新生代黄骅坳陷构造伸展、沉积作用和岩浆活动

TECTONIC EXTENSION,SEDIMENTATION AND MAGMATISM OF THE CENOZOIC HUANGHUA DEPRESSION

黄骅坳陷是一典型的张性盆地,其构造主要受西缘向东倾的拆离断层的控制。构造沉降分析显示盆地西部以裂谷沉降为主,而向东热沉降逐渐增大,反映了上地壳的强烈裂谷区与岩石圈深部强烈减薄区发生了错位。沉积作用在裂谷期和热沉降期具明显不同的特点,指示了早期基底快速不均匀的下沉和晚期基底广泛缓慢下沉的过程。玄武岩的形成和演化也与盆地构造发展过程相一致。研究结果表明,黄骅坳陷是在下伏岩石圈上部发生简单剪切和下部发生纯剪切的构造伸展过程中形成的。

The Huanghua Depression,as a part of the North China Cenozoic rift system. is a typical extentional basin.The formation and evolution of the basin clearlyshows that the underlying lithosphere was stretched by means of combined simpleshear and pure shear processes. The Cangdong fault, as an eastdipping detachmentextends downward in a step pattern and soles out in an intracrustal weak lay-er. The tectonic development of the Huanghua Depression was controlled mainlyby the detachment fault and NW-SE-trending transfer faults. Tectonic subsidenceanalyses demonstrate that the basement of the Huanghua Depression has been sub-siding 1 000--2 400 m only under the tectonic-therm-dynamic effects. The subsid-ence clearly underwent the early rifting phase and the late thermal contractingphase, but the sunsiding process was quite diverse in different partas of the basin.The rift subsidence predominated in the west part of the depression but decreasedtoward the east area, where thermal subsidence became pronounced. Spatial varia-tions of tectonic subsidence resulted from disparity of intensely rifted zones in theupper crust and strongly thinned zones in the deep continental lithosphere. The spatial and trmporal variations of sedimentary facies and environmentswere closely related to basinal development. Alluvial,salt-lacustrine, fan deltaic,anddeep-water gravity flow deposits are dominant in the rift phase of the subsidence,indicative of fault-controlled rapid and complicated deposition. The thermal subsi-dence phase was marked by extensive fluvial deposits during the late Tertiaryand Quaternary, suggestive of widespread and relatively slow subsidence. Basalticmagmatism prevailed during the early Tertiary and was consistent with tectonicdevelopment of the Huanghua Depression. Basalt formation is ascribed to partialmelting of the deep lithosphere under the combined action of simple/pure shear.It is balieved that the thermal anomaly of the asthenosphere could also promotethe generation of basaltic magma under the conditions of small stretching amountand low stretching rates. A combined simple shear-pure shear model is proposed in this study for theformation of the Huanghua Depression. This model not only reasonably accountsfor the evolutionary features of the Huanghua Depression but can be extendedand applied to interpret the development of the whole North China rift systemas well.