宮守超塩基性岩体の迸入機構について

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  • ミヤモリ チョウ エンキセイガンタイ ノ ホウニュウ キコウ ニ ツイテ
  • Emplacement Mechanism of the Miyamori Ultrabasic Rock Body, Kitakami Mountainland, Northeast Japan

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The Miyamori ultrabasic rock body which intrudes the Paleozoic strata in the Kitakami mountainland, Northeast Honshu, Japan, is examined from deformation mechanism and kinematical point of view. Peridotites in this ultrabasic rock body show textures of a tectonite, and have a strong lattice preferred orientation. Layerings (S_0), mineral lineations (L_0), schistosities (S_1) and lineations (L_1) defined by the elongation of olivine crystals are all well developed. S_0 and L_0 are parallel to S_1 and L_1, respectively. The dimensional fabric of olivine is classified into two types, S-type and B-type; they are approximated a flattening type of Flinn's strain-ellipsoids and a constriction type, respectively. The latter is dominant, and often shows a feature of B-tectonite. No marked rotation of the ultrabasic rock body during and after serpentinization was corroborated by the measurment of CRM. A detailed analysis of the internal structure was mainly made in the southeastern area of the rock body (i. e. subarea 1) where the degree of serpentinization is minimal and the deformation structure is well preserved extensively. The slip system determined by kink band in olivines is {0kl} [100]. This slip system corresponds to so-called "pencil glide", and is in harmony with the B-tectonitic feature of the dimensional fabric. The slip direction is always close to the elongation of olivine, with a small angle. The statistically determined preferred orientation of [100], Z-max, is also often oblique to L_1 at a small angle. This deviation may be regarded as a result of the simple shear flow. Accordingly, the flow elements are assimilated into the lattice and the dimensional fabric elements. The plane, direction and sense of shearing movement in each portion of the body, were determined from their geometrical relations. As shown in Fig. 6, the sense of shearing movement thus determined from the northern part of subarea I is of "thrust type", while, that of the southern part is of "normal-fault type". The β-diagram of S_1 and the L-diagram of L_1 show an elliptic conical fold-like feature; its central axis gently plunges toward the south and the short axis of elliptic section is nearly horizontal. Combining with the disposition of S_1 and L_1 on the structural map, an elliptic paraboidal structure was obtained. Its principal axis may coincide with the mean direction of movement. Such a paraboidal structure is regarded as a structure made by putting the elliptic conical anticline on the elliptic conical syncline. In synthesizing the patterns of S_1 and L_1 with the inverse relation of the shear sense, the Hagen-Poiseuille laminar flow model was adopted to deduce the kinematics of emplacement at subarea I. This flow can only be generated by a diapiric upwelling, not a lateral compression. Inclined axis of emplacement, obtained from the β-diagram of Sc and the L-diagram of Lc, is N20° W, 30°S. It is interpreted that the Miyamori body is probably a fragment of mantle diapir which originated at the low-velocity layer of the upper mantle. Dislocation substructures in olivine were investigated by the use of the oxidation decoration method. The most characteristic feature of dislocation in olivine of the Miyamori peridotite is the coexistence of slip band and kink walls. All small loops, rectangular loops, tanglings and dipoles were observed. An edge component of dislocation is dominant over a screw component. Dislocation densities and wall spacings are 1.0×10^7/ cm^2 and 150μm, on the average throughout the body. Deformation conditions were estimated by comparison of the dislocation substructures with the results of recent experimental studies made by several workers. The flow stress was about 250 bars, as derived from dislocation density. This corresponds to 10 bars/km of buoyancy gradient, and 0.1 g /cm^2 of density difference, assuming that the diameter of the diapir is 100 km. This difference in density may be attributed to super-heating and/or inclusion of liquid phase within the diapir. Judging from the dislocation substructures, the deformation-rate controlling mechanism is slip + edge climbing. Therefore, the application of Weertman's creep theory (high temperature recovery creep theory) can be approved. T-γ curve, calculated from his theoretical equation, is concordant with the deformation conditions estimated by the observation of dislocations.

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