Paleomagnetic constraints on neotectonic deformation in the Kashi depression of the western Tarim Basin, NW China

Abstract

A paleomagnetic study is reported of Eocene to Pliocene formations from the Kashi depression, which aims to constrain the pattern of neotectonic deformation within the western sector of the Tarim Basin in northwest China. With the exception of Pliocene specimens from one locality (East Kulukeqiati) which show large within sitemean variations in declination, most sites from five sampled formations yield well-grouped characteristic remanent magnetizations and positive fold tests and are of probable post-depositional detrital origin. First-order consistency of paleomagnetic results from a range of rock ages and localities demonstrates that only small inter-locational vertical-axis rotation has occurred here and indicates that the Kashi depression is decoupled from the remainder of Tarim to the east and has behaved as a quasi-rigid block which has rotated by 20-30°counterclockwise relative to Eurasia and North China since the late Pliocene. The crustal-scale Talas-Ferghana Fault cuts the Tian Shan and meets the Kashi depression in the region immediately to the northwest of the study region and we find no paleomagnetic evidence for differential rotations to suggest that this fault zone extends southwards across the Kashi depression to link with the North Pamir Thrust Fault (NPTF). Instead, we argue that the southern extension of this zone is a transform-orogen junction with southward motion of the eastern wall accommodated by southward thrusting at the margins of the south Tian Shan and the Tarim Basin. We propose that dextral transpression around the margins of the crustal block incorporating the Kashi depression was responsible for the contrasting amounts of thrusting on the NPTF in the southwest and the South Tian Shan Thrust Fault in the north. Extensive evidence for neotectonism in the bordering zones of this block, as well as some paleomagnetic evidence from low unblocking temperature components, indicates that the deformation produced by block rotation is ongoing.

Neotectonic Deformation in the Southwestern Tian Shan, Western China: Evidence From Paleomagnetic Study of Quaternary Sediments From the Mingyaole Anticline

“…Rotational differences between the results from north of Kashi and north of Atushi (NKS and NAT) are within the error bounds and indicate that there has been no substantial internal deformation across the Kashi‐Atushi fold‐and‐thrust system that might otherwise be anticipated from small scale relative rotations since the Miocene. However, the 15.3° CCW vertical‐axis rotation relative to Eurasia compares with the somewhat larger 28.2°, 22.9°, and 17.4° CCW vertical‐axis rotations observed from both Miocene and Pliocene strata in the NAT and NKS (Huang et al, , and see Figure ). Furthermore, the amount of rotation of each unit of the fold‐and‐thrust system appears to increase with the age of the rocks because the largest CCW rotation is observed in Miocene rocks at the northernmost locality NAT while the smallest CCW rotation is observed in the Pliocene‐Pleistocene rocks from southern locality Mingyaole (Figure ).…”

“…Tectonics the 15.3°CCW vertical-axis rotation relative to Eurasia compares with the somewhat larger 28.2°, 22.9°, and 17.4°CCW vertical-axis rotations observed from both Miocene and Pliocene strata in the NAT and NKS (Huang et al, 2009, and see Figure 9). Furthermore, the amount of rotation of each unit of the fold-andthrust system appears to increase with the age of the rocks because the largest CCW rotation is observed in Miocene rocks at the northernmost locality NAT while the smallest CCW rotation is observed in the Pliocene-Pleistocene rocks from southern locality Mingyaole (Figure 10).…”

“…The TFF is classified as an indent-linked strike-slip fault (Cunningham & Mann, 2007) with a slip rate averaging 11-16 mm per year over recent millennia (Figure 11; Rust et al, 2018). However, paleomagnetic evidence defines CCW rotation of 15°-30°in the Tarim Basin (this study and Huang et al, 2009) contrasting with 25°-35°clockwise vertical-axis rotation in the Tuoyun Basin (Gilder et al, 2003;Huang et al, 2005), all relative to Eurasia. Huang et al (2005) concluded that the clockwise rotation of the Tuoyun Basin was essentially local in its impact and has resulted from block rotations close to the TFF transform ( Figure 11).…”

“…The Pliocene-Pleistocene paleofield directions and poles from the Mingyaole Anticline at this northwest extremity of the Tarim Basin are compared in Figure 9 and Table 3 with Miocene-Pliocene results from adjoining blocks and with the predicted field directions anticipated in this region from the APWP of stable Eurasia since the Pliocene (Besse & Courtillot, 2002;Huang et al, 2009). This comparison identifies rotation of the Mingyaole Anticline of 15.3°± 7.1°with respect to Eurasia, −3.9°± 9.0°with respect to North Kashi (NKS) and 4.2°± 12.6°with respect to North Atushi (NAT), respectively (Table 3).…”

“…Deformation resulting from block rotations is ongoing between the south Tian Shan and Northern Tarim (Huang et al, 2009), and we infer that the rotation resolved in this study area is attributable to the prolongation of the right lateral TFF and ongoing the NW-WNW dextral slip motion (Fu et al, 2010). The common CCW rotation observed from the present study in the Tarim Basin and recognized in the Tian Shan foreland is then interpreted to be a regional response to rotation of the Tarim Basin resulting from ongoing northward motion of the larger Tibet Block to the south east into the foreland of orogen (Figure 11).…”