Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugen-dake, Unzen Volcano, Japan - Watanabe, Kazunori; Ono, Koji; Sakaguchi, Keiichi; Takada, Akira; Hoshizumi, Hideo (Journal of Volcanology and Geothermal Research, 1999)
Fugen-dake, the main peak of Unzen Volcano, began a new eruption sequence on November 17, 1990. On May 20, 1991, a new lava dome appeared near the eastern edge of the Fugen-dake summit. Small-scale, 104-106 m3 in volume, Merapi-type block and ash flows were frequently generated from the growing lava dome during May-June, 1991. These pyroclastic flows were accompanied by co-ignimbrite ash plumes that deposited ash-fall deposits downwind of the volcano. Three examples of co-ignimbrite ash-fall deposits from Unzen pyroclastic flows are described. The volume of fall deposits was estimated to be about 30% by volume of the collapsed portions of the dome that formed pyroclastic flows. This proportion is smaller than that described for other larger co-ignimbrite ash-fall deposits from other volcanoes. Grain size distributions of the Unzen co-ignimbrite ash-fall deposits are bi-modal or tri-modal. Most ashes are finer than 4 phi and two modes were observed at around 4-7 phi and 9 phi. They are composed mainly of groundmass fragments. Fractions of another mode at around 2 phi are rich in crystals derived from dome lava. Some of the fine ash component fell as accretionary lapilli from the co-ignimbrite ash cloud indicating either moisture or electrostatic aggregation. We believe that the co-ignimbrite ash of Unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope. These ashes were entrained into the convective plumes generated off the tops of the moving flows.
Keywords: Japan; ash flow; ignimbrite; pyroclastic flow; volcanic ash; volcanic eruption

Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan - Yamashina, Ken'Ichiro; Shimizu, Hiroshi (Journal of Volcanology and Geothermal Research, 1999)
Tilt observations at FG1 station located about 680 m west of the central crater of Unzen volcano, southwestern Japan, revealed details about the process of magma ascent during the mid-May 1991 crisis, just before the appearance of a lava dome. Crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike, modeled by a vertical line source and a planer tensile dislocation in an elastic half space. Marked change in tilt was observed at first in the EW direction at FG1 during May 11-14, suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater. After the interruption of the rise of the magma column, magma began to intrude laterally as a dike on May 15 toward approximately the N80°W direction. This resulted in a large tilt change essentially in the NS component at FG1. The horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m. On May 17, the magma column started to rise again, probably because the impediment to magma ascent was fractured by the growth of the dike. The top of the magma column finally reached to the crater bottom probably on May 19, and a new lava dome was actually observed on May 20. After which, the remarkable upward tilt to the south at FG1 due to the dike intrusion decreased rapidly. The deformation after May 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of May 1991. Although large errors are inevitable in estimated values because of a lack of sufficient data, the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions.
Keywords: Japan; crustal deformation; dacite; lava dome; tilt; volcanic eruption

Detailed evaluation of gas hydrate reservoir properties using JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well downhole well-log displays - Collett, T.S. (Bulletin of the Geological Survey of Canada, 1999)
The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well project was designed to investigate the occurrence of in situ natural gas hydrate in the Mallik area of the Mackenzie Delta of Canada. Because gas hydrate is unstable at surface pressure and temperature conditions, a major emphasis was placed on the downhole logging program to determine the in situ physical properties of the gas-hydrate-bearing sediments. Downhole logging tool strings deployed in the Mallik 2L-38 well included the Schlumberger Platform Express with a high resolution laterolog, Array Induction Imager Tool, Dipole Shear Sonic Imager, and a Fullbore Formation Microlmager. The downhole log data obtained from the log- and core-inferred gas-hydrate-bearing sedimentary interval (897.25-1109.5 m log depth) in the Mallik 2L-38 well is depicted in a series of well displays. Also shown are numerous reservoir parameters, including gas hydrate saturation and sediment porosity log traces, calculated from available downhole well-log and core data. The gas hydrate accumulation delineated by the Mallik 2L-38 well has been determined to contain as much as 4.15109 m3 of gas in the 1 km2 area surrounding the drill site.
cited By 55

Dinoflagellate cysts from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Kurita, H. (Bulletin of the Geological Survey of Canada, 1999)
Sparse occurrences of organic-walled dinoflagellate cysts were recorded from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Mackenzie Delta area, Northwest Territories, Canada. Some of the recorded taxa are indicative of a Paleocene-Eocene age, while others are considered to be of Cretaceous origin. Because the deepest parts of the section are correlated to the Oligocene Kugmallit Sequence, all the dinoflagellate cysts are interpreted to be reworked. According to this interpretation, the total absence of in situ marine dinoflagellate cysts suggests that the studied samples were deposited under nonmarine conditions, most likely within a fluvial system.
cited By 4
Keywords: Dinophyceae

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 - Uchida, T. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 1

Drilling operations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Ohara, T. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 5

Estimating in situ gas hydrate saturation from core temperature observations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Wright, J.F. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 8

Gas composition and isotopic geochemistry of cuttings, core, and gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Lorenson, T.D. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 33

Amount of gas hydrate estimated from compressional- and shear-wave velocities at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Lee, M.W. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 30

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan - Hoshizumi, Hideo; Uto, Kozo; Watanabe, Kazunori (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; historical perspective; lava dome; pyroclastic flow; volcanic eruption

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 - Clark, I.D. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 0

Methane and carbon dioxide gas-generation kinetics, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Snowdon, L.R. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 3

Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure - Kagiyama, T.; Utada, H.; Yamamoto, T. (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; magmatism; phreatomagmatism; resistivity; volcanic eruption

Long Valley Coring Project, Inyo County, California, 1998, preliminary stratigraphy and images of recovered core - Sackett, Penelope C; McConnell, Vicki S; Roach, Angela L; Priest, Susan S; Sass, John H (US Geological Survey, 1999)

Landsat infrared analysis of fumarole activity at Unzen Volcano: Time-series comparison with gas and magma fluxes - Kaneko, Takayuki; Wooster, Martin J. (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; fumarole; Landsat thematic mapper; magma; volcanic eruption; volcanic gas

Geomagnetic field intensity at Hawaii for the last 420 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii - Laj, C.; Kissel, C. (Journal of Geophysical Research: Solid Earth, 1999)
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.
cited By 38
Keywords: magnetic intensity; paleomagnetism; Quaternary; Hawaii; United States

Juvenile volcanic glass erupted before the appearance of the 1991 lava dome, Unzen volcano, Kyushu, Japan - Watanabe, Koichiro; Danhara, Tohru; Watanabe, Kazunori; Terai, Kunihisa; Yamashita, Tohru (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; lava dome; magma chemistry; volcanic eruption; volcanic glass

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 - Wright, J.F. (Bulletin of the Geological Survey of Canada, 1999)
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.
cited By 29

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) - Schmid, J.C.; Ratschbacher, L.; Hacker, B.R.; Gaitzsch, I.; Dong, S. (Terra Nova, 1999)
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.
Keywords: China; Dabie Shan; collision zone; crustal deformation; exhumation; fold and thrust belt; foreland basin; subduction; Triassic

Heat flow and geothermal field of the Baikal region - Duchkov, A.D.; Lysak, S.V.; Golubev, V.A.; Dorofeeva, R.P.; Sokolova, L.S. (Geologiya i Geofizika, 1999)
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.

Groundmass pargasite in the 1991-1995 dacite of Unzen volcano: Phase stability experiments and volcanological implications - Sato, Hiroaki; Nakada, Setsuya; Fujii, Toshitsugu; Nakamura, Michihiko; Suzuki-Kamata, Keiko (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; crystallization; dacite; pargasite; volcanic eruption

Ground temperature change observed at Unzen Volcano associated with the 1990-1995 eruption - Yamashina, Ken'Ichiro; Matsushima, Takeshi (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; prediction; surface temperature; volcanic eruption

Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: The 1990-1995 eruption at Unzendake volcano, Kyushu, Japan - Nishi, K.; Ono, H.; Mori, H. (Journal of Volcanology and Geothermal Research, 1999)
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.
Keywords: Japan; crustal deformation; GPS; ground motion; lava flow; magmatism; volcanic earthquake; volcanic eruption

Long Valley Coring Project - Sass, John; Finger, John; McConnel, Vicki (Bulletin. Geothermal Resources Council, 1998)
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.
cited By 0
Keywords: Costs; Exploratory boreholes; Geothermal wells; Project management; Exploratory wells; Core drilling

Osmium-isotope variations in Hawaiian lavas: Evidence for recycled oceanic lithosphere in the Hawaiian plume - Lassiter, J.C.; Hauri, E.H. (Earth and Planetary Science Letters, 1998)
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.
cited By 341
Keywords: hot spot; isotopic composition; oceanic crust; osmium; Pacific Ocean

Scientific drilling continues in Long Valley Caldera, California - Hill, David P.; Sorey, Michael L.; Ellsworth, William L.; Sass, John (Eos, Transactions American Geophysical Union, 1998)
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.

U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China - Hacker, Bradley R.; Ratschbacher, Lothar; Webb, Laura; Ireland, Trevor; Walker, Doug; Shuwen, Dong (Earth and Planetary Science Letters, 1998)
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.
Keywords: China; geochronology; Triassic; ultrahigh pressure metamorphism

Unconformity-like Reflection Pattern under the Moho in the Sulu Area - Wencai, Yang (1997)

Crustal structure and development of Sulu UHPM Terrane in east-central China - Yang, Wencai (Episodes, 1997)
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.

Preliminary results of the first scientific drilling on Lake Baikal, Buguldeika site, southeastern Siberia - Colman, S.; Grachev, M.; Hearn, P.; Horie, S.; Kawai, T.; Logachov, N.; Antipin, V.; Fialkov, V.; Gorigljad, A.; Tomilov, B.; Khakhaev, B.; Kochikov, S.; et al. (Quaternary International, 1997)
The Baikal Drilling Project (BDP) is a multinational effort to investigate the paleoclimatic history and tectonic evolution of the Baikal sedimentary basin during the Late Neogene. In March 1993 the Baikal drilling system was successfuly deployed from a barge frozen into position over a topographic high, termed the Buguldeika saddle, in the southern basin of Lake Baikal. The BDP-93 scientific team, made up of Russian, American and Japanese scientists, successfully recovered the first long (>100 m) hydraulic piston cores from two holes in 354 m of water. High quality cores of 98 m (Hole 1) and 102 m (Hole 2), representing sedimentation over the last 500,000 years, were collected in 78 mm diameter plastic liners with an average recovery of 72% and 90%, respectively. Magnetic susceptibility logging reveals an excellent hole-to-hole correlation. In this report the scientific team describes the preliminary analytical results from BDP-93 hole 1 cores. Radiocarbon dating by accelerator mass spectrometry provides an accurate chronology for the upper portion of Hole 1. Detailed lithologic characteristics, rock magnetic properties and inorganic element distributions show a significant change to the depositional environment occuring at 50 m subbottom depth, approximately 250,000 BP. This change may be due to uplift and rotation of the horst block in the Buguldeika saddle. The sedimentary section above 50 m is pelitic with varve-like laminae, whereas the section below 50 m contains a high proportion of sand and gravel horizons often organized into turbidite sequences. Accordingly, high resolution seismic records reveal a change in sonic velocity at this depth. It is inferred that sedimentation prior to 250 ka BP was from the west via the Buguldeika river system. After 250 ka BP the Buguldeika saddle reflects an increase in hemipelagic sediments admixed with fine-grained material from the Selenga River drainage basin, east of Lake Baikal. Variations in the spore-pollen assemblage, diatoms, biogenic silica content, rock magnetic properties, clay mineralogy and organic carbon in the upper 50 m of BDP-93-1 reveal a detailed record of climate change over approximately the last 250,000 years. These variables alternate in a pattern characteristic of glacial/interglacial climatic fluctuations. The present age model suggests that the climate signal recorded in Lake Baikal sediments is similar to Late Quaternary signals recorded in Chinese loess sections and in marine sediments. Copyright © 1996 INQUA/ Elsevier Science Ltd.
Keywords: Russian Federation; Siberia; Lake Baikal; Buguldeika Saddle; borehole logging; palaeoclimate; sedimentation; tectonics

Continuous paleoclimate record recovered for last 5 million years - Members, Baikal Drilling Project BDP-96 (Leg II) (Eos, Transactions American Geophysical Union, 1997)
From January to March 1996, an ice-based drilling platform (Figure 1) was positioned in 321 m of water above a submerged topographic high known as the Academician Ridge of Lake Baikal, south-central Siberia (Figure 2). The Russian Nedra drilling team expertly used this system to recover the first sediment cores from this region to span the last 5 m.y. before present (Ma). This operation was complicated and dangerous: there were constantly shifting blocks of ice nearly 1 m thick, the location was surrounded by wilderness areas nearly 400 km from the nearest population center (Irkutsk), and supplies and personnel had to travel 90 km over the ice pressure ridges in 4-wheel motorcycles and small vehicles.

Hydrogeology of the Hawaii Scientific Drilling Project borehole KP-1 1. Hydraulic conditions adjacent to the well bore - Paillet, F.L.; Thomas, D.M. (Journal of Geophysical Research: Solid Earth, 1996)
Temperature and formation resistivity logs obtained in borehole KP-1 of the Hawaii Scientific Drilling Project indicate that the adjacent formation is characterized by several zones of distinctly different average temperature and water salinity. A series of hydraulic analyses and water sampling programs were conducted to rule out the possibility of local hydraulic effects associated with the presence of the borehole in the generation of these apparent groundwater zones. Hydraulic tests and sampling with the borehole cased to a depth of 710 m and open below that depth indicate that the deep aquifer contains seawater at a temperature nearly identical to that of the open ocean at the same depth. Various analyses give estimates of aquifer transmissivity of about 10-3 m2/s in the vicinity of the borehole. Isolation of this deeper aquifer from the overlying groundwater zones was investigated by perforating the casing at six locations and then measuring the changes in water level in the borehole, in the salinity of the fluid column, in the temperature profile of the fluid column, and in the rate of flow in the fluid column induced by the perforations. These results positively confirm that the zones of distinctly different formation properties indicated on the temperature and resistivity logs are not caused by flow in or around casing. Flow and fluid column salinity induced by the perforations also confirm significant differences between the hydraulic heads and geochemistry of the different groundwater zones inferred from the well logs.
cited By 7
Keywords: aquifer; borehole logging; groundwater; hydrogeology; salinity; temperature; Hawaii; USA

The 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, Hawaii Scientific Drilling Project - Sharp, W.D.; Turrin, B.D.; Renne, P.R.; Lanphere, M.A. (Journal of Geophysical Research: Solid Earth, 1996)
Mauna Kea lava flows cored in the HilIo hole range in age from <200 ka to about 400 ka based on 40Ar/39Ar incremental heating and K-Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar, and redistribution of 39Ar by recoil. Comparison of K-Ar ages with 40Ar/39Ar integrated ages indicates that some of these samples have also lost 39Ar. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has an mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4, and 199 ± 9 ka for depths of -415.7 m to -299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at -280 m must be older than the plateau age of 132 ± 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.
cited By 88
Keywords: accumulation rate; alkali basalt; argon-40/argon-39 dating; lava flow; petrology; potassium/argon dating; recurrence interval; tholeiite; USA; Hawaii; Hawaii

Mauna Loa lava accumulation rates at the Hilo drill site: Formation of lava deltas during a period of declining overall volcanic growth - Lipman, P.W.; Moore, J.G. (Journal of Geophysical Research: Solid Earth, 1996)
Accumulation rates for lava flows erupted from Mauna Loa, as sampled in the uppermost 280 m of the Hilo drill hole, vary widely for short time intervals (several thousand years), but overall are broadly similar to those documented elsewhere on this volcano since 100 ka. Thickness variations and accumulation rates for Mauna Loa lavas at the Hilo drill site have been strongly affected by local paleotopography, including funneling and ponding between Mauna Kea and Kilauea. In addition, gentle submerged slopes of Mauna Kea in Hilo Bay have permitted large shoreline displacements by Mauna Loa flows. Ages of eruptive intervals have been determined from published isotopic data and from eustatic sea level curves modified to include the isostatic subsidence of the island of Hawaii at 2.2-2.6 mm/yr. Prior to 10 ka, rates of Mauna Loa lava accumulation at the drill site varied from 0.6 to 4.3 mm/yr for dateable intervals, with an overall rate of 1.8 mm/yr. Major eruptive pulses at about 1.3 and 10 ka, each probably representing a single long-lived eruption based on lack of weathering between flow units, increase the overall accumulation rate to 2.4 mm/yr. The higher rate since 10 ka reflects construction of thick near-shoreline lava deltas as postglacial sea levels rose rapidly. Large lava deltas form only along coastal segments where initially subaerial slopes have been submerged by the combined effects of eustatic sea level rise, isostatic subsidence, or spreading of volcano flanks. Overall accumulation of 239 m of lava at the drill site since 100-120 ka closely balances submergence of the Hilo area, suggesting that processes of coastal lava deposition have been modulated by rise in sea level. The Hilo accumulation rate is slightly higher than average rates of 1-2 mm/yr determined elsewhere along the Mauna Loa coast, based on rates of shoreline coverage and dated sea cliff and fault scarp exposures. Low rates of coastal lava accumulation since 100 ka, near or below the rate of island-wide isostatic subsidence, indicate that Mauna Loa is no longer growing vigorously or even maintaining its size above sea level.
cited By 33
Keywords: accumulation rate; coastal environment; lava delta; temporal variation; volcanic eruption; USA; Hawaii; Hawaii

Introduction to special section: Hawaii Scientific Drilling Project - Stolper, E.M.; DePaolo, D.J.; Thomas, D.M. (Journal of Geophysical Research: Solid Earth, 1996)
cited By 44

Hydrogeology of the Hawaii Scientific Drilling Project borehole KP-1 2. Groundwater geochemistry and regional flow patterns - Thomas, D.M.; Paillet, F.L.; Conrad, M.E. (Journal of Geophysical Research: Solid Earth, 1996)
A series of downhole and surface water samples were taken from the 1-km-deep KP-1 borehole located on the eastern flank of the island of Hawaii. Early samples from depths of more than 700 m showed salinities nearly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and residual drilling mud clays. Later deep samples found only minor variations from seawater cation chemistry that are consistent with low-temperature weathering of basalts; δ18O values are equivalent to seawater values and are consistent with this interpretation. Carbon 14 activities of dissolved inorganic carbonate indicate a water age ranging from 5890 to 7170 years B.P. and fluid transport rates of 1.8 to 2.2 m/yr. Fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the Mauna Kea/Mauna Loa interface; borehole resistivity logs indicate that it is ∼200 m thick. Although it has not yet been possible to obtain samples of the freshwater zone without contamination from the deep saline fluids, the chloride concentrations of the low-salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg/L. Light stable isotope data indicate that the fresh water at 320 m is derived from recharge entering the island at an average elevation of 2000 m. Inferred 14C activities of the dissolved bicarbonate in the freshwater zone indicate an average calibrated age of 2200 years B.P. and an average fluid velocity of at least 14 m/yr. A regional water flow model is proposed that suggests that the fresh water found at the 320-m depth is derived from rainfall recharge from the middle elevations of Mauna Kea volcano. This rainfall is channeled beneath the Mauna Loa lavas by the thick soil layer separating the two volcanoes. A second shallow fresh-to-brackish water zone, derived from Mauna Loa recharge, is also inferred to exist below the carbonate formation that underlies the shallow basal lens. The results of our preliminary study of the groundwater system below the KP-1 drill site demonstrate that intervolcano and interflow aquicludes can have a substantial impact on water circulation and discharge from young island volcanoes.
cited By 29
Keywords: fluid flow; freshwater; geochemistry; groundwater; hydrogeology; salinity; USA; Hawaii; Hawaii

Hawaii scientific drilling protect: Summary of preliminary results - DePaolo, D.; Stolper, E.; Thomas, D.; Albarède, F.; Chadwick, O.; Clague, D.; Feigenson, M.; Frey, F.; Garcia, M.; Hofmann, A.; Ingram, B.L.; Kennedy, B.M.; et al. (GSA Today, 1996)
Petrological, geochemical, geomagnetic, and volcanological characterization of the recovered core from a 1056-m-deep well into the flank of the Mauna Kea volcano in Hilo, Hawaii, and downhole logging and fluid sampling have provided a unique view of the evolution and internal structure of a major oceanic volcano unavailable from surface exposures. Core recovery was ∼90%, yielding a time series of fresh, subaerial lavas extending back to ∼400 ka. Results of this 1993 project provide a basis for a more ambitious project to core drill a well 4.5 km deep in a nearby location with the goal of recovering an extended, high-density stratigraphic sequence of lavas.
cited By 14
Keywords: geochemistry; geomagnetic field; hotspot; petrology; volcanism; volcano; USA; Hawaii; Hawaii; Mauna Kea

Geochemical stratigraphy of lava flows sampled by the Hawaii Scientific Drilling Project - Rhodes, J.M. (Journal of Geophysical Research: Solid Earth, 1996)
Geochemical discriminants are used to place the boundary between Mauna Loa flows and underlying Mauna Kea flows at a depth of about 280 m. At a given MgO content the Mauna Kea flows are lower in SiO2 and total iron and higher in total alkali, TiO2, and incompatible elements than the Mauna Loa lavas. The uppermost Mauna Kea lavas (280 to 340 m) contain alkali basalts interlayered with tholeiites and correlate with the postshield Hamakua Volcanics. In addition to total alkalis, the alkali basalts have higher TiO2, P2O5, Sr, Ba, Ce, La, Zr, Nb, Y, and V relative to the tholeiites and lower Zr/Nb and Sr/Nb ratios. Some of the alkali basalts are extensively differentiated. Below 340 m all the flows are tholeiitic, with compositions broadly similar to the few fresh subaerial shield-building Mauna Kea tholeiites studied to date. High-MgO lavas are unusually abundant, although there is a wide range (7-28%) in MgO content reflecting olivine control. FeO/MgO relationships are used to infer parental picritic magmas with about 15 wt % MgO. Lavas with more MgO than this have accumulated olivine. The Mauna Loa lavas have compositional trends that are controlled by olivine crystallization and accumulation. They compare closely with trends for historical (1843-1984) flows, tending toward the depleted end of the spectrum. They are, though, much more MgO-rich (9-30%) than is typical for most historical and young ( <30 ka) prehistoric lavas. The unusual abundance of high-MgO and picritic lavas is attributed to the likelihood that only large-volume, hot, mobile flows will reach Hilo Bay from the northeast rift zone. FeO/MgO relationships are used to infer parental picritic magmas with about 17 wt % MgO. Again, lavas with more MgO than this have accumulated olivine. Systematic changes in incompatible element ratios are used to argue that the magma supply rate has diminished over time. On the other hand, the relatively constant Zr/Nb and Sr/Nb ratios that compare closely with historical and young ( <30 kyr) prehistoric flows are used to argue that the source components for these lavas in the Hawaiian plume have remained relatively uniform over the last 100 kyr.
cited By 141
Keywords: basalt; geochemistry; lava flow; petrogenesis; stratigraphy; USA; Hawaii; Hawaii

Evolution of Mauna Kea volcano: Inferences from lava compositions recovered in the Hawaii Scientific Drilling Project - Yang, H.-J.; Frey, F.A.; Rhodes, J.M.; Garcia, M.O. (Journal of Geophysical Research: Solid Earth, 1996)
The lower 776 m of core recovered during the initial phase of the Hawaii Scientific Drilling Project (HSDP) contains lavas erupted from Mauna Kea volcano. Tholeiitic and alkalic basalts, including an Fe-Ti rich flow, are intercalated in the upper 58 m of Mauna Kea lavas. Similar basaltic sections are subaerially exposed on the lower east flank of Mauna Kea. The Fe-Ti rich lavas reflect large amounts of clinopyroxene, plagioclase, and olivine fractionation within the crust and upper mantle, but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet-bearing source. Based on abundances of incompatible elements, the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas. All flow units in the lower 718 m of the HSDP core are tholeiitic lavas. Their variability in major element compositions reflect variable accumulation of olivine. Incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting. Within the tholeiitic part of the core, lavas from 800 m to 950 m formed by the largest extent of melting, whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting. Inferred melt compositions at 16% MgO show that the ∼200 to 400 ka Mauna Kea lavas from the HSDP core and the <250 ka subaerial exposures define an inverse correlation between SiO2 and FeO contents. Based on experimental studies, this correlation is caused by differing pressures of melt segregation. Furthermore, abundances of Nb and SiO2 are also inversely correlated in these calculated melts. In general, the younger lavas are relatively enriched in FeO and incompatible elements but are depleted in SiO2. These trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age. There are, however, geochemical variations which indicate short-term reversals in this long-term trend. Previously, the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at Hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis. The long-term geochemical trends of tholeiitic lavas in the HSDP core also reflect migration of Mauna Kea away from the Hawaiian plume.
cited By 50
Keywords: basalt; geochemistry; lava flow; petrogenesis; petrology; temporal evolution; volcano; USA; Hawaii; Hawaii; Mauna Kea

Coral ages and island subsidence, Hilo drill hole - Moore, J.G.; Ingram, B.L.; Ludwig, K.R.; Clague, D.A. (Journal of Geophysical Research: Solid Earth, 1996)
A 25.8-m-thick sedimentary section containing coral fragments occurs directly below a surface lava flow (the ∼1340 year old Panaewa lava flow) at the Hilo drill hole. Ten coral samples from this section dated by accelerator mass spectrometry (AMS) radiocarbon and five by thermal infrared multispectral scanner (TIMS) 230Th/U methods show good agreement. The calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef. This sedimentary unit is underlain by a 34-m-thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 14C date of 10,340 years. The age-depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole or 2.5-2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age-depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30.9 m of lava of which 11 m was above sea level. This surface has now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the coastline but could extensively modify the offshore shelf.
cited By 43
Keywords: age determination; coral; lava flow; radiocarbon dating; subsidence; thorium/uranium dating; USA; Hawaii; Hawaii

Analysis of fractures intersecting Kahi Puka Well 1 and its relation to the growth of the island of Hawaii - Morin, R.H.; Paillet, F.L. (Journal of Geophysical Research: Solid Earth, 1996)
As part of the Hawaii Scientific Drilling Project, Kahi Puka Well 1 penetrated about 275 m of Mauna Loa basalts overlying a sequence of Mauna Kea flow units as it was drilled and cored to a total depth of 1053 m below land surface. A borehole televiewer (BHTV) was run in most of the well in successive stages prior to casing in order to obtain magnetically oriented acoustic images of the borehole wall. A total of 283 individual fractures were identified from this log and characterized in terms of strike and dip. These data are divided into three vertical sections based upon age and volcanic source, and lower hemisphere stereographic plots identify two predominant, subparallel fracture subsets common to each section. Assuming that most of the steeply dipping fractures observed in the BHTV log are tensile features generated within basalt flows during deposition and cooling, this fracture information can be combined with models of the evolution of the island of Hawaii to investigate the depositional history of these Mauna Loa and Mauna Kea basalts over the past 400 kyr. The directions of high-angle fractures appear to be generally parallel to topography or to the coastline at the time of deposition, as is supported by surface mapping of modern flows. Consequently, an overall counterclockwise rotation of about 75° in the strike of these fractures from the bottom to the top of the well represents a systematic change in depositional slope direction over time. We attribute the observed rotation in the orientations of the two predominant fracture subsets over the past 400 kyr to changes in the configurations of volcanic sources during shield building and to the structural interference of adjacent volcanoes that produces shifts in topographic patterns.
cited By 3
Keywords: borehole logging; borehole televiewer; fracture geometry; tensile fracture; volcano growth; USA; Hawaii; Hawaii

An 84-kyr paleomagnetic record from the sediments of Lake Baikal, Siberia - Peck, J.A.; King, J.W.; Colman, S.M.; Kravchinsky, V.A. (Journal of Geophysical Research: Solid Earth, 1996)
We have conducted a paleomagnetic study of sediment cores obtained from the Selenga prodelta region of Lake Baikal, Russia. This record, which spans approximately the last 84 kyr, contributes to a better understanding of the nature of geomagnetic field behavior in Siberia and is a useful correlation and dating tool. We demonstrate that the Lake Baikal sediments are recording variations in the geomagnetic field. The directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large-amplitude secular variation at 41, 61, and 67 ka. Smoothing of the geomagnetic excursion behavior occurs in Lake Baikal sediments owing to the intermediate sedimentation rate (13 cm kyr-1). The Lake Baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the Mediterranean Sea and Indian Ocean for the last 84 kyr. This correlation suggests a strong global (i.e., dipole) component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity. We show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records. The geomagnetic intensity stratigraphy helps constrain the age of the difficult to date Lake Baikal sediments.
Keywords: Russian Federation; Lake Baikal; geomagnetic field; palaeointensity; palaeomagnetism; Quaternary; secular variation

Continental climate response to orbital forcing from biogenic silica records in lake baikal - Colman, S.M.; Peck, J.A.; Karabanov, E.B.; Carter, S.J.; Bradbury, J.P.; King, J.W.; Williams, D.F. (Nature, 1995)
CHANGES in insolation caused by periodic changes in the Earth's orbital parameters provide the primary forcing for global ice ages1 –6. But it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets. Here we investigate this question using the record of biogenic silica in Lake Baikal as a proxy for climate change in this high-latitude mid-continental region. We find a good correlation between this record and that of marine oxygen isotopes4. Over the past 250 kyr the Baikal record exhibits both a strongly nonlinear component (manifested in a 100-kyr periodicity) and weaker direct-insolation components (manifested in the 41-kyr (obliquity) and 23- and 19-kyr (precession) orbital cycles). These results show that even though extreme continental climates such as this are influenced directly by insolation variations, they are dominated by the nonlinear rhythm of the oceans and ice sheets. © 1995 Nature Publishing Group.
Keywords: Russian Federation; Lake Baikal; silicon dioxide; biogenic silica; climate response; continental climate; ice sheet dynamics; ocean dynamics; orbital forcing; climate; climate change; custody; dynamics; heat stroke; lake; orbit; rhythm; Russian Federation; sea

What brought them up? Exhumation of the Dabie Shan ultrahigh- pressure rocks - Hacker, B.R.; Ratschbacher, L.; Webb, L.; Shuwen, Dong (Geology, 1995)
Metamorphic coesite and diamond in the Dabie Shan, eastern China, testify to subduction of continental crust to >100 km depth. Exhumation of these ultrahigh-pressure rocks through the crust encompassed two stages. 1) South-dipping foliation, southeast-plunging stretching lineation, lineation-parallel isoclinal folds, and boudins indicate extreme subhorizontal shortening and subvertical extension during top-to-northwest shearing at 200-180 Ma. 2) Northwest-southeast sub-horizontal extension from 133 to 122 Ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the Dabie Shan. The preferred exhumation model involves two stages: Triassic identation - vertical extrusion and erosion - followed by Cretaceous plate margin transtension. -from Authors
Keywords: China; Dabie Mountains; Geology; Continental crusts; Eastern China; Identation; Magmatic complexes; Stretching lineations; Transtension; Triassic; Ultrahigh pressure rocks; Cretaceous; exhumation; tectonics; transtension; Triassic; ultra high pressure metamorphism; Silica

A rock-magnetic record from Lake Baikal, Siberia: Evidence for Late Quaternary climate change - Peck, J.A.; King, J.W.; Colman, S.M.; Kravchinsky, V.A. (Earth and Planetary Science Letters, 1994)
Rock-magnetic measurements of sediment cores from the Academician Ridge region of Lake Baikal, Siberia show variations related to Late Quaternary climate change. Based upon the well-dated last glacial-interglacial transition, variations in magnetic concentration and mineralogy are related to glacial-interglacial cycles using a conceptual model. Interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals. Glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals. The variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods. We also infer an increased contribution of eolian sediment during the colder, windier, and more arid glacial conditions when extensive loess deposits were formed throughout Europe and Asia. Eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals. Variations in magnetic concentration and mineralogy of Lake Baikal sediment correlate to the SPECMAP marine oxygen-isotope record. The high degree of correlation between Baikal magnetic concentration/mineralogy and the SPECMAP oxygen-isotope record indicates that Lake Baikal sediment preserves a history of climate change in central Asia for the last 250 ka. This correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method. Future work must include providing better age control and additional climate proxy data, thereby strengthening the correlation of continental and marine climate records. © 1994.
Keywords: Russian Federation; Lake Baikal; Russian Federation; Siberia; Lake Baikal; aeolian transport; climate change; marine oxygen isotope record; oxygen isotope; Quaternary; rock magnetic record

Radiocarbon dating of Lake Baikal sediments–A progress report - Colman, S. M.; Kuptsov, V. M.; Jones, G. A.; Carter, S. J. (Russian Geology and Geophysics, 1993)
id: 191

Initial results of U.S.‐Soviet paleoclimate study of Lake Baikal (Eos, Transactions American Geophysical Union, 1992)
Lake Baikal, a Miocene‐age rift lake in southeastern Siberia, is an especially promising site for paleoclimate studies. Its high‐latitude location (52° –56°N) makes it particularly sensitive to changes in solar insolation due to long‐period variations in the Earth's orbital parameters. These variations are widely believed to be the main forcing functions of climate change in the Quaternary [Hays et al., 1976; Imbrie et al., 1984. The extreme continentality of the climate in southeastern Siberia makes Baikal an ideal location to study temporal changes in seasonality. Baikal is also one of the few high‐latitude lakes that has not been glaciated during the last 1 –2 million years [Grosswald, 1980], although a record of glaciation in its drainage basin is preserved in the lake sediments. Finally, Lake Baikal is the largest (23,000 km3), the deepest (1640 m), and one of the oldest extant lake systems in the world. The sedimentary section in the Baikal depression is more than 7 km thick and probably spans more than 15 million years [Hutchinson et al., 1992]. Accordingly, Lake Baikal sediments represent one of the longest and most complete continental climate records available anywhere in the world. ©1992. American Geophysical Union. All Rights Reserved.
Keywords: Russian Federation; Lake Baikal; age determination; dating; palaeoclimate; Pleistocene/Holocene boundary; sediment core; seismic reflection profile

Sedimentation in Lake Malawi (East Africa) during the past 10,000 years: a continuous paleoclimatic record from the southern tropics - Finney, B.P.; Johnson, T.C. (Palaeogeography, Palaeoclimatology, Palaeoecology, 1991)
A suite of piston cores recovered from Lake Malawi (9-14°S, 34-35°E), east Africa in 1986 has been analyzed for major and minor elements, organic C and N, calcium carbonate and diatoms. An internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera, Stephanodiscus and Melosira, with age control obtained primarily from 14C dating of the carbonate. Differences with time in Fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth. The depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr B.P., indicating less seasonality. Carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column. The carbonate, which precipitates from surface waters, is most abundant during the interval from about 10,000 to 6000 yr B.P. This micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced. Sedimentary evidence suggests that lake levels were 100-150 m lower than present during this period. This record is different from climatic trends in northern intertropical Africa, but appears to also be related to changes in insolation and monsoon circulation. This is the northernmost basin in Africa reported to exhibit a southern hemisphere response to the early Holocene northern hemisphere summer insolation maximum. The climatic hingeline north of Lake Malawi (∼ 9°S) implied by our results is significantly south of that indicated by general-circulation model simulations, however. The cores show evidence for periods of abrupt climate change during the interval of generally arid climate. © 1991.
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Keywords: carbonate precipitation; chemocline depth; climate; climate trend; diatom; Holocene; lake level; micrite; palaeoclimate; palaeoclimate record; salinity; sedimentation; Malawi; Lake Malawi