Evolution of fibrils with special reference to flare activity

We show observational results on the pre-flare evolutions of H structures as well as the developments of H flares. It is shown that the chromospheric features are brought to a sheared state before flares due to motions of footpoints which correspond to particular sunspot motions. Generally in evolutions of the chromospheric features it is found that motions and reconnections of the footpoints play essential roles. The following three stages are found for development of the neutral line filament before flares: (1) formation of a filament as a result of reconnection; (2) increase of the shear of the filament due to the shear motion; and (3) reconnection of fine components of the filament to form an elongated component immediately before flares. We further show developments of two particular flares with and without the filament, and point out basic release processes of flares. The flare that occurred at the filament (July 5, 1974) started with the activation of the elongated component of the filament after the process (3). The main phase of a two-ribbon flare is considered as the rises of short components of the filament triggered by the rising motion of the elongated component. The flare of September 10, 1974 occurred at the region where fibrils connect the sunspots in distorted form. Pre-flare distortion was produced by translational rotation of the sunspot. Development of this two-ribbon flare is interpreted as being due to successive rises of the fibrils with a self-trigger mechanism.On leave from Tokyo Astronomical Observatory (present address).     

Characteristics of Porphyry Cu Mineralization at Waisoi (Namosi District), Viti Levu, Fiji

The porphyry Cu deposits at Waisoi in Namosi district, Viti Levu are separated into two deposits: the Waisoi East deposit and the Waisoi West deposit. In the Waisoi East deposit, quartz porphyry is exposed and in the Waisoi West deposit, diorite porphyry is sporadically exposed in addition to a small body of quartz porphyry. The mineralization in the Waisoi East deposit is characterized by the bornite–chalcopyrite–pyrite assemblage associated with traces of molybdenite and native gold. Polyphase fluid inclusions in stockwork quartz veinlets show homogenization temperatures ranging from 210 to >500°C. The high‐grade Cu mineralization in the Waisoi West deposit is characterized by the bornite–chalcopyrite–pyrite assemblage accompanied with sheeted and stockwork quartz veinlets. Polyphase fluid inclusions occasionally containing hematite flakes in quartz veinlets in the center of the Waisoi West deposit homogenize at temperatures ranging from 450°C to >500°C. However, fluid inclusions in stockwork quartz veinlets in the periphery, homogenize at lower temperatures around 210°C. Both in the Waisoi East and Waisoi West deposits, primary bornite–chalcopyrite–pyrite assemblage in the high Cu‐grade zone was deposited at the upper stability limit of chalcopyrite with respect to sulfur fugacity. Thus, the principal Cu mineralization at the Waisoi deposits occurred at a relatively high sulfur fugacity, that is, in a high‐sulfidation environment.     

Deposition rates of polysilicic acid with up to 10 M calcium ions

Cementitious materials used for radioactive waste repository construction complicate the performance assessment of radioactive waste systems because the use of cement may greatly alter the pH (8–13) of groundwater and release constituents such as calcium ions. Under such conditions, it is important to clarify also the dynamic behavior of silica (silicic acid), in order to evaluate the alteration in the chemical and physical properties of the fractured layer or the host rock surrounding the repository. Since silica undergoes polymerization, precipitation or dissolution depending on the pH and/or temperature, the behavior of silica would be greatly complicated in the presence of other ions. This study is focused on the deposition rates of polysilicic acid and soluble silicic acid with up to 10⁻³ M Ca ions. In the experiment, Na₂SiO₃ solution (250 mL, pH > 10, 298 K) was poured into a polyethylene vessel containing amorphous silica powder (0.5 g), and a buffer solution, HNO₃, and CaNO₃ as Ca ions were sequentially added into the vessel. The pH of the solution was set to 8. The silica, initially in a soluble form at pH > 10 (1.4 × 10⁻² M), became supersaturated and either deposited on the solid surface or changed into the polymeric form. Then the concentrations of both poly- and soluble silicic acid were monitored over a 40-day period. The decrease of polysilicic acid became slow with an increase in the concentration of Ca ions in the range of up to 10⁻³ M. In general, the addition of electrolytes to a supersaturated solution accelerates the aggregation and precipitation of polymeric species. However, the experimental result showed that polysilicic acid in the presence of Ca ions is apparently stable in solution, compared with that under a Ca-free condition. On the other hand, the concentration of soluble silicic acid in the presence of Ca ions immediately became metastable, that is, slightly higher than the solubility of soluble silicic acid. Its dynamic behavior was similar to that in the Ca-free condition.     

Fluid infiltration associated with seismic faulting: Examining chemical and mineralogical compositions of fault rocks from the active Chelungpu fault

In order to understand the fault zone architecture and mechanisms that caused the Chi-Chi earthquake, the Chelungpu drilling project was conducted during April 2000 through a collaborative project between Japan and Taiwan. In this study, chemical and mineralogical variations within the overall Chelungpu fault zone, including variations between less damaged host rocks, damaged zones, and fault cores caused by the Chi-Chi earthquake were examined. Slopes of TiO₂ immobile isocons were consistently > 1 for analyses comparing host rocks with rocks from damaged zones or with gouges from fault cores, indicating that volume loss occurred in damaged zones and the fault cores. These results strongly imply that pervasive fluid infiltration occurred within the fault zone. Volume loss within the damaged zone and fault core is interpreted to result from a two-stage process involving: (i) coseismic mechanical wearing and/or dissolution in the fault core, and (ii) fluid infiltration within the fault zone during postseismic and interseismic periods along cracks caused by seismic failure. Semi-quantitative XRD analysis indicates that the kaolinite content consistently increases from the less damaged host rocks to the damaged zone and gouges in each fault core. Mineralogic changes indicate that pervasive acidic fluid infiltration occurred within the fault zones and reacted with the feldspars or muscovite to form kaolinite. Enrichment of kaolinite and illite found in the fault zones of southern drilling site could play some role on the slipping behavior of the southern part of the Chelungpu fault. Greater volume loss in the fault core may have resulted from moderate permeability, combined with the very fine grain nature of pulverized material in the fault core, which enhanced chemical reactions including transformation of feldspars and muscovite to clay minerals. The study results indicate that pervasive fluid infiltration occurred and changed the mineralogical and chemical architecture of fault zones caused by the cyclic earthquakes.     

In situ hydraulic tests in the active fault survey tunnel, Kamioka Mine, excavated through the active Mozumi-Sukenobu Fault zone and their hydrogeological significance

Abstract   The spatial hydrogeological and structural character of the active Mozumi-Sukenobu Fault (MSF) was investigated along a survey tunnel excavated through the MSF in the Kamioka Mine, central Japan. Major groundwater conduits on both sides of the MSF are recognized. One is considered to be a subvertical conduit between the tunnel and the surface, and the other is estimated to be a major reservoir of old meteoric water alongside the MSF. Our studies indicate that part of the MSF is a sub-vertical continuous barrier that obstructs younger meteoric water observed in the south-eastern part of the Active Fault Survey Tunnel (AFST) and water recharge to the rock mass intersected by the north-western part of the AFST. The MSF might be a continuous barrier resulting in the storage of a large quantity of older groundwater to the northwest. The observations and results of in situ hydraulic tests indicate that the major reservoir is not the fault breccia associated with the northeast–southwest trending faults of the MSF, but the zone in which blocks of fractured rocks occur beside high angle faults corresponding to X shears whose tectonic stress field coincides with the present regional stress field and antithetic Riedel shears of the MSF. The results from borehole investigations in the AFST indicate that secondary porosity is developed in the major reservoir due to the destruction of filling minerals and fracture development beside these shears. The increase in hydraulic conductivity is not directly related to increased density of fractures around the MSF. Development of secondary porosity could cause the increase in hydraulic conductivity around the MSF. Our results suggest that minor conduits of the fracture network are sporadically distributed in the sedimentary rocks around the MSF in the AFST.     

Fracture-zone conditions on a recently active fault: insights from mineralogical and geochemical analyses of the Hirabayashi NIED drill core on the Nojima fault, southwest Japan, which ruptured in the 1995 Kobe earthquake

An 1800-m-deep borehole into the Nojima fault zone was drilled at Nojima-Hirabayashi, Japan, after the 1995 Hyogo-ken Nanbu (Kobe) earthquake. Three possible fracture zones were detected at depths of about 1140, 1300, and 1800 m. To assess these fracture zones in this recently active fault, we analyzed the distributions of fault rocks, minerals, and chemical elements in these zones. The central fault plane in the shallowest fracture zone was identified by foliated blue-gray gouge at a depth of 1140 m. The degree of fracturing was evidently greater in the hanging wall than in the footwall. Minerals detected in this zone were quartz, orthoclase, plagioclase, and biotite, as in the parent rock (granodiorite), and also kaolinite, smectite, laumontite, stilbite, calcite, ankerite, and siderite, which are related to hydrothermal alteration. Biotite was absent in both the hanging wall and footwall across the central fault plane, but it was absent over a greater distance from the central fault plane in the hanging wall than in the footwall. Major element compositions across this zone suggested that hydrothermal alteration minerals such as kaolinite and smectite occurred across the central fault plane for a greater distance in the hanging wall than in the footwall. Similarly, H₂O+ and CO₂ had higher concentrations in the hanging wall than in the footwall. This asymmetrical distribution pattern is probably due to the greater degree of wall–rock fracturing and associated alteration in the hanging wall. We attributed the characteristics of this zone to fault activity and fluid–rock interactions. We analyzed the other fracture zones along this fault in the same way. In the fracture zone at about 1300 m depth, we detected the same kinds of hydrothermal alteration minerals as in the shallower zone, but they were in fewer samples. We detected relatively little H₂O+ and CO₂, and little evidence for movement of the major chemical elements, indicating little past fluid–rock interaction. In the fracture zone at about 1800 m depth, H₂O+ and CO₂ were very enriched throughout the interval, as in the fracture zone at about 1140 m depth. However, smectite was absent and chlorite was present, indicating the occurrence of chloritization, which requires a temperature of more than 200 °C. Only smectite can form under the present conditions in these fracture zones. The chloritization probably occurred in the past when the fracture zone was deeper than it is now. These observations suggest that among the three fracture zones, that at about 1140 m depth was the most activated at the time of the 1995 Hyogo-ken Nanbu (Kobe) earthquake.     

Comparative Study of Cyclicity of Lithofacies in Lower Gondwana Formations of Talchir Basin, Orissa, India: A Statistical Analysis of Subsurface Logs

Cyclic characters of Karharbari, Barakar and Barren Measures Formations of the Talchir Gondwana basin have been studied in the subsurface logs statistically using first order Markov chain and entropy analyses. Results strongly suggest that the sediments of these formations were deposited by Markovian mechanism and all the three formations represent cyclic sedimentation. The complete cycles of all the three formations are identical and exhibit fining-upward character. Each complete cycle starts with a thin conglomerate or pebbly to coarse-grained sandstone at the base and successively followed by medium- and fine-grained sandstones, interbedded sandstone-shale, shale and terminates with a coal seam at the top. There are, however, minor variations of facies transition in different formations. Entropy analysis also corroborates these findings. The upward sequence of facies states, which is stationary at individual localities, is non-stationary over the entire area. Broad regional variations in the depositional environment, that are not significant at the local scale, may be the plausible explanation. The Karharbari, Barakar and Barren Measures sediments of the Talchir Gondwana basin fit suitably into the concept of fluvial cycles.     

Alluvial Fan-lacustrine Sedimentation and its Tectonic Implications in the Cretaceous Athgarh Gondwana Basin, Orissa, India

The Athgarh Formation is the northernmost extension of the east coast Upper Gondwana sediments of Peninsular India. The formation of the present area is a clastic succession of 700 m thick and was built against an upland scarp along the north and northwestern boundary of the basin marked by an E-W-ENE-WSW boundary fault. A regular variation in the dominant facies types and association of lithofacies from the basin margin to the basin centre reveals deposition of the succession in an alluvial fan environment with the development of proximal, mid and distal fan subenvironments with the distal part of the fan merging into a lake. Several fans coalesced along the basin margin, forming a southeasterly sloping, broad and extensive alluvial plain terminating to a lake in the centre of the basin. Aggradation of fans along the subsiding margin of the basin resulted in the Athgarh succession showing remarkable lateral facies change in the down-dip direction. The proximal fan conglomerates pass into the sandstone-dominated mid-fan deposits, which, in turn, grade into the cyclic sequences of sandstone-mudstone of the distal fan origin. Further downslope, thick sequence of lacustrine shales occur. The faulted boundary condition of the basin and a thick pile of lacustrine sediments at the centre of the basin suggest that tectonism both in the source area and depositional site has played an important role throughout the deposition of the Athgarh succession of the present area. The vertical succession fines upward with the coarse proximal deposits at the base and fine distal deposits at the top, suggesting deposition of the succession during progressive reduction of the source area relief after a single rapid uplift related to a boundary fault movement.The NW-SE trending fault defining the Son-Mahanadi basin of Lower Gondwana sediments are shear zones of great antiquity and these were rejuvenated under neo-tensional stress during Lower Gondwana sedimentation. The E-W-ENE-WSW trending fault of the Athgarh basin, on the other hand, define tensional rupture of much younger date. In the Early Cretaceous period, there was a reversal of palaeoslope in the Athgarh basin (southward slope) with respect to the Son-Mahanadi basin (northward slope). During the phase drifting of the Indian continent and with the evolution of Indian Ocean in the Early Cretaceous period, the tectonic events in the plate interior was manifested by formation of new grabens like the Athgarh graben.