三江北段东莫扎抓矿区构造变形特征

Structural deformation in the Dongmozhazhua deposit, northern segment of the Sanjiang belt

已有关于青藏高原隆升的各种构造模型多重视新生代变形而忽略了早期构造变形的限制。本文以三江北段东莫扎抓矿区为研究对象,通过详细的构造-岩相填图,恢复了矿区二叠纪以来变形序列,结合区域资料讨论了变形事件的大地构造背景。研究表明矿区发育中-下二叠统九十道班组灰岩、上二叠统那益雄组碎屑岩、上三叠统结扎群甲丕拉组碎屑岩和上三叠统结扎群波里拉组灰岩4套地层系统,二叠系与三叠系之间为不整合接触,局部被近南北向逆断层代替。北西向逆断层横亘矿区,断层上盘三叠纪碎屑岩和灰岩整体北倾,断层下盘三叠纪岩石被左右两条走滑断层夹持向南挤出。在图面和露头尺度上矿区叠加褶皱明显,南北向剖面上多见紧闭的倾伏褶皱,近东西向剖面上则为开阔水平的斜歪褶皱,表明南北向剖面上观察到的是已被叠加的早期褶皱,为矿区第一期变形,其形成与三叠纪末古特提斯洋盆闭合有关。始新世晚期印-亚大陆碰撞地壳缩短形成矿区第二期构造,即北西向逆断层和褶皱叠加。第三期近南北向逆断层可能形成于始新世末,与印-亚大陆碰撞引起的侧向旋转有关。

The uplift models of Qinghai-Tibetan plateau are mostly based on Cenozoic deformation but lack of the information from Paleozoic and Mesozoic structures, the later is important to understand the uplift process as the early structures always providing a framework. In this paper, we document the deposit-region structures and then try to analyze the deformation sequence in the Dongmozhazhua area, northern segment of the Sanjiang belt. Detailed geological mapping reveals four lithostratigraphic associations in Dongmozhazhua area, i.e., the Lower to Middle Permian Jiushidaoban Formation, consisting mainly of limestones; the Upper Permian Nayixiong Formation composed of clastic and volcanic rocks; the Upper Triassic Jiapeila Formation comprising clastic rocks intercalated with volcanics; and the Upper Triassic Bolila Formation of carbonate rocks. Between the Permian and Triassic is a regional angular unconformity, but this contact interface is replaced by nearly north-south trend thrust locally. Northwestward thrust in Dongmozhazhua dips 40°～50° to the north and juxtaposes Triassic carbonate and clastic sequence over Permian-Triassic strata. The Triassic rocks in the footwall are squeezed out by right-lateral and left-lateral strike-slip faults both sides. Two stages of folding that deformed the Permian-Triassic strata in the map and outcrop view. They were represented by tight plunging folds in NS section and open inclined folds in EW section. Then deformation history in Dongmozhazhua can be rebuilt： the first stage （D1） is characterized by nearly E-W trend folds and formed during the close of Paleo-Tethyan oceans in Late Triassic; the second stage （D2） is associated with a NE directed shortening in Late Eocene that produced NW trend thrusts and open folds and caused by the continent collision between India and Eurasian; the third stage （D3） is characterized by nearly N-S trend thrust and possibly triggered by the left-slip faulting in the eastern Indo-Asian collision zone.