Discordant paleomagnetic direction in Miocene rocks from the central Tarim Basin: evidence for local deformation and inclination shallowing

摘要

From exposures at the southeastern end of the Maza Tagh range in the central Tarim Basin (latitude: 38.5°N; longitude: 80.5°E), 55 paleomagnetic sites were collected from red mudstones and sandstones of the Miocene Wuqia Formation. Thermal demagnetization revealed a high unblocking temperature characteristic remanent magnetization (ChRM). Five sites collected across a link fold yield a positive fold test at 99% confidence level. The mean directions computed from normal and reversed polarity sites are antipodal suggesting a primary origin for the ChRM. In stratigraphic coordinates, the final set of 30 site-mean ChRM directions yields a section-mean direction: inclination (I) = 29.4°; declination (D) = 24.7°; α_(95) = 6.2°. When compared to the Miocene expected direction (at 20 Ma), the observed direction indicates 30.8 ± 5.5° flattening of inclination and 15.3 ± 6.7° clockwise vertical-axis rotation. Anisotropy of magnetic susceptibility measurements on 155 samples show a strong foliation of 1.092 with a subvertical minimum susceptibility axis. These observations indicate a rock-magnetic (depositional or compaction shallowed) origin for the inclination flattening. The clockwise deflection of the observed declination can be interpreted as either: (1) 15.3 ± 6.7° clockwise rotation of the entire Tarim Basin since the Miocene; or (2) a local km-scale structural deformation. It is not a simple matter to discard the interpretation of 15.3 ± 6.7° clockwise rotation of the Tarim Basin because the fastest rates of rotation determined from global positioning system and slip-rate studies of Quaternary faults could produce such a rotation of extrapolated to 20 Ma. Nevertheless, we argue that local deformation is the preferred interpretation because the map pattern of local structures shows ～20° clockwise deflection toward the southeastern end of the Maza Tagh range where the paleomagnetic samples were collected.