恆春半島西側海域紅柴峽谷之成因探討

Abstract

臺灣南部及近海海域上覆著一系列廣闊的增積岩體，係因隱沒板塊上覆之海洋沉積物不斷被刮起、堆積並推擠，產生大量褶皺及逆衝斷層等構造特徵。而臺灣最南端之恆春半島為研究臺灣地區新期構造活動的極佳之處，其東側是中央山脈之南延，西側則為擠壓碰撞而隆起之恆春西臺地。該臺地與其向海之海床於地形地貌上儼然為關係密切的一背斜構造，然臺地西側近海卻出現一弧形凹陷谷地─「紅柴峽谷」。本研究認為此弧形凹陷谷地是受正斷層作用所影響，致使恆春西臺地以西之構造塊體崩移，造就出類似海底峽谷之弧形海底水道。 本研究利用國內外海洋研究船多年來蒐集之十餘條反射震測剖面資料，經處理與雜訊抑制後，分析並解釋恆春西臺地周遭海域之構造型態。在研究區域北側可明顯判識出一系列平行弧形凹陷的正斷層，顯示此海底峽谷與斷層構造關係緊密，隨著正斷層的作用，海底峽谷也有向西逐漸變寬的趨勢；至於研究區域中部與南側因水深過淺與離岸太近，加上岩層受強烈擠壓且海床表面遍布生物造礁而成之碳酸鹽岩體，使得震測剖面成像不佳，不易觀察峽谷與構造間的關係。但依據地層形貌與可辨識之背斜及古水道侵蝕等震測相，在在都支持著此一凹陷海底水道是因塊體的滑落而形成。 水深資料與震測剖面之比對結果顯示，此弧形凹陷是塊體順著一條可能帶有橫移分量之正斷層向西南滑落，而後受到水流攜帶沉積物之沖蝕與堆積而漸漸形成的海底水道。至於在臺灣西南部擠壓碰撞架構下，卻出現這樣帶有拉張陷落之正斷層活動，本研究認為它除了背斜構造頂部張裂的影響外，也可能受到臺灣西南部的構造逃脫所驅動，致使此區域受到張扭作用，產生裂陷與滑移構造。

Southern Taiwan and its offshore area is situated in a convergent plate boundary and have been suggested to be an accretionary wedge, which is characterized by numerous folds and thrusts. The unique geological setting makes Hengchun Peninsula, which is the southernmost tip of Taiwan island, a great place to study the neotectonics of Taiwan. The eastern part of the Hengchun Peninsula is the southern continuation of the Central Range; the western part of the Hengchun Peninsula lies the West Hengchun Tableland that was formed by collision uplifting. It seems that the morphology of the western Hengchun Peninsula and its offshore area are related to a large anticline. A curved depression valley, called 'Hongtsai Canyon', was found on the west side of the tableland. This study suggests that this arc-shaped small submarine channel could be associated with collapsing blocks of western part of West Hengchun tableland formed by a normal faults. In this study, we first reprocess more than 10 seismic reflection profile data for signal enhancement offshore Southern Taiwan. We then analyze the seismic data for understanding the structural styles of the West Hengchun tableland offshore area. In the northern Hongtsai Canyon, a series of normal faults that are parallel arc-shaped depression have been recognized, and the channel widens westward. In the central and southern Hongtsai Canyon, shallow water depth coupled with strong deformed strata and overlying coral reefs make it difficult to clarify how the the sedimentary and structural processes affect the development of the Hongtsai Canyon from seismic profiles. Nevertheless, seafloor features such as anticlines and eroded channels imply that this depressed channel is formed by block sliding. By comparing the bathymetric and seismic data, we suggest that this arc-shaped depression is a block sliding along a normal fault with a strike-slip component, and then this arc-shaped depression has been eroded and deposited by turbidity currents. As southern Taiwan is situated in the active collision stage, extensional normal faults are not expected to appear in this area. This study suggests that the extensional strike-slip faults may related to anticline top extention and the tectonic escape in southwestern Taiwan, which can lead to the surface rifting and sliding structures by transtension.