下扬子独立地块与中生代改造型残留盆地

下扬子地块在古纬度、火山岩和花岗岩时空发育、区域成矿专属性、沉积建造和构造变形等方面具独特性,与扬子地块有显著差异,是一个晚古生代——中生代早期的独立地块。受燕山期苏鲁板间造山作用影响,下扬子地块直至燕山中期才“楔入”在印支期已拼合的扬子与华北地块,从而演化成独特的下扬子改造型残留叠合盆地。这种改造型叠合盆地的基本特点是:晚印支-早燕山同造山期前陆盆地沉积层和上叠的中燕山期陆相沉积层遭受大量剥蚀,并与古生-中生界海相沉积层一起被卷入强烈的多期次的燕山造山运动,晚白垩世——古近纪又强烈反转成为断陷盆地,形成鲜明的上部地壳的双层结构。这是造成下扬子地区海相古生-中生界油气地质勘探高复杂性、高难度性和高风险性的根本原因,也造就了下扬子区海相烃源岩的二次生烃和晚期成藏的优势。

THE INDEPENDENT LOWER YANGTZE BLOCK AND MESOZOIC REFORMED RESIDUAL BASINS

There was a lot of particularities in the Lower Yangtze Block, evidenced from the paleolatitudes, occurring of volcanic rocks and granitoids, regional mineralization, sedimentary formation and tectonic deformation, which meant that the Lower Yangtze Block must be an independent tectonic unit during Late Paleozoic and Early Mesozoic, separated from the Yangtze craton. It collided the Jiao-Liao craton in the Yanshanian orogeny which created the Su-Lu orogen, and indented the Yangtze North China Block which amalgamated together in the Indosinian orogeny. So some reformed residual superimposed basins developed in the Lower Yangtze area during Mesozoic. The main characteristics of the basins were as follows. 1)The strata of foreland basins in age of Late Indosinian and Early Yanshanian periods and the strata of Middle Yanshanian continental facies were denuded strongly. 2)The strata mentioned above, together with the Paleozoic-Triassic marine facies strata, were evolved into the Yanshanian movement and experienced multi-phase deformation in Mesozoic. 3)The area reversed intensively in Late Cretaceous and Paleocene, so as to form a series of fault basins, which created a two-layer structure in the upper crust of the Lower Yangtze area. It resulted in complexity, difficulty and high risk of oil-gas exploration in the marine facies Paleozoic-Triassic and continental facies Mesozoic strata in the Lower Yangtze area, in addition, it brought up a situation that the secondly hydrocarbon generation and late pool formation could occur in the marine facies source beds.