All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
794.
Groundwater geochemistry of the Yucatan Peninsula, Mexico: Constraints on stratigraphy and hydrogeology
Journal of Hydrology,
367
(1-2)
27-40
2009
Abstract: ▾ We report 87Sr/86Sr and ion concentrations of sulfate, chloride, and strontium in the groundwater of the northern and central Yucatan Peninsula, Mexico. Correlation between these data indicates that ejecta from the 65.95 m.y. old Chicxulub impact crater have an important effect on hydrogeology, geomorphology, and soil development of the region. Ejecta are present at relatively shallow subsurface depths in north-central Yucatan and at the surface along the Rio Hondo escarpment in southeast Quintana Roo, where they are referred to as the Albion Formation. Anhydrite/gypsum (and by inference celestite) are common in impact ejecta clasts and in beds and cements of overlying Paleocene and Lower Eocene rocks cored around the margin of the crater. The sulfate-rich minerals that are found in rocks immediately overlying the impact ejecta blanket, may either be partially mobilized from the ejecta layer itself or may have been deposited after the K/T impact event in an extensive pre-Oligocene shallow sea. These deposits form a distinctive sedimentary package that can be easily traced by the Eocene-Cretaceous 87Sr/86Sr signal. A distinct Sr isotopic signature and high SO4/Cl ratios are observed in groundwater of northwestern and north-central Yucatan that interacts with these rocks. Moreover, the distribution of the gypsum-rich stratigraphic unit provides a solution-enhanced subsurface drainage pathway for a broad region characterized by dissolution features (poljes) extending from Chetumal, Quintana Roo to Campeche, Campeche. The presence of gypsum quarries in the area is also consistent with a sulfate-rich stratigraphic "package" that includes ejecta. The distinctive chemistry of groundwater that has been in contact with evaporite/ejecta can be used to trace flow directions and confirms a groundwater divide in the northern Peninsula. Information about groundwater flow directions and about deep subsurface zones of high permeability is useful for groundwater and liquid waste management in the area. Where it discharges at the coast, the unique chemistry of the groundwater that has interacted with the evaporite/ejecta strata may also have significant geomorphologic implications. While groundwater-seawater mixing at the coast has been shown to dissolve and erode limestone, PHREEQC modeling shows that mixing of water nearly saturated in CaSO4 with seawater has a less vigorous dissolution effect due to its high Ca content. © 2009 Elsevier B.V. All rights reserved.
793.
High-impact reservoirs
Oilfield Review,
21
(4)
14-29
2009
Abstract: ▾ The processes that accompany asteroid impact and the roles they play in creating or destroying petroleum source rocks, reservoirs and traps are discussed. Most asteroids orbit the Sun in the asteroid belt, which lies between Mars and Jupiter but some come closer to the Earth. Excavation produces a bowl-shaped transient crater. Craters larger than a few kilometers in diameter usually have complex morphologies characterized by an uplift central area. The central high consists of shocked target rock that has been structurally uplifted by rebound. The bottom of the crater filled with melt brecias, mixtures of granite, carbonate, and dolomite in a spherulitic matrix. The impact that has drawn the most attention is the collision of the Chicxulub impactor with what is now the Mexican Yucatán Peninsula. A direct hit by an asteroid can also cause the demise of an hydrocarbon accumulation.
792.
High-K alkali basalts of the Western Snake River Plain (Idaho): Abrupt transition from tholeiitic to mildly alkaline plume-derived basalts
Journal of Volcanology and Geothermal Research,
188
(1-3)
141-152
2009
ISSN: 03770273Keywords:▾
Abrupt change; Abrupt transition; Alkali basalt; Alkaline plumes; Basaltic volcanism; Hotspots; Idaho Batholith; Isotopic characteristics; Lithospheric; Major elements; Mantle lithosphere; Mass flux; Rayleigh-Taylor instabilities; Snake River; Snake river plains; SRP; Tholeiitic basalts; Yellowstone plume; Yellowstones, Isotopes; Oceanography; Olivine; Rivers; Sedimentary rocks; Silicate minerals; Trace elements, Basalt, alkali basalt; alkaline rock; hot spot; mantle plume; ocean island basalt; tholeiite; trace element; volcanism, Idaho; North America; Snake River Plain; United States
Abstract: ▾ Basaltic volcanism in the western Snake River Plain underwent an abrupt change circa ~ 700 ka to 900 ka, from low-K tholeiitic basalt and ferrobasalt to high-K transitional alkali basalt. The low-K tholeiitic basalts share major element, trace element, and isotopic characteristics with olivine tholeiites of the eastern Snake River Plain, and must have been derived by similar processes from similar sources. In contrast, the younger high-K alkali basalts share major element, trace element, and isotopic characteristics with plume-derived alkali basalts of ocean islands suites like Hawaii. We conclude that this abrupt transition reflects either or both the erosion of pre-existing mantle lithosphere in the wake of the Yellowstone-Snake River plume, or the depletion of this lithosphere in fusible components so that it no longer contributed to the overall mass flux of magma. The abruptness of the transition implies that it may have a catastrophic origin, such as lithospheric delamination caused by a Rayleigh-Taylor instability beneath the Idaho batholith. © 2009 Elsevier B.V. All rights reserved.
791.
High-resolution seismic-reflection images across the ICDP-USGS Eyreville deep drilling site, Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458209-233
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
C (programming language); Compaction; Infill drilling; Seismic waves; Seismology; Stratigraphy, Chesapeake bay impact structures; Differential compaction; High resolution image; High resolution seismic; Pleistocene sediments; Reflection amplitude; Seismic reflections; U.s. geological surveys, Sediments, deep drilling; impact structure; lithostratigraphy; seismic reflection; seismic tomography; sequence stratigraphy, Chesapeake Bay; United States
Abstract: ▾ The U.S. Geological Survey (USGS) acquired two 1.4-km-long, high-resolution (̃5 m vertical resolution) seismic-reflection lines in 2006 that cross near the International Continental Scientifi c Drilling Program (ICDP)-USGS Eyreville deep drilling site located above the late Eocene Chesapeake Bay impact structure in Virginia, USA. Five-meter spacing of seismic sources and geophones produced high-resolution images of the subsurface adjacent to the 1766-m-depth Eyreville core holes. Analysis of these lines, in the context of the core hole stratigraphy, shows that moderateamplitude, discontinuous, dipping reflections below ̃527 m correlate with a variety of Chesapeake Bay impact structure sediment and rock breccias recovered in the cores. High-amplitude, continuous, subhorizontal reflections above ̃527 m depth correlate with the uppermost part of the Chesapeake Bay impact structure crater-fi ll sediments and postimpact Eocene to Pleistocene sediments. Refl ections with ̃20-30 m of relief in the uppermost part of the crater-fi ll and lowermost part of the postimpact section suggest differential compaction of the crater-fi ll materials during early postimpact time. The top of the crater-fi ll section also shows ̃20 m of relief that appears to represent an original synimpact surface. Truncation surfaces, locally dipping reflections, and depth variations in reflection amplitudes generally correlate with the lithostratigraphic and sequence-stratigraphic units and contacts in the core. Seismic images show apparent postimpact paleochannels that include the fi rst possible Miocene paleochannels in the Mid-Atlantic Coastal Plain. Broad downwarping in the postimpact section unrelated to structures in the crater fi ll indicates postimpact sediment compaction. © 2009 The Geological Society of America.
790.
Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, based on combined geophysical and geological surveys
Journal of Volcanology and Geothermal Research,
182
(1-2)
97 – 112
2009
ISSN: 03770273Keywords:▾
Bohemian Massif; Central Europe; Eurasia; Europe; Debris; Earthquakes; Electric conductivity; Geological surveys; Geophysics; Gravitational effects; Hazards; Lakes; Minerals; Restoration; Sedimentology; Structural geology; Volcanoes; Eger Rift; geophysical survey; Quaternary maar volcanism; Saalian lake sediments; volcanic hazard potential; diatreme; earthquake swarm; electrical conductivity; geophysical survey; gravimetry; gravity anomaly; maar; magnetic anomaly; magnetometer; paleoclimate; paleoseismicity; Quaternary; Saalian; tephra; tuff; volcaniclastic deposit; Core analysis
Abstract: ▾ Based on results of previous investigations of tephra-tuff volcaniclastic deposits and a geophysical survey in the surroundings of the Železná hůrka Quaternary volcano, West Bohemia, we performed detailed geophysical surveys using gravimetry, magnetometry and electrical conductivity techniques. Striking anomalies were revealed in a morphological depression near Mýtina, West Bohemia, as a strong evidence of the assumed maar-diatreme structure. The sharp isometric gravity low of - 2.30 mGal, as well as the corresponding positive magnetic anomaly of 200 nT with a negative rim on its northern side indicate a steeply dipping geological body of low density and containing magnetic rocks/minerals. Magnetic survey also showed pronounced local anomalies outside the depression that can reflect relicts of the tephra rim of the maar. This geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly. Macroscopic on-site evaluation of the core, and more detailed sedimentological, petrochemical, palynological and microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m of lake sediments reflecting a succession of several warm and cold climatic periods. Results of palynological analyses confirm the presence of a continuous palaeoclimate archive, with at least three successive warmer periods of most probably interstadial character from the upper Quaternary Saalian complex. Therefore, the recovered sediment sequence holds strong potential for in-depth paleoclimate reconstruction and deep biosphere studies. At the bottom of the Mýtina-1 (MY-1) borehole (84-85.5 m), country rock debris was found, containing also volcanic bombs and lapilli. The discovered volcanic structure is considered to be the first known Quaternary maar-diatreme volcano on the territory of the Bohemian Massif. Because of hidden active magmatic processes in combination with earthquake swarm seismicity ca. 20-30 km north of the Mýtina maar, reconstruction of the palaeovolcanological evolution is important for evaluation of hazard potential of the NE and E part of the Cheb Basin. © 2009 Elsevier B.V. All rights reserved.
789.
Geochemical characteristics of basement target rocks, suevitic glasses from the Eyreville B drill core, Chesapeake Bay impact structure, and three bediasites
Special Paper of the Geological Society of America,
458435-445
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Buildings; Cluster analysis; Crystalline materials; Crystalline rocks; Glass; Hierarchical systems; Infill drilling; Oxide minerals; Titanium dioxide, Chesapeake bay impact structures; Continental scientific drillings; Crystalline target rocks; Display characteristics; Geochemical characteristic; Hierarchical cluster analysis; Microchemical analysis; U.s. geological surveys, Core drilling, breccia; chemical composition; crater; impact structure; marine sediment; sediment chemistry; suevite, Chesapeake Bay; United States
Abstract: ▾ The International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville B core hole, drilled into the 35.5-Ma-old Chesapeake Bay impact crater, Virginia, has recovered postimpact sediments, crater-fill breccias, megablocks of the crystalline basement, and suevites with fresh glass shards. Bulk rock analyses of 2 glass shards, 21 crystalline target rocks, and microchemical analyses of 7 glass shards and 3 bediasites (tektites of the North American strewn field) were performed in order to contribute to the understanding of formation processes and to better constrain the precursor materials of these glasses as well as of the bediasites. Statistical treatment (hierarchical cluster analyses) yielded an assignment of the data for the crystalline basement samples into four groups; two of those (various schists, meta-graywackes, and gneisses) display characteristics similar to the impact glasses in the suevites and the bediasites. However, the suevitic glasses show a broad range in composition at the micrometer scale. These data show the frequent presence of schlieren, and in particular, enhanced TiO2 contents that require admixture of an "amphibolitic component" to the melt. Evidence for such a process is provided by the occurrence of relict, in-part thermally corroded grains of rutile and ilmenite, and by formation of Ti-rich tiny mineral aggregates in the glass. The three studied bediasites show only minor inter- and intrasample heterogeneity, and their chemical composition agrees well with previously published data. The new data for the bediasites are compatible with heating of the "tektite melt" to extreme temperatures, followed by quenching. © 2009 The Geological Society of America.
788.
Geologic columns for the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure: Impactites and crystalline rocks, 1766 to 1096 m depth
Special Paper of the Geological Society of America,
45821-49
2009
Abstract: ▾ The International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville drill cores from the Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure. This paper presents a series of geologic columns and descriptive lithologic information for the lower impactite and crystalline-rock sections in the cores. The lowermost cored section (1766-1551 m depth) is a complex assemblage of mica schists that commonly contain graphite and fibrolitic sillimanite, intrusive granite pegmatites that grade into coarse granite, and local zones of mylonitic deformation. This basement-derived section is variably overprinted by brittle cataclastic fabrics and locally cut by dikes of polymict impact breccia, including several suevite dikes. An overlying succession of suevites and lithic impact breccias (1551-1397 m) includes a lower section dominated by polymict lithic impact breccia with blocks (up to 17 m) and boulders of cataclastic gneiss and an upper section (above 1474 m) of suevites and clast-rich impact melt rocks. The uppermost suevite is overlain by 26 m (1397-1371 m) of gravelly quartz sand that contains an amphibolite block and boulders of cataclasite and suevite. Above the sand, a 275-m-thick allochthonous granite slab (1371-1096 m) includes gneissic biotite granite, fine- and medium-to-coarse-grained biotite granites, and red altered granite near the base. The granite slab is overlain by more gravelly sand, and both are attributed to debris-avalanche and/or rockslide deposition that slightly preceded or accompanied seawater-resurge into the collapsing transient crater. © 2009 The Geological Society of America.
787.
Frictional and transport properties of the Chelungpu fault from shallow borehole data and their correlation with seismic behavior during the 1999 Chi-Chi earthquake
Journal of Geophysical Research: Solid Earth,
114
(1)
2009
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
borehole; Chi-Chi earthquake 1999; earthquake rupture; fault zone; fluid flow; friction; high pressure; numerical model; seismicity; slip, Asia; Chelungpu Fault Zone; Eurasia; Far East; Taiwan
Abstract: ▾ We carried out low- and high-velocity friction tests on fault rock samples from shallow boreholes on the Taiwan Chelungpu fault and measured their fluid transport properties under high pressure, with the objective of explaining the different seismic behaviors in the northern and southern sections of the fault during the 1999 Chi-Chi earthquake. Our results of low-velocity friction tests demonstrate that fault gouge from the southern section of the fault exhibits velocity-weakening frictional behavior, whereas gouge from the northern section exhibits velocity-strengthening friction. Friction in the northern gouge decreased strongly with increasing wetness, whereas friction in southern gouge samples was not affected by wetness. A rapid reduction of friction was observed immediately after the onset of slip in high-velocity friction tests. The results of high-velocity friction tests were similar for all fault gouge samples tested, although permeability in the northern fault zone was lower than that in the south. Numerical modeling indicated that thermal pressurization in the northern fault zone promoted stress reduction and fault instability during slip, whereas it did not in the south. This contrasting seismic behavior between north and south is caused mainly by differences in fluid transport properties of the slip zones. More efficient thermal pressurization in the north explains the large slip displacement there. The results of our low-velocity friction tests are consistent with nucleation of the Chi-Chi earthquake in the south and propagation of the rupture from south to north. Copyright 2009 by the American Geophysical Union.
786.
Efficiency of individual CRL stations in seismic monitoring of the western gulf of corinth
Acta Geodynamica et Geomaterialia,
6
(2)
135 – 142
2009
ISSN: 12149705Abstract: ▾ We study the efficiency of individual stations of the CRL seismic network in recording the seismic activity in the western Gulf of Corinth, Greece. The stations are located on both the northern and southern coast of the Gulf. The study is based on 5027 earthquakes recorded in 2001, separated into three groups, the southern, central and the northern one. The events were located using the HYPO71PC algorithm. It is shown that the stations significantly differ in their monitoring ability.
785.
Distributions of branched GDGTs in a tropical lake system: Implications for lacustrine application of the MBT/CBT paleoproxy
Organic Geochemistry,
40
(9)
1032 – 1036
2009
ISSN: 01466380Keywords:▾
Asia; Eurasia; Indonesia; Lake Towuti; Malili Lakes; South Sulawesi; Southeast Asia; Alkylation; Glycerol; Lakes; Methylation; Sedimentology; Soils; Aquatic environments; Degree of methylations; Freshwater environments; In-situ production; Indonesia; Lake sediments; River sediments; Soil sample; Tropical lakes; ether; fluvial deposit; lacustrine deposit; lacustrine environment; lipid; methylation; tropical environment; Sediments
Abstract: ▾ Branched glycerol dialkyl glycerol tetraether (GDGT) lipids are abundant and ubiquitous in lake sediments, potentially allowing for a paleolimnological application of the so-called MBT/CBT proxy (methylation index of branched tetraethers/cyclization ratio of branched tetraethers). To investigate the origin and characteristics of these compounds in lacustrine environments, we examined the distributions of GDGTs in soils, river sediments and lake sediments from Lake Towuti on the island of Sulawesi, Indonesia. We found significant differences in the degree of methylation and cyclization (expressed by way of the MBT and CBT indices) between the soil samples and the aquatic samples, suggesting that there may be in situ production of GDGTs in the aquatic environment. Based on these findings, we urge caution in the application of the MBT/CBT paleoproxy to lake sediments and encourage more rigorous study of these compounds in freshwater environments. © 2009 Elsevier Ltd. All rights reserved.
784.
Fission track analysis and thermotectonic history of the main borehole of the Chinese Continental Scientific Drilling project
Tectonophysics,
475
(2)
318-326
2009
ISSN: 00401951Keywords:▾
CCSD; Donghai; Fission track dating; Thermochronology; Ultrahigh-pressure metamorphic belt, Apatite; Cooling; Drilling; Fission reactions; Metamorphic rocks; Phosphate minerals; Statistical tests, Boreholes, age determination; apatite; borehole geophysics; Chinese Continental Scientific Drilling Project; displacement; fission track dating; geochronology; normal fault; thermochronology; ultrahigh pressure metamorphism; uplift, Asia; China; Dabie-Sulu Belt; Donghai [Jiangsu]; Eurasia; Far East; Jiangsu
Abstract: ▾ The Chinese Continental Scientific Drilling (CCSD) project, part of the International Continental Drilling Program (ICDP), has completed drilling a 5158 m hole in the eastern part of the Dabie-Sulu ultrahigh-pressure metamorphic belt. This study reports on an apatite fission track analysis of core samples from 0 to 4000 m depth in the CCSD main hole (CCSD-MH). We determined the fission track ages of 38 apatite samples from different depths. The ages range between 98.6 ± 17.0 and 3.2 ± 1.3 Ma, showing a general decreasing trend with depth, from 87.1 ± 11.2 Ma at the surface to 3.2 ± 1.3 Ma at 3899 m depth. As a first approximation, an average uplift rate of ~ 35 m/Ma is calculated for the period 90-30 Ma. The trend in ages within the borehole shows some fluctuations, and indicates movements along major faults. It is inferred that the highest-level major normal fault occurs at a depth of ~ 350 m, recording a vertical displacement of ~ 400 m. Movement along another prominent normal fault at a depth of ~ 2150 m occurred subsequent to ~ 25 Ma. Three major reverse faults occur at about 2450, 3050 and 3250 m depth. Testing geological constrains against the fission track data set indicated an agreement with a reheating of the area during the late Cretaceous and Eocene, followed by cooling to ~ 80 °C during the Eocene and a low cooling until the samples reached their present-day position in the Donghai area. © 2009 Elsevier B.V. All rights reserved.
783.
Drilling deep into the ultrahigh pressure (UHP) metamorphic terrane
Tectonophysics,
475
(2)
201-203
2009
ISSN: 0040-1951782.
Drilling into the El'gygytgyn Impact Crater, Arctic Russia: The 2009 ICDP Project
Meteoritics and Planetary Science Supplement,
725014
2009
781.
Dynamic fault weakening and the formation of large impact craters
Earth and Planetary Science Letters,
287
(3-4)
471-482
2009
Abstract: ▾ Impact craters are the most common landform on planetary surfaces; however, the mechanics of the end stages of their formation are not fully understood. The final stage of crater formation involves the collapse of a hemispherical transient cavity. Around small craters, the limited amount of collapse preserves a bowl-shaped cavity. In contrast, the observed shallow depths and complex inner morphologies of large craters require very low shear strength in the collapsing material. Because the observed amount of collapse cannot be reproduced using quasi-static values for the frictional strength of fractured rock, a temporary weakening mechanism is necessary. Here, we investigate the hypothesis that craters collapse along a network of impact-generated faults that weaken during long displacements at high slip velocities via, for example, frictional melting. Using the CTH shock physics code, we simulate the formation of about 100-km diameter impact craters using a simple strain-rate weakening model with parameters constrained by fault friction experiments on crystalline rocks. The model reduces the coefficient of friction from a quasi-static value (0.6-0.85) to a weakened value (0.1-0.2) when a parcel of fractured material exceeds thresholds for cumulative plastic shear strain (a proxy for slip distance) and shear strain rate (a proxy for slip velocity). During crater formation, the strain-rate weakening model leads to strain localizations that are interpreted to be fault zones. Fault zones are spontaneously created and slip over discrete time intervals during collapse. The strain-rate weakening model reproduces the major geologic features observed around the largest terrestrial craters (Vredefort, Sudbury, and Chicxulub), including shallow depths, fault structures, frictional melt distributions, and deep-seated central uplifts. The good agreement between calculations and observations supports the hypothesis that small volumes of transiently weakened material in fault zones control the collapse of large impact craters. © 2009 Elsevier B.V. All rights reserved.
780.
Earthquake swarms in non-volcanic regions: What fluids have to say
Geophysical Research Letters,
36
(17)
2009
ISSN: 00948276
Publisher: American Geophysical Union
Keywords:▾
Central Europe; Eger; Eurasia; Europe; Heves; Hungary; Degassing; Helium; Oceanography; Volcanoes; Central Europe; Earthquake swarms; Focal zones; Free gas; Lower crust; Magma intrusion; Magmatic activity; Upper mantle; Volcanic region; carbon dioxide; degassing; earthquake swarm; helium; lower crust; magmatism; rift zone; upper mantle; Earthquakes
Abstract: ▾ The detailed processes generating earthquake swarms are complex and not fully understood. Most earthquake swarms occur in volcanic regions and mid-ocean rifts. Here, we report new 3HeZ4He data of free gases monitored at CO2rich degassing locations close to the Nový Kostel focal zone (NKFZ) located in the western Eger rift. The NKFZ is known for the recurrence of earthquake swarms at which the focal zone ranges between 6 and 12 km depth. At degassing locations neighboring to the NKFZ a progressive increase of mantle-derived helium has been observed during the last 15 years - actually the highest 3He/4He ratios (>6 Ra) in Central Europe. The 3HeZ4He anomalies indicate hidden magmatic activity. We assume that the latest strong earthquake swarm in October 2008 was initiated by a hidden magma intrusion process from the upper mantle into the lower crust that has been indicated by a three month lasting increase of the 3HeZ4He ratios in spring 2006 at all degassing locations near the NKFZ. Copyright 2009 by the American Geophysical Union.
779.
Effect of thermal refraction on heat flow near the San Andreas Fault, Parkfield, California
Journal of Geophysical Research: Solid Earth,
114
(6)
2009
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
basement rock; coastal zone; data acquisition; fault slip; flow measurement; friction; heat flow; numerical model; shear stress; slip; subsurface flow; thermal conductivity, California; North America; San Andreas; United States, Calluna vulgaris
Abstract: ▾ Heat flow data near the San Andreas Fault (SAF) do not reveal a near-fault anomaly as expected from frictional heat generation, an observation interpreted to indicate that the fault slips at a depth-averaged shear stress <20 MPa. The data also contain large unexplained scatter, which has been a separate major issue in the analysis of heat flow within the California Coast Ranges. Here we use numerical models of heat conduction to evaluate the hypothesis that thermal refraction, due to contrasts in thermal conductivity in the subsurface, both produces the observed scatter in heat flow and as a result obscures the thermal signature from frictional heating on a fault that supports large shear stress during slip. Our study focuses on the region around the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, California. Our results show that surface heat flow is most sensitive to the contrast between Tertiary sediments and basement rocks and to wavelengths of basement topography of ∼10 km. With realistic thermal conductivity contrasts and a reasonable interpretation of this geologic contact, we show that thermal refraction is a plausible explanation for the observed heat flow scatter. However, refraction effects are unable to mask frictional heat generation in a manner consistent with observations. We show that even with large refraction effects, low background heat flow, a regional NW-SE decrease in heat flow, or nonsteady state heat conduction, the data are most consistent with a fault that produces little to no frictional heat. Copyright 2009 by the American Geophysical Union.
778.
Dynamics and internal structure of a lower mantle plume conduit
Earth and Planetary Science Letters,
282
(1-4)
314 – 322
2009
ISSN: 0012821XKeywords:▾
Hawaii [United States]; North America; United States; Aspect ratio; Filaments (lamp); Lenses; Locomotives; Optical instruments; Railroad cars; Strain rate; Transport properties; Axi-symmetric; Conduit structures; Geochemical fingerprints; geochemical heterogeneity; Geochemical variations; Hawaiian plume; Heterogeneous architectures; High strain rates; High-resolution numerical simulations; Hot spots; Internal structures; Lower mantles; mantle plumes; Radial distances; Scientific drillings; Small scale; Source regions; Thermal boundary layers; Time-scales; Vertical sections; Vertical velocities; isotopic composition; lower mantle; mantle chemistry; mantle plume; numerical model; Thermal plumes
Abstract: ▾ Geochemical studies, including those made possible by the Hawaiian Scientific Drilling Project, have revealed the chemically and isotopically heterogeneous nature of hotspot lavas, yet their interpretation is highly controversial and there is little agreement as to how geochemical heterogeneities might be spatially arranged within the plume conduit. To address this issue we conduct high resolution numerical simulations of an axisymmetric purely thermal plume, focusing on the lower mantle part of the conduit and on the thermal boundary layer (TBL) feeding the plume. We explore the relation between length-scales of heterogeneities across the source region and the length- and time-scales of geochemical variations in the plume conduit. The vertical velocity inside the conduit decreases exponentially with the square of radial distance generating high strain rates (order 10- 13-10- 14 s- 1) that modify the shape of upwelling heterogeneities into elongated and narrow filaments. Therefore, the preservation of 'blob-like' heterogeneities (i.e., with a 1:1 aspect ratio in a vertical section) is quite unlikely, even in the central part of the plume. For example, initial lenses of size 100 × 10 km in the TBL are stretched into filaments 500-1000 km long. These filaments constitute 'long-lived' structures in a rising plume, and their geochemical fingerprints may be registered at a given radial distance for several millions of years. We also consider an idealized heterogeneous architecture inside the TBL, consisting of 'trains' of small scale lenses. When such trains upwell in the conduit, they form high radial geochemical gradients. Their 'geochemical record', registered over time at a given depth and radial distance, will fluctuate over time, with shorter period and a larger amplitude at the conduit center than at its periphery. Finally, we demonstrate that material existing 'side by side' in the conduit originated from regions in the TBL that are separated by distances of several hundred kilometers. This implies that vigorous plumes are able to sample, and to bring side by side, very distant portions of their source region. Our results provide a fluid dynamically consistent framework to discuss the main aspects of the different (and to some extent mutually exclusive) models of conduit structure used to interpret the geochemical observations of the Hawaiian lavas. © 2009 Elsevier B.V. All rights reserved.
777.
Elastic wave velocities, chemistry and modal mineralogy of crustal rocks sampled by the Outokumpu scientific drill hole: Evidence from lab measurements and modeling
Physics of the Earth and Planetary Interiors,
175
(3-4)
151 – 166
2009
ISSN: 00319201Keywords:▾
Anisotropy; Crystallography; Curve fitting; Elasticity; Fatigue of materials; Granite; Intercalation; Lithology; Mica; Mineralogy; Mining; Rock drills; Rocks; Seismic waves; Seismology; Shear waves; Silica; Silicate minerals; Single crystals; Three dimensional; Velocity; 3d velocities; Biotite gneiss; Bulk rocks; Crustal rocks; Crystallographic orientations; Crystallographic preferred orientations; Depth ranges; Elastic wave velocities; In-situ; Intrinsic velocities; Least squares fittings; Lithologic controls; Measured and calculated elastic properties; Mica schists; Micro probes; Mineral chemistries; Modal composition; Nature of seismic reflections; Outokumpu deep drill hole; Precambrian; Rock types; S-wave velocities; Seismic anisotropies; Seismic measurements; Shallow depths; Shear-wave splitting; Single-crystal properties; Strong anisotropies; Texture measurements; Velocity anisotropies; Wave trains; crustal structure; crystallography; drilling; elastic wave; granite; laboratory method; least squares method; mica; neutron diffraction; numerical model; P-wave; S-wave; schist; seismic anisotropy; seismic reflection; wave velocity; Minerals
Abstract: ▾ The Outokumpu scientific deep drill hole intersects a 2500 m deep Precambrian crustal section comprising a 1300 m thick biotite-gneiss series (mica schists) at top, followed by a 200 m thick meta-ophiolite sequence, underlain again by biotite gneisses (mica schists) (500 m thick) with intercalations of amphibolite and meta-pegmatoids (pegmatitic granite). From 2000 m downward the dominating rock types are meta-pegmatoids (pegmatitic granite). Average isotropic intrinsic P- and S-wave velocities and densities of rocks were calculated on the basis of the volume fraction of the constituent minerals and their single crystal properties for 29 core samples covering the depth range 198-2491 m. The modal composition of the rocks is obtained from bulk rock (XRF) and mineral chemistry (microprobe), using least squares fitting. Laboratory seismic measurements on 13 selected samples representing the main lithologies revealed strong anisotropy of P- and S-wave velocities and shear wave splitting. Seismic anisotropy is strongly related to foliation and is, in particular, an important property of the biotite gneisses, which dominate the upper and lower gneiss series. At in situ conditions, velocity anisotropy is largely caused by oriented microcracks, which are not completely closed at the pressures corresponding to the relatively shallow depth drilled by the borehole, in addition to crystallographic preferred orientation (CPO) of the phyllosilicates. The contribution of CPO to bulk anisotropy is confirmed by 3D velocity calculations based on neutron diffraction texture measurements. For vertical incidence of the wave train, the in situ velocities derived from the lab measurements are significantly lower than the measured and calculated intrinsic velocities. The experimental results give evidence that the strong reflective nature of the ophiolite-derived rock assemblages is largely affected by oriented microcracks and preferred crystallographic orientation of major minerals, in addition to the lithologic control. © 2009 Elsevier B.V. All rights reserved.
776.
Evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas: The modern Qinghai Lake, Qinghai-Tibet Plateau
Chemical Geology,
268
(1-2)
126 – 136
2009
ISSN: 00092541Keywords:▾
Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; Ostracoda; arid environment; carbonate sediment; fine grained sediment; isotopic composition; lacustrine deposit; lake evolution; ostracod; oxygen isotope; salinity; water depth; water temperature
Abstract: ▾ The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in arid or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (< 60 μm). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai-Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the lakes and ponds shows a clear response to lake water δ18O values, and these vary with water salinity. The relationship between lake water δ18O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small 'vital offsets' of ostracods. Finally, the effect of water depth on temperature leads to increasing δ18O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface. For arid, high-altitude Lake Qinghai, our results suggest that variations in the δ18O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive δ18O values of ostracod and bulk carbonate. © 2009 Elsevier B.V. All rights reserved.
775.
Evolution of crystalline target rocks and impactites in the chesapeake bay impact structure, ICDP-USGS eyreville B core
Special Paper of the Geological Society of America,
458277-316
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Buildings; Corundum; Feldspar; Glass; Granite; Lead alloys; Mica; Quartz; Silicate minerals; Structural geology; Tectonics; Textures; Zircon, Amphibolite facies; Chesapeake bay impact structures; Crystalline target rocks; Detachment fault; High temperature crystallization; Sensitive high-resolution ion microprobe; Shocked quartz; Undersaturation, Crystalline rocks, breccia; crystalline rock; crystallization; Eocene; granite; impact structure; impactite; marine sediment; SHRIMP dating, Chesapeake Bay; United States
Abstract: ▾ The 1766-m-deep Eyreville B core from the late Eocene Chesapeake Bay impact structure includes, in ascending order, a lower basement-derived section of schist and pegmatitic granite with impact breccia dikes, polymict impact breccias, and cataclas tic gneiss blocks overlain by suevites and clast-rich impact melt rocks, sand with an amphibolite block and lithic boulders, and a 275-m-thick granite slab overlain by crater-fill sediments and postimpact strata. Graphite-rich cataclasite marks a detachment fault atop the lower basement-derived section. Overlying impactites consist mainly of basement-derived clasts and impact melt particles, and coastalplain sediment clasts are underrepresented. Shocked quartz is common, and coesite and reidite are confirmed by Raman spectra. Silicate glasses have textures indicating immiscible melts at quench, and they are partly altered to smectite. Chrome spinel, baddeleyite, and corundum in silicate glass indicate high-temperature crystallization under silica undersaturation. Clast-rich impact melt rocks contain α- cristobalite and monoclinic tridymite. The impactites record an upward transition from slumped ground surge to melt-rich fallback from the ejecta plume. Basement-derived rocks include amphibolite-facies schists, greenschist(?)-facies quartz-feldspar gneiss blocks and subgreenschist-facies shale and siltstone clasts in polymict impact breccias, the amphibolite block, and the granite slab. The granite slab, underlying sand, and amphibolite block represent rock avalanches from inward collapse of unshocked bedrock around the transient crater rim. Gneissic and massive granites in the slab yield U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon dates of 615 ± 7 Ma and 254 ± 3 Ma, respectively. Postimpact heating was 7lt;~350 °C in the lower basementderived section based on undisturbed 40 Ar/ 39 Ar plateau ages of muscovite and <~150 <C in sand above the suevite based on 40 Ar/ 39 Ar age spectra of detrital microcline. © 2009 The Geological Society of America.
774.
Evolution of crystalline target rocks and impactites in the chesapeake bay impact structure, ICDP-USGS eyreville B core
Special Paper of the Geological Society of America,
458277-316
2009
Abstract: ▾ The 1766-m-deep Eyreville B core from the late Eocene Chesapeake Bay impact structure includes, in ascending order, a lower basement-derived section of schist and pegmatitic granite with impact breccia dikes, polymict impact breccias, and cataclas tic gneiss blocks overlain by suevites and clast-rich impact melt rocks, sand with an amphibolite block and lithic boulders, and a 275-m-thick granite slab overlain by crater-fill sediments and postimpact strata. Graphite-rich cataclasite marks a detachment fault atop the lower basement-derived section. Overlying impactites consist mainly of basement-derived clasts and impact melt particles, and coastalplain sediment clasts are underrepresented. Shocked quartz is common, and coesite and reidite are confirmed by Raman spectra. Silicate glasses have textures indicating immiscible melts at quench, and they are partly altered to smectite. Chrome spinel, baddeleyite, and corundum in silicate glass indicate high-temperature crystallization under silica undersaturation. Clast-rich impact melt rocks contain α- cristobalite and monoclinic tridymite. The impactites record an upward transition from slumped ground surge to melt-rich fallback from the ejecta plume. Basement-derived rocks include amphibolite-facies schists, greenschist(?)-facies quartz-feldspar gneiss blocks and subgreenschist-facies shale and siltstone clasts in polymict impact breccias, the amphibolite block, and the granite slab. The granite slab, underlying sand, and amphibolite block represent rock avalanches from inward collapse of unshocked bedrock around the transient crater rim. Gneissic and massive granites in the slab yield U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon dates of 615 ± 7 Ma and 254 ± 3 Ma, respectively. Postimpact heating was 7lt;~350 °C in the lower basementderived section based on undisturbed 40 Ar/ 39 Ar plateau ages of muscovite and <~150 <C in sand above the suevite based on 40 Ar/ 39 Ar age spectra of detrital microcline. © 2009 The Geological Society of America.
773.
Experimental alteration of artificial and natural impact melt rock from the Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458559-569
2009
Abstract: ▾ The alteration or transformation of impact melt rock to clay minerals, particularly smectite, has been recognized in several impact structures (e.g., Ries, Chicxulub, Mjølnir). We studied the experimental alteration of two natural impact melt rocks from suevite clasts that were recovered from drill cores into the Chesapeake Bay impact structure and two synthetic glasses. These experiments were conducted at hydrothermal temperature (265 °C) in order to reproduce conditions found in meltbearing deposits in the first thousand years after deposition. The experimental results were compared to geochemical modeling (PHREEQC) of the same alteration and to original mineral assemblages in the natural melt rock samples. In the alteration experiments, clay minerals formed on the surfaces of the melt particles and as fine-grained suspended material. Authigenic expanding clay minerals (saponite and Ca-smectite) and vermiculite/chlorite (clinochlore) were identified in addition to analcime. Ferripyrophyllite was formed in three of four experiments. Comparable minerals were predicted in the PHREEQC modeling. A comparison between the phases formed in our experiments and those in the cores suggests that the natural alteration occurred under hydrothermal conditions similar to those reproduced in the experiment. © 2009 The Geological Society of America.
772.
Experimental alteration of artificial and natural impact melt rock from the Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458559-569
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Chlorite minerals; Clay alteration; Core drilling; Infill drilling; Rocks; Zeolites, Chesapeake bay impact structures; Geochemical modeling; Hydrothermal conditions; Hydrothermal temperature; Impact structures; Mineral assemblage; Suspended material; Synthetic glass, Clay minerals, authigenic mineral; experimental study; hydrothermal activity; hydrothermal alteration; hydrothermal deposit; hydrothermal system; impact structure; numerical model; smectite; suevite, Chesapeake Bay; United States
Abstract: ▾ The alteration or transformation of impact melt rock to clay minerals, particularly smectite, has been recognized in several impact structures (e.g., Ries, Chicxulub, Mjølnir). We studied the experimental alteration of two natural impact melt rocks from suevite clasts that were recovered from drill cores into the Chesapeake Bay impact structure and two synthetic glasses. These experiments were conducted at hydrothermal temperature (265 °C) in order to reproduce conditions found in meltbearing deposits in the first thousand years after deposition. The experimental results were compared to geochemical modeling (PHREEQC) of the same alteration and to original mineral assemblages in the natural melt rock samples. In the alteration experiments, clay minerals formed on the surfaces of the melt particles and as fine-grained suspended material. Authigenic expanding clay minerals (saponite and Ca-smectite) and vermiculite/chlorite (clinochlore) were identified in addition to analcime. Ferripyrophyllite was formed in three of four experiments. Comparable minerals were predicted in the PHREEQC modeling. A comparison between the phases formed in our experiments and those in the cores suggests that the natural alteration occurred under hydrothermal conditions similar to those reproduced in the experiment. © 2009 The Geological Society of America.
771.
Extension of Gutenberg-Richter distribution to Mw -1.3, no lower limit in sight
Geophysical Research Letters,
36
(10)
L10307
2009
ISSN: 00948276Keywords:▾
Break down; Earthquake process; Earthquake source; Laboratory experiments; Lower limits; Moment magnitudes; Natural earthquake; Nucleation zone; Seismic arrays; Seismic data; Shear failure; South Africa, Mines; Mining, Earthquakes, earthquake; gold mine; mining-induced seismicity; nucleation; seismic data; spatial distribution, Africa; South Africa; Southern Africa; Sub-Saharan Africa
Abstract: ▾ [1] With twelve years of seismic data from TauTona Gold Mine, South Africa, we show that mining-induced earthquakes follow the Gutenberg-Richter relation with no scale break down to the completeness level of the catalog, at moment magnitude Mw -1.3. Events recorded during relatively quiet hours in 2006 indicate that catalog detection limitations, not earthquake source physics, controlled the previously reported minimum magnitude in this mine. Within the Natural Earthquake Laboratory in South African Mines (NELSAM) experiment's dense seismic array, earthquakes that exhibit shear failure at magnitudes as small as Mw -3.9 are observed, but we find no evidence that Mw -3.9 represents the minimum magnitude. In contrast to previous work, our results imply small nucleation zones and that earthquake processes in the mine can readily be scaled to those in either laboratory experiments or natural faults.
770.
First results of geothermal investigations, Chesapeake Bay impact structure, Eyreville core holes
Special Paper of the Geological Society of America,
458931-940
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Heat transfer; Infill drilling; Lithology; Petrophysics; Thermal conductivity; Wellheads, Chesapeake bay impact structures; Continental scientific drillings; Continental shelves; Equilibrium temperatures; Lateral heterogeneity; Microclimatic effects; Petrophysical properties; Temperature profiles, Core samples, borehole; continental shelf; crater; Eocene; estimation method; geothermal system; heat flow; impact structure; lithology; numerical model; temperature profile; thermal conductivity; thermal regime, Chesapeake Bay; United States; Virginia, Calluna vulgaris
Abstract: ▾ The Chesapeake Bay impact structure is a late Eocene complex crater that was excavated -35 Ma ago in a continental shelf environment at the Atlantic margin, in Virginia. It is the largest impact structure in the United States and the seventh largest on Earth. It has an average diameter of -85 km and is centered near Cape Charles. The scientific well Eyreville B drilled within the framework of the International Continental Scientific Drilling Program (ICDP) penetrated the deep crater moat -9 km from the center of the structure. Core holes drilled in impact structures are especially suited for investigations of the infl uence of lithological heterogeneities on petrophysical properties and the thermal field. In the Eyreville B core hole, two high-resolution temperature-logging campaigns and a petrophysical profile measured on core samples spaced at -10 m intervals were recorded. The temperature values of the first campaign in December 2005 were heavily disturbed by outflow of artesian water and could only be used for an estimation of the depth where the fluid originated. For the second campaign in May 2006, a riser was constructed to enable measurements in standing (equilibrated) fluid of the well without opening the well head. This construction yielded a measurement of the undisturbed temperature profile as well as recognition of thermal relaxation after some outflow of artesian water, which wellheated the surrounding rock. The data allowed determination of (1) the origin of the artesian water, (2) equilibrium temperatures derived from the relaxation process, (3) microclimatic effects at the nearby test well STP2, (4) lateral heterogeneities in the core holes STP2 and Eyreville B, and (5) a profile of vertical heat-flow density in the Eyreville B core. From the calculated vertical component of the thermal gradient and the thermal conductivity measured on core samples, a mean heat-flow density of 65 ± 6 mW/m2 in the 440-1100 m depth interval was determined. These data and results are now available for application in numerical models of the local and regional geologic, hydrologic, and geothermal regimes. © 2009 The Geological Society of America.