|
Hokkaido University Collection of Scholarly and Academic Papers >
Theses >
博士 （理学） >
Cretaceous forearc tectonics in central Hokkaido : With special reference to accretion and exhumation processes of greenstones
| Title: | Cretaceous forearc tectonics in central Hokkaido : With special reference to accretion and exhumation processes of greenstones |
| Other Titles: | 北海道空知-エゾ帯南部地域における白亜紀前弧テクトニクス : とくに緑色岩類の付加・上昇プロセスについて |
| Authors: | Ueda, Hayato1 Browse this author |
| Authors(alt): | 植田, 勇人1 |
| Issue Date: | 23-Mar-2001 |
| Publisher: | Hokkaido University |
| Abstract: | New geological maps and descriptions on radiolarian ages, metamorphism, and mode of occurrences and geochemistry of greenstones are provided for a very low-grade metamorphosed accretionary complex (Idonnappu Zone: Chapter II) and a high-P/T metamorphosed accretionary complex (Iwashimizu Complex: Chapter III) in southern part of the Sorachi-Yezo Belt. Unconformity and clastic composition in the Yezo Supergroup are also described (Chapter IV). Cretaceous forearc structure and its tectonic evolution are reconstructed based on these informations. Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area consist of five lithological units, which are combined into two geologic complexes: the greenstone-dominant Naizawa Complex and the clastic-dominant Horobetsugawa Complex. Radiolarian ages were determined for each unit. The Naizawa Complex is an alternating stack of slabs of depleted tholeiitic greenstone, and fragments of Triassic seamount volcanics with Triassic-Lower Cretaceous ocean floor to trench-fill deposits. The Horobetsugawa Complex is a pile of Upper Cretaceous ocean floor to trench-fill deposits, which intercalates sheared MORB-type greenstones. Lower Cretaceous to Upper Cretaceous submarine slide and debris flow deposits and latest Cretaceous-Paleocene slope basin deposits (T-Unit) are also intercalated. The Lower Cretaceous Iwashimizu Complex in the southern Kamuikotan Zone (Mitsuishi-Shizunai area) comprises a pile of four nappe sheets (＝units), each of which represents a stacks of greenstone slices with minor chert and clastic rocks. Aii units suffered a high-P/T metamorphisni. An inverted distribution of metamorphic grade is recognized between the lower three units. A gap of metamorphic grade exists at the structural top, where blueschist facies metabasites are overlain by zeolite to lawsonite-albite facies rocks. Most of greenstone slices are of oceanic island type basalts (OIB), except a few slices and blocks of depleted tholeiite. The Middle Yezo Group in the Mitsuishi area unconformably overlies both the Iwashimizu Complex and the Lower Yezo Group. Sedimentary structures and rootlets show that the Albian basal conglomeratic member (M0 member) is fluvial deposits. It contains abundant greenstone clasts, part of which are accompanied by high-P/T metamorphic minerals. These stratigraphic and clast composition analysis lead a consequence that the Iwashimizu Complex was rapidly exhumed and subaerially exposed until the Albian time. Accretion styles of greenstones changed from the Early Cretaceous to the Late Cretaceous, as the subducted oceanic crust got younger, sediment supply into the trench increased, and subduction direction changed from highly oblique to near orthogonal. Seamounts subduction drove an abundant accretion of OIB greenstones in the Early Cretaceous. Whereas thick terrigenous sediments with minor abyssal basalts accreted in the Late Cretaceous. Depleted tholeiitic greenstone in the Lower Cretaceous Naizawa and Iwashimizu Complexes are regarded as fragments of the Jurassic Lower Sorachi Group, which represents the basal ophiolite of the Cretaceous forearc basin. Co-occurrence of OIB and depleted tholeiite suggests that a pait of the forearc basin was fragmented and mixed with the materials accreted from oceanic plate during the seamount subduction. The mid-Cretaceous exhumation event of the Iwashimizu Complex is probably caused by the younging of subducted oceanic slab and abundant accretion of seamount volcanics in the late Early Cretaceous. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第5387号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/51496 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （理学）
|
Submitter: 植田 勇人
|
