台灣西北部內麓山帶位於早期沈積盆地中心，由於複雜的地形和地質構造，以及解析度有限的震測資料，本區域仍有部分地質構造問題尚未釐清。本研究選定三義及其附近地區，北部緊密的出磺坑背斜和八卦力背斜於南端轉變成三義斷層上盤較寬廣的構造形式，兩者之間存在一東北東-西南西走向之橫移構造，東鄰數條逆斷層所截切之細道邦構造。本研究目的為瞭解東北東-西南西走向之桂竹林斷層南北兩側之構造特性以及討論三義斷層與小南勢斷層在區域地質構造發育中所扮演之角色。
本研究利用Geosec 2D構造模擬分析軟體繪製一條近東-西向之構造平衡剖面，再綜合前人所繪製之數條構造平衡剖面來深入瞭解地質構造及地下幾何形貌，進而利用砂盒物理模型模擬區域地質構造發育的現象。
研究結果發現三義斷層往北延伸的桂竹林斷層於地底下會與小南勢斷層匯聚在一起，暗示著小南勢斷層並沒有向下切穿三義斷層的下盤。另外，於砂盒物理模型中成功模擬出與桂竹林斷層斷層北段類似之橫移構造，此一橫移構造北側發育出以傾角朝東之逆衝斷層及伴生之斷層拓展褶皺，南側則以逆衝斷層及斷層彎曲褶皺之發育為主，此一特性亦符合目前地下地質構造之解釋。
本研究顯示東北東-西南西走向桂竹林斷層之發育，於地表下並不一定需要側向斷坡之存在，透過前向斷坡之側向尖滅亦可在地表上產生如同桂竹林斷層之橫移構造。此外，於砂盒物理模型中得知地底下構造發育存在未出露之反向逆衝斷層的可能性，在繪製構造平衡剖面時可考慮此情況以獲得不同之解釋。

The inner foothills belt of northwestern Taiwan was a part of the center of the pre-orogenic sedimentary basin, initially. Because of rugged topography, complex geological structures and poor resolution of seismic profiles, there still are some ambiguous features for the regional geology. The Chuhuangkeng anticline and the Pakauli anticline are tight structures of the north of the Miaoli-Sanyi area, and the hanging wall of the Sanyi thrust is a gentle structure of the south; therefore, there exists a structural transfer zone in the study area. The objects of this study are to clarify the significant difference in geological structure between the northern and southern of the Kueichulin fault, and to understand the development of and correlation between the Sanyi thrust and the Hsiaonanshih fault.
In this study, we used GeoSec2D, a software for constructing balanced cross-sections, to construct a E-W trending balanced cross section combined with other balanced cross sections to investigate the subsurface geometry. We also run several sandbox models to simulate the development of the geological structures.
The study results indicate that the Hsiaonanshih fault merges with the Kueichulin fault at depth, namely, the footwall of the Sanyi thrust is not cut across by the Hsiaonanshih fault. A transverse segment of the strike slip fault, which separates two regimes of contrasting structural style—a fault-bend fold in the south and a fault-propagation fold in the north, appears in the sandbox models and the result is consistent with the subsurface geological structrue.
The study results indicate that creating a structural transfer zone does not always need a lateral ramp at depth, only if the presence of a frontal ramp is laterally terminated. In addition, it may be neccessary to consider the existence of blind backthrust that also appears in the sandbox model; it would provide additional clues for the interpretation of any balanced cross-sections.

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