Shortening Accommodated by Thrust and Strike‐Slip Faults in the Ili Basin, Northern Tien Shan

Abstract

The Tien Shan accommodates a significant portion of the India‐Eurasia N‐S convergence. In its northern part a zigzag pattern of mountain ranges bounds the western Ili Basin. The role of this basin in the overall shortening and the regional kinematics is not well understood. Geodetic data and instrumental seismicity are not sufficient to infer the role of individual faults and fault systems. We analyze GPS data and earthquake slip vectors and present the results of fault mapping based on remote sensing and field campaigns in the western Ili Basin. These observations indicate that E‐W thrust faults are active at the basin margins, and oblique and strike‐slip faults, both in the basin and in the Paleozoic rocks within the mountain ranges, have been active in the Late Quaternary. We propose a regional tectonic model in which the left‐lateral strike‐slip faults at the NW margin of the basin accommodate ~3‐mm/year NE‐SW shear. Smaller right‐lateral oblique faults transfer the motion in between the left‐lateral faults, and also take up shortening by rotations about vertical axes. We see the onset of internal deformation within the Ili Basin, although it has a strong basement. Our kinematic model is consistent with geodetic data, earthquake seismology, historical, and prehistorical surface faulting, and describes the first‐order features of active deformation that can be observed in the northern Tien Shan. Our study illustrates the importance of combining these different data sets to understand the regional tectonics.

Oblique Right‐Lateral Faulting Along the Northern Margin of the Ili Basin in the Northern Tian Shan, Northwest China

“…The southern margin of the Ili Basin is subjected to oblique convergence, as revealed by current GPS measurements (Zubovich et al, 2010) and geological studies (Yang et al, 1988). Previous studies (e.g., Grützner et al, 2019; Luo et al, 2003) have demonstrated that the northern margin of the Usun Range is controlled by a thrust fault. Therefore, the sinistral faulting should be accommodated at the interior or the southern margin of the Usun Range.…”

“…The KSHF extends toward the WNW along the northern margin of the basin, and the expression of the dextral slip faulting disappears at the Kazakh Platform. Several NE‐ or ENE‐striking folds and thrust faults have developed at the western end of the KSHF (Figure 1b; Cording et al, 2014; Grützner et al, 2019; Kober et al, 2013). The crustal shortening rate of the Usek thrust fault, one of the ENE‐striking structures at the tip of the KSHF, is approximately 0.25 mm/yr, and the total shortening rate of several subparallel NE‐striking structures may be 1–3 mm/yr (Cording et al, 2014).…”

“…The triangular Ili Basin with a Precambrian basement (Pelsmaeker et al, 2018) and thick Cenozoic sediments is a Paleozoic microcontinent (Charvet et al, 2011) that separates the Tian Shan into northern and southern parts (Figure 1b). Although the Ili Basin has some internal deformation structures such as the Shonzhy fault (Grützner et al, 2019; Mackenzie, 2016), most of the active deformation is accommodated at its boundaries. Therefore, the Ili Basin behaves more or less as a rigid block.…”

“…Therefore, the Cenozoic deformation of the Tian Shan has been interpreted as thrust faulting and crustal shortening in a series of block‐like patterns that are separated by elongated and rigid intermontane basins that are roughly oriented east‐west (E‐W) (Figure 1b; Thompson et al, 2002). However, coseismic surface ruptures of historical large earthquakes (Abdrakhmatov et al, 2016; Crosby et al, 2007; Delvaux et al, 2001; Yang et al, 1988) and geodetic measurements (e.g., Yang et al, 2008; Zubovich et al, 2010) show that the Tian Shan involves clear strike‐slip faulting within the range (Grützner et al, 2019; Molnar & Tapponnier, 1975; Tapponnier & Molnar, 1979; Wu et al, 2019). At present, strike‐slip faulting along intermontane basins is rarely reported.…”

“…In contrast to other intermontane basins that accommodate N‐S convergence by E‐W‐striking thrust faulting, the Ili region undergoes obvious sinistral shearing in addition to N‐S compression, as revealed by GPS measurements (e.g., Grützner et al, 2019; Zubovich et al, 2010). Based on the investigations of active faulting and GPS studies at the southern margin and NW margin of the Ili Basin, Grützner et al (2019) proposed a deformation model of NE‐SW left‐lateral shearing and counterclockwise rotation between the left‐lateral faults to define the deformation pattern at the western margin of the Ili Basin. However, as a major boundary fault of the Ili Basin, the KSHF currently has no reported credible kinematic information; therefore, the deformation pattern and the associated strain accommodation in this area are still not well understood.…”