南海西北部珊瑚礁记录所反映的新构造运动*

珊瑚礁是发育于热带海洋环境中由生物作用和地质作用共同形成的地质体,具有独特的新构造运动意义。南海西北部珊瑚礁记录所反映的新构造运动主要有火山活动、地壳升降运动和地震活动等。珊瑚礁区第四纪火山活跃,到现代已停止活动,部分火山构成珊瑚礁的基座,个别出露海面为火山岛;地壳升降运动差异较大,雷州半岛西南部珊瑚礁呈上升趋势,上升率为0.02~0.05mm/a;西沙群岛等岛礁地壳运动则呈下降趋势,下降率为-0.07~-0.10mm/a,岛礁中的造礁石珊瑚生长率、礁顶和灰沙岛的堆积率均相当于或大于地壳下降率与现代海平面上升率的总和;南海西北部珊瑚礁区内地震活动较强,尤其是1994年12月31日和1995年1月10日在雷州半岛西南部海域发生了6.1级和6.2 级地震,这两次地震对该区珊瑚礁的发育有较大的影响。

NEOTECTONIC MOVEMENT RECORDED IN CORAL REEFS IN THE NORTHWESTERN SOUTH CHINA SEA

The coral reef in northwest of the South China Sea is one of the typical coral reef areas since Holocene, thus is ideal to study the neotectonic movement and environmental changes recorded in coral reef. Through field investigation and sampling in this area as well as laboratory analysis, and based on former researches, the detailed neotectonic movements, evolvement process of coral reef, and the relative sea level change during Holocene and the modern earthquake recorded in coral reef are discussed in this paper. Coral reefs, formed in the process of biological-geological interaction in tropic oceanic environment, have recorded particular characteristics of neotectonic movement. The types and characteristics of neotectonic movement in this area are as follows: Most of the rifts were normal faults, forming small-sized grabens, fault basins and buried coral reefs. Marine volcanoes were active in Quaternary, although are extinct now. Some of them formed substrates of coral reefs, with only one forming a volcanic island above the sea. Earthquake activities in special areas are intense. Crust has been subsiding since Late Oligocene, but the rate is very low, only at about 0.1mm/a on an average. All growing rates of reef-building corals and the deposition rates of reef flats and limesand islets are equal to or exceed the sum total of crustal subsident rate and modern sea level rise rate in this area. Consequently, the numbers of dried reefs and limesand islets increase on South China Sea islands. The neotectonic movement of this area is feeble and it is favorable for the engineering construction on the islets and reefs.Because of a similar growth environment in Southwestern Leizhou Peninsula, assuming that the 1.88m thickness of the Goniopora reef reflects the magnitude of crustal subsidence, the subsidence rate was calculated as 9.4mm/a. During this subsidence period (6460aB.P. to 6670aB.P.), four rapid cooling events occurred, leading to growth discontinuities. The fifth layer was raised above the low spring tidal level, and its surface was eroded. An apparent crustal subsidence rate of 3.8mm/a was implied during 6310aB.P. to 6460aB.P. Considering the eroded surfaces, the subsidence rate might be even higher. During this period, two rapid cooling and sea-level dropping events occurred, resulting in the formation of eroded growth discontinuities. During the last 150a, a mean apparent subsidence rate is 5.9mm/a. Since 6200aB.P., the crust has been raised and seal level dropped, and the Goniopora reef surface emerged 30cm above the low spring tidal. This corresponds to a mean apparent crustal uplift rate of 0.05mm/a.In Northwestern South China Sea, neotectonic movement recorded in coral reefs mainly includes volcanic activity, vertical movement and earthquake activity. In reef region, volcanism was active in Quaternary, but has ceased now. Some volcanoes became the reef substrates, and few emerged to become volcanic islands. The discrepancy between uplift and subsidence is conspicuous: coral reefs in Southwestern Leizhou Peninsula tend to uplift, at rates of 0.02~0.05mm/a; and coral reefs in Xisha Islands tend to subside, at rates of -0.07~-0.10mm/a, where the growth rate of reef building corals and the deposition rate of reef flats and sand cays are equal to or exceed the sum total of crustal subsiding rate and sea-level rising rate. Earthquakes in reef region of Northwestern South China Sea are intense, especially these two taking place on December 13 in 1994 and October 1 in 1995 in sea area of Southwestern Leizhou Peninsula, with magnitudes of 6.1 and 6.2 respectively, which have influenced the growth of coral reef greatly. Therefore, the various types of neotectonic movement recorded in coral reefs in Northwestern South China Sea are controlled by geological tectonic actions.