台灣東部石梯坪地區變形條帶之研究

Study of deformation bands in Shihtiping, eastern Taiwan

林詩婷(Shih-ting Lin)

指導教授 ： 黃文正

國立中央大學/地球科學學院/應用地質研究所/碩士(2015年)

摘要

變形條帶為高孔隙岩石在外力作用下應變局部化形成的平板狀構造。在花蓮石梯坪海岸地區廣泛分佈著變形條帶，出露岩石主要岩性為中酸性凝灰岩，層狀構造明顯，為一向斜構造，向斜軸為N14°E。石梯坪屬都巒山層的石梯坪凝灰岩相，西側整合與石門火山角礫岩相之礫岩層相鄰，此礫岩層西側則以大港口斷層與八里灣層接觸礫岩。經由高解析度空照圖判釋及野外實查，變形條帶以束狀為主，多凸出圍岩，平均寬0.1-15公分，長10公尺以上，總錯距1-20公分不等。全區分佈的變形條帶有東北東與西北走向兩組，僅部分區域出露的有東西與南北走向，四組皆為高傾角，不似受區域褶皺作用影響。透過影像分析得知圍岩孔隙率約16%，變形條帶內約3.9%。藉由滲透率試驗，變形條帶的滲透係數為0.22和0.52毫達西與圍岩相比，下降2-3個數量級。在偏光與電子顯微鏡下，變形條帶與圍岩的組成礦物雖然相同，但與圍岩相比，條帶內顆粒間排列十分緊密，顆粒平均較小且完整，不似圍岩顆粒多有破碎情形。
綜合以上結果，推斷石梯坪的變形條帶形成機制包含壓實、壓碎與剪動作用，從運動學角度將其歸類在壓密剪力型的變形條帶(Compactional shear band)，以機制分類則屬於壓碎型變形條帶(Cataclastic band)。從變形條帶的分佈情況與岩層位態之關係分析，研判其形成可能與區域向斜同期或之後的產物，非伴隨鄰近的大港口斷層生成。

關鍵字

變形條帶 ； 石梯坪 ； 高孔隙岩石 ； 中酸性凝灰岩

並列摘要

Deformation band, a tabular structure, pervades in the Shihtiping marine plateform composed of mainly Miocene ignimbrite. The older volcanic breccia composing the hills west of Shitiping conformably tilted against the ignimbrite. The north-south-trending Takangkou Fault thrusts the Miocene volcanic breccia onto the Pleistocene sedimentary rocks at about 1-2 km west of Shihtiping. There is a N14°E-trending syncline cropped out in Shihtiping. The deformation bands are commonly exposed as cluster zone composed of several individuals in Shihtiping. They can be traced easily because they are protruding on the ground surface. The cluster zones range from 0.1 to15 cm in width, up to 10 m in length and from 1 to 20 cm in total separation. The deformation bands with orientations of ENE and NW are widely distributed while those with orientations of NS and EW are locally found. The dip angles of all sets range from 50° to 90°. The syncline seems to have no effect to the deformation bands. The porosities of host rock and deformation band are respectively 16% and 4% calculated from image analysis. Through permeability test, conductivities in deformation bands of two samples are 0.22 and 0.52 md which reduce 2 to3 orders. Based on microscopic examination, minerals in the deformation bands usually include plagioclase, hornblende and augite. The Mineral assemblage is the same as the one in host rock but grains in the band are relatively smaller and more intact (i.e. not fractured). Thus, it results in tighter packing and lower porosity within deformation bands.
To sum up my findings, the deformation bands in Shihtiping were formed by compaction, shear and cataclasis after the syncline or simultaneously. They can be classified as compactional shear band in terms of kinematics and cataclastic band in terms of mechanics. They do not accompany the Takangkou Fault but reflect the regional paleostress.

並列關鍵字

deformation band ； Shihtiping ； porous rock ； ignimbrite

參考文獻

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