象背山變質岩帶 (Day Nui Con Voi metamorphic belt) 位於越南北部，為哀牢山紅河剪切帶 (Ailao Shan-Red River shear zone) 之最南段。變質岩帶東北、西南兩側各以齋河斷層 (Song Chay Fault) 與紅河斷層 (Song Hong Fault) 為界，主要葉理面呈西北-東南走向，傾向東北，野外測量之傾角約為70~80°，水平線理發育良好，運動特徵為左移剪切。前人研究認為哀牢山紅河剪切帶的左移活動引發南海的張裂，因此象背山變質岩帶受到橫移拉張的應力所控制。為深入探討其應變模式，本研究於象背山變質岩帶採集26顆定向岩石標本，並且製作XZ與YZ面之顯微薄片，測量岩石中變形之石英與長石礦物進行Rf/ψ應變分析，配合上立體應變橢球的重建與渦度的計算，分別以橫剖面與縱剖面進行討論。結果顯示象背山變質岩帶受到純剪 (pure shear) 為主的橫移壓縮影響，應變量集中在背斜構造之兩翼，呈現雙剪切的構造，立體應變橢球型態則以扁圓形為主(k<1)。主要應變三軸位態大致相互垂直，最大拉張方向之X軸在全區皆呈現西北-東南向近水平之位態，惟Z軸之方向由北段東北-西南向近水平的擠壓，向南逐漸轉為近垂直之擠壓力，顯示象背山變質岩帶於河內以南所受的應力以重力為主，側向之擠壓力則較弱。根據本研究結果重組之3D應變與應力史顯示，象背山變質岩體應先受到垂直之純剪導致水平剪切帶的發育，再經由東北西南向的擠壓伴隨哀牢山紅河剪切帶的左移與穹丘構造事件抬升至地表。

The Day Nui Con Voi (DNCV metamorphic belt) in North Vietnam is the southernmost high grade metamorphic zone along the NW-SE trending Ailao Shan-Red River Shear Zone (ASRR). It is bounded by the steeply (ca. 70˚) NE dipping, Song Hong and Song Chay sinistral strike-slip faults. The classical model considers the ASRR as a left-lateral transtension strike-slip fault which cuts through the entire lithosphere, which resulted in the spreading of South Chna Sea. However, the strain pattern of the DNCV metamorphic belt reconstructed from 26 oriented samples revealed that the tectonic mechanism is transpression dominated by pure shear stress rather than transtension as previous studies had assumed. By applying the Rf/ψ method on deformed quartz and feldspar on XZ and YZ planes, 3D stress and strain ellipsoid were reconstructed. All samples except RR2-2 showed vorticity value-Wk<0.71 which indicated the DNCV experienced pure shear deformation. The strain pattern reconstructed from Bao-Yen to Bao-Ha transection indicated the existence of two highly strain zones along the SW and NE limbs of the DNCV dome, while the hinge region experienced little strain. The strain pattern reconstructed form Lao Cai to Nam Dinh displayed similar strain condition along the ASRR and the k value of all samples are on the whole less than 1; showing oblate deformation. The Z axes in the north part of DNCV trend NE-SW subhorizontally but change to subvertically in the south part, an indication that gravity control is more significant than lateral compression.

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