Paleomagnetic evidence for Cenozoic block rotations in the Tadjik depression (Central Asia)

Abstract

This paper presents results of a paleomagnetic study of Oligo‐Miocene red beds of the Tadjik depression in Central Asia. We sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high‐temperature components appear to predate folding of upper tertiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the western margin of the depression. However, they are shallower by about 30° than the inclination predicted from the reference apparent polar wander path. This appears to indicate a 23° difference in latitude, which is incompatible with paleogeographic reconstructions for the Tertiary. A sound interpretation of this anomaly would require a better‐constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significantly rotated, counterclockwise with respect to those measured on the Turan platform. The eastern part of the depression is a domain of large rotation (52°±13° to 46°±15°), whereas smaller amounts of rotation have occurred in the western part (27°±14° to 14°±15°). The similarity between Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression. Paleomagnetic and structural data suggest that block rotations in the Tadjik depression are associated with indentation of the Pamirs into stable Asia. At a larger scale, observed rotations are compatible with a model of regional sinistral wrenching, along a strip running from the Gulf of Oman to Lake Baikal.

The interaction of two indenters in analogue experiments and implications for curved fold-and-thrust belts

“…• Counter-clockwise rotation has been reported within the Tajik basin, with the largest amounts close to the Pamir (Bazhenov and Burtman, 1986;Bazhenov et al, 1994;Thomas et al, 1994a). Our experimental results show similar counter-clockwise rotations in the transfer zone, close to the FI.…”

“…16; Bekker, 1996;Bourgeois et al, 1997;Thomas et al, 1994a) indicate that folds and thrusts verge north-west in the eastern part of the Tajik basin and southeast in its western part (Fig. 16, cross-section C-C′); this suggests a vergence toward to the centre of the basin.…”

“…It was suggested that the sediment stack in the Tajik basin has a prismatic shape and becomes thicker to the south, due to the protrusion of the Pamir and the HinduKush (Leith, 1982). Besides the poor knowledge of the southern Tajik basin, one explanation for the apparent absence of a FTB at the southern margin of the basin might be that it is covered by thick Quaternary deposits (Schwab et al, 1980;Thomas et al, 1994a). As the drainage systems merge and the major trunk stream (the Amu Darya) is located in the southern Tajik basin, the thickest Quaternary deposits accumulated there.…”

“…Furthermore, deformation occurred by thrusting along northeast-trending ranges within the basin, counter-clockwise block rotations, and intra-continental subduction along the Pamir front ( Fig. 1C; Hamburger et al, 1992;Leith and Alvarez, 1985;Thomas et al, 1994aThomas et al, , b, 1996. Seismicity suggested active intra-continental subduction along Hindu-Kush and the Pamir front (Billington et al, 1977;Chatelain et al, 1980;Negredo et al, 2007;Roecker et al, 1980).…”

Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China)

“…Paleomagnetic data suggest the Tajik depression off the Pamir's western margin was rotated $52°counter-clockwise since the Miocene (Thomas et al, 1994;Burtman, 2000). Sinistral strike-slip movement along the Darvaz fault in the west may have translated the western Pamir northward relative to the Tadjik basin (Burtman and Molnar, 1993), though Thomas et al (1994) show the Darvaz as a thrust fault. East-west extension began in the late Miocene along the Kongur Shan normal fault (Robinson et al, 2004), possibly the result of radial expansion of thickened crust under the plateau.…”