Xuanhua Chen

Cenozoic tectonic evolution of the Qaidam basin and its surrounding regions (Part 3): Structural geology, sedimentation, and regional tectonic reconstruction

The Qaidam basin is the largest topographic depression inside the Tibetan plateau. Because of its central position, understanding the tectonic origin of the Qaidam basin has important implications for unraveling the formation mechanism and growth history of the Tibetan plateau. In order to achieve this goal, we analyzed regional seismic-reflection profiles across the basin and a series of thickness-distribution patterns of Cenozoic strata at different time slices. The first-order structure of the basin is a broad Cenozoic synclinorium, which has an amplitude ranging from >16 km in the west to 48% in the west to −15 s −1 to 1.3 × 10 −17 s −1 . The eastward decrease in upper-crustal shortening requires a progressive shift in crustal-thickening mechanisms across Qaidam basin, from dominantly upper-crustal shortening in the west to dominantly lower-crustal shortening in the east. Although sedimentation began synchronously at 65–50 Ma across the entire basin, the initiation ages of the southern and northern basin-bounding structures are significantly different; deformation started at 65–50 Ma in the north and at 29–24 Ma in the south. This information and the existing inference that the uplift of the Eastern Kunlun Range south of Qaidam basin began after 30–20 Ma imply that the Paleogene (65–24 Ma) Qaidam and Hoh Xil basins on both sides of the Eastern Kunlun Range may have been parts of a single topographic depression, >500 km wide in the north-south direction between the Qilian Shan and Fenghuo Shan thrust belts in the north and south. The development of this large Paleogene basin in central Tibet and its subsequent destruction and partitioning by the Neogene uplift of the Eastern Kunlun Range requires a highly irregular sequence of deformation, possibly controlled by preexist-ing weakness in the Tibetan lithosphere.

Early Paleozoic Tectonic and Thermomechanical Evolution of Ultrahigh-Pressure (UHP) Metamorphic Rocks in the Northern Tibetan Plateau, Northwest China

Coesite- and diamond-bearing ultrahigh-pressure (UHP) metamorphic rocks represent continental materials that were once subducted to depths of >90 km. Identifying how these rocks were subsequently returned to Earths surface has been a major challenge. Opinions on this matter vary widely, ranging from vertical extrusion of a coherent continental slab to channel flow of tectonically mixed mélange. To address this problem, we conducted integrated research across the North Qaidam UHP metamorphic belt using structural mapping, petrologic studies, and geochronologic and thermochronologic analyses. Our regional synthesis indicates that the early Paleozoic Qilian orogen, within which the North Qaidam UHP metamorphic belt was developed, was created by protracted southward oceanic subduction. The process produced a wide mélange belt and the Qilian magmatic arc. Arc magmatism was active between 520 and 400 Ma, coeval with North Qaidam UHP metamorphism. The North Qaidam UHP metamorphic belt also spatially overlaps the early Paleozoic Qilian magmatic arc. Petrologic, geochronologic, and geochemical studies indicate that the protolith of the UHP metamorphic rocks was a mixture of continental and mafic/ultramafic materials, derived either from oceanic mélanges or pieces of a rifted continental margin tectonically incorporated into an oceanic subduction channel. These observations require that the North Qaidam UHP metamorphic rocks originated at least in part from continental crust that was subducted to mantle depths and then transported across a mantle wedge into a coeval arc during oceanic subduction. Upward transport of the UHP rocks may have been accommodated by rising diapirs launched from a mélange channel on top of an oceanic subducting slab. To test this hypothesis, we developed a quantitative model that incorporates existing knowledge on thermal structures of subduction zones into the mechanics of diapir transport. Using this model, we are able to track P-T and T-t paths of individual diapirs and compare them with the observed P-T and T-t paths from North Qaidam. The main physical insight gained from our modeling is that the large variation of the observed North Qaidam P-T paths can be explained by a combination of temporal and spatial variation of thermal structure and mechanical strength of the mantle wedge above the early Paleozoic Qilian subduction slab. Hotter P-T trajectories can be explained by a high initial temperature (~800°C) of a diapir that travels across a relatively strong mantle wedge (i.e., activation energy E = 350 kJ/mol for dry olivine), while cooler P-T paths may be explained by a diapir with initially low temperature (~700°C) that traveled through a weaker mantle wedge, with its strength at least two orders of magnitude lower than that of dry olivine. This latter condition could have been achieved by hydraulic weakening of olivine aggregates in the mantle wedge via fluid percolation through the mantle wedge during oceanic subduction.

Paleomagnetism indicates no Neogene rotation of the Qaidam Basin in northern Tibet during Indo-Asian collision

Paleomagnetic data were obtained from Tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the Qaidam Basin. In the east-central part of the basin, 30 sites from the lower Pliocene Youshashan Formation yielded characteristic remanent magnetization (ChRM) directions with intermediate unblocking temperatures (100‐600 8C); ChRM with high unblocking temperatures (to 680 8C) was isolated from 14 sites. In the same area, ChRM directions were obtained from six sites within the Oligocene Lower Gancaigou Formation. Characteristic magnetization was also determined from 16 sites within the Lower Gancaigou Formation exposed in the E Bo Liang range of the north-central Qaidam Basin. When compared with equivalentage expected directions for Eurasia, the mean paleomagnetic directions indicate no Neogene vertical-axis rotation of the Qaidam Basin or the Altyn Tagh fault. The Qaidam Basin may act as an indentor translating without rotation toward the North China block and driving clockwise vertical-axis rotations by differential shortening within the Nan Shan fold-and-thrust belt.

Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet

The active, left-lateral Altyn Tagh fault defines the northwestern margin of the Tibetan Plateau in western China. To clarify late Quaternary temporal and spatial variations in slip rate along the central portion of this fault system (85°–90°E), we have more than doubled the number of dated offset markers along the central Altyn Tagh fault. In particular, we determined offset-age relations for seven left-laterally faulted terrace risers at three sites (Kelutelage, Yukuang, and Keke Qiapu) spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys (total station and tripod–light detection and ranging [T-LiDAR]), and geochronology (radiocarbon dating of organic samples, 230 Th/U dating of pedogenic carbonate coatings on buried clasts, and terrestrial cosmogenic radionuclide exposure age dating applied to quartz-rich gravels). At Kelutelage, which is the westernmost site (37.72°N, 86.67°E), two faulted terrace risers are offset 58 ± 3 m and 48 ± 4 m, and formed at 6.2–6.1 ka and 5.9–3.7 ka, respectively. At the Yukuang site (38.00°N, 87.87°E), four faulted terrace risers are offset 92 ± 12 m, 68 ± 6 m, 55 ± 13 m, and 59 ± 9 m and formed at 24.2–9.5 ka, 6.4–5.0 ka, 5.1–3.9 ka, and 24.2–6.4 ka, respectively. At the easternmost site, Keke Qiapu (38.08°N, 88.12°E), a faulted terrace riser is offset 33 ± 6 m and has an age of 17.1–2.2 ka. The displacement-age relationships derived from these markers can be satisfied by an approximately uniform slip rate of 8– 12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this data set.

Cenozoic Left-Slip Motion along the Central Altyn Tagh Fault as Inferred from the Sedimentary Record

Several Cenozoic sedimentary basins are present along the central segment of the Cenozoic Altyn Tagh fault (ATF) that marks the northern boundary of the Tibetan Plateau. Field investigations reveal that basin sedimentation and subsequent deformation are controlled by left-slip motion along the Cenozoic ATF. In order to better understand the temporal and spatial interactions between the Altyn Tagh fault and the development of adjacent basins, we divided Cenozoic sedimentary sequences into three subunits based on lithologic variation and the presence of unconformities. From our own observations and regional correlations, we suggest that slip along the central ATF began in the Early Oligocene. Reconstruction of sedimentary relationships among the basin, slip along the fault, and offset topography suggests that the ATF experienced four stages of Cenozoic leftslip motion. The offset of a Late Miocene sequence and its possible correlation across the Altyn Tagh fault suggests 80-100 km left slip. This yields an average slip rate of 10-12.5 mm/yr assuming that the sequence was deposited at ~8 Ma.