All ICDP Publications with Abstracts
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1069.
Hydrogeological characteristics of the Outokumpu Deep Drill Hole
Special Paper of the Geological Survey of Finland,
2011
(51)
151 – 168
2011
ISSN: 07828535ISBN:
978-952217152-8
Abstract: ▾ Extensive hydrogeological studies on the Outokumpu Deep Drill Hole R2500, drilled in 2004-2005, have been carried out during both the drilling phase as well as the post-drilling period. The present paper introduces the main results and characteristics of deep fluids and gases in the 2516 m deep hole drilled into a Paleoproterozoic formation of metasediments, ophiolite-derived altered ultrabasic rocks and pegmatitic granite. The main hydrogeological experiments during drilling were the daily monitoring of drilling fluid electrical conductivity, pH, composition and consumption (loss) of drilling fluid, as well as targeted fluid sampling and hydraulic testing during drilling breaks with the drill stem method. Hydrogeological sampling of the drill hole water with a tube method has been carried out three times to up to 1500-2350 m depths in the post-drilling period, and undisturbed formation fluid was pumped for several weeks from a packer-isolated fracture system at 967 m. The loss of drilling water during drilling was very heavy in the uppermost 1000 m of the hole (1-4 m3 of water per 1 m of drilling), but it decreased to a low level (<1 m3/m) at lower depths, indicating that hydraulically conductive fractures are more frequent in the first kilometre of bedrock than beneath. The hydraulic testing carried out at approximately 500-m depth intervals in 40-70 m thick sections indicated a similar pattern of hydraulic conductivity decreasing with depth: about 7.5 · 10-6 m/s at 500 m, 5.3 · 10-7 m/s at 1000 m, and practically impermeable rock at deeper levels. The electrical conductivity of the drill hole fluid rapidly increased in the post-drilling period due to the discharge of saline water from several fracture systems, and was monitored with repeated down-hole logs. In the uppermost 1000 m, fluid salinity has been in a semi-stable condition since about 2006, but in the deeper parts of the hole electrical conductivity continued to gradually increase in 2008 and 2009, when the most recent downhole logs and fluid sample profiles were obtained. The fluids are Ca-Na-Cl fluids with elevated Mg concentrations at the depths of the ophiolite-derived rocks of the Outokumpu assemblage. The fluids contain abundant gases, with methane and nitrogen being the main components. The stable isotope compositions (δ2H, δ18O) of the saline fluids indicate that they are not meteoric fluids but probably result from long-term water-rock interaction. The results indicate distinct water bodies isolated in fracture zones with minimal hydraulic connections.
1068.
Estimation of q from zero-offset vsp data in gas hydrate-bearing zone
Geosystem Engineering,
14
(2)
59-64
2011
ISSN: 12269328Abstract: ▾ Q-factor (or Q) that quantifies the attenuation, the intrinsic property of the material, is a very important required factor in extracting useful subsurface material properties such as lithological information, porosity, permeability, viscosity, and the degree of the saturation from the seismic data. When seismic energy propagates through the gas hydrate-bearing zone and a free gas layer below it, the considerable attenuation occurs and affects the amplitude and wavelet shape of recorded seismic data. Thus Q- factor extracted from seismic data can be used to locate the gas hydrate bearing zone and estimate its reserves. The spectral-ratio method has been widely used in computing the frequency-independent Q's from the zero-offset VSP data because of its ease and fastness. We developed a module of the spectral ratio method, and applied it to the synthetic zero-offset VSP data set and field zero-offset VSP data set. The field data were acquired at Mallik 3L-38 gas hydrate research well in Canada. The Q-factors calculated from the synthetic zero-offset VSP data using the spectral ratio method approached closer to the true values for the medium with low Q-factor than high Q-factor. The changes in the Q-factors extracted from the Mallik zero-offset data using the spectral ratio method agreed well with the boundaries of the layers, including gas hydrate zone, depicted in a reflection image. © 2011 Taylor & Francis Group, LLC.
1067.
Evidence of transient increases of fluid pressure in SAFOD phase III cores
Geophysical Research Letters,
38
(3)
2011
ISSN: 00948276
Publisher: Blackwell Publishing Ltd
Keywords:▾
Carbonate minerals; Creep; Drilling fluids; Structural geology, California; Damaged zones; Deforming zone; Fault activity; Fault creep; Fault strands; Fault zone; Fluid circulation; Fluid pressures; Localized failure; Micro-earthquakes; Microstructural analysis; North American; Pacific plates; Permeability barriers; Permeability reduction; Pore fluid pressure; Potential mechanism; San Andreas Fault; Siltstones; Solution process, Shale, creep; fault zone; fluid pressure; microearthquake; microstructure; North American plate; Pacific plate; permeability; San Andreas Fault; shale; siltstone, California; Parkfield; United States
Abstract: ▾ The San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, central California, has been drilled through a fault segment that is actively deforming through creep and microearthquakes. Creeping is accommodated in two fault strands, the Southwest and Central Deforming Zones, embedded within a damaged zone of deformed shale and siltstone. During drilling, no pressurized fluids have been encountered, even though the fault zone acts as a permeability barrier to fluid circulation between the North American and Pacific plates. Microstructural analysis of sheared shales associated with calcite and anhydrite-bearing veins found in SAFOD cores collected at 1.5m from the Southwest Deforming Zone, suggests that transient increases of pore fluid pressure have occurred during the fault activity, causing mode I fracturing of the rocks. Such build-ups in fluid pressure may be related to permeability reduction during fault creep and pressure-solution processes, resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the SAFOD site. Copyright © 2011 by the American Geophysical Union.
1066.
Fish fossils as paleo-indicators of ichthyofauna composition and climatic change in Lake Malawi, Africa
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
126-132
2011
ISSN: 00310182Keywords:▾
carbon isotope; cichlid; climate change; cyprinid; fossil assemblage; fossil record; ichthyofauna; lake level; paleoclimate; paleolimnology; pelagic fish; sediment core; stable isotope, Africa; East African Lakes; Lake Malawi, Cichlidae; Cyprinidae; Cypriniformes; Vertebrata
Abstract: ▾ Numerous biological and chemical paleorecords have been used to infer paleoclimate, lake level fluctuation and faunal composition from the drill cores obtained from Lake Malawi, Africa. However, fish fossils have never been used to examine changes in African Great Lake vertebrate aquatic communities nor as indicators of changing paleolimnological conditions. Here we present results of analyses of a Lake Malawi core dating back ~144ka that describe and quantify the composition and abundance of fish fossils and report on stable carbon isotopic data (δ13C) from fish scale, bone and tooth fossils. We compared the fossil δ13C values to δ13C values from extant fish communities to determine whether carbon isotope ratios can be used as indicators of inshore versus offshore pelagic fish assemblages. Fossil buccal teeth, pharyngeal teeth and mills, vertebra and scales from the fish families Cichlidae and Cyprinidae occur in variable abundance throughout the core. Carbon isotopic ratios from numerous fish fossils throughout the core range between -7.2 and -27.5%, similar to those found in contemporary Lake Malawi benthic and pelagic fish faunas. These results are the first paleo-record of fish fossils from a Lake Malawi sediment core and the first reported δ13C values from Lake Malawi fish fossils. This approach provides a new methodology and framework for interpreting pelagic versus inshore fish faunas, lake level fluctuations and the evolution of the Lake Malawi fish assemblages. © 2010 Elsevier B.V.
1065.
Fluid inclusions in the Outokumpu Deep Drill Core: Implications for palaeofluid evolution and the composition of modern deep saline fluids
Special Paper of the Geological Survey of Finland,
2011
(51)
169 – 180
2011
ISSN: 07828535ISBN:
978-952217152-8
Abstract: ▾ Fluid inclusions in quartz veins in the Outokumpu Deep Drill Core are characterised by high salinities and high homogenisation temperatures in excess of 330 °C. In addition to an aqueous phase, fluid inclusions also contain gaseous phases such as CO2 and CH4. The cation ratios of the dissolved salts are higher than seawater, with Li/Na ratios indicating the influence of magmatic water in deeper parts of the drill hole. Stable isotope (δD, δ18O) signatures point to a metamorphic origin of the palaeofluids. Deep groundwaters in the Outokumpu crystalline basement deviate significantly from fluid inclusions in their stable isotope ratios, which plot to the right of the global meteoric water line in a δD, δ18O diagram. This suggests that they may have formed as a mixture of meteoric and saline waters. In addition, Cl/Br and Na/Br ratios point to chemical exchange with the host rocks. Many models have been proposed to account for the enhanced salinity of deep groundwaters and the shift in the stable isotopes, but our data indicate that the saline fluids are primarily derived through water-rock interaction. The role of fluid inclusions as important contributors to the saline fluids is not supported.
1064.
Geogas transport in fractured hard rock - Correlations with mining seismicity at 3.54km depth, TauTona gold mine, South Africa
Applied Geochemistry,
26
(12)
2134-2146
2011
ISSN: 08832927Keywords:▾
Concentration variation; Cross-correlation analysis; Fault zone; Gas component; Gas compositions; Gas concentration; Gas monitoring; Gas monitoring systems; Hard rocks; Permeability enhancement; Seismic datas; Seismic moment; Seismic monitoring; South Africa, Boreholes; Carbon dioxide; Earthquakes; Fracture; Gas detectors; Gold; Gold mines; Time series; Time series analysis, Gases, borehole; correlation; fault zone; fractured medium; gas transport; gold mine; hard rock; induced seismicity; mining industry; radon isotope; seismic data; time series, South Africa
Abstract: ▾ An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (The International NELSAM-DAFGAS projects) at the TauTona gold mine, South Africa. Five boreholes up to 60m long were drilled at 3.54km depth into the highly fractured Pretorius Fault Zone and instruments for chemical and seismic monitoring installed therein. Over the span of 4years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and air-like with about 78% N2, 21% O2, 1% Ar. The geogas components CO2, CH4, He and H2 show the most interesting trends and variations on the minute-by-minute basis and significantly correlate with seismic data, while the 222Rn activity remains constant. Time series and cross correlation analysis allow the identification of different gas components (geogas and tunnel air) and the identification of two processes influencing the borehole gas composition: (1) pumping-induced tunnel air breakthrough through networks of initially water-saturated fault fractures; and (2) seismicity induced permeability enhancement of fault fractures to above ∼5×10-10m2. The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ≤1×109Nm, located within 1000m of the cubby) and, it is suggested, also during induced earthquakes, a final goal of the project. © 2011 Elsevier Ltd.
1063.
Fault rocks from the SAFOD core samples: Implications for weakening at shallow depths along the San Andreas Fault, California
Journal of Structural Geology,
33
(2)
132-144
2011
ISSN: 01918141Keywords:▾
Fault zone; Phyllosilicate; SAFOD; San Andreas Fault; Smectites, Calcite; Carbonate minerals; Clay minerals; Core drilling; Core samples; Fluids; Sedimentary rocks; Silicate minerals, Structural geology, active fault; core analysis; creep; deformation mechanism; drilling; fault zone; foliation; overpressure; phyllosilicate; San Andreas Fault; shear zone; smectite; stress change; stress field, California; Parkfield; San Andreas; United States
Abstract: ▾ The drilling of a deep borehole across the actively creeping Parkfield segment of the San Andreas Fault Zone (SAFZ), California, and collection of core materials permit direct geological study of fault zone processes at 2-3 km depth. The three drill cores sample both host and fault rocks and pass through two currently active, narrow (1-2 m wide) shear zones enclosed within a broader (ca. 240 m wide) region of inactive foliated gouges. The host rocks preserve primary sedimentary features and are cut by numerous minor faults and small, mainly calcite-filled veins. The development of Fe-enriched smectitic phyllosilicate networks following cataclasis is prevalent in the presently inactive foliated gouges of the main fault zone and in minor faults cutting clay-rich host rocks. Calcite, anhydrite and minor smectitic phyllosilicate veins are interpreted to have formed due to local fluid overpressuring events prior to, synchronous with and after local gouge development. By contrast, the active shear zone gouges lack mineral veins (except as clasts) and contain numerous clasts of serpentinite. Markedly Mg-rich smectitic phyllosilicates are the dominant mineral phases here, suggesting that the fault zone fluids have interacted with the entrained serpentinites. We propose that weakening of the SAFZ down to depths of at least 3 km can be attributed to the pervasive development of interconnected networks of low friction smectitic phyllosilicates and to the operation of stress-induced solution-precipitation creep mechanisms. © 2010 Elsevier Ltd.
1062.
Geothermal studies of the Outokumpu Deep Drill Hole
Special Paper of the Geological Survey of Finland,
2011
(51)
181 – 198
2011
ISSN: 07828535ISBN:
978-952217152-8
Abstract: ▾ We present high resolution geothermal results from the 2516 m deep Outokumpu Deep Drill Hole in eastern Finland drilled in 2004-2005 into a Palaeoproterozoic formation with metasedimentary rocks, ophiolite-derived altered ultramafic rocks and pegmatitic granite. Detailed geothermal studies of deep drill holes provide insights into heat transfer processes in the crust, and allow the separation of different factors involved, such as palaeoclimatic and structural conductive effects as well as advective fluid flow effects. The down-hole temperatures of the Outokumpu hole have been logged five times following the end of drilling and extend to day 948 after drilling. The hole was continuously cored (79% core coverage) and thermal conductivity was measured at 1-m intervals. The temperature gradient, thermal conductivity and heat flow density results yield an exceptionally detailed geothermal data set and indicate significant vertical variation in the gradient and heat flow density. This result has important implications for correcting shallow (<1 km) heat flow data in the Fennoscandian Shield. The heat flow density was determined to increase from about 28-32 mW m-2 in the uppermost 1000 m to 40-45 mW m-2 at depths exceeding 2000 m. The estimated undisturbed surface heat flow value was 42 mWm-2. We present results based on inverse transient conductive models that suggest that the vertical variation in heat flow can mostly be attributed to a palaeoclimatic effect due to ground surface temperature (GST) variations during the last 100,000 years. The modelling suggests that the average GST was about -3...- 4 °C during the Weichselian glaciation. Holocene GST values are within ±2 degrees of the present average GST in Outokumpu (5 °C). The topographic hydraulic heads and hydraulic conductivity of crystalline rocks are low, which suggests that advective heat transfer in the formation is not significant. The slow replacement of fresh flushing water by saline formation fluids is observed in the hole, but it does not generate significant thermal disturbances in the logs. On the other hand, free sluggish thermal convection is present in the large diameter (22 cm) borehole, but it generates only local thermal 'noise' in the range of 0.001-0.01 K not affecting the general geothermal results.
1061.
High resolution reflection seismics integrated with deep dril hole data in Outokumpu, Finland
Special Paper of the Geological Survey of Finland,
2011
(51)
105 – 118
2011
ISSN: 07828535ISBN:
978-952217152-8
Abstract: ▾ The Outokumpu area, located in eastern Finland, is well known for its Precambrian Cu-Zn-Co-Ni-Ag-Au sulphide deposits hosted by ophiolite-derived altered ultramafic rocks. In 2004-2005, a 2.5 km deep research borehole was drilled on the south-east side of the main ore belt. The ophiolite-related Outokumpu-assemblage rocks were penetrated at depths of 1.3-1.5 km. The other main lithologies observed in the Outokumpu Deep Drill Hole were mica schist with biotite-gneiss layers (upper 2 km) underlain by pegmatitic granite. In May 2006, high resolution reflection seismic data with 4 m receiver spacing were acquired at the drilling site along two crooked lines to further refine the geological model of the area. The Outokumpu Deep Drilling Project provided an excellent opportunity to correlate high resolution seismic data with drilling results. The main emphasis in the processing of the reflection seismic data was put on static corrections. Substantial topographic variation and a significant velocity contrast between the glacially deposited overburden and the bedrock caused severe travel time variations in the near surface. Results achieved using static corrections carried out with the standard refraction method and using a tomographic approach were compared. Sonic velocity and density logs were used to calculate acoustic impedances and a synthetic seismogram, and theoretical calculations were compared with reflectivity observed in the seismic section. The results indicate that the host rocks of the Outokumpu type deposits are bright reflector packages that can be observed with reflection seismic techniques. The pegmatitic granite shows only weak reflection contrast with the mica schist, but it can be delineated as homogeneous, transparent domains, whereas the mica schist is internally more heterogeneous. A fracture zone at the depth of 967 m can be observed as a sharp sub-horizontal reflector.
1060.
High-grade contact metamorphism in the Reykjanes geothermal system: Implications for fluid-rock interactions at mid-oceanic ridge spreading centers
Geochemistry, Geophysics, Geosystems,
12
(8)
2011
ISSN: 15252027
Publisher: Blackwell Publishing Ltd
Keywords:▾
Feldspar; Geothermal fields; Geothermal prospecting; Hydraulic structures; Igneous rocks; Oceanography; Offshore oil wells; Olivine; Recrystallization (metallurgy); Seawater; Submarine geology, alteration; Average composition; Clinopyroxenes; contact metamorphism; Fluid-rock interaction; Geothermal systems; Geothermometers; Greenschist; Hydrothermal alterations; Hydrothermal system; hydrothermal systems; Hydrothermally; Magma sources; Metamorphic reactions; Mid-ocean ridges; Mid-oceanic ridges; Ocean drilling programs; Oman ophiolite; Orthopyroxene, Metamorphic rocks, clinopyroxene; contact metamorphism; fluid-structure interaction; geothermal system; geothermometry; greenschist facies; hydrothermal alteration; hydrothermal system; mid-ocean ridge; Ocean Drilling Program; P-T conditions; recrystallization; spreading center, Atlantic Ocean; Reykjanes Ridge
Abstract: ▾ Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo49En3Fs48 to Wo 53En30Fo17; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo48En 27Fs25. The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927°C to 967°C. Redox estimates based on the 2-oxide oxybarometer range from log fO2 of -13.4 to -15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems. Copyright 2011 by the American Geophysical Union.
1059.
Geothermal studies of the Outokumpu Deep Drill Hole, Finland: Vertical variation in heat flow and palaeoclimatic implications
Physics of the Earth and Planetary Interiors,
188
(1-2)
9 – 25
2011
ISSN: 00319201Keywords:▾
Baltic Shield; Finland; Calluna vulgaris; Boreholes; Climatology; Crystalline rocks; Flow of fluids; Glacial geology; Heat transfer; Hydraulics; Rock drills; Saline water; Thermal conductivity; Thermal logging; Conductive effects; Conductive model; Convection cell; Data sets; Drill hole; Eastern Finland; Fennoscandian Shields; Finland; Fluid flow effect; Geothermal gradients; Ground surface temperature; Heat flow density; Heat flows; Heat transfer process; High resolution; Holocenes; Hydraulic heads; Inverse transient; Large diameter; Metasedimentary rocks; Paleoclimatology; Saline formation; Surface heat flow; Temperature gradient; Temperature variation; Thermal conductivity measurements; Thermal convections; Thermal disturbance; Ultramafic rocks; Vertical variation; drilling; geothermal gradient; granite; heat flow; heat transfer; hydraulic conductivity; hydraulic head; paleoclimate; Proterozoic; thermal conductivity; thermal convection; ultramafic rock; Geothermal energy
Abstract: ▾ Detailed geothermal studies of deep drill holes provide insights to heat transfer processes in the crust, and allow separation of different factors involved, such as palaeoclimatic and structural conductive effects as well as advective fluid flow effects. We present high resolution geothermal results of the 2516m deep Outokumpu Deep Drill Hole in eastern Finland drilled in 2004-2005 into a Palaeoproterozoic formation with metasedimentary rocks, ophiolite-derived altered ultramafic rocks and pegmatitic granite. The down-hole temperatures have been logged five times after end of drilling and extend to day 948 after drilling. The hole is completely cored (79% core coverage) and thermal conductivity measurements were done at 1m intervals. The geothermal results on temperature gradient, thermal conductivity and heat flow density yield an exceptionally detailed data set and indicate a significant vertical variation in gradient and heat flow density. Heat flow density increases from about 28 - 32mWm-2 in the uppermost 1000m to 40-45mWm-2 at depths exceeding 2000m. The estimated undisturbed surface heat flow value is 42mWm-2. We present results on forward and inverse transient conductive models which suggest that the vertical variation in heat flow can mostly be attributed to a palaeoclimatic effect due to ground surface temperature (GST) variations during the last 100,000years. The modeling suggests that the average GST was about -3 to -4°C during the Weichselian glaciation. Holocene GST values are within ±2° from the present average GST in Outokumpu (5°C). The topographic hydraulic heads and hydraulic conductivity of crystalline rocks are low which suggests that advective heat transfer in the formation is not significant. The slow replacement of fresh flushing water by saline formation fluids is observed in the hole, but it does not generate significant thermal disturbances in the logs. On the other hand, free sluggish thermal convection is present in the large diameter (22cm) borehole, and temperature variations in the range of few mK to 0.01K occur over times of minutes to tens of minutes. Theory suggests that convection cells are about as tall as the drill hole diameter, and thus the free convection is expected to generate only local thermal 'noise' not affecting the general geothermal results. © 2011 Elsevier B.V.
1058.
High-frequency Born synthetic seismograms based on coupled normal modes
Geophysical Journal International,
187
(3)
1420-1442
2011
ISSN: 0956540XKeywords:▾
Aspherical; Coupled integral equations; Earth models; Earthquake source; Frequency domains; Full-waveforms; High frequency HF; Inverse Fourier transforms; Normal modes; San Andreas Fault; Seismic wavefields; Structural heterogeneity; Synthetic seismogram; Time domain; Wavefields, Born approximation; Seismology; Three dimensional; Wave propagation, Integral equations, Fourier transform; seismic source; seismic wave; synthetic seismogram; wave field; wave propagation; waveform analysis
Abstract: ▾ High-frequency and full waveform synthetic seismograms on a 3-D laterally heterogeneous earth model are simulated using the theory of coupled normal modes. The set of coupled integral equations that describe the 3-D response are simplified into a set of uncoupled integral equations by using the Born approximation to calculate scattered wavefields and the pure-path approximation to modulate the phase of incident and scattered wavefields. This depends upon a decomposition of the aspherical structure into smooth and rough components. The uncoupled integral equations are discretized and solved in the frequency domain, and time domain results are obtained by inverse Fourier transform. Examples show the utility of the normal mode approach to synthesize the seismic wavefields resulting from interaction with a combination of rough and smooth structural heterogeneities. This approach is applied to an ~4 Hz shallow crustal wave propagation around the site of the San Andreas Fault Observatory at Depth (SAFOD). © The Author Geophysical Journal International © 2011 RAS.
1057.
Erratum: Intrabasin paleoearthquake and quiescence correlation of the late Holocene Dead Sea (Journal of Geophysical Research (2011) (116) DOI: 10.1029/2011JB008870)
Journal of Geophysical Research: Solid Earth,
116
(11)
2011
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
1056.
High pore pressure, or its absence, in the San Andreas fault
Geology,
39
(11)
1047-1050
2011
ISSN: 00917613DOI:10.1130/G32294.1
Keywords:▾
Active fault; California; Drilling projects; Fault zone; Long term monitoring; San Andreas Fault, Earthquakes; Strike-slip faults, Pore pressure, active fault; earthquake mechanism; fault zone; pore pressure; San Andreas Fault, California; San Andreas; United States
Abstract: ▾ High pore pressure in the San Andreas fault (California) was hypothesized to explain the prevailing weakness of the fault and may have major implications on the mechanics of earthquakes. However, no evidence of high pore pressure was found in the latest drilling into the San Andreas fault (SAFOD: San Andreas Fault Observatory at Depth) in central California (Zoback et al., 2010). If widely applicable, this result would impact our understanding of earthquake mechanisms on this and other active faults around the world. Here, however, I show that the available evidence from the latest SAFOD drilling may not be sufficient to reject the high pore-pressure hypothesis, and that definite knowledge of pore pressure in the fault zone may require long-term monitoring at the SAFOD site. The inference may also be useful for interpreting results from drilling projects on other active faults. © 2011 Geological Society of America.
1055.
Helium measurements of pore fluids obtained from the San Andreas Fault Observatory at Depth (SAFOD, USA) drill cores
Hydrogeology Journal,
19
(1)
237-247
2011
ISSN: 14312174Keywords:▾
concentration (composition); diffusion; fluid dynamics; geoaccumulation; helium; hydrogeology; isotopic ratio; porosity; tortuosity; tracer
Abstract: ▾ 4He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4-6 times on each sample, and indicate a bulk 4He diffusion coefficient of 3.5 ± 1.3 × 10-8 cm2 s-1 at 21°C, compared to previously published diffusion coefficients of 1.2 × 10-18 cm2 s-1 (21°C) to 3.0 × 10-15 cm2 s-1 (150°C) in the sands and clays. Correcting the diffusion coefficient of 4Hewater for matrix porosity (~3%) and tortuosity (~6-13) produces effective diffusion coefficients of 1 × 10-8 cm2s-1 (21°C) and 1 × 10-7 (120°C), effectively isolating pore fluid 4He from the 4He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8 ± 0.4% (SD, n = 4) and mudstones 3.1 ± 0.8% (SD, n = 4). © 2010 Springer-Verlag.
1054.
Extant freshwater ostracodes (Crustacea: Ostracoda) from Lago Peten Itza, Guatemala.
Revista de Biologia Tropical,
58871 - 895
092010
ISSN: 0034-7744
Publisher: scielo
1053.
Scale-invariant stress orientations and seismicity rates near the San Andreas Fault
Geophysical Research Letters,
37
(24)
2010
ISSN: 00948276
Publisher: Blackwell Publishing Ltd
Keywords:▾
Compressive stress; Earthquakes; Horizontal wells; Wells, Active fault; Earthquake frequency; Earthquake magnitudes; Loading condition; San Andreas Fault; Scale-invariant; Scaling parameter; Scientific researches; Southern California; Stress orientations; Stress perturbations, Stress analysis, active fault; compression; creep; earthquake magnitude; earthquake recurrence; fault zone; seismicity, California; San Andreas; United States
Abstract: ▾ We analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the San Andreas Fault in central and southern California. We found that the stress orientations exhibit scale-invariant fluctuations over intervals from tens of cm to several km. Similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically-stressed crust adjacent to the fault. The apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude-frequency statistics for the creeping versus locked sections of the fault along which these two wells are located. This suggests that stress heterogeneity adjacent to active faults like the San Andreas may reflect variations in stresses and loading conditions along the fault. © 2010 by the American Geophysical Union.
1052.
Single crystal U-Pb zircon age and Sr-Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites
Chemical Geology,
275
(3-4)
254 – 261
2010
ISSN: 00092541Keywords:▾
Cote d'Ivoire; Ghana; comparative study; country rock; geochronology; graywacke; impact structure; impactite; isotopic composition; isotopic ratio; lithology; metamorphism; neodymium isotope; phyllite; strontium isotope; tektite; uranium-lead dating; zircon
Abstract: ▾ The 1.07. Myr old Bosumtwi impact structure (Ghana), excavated in 2.1-2.2. Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U-Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca 2145 ± 82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb-Sr and Sm-Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb-Sr and Sm-Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible "metamorphic age" of ~. 1.8-1.9. Ga tentatively derived from our U-Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb-Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event. © 2010 Elsevier B.V.
1051.
Siderophile elements from the Eyreville drill cores of the Chesapeake Bay impact structure do not constrain the nature of the projectile
Special Paper of the Geological Society of America,
465395-409
2010
Abstract: ▾ Fifteen impactites from various intervals within the Eyreville cores of the Chesapeake Bay impact structure were sampled to measure siderophile element concentrations. The sampled intervals include basement-derived rocks with veins, polymict impact breccias and associated rocks, and crater-fill sediments. The platinum group element (PGE) concentrations obtained are generally low (e.g., iridium concentrations less than 0.1 ng/g) and are fractionated relative to chondrites. There is no clear distinction in concentration between the different impactite units. So far in the Chesapeake Bay material, only the impact melt rocks from the 823-m-deep Cape Charles test hole, drilled over the central uplift of the structure, have generated a bulk chondritic signature of 0.01-0.1 wt% meteoritic contribution based on a mixing model of 187 Os/ 188 Os isotopic ratios and Os concentrations. However, none of the samples studied shows PGE abundances that enable identification of the type of projectile responsible for the formation of the structure. Hence, it is at present not possible to link the Chesapeake Bay impact to the proposed ordinary chondrite falls by projectiles recorded for other late Eocene craters, namely the 100-km-diameter Popigai impact structure in Siberia and 7.5-km-diameter Wanapitei structure in Canada. The absence of a clear projectile signature hinders further discussions on the existence and the nature of the late Eocene shower event (asteroid versus comet). © 2010 The Geological Society of America. All rights reserved.
1050.
Seismotectonic setting of the Karadere-Düzce branch of the North Anatolian Fault Zone between the 1999 Izmit and Düzce ruptures from analysis of Izmit aftershock focal mechanisms
Tectonophysics,
482
(1-4)
170-181
2010
ISSN: 00401951Keywords:▾
Fault plane solutions; Izmit earthquakes; North Anatolian Fault Zone; Seismotectonics; Stress tensors, Earthquakes; Focusing; Tensors, Aircraft accidents, aftershock; earthquake catalogue; earthquake hypocenter; earthquake magnitude; earthquake rupture; extensional tectonics; fault zone; faulting; focal mechanism; seismotectonics; spatial variation; strike-slip fault; tectonic setting, Anatolia; Duzce; Izmit; Kocaeli [Turkey]; Turkey
Abstract: ▾ We investigate aftershock focal mechanisms along the eastern part of the Izmit Mw = 7.4 August 17, 1999 rupture zone during the time period August 22, 1999-October 17, 1999. Two spatial clusters of aftershock activity are analyzed representing the Karadere Fault (KF) and the Düzce Area (DA). Based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors < 2 km, we determine fault plane solutions for 221 events. The high number of focal mechanisms at the eastern Izmit rupture zone could be determined only due to the low magnitude-detection threshold of the seismic network and allows to resolve the local deformation pattern with unprecedented precision. Focal mechanisms along the Karadere Fault allow us to identify dominantly dextral strike-slip mechanisms with normal faulting components on NE-SW trending fault planes. Focal mechanisms in the Düzce Area predominantly exhibit NE-SW extensional normal faulting but also a substantial part of strike-slip faulting. Further subdivision of the data set slightly decreases for the misfit for deeper (z > 10 km) events. North and east of the easternmost end of the Karadere Fault we observe a high variance in stress field orientation correlated with lower b-values. While the Karadere Fault reflects a predominant dextral strike-slip regime with normal faulting components, the Düzce Area further to the East that also hosted the forthcoming Mw = 7.2 mainshock 87 days after the Izmit earthquake can be subdivided into a dominantly NE-SW extensional normal faulting regime below the Düzce Basin (DB) and a first-order strike-slip regime along the western Düzce Fault (DF). We conclude that the Düzce Basin was set under tension by the Izmit rupture and partly released the slip deficit by extensional faulting on Karadere Fault parallel to the coseismic displacement. At the same time this area and in particular the Düzce Fault that bounds the Düzce Basin to the south reflects mostly strike-slip events representing a major asperity along the North Anatolian Fault Zone (NAFZ) before initiating the Düzce rupture 87 days after the Izmit event. © 2009 Elsevier B.V. All rights reserved.
1049.
Seismic imaging using microseismic events: Results from the San Andreas Fault System at SAFOD
Journal of Geophysical Research: Solid Earth,
115
(12)
2010
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
algorithm; image resolution; imaging method; microearthquake; San Andreas Fault; seismic data; seismic reflection; seismic source; seismology; stacking; wave field
Abstract: ▾ We have developed a new passive seismic imaging approach that consists of two steps. First, the hypocenter of the microseismic event is precisely located. Second, this event is treated as a "pseudo-active" seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high-resolution image of the illuminated subsurface region. In this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the San Andreas Fault Observatory at Depth main hole. Results obtained were high-resolution 3-D images of different SE-NW-oriented reflectors related to the San Andreas Fault (SAF) system in the close vicinity of the borehole. To support our approach, we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology. We revealed a predominantly satisfactory agreement for both juxtapositions. Furthermore, the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the SAF, with much higher resolution than can be obtained from surface seismic reflection data. Copyright 2010 by the American Geophysical Union.
1048.
Site selected for colorado plateau coring
Eos,
91
(14)
128
2010
ISSN: 00963941
Publisher: American Geophysical Union
Keywords:▾
conference proceeding; plateau; site selection, Colorado; United States
1047.
Seismic images of Chicxulub impact melt sheet and comparison with the Sudbury structure
Special Paper of the Geological Society of America,
465103-113
2010
Abstract: ▾ Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ∼1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet. © 2010 The Geological Society of America. All rights reserved.
1046.
Sedimentary facies and cyclostratigraphy of the Cretaceous first member of Nenjiang Formation in the Southeast uplift zone, Songliao Basin and its correlation with the CCSD-SK-I.
Acta Petrologica Sinica,
26
(1)
99-108
2010
1045.
Sediment core fossils in ancient Lake Ohrid: Testing for faunal change since the Last Interglacial
Biogeosciences,
7
(11)
3435 – 3446
2010
ISSN: 17264189Keywords:▾
Lake Ohrid; Bivalvia; Gastropoda; Mollusca; biostratigraphy; deep drilling; fossil record; interglacial; mollusc; paleoenvironment; paleontology; sediment core
Abstract: ▾ Ancient Lake Ohrid is probably of early Pleistocene or Pliocene origin and amongst the few lakes in the world harbouring an outstanding degree of endemic biodiversity. Although there is a long history of evolutionary research in Lake Ohrid, particularly on molluscs, a mollusc fossil record has been missing up to date. For the first time, gastropod and bivalve fossils are reported from the basal, calcareous part of a 2.6 m long sediment succession (core Co1200) from the north-eastern part of Lake Ohrid. Electron spin resonance (ESR) dating of mollusc shells from the same stratigraphic level yielded an age of 130 ± 28 ka. Lithofacies III sediments, i.e. a stratigraphic subdivision comprising the basal succession of core Co1200 between 181.5-263 cm, appeared solid, greyish-white, and consisted almost entirely of silt-sized endogenic calcite (CaCO3>70%) and intact and broken mollusc shells. Here we compare the faunal composition of the thanatocoenosis with recent mollusc associations in Lake Ohrid. A total of 13 mollusc species (9 gastropod and 4 bivalve species) could be identified within Lithofacies III sediments. The value of sediment core fossils for reconstructing palaeoenvironmental settings was evaluated and the agreement between sediment and palaeontological proxies was tested. The study also aims at investigating major faunal changes since the Last Interglacial and searching for signs of extinction events. The combined findings of the ecological study and the sediment characteristics suggest deposition in a shallow water environment during the Last Interglacial. The fossil fauna exclusively included species also found in the present fauna, i.e. no extinction events are evident for this site since the Last Interglacial. The thanatocoenosis showed the highest similarity with recent Intermediate Layer (5-25 m water depth) mollusc assemblages. The demonstrated existence of a mollusc fossil record in Lake Ohrid sediment cores also has great significance for future deep drilling projects. It can be hoped that a more far reaching mollusc fossil record will then be obtained, enabling insight into the early evolutionary history of Lake Ohrid. © Author(s) 2010.