| The northeastern margin of Tarim basin is located along the southern margin of the Central Asia Orogenic belt, has undergone an accretional orogenic process, connects the East Tianshan Orogen and Tarim Basin, it is an important area that geologists study accretion-type orogens and basin-mountain coupling relations.With field investigation and lab study, the author analyzed the orogenic process of orogenic belt and its relations with Tarim Basin combining studies on structural geology, chronology, petrology, geochemistry, stratigraphy and sedimentology.1) Establish structural framework of the orogenic belt on the northeastern margin of Tarim basinAccording to the locations of Yushugou-Tonghuashan suture zone in South Tishan and large strike-slip ductile shear zones between neighboring units, orogenic belt in the northeastern margin of Tarim is divided into several parts including the North Tianshan unit, the Central Tianshan unit, the South Tianshan unit and the Kuruktag unit with the boundaries:the north Central Tianshan shear zone, the south Central Tianshan shear zone, Yushugou-Tonghuashan suture zone, Xinger shear zone and Xingdi shear zone.2) Identify pre-Sinian metamorphic and magmatic events in Kuruktag, discover a large ductile detachment shear zone in the basement of the orogenic belt at the northeast Margin of Tarim Basin.The basement rocks outcrop widely in Kuruktag. With zircon dating result of 2505 Ma and 1161-811 Ma and other previous isotope dating results, it is believed that Kuruktag terrane has late archean-early proterozoic (2565-2296 Ma) or older basement and experienced major metamorphic and magmatic events of proterozoic period at 1880-1817 Ma and 1161-811 Ma. (2) A decollement exists between the cover rocks and the basement stretches in E-W direction with a width of 2 km, is characterized by "A" type fold, mylonites, gently mylonite foliation, stretching lineation plunging to northwest or southeast. Studies on asymmetrical rotational structure and crystallographic preferred orientations of quartz in mylonites (EBSD analysis) indicate a movement of the ductile decollement in a top-to SE direction. (3) Ar-Ar dating result of the muscovite in the mylonite from the ductile decollement shows the decollement formed before 383 Ma.3) Sinian-early Paleozoic rocks in orogenic belt in the northeastern margin of Tarim experienced several deformation envents.The deformation events can be divided into 4 periods, including (1) N-S compressional deformation, (2) dextral strike-slip ductile-brittle deformation, (3) NE-SW compressional deformation and sinistral strike-slip ductile-brittle deformation in the east of this area, (4) intra-continental thrusting and gently folding deformation occurred in Cenozoic.4) Early Paleozoic cap rocks thrusting southward spread sheet-like from N to S and are deformed weaker increasingly from bottom to top.5) Redefine four large dextral strike-slip shear zones in orogenic belt of the northeastern margin of Tarim Basin and identify their formation duration time.The north Central Tianshan shear zone, the south Central Tianshan shear zone, Xinger shear zone and Xingdi shear zone are identified as ductile-brittle dextral shear zones. These dextral strike-slip shear zones formed before 368-340 Ma (Ar-Ar dating results), and large scale Intra-continental ductile dextral strike-slipping event occurred at 290-241 Ma (Ar-Ar dating results).6) Identify Caledonian orogenic event on the orogenic belt in the northeastern margin of Tarim, the northeastern margin of Tarim Basin.The evidences are:(1) An early Paleozoic suture zone (Yushugou Tonghuashan suture zone) exists in the south Tianshan; (2) The Cambrian and Ordovician rocks in Kuruktag (south part of the orogeny) have the feature of passive continental margin abyssal and slope facies. (3) Early Paleozoic abyssal sedimentary accretionary wedge, the product of paleo South Tianshan Ocean subduction, outcrops in Kumish (in the south Tianshan). (4) Early Paleozoic magmatic arc-type granites outcrop widely in Central Tianshan. (5) Fold-thrust belt thrusting southward formed at early Paleozoic in Kuruktag. (6) Silurian-Devonian conglomerates develop widely in orogenic belt and basin, northeastern margin of Tarim Basin. (7) Carboniferous rocks and Ordovician-Silurian rocks contacted with an angular unconformity have different structural features. Deformation in Carboniferous rocks occur broad and gentle upright folds and Ordovician-Silurian upright compact folds with flow cleavage. (8) The spade-like cap rocks and the decollement bellow them is the product of the intercontinental subduction after the collision event (before 383 Ma). (9) Dextral strike-slip fault system in Central Tianshan and Kuruktag, the product of intra-continental deformation after early Paleozoic close of the south Tishan Ocean, formed at 368-340 Ma (Ar-Ar dating results).7) The relation between Tarim Basin and the northeastern margin orogenic belt at the early paleogenic.After early Paleozoic orogeny foreland basin formed and massive sandstone, sandy conglomerate and mudstone deposited. After collisional orogeny intra-continental translation and compression happened in basin-mountain binding site. Apparent subsidence occurred in basin.8) Propose a structural evolution pattern of the orogenic belt on the northeastern margin of Tarim(1) At Sinian the South Tianshan Ocean opened. (2) Paleo South Tianshan ocean shrinked at Sinian-early Paleozoic period and the Tarim block subducted underneath the Jungar block resulting accretionary arc formed at Central Tianshan. (3) At Caledonian Tarim block and Central Tianshan accretionary magma arc collided which lead to formation of a series of asymmetric similar folds and flow cleavages. Molasses basins formed at the same time, the majority part of which was still under the sea level. (4) After the end of arc-continental collision, intra-continental translational thrusting occurred at orogenic belt, and large ductile detachment belt formed between cap and basement. (5) Carboniferous igneous rocks, indicating formation setting of extensional cracking at back-arc, develop widely in northeastern margin of Tarim due to subduction southward of paleo Asia Ocean at north. (6) Early Permian massive continental red molasses along orogenic belt are the signal that molasse basins began to change from marine to continental. (7) At Permian-early Triassic, large dextral ductile-brittle strike slip happened in northeastern margin of Tarim. (8) The east part of orogenic belt in northeastern margin of Tarim shows deformation characteristics of sinistral strike slip. (9) During Mesozoic period large uplift erosion occurred commonly in the orogenic belt of northeastern margin of Tarim. (10) At Cenozoic, the northeastern margin of Tarim experienced intra-continental compressinal orogenic process again due to the compression of India plate towards Tibetan plateau.
|
PDF Full Text Request