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

From exposures at the southeastern end of the Maza Tagh range in the central Tarim Basin (latitude: 38.5degreesN; longitude: 80.5degreesE), 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 kink 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 (1) = 29.4degrees; declination (D) = 24.7degrees; alpha(95) = 6.2degrees. When compared to the Miocene expected direction (at 20 Ma), the observed direction indicates 30.8 +/- 5.5degrees flattening of inclination and 15.3 +/- 6.7degrees clockwise vertical-axis rotation. Anisotropy of magnetic susceptibility measurements on 155 samples show a strong foliation of 1.092 with a sub-vertical 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 +/- 6.7degrees 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 +/- 6.7degrees 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 if extrapolated to 20 Ma. Nevertheless, we argue that local deformation is the preferred interpretation because the map pattern of local structures shows similar to 20degrees clockwise deflection toward the southeastern end of the Maza Tagh range where the paleomagnetic samples were collected. (C) 2002 Elsevier Science B.V. All rights reserved.