5-2 MA HIGH-RESOLUTION STABLE ISOTOPE RECORD FROM CARBONATES FORMED IN THE QINGHAI LAKE DURING OUTWARD GROWTH OF THE QILIAN SHAN-NAN SHAN OROGENIC BELT AND GLOBAL COOLING

The Qinghai basin is located in the southeastern part of the Qilian Shan-Nan Shan orogenic belt, Northeastern Tibetan Plateau (NETP), and contains the largest lake in China, the Qinghai Lake (QL, elevation of 3200 m.a.s.l). The modern basin formed after the growth of Qinghai Nanshan mountain to its south, whereas the lake formed at ~4.6 Ma, as indicated by the Yilang sediment core record. Spanning the past 5.11 My, the Yilang core records QL sediment deposition that was coeval with late Plio-Pleistocene global cooling; Asian monsoonal changes; and late Cenozoic outward growth of the NETP. The published age model based on magnetostratigraphy indicates the core contains the following units from 4.63 to 1.8 Ma: [4.63-4.42 Ma] lake formation and rising lake level; [4.42-3.58 Ma] deep lacustrine; [3.58-3.06 Ma] transition to shallow lacustrine; [3.06-2.15 Ma] shallow lacustrine; and [2.15-1.8 Ma] transition to marginal lacustrine. The goal of this study is to understand changes in late Cenozoic tectonics and climatic evolution of the NETP region recorded by Qinghai Basin seismic profiles and carbon and oxygen isotopic values in lacustrine authigenic carbonates collected from the Yilang core spanning 4.6 to 2.0 Ma. Analysis of authigenic carbonates (n=788) yield mean isotopic values of: [4.63-4.42 Ma] δ¹³C= -5.6‰, δ¹⁸O= -6.2‰; [4.42-3.75 Ma] δ¹³C=-3.4‰, δ¹⁸O= -4.6‰; [3.75-3.50 Ma] δ¹³C= -1.6‰, δ¹⁸O= -4.0‰; [3.50-3.06 Ma] δ¹³C= -4.2‰, δ¹⁸O= -7.1‰; [3.06-2.15 Ma] δ¹³C= -3.9‰, δ¹⁸O= -6.3‰. Our data shows: 1) a positive excursion from 3.75 to 3.5 Ma, when mean values increase by 1.8‰ (δ¹³C) and 0.6‰ (δ¹⁸O), then decrease by 2.6‰ (δ¹³C) and 3.1‰ (δ¹⁸O); 2) no significant shift at the onset of Northern Hemisphere Glaciation, at ~2.75 Ma; and 3) more depleted δ¹⁸O values than Holocene authigenic carbonate values of δ¹⁸O= -2 to 2‰. Evaluation of published and unpublished Qinghai basin seismic profiles indicate that normal faulting, an angular unconformity and growth strata occurred at 3.6 Ma in the basin. Several basins on the margins of the Qilian Shan show thrust faulting, coarsening, angular unconformities and growth strata at ~3.6 Ma. Thus we link the 3.75 to 3.5 Ma isotopic excursion to outward growth of the Qilian Shan-Nan Shan orogenic belt, which may be related to overall elevation change of the NETP in the late Pliocene.