基於薄皮逆衝理論，基底滑脫面與岩楔本身強度控制了加積岩楔之幾何形貌，本研究將探討基底滑脫面之摩擦特性。因深部基底滑脫面之剪動材料取樣不易，本研究乃針對台灣中部基底滑脫面分支斷層－雙冬斷層地表露頭進行斷層泥取樣，以作為基底滑脫面剪動材料之代表試體。利用為現地含水量之重模試體，於不同正向應力、轉速條件下進行旋剪試驗，探討斷層泥於不同岩覆與剪動速度下摩擦特性之變化趨勢與差異，並將其結果推衍至基底滑脫面之摩擦行為。研究結果指出，旋剪速度小於1 m/s之試驗均呈現位移強化；而旋剪速度1 m/s之試驗則呈現位移弱化，當速度越快時其穩態摩擦係數( )將越低，此趨勢代表基底滑脫面之剪力強度將隨剪動速度增加而降低(即速度弱化)。而於10-1與10-2 m/s之速度條件、正向應力介於0.5~2.5 MPa範圍內， 變化不明顯。換言之，於本研究應力範圍內所對應之加積岩楔厚度變化，對基底滑脫面摩擦係數影響有限。另外，根據微觀組構之觀察發現，旋剪速度1 m/s (較接近同震時斷層剪動速度)之試體，其於旋轉端處Y剪切面上方區域，顆粒粒徑大多較小(<0.004 mm)，且此一區域發現部分摩擦融熔(partial frictional melting)現象，此現象伴隨著摩擦係數隨位移弱化之情形出現，因此，基底滑脫面於同震時摩擦行為可能與間震時期相當不同。微觀剪切構造觀察結果發現，正向(軸向)應力由0.5 MPa增加至2.5 MPa，R1剪切面、Y剪切面與P葉理之延續性亦隨之增加，此一結果暗示，加積楔厚度可能控制基底滑脫面微觀剪切構造之發育，因剪切構造可能影響孔隙壓力消散過程，故可能間接影響加積岩楔基底滑脫面之強度。

Basal décollement and wedge strength domain the geometry of accretionary wedge based on thin-skin thrust theory. Therefore, we will research the frictional characteristic of décollement fault that to understand the geometry of accretionary wedge. Due to the sampling of décollement fault was hardly, there had not been to research representative specimen for frictional experiments in the past. We use Shuangdong fault gouge to substitute basal décollement material, because Shuangdong fault is the main branch of basal décollement in central Taiwan. To understand the gouge frictional characteristics in different depths and slip velocity, we use remolded gouge sample with different normal stress and velocity to conduct rotatory shear experiments, and in-situ water content used with all tests. And the results can evaluate basal décollement mechanical behavior. Research results indicate that, when the rotating shear velocity is less than 1 m/s of the test, it’s showed slip strengthening. While 1 m/s rotating shear test result shows slip weakening. We also find out that the steady-state friction coefficient( ) will decrease with increasing velocity(velocity weakening), when the velocity of rotating shear is within the range of 10-4~1 m/s. This result representing the basal décollement strength will be reduced as the velocity increases. change slightly in the same velocity(10-1、10-2 m/s), normal stress between 0.5~2.5 MPa. In other words, the variation thickness of the accretionary wedge impact limited frictional coefficient on the basal décollement. In addition, according to the observation of the microstructure, found that rotating shear velocity 1 m/s(closer to the velocity of coseismic) of the specimen, there are almost all extremely fine grained(<0.004 mm) develop above Y-shear(along rotational side). This area has discovered partial frictional melting phenomenon, which accompany with the displacement weakening of frictional coefficient. Therefore, the frictional behavior of basal décollement may be quit different between coseismic and interseismic condition. Microstructure comparisons R1-shear, Y-shear, and P-foliation continuities are mainly controlled by normal stress(from 0.5 MPa increased to 2.5 MPa). It implies that, the thickness of accretionary wedge is possible to control the basal décollement’s development of microstructure. Microstructure may affect the pore pressure dissipation process, so the thickness of the accretionary wedge may indirectly affect the strength of the basal décollement.

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