摘要:

AbstractSlab detachment is a geophysical instability whose manifestation can be revealed by seismic tomography. Evidence of this phenomenon is in the Dinarides/Hellenic and the New Hebrides subduction zones. Subducted slabs in these regions are torn horizontally at depths ranging from 100 to 300 km. We constructed a viscoelastic three‐dimensional finite element model and investigated the state of stress. We found that an area with high stress concentration of the order of several hundred MPa is formed near the tip of the tear inside the slab, which can cause lateral migration of the tear. Favorable conditions for slab detachment are characterized by large interplate frictional force at a subduction zone and small slab resistance force deeper down. Stress concentration increases with the down‐dip tension inside the slab. The phenomenon of slab weakening has also been studied from a thermal‐mechanical standpoint, using a two‐dimensional convection model with non‐Newtonian, temperature‐dependent rheology. The stress‐dependent rheology plays an important role in causing local weakening of the descending slab. In strongly time‐dependent situations the fast descending slab is not strong everywhere but has a weak region in the middle, making it vulnerable to slab detachment. The presence of viscous heating will enhance slab detachment tendency by further weakening the interior by the frictional heating. Besides these effects, there are other mechanisms which can also weaken the slab interior and help to make slabs more pliable and susceptibl

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00134.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe sub‐arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc‐related primitive magmas. This almost excludes the possibility of their sub‐oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr#<0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high‐field strength elements (HFSE) to large‐ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc‐type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00135.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractX‐ray diffraction (XRD) analyses of carbonaceous materials were carried out in conjunction with petrological studies for selected metamorphic rocks in order to compare the structural state of carbonaceous materials between contact and regional metamorphic rocks. The most extensive study was done for the Daimonji contact aureole in the eastern part of Kyoto city, Japan. The Daimonji contact aureole can be divided into three mineral zones using mineral parageneses of pelitic rocks: chlorite, biotite and cordierite zones. The cordierite zone can be further subdivided into lower‐ and higher‐grade subzones. Petrological considerations allow the two isograd reactions that define the lower‐ and higher‐grade cordierite subzones to be determined and suggest these reactions occurred at 510–560°C and 560–590°C per 2.0‐2.3 kbar, respectively. A combination of the petrological studies and the XRD data of carbonaceous materials suggest that fully ordered graphite (FG; defined by d(002) ≤ 3.360 Å following the convention used by many workers), appears around 560–590°C in the Daimonji contact aureole. This data and refinement of geothermometer for published data confirmed that the FG appears at 400–500°C in regional metamorphic rocks, but at higher than 530°C in contact aureoles. One possible explanation for such a temperature difference is the duration of heating. However, the width at half height (WH) of the graphite peak attains a similar value of 0.30° at around 500°C both in contact and regional metamorphic rocks, suggesting that WH value is a more reliable indicator of metamorphic grade than the change of d(002) value. Furthermore, the depressed d(002) data of graphite was observed locally in the higher grade part (≥ 500°C) of the Ryoke regional metamorphic belt, where granitic intrusions exist within a few km distance. These facts indicate that the duration of heating is not an important factor controlling the change of d(002) value. It is possible that interlayered impurities, such as chlorine, which was derived from igneous intrusions, may be an important factor in suppressing the reduction in d(002) at

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00136.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Cretaceous geodynamic and paleogeographic evolution of the northeastern Asian continental margin is summarized in five maps for time‐intervals of 115‐110, 100‐90, 90‐85, 85‐74 and 70‐50 Ma. Four major evolutionary stages are recognized: (i) the moderate extensional tectonic regime and origination of a system of island arcs in the Jurassic‐Neocomian stage; (ii) Middle Cretaceous strong collisional and accretional processes resulting in the Asian continent formation in the Barremian‐Early Albian stage; (iii) the origination of the subduction‐related Okhotsk‐Chukotka continental marginal volcanic belt in relation to the newly formed convergent plate boundary in the Late Albian‐Senonian stage; and (iv) the next collision accompanied by the extinction of the subduction‐related volcanism in the Late Cretaceous‐Early Paleocene stage (Laramian orogeny) and displayed rift‐related processes with the intraplate‐type volcanism on the Asian continental margin. Those stages had been established through the whole Pacific ring and had close genetic relationships with mid

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00137.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA radiolarian zonal scheme for the entire Jurassic and Lower Cretaceous using biostratigraphic data from both Japanese Island sections and the western Pacific seafloor is documented. The zonation is applicable to low and middle paleolatitude portions of the Paleo‐Pacific ocean. Radiolarian bio‐events such as the evolutionary first appearance biohorizon, first occurrence biohorizon, and last occurrence biohorizon were used to define zones. The 11 zones proposed are, in ascending order,Parahsuum simplum, Trillus elkhornensis, Laxtorum(?) jurassicum, Tricolocapsa plicarum, Tricolocapsa conexa, Stylocapsa(?) spiralis, Hsuum maxwelli, Pseudodictyomitra primitiva, Pseudodictyomitra carpatica, Cecrops sep‐temporatus, andAcanthocircus carinatuszones. Preliminary age assignments for these zones are pres

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00138.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY