台灣南部外海、北部馬尼拉增積岩體之墾丁高原地形演育、沉積以及海洋作用研究;Morphological evolution, sedimentary and oceanographic processes around Kenting Plateau in the northern Manila accretionary wedge, offshore southern Taiwan

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題名:	台灣南部外海、北部馬尼拉增積岩體之墾丁高原地形演育、沉積以及海洋作用研究;Morphological evolution, sedimentary and oceanographic processes around Kenting Plateau in the northern Manila accretionary wedge, offshore southern Taiwan
作者:	達斯;Das, Prabodha
貢獻者:	地球科學學系
關鍵詞:	黑潮;海底侵?;台?
日期:	2021-10-26
上傳時間:	2021-12-07 11:51:36 (UTC+8)
出版者:	國立中央大學
摘要:	墾丁高原為橫跨台灣南部的馬尼拉北部增積岩體之地形峰頂，其特徵是不尋常的低地勢表面，水深約為400-700 米。本研究於墾丁高原區域內收集許多數據與資料，包含多音束水深、反射震測、聲學多普勒流譜剖面(ADCP) 、聲學分束測深(EK60)、海床沉積物等。以上述資料來佐證墾丁高原有明顯的侵蝕作用，並了解海底侵蝕如何影響地質時間尺度上之形態演變。墾丁高原最顯著的特徵是藉由多音束水深所顯示的 3 公里 x 7 公里之豆形平坦高架平台（本研究命名為黑潮海丘）。反射震測資料顯示，大部分的墾丁高原海床下幾乎沒有反射，海床也被侵蝕截斷，特別是在高原的東半部，此現象證明了該區域被廣泛的侵蝕。從高原頂部採集的礫石，其P波速度範圍為介於 2.2 至 4 公里/秒間。與附近盆地的鑽井數據對比，推測該礫石母岩的埋藏深度為海床底下約2～4公里，此說明了母岩受高原的侵蝕而被抬升且形成礫石。沉積物岩心的含砂量隨著遠離高原而減少，說明此區域的沉積物傳輸為高能量沉積，從而在高原上堆積粗粒沉積物，並帶走細顆粒沉積物。與黑潮平行且向高原北方遷移之沙丘的現象也證明了該地區活躍的沉積物搬運現象。從 ADCP 數據中觀測到的黑潮流速非常高，在黑潮海丘頂部達到 1.8 m/s（在東南方）。因此，我們認為強烈侵蝕作用乃由黑潮所引起。藉由MD97- 2145 的沉積物岩心分析，可發現低位海平面時期（20,000-12,000 yrs BP）的沉積速率較高且粒度較粗，代表與融冰期（<12,000 yrs BP）相比，冰期的侵蝕作用更為強烈。高原北部有眾多的泥火山與泥貫入體發育，而高原南部海床主要由侵蝕面和岩化岩石組成。其中高原北部的多處泥火山和泥貫入體（>10個），形成不規則的地形，與高原南部較為平整的地形，形成了鮮明的對比。泥火山的底部和頂部分別位於海平面以下約 450-900 m 和 260-400 m 之間。一些泥火山也顯示8~20 度之陡坡，表明其泥流非常活躍且粘稠。其中EK60 數據也顯示某一泥火山頂部有200公尺高的噴氣柱，代表有些泥火山是活躍的。另外，研究區域內也發現幾座泥火山有崩塌現象，並有被高原上方強勁黑潮侵蝕的現象。研究區泥貫入體和泥火山的形成與沈積層超壓、擠壓構造應力和含氣流體有關。墾丁高原的隆升主要因構造壓所作用所形成，部分原因則是侵蝕所引起的地殼回彈。海底侵蝕在整個高原占主導地位，與貫入體侵入一起控制著高原的地貌發育。 ;The Kenting Plateau is characterized by unusual low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism off southern Taiwan at 400-700 m water depth. Multibeam bathymetric data, reflection seismic data, Acoustic Doppler Current Profiler (ADCP) data, Acoustic Split-Beam Echosounder (EK60) data, surface grab samples, and sediment cores were collected in and around the Plateau to identify evidence of erosion in the Kenting Plateau and understand how the morphological evolution has been influenced by submarine erosion over geological time scales. The most distinctive feature on the Kenting Plateau is a 3 km x 7 km bean-shaped flat elevated platform (Kuroshio Knoll) revealed by multibeam bathymetry. Seismic data show almost no reflections beneath the seafloor and erosional truncations at the seafloor, especially in the Plateau′s eastern half, evidencing widespread erosion. The P-wave velocity of the gravels recovered from the top of the Plateau ranges from 2.2 to 4 km/s. After comparing the velocity with the borehole data from nearby basin the burial depth of the parent rocks was found to be around 2 to 4 km below the seafloor, indicating that the parent rocks have been uplifted and gravels were formed due to erosion of the Plateau. Sand content of the sediment cores decreases away from the Plateau, suggesting that sediment transport is effective in this area with high energy deposition, thereby accumulating coarse sediments on the Plateau and removing fine particles away from it. The presence of a dune field migrating northward of the Plateau, parallel to Kuroshio Current also evidences active sediment transport in the area. The Kuroshio Current velocity observed from the ADCP data is very high, reaches up to 1.8 m/s on top of the Kuroshio Knoll (SE domain). We thus interpret that the observed intense erosion is caused by the Kuroshio Current. The higher sedimentation rate and coarser in grain size during sea level lowstand (20,000-12,000 yrs BP) suggests that the erosion was more intense during the glacial period in compared to that of deglacial period (< 12,000 yrs BP) as seen from the MD97-2145 core. There is a prevalent development of mud volcanoes and mud diapirs in the northern part of the plateau whereas the southern part is mostly comprised of erosional surfaces with lithified rocks. Numerous mud volcanoes and mud diapirs (>10) are found in the northern domain of the plateau, leading to an irregular topography which is in sharp contrast with the southern domain. The base and top of the mud volcanoes range from 450-900 m and 260-400 m, respectively, below the sea level. A few mud volcano edifices show abrupt slope ranging from ~8 to ~20 degree suggesting the mud flow is active and viscous. EK-60 data demonstrate a 200 m gas flare through the top of one of the mud volcanos, revealing that some of the mud volcanos are active. A few mud volcanos are collapsed and further eroded away by the strong Kuroshio current around the plateau. The formation of mud diapirs and volcanoes in the study area is attributed to the overpressure in sedimentary layers, compressional tectonic forces and gas-bearing fluids. The uplift of the Kenting Plateau is partially due to isostatic rebound caused by sediment removal through erosion and compression of the accretionary wedge. Submarine erosion is predominant throughout the Plateau, and along with the diapiric intrusion, it controls the geomorphology of the Plateau.
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