Granitic Layer Research Articles

Distribution of radioactive heat production around an active fault and in accretionary prisms of southwest Japan

Contents of radioactive heat-producing elements (U, Th and K) were measured on core samples from a borehole drilled into the shear zone of the active Nojima fault, southwest Japan. The heat production rate is rather uniform in a granite layer below 560 m with an average of 1.6 μW/m 3. In the lower part of the granite layer, which is close to the fault zone, the U/Th and U/K ratios seem to be lower than those in the upper part. A possible cause of this anomaly is redistribution of U by groundwater flows in highly fractured layers. Through monitoring of the temperature profile in the borehole, a local anomaly, which may be associated with groundwater flow along a fracture zone, was detected in the uppermost part of the hole. The conductive heat flow was found to increase with depth, which is possibly due to downward fluid flow in the surrounding formations. Measurements of heat production were also made on rock samples from Permian to Cretaceous accretionary prisms in southwest Japan. The heat production of sandstone and mudstone, major components of the accretionary prisms, is about 1.5 μW/m 3 on average, which is similar to the average value for granitic rocks in southwest Japan. These values are significantly higher than those reported in accreted terranes in the Cascadia subduction zone.

Long‐term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan

Abstract Long‐term monitoring of temperature distribution in an active fault zone was carried out using the optical fiber temperature‐sensing technique. An optical fiber cable was installed in a borehole drilled into the Nojima Fault in Awaji Island, south‐west Japan, and the temperature profile to a depth of 1460 m had been measured for 2.5 years (July 1997–January 2000). Although the obtained temperature records showed small temporal variations due to drifts of the measurement system all along the cable, local temperature anomalies were detected at two depths. One at around 80 m seems to correspond to a fracture zone and may be attributed to groundwater flow in the fracture zone. This anomaly had been stable throughout the monitoring period, whereas the other anomaly at around 500 m was a transient one. The water level in the borehole could be estimated from the diurnal temperature variations in the uppermost part of the borehole and may provide information on the hydrological characteristics of the fault zone, which is connected to the borehole through perforations on the casing pipe. Except for these minor variations, the temperature profile had been very stable for 2.5 years. The conductive heat flow calculated from this profile and the thermal conductivity measured on core samples increases with depth, probably resulting from errors in thermal conductivity due to sampling problems and/or from advective heat transfer by regional groundwater flow. Assuming that the middle part of the borehole (less fractured granite layer) is least affected by these factors, heat flow at this site is estimated to be approximately 70 mW/m2.

Sedimentary Basins and the Blockage of Lg Wave Propagation in the Continents

— The phenomenon of “Lg blockage,” where Lg is strongly attenuated by crustal heterogeneities, poses a serious problem to CTBT monitoring because Lg is an important seismic phase for discrimination. This paper examines blockage in three continental regions where the Lg blockages may be caused by large, enclosed sedimentary basins along the propagation path. The Barents Sea Basin blocks Lg propagation across the Barents Sea from the Russian nuclear test sites at Novaya Zemlya to Scandinavian stations. Also, “early Lg” waves are observed in Sn codas on NORSAR, NORESS, and ARCESS recordings of Novaya Zemlya explosions where direct Lg is blocked. Early Lg waves may have resulted from Sn-to-Lg mode conversion at the contact between the Barents Basin and the Kola Peninsula. The Northern and Southern Caspian Sea Basins also block Lg waves from PNEs and earthquakes, perhaps due to thick, low-velocity, low-Q sediments replacing the granitic layer rocks in the crust. Lg blockage has also been observed in the Western Mediterranean/Levantine Basin due to low-Q sediments and crustal thinning. A “basin capture” model is proposed to explain Lg blockage in sedimentary basins. In this model, shear waves that reverberate in the crust and constitute the Lg wave train are captured, delayed, and attenuated by thick, low-velocity sediments that replace the “granitic” layer rocks of the upper crust along part of the propagation path. Sn waves, which propagate below the basin, would not be blocked and in fact, the blocked Lg waves may be diverted downward into Sn waves by the low velocity sediments in the basin.

Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma

Many granitic plutons contain sheet-like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature, more mafic magma during crystallization of the granitic plutons. Small-scale structures associated with these bodies (e.g. load-cast and compaction features, silicic pipes extending from granitic layers into adjacent gabbroic sheets) indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber (on granitic crystal mush and beneath crystal-poor magma) while the mafic magma was incompletely crystallized. These structures indicate ‘way up', typically toward the interior of the intrusions, and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition. Hence, these plutons preserve a stratigraphic history of events involved in the construction (filling, replenishment) and crystallization of the magma chamber. The distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions, which usually preserve mutual intrusive contacts and ‘dike–sill’ relations of different magma types. The considerable thickness of material that can be interpreted as depositional, and the evidence for replenishment, suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton. Thus, magma chambers may be constructed much more slowly than presently envisaged. The present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber (along with the underlying solidified granite and country rock) sank during continuing episodes of magma chamber replenishment. These internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber.

Modélisation de la structure fractale d'un milieu géophysique

A study has been conducted about the fractal structure (FS) of the seismic rate in block rocks constituted by a granitic layer heated to 400 °C and doubled by a sandstone layer. A pair of stress concentrators has been inserted within the center of the block under an angle of 35° with the vertical axis of the pattern, in order to favour the shearing rupture. This construction is a model of one part of the crust where we can have a releasing of normal seisms. We have applied a new technique of biaxial loading of samples. This has permitted, contrary to traditional experiments, to prolong in time the stage of formation and evolution of the rupture and to study in detail the step of intense interaction of acoustic events. The acoustic rate consists of three main steps: dispersive releasing of acoustic emission (AE); critical concentration of acoustic events and the formation of the focus; lastly, the evolution of this focus in order to give birth to the main rupture. The parameter K T , of rupture concentration decreases until 5–8, the b value, the dimension fractal D decreases and the relation D≈3b is interrupted during the FS genesis.

Crustal Structure in the Southern Kanto-Tokai Region Derived from Tomographic Method for Seismic Explosion Survey.

The detailed P-wave velocity structure of the crust in the southern Kanto-Tokai region was analyzed using the tomographic method for seismic refraction survey in this paper. A total of 332 P-wave arrival times received from 13 seismic explosion surveys were used in the analysis. The results indicate that analyses of travel-time curves are probably useful for the evaluation of inverted structures. The lateral heterogeneity of the velocity structure is obviously related to tectonics. The crust in the eastern region is thinner than that in the western region. The Conrad discontinuity obviously fluctuates. The granitic layer is thinner beneath the oceanic region to the east of Oshima. The layer becomes about 16 km thick beneath Suruga Bay. The Conrad discontinuity drops nearly 17 km in depth beneath Suruga Bay, and velocity is relatively low there. The Conrad discontinuity rises 6 km beneath MTL and its vicinity. The Moho discontinuity is located at a depth of around 34 km beneath the region to the west of ISTL and roughly coincides with the upper boundary of the seismic zone due to subduction of the Philippine Sea Plate under the Eurasian Plate. It becomes shallow across the Suruga trough toward the eastern region. The discontinuity is located about 27 km in depth beneath the oceanic region east of Oshima.

Numerical Modelling of Melting Processes and Induced Diapirism In the Lower Crust

SUMMARY The processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model. Fully molten basalt (T= 1200°C) is emplaced at the Moho beneath a solid granite (T= 750OC) in order that a melt front grows into the granite. If diapirism does not occur, this melt front in the granite reaches a minimal depth in the crust before (like in the molten basalt) crystallization takes place. The density contrast between the partially molten granite layer and the overlying solid granite can lead to a Rayleigh-Taylor instability (RTI) which results in diapiric rise of the partially molten granite. Assuming a binary eutectic system for both the granite and the underplating basalt and a temperature- and stress-dependent rheology for the granite, we numerically solve the governing equations and find (a) that diapirism occurs only within a certain but possibly realistic range of parameters, and (b) that if diapirs occur, they do not rise to levels shallower than 15 or perhaps 12km. The growth rate depends on the degree of melting and the thickness of the partially molten layer, as well as the viscosity of the solid and the partially molten granite. From a comparison of the growth rate with the velocity of a Stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created, which remains at depth. We carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism.

Thermal regimes of the Southern Rocky Mountains and Wyoming Basin in Colorado and Wyoming in the United States

Heat flow in the eastern ranges of the Southern Rocky Mountains in Colorado substantially exceeds that in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. The transitions between these areas are narrow (≤ 60 km wide), estimated near-surface crustal radiogenic heat productions are different, and there is no evidence for young magmatism in the easternmost mountains in northern Colorado and southern Wyoming. Therefore, radioactive heat contrasts in the upper crust are used to explain significant amounts of the heat-flow differences in these regions. In southern Wyoming, normal heat flow in Archean and Proterozoic terranes probably reflects a deeply eroded, thin (7–15 km) granitic layer that overlies low-radioactivity units in a 37–40 km thick crust. In the Colorado mountains to the south, silicic metamorphic and igneous rocks with relatively enriched radiogenic heat could comprise a 20–30 km thick granitic layer in the upper parts of the 50–52 km thick crust, and explain much of the high reduced heat flow and isostatic equilibrium. Areas of unusually high heat flow occur in the Rio Grande rift zone in the environs of the Colorado Mineral Belt in the Leadville-northern Sawatch Range region, eastern parts of the San Juan Mountains in southern Colorado, and in Park Range-Middle Mountain Park areas near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crust-mantle boundary, and the narrow borders (50–60 km wide) of the Leadville-northern Sawatch Range heat-flow anomaly must be caused by sources in the upper crust. Hence, young (10-1 Ma) intrusions in a late Tertiary rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the excess heat flux in the Leadville-northern Sawatch Range area. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system in Colorado.

爆破地震動観測による甲府盆地の地下構造

Seismic refraction and bore hole data are used to study the structure of the Kofu Basin, which is located in the South Fossa Magna. Observing seismic waves at 33 stations which were aligned along N-S and E-W profiling lines in the basin, we obtained clear seismic signals from the shot, which was detonated near the crossing point of the two lines. There is considerable complexity of the structure throughout the basin. The main features of the basin structure are as follows. The basement with a P-wave velocity of about 5.8km/s is a granitic layer and tilted to the southwestwards. The depth of the layer is more than 2km at the shot point and the basement is overlain by thick sediments from the late Pliocene to the Holocene with relatively low velocities of 3.0-2.1km/s. Only in the western part of the basin, the so-called “Green tuff” layer with a velocity of about 5km/s is distributed at the depth of about 1km.

岡山市周辺の環境放射線の調査 等価線量率の平面分布について

Environmental ionizing radiations were surveyed in Okayama city during December of 1992, using a NaI (Tl) scintillation survey meter (TCS-161; Aloka, Japan) and a NaI (Tl) γ spectrometer (JSM-102; Aloka, Japan) ; measerments were carried out in fine days.Equivalent dose rate of environmental ionizing radiations was in the range of 0.048-0.171μSv/h, and the average was 0.082±0.019μSv/h. The dose rate was higher in hills than in fields. Therefore, it appears as if rocks have more radioactive materials than soils do.The distribution of the dose rates was analyzed from geological points of view. Mesozoic and Paleozoic layer was in the range of 0.067-0.095μSv/h, and the average was 0.080±0.008μSv/h. Rhyolitic layer was in the range of 0.076-0.105μSv/h, and the average was 0.088±0.010μSv/h. Granitic layer was in the range of 0.057-0.171μSv/h, and the average was 0.098±0.022μSv/h. Neogene Period layer was in the range of 0.057-0.114μSv/h, and the average was 0.076±0.015μSv/h. Quaternary Period layer was in the range of 0.048-0.114μSv/h, and the average was 0.076±0.013μSv/h.By γ spectrometry, 40K, 208Tl, 212Bi, 214Bi, and 228Ac were detected at Mt. Kaigara (0.171μSv/h), Tsushima (0.076μSv/h) and Fujita (0.048μSv/h), especially, 226Ra was detected at Mt. Kaigara. It seems that 40K was distributed uniformly in Okayama city area. Thas, the difference in the dose range is attributable to nuclides other than 40K.Now, we conclude that there has been no change of the dose rates of environmental radiation in Okayama city these 10 years.

Age, thermal evolution and history of the Black Sea Basin based on heat flow and multichannel reflection data

Multichannel reflection data (Tugolessov et al., 1985) have revealed two deeps in the basement topography of the Black Sea which are filled with sediments from 12 to 15 km thick. The deeps lack the “granitic layer” and are underlain by oceanic-type crust which we assume to be generated by seafloor spreading processes. The age of the deeps was interpreted previously, in a highly controversial manner, as being from the Paleozoic — Early Mesozoic to the Recent. In the paper, age estimations were undertaken using surficial heat flow data, assuming that they are related to deep-seated age-dependent heat flow generated by the cooling oceanic lithosphere, but that they are strongly distorted by the heating of continuously accumulating sediments as well as by additional heat input from radiogenic production within sediments. Using reliable thermophysical parameters of compacting sediments, the distorted heat flow in the sediments was evaluated numerically. This allowed us to estimate the age of the Black Sea deeps floor. The results show that the West Black Sea deep is 130 to 95 m.y. old, and the East Black Sea deep is nearly 110 m.y. old. These figures support an interpretation of the Black Sea deeps as remnants of a Late Mesozoic back-arc basin that evolved behind the Lesser Caucasian — Pontide island arc. The inferred Middle Cretaceous age of the deeps is the first estimate obtained quantitatively, and corresponds well with available heat flow and multichannel reflection data.

Bombay gravity high and eruption of Deccan flood basalts (India) from a shallow secondary plume

The western margin of India, especially the area surrounding the gravity high of Bombay, is covered by a thick pile of Deccan flood basalts. The massive eruption is one of the largest continental volcanic events at the Cretaceous/Tertiary biological mass extinction boundary ( ∼ 65 Ma ago). The area is characterized by several hidden tectonic features, and their origin and evolution have remained controversial. In the search for magma source structure, attention was focused on an unusual oval-shaped positive gravity anomaly over the Bombay coastal zone. The gravity analysis indicates the presence of two subsurface structures at depths of 4.5 ± 0.5 km and 18 ± 2 km. These depths correspond to the top of the crustal basaltic layer and an upwarped Moho discontinuity, respectively. The positive gravity anomaly in three-dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer. The unusually shallow Moho layer coincides with the region of a broad high heat flow anomaly (average heat flow more than 80 mW/m 2). The geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km. High heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths. It is suggested that the highly differentiated magma associated with the major eruptive phase of Deccan Traps did not originate at the D′' layer. It is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust-mantle boundary, as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of India during the late Cretaceous and early Tertiary. Substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre-existing fracture zone near the continental margin. The presence of a magmatic conduit structure above the Moho, derived from the inversion of gravity data and other tectonic conditions, suggests a possible triggering of the magma reservoir by a large bolide impact ( ∼ 10 km diameter) at the Cretaceous/Tertiary boundary near Bombay.

The Origin of the Mesozoic Associated Multiseries Granitoid Magma in the Shandong Peninsula

Abstract Three melting events of the eartr's crust occurred during the period of 220‐120 Ma in the Shandong Peninsula. Three subcycles of granitoid magma including six rock series were generated in the faulted granitoid magma belt. The parent magma of several rock series formed earliest originated from the lower crust of granulite facies; following the increase of geothermal temperature the source magma would migrate into the middle crust of amphibolite facies! In the diapiric granitoid magma belt, the granitoid magma was formed first in granitic layer of the upper crust, and then in the middle crust. In each subcycle the generation of magma started with the generation of more mafic one and finished with low eutectic one; they were formed in the form of layered melting in a particular position of the crust.

Contents and distribution of carbon, sulfur and phosphorus in the lithospheric layers of China’s Continent

Based on the most recently available petrochemical data from China’s Continent and the thickness of the crust, the authors have calculated the contents of carbon, sulfur and phosphorus in the lithosphere of China’s Continent (including the sedimentary metamorphic structural layer, the granite layer and the basalt layer) and their distribution in the major types of rocks. In addition, a discussion is made on the significance of the results in the study of global circulation of carbon, sulfur and phosphorus.

Modeling of the geothermal history of the Okhotsk Sea lithosphere

Abstract A method of determination of evolution of the partial melting zone within the lithosphere using the observed heat flow data on the Earth's surface is presented. Modeling of the evolution of the partial melting zone within the Okhotsk Sea lithosphere is carried out. It is determined that the partial melting zone began developing 40–45 m.y. ago in the south of the Okhotsk Sea and 30–35 m.y. ago in the north. The location of the upper boundary of the partial melting zone at the initial stage of its development is mapped. The decrease with time in the thickness of the granitic (dioritic) layer above the partial melting zone is recorded.

Age and origin of granitic rocks in the kalgoorlie-norseman region of Western Australia: Implications for the origin of archaean crust

Three granites from the Norseman area (at Stennet Rocks, Buldania Rocks and east of Lake Johnston) have U-Pb zircon ages of 2691 ± 8 (95% C.L.), 2689 ± 22 and 2661 ± 8 Ma, respectively. One (Stennet Rocks) contains 3100–3200 Ma cores, providing support for Oversby's (1975) argument that all three plutons were derived by anatexis of much older basement. “Common” leads within the zircons show evidence of having been derived from within the rock, and represent mixtures of initial rock leads and radiogenic leads. The unusually 208Pb-rich compositions of the common leads suggest the breakdown of a Th-rich phase, perhaps uranothorite, as a major contributor to the radiogenic component. Available zircon, geochemical and isotopic data are used to develop a two-stage model for Archaean crustal development in this region, with formation of primitive crust of broadly andesitic composition from depleted mantle (or basalt) beginning at 3400 Ma, followed by extensive thermal reworking and possibly continued growth over the period 3000–3300 Ma. In the second stage, at 2700 Ma, anatexis resulted in large scale intra-crustal differentiation, giving a thick upper granitic layer and a lower crust residual from anatexis. It is this second major thermal event that is recorded by most geochronometers. Implicit in the andesitic crustal growth model is that because the Sm/Nd and Rb/Sr ratios of newly formed Archaean crust are close to mantle values, this process is transparent to the Sm-Nd and Rb-Sr isotope systems until recycling of incompatible-element-rich sediment becomes important following the Late Archaean production of a wide-spread differentiated upper crust.

Significance of past and recent heat-flow and radioactivity studies in the Southern Rocky Mountains region

As reported in The Geological Society of America Bulletin of 1950, Francis Birch's innovative heat-flow research in the Colorado Front Range introduced frequently used terrain correction and thermal-conductivity measurement methods. That report also presented the first empirical evidence for a positive correlation between above-normal flux and radioactive roots in an isostatically compensated mountain area with a thick crust, an observation that strongly influenced continental heat-flow studies for the next 18 years. Birch's Front Range study clearly showed that reliable continental heat-flow research requires knowledge of bedrock radioactivity. This concept reached an unprecedented level of acceptance in 1968, when linear relations between heat flow and radiogenic heat production were discovered for three contrasting provinces in the United States. Subsequently, such lines have been central to virtually all heat-flow and radioactivity research on land. Heat-flow (Q) data for 139 boreholes provide new detail on thermal regimes in the Southern Rocky Mountains and bordering areas, as do radiogenic heat-production (A) data for 60 locales. Interpretations also emphasize reduced and residual heat-flow values; reduced values correspond to the intercepts of regional Q-A lines, whereas a residual value is the flux at a locale after the probable effect of near-surface heat production has been subtracted. The observed Q-A lines demonstrate that the Front Range and other easterly frontal ranges of the Southern Rocky Mountains in Colorado and northern New Mexico are characterized by a reduced flux (54-58 mWm-2) that is dramatically higher than that (∼27 mWm-2) in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. Because the transitions between these provinces are narrow (≤50-60 km), sources in the upper crust must explain some of the contrasting reduced heat-flow values. In southeastern Wyoming, normal heat flow in Archean and early Proterozoic basement terranes probably reflects deep erosion that produced a thin (∼7 km) near-surface granitic layer that overlies a low-radioactivity lower crust. In the Colorado Front Range, Proterozoic, Mesozoic, and Cenozoic silicic rocks with relatively enriched radiogenic heat could comprise a 20- to 25-km-thick granitic layer in the upper crust that produces a large part of the above-normal reduced and residual flux. Here, partial melting of deep protolithic rocks in late Mesozoic and Cenozoic times could have produced a lower crust with low radiogenic heat production. By these views, Birch's heat flow-radioactive mountain root model of the Colorado Front Range is confirmed if a large part of the topography is isostatically compensated by low-density pre-Miocene crystal-line masses in the upper crust. Background reduced heat flow in Colorado parts of the Southern Rocky Mountains and the eastern Colorado Plateau is high (54-68 mWm-2). Zones of unusually high residual flux (88-118 mWm-2) occur in the Rio Grande rift zone in the environs of the Colorado mineral belt in the Leadville-northern Sawatch Range region, eastern parts of the San Juan Mountains in southern Colorado, and in the Park Range-Mountain Parks area near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crustmantle boundary, and the 50- to 60-km-wide borders of the Leadville-northern Sawatch Range residual heat-flow anomaly must be caused by sources in the upper crust. There-fore, young (10- to 1-m.y. -old) intrusions in a late Cenozoic rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the high residual flux in the Leadville-northern Sawatch Range area. Similar models may apply elsewhere in the northern rift zone. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system. Because very high regional flux in the northern rift zone and the San Juan Mountains areas implies above-liquidus equilibrium temperatures in the lower crust and upper mantle, serious inconsistencies arise when steady-state thermal models are examined relative to volumes of late Tertiary volcanism, uniform crustal thickness, and the absence of unusually low P-wave velocities in the mantle in the Southern Rocky Mountains.

Origin of rhythmic layering in the Calamity Peak satellite pluton of the Harney Peak Granite, South Dakota: The role of boron

The Calamity Peak satellite granite pluton on the flanks of the Harney Peak Granite, South Dakota, displays prominent mineral and textural layering, the latter defined by alternating fine-grained and pegmatitic granite. Superimposed on the fine-grained granite layers is a fine-scale, 0.1–5 cm, rhythmic layering defined by modal differences in tourmaline, microcline, albite and muscovite. There is very little variation in the major element mineral compositions in the whole complex. Analyses by Prompt-Gamma Neutron Activation gave a B 2O 3 content of 0.07% in the light layers (albite, quartz, and muscovite) and 0.65% in the dark layers (tourmaline, microcline, quartz, and albite); these contents are essentially proportional to tourmaline abundance in each layer. The textural and mineralogic layering is the result of the interplay of tourmaline crystallization and shifts in phase equilibria due to changes in B concentration in the melt. Initial cotectic crystallization of albite, quartz and muscovite increased the activity of Fe and Mg in the adjacent melt to levels sufficient for tourmaline to form. Tourmaline crystallization depleted B and decreased the solubility of H 2O in the melt, causing exsolution of H 2O from the melt and further removal of B due to high fluid melt partition coefficient. This process also resulted in the precipitation of K-feldspar, partly at the expense of muscovite and partly due to a slight increase in the solidus temperature. Once the crystallization front moved to an Fe and Mg-poor region of the melt, tourmaline precipitation ceased and the equilibrium shifted back toward quartz, muscovite, and albite. The pegmatitic layers, some of which cut across previously formed rhythmic layers and commonly have tourmaline along their borders, appear to have crystallized from melt enriched in the exsolved fluid phase.

関東平野の地殻構造とテクトニクス

Studies of the crust and upper mantle structure in and around the Kanto Plain was reviewed. It has been revealed that the crust under the Kanto Plain has no distinct granitic layer and that the crust and the upper mantle under the southern half of the Kanto Plain is the Philippine Sea Plate which is subducting under the North American Plate. It is supposed that the anomalous crustal structure, i.e., lack of the granitic layer, is a cause of low activity of shallow earthquake under the Kanto Plain. The tearing of the Philippine Sea Plate under the Tokyo Bay is considered to be the cause of the occurrence of destructive earthquakes in the metropolitan Tokyo.

Dynamics of Reproduction by Hatchery Lake Trout on a Man - Made Spawning Reef

Reproduction by hatchery lake trout, critical for rehabilitation of lake trout stocks in the Great Lakes, had not been previously described and measured. Reproduction by hatchery lake trout on a man-made spawning reef in Presque Isle Harbor, Lake Superior, in 1977-80 was qualitatively and quantitatively described using gill nets, egg traps, and fry traps. Scuba divers measured physical parameters of the reef. Lake trout spawned during a 15- to 28-day period between 12 October and 14 November mainly during 1800–2000 hours. The Petersen single census was a better method of estimating adults than either multiple-census or fecundity-egg deposition methods. The Petersen estimate of adults was nearly 4,000 males and 1,900 females in 1979. Egg deposition and swim-up fry production ranged from 122 to 518/m 2 and 20 to 46/m 2, respectively. Substrate on the man-made reef was a 27- to 42-cm thick layer of granite and limestone cobbles 6 to 20 cm in diameter. Spawning behavior and quantitative aspects of reproduction by hatchery lake trout were similar to that previously reported for native lake trout in the Great lakes and elsewhere. Man-made reefs may be a valuable lake trout management tool.