南段花東縱谷之新期構造研究─利吉斷層與鹿野斷層的活動特性

Abstract

為了更加了解花東縱谷南段的新期構造運動，本研究於鹿野的瑞源到台東市之間的縱谷區域進行地質製圖、斷層調查與階地地形的研究。本研究根據礫石的岩性、沉積特徵與層位關係，將卑南山礫岩上部的地層劃分為「初鹿礫石」。斷層調查發現利吉斷層於部分地區已停止活動，推測近期的活動向西遷移，分支斷層切穿卑南山礫岩，位置應位於卑南大溪中。鹿野斷層造成紅土面與全新世階地的變形，斷層未出露地表。新良地區的斷層造成階地抬升並形成三條明顯的斷層崖，地質特徵為利吉層逆衝至河階礫石層之上，地表建物的擠壓變形顯示斷層可能以潛移的方式活動，與池上斷層有相似的特徵，因此將新良地區劃分為池上斷層。階地定年顯示鹿野斷層的垂直變動量約為3 mm/yr，利吉斷層的垂直變動量約為17-20 mm/yr，根據GPS速度場變化特性推測，利吉斷層與鹿野斷層皆為潛移活動為主的逆衝斷層。

In order to fully understand the neotectonics in the southernmost part of Longitudinal Valley, eastern Taiwan, we mapped the geology and surveyed the fault traces and terraces in Longitudinal Valley from Juiyuen to Taitung city. In this study we distinguished a new formation, Chulu Gravel, from Pinanshan Conglomerate based on its gravel lithology, sedimentary character and spatial relationships between two formations. In fault traces survey, we found that slip along Lichi fault has stopped in some area and inferred the recent faulting probably branches to the west. This branch fault may propagate into Pinanshan Conglomerate and its trace should be in the Peinantahsi river. Luyeh fault deformed the lateritic and Holocene terraces and fault tip didn’t cut through surface. In Shinlian area, the faults caused the uplifting of terraces and formed three obvious scarps. It is observed the Lichi Fm. thrust on terrace gravels in outcrop, and the rupturing of the artificial structure may represent the fault is creeping. All the features of the faults in Shinlian area are similar to Chishang fault, so we classified the faults in this area as Chishang fault.Based on the dating of the terraces we estimate the vertical deformation rate of the Luyeh fault is about 3 mm/yr, and the Lichi fault is about 17-20 mm/yr. Analysis of the pattern of GPS velocity field, we inferred the Lichi fault and the Luyeh fault behave as creeping thrust faults.