Flow Phenomena within the Jurassic Accretionary Complexes of Northeast Japan.

1992 Fiscal Year Final Research Report Summary

• Project/Area Number: 02640591
• Research Category: Grant-in-Aid for General Scientific Research (C)
• Allocation Type: Single-year Grants
• Research Field: 地質学一般
• Research Institution: Faculty of Science, Hokkaido University
• Principal Investigator: KAWAMURA Makoto Hokkaido University, Faculty of Science ; Assistant Staff, 理学部, 助手 (80186147)
• Project Period (FY): 1990 – 1992
• Keywords: Accretionary complex / Mud injection / Sandstone sill-dyke / Pore-fluid / Melange

Research Abstract

The intention of this study is to exhibit the fluid-involving flow phenomena, such as mud-injection, sand sill - dyke, fluidized structure noted in the Mesozoic accretionary complexes (the Oshima Belt, Idon'nappu Belt and the Northern Kitakami Belt) of Northeast Japan, and to interpret the significance of the pore-fluid behavior in accretionary complex.
Mud Injection : Pebbly mudstone bodies as a result of mud injection is observed in the Cape Shiragami Area on a small scale. It intrudes and injected into the turbiditic alternation and green rocks. Geometry of the mud injection is generally thin veinlet (-film) sometimes branched, but shows irregularly impregnating network-form in a case. Host rocks of clastic alternation were tectonically deformed clearly before the injection. Jigsaw-puzzle structure is occasionally recognized in the inclusion. Such intrusion/injection were caused by the fluidization of underconsolidated muddy sediments under high confining pressure and excess pore-flu id pressure within relatively consolidated host rocks in the decollement/thrust zone of comparatively deep part of accretionary complex.
Sand Sill - dyke : Sills/dykes of coarse-grained sandstone are recognized in the turbiditic alternation in the Orito-hama coast, Oshima Belt.They are classified as follows : dykesill transitional type, multi-branching type, pinch-and-swell veinlet swarm type and thick planar type. Peculiar structure as pod-like "reservoir" in a sill, which "injects" several thin branched veins, is occasionally observed in this type. It is concluded that the "fluidized sand" with high pore-pressure was pushed into the semi-consolidated host rock like syringe, and injected as branched veins in consequence of hydraulic fracturing induced when the pore-pressure of the sand exceeded the strength of wall-rock.
Fluidization - injection phenomena described above reveal that the flow (-intrusion) process of wet-sediment take place extensively in accretionary complex. And it suggests that the behavior of pore-fluid is deeply concerned with the formative mechanism of the mixed facies characteristic of accretionary complex.