印支期龙门山造山楔推进作用与前陆型礁滩迁移过程研究

马鞍塘期龙门山前陆盆地是印支期造山楔加载于扬子地台西缘而形成的挠曲前陆盆地。根据地表露头、钻孔剖面和地震反射剖面资料,本文通过分析前陆盆地早期前陆缓坡型鲕粒滩-硅质海绵礁组合在时间和空间上的迁移规律,标定了卡尼期龙门山造山楔的推进速率。结果表明:卡尼期马鞍塘组是分布于底部不整合面之上的第一套地层单元,在垂向上前陆型鲕粒滩-硅质海绵礁组合显示为鲕粒灰岩滩-生物碎屑滩-硅质海绵礁灰岩-泥页岩的向上变细的沉积序列,记录了前缘隆起边缘碳酸盐缓坡和海绵礁的构建和淹没过程,反映了在相对海平面的持续上升中鲕粒滩-硅质海绵礁被淹没致死的过程。在横向上,盆地结构显示为西厚东薄,并向西倾斜的不对称盆地,由西向东依次分布了深水盆地、碳酸盐缓坡和海绵礁和浅水滨岸带等沉积物类型,显示了从龙门山造山楔向前陆一侧具有泥页岩向鲕粒滩-硅质海绵礁的变化特征。其中鲕粒滩-硅质海绵礁丘组合发育于15～30m深度的前陆同斜缓坡上,呈面向西的条带状展布,其走向线与龙门山冲断带的走向大致平行。并可将其划分为7个鲕粒滩-硅质海绵礁相带,表明卡尼期硅质海绵礁丘和滩沿底部不整合面向南东方向的前陆缓坡超覆,其超覆线和相带的走向与龙门山冲断带的走向平行,显示了7条硅质海绵礁丘和滩是随着相对海平上升过程而向南东方向的前陆缓坡超覆过程中逐次形成的。卡尼期硅质海绵礁迁移速率为18mm·yr-1,其与龙门山造山楔推进速率(15mm·yr-1)基本一致,表明印支期龙门山逆冲楔推进速率与前陆鲕粒滩-硅质海绵礁丘迁移速率具有明显的耦合关系。据此,本次提出了龙门山前陆盆地早期前陆型碳酸盐缓坡和硅质海绵礁的迁移模式,其形成的过程为:龙门山造山楔于卡尼期初始构造负载于扬子板块西缘,导致了前陆地区的挠曲沉降,形成了前陆盆地,驱动了相对海平面的持续上升,前陆盆地处于欠补偿状态,当相对海平面上升速率与硅质海绵礁生长速率相同时,在15～30m深度的前陆同斜缓坡上发育了鲕粒滩-硅质海绵礁丘组合,随着龙门山造山楔不断地的向前陆地区推进,前陆盆地内相对海平面持续上升,逐次在前陆缓坡上15～30m深度的的位置开启了新的硅质海绵礁群的生长窗,形成了本区卡尼期7条带状展布的鲕粒滩-硅质海绵礁丘组合。因此,硅质海绵礁的淹没过程和迁移过程是龙门山造山楔向扬子克拉通推进过程的沉积响应,显示了在卡尼期-诺利期松潘-甘孜残留洋盆的迅速闭合和逆冲楔构造负载向扬子板块推进的动力学过程。

The advance of Longmen Shan orogenic wedge and migration of the foreland reef and shoal during the Indosinian orogeny, China

The Longmen Shan foreland basin developed as a flexural foredeep at western Yangtze craton driven by the orogenic load system during the Late Triassic Indosinian orogeny. This paper will discuss the advance of Longmen Shan orogenic wedge after analyzing the migration pattern of sponge reefs and oolitic limestone. The results show as follow: The Maantang Formation is the first stratigraphic unit overlies a flexural basal unconformity and is an upward-finingand deepening sedimentary sequence made up of oolitic limestone-bioclastic-sponge reefs-shales and records the establishment and drowning of a carbonate ramp and sponge build-up along the forebulge margin of the marine basin. In a transverse section, the geometry of the basin is wedge-shaped, thinning southeastward with shale grades cratonward into carbonate rock, there was a deep-water basin in the west, shoal facies and patch reefs which were on the edge of a carbonate ramp in the middle and the eastern part of the foreland basin. The basal Early Carnian limestone and sponge built-up of the Maantang Formation was deposited on a westward, NE-SW orientated carbonate ramp in the east of distal margin of the early foreland basin, and can be divided into seven zones. On the basis of previous studies on the subsidence rate, the growth rate of sponge reefs and the rising rate of relative sea level of the foreland basin in Carnian, (Li et al., 2003, 2011a, b, in press), we have done the calculation of the migration rate of sponge reefs and oolitic limestone. The results show that: the migration rate of sponge reefs and oolitic limestone is 18mm·yr^-1 (Li et al., 2003), and the migration rate of sponge reefs and oolitic limestone was equivalent to the advance rate of orogenic wedge, leading to the establishment of an upward-fining and deepening sedimentary sequence made up of oolitic limestone-bioclastic-sponge reefs-shales and the migration of sponge reefs and oolitic limestone. This paper suggests that the migration of sponge reefs and oolitic limestone result from the flexural deformation driven by the orogenic load system advanced rapidly towards the Yangtze craton in the Carnian, associated with the rapid closure of Songpan-Ganzi residual ocean basin.