Transition from Flexural—Flow Folding to Flexural—Slip Folding in the Sambagawa Belt

The styles and physical condition of the folding of the Hijikawa—Oboke phase (Dh phase) have been described and discussed in this paper. The physical condition has been analyzed on the basis of quartz microtextres such as c — axis fabrics and deformation lamellae and of homogenization temperature of fluid inclusion in quartz. When the Dh phase folding occurred in multilayered rock types with extremely thin incompetent layers (films), it shows a transformation from a flexural—flow type during the early stage to a flexural—slip type during the later stage. The flexural—flow folding formed the axial plane cleavage (quartz shape orientation) in competent layers (quartz—rich layers) converging toward the fold core, resulting in the Class 1C type thickness variation for all competent layers, though the thickness variation is as near to Class 1B in the outermost knee and to Class 2 in the fold core. While the flexural—slip folding resulted in the Class 1C thickness variation for the competent layers involved in the outer knee and the fold core and around the inflection points of these involved in the middle knee and in the Class 2—Class 3 thickness variation around the axial zones of these involved in the middle knee. The fault system consisting of R1 set, Y set and P set developed along the layer boundaries and in competent layers during the flexural—slip folding has been also described and discussed, clarifying the relationship between the thickness variation and its related fault system: The Class 2—Class 3 thickness variation is related with the P set (thrust set) which converges toward the top of fold, while the Class 1C thickness variation around the inflection points in the middle knee with the R1 set and Y set

Nappe Boundary Migration during the Subcretion: Exhumation Processes of the Sambagawa Schists

The Sambagawa schists consist of some successively subcreted units as nappes, Saruta unit as Saruta nappe II and Saruta nappe I, Fuyunose unit (Fuyunose nappe) and Sogauchu unit (ST subunit, KAT subunit and NOM subunit) etc. in descending order of structural level. The coupling of the Saruta nappe II and the Saruta nappe I occurred accompanying the underplating of the Fuyunose unit schists. When the underplating of the Sogauchi unit occurred accompanying the exhumation of the previously subcreted units, the latter was highly deformed, accompanying the formation of large—scale recumbent folds such as Shirataki II fold and Suryo fold and the nappe boundary migration from the Saruta I —II boundary and Saruta I — Fuyunose boundary to the Kuwanokawa thrust

Palaeo — Stress Analysis of the Tsuji Overturned Fold in the Sambagawa Belt

Quartz microtextures of the Tsuji nappe with the Tsuji overturned fold in the Sambagawa belt, eastern Shikoku, have been analyzed to understand the movement picture of the Sambagawa schists which were exhumed into shallower tectonic postion. The stress picture related to the formation of the Tsuji overturned fold has been clarified from deformation lamellae in quartz

Some Considerations on Research History of Geological Structure of the Sambagawa Belt in Central Shikoku : Methodology of Regional Structural Geology

Tectonics of the Sambagawa Belt was first comprehensively studied by Hara et al.(1977), distinguishing four main tectonic phases (in chronological order): formation of Iithologic layering-parallel schistosity, Nagahama phase of folding with southward vergence associating formation of nappes, Ozu phase of folding with southward vergence, and Hijikawa phase of sinistral en echelon upright folding. The metamorphic facies analysis of the schists of the Sambagawa Belt and Northern Chichibu Belt in central Shikoku was first performed by Kurata & Banno (1974) and Higashino (1975), clarifying that the facies series of pelitic schists defined by minerals crystallized during the highest-temperature phase of metamorphism can be explained in terms of chlorite zone, garnet zone and biotite zone. Banno et al. (1978), Banno and Sakai (1989), Wallis (1990,1995) and Wallis et al.(1992) have been regarded the distribution of the mineral zones as a thermo-mechanical continuum throughout all phases of deformation. However, Hara et al. (1988, 1990, 1992) and Hara and Shiota (1996) have pointed out that individual mineral zones are nappes which show distinct deformational and metamorphic histories, and that the highest-temperature metamorphism of the garnet zone nappe (Fuyunose nappe) occurred when the Fuyunose nappe schist had been coupled with the biotite zone nappes (Saruta nappe I and II) which were exhuming, and in addition, that the highest-temperature metamorphism of the chlorite zone nappe (Sogauchi nappe) occurred when the Sogauchi nappe schist had been coupled with the Fuyunose and Saruta nappe schists which were exhuming, and furthermore, that the Saruta ～ Sogauchi nappe schists had been exhumed onto an off-scraped sequence (Northern Chichibu Zone) along the mechanical boundary of consuming plates. The methodological differences between the students of the former and the students of the latter have been investigated in this paper with respect to Shimizu and Yoshida's (2004) comments on the research history of tectonics of the Sambagawa Belt

Deformation and Recrystallization of Amphiboles in Sambagawa Schist with Special Reference to History of Sambagawa Metamorphism

The amphibole grains in the Shirataki hornblende-schist, which has been collected from an outcrop in the biotite zone of the Sambagawa belt of the Shirataki district; Central Shikoku, are divided into three distinguishable populations which are different from each other in generation age: amphibole grains (Si-amphibole) included in cores of plagioclase porphyroblasts, and their matrix amphibole grains which consist of two populations, old hornblende grains (porphyroblasts) and new hornblende grains. The Si- amphibole grains, which recrystallized during growth of the cores of plagioclase porphyroblasts, belong to the actinolite - common hornblende group with Si content of 7.36-6.95, showing that the plagioclase cores grew during progressive increase of temperature. The old horriblende grains (Si content=6.97-6.66), which are of the same generation as the inner zones of mantles of plagioclase porphyroblasts, appeared under non-deformational condition and progressive increase of temperature until the highest tempeature. The new hornblende grains (Si content=6.87-6.79) grew during growth of the outer zones of mantles of plagioclase porphyroblasts and the deformation (Se-deformation) of the beginning stage of retrogressive metamorphism. The lattice fabric of the old hornblende grains, which was produced by the Se-deformation, is characterized by preferred orientation of crystallographic axes c forming a single set of lineation (L), though poles of (100) planes form a great-circle girdle normal to L. While that of the new hornblende grains, which grew during the Se-deformation, is characterized by preferred orientation of c axes parallel to L and of (100) planes forming a single set of schistosity. On the basis of informations given by HARA et al. (1977), TAKAGI and HARA (1979), HARA et al. (1980), MAEDA and HARA (1983a and b), MAEDA et al. (1983) and HARA et al. (1983), as well as the present authors, the time-relationship between deformation and metamorphism in the biotite zone of the Sambagawa belt of Central Shikoku has been also briefly discussed in this paper, showing a result of Table I and that the deformation styles of the Sambagawa schists changed cyclically from ductile deformation (=folding and formation of schistosity) to brittle deformation

Three-Dimensional Size-Analysis of Folds of Quartz Veins in the Psammitic Schist of the Oboke District, Shikoku

Three-dimensional size-analysis of folds of quartz veins in the psammitic schist of the Oboke district, Shikoku, has been performed. Many of fold-forms observed on the facies of quartz veins show lens-like form elongated along the fold axis, associating bifurcation of folds. A linear relationship is found, between layer-thickness (T) and arc-length (La) as measured on a given joint surface normal to the fold axis and La increases with increase of T. The mode value of La/T ratios is 8-8.5. The La values on individual folds are not always constant as measured on several sections normal to the fold axis, because the folds show commonly lens-like form elongated along the fold axis. The ratios between axial length of fold (Lfa) and T (Lfa/T) appear to show a bimodal distribution, through Lfa tends to increase with increase of T. One of the mode values of Lfa/T ratios is 35 and the other is 65. The small values of Lfa/T ratio tend to be more frequently found on the folds with larger interlimb angles rather than on those with smaller interlimb angles. During the process of growing of amplitude of fold appears to occur frequently the phenomenon that adjacent folds approximately oriented on a straight line are combined with each other into one fold, associating change of La value

Discovery of Biotite-Bearing Schists Blocks in the Garnet Zone of the Sambagawa Belt of the Asemi District: an Evidence of Tectonic Erosion of Hanging Wall Rocks by Subducting Sediments

Biotite-bearing schists have been discovered in the garnet zone (Fuyunose nappe) of the Sambagawa belt of the Asemi district, central Shikoku, which is covered by the biotite zone (Saruta nappe II and Saruta nappe I). The biotite-bearing schists (subunit II schists of the Fuyunose nappe) are pelitic schists, siliceous schists and basic schists and have plagioclase porphyroblasts, which crystallized during the prograde phase of metamorphism, like the case of the Saruta nappe (I +II) schists. They occur as lenses in the biotite-free schists (subunit I schists of the Fuyunose nappe) which have plagioclase porphyroblasts of the rerograde phase. Amphibole, which crystallized in hematite-bearing basic schists of the subunit I of the Fuyunose nappe during the peak metamorphism, is glaucophane. Biotite of the subunit II schists is commonly found only in plagioclase porphyroblasts, and the inclusion biotite in hematite-bearing siliceous schists of the subunit II occurs together with barroisite, katophorite and taramite. Barroisite of the subunit II schists, which crystallized together with biotite, have distinctly lower values of NaB content than that of the prograde phase of the Saruta nappe (I +II) schists (biotite zone schists) and than that of the retrograde phase of the subunit ( I +II ) schists, showing that the subunit II schists were derived from shallow tectonic positions of subduction zone. The subunit II schists had already been intermingled with the subunit I schists when the peak metamorphism of the latter had begun. It has been concluded in this paper that the origin of the subunit II schists is ascribed to the tectonic erosion and subduction of the hanging wall rocks [probably low pressure parts of the Saruta nappe ( I +II ) schists] of the subduction zone during the subduction of the original sediments for the subunit I schists,which induced great decrease of temperature along the subduction channel

P-t Path of Sediment Subduction-Underplating-Exhumation Process Related to the Formation of the Sambagawa Schists

The Valanginian accretionary complex and the Barremian accretionary complex of the Chichibu megaunit I of the Southern Chichibu belt in east Shikoku, which consist of prehnite-pumpellyite facies rocks and overlie the Albian accretionary complex of the Chichibu megaunit I and the Cenomanian-Turonian accretionary complex of the Shimanto megaunit, have been clarified by Hara et al. (1992) to be of the same age with reference to the subduction beginning age (youngest fossil age) as the Saruta nappe (I+II) schists and the Fuyunose nappe schists of the Sambagawa megaunit as high P/T type metamorphic rocks respectively. K-Ar ages of muscovites from the former two accretionary complexes, which are considered to have been roughly comparable with the exhumation beginning age, were determined in this paper to be 114 ± 6Ma and 108 ± 5Ma respectively. The exhumation beginning age appears to have been different by ca. 20Ma between the Chichibu megaunit I of subcretion depth of a few kilobars (less than 4kb) and the Sambagawa megaunit of subcretion depth of ca. 10kb with the same subduction beginning age. It would said that this is a rough estimate of P-t path of sediment subduction-underplating-exhumation process related to the formation of the Sambagawa megaunit

The Baric Structures and Exhumation Processes of the Sogauchi Unit in the Sambagawa Belt

The Sogauchi unit is developed as the Sogauchi nappe as a member of primary structure and as the Omogiyama nappe, Terano—Isozu nappe and Saredani—Kabayama—Izushi nappe as members of secondary structure. The baric structures of these nappes have been analyzed on the basis of chemical composition of amphibole in hematite—bearing basic schist. The Sogauchi nappe consists of three subunits as nappes, showing increase of pressure from the lower nappe to the upper nappe and northward increase of pressure for each nappe. The assumed isobaric lines appear to be running in WNW—ESE trend, which is slightly oblique to the general trend of mineral lineation (Lm), and the lower pressure part of each nappe appears to be placed on the western side on the line along Lm. The displacement of the nappes during their subcretion— exhumation appears to have been of westward sense judging from quartz microtextures. The Omogiyama nappe and Saredani—Kabayama —Izushi nappe have been assumed to have been derived as nappes from the northwestern extension (higher pressure parts) of the Sogauchi nappe. However, an alternative model has also been shown for the root of the Saredani — Kabayama —Izushi nappe

Sinistral En Echelon Folding of the Sambagawa Schists and Its Tectonic Implication

The folds of the Sambagawa schists, which were produced during the last phase (Hijikawa—Oboke phase = Dh phase) of their folding history, are developed as a series of sinistral en echelon upright folds with half wavelength of less than 20 Km (Hara et al.,1977,1992). The Dh phase folds in Shikoku are accompanied with two culminations, Oboke culmination and Nakashichiban culmination, placed near the MTL. Their movement picture during the formation process of such the Dh phase folds has been analyzed on the basis of orientation pattern of parasitic folds and quartz microtextures. It has been clarified that the Dh phase folds were produced by left—lateral shear under N —S compression, being accompanied by the southward tectonic emplacement of two rigid bodies which gave rise to the Oboke and Nakshichiban culminations. These bodies can be assumed to be granitic and/or high—temperature metamorphic rocks tectonically derived from the Kurosegawa—Koryoke continent, as judged from the seismic refraction data in the Oboke district after Ichikawa (1968)