臺灣西北部麓山帶為整段褶皺衝斷帶中的一個突出處，主要逆衝斷層及其相關的褶皺構造呈現不連續的片段並以轉換構造相銜接。本研究彙整了數條橫跨此褶皺衝斷帶已建立的構造平衡剖面資料，利用地下三維構造模型分析地下逆衝斷層形貌，包括斷層傾角及層間滑移面的深度，在側向變化的特性。根據地表地質幾何學上的限制修正剖面，本研究提出此褶皺-逆衝系統三維形貌之合理解釋。
褶皺衝斷帶突出處北側的構造轉換帶具有以下特徵形貌: (1)竹東-北埔斷層向南與斗換坪斷層銜接以及軟橋斷層向南延伸連接至竹湖斷層、D斷層或是A斷層，這些主要逆衝斷層都在內麓山帶朝轉換帶匯聚；(2)在外麓山帶，主要逆衝斷層被一系列東西向再復活高角度正斷層所截切且低角度逆衝斷層利用高角度逆衝斷層作為其側斷坡(lateral ramp)滑移面，發生於高角度逆衝斷層的東南側，作為分隔低角度逆衝斷層的剪切斷層(tear fault)，總是發生在高角度逆衝斷層的西北側；(3)內麓山帶之高角度主要逆衝斷層向南轉變為低角度的斷層爾後受到上盤背斜構造影響轉為高角度的斷層；(4)三義斷層向北延伸至出磺坑背斜東側的構造轉換帶特徵為低角度逆衝斷層形成間隔較為寬廣的斷層彎曲褶皺關係轉變為高角度逆衝斷層向上滑移形成間隔狹窄的斷層延展褶皺關係的構造轉換帶特性。此外，在突出處的主要部分內，地下斷層與褶皺構造彼此交錯，呈現特定形式的轉換構造。某些主要逆衝斷層向南消失轉變為向斜與背斜構造。
根據上述觀察所得的突出處中構造轉換帶的幾個共同特徵是由逆衝斷層的側向斷坡與不同形式的斷層相關褶皺來呈現，且由地下三維斷層形貌顯示，側向斷坡均為西北-東南走向。

關鍵字：台灣西北部、褶皺衝斷帶、構造突出處、三維構造形貌、構造平衡剖面

SUMMERY
Foothills belt in NW Taiwan is characterized by the salient in the fold-and-thrust belt. This study compiled several constructed structural profiles across the fold-and-thrust belt in NW Taiwan to analyze the characteristics of variation in thrust fault shape, especially the dipping angle and depth of bedding slip surface, in the subsurface. Based on the constraints of surface geometry on the subsurface features in the geological profiles, this study proposes reasonable interpretations on 3D geometry of the fold-and-thrust system.
The structural transferring in the northern part of the salient is characterized by 1) major thrusts converging toward to the transfer zone in the inner part of the fold -thrust belt, and 2) appearance of E-W reactivated normal fault with high-angle dip to the south. Also in the inner part of the belt the high-angle major thrusts transfer into the low-angle ones to the south. In the main part of the salient the subsurface fault and fold structures intertwined with each other but can be clearly traced and show a specific style of transfer structure in the segment. In the outer part of the fold-thrust belt the major thrusts are segmented by a series of E-W reactivated normal fault. Transfer zone between narrow-spaced and high-angle thrust related structures (fault-propagation folds) and wide-spaced and low-angle thrust related structures (fault-bend folds) appears in the southern part of the salient.

Keyword: NW Taiwan, fold-and-thrust belt, salient, 3-D structural geometry, balanced cross section.

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