Permalink : http://hdl.handle.net/2115/38516

KEYWORDS : Stratigraphy;Thermochronology;Uplift;Hidaka collisional orogen;Hokkaido

This paper reviews the stratigraphic and thermochronologic data concerning the exhumation history of the Hidaka collisional orogen, Hokkaido, Japan. The Neogene sedimentary record of foreland basin suggests that the topographic uplift of the orogen occurred in the Middle Miocene. The first occurrence of large amount of orogen-sourced granite clasts has been found in the Middle Miocene turbidites around 15 Ma. These granite clasts are S-type monzogranite-granodiorite, and show the Paleogene biotite K-Ar ages of 29-46 Ma. These data suggest that they are originated from Paleogene granitic plutons emplaced in the shallow part of the Hidaka crust. On the other hand, the overlying Late Miocene deposits contain clasts of metamorphic rocks and tonalite which are characterized by Miocene biotite K-Ar ages of 16-19 Ma. These clasts of syn-uplift reset ages represent exhumation of the middle part of the crust. The present-day outcrops of the crystalline core zone of the Hidaka orogen show that the upper and middle crustal rocks have Paleogene and Miocene biotite K-Ar ages, respectively. Thus, the diachronic change of detrital materials in the foreland basin together with the above-mentioned thermochronologic contrast simply responds to a normal exhumation of the orogen. To explain the change of detrital assemblages, it was suggested that the different areas of the Hidaka orogen have undergone a significant denudation through two uplift events at Middle and Late Miocene. Our data, however, show a successive exhumation occurred in the southern Hidaka orogen where deepseated crustal rocks now crop out.