Jp. Cogne et al., A PALEOMAGNETIC STUDY OF MESOZOIC SEDIMENTS FROM THE JUNGGAR AND TURFAN BASINS, NORTHWESTERN CHINA, Earth and planetary science letters, 133(3-4), 1995, pp. 353-366
Mesozoic sedimentary rocks have been collected from the Karamay area (
northwestern Junggar block) and in the Turfan basin (northeastern Tari
m block) for a paleomagnetic study with the aim of testing the rigid n
ature of these blocks and to better constrain their paleogeographic re
lationships within central Asia prior to the collision with India. The
results from the Karamay area are disappointing, Lower Cretaceous gre
y sandstones having apparently been recently remagnetized. This remagn
etization is carried by magnetite, and yields highly clustered normal
polarity directions. Both a medium (MTC) and a high (HTC) unblocking t
emperature component are isolated from flat-lying or weakly tilted Jur
assic red sandstones. The MTC could be carried by maghemite and the HT
C is carried by hematite. The two components are statistically identic
al, and seem to correspond to an ancient magnetization. However, there
is no fold test and comparison with previous data from Junggar and th
e reference APWP of Siberia does not allow us to decide between a Midd
le Jurassic primary magnetization and an Upper Jurassic or Cretaceous
remagnetization. Stronger constraints are obtained from Upper Jurassic
to lower Eocene fine-grained red sandstones from anticlines that have
been thrust southwards, in the Turfan area of South Tien Shan. After
the elimination of a recent overprint that is probably related to weat
hering, a high coercivity, high unblocking temperature component carri
ed essentially by hematite is isolated. All directions from the Jurass
ic to the lower Eocene, which yield positive fold and reversal tests,
are statistically indistinguishable. This is compatible with a primary
origin for these magnetizations, similar to what has previously been
found in Junggar, Tarim and western Tibet. The corresponding Turfan po
le is not statistically distinct from that of Tarim for the same perio
d, in accordance with the notion of a large, quasi-rigid Tarim block r
emaining attached to Eurasia in a rather constant position and at a co
nstant latitude throughout the second half of the Mesozoic. This new d
atum can therefore be included to derive a new mean pole for the Tarim
that is valid for the period from the Middle Jurassic to the end of t
he Cretaceous: 68.4 degrees N, 224.5 degrees E (A(95) = 3.1 degrees).