Zongxiu Wang, Chinese Academy of Geological Sciences (China)
Tao Li, China Seismological Bureau, Beijing (China)
Yongqing Liu, Chinese Academy of Geological Sciences (China)
The Tien Shan orogen underwent initial orogeny, extension adjusting and re-orogeny since Later Paleozoic, especially its re-orogeny and uplifting in Cenozoic is one landmark event in the geological evolution of the Eurasian Continent, which results in the present basin-orogen landscape.
76 apatite fission track ages show that there were three conspicuous uplifting stages of Bogad range in Cenozoic: ¢Ù5.6£19Ma,¢Ú20£30Ma,¢Û42£47Ma. By the systematic analysis of the thermochronological data and the sedimentary response in foreland basins along north and south sides of Bogad range , the re-orogenic process of the eastern part of Tien Shan (Bogad¡ªBalikun range) in Cenozoic was established:
(1)the initial uplifting of Bogad range was no later than Paleocene (65ma); during Eocene and Oligocene,one stronger uplifting event took place; during Miocene (about 24.4ma ago) the main stage of uplifting occurred; from 20ma to 5.6ma the range chain evolved into the stage that the uplifting of range was not uniform. The beginning of uplifting 65ma ago may be the initial time of re-orogeny of Bogad range; this is basically consistent with the time of re-orogeny shown by the sediments in the foreland basins.
(2)along the east-west and south-north directions, the Cenozoic uplifting of Bogad-Balikun range chain shows conspicuous differences, the cooling ages became younger gradually from west to east; in Bogad range three uplifting stages can be identified, well in Balikun range only the later two stages can be identified, the uplifting event of area between Bogad and Balikun range (Qijiaojin) was mainly the second one. Along the north-south direction, apatite fission track ages show that the uplifting of the northern part was older that that of the southern one, reflecting that the time and strength of uplifting between northern and southern parts of Bogad range were different, namely, the uplifting began firstly in the northern part, then spread to the south. This sequence of uplifting was consistent with the present arcuate style of Bogad range convexing to the south, the heavily erosion of the alluvial plain along the northern Tulufan basin, and the untypical foreland basins formed to the north of the range in Cenozoic. These were also coincident with the characteristics of Bogad range, which is one anticlinorium as a whole with axial plane dipping to the north with large angle, and with the geometry and kinemics of the piggyback propagation of thrusting into the Tu-Fan basin in Cenozoic.
(3)the fission track ages with length on shorter than 14¦Ìm concentrate on 24ma, meaning the quick uplifting during the Miocene, furthermore, the velocity of uplifting became faster along with the time; the length of fission track with age of 65.3ma is 13.61¦Ìm, meaning the velocity of cooling and uplifting of the range in the first stage was slower than that of the second one when passing through the anneal zone.