Vol.3 No.1 2010

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Research paper : The advanced geological researches and fundamental national land information (M. Saito)−17−Synthesiology - English edition Vol.3 No.1 (2010) basis of observations, and the model is progressively updated as the survey proceeds. The accuracy of the information gathered in the field is heavily dependent on the skills and knowledge of the researcher conducting the survey. Careful and knowledgeable observation in the field is the most important process in the creation of a geological map, and also can yield new discoveries. New maps produced using more advanced methods to resurvey previously mapped areas may produce considerable changes. Improved accuracy may be the result of finding new areas of outcrop (places where rocks are exposed at the surface), or because observation methods have improved and geological theory has advanced since the original survey. It is similar to the case where noise may be reduced due to the advancement of the measuring technology and therefore precise measurement becomes possible. This is clearly shown by comparison of previous geological maps of a particular area with a current geological map (Fig. 3).Therefore, the GMJ 1:50,000 “Tomochi” was expected to become the standard for determining the strata and rocks based on the latest geological knowledge of this district. Also, we employed the following three research methods to obtain advancements in this field as indicated in the research objective.1) The researchers in charge collected the geological information based on the latest geological findings, and completed the geological map of the district in charge. Taking the example of the strata and rocks of the accretionary complex that the author was in charge, it is currently accepted that structures within an accretionary complex are nearly horizontal and are controlled largely by thrust faults. Within the complex, the rocks above thrust faults are older than those underlying the faults (Fig. 4). We used age data from abundant microfossils (plankton) to establish that the sediments above the thrust faults were older than those below it. Current geological theory also suggests that an accretionary complex in this district should be overlain by a rock sequence that includes serpentinite. During our field survey, we identified the rocks overlying the accretionary complex and noted their structural relationship with the accretionary complex (Fig. 5).2) Older strata and rocks in their present position have been affected by repeated later episodes of folding and faulting. Therefore, for example, if the deformation that occurred after the formation of an accretionary complex is recognized, by comparing the structure within the complex with the present geological structure it is possible to understand the deformation that occurred both before and during the formation of the accretionary complex. We shared the geological survey data collected by the four researchers at the Example of formation of an undeformed sedimentary sequenceExample of formation of an accretionary complexDifference between an undeformed sedimentary sequence and an accretionary prismLaw of superpositionDeposited in order a, b, and c → Lower formation is olderRiverc: Coarse-grained terrigenous sediment deposited in trench (turbidite)b: Fine-grained hemipelagic sediment in the ocean side of trench (e.g., mudstone)a: Very fine-grained pelagic sediment deposited in area away from continental margin (e.g., chert)Oceanic plate below the very fine-grained pelagic sediment is basalt→ Lower one is newer accretionary complexAccretion segments in order from 1to 7Coarse-grained terrigenous sedimentTrench← Oceanic plateAccretionarycomplexFaultFaultFaultFaultOceanic sideContinental sideNewOldFig. 4 Difference between an undeformed sedimentary sequence and an accretionary complex.In an undeformed sedimentary sequence, the age of layers increases with depth (c to a). In an accretionary complex, there is an a-b-c sequence separated by low-angle faults. In each sequence, the age of layers increases in order of c to a, and sequence 1 (uppermost) is the oldest a–b–c sequence, and sequences 2 to 4 are successively younger. The stratification of the accretionary complex cannot be understood if considered in terms of an undeformed sedimentary sequence.

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