Seismic Constraints on Central Asian Evolution of Late Cenozoic: Uplift Rates and Two-Stage Evolution of the Northern Tibetan Plateau, China

The Tibetan Plateau, also known as the Qinghai-Tibetan (Qingzang) Plateau, is the world's highest and largest plateau with an average elevation exceeding 4,500 meters (14,800 ft) and an area of 2,500,000 square kilometers (970,000 sq mi) (Figure 1). The uplift history of this Plateau is important because of its effect on regional and global climate during Cenozoic time [1], and its uplift dynamics related to the Indo-Asian collision [2,3]. The uplift of the Tibetan plateau is a multi-stage process, including three stages of slow uplift (10.0-6.0 Ma), transitional uplift (6.0-2.5 Ma) and rapid uplift (since 2.5 Ma) [4,5]. During Eocene time, the northern boundary of this plateau was in the Tanggula Shan, and expanded throughout the Neogene toward its present northern margins in the Qilian Shan [6], and the southern Tibet Plateau began uplift at similar to 5.89 Ma, later than the northern Tibetan Plateau, and the strong uplift took place at similar to 5.15 and similar to 2.71 Ma, with the uplift peaking at similar to 2.71 Ma [7]. The Tibetan plateau grew in somewhat by accretion of Cenozoic foreland basins such as the Hoh Xil basin, Qaidam basin and Jiuquan basin during 49-23 Ma, 46-2.45 Ma and 29.5-0 Ma respectively to its northern margin [8]. Along the Allyn Tagh Fault the accumulation of molasses deposits began ca. 13.7 Ma ago due to the tectonic uplift of the Tibetan Plateau, a process that continued until at least 9 Ma ago [9].The change in depositional facies was accompanied by an increase in sedimentation rate from an average similar to 0.15 mm/yr between the earliest Oligocene and the earliest Pliocene to 1.4 mm/yr in the Gauss normal Chron (3.6-2.6 Ma), indicating that the main uplift of the northwestern Tibetan Plateau began ca. 4.5 Ma [10].

Figure 1: Geological sketch map of the study area and nearby regions.

However, the Late Cenozoic uplift history of this Plateau is still controversial for lack of convincing quantitative evidence. In this paper, the 2D seismic profiles for petroleum exploration across the Northern Qilian Thrust Belt near/within the Hexi Corridor, northern margin of the Tibetan Plateau were used to trace the quantitative uplift of the Tibetan Plateau, western China. The strata on the seismic profiles is calibrated by the data from the nearby drilling holes and geological outcrop.

The Hexi Corridor is the north-most foreland basin of the Tibetan Plateau with a length of about 1000-1200 km and its formation is controlled by the Altyn Tagh Fault, northern marginal faults of North Qilian Orogenic Zone, and the Kuantai Shan-Longshou Shan Fault (Figure 1). In Late Cenozoic, the Northern Qilian Orogenic Zone (NQOZ) thrust violently over the Hexi Corridor, and a Northern Qilian Thrust Belt developed within the Hexi Corridor. The Hexi Corridor and Northern Qilian Thrust Belt contain rocks ranging in age from Archean to Pliocene. The Archean-Lower Paleozoic basement units are composed of the Metamorphic or marine clastic rocks, carbonate and Volcano rocks. Sedimentary sequences in the Hexi Corridor and Northern Qilian Thrust Belt include Lower-Middle Jurassic sandstone-pebble conglomerate (J_1-2), widespread Lower Cretaceous sandstone and mudstone, Tertiary sandstone and conglomerate, and the loose deposits of the Quaternary (Q). From the Lower to the Upper, the Lower Cretaceous includes Chijinbao (K_1c), Xiagou (K_1g) and Zhonggou(K_1z) Formations respectively. The Tertiary is composed of the Paleocene-Eocene Liugouzhuang (E_1-2L), Oligocene Huosaogou (E_3h) and Baiyanghe (E_3b) Formations, Miocene Gongxingshan (N_1g) and Getanggou (N_1t) Formations, Pliocene Niugetao Formation (N_2n), Lower-Middle Pleistocene (Q_1y) and Jiuquan (Q_2J) Formations, as well as Upper Pleistocene and Holocene aeolian and glacial deposits (Q₃₊₄).

The Northern Qilian thrust belt is particularly a good place to trace the Late Cenozoic uplifting history of the Tibetan Plateau because of its location of the northern margin of the Tibetan Plateau.

The Northern Qilian Orogenic Zone (NQOZ), located between the Qaidam basin and the Hexi Corridor, is a Caledonian, Indosinian and Himalayan intraplate orogenic zone in NW China (Figure 1). In Late Cenozoic, it uplifted rapidly and thrust northward over the Hexi Corridor due to the Indo-Asian collision [2,3].

The Northern Qilian Orogenic Zone (NQOZ) contains rocks ranging in age from Archean to Pliocene. The Archean-Upper Paleozoic basement units are composed of the Metamorphic or marine Siliceous rocks, flysch and molasses deposits, carbonate and Volcano rocks [11].

Sedimentary sequences in the Northern Qilian Orogenic Zone (NQOZ) include Triassic, Lower to Middle Jurassic and Lower Cretaceous mudstone, coal beds and sandstone-pebble conglomerate.

Two-stage uplift

The 2D seismic profile for petroleum exploration across the Northern Qilian Thrust Belt near/within the Hexi Corridor, northern margin of the Tibetan Plateau displays clearly the growth strata of Upper Pliocene and the thrust of Lower Cretaceous over the Quaternary, indicating the two-stage uplift of the Northern Qilian Orogenic Zone in Late Cenozoic.

The growth strata of Upper Pliocene Niugetao Formation (N₂n2), developed to the north of the drilling hole named Liudong1, are expressed by the overlap sedimentary of the Upper Pliocene toward the North Qilian Orogenic Zone, indicating an uplift of the North Qilian Orogenic Zone in the Late Pliocene while the Lower Pliocene Niugetao Formation (N₂n1) shows a steady sedimentation from the north to the south, suggesting that thrust northward and uplift of the North Qilian Orogenic Zone, did not occur during the Early Pliocene (Figure 1).

Moreover, the above 2D seismic profile for petroleum exploration also displays clearly the thrust from the northern of the Lower Cretaceous (K₁) over the Lower and Upper Pleistocene (Q₁,Q₃), indicating another uplift of the Northern Qilian Orogenic Zone in Quaternary.

Uplift rates

The uplift height and rates of the Northern Qilian Orogenic Zone in Late Cenozoic can be calculated by the growth strata height and altitude of strata on the above 2D seismic profile for petroleum exploration, as well as the altitude on geological map and the data of drilling hole. For example, the smallest uplift height (H0) and rates (S1) of the Northern Qilian Orogenic Zone in Late Pleistocene can be calculated respectively.

Figure 2: Seismic profile across the Northern Qilian Thrust Belt within Hexi Corridor (For location, see Figure 1).

Figure 3: Calculated uplift height of Northern Qilian Thrust Belt within Hexi Corridor (For location, see Figure 1).

H1=3550-1460 (wellhead altitude of the drilling hole named Liudong1) =2090 m

H2=0.4s×3800 m/s (travel velocity of seismic wave)/2=760 m

H0= H1+H2=2090 m+760 m=2850 m

S1=H0/180 Ma=2850/(78.1-1) Ma=3.7 mm/a

On the other hand, the smallest uplift height (H3) and rates (S2) of the Northern Qilian Orogenic Zone in Late Pliocene can be calculated respectively.

H3= The thickness of growth strata of Upper Pliocene Niugetao Formation (N₂n2)

=0.7s×3800 m/s(travel velocity of seismic wave)/2=1330 m

S2=H3/(258.8-180.6) Ma=1330 m/78.2 Ma=1.7 mm/a

Thus, the uplift rate of the Northern Qilian Orogenic Zone was ≥1.7 mm/a during the Late Pliocene, and ≥ 3.7 mm/a during the Late Pleistocene.

Shifting of Northern margin fault of the Northern Tibetan Plateau

The above 2D seismic profile for petroleum exploration across the Northern Qilian Thrust Belt near/within the Hexi Corridor, northern margin of the Tibetan Plateau also displays clearly the different northern margin of the Northern Qilian Thrust Belt during the Late Pliocene and Late Pleistocene respectively. The northern margin of the Northern Qilian Thrust Belt of the Late Pliocene was expressed by the front thrust fault, which is located within central Hexi Corridor, north of the drilling hole named liudong (Figures 2 and 3). However, During the Late Pleistocene, the front thrust fault of the Northern Qilian Thrust Belt shifted southward within southern Hexi Corridor, near the drilling hole named liudong1 (Figures 2 and 3).

Thus, during the Pliocene, the front thrust fault of the Northern Qilian Thrust Belt reached at the central Hexi Corridor, indicating the general emergence and horizontal propagation period. However, during the Quaternary, the front thrust fault retreated back to the southern Hexi Corridor, indicating the main uplift period.

This study provides a foundation for future work to study the uplift history of the Tibetan Plateau, China. In this study, the petroleum seismic profiles across the Northern Qilian Thrust Belt near/within the Hexi Corridor, northern margin of the Tibetan Plateau were used to trace the two-stage uplift of the Northern Qilian Orogenic Zone in Late Cenozoic. The first stage uplift, occurred in the Late Pliocene, was expressed by the growth strata of the Upper Pliocene with an uplift rate of ≥ 1.7 mm/a, and the front thrust fault of the Northern Qilian Thrust Belt reached at the central Hexi Corridor, indicating the general emergence and horizontal propagation period of the Northern Qilian Orogenic Zone. These results show the two-stage uplift of the Northern Qilian Orogenic Zone because of the collision between India and Asia during the Late Cenozoic, and thus resolve the long-standing dispute over the Late Cenozoic uplift history of the Tibetan Plateau.

This study was supported by Yumen Oil field Company. We thank Yongke Han, Jianjun Chen, Jun Wei and Jianli Li at Yumen Oilfield Company of CNPC for their thoughtful reviews or kind help.