Cretaceous paleomagnetic results from western Tibet and tectonic implications

Abstract

We present paleomagnetic results obtained from samples collected in the summer of 1989 along a 1000 km traverse between Yecheng (Xinjiang) and Shiquanhe (Tibet) in the western Qinghai‐Xizang Plateau (along ∼80°E longitude). During this field trip, 480 paleomagnetic cores in Jurassic to Tertiary limestones and sandstones were drilled at 49 sites; 400 were measured. Isothermal remanent magnetization analysis indicates the existence of high coercivity minerals, and thermal, rather than alternating field demagnetization, was found to be more efficient in isolating paleomagnetic directions. The characteristic magnetization has rather low unblocking temperatures (250–560°C) in most limestone samples and higher unblocking temperatures (400°C–680°C) in the sandstone samples. About half of the measured samples show the present geomagnetic field or aberrant directions. The remainder display stable multicomponent magnetizations. The stable characteristic magnetizations indicate (1) a probable Jurassic remagnetization; (2) a Cretaceous (120‐80 Ma) magnetization which passes a fold test, yielding a mean pole at 66.2°N, 245.0°E (A95=5.1°, N=14 sites); and (3) a large scatter of Tertiary directions, which may be due to tectonic problems and/or remagnetization in the sampling zone. The Cretaceous paleodirections from Aksaichin and Longmuco (11 sites on the Qiangtang (North Tibet) block) are very similar to those from Shiquanhe (three sites on the Lhasa (South Tibet) block), separated by 300 km. This suggests that the North and South Tibet blocks formed a single unit at least as early as the Cretaceous. Comparing our results with previous paleomagnetic work on the Tibetan plateau, it is found that the paleolatitude of 10.6°±5.1° obtained from this study for a reference at Domar (33.75°N, 80.4°E) is close to that (9.6°±2.8°) obtained by Achache et al. (1984) and Lin and Watts (1988) from the Takena formation (110‐100 Ma) in the Lhasa region (30°N, 91°E). In addition, it appears that the southern margin of the Tibetan blocks (i.e., Eurasia) occupied an E‐W trending position at about 7°±6°N latitude. Paleodeclination differences between the western and eastern Tibetan blocks suggest internal deformations of the plateau at different scales (from 1 to 1000 km). According to differences in paleomagnetic declinations, the arcuate shape of the blocks would have been partly acquired after the collision.

Paleomagnetic results from the Early Cretaceous Lakang Formation lavas: Constraints on the paleolatitude of the Tethyan Himalaya and the India–Asia collision

“…Finally, the 35 reliable paleomagnetic sites from Late Cretaceous volcanics provide an inclination-only mean of 30.4 • ± 6.8 • and a corresponding paleolatitude of 16.3 • ± 4.2 • N, which are very consistent with the inclination of 30.9 • ± 2.4 • and paleolatitude of 16.7 • ± 1.5 • N from the 81 reliable Early Cretaceous volcanic sites, supporting the previous conclusion that the precollisional southern margin of Asia maintained a stable paleolatitude during the whole Cretaceous (e.g., Chen et al, 2012;Lippert et al, 2014;Ma et al, 2014;Yang et al, 2014). Considering that the Lhasa terrane accreted onto the Qiangtang terrane by the Early Cretaceous (Chen et al, 1993;Matte et al, 1996;Kapp et al, 2007) and maintained a relatively stable paleolatitude up to the India-Asia collision, a paleolatitude of 16.6 • ± 1.6 • N calculated from the inclination-only mean of all 116 Cretaceous volcanic sites should be a robust estimate for the Lhasa terrane during the entire Cretaceous period (Table 1 and Fig. 8).…”

Reconstruction of the Altyn Tagh fault based on U‐Pb geochronology: Role of back thrusts, mantle sutures, and heterogeneous crustal strength in forming the Tibetan Plateau

“…Second, as Figure 7 indicates, the extrusion model predicts ∼35° of clockwise, vertical axis rotation of the western Kunlun Shan and Karakax fault. Paleomagnetic directions measured from Cretaceous and early Tertiary strata north and south of the western Kunlun batholith (Table 4 and Figure 5) [ Chen et al , 1992, 1993; Gilder et al , 1996; Rumelhart et al , 1999; Yin et al , 2000] are inconsistent with such rotation. Because rocks to the north and south of the western Kunlun belt have not been appreciably rotated, it is likely that the intervening Kunlun belt also has not been rotated.…”

Paleomagnetism of Upper Cretaceous red-beds from the eastern Qiangtang Block: Clockwise rotations and latitudinal translation during the India–Asia collision

“…Chen et al (1993) suggested that the western part of the Qiangtang Block was situated at a paleolatitude of 11.3 ± 5.6°N in the Cretaceous, based on paleomagnetic results obtained from the Cretaceous red-beds at the western end of the Qiangtang Block, and this may indicate that western Qiangtang experienced $18.9 ± 4.9°northward latitudinal displacement relative to Eurasia since the Cretaceous. Lippert et al (2011) obtained Eocene paleomagnetic results from three volcanic sampling sections in the central part of the Qiangtang Block (Table 6 and Fig.…”

“…8A), which gave a mean paleolatitude of 28.7 ± 3.7°N at a reference point (33.0°N/88.0°E), suggesting that the central part of the Qiangtang Block experienced $6.2 ± 4.6°n orthward displacement relative to Eurasia (Lippert et al, 2014). Although inclination shallowing tests were not conducted in studies of red-beds by Chen et al (1993) and Halim et al (1998), our present Cretaceous result, and those obtained from Cretaceous volcanic rocks from the Lhasa Block and from Eocene volcanic rocks in the Qiangtang Block (Table 6), indicate the existences of inclination shallowing in the Cretaceous red-beds in the western and central part of the Qiangtang Block (Chen et al, 1993;Halim et al, 1998). Thus, the paleomagnetic data of Chen et al (1993) and Halim et al (1998) were not used in the following discussion.…”

“…Although the possible inclination shallowing of red-beds would result in an inaccurate estimation of paleo-latitudes from Cretaceous red-beds, several paleomagnetic data obtained from Cretaceous to Paleocene-Eocene volcanic rocks in the Lhasa Block can still provide reliable constraints on the paleo-positions and crustal convergence of southern Eurasia. For the Qiangtang Block, almost all the paleomagnetic data were obtained from the Cretaceous red-beds in the eastern and western parts (Huang et al, 1992;Otofuji et al, 1990;Chen et al, 1993), and none of these data were evaluated for possible inclination shallowing. Recently Lippert et al (2011) obtained paleomagnetic data from the Eocene volcanic rocks in the central part of the Qiangtang Block, and suspected the occurrence of inclination shallowing in the Cretaceous red-beds in the Qiangtang Block.…”