Yongjun Yue

Initiation and Long-Term Slip History of the Altyn Tagh Fault

New Tertiary piercing points along the eastern and central Altyn Tagh fault, the northern boundary of the Tibetan Plateau, allow construction of the first well-defined time-displacement curve for the fault. Displacement-history analysis indicates: (1) late Oligocene-earliest Miocene inception of the Altyn Tagh fault; (2) 375 ± 25 km of total left-lateral slip on the eastern and central segment of the Altyn Tagh fault; and (3) an average long-term Cenozoic slip rate of approximately 12-16 mm/year. These results demonstrate that Himalayan deformation propagated well into the interior of Asia by early Miocene time and that a significant amount of India-Asia convergence was accommodated by sinistral slip on the Altyn Tagh fault.

Two-stage evolution model for the Altyn Tagh fault, China

The northeastern extension of the Altyn Tagh fault, China, beyond the apparently truncated Qilian Shan has been controversial since the recognition of this large left-slip fault. We propose that the Altyn Tagh fault may have been contiguous with the east-northeast to northeast-trending, currently inactive (but apparently active during the Cenozoic) Alxa–East Mongolia fault; the latter truncated the Beishan orogen to the northwest and the Inner Mongolia orogen to the southeast. The Beishan orogen can be correlated with the Inner Mongolia orogen on the basis of the recognition of three offset petrotectonic units and two offset sutures. This correlation requires 400 ± 50 km of left-lateral offset along the Alxa–East Mongolia fault, identical to the slip along the northern segment of the Altyn Tagh fault inferred by P. Molnar and P. Tapponnier. We suggest a two-stage model for fault evolution: during the first stage, 400 km of displacement separated the Inner Mongolia orogen from the originally contiguous Beishan orogen along a continuous Altyn Tagh–Alxa–East Mongolia fault. During the second stage, the Alxa–East Mongolia fault became inactive, and additional offset along the Altyn Tagh fault has been mainly accommodated by shortening of the Qilian Shan and the Qaidam basin. We infer that the first stage of movement along the Altyn Tagh–Alxa–East Mongolia fault system may have begun around Oligocene time; the region transformed to the second stage of fault movement ca. 13–16 Ma, probably because of thinning of the lithospheric mantle beneath northern Tibet. The proposed model suggests that the Derbur fault in northernmost China and beyond is an extension of the Alxa–East Mongolia fault, and has a history of strike-slip movement. The nearly 400 km of offset on the Alxa–East Mongolia fault may have been accommodated mainly by subduction in the Sea of Okhotsk.

Slowing extrusion tectonics: lowered estimate of post-Early Miocene slip rate for the Altyn Tagh fault

Abstract Determination of long-term slip rate for the Altyn Tagh fault is essential for testing whether Asian tectonics is dominated by lateral extrusion or distributed crustal shortening. Previous slip-history studies focused on either Quaternary slip-rate measurements or pre-Early Miocene total-offset estimates and do not allow a clear distinction between rates based on the two. The magmatic and metamorphic history revealed by SHRIMP zircon dating of clasts from Miocene conglomerate in the Xorkol basin north of the Altyn Tagh fault strikingly matches that of basement in the southern Qilian Shan and northern Qaidam regions south of the fault. This match requires that the post-Early Miocene long-term slip rate along the Altyn Tagh fault cannot exceed 10 mm/year, supporting the hypothesis of distributed crustal thickening for post-Early Miocene times. This low long-term slip rate and recently documented large pre-Early Miocene cumulative offset across the fault support a two-stage evolution, wherein Asian tectonics was dominated by lateral extrusion before the end of Early Miocene, and since then has been dominated by distributed crustal thickening and rapid plateau uplift.

From sea level to high elevation in 15 million years: Uplift history of the northern Tibetan Plateau margin in the Altun Shan

Approximately 1300 m of Oligocene-Miocene clastic strata are exposed along the Miran River in the southeastern Tarim basin, where the adjacent Altun Shan form the topographic escarpment of the northern Tibetan Plateau. The sedimentary section is faulted against Proterozoic rocks of the Altun Shan in the footwall of the south-dipping, oblique reverse Northern Altyn Tagh fault. Oligocene-Lower Miocene strata consist of fine-grained rocks that record low-gradient depositional systems. Mid-Miocene and younger rocks consist of coarse conglomerate, derived from the Altun Shan and deposited by high-gradient depositional systems. The change to coarse, high-gradient depositional systems with detrital source areas coincident with the modern Miran River drainage is interpreted to mark the onset of uplift of the Altun Shan on the Northern Altyn Tagh fault and its erosional exhumation. The age of the change from pre-orogenic to synorogenic sedimentation is constrained by a foraminifera assemblage at the base of the conglomeratic section that includes Early-Middle Miocene planktonic foraminifera. This interpretation is also supported by apatite fission track and (U-Th)/He ages and thermal models that indicate rapid Miocene cooling, and hence, rapid exhumation of the Altun Shan. In addition to defining the age of the synorogenic section, the foraminifera assemblage contains planktonic taxa, indicating a connection to open marine waters, and benthic assemblages typical of brackish to near-sea level paleobathymetry. Thus, micropaleontologic evidence demonstrates that the Miran River locality, now at ∼1400 m elevation, was at sea level approximately 15 million years ago. Thus, in addition to constraining the age of surface uplift and exhumation of the Altun Shan, the principal mountain range of the Tibetan Plateau in this region, as ∼15 to 16 Ma, the foraminifera assemblage indicates that the SE Tarim basin, off the northern edge of the plateau, had an average surface uplift rate of nearly 100 m/m.y. for the past 15 million years. These results suggest that shortening in the Altun Shan and uplift of the range significantly post-dated the initiation of large-scale strike-slip on the Altyn Tagh fault, and that regional surface uplift mechanisms operated in the Tarim basin, beyond the margins of the Tibetan Plateau.

Oligocene‐Miocene Tectonics and Sedimentation along the Altyn Tagh Fault, Northern Tibetan Plateau: Analysis of the Xorkol, Subei, and Aksay Basins

Tertiary basins adjacent to the northern edge of the Tibetan Plateau include both giant intracontinental sedimentary basins such as Tarim and Qaidam and smaller basins directly related to the evolution of the Altyn Tagh strike‐slip fault system and nearby fold‐thrust belts. Three examples of these smaller basins that formed during early evolution of the Altyn Tagh fault system are the Xorkol, Aksay, and Subei basins. The Xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan, playa, and fluvial environments from at least the mid‐Oligocene through the Miocene. Sediment was delivered to the basin from source areas uplifted by basin‐bounding thrust faults and from the opposite side of the Altyn Tagh fault. The age of the Aksay basin is more poorly constrained, but it also contains Oligocene(?)‐Miocene(?) nonmarine clastic strata that were derived from the southern side of the Altyn Tagh fault and adjacent thrust‐fault‐bounded uplifts. The Subei basin is characterized by similar Oligocene‐Miocene fluvial, playa, and alluvial fan deposits; however, in this basin, sediment was derived only from adjacent reverse‐fault‐bounded highlands rather than from across the Altyn Tagh fault. Differences in the lithostratigraphy, depositional environments, clast composition, and paleocurrent directions demonstrate that these basins formed as individual entities and do not represent remnants of a regionally extensive basin that was later dislocated by strike‐slip faulting. Thus, these basins do not represent piercing point pairs, as was previously suggested. However, because they are syn‐slip basins, they do record the relative timing of strike‐slip and reverse faulting and the surface response to deformation, and they provide fault offset data in the provenance relationships between sediment and source terranes on opposite sides of the fault.