Vol.3 No.1 2010

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Research paper : The advanced geological researches and fundamental national land information (M. Saito)−21−Synthesiology - English edition Vol.3 No.1 (2010) 6 Creation of the GMJ by integration of research elementsThe GMJ Tomochi 1:50,000 sheet (Fig. 6) was finally compiled from the individual research elements described above. We found that the rocks exposed on the Tomochi sheet area ranged in age from the late Cambrian Period (at the beginning of the Paleozoic Era about 500 million years ago) to the present day. Classifying all of the geological features shown on the final map required 149 entries in the map legend. This is the largest number of legend entries to date for a published GMJ 1:50,000 sheet. Our map incorporated the concept of an accretionary complex and took into account the latest available information from the research papers listed in the previous section, as well as other research in the district (e.g., reports of large fossils in the area). The integration of all these sources of information produced a very detailed geological map. A report containing the data and analyses that support the final map was also published (Fig. 6). This report includes the route maps that provided the basis of the classification of the rocks and strata and the microfossil data used to determine the age of the rocks of the accretionary complex. The research that led to the final Tomochi 1:50,000 sheet provided the basic information and analysis that were used to create the regional GMJ Yatsushiro and a part of Nomo Zaki 1:200,000 sheet [19].As previously mentioned, in creating a geological map it is important that the evolutionary model of the area is consistent with the distribution and relationships of the present-day rock units. The four researchers involved in mapping the Tomochi sheet engaged in long discussions to ensure this was the case. For example, we discussed the relationships of the Jurassic accretionary complex with serpentinite and other metamorphic rocks, with the shallow-marine deposits covered by low-angle thrust faults, and the overall geological evolution of the area. Through these discussions we identified positive evidence that the rocks of the previously identified “Kurosegawa Belt” overlie Jurassic rocks above a low-angle thrust fault. This important finding established the existence of the Jurassic accretionary complex and is an important finding in the context of understanding the geological structure and evolution of southwestern Japan. We created a cross sectional diagram to illustrate this (Fig. 7). Moreover, through our discussions we were better able to understand the spatial and temporal relationships of the rocks of the “Kurosegawa Belt”, and also able to obtain results on the evolutionary history of the geological structure from the time it overlapped onto the Jurassic accretionary complex as a low-angle fault to present.We also presented details of the evidence for faults in outcrops in the report that accompanies the published Tomochi sheet. This information can be used to verify our interpretation of the position, extent, and continuity of the faults. During our discussions of the faults, we were able to confirm the position of the concealed fault (i.e., where it is buried by younger sediments) and identify the faults that we could not confirm as active faults but which could be geologically recognized as active faults.Fig. 7 Schematic cross section for the south to central area of the GMJ Tomochi 1:50,000 sheet area.Serpentinite (serpentinite melange in the diagram), normal sediments, and the Permian accretionary complex (P2) overlie the Jurassic accretionary complexes (J1, J1-2, J2-3, J3-K1). The boundary between the Jurassic accretionary complexes and overlying rocks and strata was originally an almost horizontal fault, but was subsequently folded. This diagram summarizes the geological theory and evolution of the Tomochi district.S, D, C, P, Tr, J, K1= Forearc, intra-arc or slope basin depositsP2, J1, J1-2, J2-3, J3-K1= Sedimentary complexes ([m]melange, [c]coherent)?NRyuhozan Gr.Usuki-Yatsushiro Tectonic LineTomochiFm.P2[m]P2[m]CSDJ1[c]TrDCSJ1[m]J1[m]J1[m]PJK1K1-2[c]J3-K1[m][c][c]J1-2J2-3SK1Hirodaira U.(Monobegawa Gr.)Serpentinite melangeSerpentinite melangeOtao U.Miyama U.Kuriki SynclineTakadake U.Hashirimizu U.Yonagu U.Gokanosho Metamorphic RocksNitao U.Momigi U.Butsuzo Tectonic LineOmae U.Cretaceous sedimentary complexes in the Shimanto terraneKawaguchi, Hachiryuzan, Kesado and Imaizumigawa Fms.(“Pre-Sotoizumi Group”)

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