ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00062.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Nankai accretionary prism, off southwest Japan represents one of the best developed clastic prisms in the world. A combination of swath mapping including Sea Beam and ‘IZANAGI’ sidescan sonar and closely spaced seismic reflection data was used to investigate the relationship between the progressive landward change in surface morphology and the internal structural evolution of the prism. The prism surface is divided into three zones sub‐parallel to the trough axis on the basis of the IZANAGI backscattering image. The frontal part of the prism is characterized by several continuous lineaments that are approximately perpendicular to the plate convergence direction. These lineaments correspond to anticlinal ridges caused by active imbricate thrusting. Landward, these anticlinal ridges become progressively masked by fine‐grained hemipelagic slope sediments that are constantly supplied to the entire prism slope. However, these overlying sediments show little deformation. This implies a change in deformation style from frontal thrusting with fault‐bend folds to internal refolding of thrust sheets. In the middle to upper prism slope, the IZANAGI image shows numerous landslide features and large fault scarps, suggesting that exposed sediments are lithified enough to fail in brittle mode compared with the wet sediment deformation at the prism toe. Prism evolution is strongly affected by the decollement depth which may be indirectly controlled by oceanic basement relief; a topographic embayment coincides with a regional minimum of sediment offscraping where a basement high has been subducted. The small tapered prism observed in the embayment may be due to lateral supply of overpressured pore fluids from the adjacent prism. Strain caused by the differential rate of prism growth across the basement relief forms faults trending at high angles to the tr

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00063.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub‐belt (Eocene‐Oligocene) and the Nabae Sub‐belt (Oligocene‐Miocene). Both sub‐belts have been affected by thermal overprints following the peak of accretion‐related deformation. Palaeotemperatures for the entire Tertiary section range from ∼ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%Rm. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b0lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub‐belt than for the Nabae Sub‐belt. The Eocene‐Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional‐scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub‐belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ∼ 90°C across the boundary between the Murotohanto and Nabae Sub‐belts (Shiina‐Narashi fault), and this contrast is consistent with approximately 1200 m of post‐metamorphic vertical offset.Subduction prior to Middle Miocene probably involved the Kula or fused Kula‐Pacific plate and the background geothermal gradient during the Eocene‐Oligocene phase of accretion was ∼ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ∼ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ∼ 70°C/km. Whether or not all portions of the inherited (Eocene‐Oligocene) palaeothermal structure were overprin

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00064.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Late Oligocene‐Early Miocene Nabae Sub‐belt of the Shimanto Accretionary Prism was created coevally (ca25‐15 Ma) with the opening of the Shikoku back‐arc basin, located to the south of the southwest Japan convergent margin. The detailed geology of the sub‐belt has been controversial and the interaction of the Shimanto accretionary prism and the opening of the Shikoku Basin has been ambiguous. New structural analysis of the sub‐belt has led to a new perception of its structural framework and has significant bearing on the interpretation of the Neogene tectonics of southwest Japan.The sub‐belt is divided into three units: the Nabae Complex; the Shijujiyama Formation; and the Maruyama Intrusive Suite. The Nabae Complex comprises coherent units and mélange, all of which show polyphase deformation. The first phase of deformation appears to have involved landward vergent thrusting of coherent units over the mélange terrane. The second phase of deformation involved continued landward vergent shortening. The Shijujiyama Formation, composed mainly of mafic volcanics and massive sandstone, is interpreted as a slope basin deposited upon the Nabae Complex during the second phase of deformation. The youngest deformational pulse involved regional flexing and accompanying pervasive faulting. During this event, mafic rocks of the Maruyama Intrusive Suite intruded the sub‐belt. Fossil evidence in the Nabae Complex and radiometric dates on the intrusive rocks indicate that this tectonic scheme was imprinted upon the sub‐belt between ∼23 and ∼14 Ma.The timing of accretion and deformation of the sub‐belt coincides with the opening of the Shikoku Basin; hence, subduction and spreading operated simultaneously. Accretion of the Nabae Sub‐belt was anomalous, involving landward vergent thrusting, magmatism in newly accreted strata and regional flexing. It is proposed that this complex and anomalous structural history is largely related to the subduction of the active Shikoku Basin spreading rid

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00065.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractWe propose that a change in convergence between the Pacific and Eurasian plates and the demise of the Kula‐Pacific spreading centre atca43 Ma resulted in an ∼40° counterclockwise rotation in shortening direction within the Eocene Shimanto accretionary prism of southwest Japan. Evidence for this interpretation comes from: (1) structural studies of the accreted, deep‐sea rocks of the Eocene Shimanto Belt from four widely separated localities; and (2) new plate reconstructions that incorporate the geological history of east Asia as well as the recently recognized reorganization of the Kula and Pacific plates at the time of anomaly 24. These reconstructions suggest that the Philippine Sea plate formed as the Kula‐Pacific spreading centre reoriented at the time of anomaly 24 and that the Kula plate was being subducted beneath southwest Japan untilca43 Ma. Our reconstructions and structural studies suggest that afterca43 Ma, plate convergence in southwest Japan was oblique to the trend of the continental margin. Oblique convergence was apparently recorded at this time because arc volcanism had decreased and the accretionary prism was not detached from the arc massif. Moreover, the transition from cataclasis and faulting to pressure solution within the accreted sediments may have resulted in a stronger basal décollément, resulting in higher shear stresses along this boundary. We therefore propose that where the arc region and the décollément are of similar strengths, structures within accretionary prisms may record changing plate motions, including oblique

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00066.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA deep section of accretionary complex, the metamorphosed Susunai Complex, is observed on Sakhalin Is., Russia. High pressure part of pumpellyite‐actinolite facies metavolcanics, metacherts and metapelites are well exposed and constitute a tectonic pile preserving primary structures related to underplating of the oceanic crust. Three stages of deformation, D1through D3, suggest successive deformation during subduction, underplating and exhumation of the complex. Oceanic material in the complex is more abundant than other well documented ancient accretionary complexes (e.g. the Shimanto Belt in southwest Japan and the Ghost Rocks Formation in Alaska), which were shallowly underplated. At Susunai, deep down‐stepping of a décollément has scraped off the upper part of the oceanic crust, primarily the pillowed basalt horizon. This down‐stepping results from crustal weakening as overpressured water is released from the fractured oceanic crust during metam

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00067.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractDeformation in the Sanbagawa Belt is characterized by ductile flow in an east‐west direction sub‐parallel to its length. The east‐west flow (D1) caused large‐scale recumbent folding of the metamorphic sequence in central Shikoku, which can explain the inverted thermal structure of this region. Chemical zoning of metamorphic minerals associated with D1microstructures also suggest that the east‐west flow developed under retrograde conditions. D1is therefore related to exhumation rather than subduction processes. A variety of kinematic indicators show that during the east‐west flow, deformation was partitioned into structurally continuous domains with opposed senses of shear. This suggests that bulk deformation was not simple shear but included a component of

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00068.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractQuartz c‐axis fabrics of the Sambagawa schists produced along a late Mesozoic convergent plate margin were analysed so that their tectono‐metamorphic history could be clarified. It has been noted by many authors that quartz fabrics produced by earlier phase deformation are easily modified by strain increment during later phase deformation. This paper attempts to elucidate the high‐temperature phases of prograde metamorphism (Sim‐Bim phase) and of retrograde metamorphism (Sb1phase and Sb2−1phase) from quartz grains included in garnet and plagioclase porphyroblasts. Quartz c‐axis fabrics for all these phases are explained in terms of a type I crossed girdle, without (only rarely with) higher concentration in the principal axis of strain Y (X>Y>Z), that must have been produced by the activity of a dominant slip system such as rhomb and basal. As a result, the plastic deformation of quartz, which was responsible for the formation of the type I crossed girdle, occurred even under temperatures greater than 500°C and pressures a little greater than 10–11 kb, which correspond to the physical condition of the Sim‐Bim phase. It has been assumed that a high strain rate (and/or low H2O content) caused rhomb and basal to be active as dominant slip systems in the subduction zone related to the formation of the Sambagawa schists even under high tempe

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1992.tb00069.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY