碩士 / 國立成功大學 / 地球科學系碩博士班 / 99 / An oblique collision between the Luzon volcanic arc and Asian continental margin results in accretions and deformations of the North Luzon Trough forearc basin and the North Luzon Arc as the Coastal Range in eastern Taiwan. Now the Coastal Range is composed of four remnant forearc basins and two volcanic islands. The Loho remnant forearc basin in the middle part of the Coastal Range is located at a special position that connects the Shulien remnant forearc basin to the north and the Taiyuan remnant forearc basin to the south, and in the junction between Chimei volcanic island to the north and the Chengkuangao volcanic island to the south. For its special position, the sedimentation and structures of the Loho basin plays an important role for understanding the geohistory of the Coastal Range. The Lithostratigraphic units of Loho basin include the Tuluanshan Formation (volcanic sequence), Takangkou Formation (stratified forearc turbidites) and Lichi M?lange (sheared sequences).
Field survey, clay mineral analysis and micropaleontology of planktic foraminifers and calcareous nannofossils are studied to explore the stratigraphy and structure, especially the mechanism responsible for formation of the Lichi M?lange, of the Loho Basin:
1. In Loho basin, both the Takangkou Formation and the Lichi M?lange have the same age of Late Early Pliocene (N19-N20 or NN14-NN15; 4.2-3.65 Ma). The Lichi M?lange is characterized by occurrence of broken formation facies and m?lange facies of intensively shearing and mixing of polygenetic origins of tuff/sandstone/mafic/ultramafic blocks with discernible stratification. No syncline structure is found in the Loho basin.
2. Both Takangkou Formation (bedded turbidites) and Lichi M?lange contains illite and chlorite and rare semectite, however, the Takangkou Formation contains kaolinite less than 3% while the Lichi M?lange contains much rich kaolinite (13-17%). Both the Takangkou Formation and Lichi M?lange have the same age but contain different clay mineral assemblage, indicating that the Lichi M?lange is unlikely to be a submarine-slumping facies coeval to the Takangkou Formation.
3. The m?lange exposed in the Loho basin has east of Tuluanshan fault the same depositional age (NN14-15 Zone), microfossil assemblages and clay mineral composition as the type Lichi M?lange west of the Tuluanshan fault in the southern Coastal Range, suggesting a similar stratigraphic unit.
4. Clay minerals in volcanic blocks of the Lichi M?lange are predominated by semectites (83 ~ 100%), which are almost absent in both the Takangkou Formation and the sheared mudstones of the Lichi M?lange. This indicates that the source of kaolinite in the Lichi M?lange is unlikely derived from the volcanic blocks as it was suggested. Distribution patterns of clay minerals in the South China Sea indicate that kaolinites (10-20%) in the modern hemipelagic muds of the Central Basin above the oceanic crust are primarily derived from the laterite soils weathered from granites in southern China. Taking the clay mineral distribution pattern of the South China Sea as an analog, we interpret that in the early Pliocene similar hemi-pelagic muds might have been entered the subduction channel when the South China Sea oceanic crust was subducted beneath the Taiwan accretionary prism since the late Miocene. They were then tectonically moved into Lichi M?lange by back-thrusting in the western forearc basin during the incipient arc-continent collision at ~3.5 Ma.
5. The structure evolution of the Loho basin has involved several geodynamic processes:
(1) In the Late Miocene, the SCS oceanic crust was subducted eastward beneath the Philippine Sea plate along the Manila Trench. The subduction was followed by an oblique arc-continent collision in early Pliocene. When the Philippine Sea Plate continuously moved westward, back-thrusting occurred along the western part of North Luzon forearc basin and the lower forearc sequences (4.3 ~ 3.6 Ma B.P.) were deformed to develop the incipient Lichi M?lange (~3.5 Ma) like to modern Huatung Ridge.
(2) Meantime, a clockwise rotation of the Chimei volcanic island in the north and the uplifting of the Huatung Ridge by end of Pliocene separated the North Luzon Trough into three remnant forearc basins, from north to south: Shulien basin, Loho basin, Taiyuan basin and Taitung basin. Deformation and thrusting-uplifting of the Huatung Ridge at the junction between the Chimei volcanic island and the Chengkuangao volcanic island would play like a sedimentary dame to block more than 3 km-thick Late Pliocene - Pleistocene deep-sea conglomerates and the associated low fan turbidites derived from the north in the Shuilien remnant forearc basin, and therefore only Late Early Pliocene turbidites were found in the Loho Basin.
(3) At the early stage of the advanced arc-continent collision (~1.5 Ma B.P.), the Huatung Ridge was furthermore westward thrust and deformed to become the modern Lichi M?lange. The Shuilien remnant forearc basin-Chimei volcanic island and Loho basin were thrust over the Lichi M?lange along a low-angle decollement as the modern northern Coastal Range. At the late stage of the advanced arc-continent collision (＜ 1 Ma B.P.), the Taiyuan basin, the Chengkuangao volcanic island and the Taitung basin were further thrust and accreted westward onto the Lichi m?lange to develop the southern Coastal Range.
(4) During the late Pleistocene, the Coastal Range was uplifted. Active river erosions in the Loho basin resulted in exposures of the Lichi M?lange beneath the low-angled decollement to appear a “structure window” in the center of the Loho basin.
| Identifer | oai:union.ndltd.org:TW/099NCKU5135002 |
| Date | January 2011 |
| Creators | Yen-ChunLin, 林彥均 |
| Contributors | Chi-Yue Huang, 黃奇瑜 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 58 |
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