Holocene sea-level reconstruction for Guangdong coast and a comparison with GIA model outputs

Abstract

This study aims to investigate the influence of tectonic activity in the Holocene sea-level records. The coast of Guangdong was selected because of its active tectonics during the Late Quaternary. Due to the fact that the tectonic movement during the Holocene has been very small in magnitude, a careful sea-level reconstruction is needed. With the high precision reconstruction, this study is able to reveal some important features of tectonic movement and evaluate its impacts on the sea-level history. Most important in sea-level reconstructions is the ability to precisely determine the vertical relationship(also called indicative meaning)of a sea-level index point to its associated reference water levels. This study employed a scheme to help quantify this relationship for 15 types of sea-level indicators, which are commonly obtained from this coast. Once the palaeo-sea-level height was established by the above method, a sea-level curve for a small area was developed. To identify tectonic component within the sea-level records, the newly developed global eustatic history and a laterally heterogeneous earth model coupled with a deglacial model were used to generate predictions for the Holocene eustatic sea-level history and the effects of glacial-isostatic adjustments(GIA)for the study area. A comparison between the reconstructed sea-level history and the GIA history shows valuable information about the influence of tectonic movement. Results show that along the west coast of Guangdong there are few good quality sea-level data for the period before 7700 year. A group of high quality data from coral samples of Leizhou Peninsula suggests a small magnitude of uplift, whilst data from estuarine environments of this coastal area show the coast was tectonically stable and the GIA was the main process that determines the mid-late Holocene sea-level history. Within the Pearl River deltaic basin, sea-level data suggest subsidence and possible sedimentary compaction. Further study is needed to quantify the tectonic component in the sea-level history. In the case of East Guangdong, the sea-level data scatter around the eustatic sea-level curve, with some data points(in Mid-Holocene)appearing below the eustatic curve and some others(in Late Holocene), above the GIA curve, implying a complex history of tectonic movement. This study concludes:(1)The Guangdong coastal area in general has experienced a slight subsidence during the Holocene, possibly a result of the long-term continental extension that has caused the subsidence of northern continental shelf of the South China Sea;(2)Due to a change of the stress field in Late Quaternary, many NW-SE faults became active in this coast, resulted in differential, local subsidence and uplift, with estuaries and deltaic basins being mainly subsiding;(3)Leizhou Peninsula, an tectonically active area with the Hainan Island, shows evidence of uplift during the Holocene, reflecting the fact that this area has a long history of volcanic activity and earthquake throughout the Quaternary;(4)The data from the Pearl River Delta may show a combination of tectonic subsidence and sedimentary compaction due to dewatering of the sedimentary sequence.

中國廣東沿岸作為一個冰川均衡調整(GIA)'遠場'地區,按照水動型均衡理論,應存在全新世中期高海平面.本文通過海平面標誌物重建了廣東古海平面的歷史,結果顯示出在粵東、粵西與珠江三角洲的海平面歷史存在明顯的差異.若將海平面歷史與GIA模型結果對比,可以看出廣東沿岸在全新世以來具有微小的區域性地殼沉降.這種大區域的沉降可歸因於華南陸塊向南延伸所引起.此外,粵西雷州半島因新構造運動,於全新世早中期有輕微抬升.珠江三角洲盆地則因斷裂構造沉降和沈積物壓實作用,重建的中全新世海平面明顯低於GIA結果.