日高山脈トッタベツ深成岩体の重力異常と貫入形態

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An exposed cross section of the tilted Tottabetsu plutonic complex allows direct observation of its 2-D cross-sectional shape and pretilting vertical variation in lithology and small-scale internal structures. Furthermore, the cross-sectional exposure allows directevaluation of pretilting vertical density variations in both the pluton and the country rocks, which serve as a strong constraint in gravity modeling that complements information on the‘missing’original horizontal dimension of this tilted pluton. The pluton is rectangular in cross-sectional shape, measuring ～10-Km thick by　～8-Km wide. It is stratified with the uppermost thin granitic unit (～1-Km thick) and the underlying thick gabbro-diorite units (～9-Km thick) that preserve a stratigraphic record of numerous botter replenishments in the form of alternation of originally horizontal mafic sheets and cumulate layers. The paleohorizontals (i.e. floor of the magma chamber at any one time) inferred from the sheets, pipes and cumulate layering and foliation dip steeply eastward, being roughly perpendicular to the original side walls and subparalell to the original roof of the pluton. Thus, in terms of the 2-D cross-sectional exposure, the pluton apparently had a vertically-elongated cylindrical form with vertical side walls and a flat roof before tilting.Both the pluton and the country rocks show systematic density increase with paleodepth, but density contrast of the pluton with the country rocks varies between each unit. The gabbro-diorite units show positive density contrasts (Δp=0-0.2g/㎝＾{3}) with the tonalitic and metasedimentary country rocks, while the granitic unit shows negative density contrasts with the country rocks (Δp=～0.1ｇ/㎝{3}). The new Bouguer anomaly map shows distinct correlations between the bedrock geology and local gravity field. As expected from the positive density contrasts of the gabbros and diorites relative to country rocks, a local gravity high is seen associated with the exposure of the gabbro-diorite units. At the same time, a weak gravity low is seen associated with the exposure of granite, concordant with the negative density contrasts of the granites relative to the country rocks. The gravity model shows that the subsurface contact between the pluton and the country rock gently dips westward, being roughly perpendicular to the steeply east-dipping paleohorizontals inferred from the surface geology. This strongly supports that the hidden pretilting side wall of the pluton was also nearly vertical.At first sight, the cylindrical shape with vertical side walls and a flat roof could be interpreted as being the result of emplacement at dilational sites along major faults and shear zones. However, such an emplacement mechanism invoking space creation by horizontal displacement of country rocks contradicts gradual vertical stacking of the originally horizontal mafic sheets that represent recurrent hotter injections. We suggest that space for the successive magma batches injected into the Tottabetsu magma chamber was created by vertical displacement of fault-bounded blocks of roof or floor country rocks with little horizontal displacement.
北海道大学大学院理学研究科地球惑星科学専攻（地球物理学）の論文
2006-03-15

日高山脈トッタベツ深成岩体の重力異常と貫入形態

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