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摘要:
为深入认识礼乐盆地的构造演化史,基于已有钻井资料和重新处理解释的地震数据,对区内43个代表点进行了系统的沉降史重建,发现礼乐盆地新生代3个演化阶段分别具有快速、缓慢和快速的构造沉降特点,并且总构造沉降量与地壳减薄程度密切相关;礼乐滩礁体发育区晚渐新世以来构造沉降量为580~900 m,礁体厚度与构造沉降量和下伏沉积层厚度有关。分析表明礼乐盆地构造沉降具有“先抑后扬”的特征,礼乐地块裂离和漂移阶段,构造沉降受到深部热物质上涌产生的浮力作用而出现明显亏损,拗陷阶段,礼乐盆地随着南海海底扩张停止而失去深部浮力的支持,从而发生幕式的快速构造沉降,以补偿早期亏损的构造沉降。
Abstract:In order to further understand the tectonic evolution of the Liyue Basin, we systematically rebuilt the tectonic subsidence history for 43 representative stations selected from existing drill holes with reprocessed 2D seismic data. The results suggest that the Cenozoic tectonic subsidence are characterized by rapid, slow and rapid tectonic subsidence responding to the three tectonic evolutionary stages of synrift, drift and subsidence, respectively, and the total tectonic subsidence is mainly controlled by the thinning crust. In the Reed Bank, the accumulated tectonic subsidence since Late Oligocene has reached a figure from 580 m to 900 m. However, the thickness of the reef deposits not only depended on tectonic subsidence, but also related to the thickness of the underlying Cenozoic deposits. Further analyses suggest that the tectonic subsidence history of the Liyue basin is featured by a slow early subsidence followed by a rapid subsidence, while the deficit of tectonic subsidence during the margin breakup and drift stages might be caused by the buoyancy of the upwelling thermal asthenosphere material driven by seafloor spreading. Since Middle Miocene, with the cessation of seafloor spreading in the South China Sea, the upwelling of deep hot material became so weak that could not provide enough buoyancy to support the overlying Liyue basin, episodic rapid subsidence thus occurred for compensating the earlier deficit of subsidence.
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Key words:
- tectonic evolution /
- anomalous subsidence /
- Nansha block /
- Liyue block /
- Reed bank
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图 5 地震测线G地层结构的时间剖面(a)与深度剖面(b)(G测线位置见图1c)
Figure 5.
表 1 模型参数值
Table 1. Parameter symbols and values in the model
符号/单位 参数物理含义 参数值 a/km 岩石圈初始厚度 125 tc/km 地壳初始厚度 32 ρw/kg·m−3 海水密度 1030 ρc/kg·m−3 地壳密度(0 ℃) 2800 ρm/ kg·m−3 地幔密度(0 ℃) 3330 ρa/kg·m−3 软流圈密度(1333 ℃) 3185 α/℃ −1 热膨胀系数 3.28×10-5 岩层表面孔隙度 砂岩 0.49 φ0 泥岩 0.63 灰岩 0.60 岩层压实系数 砂岩 0.27×10−3/m c 泥岩 0.51×10−3/m 灰岩 0.53×10−3/m 地层骨架密度 砂岩 2650 ρ/kg·m−3 泥岩 2720 灰岩 2710 -
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