台灣東北角海岸萊萊地區煌斑岩脈及其圍岩之構造演育探討;Structural evolution study of lamprophyric dikes and country rocks in Lailai, northeastern coast of Taiwan

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题名:	台灣東北角海岸萊萊地區煌斑岩脈及其圍岩之構造演育探討;Structural evolution study of lamprophyric dikes and country rocks in Lailai, northeastern coast of Taiwan
作者:	尤芊翔;You,Cian-siang
贡献者:	應用地質研究所
关键词:	岩脈;雁行排列;構造演育;en-echelon dike;structural evolution
日期:	2015-08-25
上传时间:	2015-09-23 15:12:55 (UTC+8)
出版者:	國立中央大學
摘要:	台灣東北海岸的萊萊海蝕平台為雪山山脈地質的延伸，煌斑岩脈(lamprophyric dikes)以右階雁行排列(right-stepping en-echelon form)的形式聳立於其上，伴隨著有不同尺度的斷層、節理及褶皺等構造的出現。煌斑岩脈約形成於九百萬年前，當時的分布形貌，記錄了中新世晚期火山活動的應力環境；岩脈及圍岩中的斷層及破裂等構造，則紀錄上新世以來蓬萊造山運動的應變史，同時反映岩石強度差異所造成的不同變形行為。本研究進行岩脈及地表構造分布及形貌之詳細記錄及精確構造作圖，輔以無人飛行載具 (UAV) 的空照正射影像，以不同比例尺(最大到1:20)的底圖進行野外工作，搭配地電阻測勘剖面確認岩脈在地下的延伸情形，藉以判定各斷層的位態及錯動性質等。煌斑岩脈呈現東西走向，高可達3公尺，總長約190公尺，由2到30餘公尺長不等的19段岩脈所組成。圍岩岩層為漸新世晚期大桶山層，以硬頁岩為主，大體上呈北北東走向，向西傾約十幾度。研究結果顯示，最晚期的斷層作用為南北向的右移斷層，明顯錯開岩脈，但錯距皆小於2米，在還原斷層錯動後，岩脈僅為三道，但仍以右階雁行排列；藉野外構造觀察的推論及地層序列的分析，研判岩漿沿當時既存的垂直裂縫爬升，侵位當時地下2.6公里深的”大桶山層”中的雁行排列節理，冷卻後形成至少三道右階雁行排列的垂直岩脈。分析岩脈與圍岩的破裂現象，岩脈上除了一系列低角度西傾的逆斷層外，另有和岩脈走向夾銳角的右移斷層，使得岩脈內出現水平向的類併置構造(duplex structures)；而圍岩則產生變形。構造演育形成順序為，晚漸新世在被動的大陸棚環境沉積，經深埋岩化後，產生差排的垂直裂隙，中新世晚期岩漿沿著裂隙侵位，形成右階雁行排列的岩脈列，上新世以來受到蓬萊運動的影響，使得岩脈產生逆斷層及類併置構造，圍岩則產生變形，而後應力場改變產生南北向的右移斷層將岩脈錯開，最後構造抬升及頂蝕作用(unroofing)，形成張裂節理，岩脈則以右階雁行排列出露於現今的海蝕平台。;Lai-Lai wave-cut platform is the geological continuation of Hsuehshan mountain range in the northeastern coast of Taiwan. The Lamprophyric dikes stand in right-stepping en-echelon form with faults, joints and folds in different scales on the platform. The dike formed in 9±1.1 Ma ago and the arrangement of the dikes records the stress state during volcanic activity in the late Miocene. Faults and fractures in the dikes and the country rocks show the strain state of Penglai orogeny since Pliocene, and also response the different deformation behavior caused by the difference of rock strength. In this study, I made detailed structural geologic maps in different scales (up to 1:20), and used unmanned aerial vehicle (UAV) to take orthophotography to display the detailed structural distribution. Electrical resistivity imaging (ERI) profile was performed to decipher the arrangement of the dikes underground and the characteristics of the faults. East-west trending dikes can reach 3 meters in maximum height and 190 meters in total length. 19 dike segments are exposed on the platform and range from 2 to 30 meters in length f. The strata of country rocks orientated north-south, dipping to west 10~20 degree are Oligocene Tatungshan formation composed mainly of argillite. My field observation reveals that N-S-trending right-lateral strike-slip faults which dislocated dikes less than 2 meters are the latest faulting event. After restoring the results from faulting, dikes remain right-stepping en-echelon arrangement in 3 segments at least. Based on field observation and stratigraphic analysis, I conclude that magma ascended along en-echelon joints and solidified into three dikes in ‘Tatungshan formation’ at depth of 2.6 kilometers. There are west dipping reverse faults and vertical right-lateral strike-slip faults in dikes. The strike-slip faults intersect with dikes in an acute angle, which makes the dikes appear as horizontal duplex structures. The structural evolution of dikes in Lailai platform are: ‘Tatungshan formation’ formed in passive continental shelf and en-echelon joints formed in late Oligocene; magma ascended along en-echelon joints and en-echelon dikes formed in late Miocene. After the Penglai orogeny took place in Pliocene, reverse faults and duplex structures took place in dikes. Then the N-S-trending right-lateral strike-slip faults cut the dikes and displaced them. At last, tectonic uplift and unroofing formed extension joints. Dikes are exposed in right-stepping en-echelon form in the platform now.
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