恆春南部外海的上部增積岩體，其構造抬升作用主要受控於分歧斷層(splay fault)的逆衝作用、構造底積作用、増積岩體後方的背衝作用。在恆春半島南方約50公里處，上部增積岩體的海底地形突然轉變成寬廣淺水的高原地形。此海底高原水深介於400-700公尺間、面積約為2,000平方公里，本計畫命名為「恆春高原」。本研究為多年期計畫的第三年，研究目的之一為探討在海洋環境增積岩體上形成海底高原的成因及地體構造的意涵。在前期利用OR1-1138航次所採岩心的研究成果，證實本海底高原海床由膠結良好的沉積岩所組成，由所採集岩石的P波速度與膠結程度，估計此出露於400-700公尺深的海床岩石，至少曾被深埋2至4公里深。前期研究成果也指出於此海底高原東南方有一水深約70公尺、面積約30平方公里的平頂尖礁，本計畫暫命名為「黑潮尖礁」。此尖礁亦由堅硬的沉積岩盤所組成。由於此平頂尖礁水深淺、地形平坦寬闊、底質硬、且位於黑潮流經區域，未來或許可成為黑潮發電的良好場址。本計畫預計於2018年8月至2019年7月期間，利用海研二號於研究區進行多音束水深測量，探測恆春高原以及黑潮尖礁的精確海底地形，由精細地形研判高原區的地質作用以及侵蝕模式。本計畫也預計用挖掘取心(dredge coring)方式，挖取較大量海床表面硬底質岩石，進行岩石組成、地質年代與鋯石來源研究。前期研究已在鄰近恆春高原的獨立盆地以及分歧斷層上下盤採集多管沉積物岩心。這些岩心皆含有多層地震濁流岩。法國R/V Marion Dufresne (MD)研究船，預計於2018年6月，前來台灣海域採集超長沉積物岩心，進行極端事件與古地震研究。本計畫另一目的為分析由MD研究船所採集的巨型岩心，解析數萬年來，馬尼拉隱沒帶極端事件與古地震的發生時間與再現率。MD研究船也將於鄰近恆春高原的獨立斜坡盆地採集數管巨型岩心，這些岩心除了可以用來研究極端事件與古地震外，預計也可利用可淘選粉砂粒徑隨地質時間的變化，推知古代黑潮洋流之流速變化。本計畫科學目的為探討：(1)恆春高原的特性與成因，上部增積岩體抬升的地體構造機制、(2) 恆春高原相鄰獨立盆地以及北呂宋弧前盆地沉積物之沉積速率以及沉積物來源、(3)流經恆春高原黑潮洋流隨地質時間的流速變化以及高原侵蝕速率、以及(4)馬尼拉隱沒帶極端事件與大地震發生時間以及再現率。 ;The tectonic uplift of the upper accretionary wedge offshore the Hengchun Peninsula is mainly governed by splay faulting, structural underplating, and back-thrusting in the rear wedge. Around 50 km offshore the peninsula, the upper accretionary wedge becomes a wide and shallow submarine plateau. This large submarine plateau lies around 400-700 m in water depths and 2,000 km2 in area, which is named the “Hengchun Plateau” in this study. One of the scientific objectives for this proposal is to understand the nature and origin of the Hengchun Plateau and its tectonic implications. This proposal is the third year project-proposal. Rocks recovered from the plateau surface, lying at 400-700 m water depths, during the OR1-1138 cruise, show high P-wave velocity, indicating that these well-lithified rocks were buried, at least, 2-4 km in depths. We also found a flat-topped pinnacle located in the southeast corner of the Hengchun Plateau. This flat-topped pinnacle is underlain by well-lithified sedimentary rocks of 70 m water depth, covering around 30 km2 in area and is informally coined “Kuroshio Knoll” in this proposal. This wide and shallow flat surface lies in the path of the strong Kuroshio Current and it may serve as good candidate site for renewable energy powered by the Kuroshio Current in the future. This project proposes to acquire multi-beam bathymetric data and to collect rock samples by using dredge coring in and around the Hengchun Plateau during the third project year (8/2018~7/2019). The detail bathymetry will shed lights on the geological and erosional processes operating on the plateau and the rock samples are to study their compositions, sources of zircon grains and possibly geological ages.There are a few sediment cores containing seismo-turbidites being retrieved from perched basins neighboring the Hengchun Plateau as well as in the hanging wall and footwall of the splay fault. The French research vessel Marion Dufresne (MD) is scheduled to acquire giant piston cores in the Taiwanese waters in June 2018 for studying extreme events and paleoseismology. One of the scientific objectives for this proposal is to use these MD cores to study the timing and recurrence interval for subduction zone great earthquakes and extreme events in the Manila subduction zone. The MD cores recovered from the perched basins around the Hengchun Plateau will be studied to characterize the temporal variations of the Kuroshio-Current flow strength over the plateau surface.The scientific objectives for this study are: (1) to understand the nature and origin of the submarine Hengchun Plateau and the mechanism that drives the uplift of the upper accretionary wedge in the study area; (2) sedimentation rates and sediment provenances for the perched basins and the Luzon forearc basin in and around the Hengchun Plateau; (3) reconstructing the past current speeds of the Kuroshio Current through analyzing the grain-size variations of sortable silts through time recovered from perched basins neighboring the Hengchun Plateau; and (4) timing and return periods for extreme events and great earthquakes in the Manila subduction zone.
