福建深沪湾地区晚更新世中晚期以来的地壳运动

CRUSTAL MOVEMENT IN SHENHUWAN BAY AREA, FUJIAN PROVINCE SINCE THE MIDDLE STAGE OF LATE PLEISTOCENE

研究表明 ,深沪湾地区晚更新世早中期地壳运动以下沉为主 ,沉积了冲洪积含砾砂粘土层。晚更新世中晚期以来地壳先升后降 ,近 15 0 0aB .P .以来地壳较为稳定。在 (2 5 5 80± 2 4 90 )～15 4 6 0aB .P .期间 ,该区地壳与水动型海平面大致同步升高 ,上升幅度为 8m左右 ,速率达 0 .78mm/a ,深沪湾内生长了古牡蛎。 15 4 6 0～ 110 0 0aB .P .期间 ,该区地壳快速上升 ,上升幅度超过 10m ,速率超过 2mm/a ,形成多个由晚更新世含砾砂粘土构成的低凹洼地 ,古牡蛎滩成为台地。其后地壳稳定 ,在低凹洼地内生长了古森林。 70 0 0aB .P .深沪湾发生了由强古地震导致的地壳快速下沉 ,古森林生长的凹陷洼地变成泻湖环境 ,古牡蛎滩台地与海面很接近。 2 0 0 0aB .P .左右 ,深沪湾又一次发生由强古地震导致的地壳快速下沉 ,古森林下沉到更深的海底 。

Crustal movement in Shenhuwan Bay area during the early and middle stages of late Pleistocene was dominated by subsidence, resulting in the deposition of alluvial and fluvial gravel-bearing sandy clays. Since the middle stage of late Pleistocene, crustal movement here has been characterized by uplift followed by subsidence. In the past 1 500 years, the crust was relatively stable. During the time period of 25.58± 2.49ka～15.46ka B.P., the crustal movement in Shenhuwan Bay was characterized by uplift simultaneous with the eustatic change of sea-level. The amplitude of uplift reached up to 8m, and the rate of uplift was about 0.78mm/yr. During that time, the coastal environment was relatively stable, and oysters were growing in Shenhuwan Bay. In 15.46～11.00ka B.P., however, the crust in Shenhuwan Bay was rapidly uplifted. The amplitude of uplift reached up to 10m or more, while the rate of uplift reached up to 2mm/yr. In that period, the Shenhuwan Bay had possibly experienced sudden uplift caused by strong earthquakes, resulting in the rapid escape of the ancient oyster bank from the tidal zone to avoid wave erosion. At that time, several low-lying depressions filled with late Pleistocene gravel-bearing sandy clays were formed in the Shenhuwan Bay, while the ancient oyster bank had become a tableland. Afterward, the crust tended to be stable, and forest was flourishing in the low-lying land of the Shenhuwan Bay. At 7 000yr B.P. a sudden crustal subsidence had occurred in the Shenhuwan Bay due to a strong earthquake, which caused the transformation of low-lying land into the lagoon environment, and hence the ancient oyster bank became closer to the sea level. Since 2,800yr B.P. the Shenhuwan Bay had been slightly uplifted again, and the lagoon had become land. A strong earthquake occurred at 2 000yr B.P. had given rise to the sudden crustal subsidence in Shenhuwan Bay, causing the submergence of the ancient forest into a deeper sea bottom, and the return of the ancient oyster bank to the tidal zone or offshore. It is concluded that in the past 45,000 years, the Shenhuwan Bay had experienced a complicated process including subsidence-uplift-sudden uplift-stable-sudden subsidence-stable-slight uplift-sudden subsidence-stable, and at least 3 strong paleoearthquakes. It seems that the Shenhuwan Bay had experienced 3 times of land-forming and 3 times of sea-forming processes.