沈み込み帯35km-10km深度のテクトニクス : 三波川帯からの情報

Tectonics in the 35km - 10km Depth of a Subduction Zone: Information from the Sambagawa Belt

原 郁夫

塩田 次男

広島大学地学研究報告

Geological report of the Hiroshima University

Tectonics in the 35km-10km depth of a subduction zone has been inferred by information from the Sambagawa belt of central Shikoku as follows:
1) The history of sediment subduction is printed as a downward younging nappe pile of high pressure metamorphic rocks [The (Saruta nappe II +Saruta nappe I), Fuyunose nappe and Sogauchi nappe (ST-subnappe, KAT subnappe and NOM subnappe) in the Sambagawa belt are a downward younging nappe pile] (Fig.2, 33, 37, 53, 54, 55&57).
2) change of temperature condition of subduction channel: The abduction of the continental wedge onto the subduction channel induces isobaric increase of temperature of the schists of low temperature-high pressure type, which were forming under the high-speed subduction of low temperature oceanic plate, and the stop of successive sediment subduction. The temperature of the subduction channel again decreases, when sediment subduction newly began after the stop of the abduction of the continental wedge [The Saruta nappe (I + II) schists changed from crossite-bearing blueschists to oligoclase-biotite-schists by nearly isobaric increase of temperature when the 17kb depth part of the Kurosegawa-Koryoke continent was uplifted to the 10kb depth (Fig.5), abducting onto them along a direction subnormal to the trench axis (Fig.22), and their metamorphic temperature again decreased under nearly isobaric condition, when sediment subduction newly began forming the Fuyunose nappe schists as glaucophane-bearing blueschists (Figs.32 & 43)]
3) The thermal structure of subduction channel is inverted at the depth zone of larger than 7kb, and Iithologic layering of sediments in the subduction channel is oriented along the XY plane of strain in the shear zone developed by plate subduction, faming bedding schistosity [The Saruta nappe (I + II), Fuyunose nappe and Sogauchi nappe schists were produced under inverted metamorphic field, and Fig.29 illustrates the orientation of the isothermal surface, isobaric surface, bedding schistosity and plate boundary for the Saruta nappe II schists during the prograde phase of metamorphism] .
4) tectonic erosion of hanging wall rocks by subducting sediments [The Saruta nappe I and the Fuyunose nappe (subunit II) are slices, which were tectonically eroded and suducted from the hanging wall, coupling with the Saruta nappe II (Fig.44) and mixing with the Fuyunose nappe (subunit I) schists respectively (Fig.39)] .
5) Sediment subduction can occur in a direction· oblique to the isobaric line ( = trench axis), probably owing to oblique subduction of the oceanic plate [The Saruta nappe I and the Fuyunose nappe-schists were subducted in a direction oblique at angles of 20~30° to the isobaric line] (Fig.44).
6) Subducted sediments show a return flow (upflow) induced by the appearance of the control point (cf. Cloos & Shreve,1988) [return flow tectonics ( = tectonics of the Sb2-l-ST phase) of the subducted sediments related to the formation of the Sogauchi nappe (ST subnappe): The return flow induces the exhumation of its overlying Saruta nappe (I+ II) and Fuyunose nappe in the same direction. The appearance of the control point corresponds in time to the beginning of the subduction of the Kula-Pacific ridge] (Fig.65-a & b).
7) The return flow and exhumation of the subducted sediments occur in a direction corresponding to an angle of reflection to the isobaric line as compared with that of the sediment subduction to the isobaric line, showing oblique upflow and exhumation and that the control point is a reflection plane [The upflow and exhumation of the geological bodies during the Sb2-l-ST phase (Fig.65) occurred in a direction corresponding to the angle of reflection (20~30°) to the isobaric line as compared with that of the subduction of the Saruta nappe I and sediments for the Fuyunose nappe schists (Fig.44)] (Fig.66).
8) The movement picture of the exhumation of the subducted sediments ( = high pressure type schists) changes with progress of the subduction of the Kula-Pacific ridge into their thrusting up along the subduction channel, which is accompanying their coupling with the continental wedge and tectonic erosion of the subduction channel-filling rocks(= underplating of film-like nappes). The direction of the exhumation of the schists is parallel to that of the return flow of the subducted sediments [tectonics related to the formation of the Sogauchi nappe (ST-subnappe, KAT-subnappe and NOM-subnappe) during the Sb2-1-KAT phase~Sb2-1-NOM phase] (Fig.65).
9) The high pressure type schists are exhumed onto an off-scraped sequence along the mechanical boundary of consuming plates with further progress of the subduction of the Kula-Pacific ridge [The Sogauchi nappe and its overlying nappes were exhumed onto the Sakamoto-Niyodo nappe schists (Middle Jurassic accretionary complex) which show a downward increase of temperature (Figs.54,67 & 68). The direction of the thrusting of those nappe during the Sb2-2 phase is parallel to that during the Sb2-1-KAT phase~Sb2-1-NOM phase] (Fig.75).
10) The high pressure type schists are not exhumed until the subduction of the Kula-Pacific ridge began [The Saruta nappe II was moved along the isobaric line accompanying the underplating of the subducted Saruta nappe I, when the sediment subduction related to the formation of the Fuyunose nappe schists occurred (Fig.44). The beginning of the exhumation of the Saruta nappes and Fuyunose nappe (Fig.65) coincides with the beginning of the subduction of the Kula-Pacific ridge, whose time has been assumed by Kiminami et al.(1991) from the intrusion time of MORB type basalt in the Shimanto megaunit] (Fig.75).
11) Fig. 88 illustrates deformation styles of minerals and rocks in the 35km-10km depth of the subduction zone related to the formation of the Sambagawa schists, showing that the relicts of seismogenic cataclastic faulting are only rarely found.

日本語

広島大学理学部地球惑星システム学教室

[ISSN] 0073-2303

[NCID] AN00213676