越南北部紅河剪切帶自中生代以來應變史之探討

Abstract

摘要 紅河剪切帶位處中南半島和南中國陸塊之間的一條地質不連續帶，自中生代以來曾受過多次變形作用。為了釐清紅河剪切帶於不同變形事件之應力所產生之應變史，我們針對從紅河剪切帶在越南北部段所採集之定向標本進行詳細的顯微構造分析。藉由測量各變形期葉理交軸之位態，進而了解各變形期應力之三維結構。顯微構造觀察分析結果顯示此區經歷有三期的塑性變形：第一期變形 (D1) 可能發生於印支期時，形成近垂直的新葉理面S1，並右旋剪切更早期的葉理S0/1；所測量出之葉理交軸 (第一組葉理軸) 為南-北向，傾角介於0˚~40˚之間，σ1為東-西向水平擠壓力。第二期變形 (D2) 可能與始新世到中新世時紅河剪切帶左移運動有關，形成近水平的新葉理面S2，並左旋剪切S1；葉理交軸 (第二組葉理軸) 為南-北向，傾角介於2˚~55˚之間，反應鉛直方向的σ1和東-西向的水平擠壓力σ2。第三期變形 (D3) 可能為開始於晚中新世與紅河剪切帶右移運動有關，形成近垂直的葉理面S3及右旋的S2；其葉理交軸 (第三組葉理軸) 為南-北向，傾角介於0˚~55˚之間，σ1為東-西向水平擠壓力。總結研究之結果，所觀測到此三期塑性變形的葉理交軸之方位一直保持南-北向，顯示其水平擠壓力一直保持東-西向，並無歷時或因地而逐漸改變方向。這一點與Huchon 等人 (1994) ，和Burchfiel (2004) 的應力場模式不符合；亦與Leloup 等人 (2001) 的拉鍊模式 (Zipper Tectonic Model) 所表示的中南半島在沿紅河剪切帶左移脫逸時，沿紅河剪切帶西北段呈滑移壓縮，東南段呈滑移伸張之現象並不符合。

ABSTRACT In order to study the strain evolution of the highly deformed Red River Shear Zone (RRSZ) in northern Vietnam, matrix Foliation Intersection/Inflection Axes (FIAs) were measured from the orientated thin sections sampled between Lao Cai and Nam Dinh along the RRSZ, northern Vietnam. Microstructural observation recorded three ductile deformation events in this region. D1, possibly occurred during the Indosinian orogeny, showed the sub-vertical S1, the dextral shearing S0/1, and sub-horizontal N-S trending FIA set 1, indicating E-W compressional σ1 . The orientations of FIA set 2 and 3 remained similar to FIA set 1 (N-S trending, sub-horizontal). D2 , possibly resulted from the left-lateral strike-slip movement of the RRSZ during Eocene-Miocene, formed the sub-horizontal S2 and sinistral shearing S1, and indicated σ1 in the vertical direction and E-W compressional σ2 . σ1 changed to E-W compression during D3 , and formed the sub-vertical S3 and the dextral shearing S2 as the right-lateral strike-slip movement of RRSZ, which possibly commenced during the Late Miocene. These results correlate well with the regional tectonic framework of the Indochina. The stress patterns didn’t change along the RRSZ as predicted by the left-lateral strike-slip movement of the RRSZ of the Zipper Tectonic Model Reference. The observed stress evolutionary history since the Cenozoic is different from previous studies, e.g. Huchon et al. (1994) and Burchfiel (2004).