Zhenyu Yang

Clockwise rotations recorded in redbeds from the Jinggu Basin of northwestern Indochina

Paleomagnetic data have been used to infer clockwise rotations and significant southward motion of the Indochina block during Cenozoic extrusion from the India-Asia collision zone. Because the Cenozoic of the Indochina block has been sparsely sampled to date and is key to determining the timing of this tectonic motion, we performed an extensive paleomagnetic study on Paleocene to Oligocene redbeds and middle Miocene sandy silts from the Jinggu Basin (23.5°N, 100.7°E), in northern Indochina. Paleomagnetic results from the redbeds pass fold tests, but they show exclusively normal polarity. There is controversy, however, on the age assignment to the lower part of the succession. If the age of the redbeds is indeed Paleogene, this indicates a prefolding remagnetization associated with Oligocene deformation of the Indochina block. If the age of the redbeds would be Late Cretaceous, their magnetization may be primary, and deposition could have taken place during the Cretaceous normal superchron. However, the abundance of secondary hematite in the redbeds in combination with the biostratigraphy, which indicates a Paleogene age for at least the upper two formations of the redbeds in the Jinggu Basin, implies a pervasive remagnetization. The middle Miocene sediments pass both the fold test and the reversals test and contain magnetite as well as hematite as carriers, suggesting a primary magnetization. Our large data set from the redbeds (>2000 paleomagnetic directions) demonstrates an ∼30°−35° clockwise rotation of the Jinggu Basin with respect to Eurasia, for both scenarios, i.e., when compared to a ca. 100 Ma pole (if the redbeds carry a primary natural remanent magnetization) or to a ca. 30 Ma pole (if the sequence is remagnetized). The middle Miocene results, however, indicate that the Jinggu Basin experienced no significant (2° ± 5.6°) rotation with respect to Eurasia. Since no major deformation has occurred within northern Indochina during Late Cretaceous to Eocene times, our results reflect a major clockwise rotation of the Indochina block during its Oligocene to early Miocene extrusion from the India-Asia collision zone.

High-resolution magnetic and palynological records of the last deglaciation and Holocene from Lake Xiarinur in the Hunshandake Sandy Land, Inner Mongolia

High-resolution mineral magnetic and pollen records from overlapping piston cores from Lake Xiarinur (42°37′N, 115°28′E) document detailed changes in environment and vegetation since the last deglaciation in the Hunshandake Sandy Land, Inner Mongolia. The formation of Lake Xiarinur commenced during the Bølling–Allerød warming, as reflected by an abrupt shift in sedimentary facies from eolian sands to lacustrine sediments at a core depth of 3.5u2009m (~14.1 cal. kyr BP). The pollen records demonstrate that desert vegetation with sparse herbs occurred before 14.1 cal. kyr BP, which was succeeded by meadow grassland vegetation from ~14.1 –13.3 cal. kyr BP (the Bølling -Allerød). A dramatic decrease in pollen concentration occurred between ~13.3 and 11.7 cal. kyr BP corresponding to the Younger Dryas. The remarkable increases in pollen and Pediastrum concentrations at ~11.7 cal. kyr BP suggest that the environment began to ameliorate, and the most humid conditions prevailed until ~8.5 cal. kyr BP. The maxima of magnetic concentration–dependent and magnetic-grain-size-dependent parameters, together with high pollen concentrations, between ~10 and 9 cal. kyr BP are interpreted as a brief interval of high annual precipitation. Our results indicate a stepwise decrease in effective moisture in the mid-Holocene and an accelerated drying trend in the late Holocene, characteristics typical of an East Asian summer monsoon–influenced region. The broad similarities between our data and other lake records from central-eastern Inner Mongolia, well-dated speleothem δ18O records from southern and central China, and summer insolation at 45°N, support the idea that vegetation and climate changes in the Hunshandake Sandy Land were controlled mainly by fluctuations in the response of the East Asian monsoon to Northern Hemisphere summer insolation.