A paleomagnetic study of Tertiary formations from the Kyrgyz Tien‐Shan and its tectonic implications

Abstract

As part of a general investigation of Cenozoic deformation in Central Asia, we studied the paleomagnetism of Tertiary red beds and lava flows from intermontane basins in the Tien‐Shan region of Kyrgyzstan. We collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them, thermally or with alternating fields. For most sites, there are magnetic components with unblocking temperatures higher than 600°C. We infer that the magnetic carriers are mainly hematite and magnetite. For most localities, the high‐temperature component appears to predate tectonic folding. For all localities, mean inclinations are shallower than expected from apparent polar wander paths. Inclination anomalies range from 16°±5° for the Issyk‐Kul basin, to 26°±7° for the Fergana basin. If due to changes in latitude, these anomalies imply at least 2000 km of northward displacement of the Tien‐Shan during the Tertiary, for which there is no tectonic evidence. We thus consider that the paleomagnetic reference directions cannot be directly compared with our Tertiary data. We explored other possible reasons for this anomaly, which has also been reported from other parts of the Alpine belt, but we could not find a satisfactory explanation. Absolute rotations cannot be accurately determined, because of problems with the reference direction. Nevertheless, the mean declination for the Fergana basin lies counterclockwise by 20°±11° from the mean declination of the Issyk‐Kul basin. This result is consistent with the counterclockwise rotation inferred for the Fergana basin from the pattern of Cenozoic faults and folds. It suggests a Cenozoic right‐lateral displacement of 110±60 km on the Talas‐Fergana fault.

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

“…As shown in Fig. 9, this mean Oligo-Miocene pole for Tarim-Turfan is not statistically different from three Paleogene to early Miocene poles derived from Issyk Kul, Naryn, and Manas in the intramontane basins, or at the northern margin, of the Tian Shan (Chen et al 1991;Thomas et al 1993). Hence no significant rotations are detected between these Paleogene to Miocene results and the 5-50 Ma mean pole for Eurasia (Besse and Courtillot 2002), or from the mean Miocene pole for North China (Huang et al 2004), or from a mid-Miocene pole from volcanic rocks at the northern margin of the Junggar Basin (Huang et al 2006a).…”

“…9). We also observe that the paleomagnetic poles obtained from Eocene to Neogene rocks in the Kashi depression and the mean Oligo-Miocene pole for TarimTurfan, as well as those from the Issyk Kul, Naryn, and Manas basins in the Tian Shan (Chen et al 1991;Thomas et al 1993) are distributed along small circles centered about the sampling sites in Tarim-Turfan (Fig. 9).…”

“…In contrast, Cretaceous to Miocene rocks to the west of the TFF have undergone a CCW rotation of *20-30°with respect to the Eurasian plate (Bazhenov et al 1993;Thomas et al 1993;Burtman et al 1996 and Fig. 1a).…”

Seismotectonic study of the Fergana Region (Southern Kyrgyzstan): distribution and kinematics of local seismicity

“…The Fergana basin is an east-west-trending intermontane basin filled with up to 8 km of Cenozoic sediments situated in the southwestern part of the Tien Shan (Bazhenov 1993;Thomas et al 1993;Cobbold et al 1996). The Tien Shan mountain belts stretch Figure 1 Map of Central Asia.…”

“…Particularly, none of the events proximal to the Talas-Fergana fault show strike-slip mechanisms, suggesting that this fault is currently locked and that the present-day activity is being accommodated along the neighboring compressional structures (Ghose et al 1998;Xu et al 2006). Paleomagnetic studies concluded that the Fergana basin and the neighboring Chatkal Range have rotated 20°to 30°counterclockwise relative to Eurasia since the Paleocene (Bazhenov 1993;Thomas et al 1993). The results of the work of Zubovich et al (2010), analyzing the regional surface velocity field using GPS data, corroborate the inference that this basin might currently still rotate.…”