All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
44.
Dissociation properties of natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well by X-ray computerized tomography (CT) experiments
Bulletin of the Geological Survey of Canada
(544)
269-279
1999
ISSN: 00687626Abstract: ▾ Core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well contained a variety of forms of gas hydrate within sands and granular sands in the interval from 896 to 926 m. A number of these samples were placed inside pressure vessels charged with nitrogen gas and subsequently transported to Japan for specialized dissociation experiments. X-Ray CT images were acquired, at constant intervals, from a granular sand (collected from 913.76 m) containing pore-space gas hydrate during warming from -35 to 4°C, and subsequently during stepped decreases in cell pressures (0.1 MPa) below assumed threshold stability conditions. Dissociated gas flow and sample temperatures were monitored continuously. Changes in CT values indicated that gas hydrate dissociated simultaneously, both on the exposed surfaces and within the pore spaces of the sample, in response to pressure changes. This suggested that pressure reductions were effectively transmitted through the sample, most likely because the samples were not fully saturated with gas hydrate. Gas-flow measurements indicated that a larger pressure drawdown caused a higher dissociation rate.
43.
Drilling operations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
19-30
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, located in the Mackenzie Delta, Northwest Territories, Canada, was completed to 1150 m on March 30, 1998, after 39 days. Operations were undertaken through a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada. Research goals included evaluation of engineering technologies used to drill and core gas-hydrate-bearing strata. Eight coring runs were conducted within the permafrost interval (0-640 m) in a surface hole drilled to 687 m. Subsequently, a 340 mm surface casing was installed and the main hole was advanced to a depth of 1150 m with 16 coring runs. A cooled (∼2°C) KCl/polymer drilling mud and Drilltreat, a chemical mud additive, successfully stabilized gas hydrate within cores and formation sediments. No serious hole problems, accidents, or mishaps occurred; however, delays caused by adverse weather and mechanical problems caused cancellation of planned production testing. Coring in the main hole was successful, allowing the evaluation of four different core barrels. Gas-hydrate-bearing cores were collected in a variety of sediments between 896 and 952 m.
42.
Estimating in situ gas hydrate saturation from core temperature observations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
101-108
1999
ISSN: 00687626Abstract: ▾ During drilling of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, core temperatures were measured immediately upon recovery in the core-logging trailer. Gas-hydrate-bearing cores were typically frozen, with temperatures as much as 6°C lower than cores containing no gas hydrate. This temperature depression is attributed to the endothermic dissociation of gas hydrate during uphole tripping, and can be used to estimate minimum in situ gas hydrate saturation. Numerical modelling of heat exchange between core and circulating mud during tripping demonstrates that cores cool to mud temperature before leaving the methane hydrate P-T stability field. Simple arguments support the hypothesis that the endothermic heat of gas hydrate dissociation is supplied largely by the release of latent heat during coincident freezing of pore waters. Assuming minimal heat exchange with circulating mud, energy-balance calculations yield estimates of the quantity of gas hydrate lost to dissociation during recovery. These estimates are comparable to the in situ gas hydrate concentrations inferred from downhole geophysical logs.
41.
Gas composition and isotopic geochemistry of cuttings, core, and gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
143-163
1999
ISSN: 00687626Abstract: ▾ Molecular and isotopic composition of gases from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well demonstrate that the in situ gases can be divided into three zones composed of mixtures of microbial and thermogenic gases. Sediments penetrated by the well are thermally immature; thus the sediments are probably not a source of thermogenic gas. Thermogenic gas likely migrated from depths below 5000 m. Higher concentrations of gas within and beneath the gas hydrate zone suggest that gas hydrate is a partial barrier to gas migration. Gas hydrate accumulations occur wholly within zone 3, below the base of permafrost. The gas in gas hydrate resembles, in part, the thermogenic gas in surrounding sediments and gas desorbed from lignite. Gas hydrate composition implies that the primary gas hydrate form is Structure I. However, Structure II stabilizing gases are more concentrated and isotopically partitioned in gas hydrate relative to the sediment hosting the gas hydrate, implying that Structure II gas hydrate may be present in small quantities.
40.
Amount of gas hydrate estimated from compressional- and shear-wave velocities at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
313-322
1999
ISSN: 00687626Abstract: ▾ The amount of in situ gas hydrate concentrated in the sediment pore space at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was estimated by using compressional-wave (P-wave) and shear-wave (S-wave) downhole log measurements. A weighted equation developed for relating the amount of gas hydrate concentrated in the pore space of unconsolidated sediments to the increase of seismic velocities was applied to the acoustic logs with porosities derived from the formation density log. A weight of 1.56 (W=1.56) and the exponent of 1 (n=1) provided consistent estimates of gas hydrate concentration from the S-wave and the P-wave logs. Gas hydrate concentration is as much as 80% in the pore spaces, and the average gas hydrate concentration within the gas-hydrate-bearing section from 897 m to 1110 m (excluding zones where there is no gas hydrate) was calculated at 39.0% when using P-wave data and 37.8% when using S-wave data.
39.
Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
81 – 94
1999
ISSN: 03770273Keywords:▾
Japan; historical perspective; lava dome; pyroclastic flow; volcanic eruption
Abstract: ▾ During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200-300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400-500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100-70 ka, 30-20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.
38.
Isotope constraints on the origin of pore waters and salinity in the permafrost and gas hydrate core intervals of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
177-188
1999
ISSN: 00687626Abstract: ▾ Pore waters have been extracted from sediments in the permafrost interval (110-176 m) and the gas hydrate interval (886-952 m) of the JAPEX/JNOC/GSC Mallik 2L-38 drill core and analyzed for δ18O,δ2H, and geochemistry. Pore waters from the permafrost interval have δ18O values of -19.5 ± 0.5‰ (upper permafrost) and -23.1 ± 1.0‰ (lower permafrost) indicating the likely origin to be local, contemporary meteoric waters infiltrating these sediments during a period of subaerial exposure. Pore waters in the subpermafrost gas hydrate zone are isotopically depleted from seawater values, with δ18O ranging between -14‰ and -8‰c. A weak correlation between δ18O and Cl- exists in the gas-hydrate-bearing sands, consistent with the combined effect of isotopic depletion during gas hydrate formation, and enrichment associated with gas hydrate decomposition. The upper silt and deeper clayey silt sections also retain a minor correlation between isotopes and Cl-, and show strong variability in both δ18O and Cl- with depth, suggesting a history of gas hydrate formation, decomposition, and fluid migration. The Cl--δ18O relationships demonstrate that the original pore waters are a mixture of seawater with greater than 50% meteoric water.
37.
Methane and carbon dioxide gas-generation kinetics, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
125-141
1999
ISSN: 00687626Abstract: ▾ Optimized, first-order, discrete Arrhenius kinetic parameters have been determined for the thermogenic generation of methane and carbon dioxide for several low-maturity, organic-rich core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Pyrolysis was carried out using a thermogravimetric analyzer heated at 10, 25, and 50°C/min. The specific products were detected using a directly coupled Fourier Transform Infrared spectrometer. Results indicated that at typical geological heating rates of 3°C/Ma, significant (about 10% of the total) thermogenic carbon dioxide was released at very low temperatures (<60°C) and would be coproduced with microbiologically mediated, diagenetic carbon dioxide. At the same geological heating rate, the first 10% of thermogenic methane was determined to have been released between about 110 and 140°C while significant methane generation from the kerogen continued beyond 250°C. The absolute kinetic parameters for methane indicate that below about 60°C essentially no thermogenic methane should be expected. Thus no in situ thermogenic methane should be expected in the Mallik 2L-38 well.
36.
Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure
Journal of Volcanology and Geothermal Research,
89
(1-4)
35 – 42
1999
ISSN: 03770273Keywords:▾
Japan; magmatism; phreatomagmatism; resistivity; volcanic eruption
Abstract: ▾ The resistivity structure of Unzen Volcano has been revealed by extensive magnetotelluric surveys since the first eruption on November 17, 1990. This structure comprises a highly resistive surface layer, a low-resistive second layer at several hundred meters depth, interpreted as a water-saturated layer, a resistive third layer, and a low-resistive fourth layer at 10 km depth, possibly related to the deep magmatic activity. The structure has influenced the volcanic activity of Unzen. This activity was characterized by a series of dramatic changes in eruption type: a minor phreatic eruption on November 17, 1990; phreatic eruptions after February 12, 1991, preceded by several weeks of volcanic tremor; phreatomagmatic eruptions after April 9, and dome effusion beginning May 19, 1991. This paper presents a hypothesis in which the top of the magma column rose about 20 m/day, reached the base of the water-saturated layer at the end of January, 1991, and approached the upper boundary of this layer on April 9. Thus, the temporal change of eruption type and associated phenomena are systematically explained by an interaction between magma and groundwater contained in the saturated layer.
35.
Long Valley Coring Project, Inyo County, California, 1998, preliminary stratigraphy and images of recovered core
US Geological Survey,
99-158378
1999
DOI:10.3133/ofr99158
34.
Landsat infrared analysis of fumarole activity at Unzen Volcano: Time-series comparison with gas and magma fluxes
Journal of Volcanology and Geothermal Research,
89
(1-4)
57 – 64
1999
ISSN: 03770273Keywords:▾
Japan; fumarole; Landsat thematic mapper; magma; volcanic eruption; volcanic gas
Abstract: ▾ We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992. We identify two discrete regions of the dome that were heated to high temperature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships between parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters derived from the radiance data closely follow those observed in the measured rate of magma effusion. The positive correlation observed between the fumarolically driven shortwave infrared flux and the magma discharge rate (r2 = 0.64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest that gas accumulation within the edifice did not occur, this conclusion agreeing with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO2 flux rates. A positive correlation (r2 = 0.56) was also found between the mean radiance of the pixels in the fumarolically heated region and the overall size of that region. This suggests a potential mechanism whereby, when gas pressure within the edifice increased, excess gas escaped through additional pathways to the surface as well through an increased flux at the main fumarolic vents.
33.
Geomagnetic field intensity at Hawaii for the last 420 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii
Journal of Geophysical Research: Solid Earth,
104
(B7)
15317-15338
1999
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
magnetic intensity; paleomagnetism; Quaternary, Hawaii; United States
Abstract: ▾ Four hundred twenty five new paleointensity (Thellier-Thellier) determinations (out of 545 analyzed samples) have been obtained from core HSDP, which penetrates about 1000 meters (208 flows) of the Mauna Loa and Mauna Kea volcanic series encompassing the last 420 kyr. Rock magnetic investigations identify pseudo-single-domain magnetite as the main magnetic mineral. Inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude. The inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events. The paleointensity record from the Mauna Loa sequence is not very detailed and does not allow precise comparison with other data in the 0-50 kyr interval. The record from the Mauna Kea sequence, on the contrary, is very detailed and documents relatively short-lived episodes of low and high field strength from 15 to 60 μT. The average virtual dipole moment (8.7 ± 3.0 1022 A.m2) is not significantly different from the value reported by Kono and Tanaka [1995] for the last 2.5 Myr. A comparison with other data from Hawaii and other geographical regions is described in detail. There are no drastic changes in paleointensity with the inclination anomaly, in agreement with previous results from Hawaii but in contrast with most published results which, however, consider data from polarity transition. Spectral analysis of a particularly detailed portion of the record, between 420 and 326 kyr, documents significant periodicities at 36, 8, 5, and 4 ka in the inclination record but not in the intensity record, suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity. Copyright 1999 by the American Geophysical Union.
32.
Juvenile volcanic glass erupted before the appearance of the 1991 lava dome, Unzen volcano, Kyushu, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
113 – 121
1999
ISSN: 03770273Keywords:▾
Japan; lava dome; magma chemistry; volcanic eruption; volcanic glass
Abstract: ▾ Fugen-dake, the main peak of Unzen volcano, began to erupt on 17 November 1990 after 198 years of dormancy. Following a period of steam-dominated tephra eruptions, a new lava dome appeared at the bottom of Jigokuato Crater in the summit region on 20 May 1991. Non-hydrated vesiculated glass was detected in the precursory tephra eruptions, beginning 12 February 1991. Determination of whether glass pyroclasts are hydrated or not appears to be an effective means of ascertaining whether the tephra contains juvenile material from ascending magma. The detection of juvenile pyroclasts at Fugen-dake more than 3 months before the appearance of the lava dome suggests that monitoring of early tephra emissions is an effective way to help forecast impending magmatic eruptions.
31.
Influences of grain size and salinity on pressure-temperature thresholds for methane hydrate stability in JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research-well sediments
Bulletin of the Geological Survey of Canada
(544)
229-240
1999
ISSN: 00687626Abstract: ▾ This paper summarizes laboratory determinations of the pressure-temperature (P-T) phase equilibrium conditions for methane hydrate stability in sediments recovered from the gas-hydrate-bearing interval at JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Three test samples consisted of quartz-rich sand with in situ pore-water salinities of 4 ppt (parts per thousand), 20 ppt, and 40 ppt. A fourth sample was dominated by silt, with a salinity of 31 ppt. Initially, methane hydrate was regrown in the sediments, followed by the determination of P-T stability thresholds between 0°C and 12°C. Comparisons with published data for methane hydrate stability in pure gas-water systems indicate no appreciable shift in P-T stability conditions in sand with salinity of 4 ppt, but suggest a progressively increasing shift towards the higher pressure, lower temperature region for sand samples with elevated salinity. Test results for the saline silt sample indicate an additional shift in the stability threshold attributed to the effect of the porous medium in fine-grained sediments.
30.
How did the foreland react? Yangtze foreland fold-and-thrust belt deformation related to exhumation of the Dabie Shan ultrahigh-pressure continental crust (eastern China)
Terra Nova,
11
(6)
266 – 272
1999
ISSN: 09544879
Publisher: Blackwell Publishing Ltd.
Keywords:▾
China; Dabie Shan; collision zone; crustal deformation; exhumation; fold and thrust belt; foreland basin; subduction; Triassic
Abstract: ▾ During the Triassic collision of the Yangtze and Sino-Korean cratons, the leading edge of the Yangtze crust subducted to mantle depths and was subsequently exhumed as a penetratively deformed, coherent slab capped by a normla shear zone. This geometry requires a reverse shear zone at the base of the slab, and we suggest that the Yangtze foreland fold-and-thrust belt constitutes this zone. Lower Triassic rocks of the eastern foreland record NW-SE compression as the oldest compressional stress field; onset of related deformation is indicated by Middle Triassic clastic sedimentation. Subsequent Jurassic stress fields show a clockwise change of compression directions. Based on nearly coeval onset and termination of deformation, and on a common clockwise change in the principal strain/stress directions, we propose that the foreland deformation was controlled by the extrusion of the ultra high-pressure slab. Widespread Cretaceous-Cenozoic reactivation occurred under regional extension to transtension, which characteristically shows a large-scale clockwise change of the principal extension directions during the Lower Cretaceous.
29.
Heat flow and geothermal field of the Baikal region
Geologiya i Geofizika,
40
(3)
287 – 303
1999
ISSN: 00167886Abstract: ▾ Data on the geotemperature field of the central part of the Baikal Rift Zone (BRZ) and adjacent regions of the Siberian Platform and Transbaikalia are summarized. About 800 measurements of heat flow were made here, 700 of them were carried out in the Baikal basin (shallow-depth measurements in the upper layer of sediments). The average heat flow through the Baikal bottom is 71±21 mW/m2. At some sites of the bottom extremely high flows (up to 6000-8000 mW/m2) confined to the local centers of discharge of underwater sources have been revealed. In the montane framing of the rift troughs heat flow is much less intense - no more than 15-40 mW/m2. In the south of the Siberian Platform and in Transbaikalia average heat flow is 40±9 and 52±11 mW/m2, respectively. This differentiated heat flow distribution in the BRZ is, probably, the result of active water exchange in intermount artesian basins, which leads to cooling of feeding zones (mountain ridges) by meteogenic waters and further warming-up of the sedimentary cover of these basins (discharge areas). Special studies in the North-Muya tunnel supported this hypothesis. The validity of numerous shallow-depth measurements of heat flow in Lake Baikal is corroborated by the temperatures measured in the first underwater boreholes BDP-93 and BDP-96. By the location of the lower boundary of the gas hydrate layer of the Baikal sediments, 500 estimates of heat flow in the Southern and Central basins were made. In accordance roith these estimates, average heat flow is 76±10 mW/m2, which also corroborates the above average heat flow value. Heat flow data were used to forecast deep temperatures by the geothermal method. The paper analyzes the forecast temperature distributions at depths of 10, 20, 30, and 40 km. The validity of a geothermal forecast for the rift troughs is corroborated by the results of thermobarometric study of mineral inclusions from deep-seated volcanites of East Sayan and the Vitim Plateau. These data also evidence that heat flow and deep temperatures within the BRZ are underestimated.
28.
Groundmass pargasite in the 1991-1995 dacite of Unzen volcano: Phase stability experiments and volcanological implications
Journal of Volcanology and Geothermal Research,
89
(1-4)
197 – 212
1999
ISSN: 03770273Keywords:▾
Japan; crystallization; dacite; pargasite; volcanic eruption
Abstract: ▾ Pargasite commonly occurs in the dacitic groundmass of the 1991-1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991-1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe-Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880 ± 30°C, suggesting that the groundmass pargasite crystallized at > 70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991-1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.
27.
Ground temperature change observed at Unzen Volcano associated with the 1990-1995 eruption
Journal of Volcanology and Geothermal Research,
89
(1-4)
65 – 71
1999
ISSN: 03770273Keywords:▾
Japan; prediction; surface temperature; volcanic eruption
Abstract: ▾ Ground temperature associated with eruptive activity of Unzen Volcano, southwestern Japan, was observed in a cave about 680 m west of the central vent. Based on the data during 1991-1996, eliminating the effects of seasonal change, the temperature rose to the highest level probably around the first half of 1992. Hereafter the temperature decreased gradually year by year at least up to the autumn of 1996, beyond the level recovered at the beginning of the lava extrusion in May 1991. Although it is not necessarily conclusive, the present observation suggests a possible rise in ground temperature prior to the commencement of the first phreatic eruption in November 1990. If so, a careful observation may help to predict future eruptions by detecting a precursory rise of the ground temperature in this volcano.
26.
Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: The 1990-1995 eruption at Unzendake volcano, Kyushu, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
23 – 34
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; GPS; ground motion; lava flow; magmatism; volcanic earthquake; volcanic eruption
Abstract: ▾ Global positioning system (GPS) measurements made around Unzendake volcano, Kyushu, Japan, since January 1991 have detected ground deformation caused by magma intrusion and lava discharge. In the intermittent phreatic and phreatomagmatic eruption stage, the ground was inflating. After growth of the lava dome and following frequent pyroclastic flows at Unzendake volcano, the ground began deflating. These ground deformations are explained by the inflation and deflation of a Mogi's source model (a point source model) located about 6 km west of the active crater at a depth of 11 km, at an aseismic region. The observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation. The horizontal displacements at the reference GPS station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other GPS baseline systems. These observations validate our estimates for the pressure source. Based on the relation between the deformation volume of the ground surface and the discharged volume of the lava, it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater. Magma is estimated to be supplied to the reservoir at an average rate of 1.1 x 105 m3/day; magma intrusion began in December 1989 at the latest and continued for 1.9 x 103 days.
25.
Long Valley Coring Project
Bulletin. Geothermal Resources Council,
27
(2)
43-46
1998
ISSN: 01607782
Publisher: Geothermal Resources Council, Davis, CA, United States
Abstract: ▾ In December 1997, the California Energy Commission (CEC) agreed to provide funding for Phase III continued drilling of the Long Valley Exploratory Well (LVEW) near Mammoth Lakes, CA, from its present depth. The CEC contribution of $1 million completes a funding package of $2 million from a variety of sources, which will allow the well to be cored continuously to a depth of between 11,500 and 12,500 feet. The core recovered from Phase III will be crucial to understanding the origin and history of the hydrothermal systems responsible for the filling of fractures in the basement rock. The borehole may penetrate the metamorphic roof of the large magmatic complex that has fed the volcanism responsible for the caldera and subsequent activity.
24.
Osmium-isotope variations in Hawaiian lavas: Evidence for recycled oceanic lithosphere in the Hawaiian plume
Earth and Planetary Science Letters,
164
(3-4)
483-496
1998
ISSN: 0012821XKeywords:▾
hot spot; isotopic composition; oceanic crust; osmium, Pacific Ocean
Abstract: ▾ Isotopic heterogeneity in Hawaiian shield lavas reflects the presence of two distinct recycled components in the Hawaiian plume, both from the same packet of recycled oceanic lithosphere. Radiogenic Os-isotopes and anomalously heavy oxygen-isotopes in Koolau lavas reflect melt generation from recycled oceanic crust plus pelagic sediment. In contrast, Kea lavas have unradiogenic Os-isotopes but anomalously light oxygen-isotopes. Oxygen-osmium-lead isotope correlations preclude generation of the Kea isotopic signature from asthenospheric upper mantle or the in situ lithospheric mantle or crust. Instead, melting of recycled, hydrothermally altered ultramafic lower crust or lithospheric mantle in the Hawaiian plume can produce Kea-type lavas. The preservation of both upper- and lower-crustal oxygen isotope signatures in plume-derived Hawaiian lavas indicates that chemical heterogeneities with length scales of only a few kilometers can be preserved in the convecting mantle for long periods of time, probably on the order of 1 Ga or more.
23.
Scientific drilling continues in Long Valley Caldera, California
Eos, Transactions American Geophysical Union,
79
(36)
429-432
1998
Abstract: ▾ The Long Valley caldera region, located between the Sierra Nevada and the Basin and Range Province in eastern California, encompasses a large volcanic complex whose eruptive history began nearly 4 m.y.a. and continues to the present, with eruptions occurring, on average, every few hundred years. Eruptive activity in the area occurred as recently as 250 years ago with small eruptions from vents on Paoha Island in the middle of Mono Lake and 550–600 years ago from three vents at the southern end of the Inyo volcanic chain in the west moat of Long Valley caldera. The current unrest in the caldera began in 1980 and has included recurring earthquake swarms and uplift of the resurgent dome in the center of the caldera by over 70 cm [Bailey and Hill, 1990]. Long Valley caldera is one of several large calderas around the world that have shown similar signs of magmatic unrest in the last few decades.
22.
U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China
Earth and Planetary Science Letters,
161
(1-4)
215 – 230
1998
ISSN: 0012821X
Publisher: Elsevier
Keywords:▾
China; geochronology; Triassic; ultrahigh pressure metamorphism
Abstract: ▾ New SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accomodated ~100% N-S- stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ~219 to ~245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.
21.
Unconformity-like Reflection Pattern under the Moho in the Sulu Area
1997
20.
Crustal structure and development of Sulu UHPM Terrane in east-central China
Episodes,
20
(2)
100 – 103
1997
ISSN: 07053797
Publisher: International Union of Geological Sciences
Abstract: ▾ This article provides a brief view of development of the Sulu ultrahigh pressure metamorphic (UHPM) terrane involved in a conceivable intracontinental collision between the Sion-Korean Plate and the Yangtze Plate during the Mesozoic Era. Integrated study of geological and geophysical data reveals crustal structures in the Sulu terrane, and provides evidence for an evolution model of the UHPM terrane called "the dual intracontinental subduction model".