宜蘭平原土壤逸氣調查及其大地構造隱示

Abstract

宜蘭平原位處沖繩海槽的西南端，目前正受到拉張及橫移的變形作用。根據前人在宜蘭平原上的調查指出，平原底下可能有一些存疑性斷層與破裂帶存在。然而，平原地區覆蓋著大量的沉積物且斷層未出露地表，因此這些構造線的位置仍不確定。此外，從地球物理、地球化學等研究認為，受到沖繩海槽擴張的影響，熔融物質與火成岩體可能已經侵入到宜蘭平原底下，然而其具體位置與流體特性仍不清楚。 土壤逸氣的方法能夠作為判斷破裂帶與斷層的位置的有效工具，這是由於斷層或破裂帶提供了地底深部氣體向上遷移的通道，使地表氣體成分異常。本研究在宜蘭平原進行土壤逸氣調查，採集樣本並進行二氧化碳通量、氦氣、氡氣與主要氣體成分的分析，來了解斷層或破裂帶的地表分布以及探討沖繩海槽的張裂對研究區域的影響程度。 分析結果顯示宜蘭平原的土壤氣體濃度大致由東往西遞減，似乎與沖繩海槽向西漸漸張裂有關。此外，氣體濃度的異常值分布位置並無明顯與前人研究所提出之構造線一致的線形，這可能是宜蘭平原底下的沉積物過厚且地下水面高，使得氣體無法順暢地沿著通道到達地表，或是斷層可能並不存在於這些位置亦或是並未切穿地表。 由氡氣與二氧化碳對比可知，山區的氡氣值背景值較高，可能是淺層地表中含有較高的放射性元素；而在平原區，二氧化碳與氡氣濃度呈現正相關，顯示二氧化碳是將氡氣由深部帶往地表的攜行氣體。氦同位素顯示，本研究大部份樣本主要成分為空氣，並混有部分地殼源成分。二氧化碳碳同位素值顯示其來源為地殼與地函的混合。 在宜蘭平原的氣體濃度可定義出兩個異常帶，一個位在平原中部，另一則在平原南側。整合其他地球物理與地球化學資料，我們認為平原中部的異常帶可能代表了平原上張裂中心的位置；而在平原南側的異常帶，可能反映了斷層的存在。 由長期的定期採樣結果發現，在特定地區的氣體成分變化，可能與當地的地震活動有關。因此在地震頻繁的東部地區，針對敏感的地點做連續性的觀察，更有利於地震的監測。

The Ilan Plain, northeast Taiwan, which is located at the western tip of the Okinawa Trough, undergoes extension and left-lateral slip. There may exist a few suspected faults and fractures underneath the Ilan Plain related to back-arc spreading of the Okinawa Trough. However, theses fault traces have not been well understood due to the thick alluvial sediments. Furthermore, previous geochemical and geophysical researches suggested melting features and igneous intrusive rocks resulting from the opening of the Okinawa Trough. The soil gas method is a useful tool to recognize the fractures and fault zones of which provide the pathways for migration of fluid from deep source toward surface. This study attempts to utilize soil gas method to survey the Ilan Plain. We measured carbon dioxide flux, helium concentration, radon concentration and gas chemical compositions to identify the distribution of faults/fractures and to verify the influence of spreading of the Okinawa Trough to the Plain. The results show that soil gas concentrations decrease apparently from east to west, which seems to relate to the progressive westward extension of the Okinawa Trough. Besides, the spatial distribution of soil gas anomalies corresponds to the specific faults and fractures pointed out by previous studies. Higher soil gas concentration appears in the southern part of the Plain, where seismic activities occurred intensively and frequently, implies the existence of fault/fractures beneath the Ilan Plain. Carbon isotopic compositions of soil carbon dioxide indicate a mixture of organic and magmatic source. Based on the radon and carbon dioxide concentration, we propose two potential gas reservoirs in studied area. One is the deep source, showing good correlation with radon and carbon dioxide concentration. The correlation also suggests that carbon dioxide is the carrier gas of radon being transported along the faults and fractures in the Ilan Plain. The other one may be the in-situ radon source, which is mostly observed in the northwestern flank of the Plain. Nevertheless, radioactive isotopes in soil samples should be undertaken to further explore this hypothesis. Results of continuous monitoring indicated temporal variations of helium, radon, and carbon dioxide concentration, which may relate to tectonic activity or seismicity. However, further observation and monitoring is needed.