Geodynamic Evolution of Central Asia in the Phanerozoic

Geodynamic Evolution of Central Asia in the Phanerozoic

622 RODINIA, GONDWANA AND ASIA The third event is related to exhumation of the thickened crust and is reflected in retrograde metamorphism accompany...

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RODINIA, GONDWANA AND ASIA

The third event is related to exhumation of the thickened crust and is reflected in retrograde metamorphism accompanying strong decompression (ca. 5 kbar or more). One of the important

tectonic implications is the connection between exhumation and the continued movement along the same thrust, which lead to crust thickening.

Geodynamic Evolution of Central Asia in the Phanerozoic I.V. Gordienko Geological Institute, Siberian Branch of the U S , Sakhyanova str. 6a, Ulan-Ude, 670047, Russia Within the folded frame of the southern part of the Siberian platform, covering the main part of the Central Asian and Mongol-Okhotsk fold belts, all known types of geodynamic settings have existed during its long history. These are: midoceanic spreading zones and zones of transform faults, active and passive continental margins, ensimatic and ensialic island arcs, marginal and innercontinental volcano-plutonic belts of subduction and riftogenic types, zones and regions of withinplate magmatism. But its role in various periods, especially in the Precambrian, was ambiguous. In the Early Precambrian (in the Archaean - Early Proterozoic) the Siberian continent together with other big Early Precambrian massives was a part of the vast supercontinent Pangea, which by the beginning of the Middle Proterozoic was broken into a number of continents and microcontinents (Kana, Gargan, Muya, Aldan Stanovoy, etc.). Along the margins of the Siberian continent appeared “narrow” basins, with oceanic crust of the Red Sea type (Muya and etc.), as well as active continental margins with bordering volcano-plutonic belts (Akitkan and

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Fig. 1.

300 km

Schematic arrangement of Neoproterozoic-Paleozoicophiolite belts and areas of the Central Asian fold belt. 1 -sedimentary cover of Precambrian platforms; 2 - marginal uplifts of the Siberian Platform basement, including pericratonic troughs (Baikal-Patom etc.); 3 - uplifts of the Precambrian basement; 4 - predominantly Caledonian and Hercynian structure-formation complexes of various geodynamic origin; 5-ophiolite belts: 1- Terekta, 2 - Kobda, 3 - Tsaganshibetin, 4 - West-Sayan, 5 Dzabkhan, 6 - Khankhukhei, 7 - South-mva, 8 - Kurtushiba, 9 - Kuznetsk Alatau, 10 - Iya, 11 - Il’chir, 12 - Dzhida (Argyingol’), 13 - Bayanulan, 14 - Abagin, 15 - Bayangol’, 16 - Shilka, 17 - Ih Bogd (Hantaishir), 18 - Bayank-Hongor, 19 - Central Gobi (Adatsag), 20 - Kerulen, 21 Undurshilin, 22 - Baikal-Muya, 23 - Solonker.

etc.), and the zones of the collisional granite magmatism appeared. Intensive development of subduction zones in the Middle Proterozoic brought to a rapid closure of the oceanic basins, to the terrane accretion and to the formation of the supercontinent Rodinia. Its breaking began in the end of the Proterozoic, and was caused first of all, by the formation of the new oceanic basins: Yapetus, Paleo-Asian and its continuation to the East (in modern coordinates) - Mongol-Okhotsk. The remains of these basins are fixed in the Neoproterozoic and the Vendian - Early Cambrian ophiolite belts and island-arc volcanic complexes (Fig. 1). The formation of the continental crust within the territory under review was completed by the end of the Early Paleozoic. As a result, by the end of the Silurian the continental margin of the ancient Siberian continent arised .The margin bordered on the Mongol-Okhotsk and Paleoasian oceans; the latter later transformed into the Tethys paleoocean. From the Early Devonian, the intensive interaction of the continental margin with oceanic plates resulted in formation of volcano-plutonic belts of different age along the active continental margin. In D-C, along the whole southern border of the Siberian continent (in moderm coordinates) strong tectonomagnetic processes brought to the formation of the extended volcanoplutonic belt. In the cross-section of this belt in the composition of these magmatic complexes one can see the lateral changes: the increasing of the total alkalinity (first of all Potassium) and the decreasing of the isotopic Strontium component towards the center of the Siberian continent active continental margin. These data as well as the calculations of the depth of subduction zones in the Devonian give an opportunity to compare the stuctural-magmatic zonality with the contemporary geodynamic settings of the Andian type. In C, 2, the geodynamic setting of the West-Pacific type (of island arc and marginal sea type) was formed along the continent. In C,-PI, this setting was substituted by setting of active continental margin (Andian type), where extent volcano-plutonic belts (Central-Mongolian, Selengino-Vitim, etc.) and big granitoid batholiths began their formation. In P,-T,, geodynamical setting became more complicated because of riftogenic volcano-tectonic structure formation in the rear of the active continental margin, like Altiplano and Kito in South America. The later, Mesozoic, history was related to the interaction of the Siberian continent with the Paleopacific. At that time volcanic island arcs (Uda-Murgal, etc.) and marginal volcano-plutonic belts of Andian and Californian type (Verkchne-Amur, Sikhoted i n , etc.) formed in the East of Asia. Gondwana Research, V. 4,No. 4,2001

RODINIA, GONDWANA AND ASIA

Thus, the geodynamic development of the region in the Phanerozoic can be treated as an irreversible directed evolution of various geodynamic settings arising a t the oceanic, transitional, and continental stages. These settings documented constructive processes of transformation of the oceanic crust into the continental one with its further repeated destruction and reconstruction (accretion and collision), which finally

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brought to the formation of a single continental massif on a vast area in the Central Asia.

Acknowledgments This work was financially supported by the Russian Foundation for Basic Research (Project No. 99-05-64268).

Cryptic Pan-African Transpression Along the Eastern Margin of the Kalahari Craton and Beyond - A Possible Suture Between East and West Gondwana G.H. Grantham Council for Geoscience, P/Bag X112, Pretoria, South Africa Pan-African deformation and metamorphism (-450-600 Ma) along the eastern margin of the Kalahari Craton is recognised at various localities. Progressing from south to north, Pan-African age deformation is recognised in Antarctica (Heimefrontfjella, Kirwanveggen, Sverdrupfjella,Ahlmannryggen) and Mozambique and Madagascar. In Heimefrontfjella, a transpressional setting has been recognised by German researchers in the Grenvillian-aged gneiss terranes (Jacobs, 1991). In southern Kinvanveggen the deformation recorded in the Pan-African age (Moyes et al., 1997,1995) Urfjell Group clastic sedimentary rocks suggests a sinistral strike-slip structural setting (Croaker, 1999). Recognition of Pan-African deformation in the gneissic terranes of Kirwanveggen is difficult because of similar strain geometries to the Grenvillian structures, however, Pan-African closure temperatures are recorded in amphiboles and phyllosilicates. In Sverdrupfjella Pan-African age, top to the SE folding and reverse faulting, is recognised and locally has facilitated syn-tectonic granite emplacement at -480 Ma (Grantham et al., 1991). Extensive data from Rb/Sr, Ar/Ar and U/Pb SHRIMP analyses from a variety of rock types confirm a strong thermal input during Pan-African times (Moyes and Harris, 1996). In Ahlmannryggen, weak folding of the rocks is reported with extensive jointing. K-Ar data from thrust faults records Pan-African ages (Peters et al., 1991). In southern Mozambique in the Manica Province, a wide sinistral shear zone is developed adjacent to the Kalahari Craton. Ar/Ar dating of phyllosilicates suggests thermal resetting related to this shearing at -450-550 Ma (Manhica, 1998). This resetting has affected both cratonic rocks on the Kalahari Craton and Grenvillian-age gneisses to the east. In central northern Mozambique in Zambesi and Nampula Provinces, the dominant structures include top to the southeast shear structures with a E-NE-N oriented lineations. Local sinistral strike-slip shears disturb the general NE striking structural grain typified by the Lurio Belt. Pan-African ages from undeformed granites as well as anatectic melts confirm the Pan-African age of deformation and metamorphism of the Grenville-aged gneisses. One aspect which bedevils the definition of the continentalscale structure proposed here is the difference in crustal levels Gondwana Research, I? 4, No. 4,2001

along the opposing margins and along the structure itself. Opposing margins in WDML have different depths of exposure -20km in Sverdrupfjella a n d probably half t h a t in Ahlmannryggen to the west whereas the level of exposures thins from Sverdrupfjella in the north to Urfjell in the south with depths varying from -20 kms to 0 at similar times. Broadly the western margin of this structure appears to reflect shallower crustal levels (where both margins are exposed), which may suggest top to the NW deformation. Structural styles however suggest top to the SE deformation, which was followed by inversion and exhumation of the eastern margin. The nature of deformation along the transpressional zone also varies from near vertical sinistral shear zones (central Mozambique, Heimefrontfjella) to localised zones of thrust-faulting (northern Mozambique) and folding (Sverdrupfjella, Ahlmanrryggen) depending on whether the orientation of plane of shear is parallel or weakly to strongly oblique to the broadly N-S, orientation of the large scale structure. Areas of obliquity include WDML and north-cental Mozambique. It is possible that continental scale structure continues into Madagascar and northwards into East Africa. It is uncertain whether this suture can viewed as representing the suture between east and west Gondwana or whether it merely represents intra-continental shearing.

References Croaker, M. (1999) Geological constraints on the evolution of the Urfjell Group, southern Krwanveggen, western Dronning Maud Land, Antarctica. Unpub. M.Sc. Thesis, University of Natal. Grantham, G.H., Moyes, A.B. and Hunter, D.R. (1991) The age, petrogenesis and emplacement of the Dalmatian Granite, H.U. Sverdrupfjella, Dronning Maud Land, Antarctica. Antarctic Sci., v. 3, pp. 197-204. Jacobs, J. (1991) Structural evolution and cooling history of the Heimefrontfjella Mountains, western Dronning Maud Land, Antarctica. Berich. Polarforschung , v. 97, p. 141. Manhica, A.S.T.D. (1998) The Geology of the Zimbabwe Craton and the Mozambique Belt in the Manica Province of Mozambique.Unpub. M.Sc. Thesis, University of Pretoria. Moyes, A.B. and Harris, ED. (1996) Geological Evolution of western Dronning Maud Land within a Gondwana framework: radiogenic isotope geology project. Final Report submitted to the South African