Tectonic Evolution of the South China Continental Margin

Tectonic Evolution of the South China Continental Margin

686 RODINIA, GONDWANA AND ASIA results could be interpreted as one of the following: (1) south China was at a different position than that in the mi...

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RODINIA, GONDWANA AND ASIA

results could be interpreted as one of the following: (1) south China was at a different position than that in the missing-link hypothesis, (2) South China was at a similar position as in the missing-link hypothesis, but with a different orientation, or (3) South China had already broken away from its Rodinia position by the time of the Liantuo Formation (fa. 750 Ma). Work is in progress to obtain a high-quality ca. 820 Ma pole from South China. SHRIMP U-Pb analysis of zircon grains from foliated granite and orthogneisses in the basement of the Hainan Island at the southwestern end of the Cathaysia Block gave (1) a concordia age of ca. 1430 Ma, interpreted as the age of its crystallisation, (2) metamorphic zircon overgrowths of ca. 1300 - 1000 Ma, and (3) an inherited zircon core of ca. 1790 Ma. On the other hand, Mesoproterozoicmetasediments underlying Neoproterozoic rift-platform successions of the Yangtze Block in southern Sichuan Province have an age spectrum very similar to those from the Cathaysia samples, including the ca. 1300 - 1000 Ma ages. In addition, a deformed granite intruding the Mesoproterozoic metasediments gives a magmatic crystallisation age of ca. 1000 Ma (Z.X. Li et al., 2001). These results are the first direct evidence for Grenville-aged collision between the Cathaysia and Yangtze Blocks. The ca. 1300 - 1000 Ma zircon overgrowths in the basement of Cathaysia are regarded as evidence for a protracted orogenic process during the collision, the 1000 Ma granite (beneath the Yangtze side of the Neoproterozoic continental rift succession) as a syn- to late-orogenic granite, and the Mesoproterozoic sediments analysed for detrital zircons as possible foreland basin deposits on the Yangtze side of the late Mesoproterozoic orogen (i.e. sediments shed from the Cathaysia side of the orogen). In view of the existence of ca. 970 Ma ophiolite complexes between the two blocks in northeastern Jiangxi and southern Anhui (X.H. Li, 1997), the collision was possibly similar to the Africa-Eurasia collision, in that small oceanic basins were generated on the overriding plate along segments of retreating plate boundaries (Royden, 1993). These new constraints of

orogenic polarity may assist correlations between the YangtzeCathaysian suture and its possible continuations as the Racklan (NW Laurentia), and Queensland (NE Australia) zones within a global pulse of late Mesoproterozoic orogenesis.

References Evans, D.A.D., Li, Z.X., Kirschvink, J.J and Wingate, M.T.D. (2000) A high-quality mid-Neoproterozoic paleomagnetic pole from South China, with implications for ice ages, regional stratigraphy, and the breakup configuration of Rodinia. Precamb. Res., v. 100, pp. 313334. Li, X.H. (1997) Geochemical and Sm-Nd isotopic study of Neoproterozoic ophiolites from southeastern China: pctrogenesis and tectonic implications. Precamb. Res., v. 81, pp. 129.144. Li, X.H. (1999) U-Pb zircon ages of granites from the southern margin of Yangtze Block and the timing of Neoproterozoic Jinning orogeny in SE China: termination of Rodinia assembly? Precamb. Res., v. 97, pp. 43-57. Li, X.H., Li, Z.X., Zhou, H., Liu, Y. and Kinny, P.D. (2001) U-Pb zircon geochronology, geochemistry and Nd isotopic s t u d y of Neoproterozoic biomodal volcanic rocks in the Kangdian Rift of South China: implications for the initial rifting of Rodinia. Precamb. Res., (in press) Li, Z.X., Li, X.H., Kinny, PD. and Wang, J. (1999) The breakup of Rodinia: did it started with a mantle plume beneath South China? Earth Planet. Sci. Lett., v. 173, pp. 171.181. Li, Z.X., Li, X.H., Zhou, H. and Kinny, ED. (2001) New SHRIMP data from South China support a Grenville-aged collision between the Cathaysia and Yangtze Blocks. Abstract Volume of the Rodinia Field Symposium, Irkutsk, Russia. Preiss, W.V. (2000) The Adelaide Geosyncline of South Australia and its significance in Neoproterozoic continental reconstruction. Precamb. Res., v. 100, pp. 21-63. Royden, L.H. (1993) Evolution of retreating subduction boundaries formed during continental collision. Tectonics, v. 12, pp. 629-638. Wang, J. and Li, Z.X. (2001) Sequence stratigraphy and evolution of the Neoproterozoic marginal basins along southeastern Yangtze Craton, South China. Gondwana Rcs., v. 4, pp. 17-26. Zhang, Q.R. and Piper, J.D.A. (1997) Palaeomagnetic study of Neoproterozoic glacial rocks of the Yangzi Block: palaeolatitude and configuration of South China in the late Proterozoic Supercontinent. Precamb. Res., v. 85, pp. 173-199.

Tectonic Evolution of the South China Continental Margin Zong-ting Liao, Chen Yiao-kun, Ma Ting-ting and Li Yu-jia Laboratory of Marine Geology, MOE, Tongji University, Shanghai, 200 092, China --

The tectonics of the south China continental margin has been one of the themes most widely discussed in recent literature. Here we discuss the characteristics of the tectonic evolution of the south China continental margin in relation to the breakup of Rodinia and Gondwana and growth of Asia. Based on regional geology, south China can been divided into four larger tectonic units which are the south China microplate, Sanjiang orogenic belt, Guangdong-Hainan orogenic belt and Philippine-sea plate by Fuzhou-Yongdingfracture zone, Ailao Mountian - Red-River fracture zone, and Great Longitudinal Valley of Taiwan fracture zone. The south China micro-plate is a part of Eurasian-Asia plate. It can again be divided into Yangtzi

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landmass, Simao mass, Huiyushan mass, Guangxi-Hunan mass, west Zhejiang mass, middle Jiangxi Mass, Yunkai mass and middle Guangdong mass. Guangdong-Hainan orogenic belt and Sanjiang orogenic belt are related to Tethys plate. GuangdongHainan orogenic belt can again be divided into southwest Fujian mass, east Fujian mass, west Taiwan mass and Hainan mass. Sanjiang orogenic belt can be divided into Nanping-Simao mass, Baoshan mass, Tengchong mass, north Vietnam mass, east Burma mass etc. Philippine-sea plate consists mainly of eastern Taiwan mass in land part. The tectonics of south China continental margin mainly represents interaction among Eurasian-Asia plate, Indian-Tethys Gondwana Research, V. 4, No. 4, 2001

RODINIA, GONDWANA AND ASIA

plate and Pacific plate. But it may have only been the form of crust movement since Mesozoic Era. Continental margin evolution in south China began at about Middle Proterozoic Era. During pre-Indosinian (or late Variscian), the tectonics of the continental margin were characterized by continental growth or accretion in the mode of ductile creeping of various masses and collision at different times. Development and subduction of the Tethyan and Pacific plate record the juxtaposition and accretions of masses. Since the tectonic deformation and the superimposition of the later tectonics on those of the former continued to go on by collision after convergence or inter-blocks

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squeezing along pre-collision belts. This is indeed a type of thincrusted collision tectonics that occurred in the new global tectonic stage of plate tectonic regime. However, the tectonics of the south China continental margin differs from both of the Alps and south Appalachian and collision took place at different times and in different styles. Moreover, it differs also from the German0 and Cordileran types of orogenic tectonics since the inter-block squeezing was accompanied by severe deformation, profuse of metamorphism, magmatism, migmatization and magma intrusion. The various highly heterogeneous masses were welded to each other in different times and styles.

The Indo-Antarctic Rift: Geochronological Evidences from the Mahanadi Basin and the Lambert Graben F. Liskerl, R. Brown2and D. Fabe12 FB Geowissenschaften, University of Bremen, PF 330440,28334 Bremen, Germany, E-mail:[email protected]

School of Earth Sciences, The University of Melbourne, Melbourne 3010, Australia Recent syntheses of isobath fits and paleomangnetic data sets, as well as the correlation of basement complexes, sedimentary basins and large-scale tectonic structures, have allowed Gondwana reconstructions at a reasonable scale. Less constrained, however, is the detailed match of the fragments of East Gondwana, in particular between India, Antarctica and Australia. A major link between the east coast of India and East Antarctica might be obtained by the correlation of intraGondwanan rift segments. Based on geophysical, stratigraphical and geochronological data and structural observations, an IndoAntarctic Rift consisting of the Mahanadi basin (eastern India) and the Lambert graben (East Antarctica) has been suggested (e.g., Hofmann, 1996). Both rift segments are characterised by varying degrees of Pan-African overprint of the adjacent Precambrian metamorphic basement and the deposition of some kilometres of sedimentary rocks during the Carboniferous to Triassic. More than 100 fission track data were obtained during the last years to investigate the long-term landscape development of the two-suspected rift segments. Apatite fission track ages from the respective graben shoulders range between -310 to 120 Ma (Mahanadi basin: Lisker and Fachmann, in press), and -310 to 90 Ma (northern Lambert graben: Arne, 1994; Lisker and Brown, in prep.). In both vicinities, apatite fission track ages correlate with the distance to the basin margin, while there is no systematic variation of apatite age with distance from the respective continental margins unlike the pattern commonly observed elsewhere in Gondwana. The pattern of the fission track data indicates the formation of an alternating half-graben with coeval rifting phases during the Permo-Carboniferous and

Gondwana Research, I/: 4, No. 4, 2001

the mid-Triassic, closely correlating with sedimentation stages in the rift basin(s). The amount of late Paleozoic exhumation can be estimated as being 2 to 3 km.The time gap between the two exhumation stages agrees with a hiatus in sedimentation not only in the Indian and Antarctic basins of East Gondwana, but in almost all major Gondwana sedimentary basins in Early to Middle Triassic time, such as the Beacon, Karoo, Parana and Perth basins. The close conformity in timing of episodic denudation and cooling of the basement around the rift shoulders and time of extension and intracontinental deformation indicates that the denudational episodes were driven by plate tectonic forces, presumably due to the enhancement of topographic relief by faulting. The apatite fission track data also indicate that further basement cooling occurred in the Late Cretaceous, and probably represents erosional unroofing and associated fault tectonics initiated due to the breakup of Gondwana. 40Ar/39Arages of mafic dykes from both rift branches indicate a common episode of magmatic activity around 120 Ma.

References Arne, D.C. (1994) Phanerozoic exhumation history of northern Prince Charlcs mountains (East Antarctica). Antarct. Sci., v. 6, pp. 69-84. Hofmann, J. (1996) Fragmente intragondwanischer Rifte als Werkzeug der Gondwana-Rekonstruktion- das Beispiel des Lambert-MahanadiRiftes (Ostantarktika-Peninsular Indian). N. Jb. Geol. Palaont. Abh., V. 199, pp. 33-48. Lisker, E and Brown, R.W. (in prep.) Lambert rifting associated with the exhumation of the Prince Charles mountains (Antarctica). Lisker, E and Fachmann, S. (in press) The Phanerozoic history of thc Mahanadi region, India. J. Geophys., Research.