新竹外海構造研究

Abstract

位於台灣西北部的新竹地區，正處於活動碰撞變形的北界，接近碰撞到伸張環境的轉換區，過去中油公司在新竹苗栗地區進行石油探勘，對於地層深部構造有相當之了解，但對於接近地表之淺層構造則報導不多。本研究為探討台灣造山運動對新竹苗栗海域影響的程度，利用高解析反射震測來分析雙程走時兩秒之內的地層構造，以辨識出近期之斷層活動。 經由震測剖面的解析，以及與相鄰剖面之比對，本研究整理出新竹外海淺部構造，認為研究區域內北部之褶皺－逆衝構造或可與陸上之湖口斷層及新竹斷層連接，且發展為海域的主要橫移斷層構造。研究區域南部與陸上構造連結之可能性則較低。本研究也依據加積現象停止的位置、斷層型態的改變，推論出整個變形前緣在海域的延伸，其位置介於北港高區及觀音高區之間。 在斷層的活動性方面，本研究發現竹南外海淺部有斷層出露於海床，再往南則只在剖面較深處觀察到明顯的正斷層，顯示研究區域北部之構造活動較活躍，而研究區域南部則已進入北港高區範圍，其構造較為穩定，不易發生斷層活動。

Hsinchu area in Northwestern Taiwan is located on the northern edge of the active collision-deformation belt, close to the transition zone between the collision environment to the east and the extension environment to the west. Different from the studies of CPC in the Hsinchu-Miaoli area which focus on deep structures, this study analyses the structures above 2 seconds of two-way travel time on seismic sections for the purpose of identifying recent fault activities. Through the seismic profiles and comparing with adjacent ones, we identified structural features in offshore Hsinchu area. The fold and fault structures in the northern study area could be connected with on land structures, such as the Hukou fault and the Hsinchu fault, which became strike-slip faults offshore. In the southern study area, the faults are less likely to have connection with on land structures. Based on the locations where aggradation ceased and the change of fault types from thrust to normal faults, we delineate the location of the deformation front in the offshore area between the Peikang Basement High and Guanyin Basement High. At shallow depths offshore southern Hsinchu, there are faults almost outcrop to the seafloor, while in the southern part of the study area normal faults appeared at some depths. This study reveals that the northern study area is tectonicly more active then the southern part of the study area, and the area of basement high is stable that faulting activities do not usually occur.