Early Cretaceous stratigraphy, depositional environments, sandstone provenance, and tectonic setting of central Tibet, western China
Early Cretaceous stratigraphy, depositional environments, sandstone provenance, and tectonic setting of central Tibet, western China
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DOI：
10.1130/b25388.1
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发表时间：
2004-09
影响因子：
4.9
通讯作者：
Kai‐Jun Zhang;B. Xia;Guangzhe Wang;Yong-tie Li;H. Ye
中科院分区：
文献类型：
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作者：
Kai‐Jun Zhang;B. Xia;Guangzhe Wang;Yong-tie Li;H. Ye
晚侏罗世大陆碰撞之后、晚白垩世-新生代弧岩浆作用之前的时期代表了中国西部西藏中部的一个重要但人们知之甚少的构造转变。西藏中部阿萨盆地（31°00'-32°40'、84°30'-87°05'）横跨羌塘地块南缘、班公怒江缝合带和拉萨地块北半部，具有长达5000 m的陆海碎屑碳酸盐沉积物，地理位置优越，具有良好的约束条件。 在这个转变上。最近实地研究的沉积和火山数据表明，收缩控制了贝里亚期-瓦兰吉尼期构造，而弧后伸展可能控制了欧特里夫期-早塞诺曼期盆地演化。 Berriasian-Valanginian 地层出现在阿萨盆地的北半部（大约在 31°40' 以北），并且不整合地覆盖在不同的基底之上。泥沙输送和细化方向一般向南。班公湖缝合带中部砂岩（Q 7 8 F 1 1 L 1 1 、Qm 7 2 F 1 1 Lt 1 7 、Qp 3 8 Lvm 8 Lsm 5 4 ）以波状消光的单晶石英颗粒为主，也含有丰富的燧石颗粒以及一些超基性岩屑和放射虫碎片。欧特里维安-下巴列姆统地层的特征是高能潮汐环境中沉积的硅质碎屑岩、障壁骨架生物碎屑颗粒岩和硬砂岩。层内石灰岩角砾岩或砾岩和双峰式火山岩位于盆地南缘沉积层序内（大约在 31°35' 以南）。砂岩（Q 5 4 F 2 0 L 2 6 、Qm 4 5 F 2 0 Lt 3 5 、Qp 2 0 Lvm 5 9 Lsm 2 1 ）的特点是来自再生造山带和火山弧物源的复杂衍生。广泛分布的厚碳酸盐岩是上巴列姆阶-下塞诺曼阶岩相的特征，可能形成于受限台地和台地内部环境中。砂岩中与碳酸盐互层的岩屑（Q 3 4 F 2 4 L 4 2 、Qm 3 3 F 2 4 Lt 4 3 、QP 3 Lvm 8 7 Lsm 1 0 ）主要反映火山源区（65%）。 Berriasian-Valanginian序列被解释为代表来自北部造山带的磨拉石沉积物，并表明羌塘和拉萨地块之间的碰撞可能已经延伸到Valanginian。欧特里阶初期砂岩从造山带向岩浆弧源区的变化，以及同时代西藏中南部广泛的海侵、正断层和丰富的双峰式火山岩夹层，被解释为与中白垩世（约120-95 Ma）冈底斯岛弧的弧后伸展有关。这可能是由于特提斯洋板块的回滚而发生的。在白垩纪中期，整个欧亚大陆南部边缘可能以一系列弧后裂谷盆地（包括科希斯坦和拉达克的盆地）为特征。一些盆地可能被洋壳覆盖，而另一些盆地则仍然被大陆或过渡基底所覆盖，就像现在的西太平洋边缘一样。
The period following Late Jurassic continental collision but preceding Late Cretaceous-Cenozoic arc magmatism represents a significant, yet poorly understood tectonic transition in central Tibet, western China. The Asa basin (31°00'-32°40', 84°30'-87°05') in central Tibet, which crosses the southern margin of the Qiangtang block, the Bangong-Nujiang suture zone, and the northern half of the Lhasa block, is characterized by up to 5000 m of continental-marine clastic-carbonate sediments and is ideally positioned to provide constraints on this transition. Sedimentary and volcanic data from recent field studies suggest that contraction controlled Berriasian-Valanginian tectonics, whereas back-arc extension may have controlled Hauterivian-Early Cenomanian basin evolution. Berriasian-Valanginian strata occur over the northern half of the Asa basin (approximately north of 31°40') and unconformably overlie a varied basement. Sediment transport and fining directions are generally toward the south. The sandstones (Q 7 8 F 1 1 L 1 1 , Qm 7 2 F 1 1 Lt 1 7 , Qp 3 8 Lvm 8 Lsm 5 4 ) in the central Bangong-Nujiang suture zone are dominated by monocrystalline quartz grains with undulose extinction but also contain abundant chert grains and some ultrabasic lithic fragments and radiolarian fragments. Hauterivian-Lower Barremian strata are characterized by siliciclastic rocks deposited in high-energy tidal environments, and barrier-bank skeletal bioclastic grainstone and wackestone. Intraformational limestone breccia or conglomerate and bimodal volcanic rocks are in places found within the sedimentary sequences in the southern margin of the basin (approximately south of 31°35'). Sandstones (Q 5 4 F 2 0 L 2 6 , Qm 4 5 F 2 0 Lt 3 5 , Qp 2 0 Lvm 5 9 Lsm 2 1 ) are characterized by complex derivations from both recycled orogen and volcanic arc provenances. Widespread thick carbonate is a characteristic feature of Upper Barremian-Lower Cenomanian lithofacies, possibly formed in both restricted platform and platform interior environments. Lithic fragments in the sandstones (Q 3 4 F 2 4 L 4 2 , Qm 3 3 F 2 4 Lt 4 3 , QP 3 Lvm 8 7 Lsm 1 0 ) interbedded with the carbonates predominantly reflect volcanic source areas (65%). The Berriasian-Valanginian sequence is interpreted to represent molasse sediments from a northern orogen and indicates that the collision between the Qiangtang and Lhasa blocks could have extended into the Valanginian. The change from orogenic to magmatic arc source areas for sandstones during the beginning of the Hauterivian, along with a coeval extensive marine transgression in central-southern Tibet, normal faulting, and rich intercalations of bimodal volcanic rocks, are interpreted to be related to back-arc extension of the Gangdese arc during the Middle Cretaceous (ca. 120-95 Ma). This may have occurred in response to a rollback of the Tethyan oceanic slab. During the Middle Cretaceous, the entire southern Eurasian margin could have been characterized by a series of back-arc rift basins (including those in Kohistan and Ladakh). Some basins may have been floored by oceanic crust whereas others remained underlain by continental or transitional basement, much like the present western Pacific margin.