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

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

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)

Tectonic Evolution of the Sambagawa Schists and its Implications in Convergent Margin Processes

The Sambagawa schists as high P /T metamorphic rocks are a member of Mesozoic accretionary complexes developed in the southern front of the Kurosegawa-Koryoke continent of Southwest Japan. The Mesozoic accretionary complexes are divided into four megaunits developed as nappes, Chichibu megaunit II, Sambagawa megaunit, Chichibu megaunit I and Shimanto megaunit in descending order of structural level. The Chichibu megaunit II consists of three accretionary units developed as nappes, late early Jurassic unit, late middle Jurassic unit and latest Jurassic unit (Mikabu unit) in descending order of structural level. The Chichibu megaunit I consists of five accretionary units developed as nappes, late middle Jurassic unit (Niyodo unit), late Jurassic unit, Valanginian unit, Barremian unit and Albian unit in descending order of structural level. The Shimanto megaunit, which just underlies the Chichibu megaunit I, is Cenomanian-Turonian accretionary unit and Coniacian-Campanian accretionary unit. The schists, which underlie the Chichibu megaunit II, all have been so far called the Sambagawa schists. These are divided into six units, Saruta unit, Fuyunose unit, Sogauchi unit, Sakamoto unit, Oboke unit and Tatsuyama unit in descending order of structural level, which show different tectono-metamorphic history and different radiometric ages from each other. The Sakamoto unit, Oboke unit and Tatsuyama unit have been assumed with reference to their radiometric ages and structural relations to belong to the late middle Jurassic accretionary unit of the Chichibu megaunit I (high pressure equivalent of the Niyodo unit), the Cenomanian-Turonian accretionary unit of the Shimanto megaunit and the Coniacian-Campanian accretionary unit of the Shimanto megaunit respectively in this paper. The upper member of the Sambagawa schists, Saruta unit, Fuyunose unit and Sogauchi unit, is therefore called the Sambagawa megaunit in this paper. The northern half and the southern half of the Sambagawa megaunit are intercalated as nappes between the Chichibu megaunit II and the Oboke unit and between the Chichibu megaunit II and the Sakamoto unit respectively. The constituent units of the Chichibu megaunit II, Sambagawa megaunit and Shimanto megaunit clearly show a downward younging age polarity, as compared with each other with reference to the oldest one of radiometric ages ( = metamorphic ages) of each unit. The Chichibu megaunit II and the Chichibu megaunit I show the same radiometric ages as compared between them with the same fossil age. The Saruta unit, Fuyunose unit and Sogauchi unit have therefore been assumed to be high pressure equivalent of Valanginian unit, that of Barremian unit and that of Albian unit of the Chichibu megaunit I respectively. These high pressure units were exhumed, separating the Chichibu supermegaunit into the Chichibu megaunit II and the Chichibu megaunit I and thrusting up onto the Chichibu megaunit I. On the basis of the growth history of amphibole in hematite-bearing basic schists of the Sambagawa megaunit, it has been assumed that the highest temperature metamorphism of the Fuyunose unit occurred, when it had been coupled with the Saruta unit which was exhuming, and that of the Sogauchi unit did through its coupling with the Fuyunose and Saruta units which were exhuming. In the subduction zone which was responsible for the formation of the Sambagawa megaunit, namely, the peak metamorphism of a newly subducted unit appears to have occurred when it had been coupled with previously subcreted units which were exhuming. It has been also clarified that the subduction of a new unit occurred mixing the lower pressure part of the pre-existing subcretion unit as tectonic blocks. There is a distinct difference in the oldest one of radiometric ages between constituent units of the Sambagawa schists, showing a downward younging age polarity. The oldest one of radiometric ages of each unit appears to approximate to the age of the ending of peak metamorphism and to the age (Eh age) of the beginning of its exhumation. Such the tectonics of the Sambagawa megaunit would be explained in term of two-way street model. Because the age (Sub age) of the beginning of the subduction of each unit can be assumed from its fossil age, the average velocity of the subduction and that of the exhumation of the Sambagawa megaunit in Shikoku have roughly been estimated to be ca. 0.9 mm/year and ca. 2.0 mm/year respectively. Deformation of quartz, whose style depends strongly upon strain rate, resulted in type I crossed girdle without conentration in Y even in the depth part of more than 10kb of the subduction zone, which was placed under temperature condition of much higher than 500°C, unlike the cases of magma-arcs where quartz c-axis fabrics with maximum concentration in Y are found in gneisses produced under temperature condition of lower than 500°C. Quartz deformation in the depth part of 15-17kb of the subduction zone appears to have occurred as dominant prism slip. The hanging wall of the Kurosegawa-Koryoke continent, which was placed at the depth of ca. 15-17 kb, thrust onto the Saruta unit at the depth of ca. 10-11 kb, accompanying intermingling of constituent rocks of the former and the latter and also mixing of various depth parts of the latter. The highest temperature metamorphism of the Saruta unit, which appears to have occurred under metamorphic condition of lower P /T than under that related to the formation of the general type of high P /T type metamorphic rocks, is ascribed to a contact metamorphism related to the overthrusting of the Kurosegawa-Koryoke continent. The thrusting of the Kurosegawa-Koryoke continent is ascribed to its collision with the Hida continent. The coupling of the previously subcreted Saruta unit with the newly subcreted Fuyunose unit occurred accompanying nearly isobaric cooling of the former. The great exhumation of the Saruta nappe (I + II) and Fuyunose nappe schists with great volume began together with the subcretion of the Sogauchi unit. The beginning age of the exhumation of the Sambagawa schists with great volume appears to coincide with that of the subduction of the Kula-Pacific ridge in Kyushu-Shikoku, which has been assumed by Kiminami et al. (1990). Namely, their great exhumation occurred with the progress of the subduction of the Kula-Pacific ridge with an eastward younging age polarity. The exhumation units, which were developed after the Mikabu unit, clearly show an eastward younging age polarity. Namely, these comparable with the Saruta unit, Fuyunose unit and Sogauchi unit are not found in central Japan and the Kanto Mountains. Rock deformation in the deformation related to the exhumation of the Sambagawa schists and their underlying schists appears to have commonly been of flattened type in mean strain. During the Ozu phase when the Kula-Pacific ridge subducted to the greater depth, the collapse of the Kurosegawa-Koryoke continent took again place, accompanying that of the pile nappe structures of the Sambagawa megaunit, Chichibu megaunit I and Oboke unit, and the thermal gradient along the plate boundary greatly changed, giving rise to medium P/T type metamorphism in the subduction zone (formation of the Tatsuyama nappe schists). The geological structures of the Sambagawa megaunit consist thus of two types of pile nappe structures, pre-Ozu phase pile nappe structures and Ozu phase pile nappe structures. The former is structures related to the coupling of the exhuming units ( = previously subcreted units) with the newly subcreted unit. The latter is structures showing the collapse of the former. The Ozu phase pile nappe structures are further divided into the pile nappe structures formed during the earlier stage (Tsuji stage) of the Ozu phase and these formed during the later stage (Futami stage). The former is disharmonic with reference to movement picture with the latter: The deformation related to the formation of the former, accompanying exhumation of the Oboke nappes, appears to contain a component of northward displacement, while that for the latter does a component of southward displacement. After the Ozu phase deformation the Sambagawa megaunit suffered the Hijikawa-Oboke phase folding, forming a series of sinistral en echelon upright folds. The relationship between the above-mentioned tectonic events of the Sambagawa megaunit and its surroundings and their radiometric ages is summarized as follows: [Original table is skipped. For more details, please refer to the full text.

Human Adrenocortical Remodeling Leading to Aldosterone-Producing Cell Cluster Generation

Background. The immunohistochemical detection of aldosterone synthase (CYP11B2) and steroid 11β-hydroxylase (CYP11B1) has enabled the identification of aldosterone-producing cell clusters (APCCs) in the subcapsular portion of the human adult adrenal cortex. We hypothesized that adrenals have layered zonation in early postnatal stages and are remodeled to possess APCCs over time. Purposes. To investigate changes in human adrenocortical zonation with age. Methods. We retrospectively analyzed adrenal tissues prepared from 33 autopsied patients aged between 0 and 50 years. They were immunostained for CYP11B2 and CYP11B1. The percentage of APCC areas over the whole adrenal area (AA/WAA, %) and the number of APCCs (NOA, APCCs/mm2) were calculated by four examiners. Average values were used in statistical analyses. Results. Adrenals under 11 years old had layered zona glomerulosa (ZG) and zona fasciculata (ZF) without apparent APCCs. Some adrenals had an unstained (CYP11B2/CYP11B1-negative) layer between ZG and ZF, resembling the rat undifferentiated cell zone. Average AA/WAA and NOA correlated with age, suggesting that APCC development is associated with aging. Possible APCC-to-APA transitional lesions were incidentally identified in two adult adrenals. Conclusions. The adrenal cortex with layered zonation remodels to possess APCCs over time. APCC generation may be associated with hypertension in adults

Arterial endothelium-specific activin receptor-like kinase 1 expression suggests its role in arterialization and vascular remodeling

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular disorder characterized by epistaxis, mucocutaneous telangiectases, and arteriovenous malformations (AVM). Two genes are linked to HHT: endoglin (ENG) in HHT1 and activin receptor-like kinase 1 (ACVRL1; ALK1) in HHT2. Although both genes are involved in the transforming growth factor β signaling pathways, the pathogenetic mechanisms for HHT remain elusive. It was shown that mutations in the Alk1 gene in mice and zebrafish resulted in an embryonic lethal phenotype due to severe dilation of blood vessels. We created a novel null mutant mouse line for Alk1 (Alk1lacZ) by replacing its exons, including the one that encodes the transmembrane domain, with the β-galactosidase gene. Using Alk1lacZ mice, we show that Alk1 is predominantly expressed in developing arterial endothelium. Alk1 expression is greatly diminished in adult arteries, but is induced in preexisting feeding arteries and newly forming arterial vessels during wound healing and tumor angiogenesis. We also show that hemodynamic changes, which require vascular remodeling, may regulate Alk1 expression. Our studies suggest the role of Alk1 signaling in arterialization and remodeling of arteries. Contrary to the current view of HHT as venous disease, our findings suggest that the arterioles rather than the venules are the primary vessels affected by the loss of an Alk1 allele, and that blood vessels with reduction in Alk1 expression may harbor defects in responding to demands for vascular remodeling