoh Xil discussed above, the imprecise age control on the Lunpola basin deposits raises concern about the inference of progressive northward growth of the Tibetan Plateau from Eocene time forward during the Indo–Asian collision [21], [23], [32]. Adding to the complexity of Tibetan paleoelevation records during the Cenozoic are data from a large sector of the northern margin of the Plateau, which indicate a major positive shift in δ18O values from Eocene to Oligocene time [24]. This isotopic shift north of the Tibetan Plateau was interpreted to result from climate change associated with the rise of the Tibetan–Himalayan region to high elevations [24].
What is clearly needed are isotopic analyses of carbonates from well-dated basins situated within the interior of the Tibetan Plateau, and δ18Omw values of locally sampled, modern waters and soils in order to calibrate the relationship between δ18Occ and paleoelevation. In this paper we focus on modern and radiometrically dated ancient paleosol carbonates, ancient lacustrine marl deposits and fossils, and modern stream waters and soil carbonates collected in the Nima basin area in the heart of the Tibetan Plateau at elevations of 4500–4700 m. We show that paleoelevation at ∼ 26 Ma in this region was not significantly different from modern elevation, which in turn provides strong constraints on tectonic models for the formation of the Tibetan Plateau. In particular, models that call for regional uplift of Tibet during Late Miocene time may need to be revised.
Section snippets
Setting of Nima Basin
Nima basin is located ∼ 240 km west of the Lunpola basin and ∼ 450 km northwest of Lhasa (Fig. 1). Like the Lunpola basin, the Nima basin is located in the southern part of the Bangong suture zone, which separates the Qiangtang and Lhasa terranes in central Tibet (Fig. 1). From mid-Cretaceous through Late Miocene time a variety of nonmarine deposits accumulated in the Nima basin, derived from high, thrust-fault-bounded ranges to the south and north that were uplifted during reactivation of the
Geochronology
For age control, 40Ar/39Ar geochronology on biotite separates from samples of six reworked tuffs in the Nima Redbed unit securely places the age of the analyzed deposits between 25 and 26 Ma (Late Oligocene; Fig. 2). 40Ar/39Ar ages on biotites were determined by M. Heizler at the New Mexico Geochronology Research Laboratory. Single-crystal laser and step-heating age results are summarized in [34]. The tuffs consist of fine-grained sand, silt, and abundant coarse-grained, euhedral,
Approach and methods
Key questions for paleoelevation reconstructions using oxygen isotopes include (1) the sensitivity, past and present, of δ18Occ values to elevation; (2) whether burial in deep sedimentary basins has led to diagenetic alteration of primary δ18Occ values; and (3) whether evaporation has increased the δ18Omw values of paleowaters, and therefore also the values of carbonates precipitated from those waters, producing underestimates of paleoelevation. In this paper, we take a fresh approach to these
Oxygen and carbon isotope results
Texturally, the Lower Cretaceous limestone clasts are a mix of micrite and sparite and commonly contain obviously recrystallized marine fossils [43], whereas the Tertiary lacustrine marls and nodular paleosol carbonates are dense, well-indurated micrite containing well-preserved gastropod, ostracode, and Chara fossils. Analysis of the reworked Cretaceous marine limestone pebbles and cobbles yielded several δ18Occ values between − 3‰ and − 5‰ (Fig. 5; Table 2), in the range expected for unaltered
Discussion
A key imponderable in any reconstruction of paleoelevation using oxygen isotopes–especially in deep time such as this case — is climate change related to such factors as major air-mass reorganizations. As an example, we noted in the Introduction that from the perspective of oxygen isotopes, Tibet can be divided into two regions: a southern region, including the Nima and Lunpola basins, where oxygen isotope lapse rates are steep (− 2.8‰/km) and δ18Omw values for mean annual rainfall are very
Conclusions
Comparison of oxygen isotope ratios in well-dated paleosol carbonates with ratios from modern soils in central Tibet indicates that paleoelevation was ∼ 4500–5000 m during Late Oligocene time. Oxygen isotope ratios in unaltered aquatic fossils also support the conclusion that the Nima basin has been at high elevation since the Late Oligocene. Oxygen isotope ratios from Oligocene lacustrine marl and δ13Cmw values from paleosol carbonate indicate high rates of evaporation and low soil respiration
Acknowledgments
We thank Shiling Yang for the assistance in the laboratory, and B. Quade and E. Quade for the assistance in the field. Research was supported by the National Science Foundation and ExxonMobil Corporation. We thank Brad Ritts, Page Chamberlain, and Peggy Delaney for the constructive reviews that helped us to substantially improve this manuscript.
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