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THE STRUCTURE OF THE CENTRAL ANGARA-VITIM BATHOLITH BY GEOPHYSICAL DATA Geological Institute SB RAS, Ulan-Ude, Russia.
The Angara-Vitim batholith more than 200 000 km2 by area occupies nearly the whole Baikal-Vitim system. It is a complex structure caused by presence of numerous relics and roof pendants of sedimentary-metamorphic rocks that were cut by various granitoids, Meso-Cenozoic depressions and other structures (Litvinovsky et al., 1993). Its thickness equals 15-20 km by geophysical data (Alakshin et al., 1988). By our data, it is 23 km in the area of the Barguzin depression. The structure of batholith, its shape and size are most fully reflected in anomalies of gravity (Fig.1). Big deep faults play a great role in batholith structure. They are borders of Earth’s crust structural blocks and clearly mapped by intense (3-10 mg/km) gravitational steps that reflect specific stratigraphical, intrusive and tectonic contacts. A general specific feature of gravitational field is: 1) exclusively high differentiation; 2) clear zonation; 3) presence of big and intense positive and negative anomalies of various forms and size; 4) presence of numerous mostly north-western gravitational steps that are confined to deep faults, being borders of structural blocks; (5) field value ∆g increases by steps within the range of field values up to 20-30 mg, i.e. from 170 to 80 mg. The Baikal-Konkuderskaya (BK), Tompudo-Nerpinskaya (TN), Dzhalokanskaya (Dzh), Dzhida-Vitim (DV), Konkudero-Mamakan (KM), Argoda-Bambujskaya (AB), Turka-Amalat (TA), Upper Zipinskaya (UZ), Barguzin (Br), Zipa-Uakit (ZU), Tulujskaya (Tu), Sunueokitskaya (Su) and other gravitational steps confined to deep faults with the same name are the largest elements of gravitational field in the studied area that form its main regional features (Fig.1). Dislocations with break in continuity are widely developed. They play a significant role in its tectonic structure and are represented by zones of smash, faults, upthrusts, fault-shifts and overthrusts. The north-eastern dislocations that likely originated in the Early Proterozoic were most widely developed. They divide the gravitational field into seven large anomalous fields and zones, i.e. Olokit or Tyya-Kholodninskaya (TKh), North Baikal (B), Baikal-Upper Angara (BU), Barguzin-Muya (A), Muya (M), Katera-Zipinskaya (E), Turka-Zipinskaya (Z), Zipa-Uakit (G), Vitim-Oleokma (Zh) and Selenga-Vitim (I) that belong to large blocks of the Earth’s crust. Anomalous zones are divided into subzones and anomalous sites, differed by specific peculiarities of morphology, mostly north-eastern direction of isoanomals and field intensity. Large blocks are divided by numerous faults of higher order into a large number of small blocks being of various shape, size and orientation. Big quantity of gravitational local maxima and minima was defined within the anomalous fields and zones. Maxima are caused by the Archean-Proterozoic and Vend-Cambrian denser sedimentary-metamorphic rocks only preserved as relics, xenoliths and roof pendants as well as massifs of basic and ultrabasic intrusions. Minima are confined to less dense granitoids of the Angara-Vitim batholith and large Meso-Cenozoic depressions. Predominant development of granitoids (~65%) determined character of gravitational field in the whole region. A sharp by step decrease in field level at contact of Tyya-Kholodninskaya zone 2,74 g/cm3 (along Baikal-Konkudera fault) dense by 90-110 mg related to the Baikal (B) and Baikal-Upper Angara (BU) zones is explained by effect of batholith granites of less density 2,62 g/cm3, with their thickness reaching 10-15 km and more. The central part of the batholith (interfluve of Barguzin, Kotera, Muya and Zipa) is differed by zone of low values ∆g (to 170 mg). The Barguzin (Br), Zipa-Uakite (ZU) and Upper Zipa (UZ) gravitational steps reflect borders of blocks that represent steep, near vertical contacts of granite massif with host rocks. The complexity and variety of morphology and structure of observed physical fields are caused by a great number of rocks from the Archean to Cenozoic age in the Baikal Fold Area. Within the studied area, a large amount of maxima and minima of various nature, form, size and strike had been distinguished (Fig.1).
1 – isolines of field ∆g in mg; 2 – gravitational fields and zones: Á – Tyya-Kholodninskaya; Á – North Baikal, ÁÂ – Baikal-Upper Angara, ÁÊ – Barguzin-Katera, E – Katera-Zipinskaya, Ã – Zipa-Uakit, A – Angara-Vitim, M – Muya, Æ – Vitim-Oleokma, Ç – Turka-Zipa, È – Selenga-Vitim; 3-5 – gravitational steps associated with faults: 3 – deep faults of mantle origin dividing the gravitational fields, zones and blocks of the Earth’s crust of I order (Àá – Abchadsky, ÁK – Baikal-Konkudera, KM – Konkudera-Mamakan, TÖ – Turka-Zipa, ÄÂ – Dzhida-Vitim); 4 – interblock crust-mantle faults of II order: KÍ – Kunerma-Lower Angara, TÍ – Tompudo-Nerpinsky, Áð – Barguzin, ÀÁ – Angara-Bambujsky, ÂÖ – Upper Zipa, ÖÓ – Zipa-Uakit, Dzh, Äæ – Dzhalokansky, Ñó – Sunueokitsky, Tó – Tulujsky; 5 – other innercrustal interblock faults of higher orders; 6 – gravitational maxima caused by relics and blocks of sedimentary-metamorphic formations of the Archean, Proterozoic, Vendian and Cambrian and massifs of basic and ultrabasic rocks: Olokitsky – 2, Frolikha-Khakusinsky – 4, Barguzin-Katera – 6, Vitim – 8, Uakit – 10,, Churovskoy – 12, Aktragda-Amalat – 14, Tildimo-Amalat – 16, Usty-Paramsky – 18, Big Amalat – 20; 7 – gravitational minima confined to massifs of granitoids and Meso-Cenozoic depressions: North Baikal – 1, Lower Angara – 3, Upper Angara – 5, North Muya – 7, Barguzin – 9, Talojsky – 11, Kydzhimitsky – 13, Zipa- Baunt – 15, including the Ozeornaya, Upper Zipa, Baunt, Shurindinskaya, Basanskaya, Lower Zipa depressions, Zaza – 17, Vekovjinsky – 19.
The largest and most intense minima are confined to the Meso-Cenozoic depressions, dome0like intrusions of granites, crush zones and reflect their spatial position. The North Baikal (1), Lower Angara (3), Upper Angara (5), North Muya (7), Barguzin (9), Talojsky (11), Kydzhimitsky (13), Zipikan and Zipa (15), Zaza (17), Vekovjinsky (19) and other minima belong to them. The Tyya-Kholodninsky (2), Frolikha-Khakusinsky (4), Barguzin-Katera (6), Vitim (8), Uakit (10), Churovsky (12), Aktragda-Amalat (14), Tildimo-Amalat (16), Usty-Paramsky (18), Upper Amalat (20) and other maxima belong to the large ones. Their nature is simply associated with massifs of basic, ultrabasic rocks and sedimentary0metamorphic rocks. It is difficult to establish exact borders of batholith, as it has numerous satellites. By the geophysical data, its western and north-western borders are along the Baikal-Konkudera fault and northern border – along Konkudera-Mamakan one. Some researchers (Lesnikova, “Irkutskgeophysics”) make the eastern border of the batholith along the Tulujsky (Sunueokitsky) fault, the other ones (Pismenny, Irkutskgeophysics”) – to the east from Vitim. Its south-eastern and southern borders can be contoured either along the Dzhida-Vitim structural suture or Tugnui-Konda regional fault. Thus, a comparison of geophysical and tectonic maps of the district allow to establish general conformity of anomalous fields both with regional structural-tectonic peculiarities of the studied area and some of their elements. At the same time, some general regularities of gravitational field are found: 1. Big structural-tectonic elements are clearly reflected in anomalies ∆g, with their strike being mostly north-eastern one. 2. Deep faults being the borders of structural blocks are distinctly mapped by intense horizontal gradients (gravitational steps). 3. Sedimentary-metamorphic rocks are determined by intense maxima, and granitoids and Meso-Cenozoic depressions – by the lower values of field ∆g. 4. Main peculiarities of geological structure and the latest tectonics (rift depressions) are clearly reflected in gravitational field. 5. Block structures are brightly reflected in the gravitational field, where volumous deep structure of the region, density irregularities of the Earth’s crust, thickness of sedimentary-metamorphic strata, character and form of contact surface are determined. 6. All determined fields, zones and subzones are as a rule compared to large structural elements of the region.
References Alakshin A.M., Pismenny B.M. (1988) About the structure of the Earth’s crust in conjugated zone of Siberian platform with folded rim // Geology and Geophysics. N11. p.24-31. Litvinovsky B.A., Zanvilevich A.N., Alakshin A.M. et al. (1993) The Angara-Vitim batholith is the largest granitoid pluton.
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