Triassic folding and thrusting in the Eastern Block of the North China Craton and the Dabie-Sulu orogen and its geodynamic

Abstract

The Dabie-Sulu orogen consists of different slices and blocks undergoing multiple-stage folding and thrusting. Particularly some eclogite blocks and jadeite quartzite units underwent two-stage pre-UHP and syn-UHP deformatiom, following boudin-making and slicing during regional first-stage （D1 ） deformation and D2 folding during the regional second-stage deformation. However, other slices or units only underwent mainly regional three-stage folding. The regional first-stage deformation is shown as intra-folial minor folds, mainly preserved in amphibolite blocks. The regional second-stage deformation is commonly outcrop-scale. Its fold axial plane is crenulation cleavage, locally being foliated composite schistosity belt due to intensive displacement. After restoration of D3 folding, it is shown that axial planes of different-scale tight, asymmetric D2 folds in different units are southeastern-overturned and NNE-striking. D2 fold asymmetry implies that it is related to southeast-directed thrusting. The third-stage folds are open folds with NWW-strike, western plunge consisting of deformed schistosity and crenulation cleavage. Unpenetrative ductile shearing related to these folding has two stages. The early ductile shear thrusting is south-directed in the north of the Xinxian dome and south-directed in the north of the Xinxian dome, cutting the second-stage folds and being associated with secondary-scale thrusts by the limb of the second-stage large-scale folds. The later ductile decolloment zones only developed in the contact belts of obvious rock types around the Xinxian dome, with extensional crenulation cleavage, obviously reforming the previous structures. The Eastern Block is an important tectonic unit of North China Craton located at the hanging wall of the Dabie-Sulu organ, of which Paleozoic to Triassic sedimentary cover underwent one-stage regional folding during Indosinian （Late Triassic）. Its axial trace is NWW-striking. It is consistent with that of the third-stage folds in the aspects of deformation time and fold style and spatial trending. This implies that they are related in geodynamics. At the same time, this study shows that the Eastern Block didn’ t undergo the other earlier regional two-stage deformation in the Dabie-Sulu orogen. Therefore, the two-stage regional Triassic folding has no influence to the inner of North China plate, only happened in the southern margin of North China plate. Only in the case of unifying of North and South China plates after the regional second-stage folding, the third-stage deformation in the Dabie-Sulu orogen induced the first-stage folding in the inner of North China Craton. Diachronous exhumation of HP-UHP rocks in the Dabie-Sulu orogen is related this geodynamic setting and processes of structural evolution.

大別-蘇魯造山帶不同巖片(塊)經歷了不同的褶皺變形。榴輝巖塊(或透鏡體)和硬玉石英巖片經歷了高壓-超高壓背景下的兩幕褶皺變形之后,在區域性第一幕變形期間主要發生透鏡化為主,后期與圍巖共同經歷緊閉同斜第二幕褶皺。而其它巖片主要經歷了現今野外可見的區域性三幕褶皺,其中區域性第一幕褶皺為片內殘留褶皺,在斜長角閃巖透鏡體中多見,宏觀規律不明。區域性第二幕褶皺在露頭尺度多見,軸面為折劈理,局部強烈置換成片理化帶(復合片理或第二期片理),恢復第三幕褶皺改造作用后,揭示出各種巖片中的各級尺度的第二幕褶皺都為軸面北西傾南東倒、軸跡走向為NNE向的緊閉不對稱褶皺,不對稱性一致反映其指向與各種巖片向南東的逆沖運動有關。第三幕褶皺為以片理或折劈理為變形面的寬緩褶皺,軸跡走向NWW,樞紐向西傾伏。韌性剪切帶為非透入性構造,分早晚兩期,早期為韌性逆沖,新縣穹隆以南,運動學標志指示向北逆沖,錯切第二幕褶皺,結合新縣穹隆北部向南的逆沖特征,反映這些韌性逆沖斷層多數為第二幕大型褶皺翼部的次級逆沖斷層;晚期為韌性滑脫帶,其發育局限于幾個巖性差異較大的接觸帶,帶內伸展型折劈理發育,并對擠壓構造樣式有重要的改造作用。華北克拉通東部地塊是華北克拉通的重要組成,其蓋層古生界和三疊系在印支運動期間經歷了一幕寬緩褶皺作用,其軸跡方向主體也為NWW向。這一褶皺構造明顯在變形時間、變形樣式和展布方向上都和大別-蘇魯造山帶中的第三幕褶皺非常一致,說明它們具有動力學上的必然聯系。同時,研究表明在華北克拉通東部地塊中沒有經歷大別-蘇魯造山帶中區域性第一、第二幕褶皺變形的記錄,故本文認為印支期這兩幕變形主要發生在華北板塊東南緣的邊界上,并沒有波及到板內,而且從東向西高壓-超高壓巖石剝露具有穿時性。只有當華北板塊和華南板塊在第二幕變形之后構成了統一塊體后,第三幕變形才波及華北板內。