利用鋯石核飛跡年代與數值模擬來探討雪山山脈中部熱力演化過程

Abstract

雪山山脈是台灣的第二大山脈，其位於台灣島北部。除了岩性上與構造上具有其獨特的特徵之外，在熱定年學上其與中央山脈一樣地反映出具有著高掘升速率。然而在大多前人探討台灣造山演化的正演模型研究當中，雪山山脈的存在在模型設置上被忽略了，因此雪山山脈的造山熱力演化過程仍然是有待探討的，其熱力演化過程應有別於整體台灣造山。此外，在探討熱定年學的研究中，也鮮少考慮降溫速率對於封存溫度所造成的效應對於熱定年年代-高度關係的影響。因此在此研究中，我們考慮此效應並將一維熱傳導模型應用雪山山脈中部，將模型結果與熱定年年代互相比較來探討該區域的抬升與剝蝕歷史。其後為了將模型推展到二維以及探討雪山山脈的形成，我們引入熱力學模型，並將其模型結果與實際雪山山脈的地球化學數據進行對比。在一維模型部分，本研究發現了一個特殊的熱定年學現象，我們稱之為「寬帶癒合」。在造山初期地層開始剝蝕冷卻時，在地底深處會形成一個因封存邊界垂直快速加深，而形成擁有相近熱定年年代的區域。將此一維模型與雪山山脈中部的熱定年資料做對比，在假設剝蝕速率不變的前提之下，模型結果得出雪山山脈是從三百一十萬年前開始以每百萬年五公里的速率剝蝕至今。在另一分面，二維模型的結果顯示最初的岩性分布設置會影響到造山帶的變形行為，進而產生不同的熱年代分布結果。模型的結果說明了雪山槽的存在影響了雪山山脈的形成，且雪山山脈東側邊界在造山初期曾經是一個活躍的構造變形帶。

Located in northern Taiwan, the Hsuehshan Range is the second largest range and has its own special lithology, structural and thermochronological features with high exhumation rate. In most previous studies of forward modeling of Taiwan mountain building, however, the existence of the Hsuehshan Range was ignored. Its thermal-mechanical evolution should be investigated in addition to the evolution of Taiwan mountain range. Moreover, in the aspect of thermochronology, most of the previous studies that discussed the age-elevation relationship do not consider the effect of cooling rate on closure temperature. In this study, we simulate this effect numerically and discuss local exhumation rate in middle Hsuehshan Range using six new zircon fission track ages together with previously published data via the 1-D thermal advection-diffusion model. Furthermore, for the purpose of applying the cooling-rate-closure-temperature relationship to 2-D model and testing the assumption of the initial tectonic framework of the Hsuehshan Range, we simulate the formation of the Hsuehshan Range by introducing the 2-D thermal-mechanical model and compared the modeling predictions with real geochemical data. In the 1-D model, we found a unique phenomenon that we call 'Wide-zoned Closure' in the initial stage of orogen. The Wide-zoned Closure signature will induce a spatially vertical zone with similar thermochronological ages. As a result, a vertical profile of thermal age would show a slowly decreasing age trend. By regulating parameters reasonably, the best-fitting 1-D model to the thermochronological data in middle Hsuehshan Range suggests that this area started to exhume with a rate of 5.0 km/Myr around 3.1 Ma. On the other hand, the 2-D modeling results imply that the initial lithology setting would influence the deformation behavior of orogen and result in different distribution of thermo-chronological ages. Additionally, the results suggest that the Hsuehshan Range has inherited influence from the Hsuehshan Basin and that the eastern part of the Hsuehshan Range was an active deformation zone in initial stage of orogen.