high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

in situ observation and evaluation of zonal disintegration affected by existing fractures in deep hard rock tunneling abstract zonal disintegration refers to the phenomenon whereby fractured zones and intact zones appear alternately in surrounding hard rocks under high stress conditions in this study zonal disintegration within hard surrounding rocks of deep tunnels at the china jinping underground laboratory phase ii cjpl ii was observed by using a digital borehole televiewer in pre drilled boreholes the results revealed that fractured zones within intact high strength marbles were small and exhibited a higher degree of fracturing whereas fractured zones within low strength low integrity marbles showed fewer yet larger fractures an evaluation method based on rock mass integrity index using digital borehole televiewer data rmibt has been proposed for the analysis of zonal disintegration process in the deep hard rocks surrounding the tunnels by using the method we were able to quantify the initiation and evolution of fractures during zonal disintegration the formation mechanism of the second and third fractured zones induced by the existing fractures was further investigated using numerical analysis the data provide an insight into the evolution of failures in and a basis for support design and stability control of deep engineering works in hard rock

detecting hydrate and fluid flow from bottom simulating reflector depth anomalies methane hydrates ice like compounds that consist of water and methane represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification however it remains unclear how much hydrate exists here using a newly developed three dimensional 3 d thermal technique we reveal a novel method for detecting and quantifying methane hydrate the analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter scale horizontal resolution our study located at hydrate ridge offshore oregon united states suggests that heat flow and hydrate concentrations are coupled and that 3 d thermal analysis can be used to constrain hydrate and fluid flow in 3 d seismic data hydrate estimates using this technique are consistent with 1 d drilling results but reveal large previously unrecognized swaths of hydrate rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies the 3 d analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three

seismic investigation of the excavation damaged zone in opalinus clay a mini sonic borehole probe was used to investigate the excavation damaged zone edz around an underground opening in the opalinus clay in the mont terri rock laboratory in switzerland the edz could be determined by using seismic parameters as interval velocities and amplitudes of first arrivals derived from measurements in eight radial drilled boreholes the extent of the edz is about 1 m but varies between 0 6 and 2 2 m

a new method for assessing slope unloading zones based on unloading strain unloading zones are formed in shallow slopes after rock mass relaxation and crack extension generally slope unloading assessment depends on the traditional methods mainly related to the phenomena however unloading is a process of energy release based on the fracture mechanics and the statistical theory of rock mass structure a theoretical analysis was performed to establish the relationship between the accumulative opening displacement of unloading cracks and unloading strain and a new method was proposed to quantify the extent and damage degree of unloading zones according to unloading strain the new method was applied to assess the slope unloading zones at a hydropower station dam site in northwest china results showed that the accumulative opening displacement curve of unloading cracks regularly varied and could be easily divided into three parts the strong and weak unloading zones could be recognized from the slope of the first two parts while their extent was limited by the two inflexions on each curve by comparison with the traditional method the new method confirmed the reasonability in the slope unloading zone assessment however with an increase in the unloading degree the difference in the zonation results obtained from the two methods may increase

asymmetric sea floor spreading caused by ridge plume interactions crustal accretion at mid ocean ridges is generally modelled as a symmetric process regional analyses however often show either small scale asymmetries which vary rapidly between individual spreading corridors or large scale asymmetries represented by consistent excess accretion on one of the two separating plates over geological time spans1 2 3 4 5 6 in neither case is the origin of the asymmetry well understood here we present a comprehensive analysis of the asymmetry of crustal accretion over the past 83 myr based on a set of self consistent digital isochrons7 and models of absolute plate motion8 9 we find that deficits in crustal accretion occur mainly on ridge flanks overlying one or several hotspots we therefore propose that asymmetric accretion is caused by ridge propagation towards mantle plumes or minor ridge jumps sustained by asthenospheric flow10 11 between ridges and plumes quantifying the asymmetry of crustal accretion provides a complementary approach to that based on geochemical12 and other geophysical data13 14 in helping to unravel how mantle plumes and mid ocean ridges are linked through mantle convection processes

seafloor spreading magnetic anomalies in the enderby basin east antarctica abstract the timing of the early separation of india from the contiguous antarctica australia is still an unresolved problem although it is well established that antarctica and india formed a single indo antarctic platform prior to the fragmentation of eastern gondwanaland inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of antarctica and india perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of india from antarctica analysis of magnetic and satellite derived gravity data in the enderby basin east antarctica reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones the observed magnetic anomalies can be interpreted as the younger sequence of mesozoic anomalies m11 m0 half spreading rates range from 6 5 to 2 8 cm yr and are comparable with those measured in the bay of bengal these similarities in the mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary a consistent plate reconstruction model can be derived from the identified conjugate patterns of mesozoic magnetic anomalies and fracture zones the occurrence of the oldest magnetic anomaly m11 close to the coasts in these two offshore basins unequivocally suggests that the break up of india from antarctica occurred before 134 ma

in situ monitoring of tunnel deformation evolutions from auxiliary tunnel in deep mine abstract deformation caused by excavation is of great effects on tunnel stability and tunnel support design monitoring using instruments that are installed in place within the examined tunnel cannot record a large amount of surrounding rock deformation that usually takes place ahead and close to the tunnel face for this important reason the overall deformation process was observed using instruments installed prior to excavation from auxiliary tunnel in a 900 m deep coal mine tunnel two monitoring methods i e multipoint extensometers and wave velocity tomography were used at two monitoring sections with different rock lithology the results showed that a large amount of deformation at both monitoring sections had already been occurred when the excavation face passed the monitoring sections at a very small distance the size of the plastic zone at monitoring section 1 is identified as 1 8 m 0 36 d d denotes the diameter of the examined tunnel and the velocities within this zone decreased from 4 5 km s to 2 km s the primary deformation was generated in a distance from 0 8 d ahead the excavation face to 2 3 d behind the excavation face the induced deformation was also observed after the primary deformation period and some additional deformation was still occurred in the plastic zone although the size of plastic zone remains constant the displacements and p wave velocities at the two monitoring sections showed little differences

deep tow magnetic anomaly study of the pacific jurassic quiet zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior 1 the jurassic quiet zone jqz is a region of low amplitude magnetic anomalies whose distinctive character may be related to geomagnetic field behavior we collected deep tow magnetic profiles in pigafetta basin western pacific where previous deep tow data partially covered the jqz sequence our goals were to extend the survey through the jqz examine anomaly correlations and refine a preliminary geomagnetic polarity timescale gpts model we collected a series of closely spaced profiles over anomaly m34 and ocean drilling program hole 801c to examine anomaly correlation in detail one profile in between previous profiles and two long profiles extending the survey deeper into the jqz anomaly features can be readily correlated except in a region of low amplitude short wavelength anomalies in the middle of the survey area low amplitude zone or laz the small multiprofile surveys demonstrate anomaly linearity implying that surrounding anomalies are also linear and likely result from crustal recording of geomagnetic field changes we constructed a gpts model assuming that most anomalies result from polarity reversals the polarity timescale is similar to the polarity sequences from previous studies but its global significance is uncertain because of problems correlating anomalies in the laz and the ambiguous nature of the small jqz anomalies overall anomaly amplitude decreases with age into the laz and then increases again implying low geomagnetic field strength perhaps related to a rapidly reversing field other factors that may contribute to the laz are interference of anomalies over narrow crustal polarity zones and poorly understood local tectonic complexities

in situ experiments on an excavation disturbed zone induced by mechanical excavation in neogene sedimentary rock at tono mine central japan abstract the excavation disturbed zone edz is defined as the rock zone where rock properties and conditions have been changed due to the processes induced by excavation such as fracturing stress redistribution and desaturation based on our results of the in situ blasting excavation disturbance experiment at the tono mines the rock mass around the drift behaved elastically and the excavation damage induced by blasting was considered to be a primary factor in causing a change in rock properties the excavation damage was considered to depend on the excavation method so we performed an excavation disturbance experiment with mechanical excavation using a boom header in the neogene sedimentary soft rock to compare the rock property change and width of the edz induced by blasting geological observation rock mass displacement and vibration measurement were performed during the drift excavation seismic refraction and tomographic surveys borehole expansion test and hydraulic test were performed before and after the drift excavation a low seismic velocity layer of the edz with a width of ca 0 3 m and a seismic velocity of 65 70 of the intact rock was detected by a seismic refraction survey along the drift wall deformability and hydraulic conductivity were not changed in the rock mass up to 0 3 m from the drift wall the change in rock properties and width of the edz was shown to depend on the excavation method in this geological setting the mechanical excavation method is more effective in limiting the extent of the edz and change in rock mass properties

excavation damaged zone division and time dependency deformation prediction a case study of excavated rock mass at xiaowan hydropower station abstract rock mass excavation is accompanied by rock deformation during unloading which degrades the quality of rock mass and significantly affects the safety and stability of rock mass engineering taking the excavated rock mass from the dam foundation of xiaowan hydropower station as the study object long time single hole acoustic tests shat were conducted in the dam abutment rock mass with the changes in the acoustic wave velocity recorded then a new method quantifying the excavation damaged zone edz was proposed according to the cumulative degradation rate of acoustic wave velocity and a time dependency model for evaluating the evolution of the excavation heavily damaged zone ehdz was established the results indicated that the cumulative degradation rate curves of the wave velocity along the boreholes exhibited significant differences in time and space the depth corresponding to the inflection point on the curve can be regarded as the bottom boundary of the ehdz which could be used to generate time dependency curves and demonstrate the spatiotemporal evolution process for the ehdz the time dependency curves can be classified into three categories the differences between the classified curves are related to the depth of the initial ehdz ehdz0 rock mass elastic modulus e and unloading stress u the time dependency model for the ehdz evolution process follows a negative exponential function the ehdz0 depth e and u can determine the coefficients of the model the model provides an effective method to predict ehdz depths at different times after rock mass excavation the prediction results are of considerable significance for researches on the unloading failure and the selection of rock mass reinforcement which would ensure the stability and safety of the rock mass engineering during construction and operation

estimating the excavation disturbed zone in the permanent shiplock slopes of the three gorges project china abstract the stability and deformation of the permanent shiplock slopes are among the key issues in the design and construction of the three gorges project the permanent shiplock slopes formed by deeply and steeply cutting into weathered and fresh granites are 1607 m in length and 50 170 m in height the disturbed zone in the permanent cut slopes induced by excavation is therefore one of the most important aspects in slope stability and deformation evaluation a comprehensive investigation including non linear finite element analysis in situ testing instrumentation and monitoring back analysis and rock mass quality rating has been carried out for the identification and evaluation of the excavation disturbed zone in the permanent cut slopes the results of the investigation confirmed the existence of such an excavation disturbed zone in the permanent cut slopes this zone is characterized by a considerable weakening in the mechanical properties of the rock mass from the cut surface to the deeper region of the permanent cut slopes the excavation disturbed zone can be further divided into a damaged zone an affected zone and a slightly affected zone according to the extent of weakening in the rock mass the damaged zone and the affected zone have thicknesses of 5 10 m and 10 20 m respectively the exact pattern of the excavation disturbed zone is variable in different parts of the permanent cut slopes and are generally similar to that of the plastic zones estimated by using the non linear finite element analysis the present investigation has provided both factual data and insights for the stability and deformation evaluation of the permanent shiplock slopes the approach and methodology developed in the paper can be used to assess similar excavation disturbed zones in other large cut slopes

geohydromechanical processes in the excavation damaged zone in crystalline rock rock salt and indurated and plastic clays in the context of radioactive waste disposal abstract the creation of an excavation disturbed zone edz or excavation damaged zone edz is expected around all man made openings in geologic formations macro and micro fracturing and in general a redistribution of in situ stresses and rearrangement of rock structures will occur in this zone resulting in drastic changes of permeability to flow mainly through the fractures and cracks induced by excavation the edz may have significant implications for the operation and long term performance of an underground nuclear waste repository various issues of concern need to be evaluated such as processes creating fractures in the edz the degree of permeability increase and the potential for sealing or healing with permeability reduction in the zone in recent years efforts along these lines have been made for a potential repository in four rock types crystalline rock salt indurated clay and plastic clay and these efforts have involved field laboratory and theoretical studies the present work involves a synthesis of the ideas and issues that emerged from presentations and discussions on edz in these four rock types at a cluster conference and workshop held in luxembourg in november 2003 first definitions of excavation disturbed and edzs that may be applicable to all four rock types are proposed then an approach is suggested for the synthesis and intercomparison of geohydromechanical processes in the edz for crystalline rock salt indurated clay and plastic clay comparison tables of relevant processes associated factors and modelling and testing techniques are developed a discussion of the general state of the art and outstanding issues is also presented a substantial bibliography of relevant papers on the subject is supplied at the end of the paper

mechanism of deep cracks in the left bank slope of jinping first stage hydropower station abstract the proposed jinping first stage hydropower station is located on the middle reach of the yalongjiang river sichuan province pr china it consists of a double curvature arch dam with a maximum height of 305 m which will be the highest in the world cracks are well developed in the left bank slope of the pusiluogou the dam site of jinping first stage hydropower station some cracks are even developed at the horizontal distance into rock face in this paper the term horizontal depth was used to describe the horizontal distance into face of the slope beyond 200 m and these cracks are here referred to as deep cracks the distribution of these deep cracks is far beyond people s experiences in common unloading zones so the mechanism of the deep cracks is put forward as a new subject since these cracks are significant for the stability of the slope their mechanism becomes one of the key engineering geology problems a comprehensive study is made in this paper on the distribution characteristics of the cracks as well as their deformation features the factors that influence their distribution and deformation feature are also analyzed by means of geology mechanics and numerical simulation an important conclusion is consequently reached that deep cracks occurred as a result of release of high ground stress including gravitational stress and tectonic stress the formation of the deep cracks in the slope is a result of unloading process under favorable geology conditions fast crust uplift and quick river cutting and high tectonic stress are indispensable regional geological condition steep and high geometry condition provides favorable geometry condition hard at the lower part and soft at the upper part of the slope offers suitable rock combination condition and back facing bending planes and consequent discontinuities supply suitable structure of the slope

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

source of tiny wiggles in chron c5 a comparison of sedimentary relative intensity and marine magnetic anomalies 1 in addition to the well established pattern of polarity reversals short wavelength fluctuations are often present in both sea surface data tiny wiggles and near bottom anomaly data while a high degree of correlation between different geographical regions suggests a geomagnetic origin for some of these wiggles anomaly data alone cannot uniquely determine whether they represent short reversals or paleointensity variations independent evidence from another geomagnetic recording medium such as deep sea sediments is required to determine the true nature of the tiny wiggles we present such independent evidence in the form of sedimentary relative paleointensity from chron c5 we make the first comparison between a sedimentary relative paleointensity record odp site 887 at 54 n 148 w and deep tow marine magnetic anomaly data 43 n 131 w bowers et al 2001 for chron c5 the sediment cores are densely sampled at 2 5 kyr resolution the inclination record shows no evidence for reverse intervals within the 1 myr long normal chron c5n 2n rock magnetic measurements suggest that the primary magnetic carrier is pseudo single domain magnetite we choose a partial anhysteretic magnetization parm as our preferred normalizer and the resulting relative paleointensity record is used as input to a forward model of crustal magnetization we then compare the results of this model with the stacked deep tow anomaly records the two records show a significant degree of correlation suggesting that the tiny wiggles in the marine magnetic anomalies are likely produced by paleointensity variations an analysis of our sampling density suggests that if any reverse intervals exist at this site they are likely to be 5 kyr in duration furthermore we suggest that reverse intervals during chron c5n 2n documented in other locations are unlikely to be global

revised pacific m anomaly geomagnetic polarity timescale summary the current m anomaly geomagnetic polarity timescale gpts is mainly based on the hawaiian magnetic lineations in the pacific ocean m anomaly gpts studies to date have relied on a small number of magnetic profiles a situation that is not ideal because any one profile contains an uncertain amount of geologic noise that perturbs the magnetic field signal compiling a polarity sequence from a larger array of magnetic profiles is desirable to provide greater consistency and repeatability we present a new compilation of the m anomaly gpts constructed from polarity models derived from magnetic profiles crossing the three lineation sets hawaiian japanese and phoenix in the western pacific polarity reversal boundary locations were estimated with a combination of inverse and forward modelling of the magnetic profiles separate gpts were established for each of the three pacific lineation sets to allow examination of variability among the different lineation sets and these were also combined to give a composite timescale owing to a paucity of reliable direct dates of the m anomalies on ocean crust the composite model was time calibrated with only two ages one at each end of the sequence these two dates are 125 0 ma for the base of m0r and 155 7 ma for the base of m26r relative polarity block widths from the three lineation sets are similar indicating a consistent pacific wide spreading regime the new gpts model shows slightly different spacings of polarity blocks as compared with previous gpts with less variation in block width it appears that the greater polarity chron irregularity in older models is mostly an artifact of modelling a small number of magnetic profiles the greater averaging of polarity chron boundaries in our model gives a gpts that is statistically more robust than prior gpts models and a superior foundation for late jurassic early cretaceous geomagnetic and chronologic studies

a method for assessment of excavation damaged zone edz of a rock mass and its application to a dam foundation case abstract excavation of slope is an unloading process and also an energy releasing process when the stress in the rock mass is unloaded leading to the rebounding deformation towards the free face and then a zone namely excavation damaged zone edz will be formed near the excavation boundary where the rock is damaged or disturbed and its physical mechanical and hydraulic properties are greatly changed this paper is to put forward a new method to quantify the extent and damage degree of the edz according to the unloading strain energy which is released in the process of excavation a simple relationship between unloading strain e and accumulative opening displacement t of cracks d t d h is proposed to describe the damage degree of rock mass in the edz where h is the depth variable in normal direction of the open cracks this method is used to assess the edz of the dam foundation at xiaowan hydropower station in southwestern china by the accumulative opening displacement curves of cracks along boreholes based on boreholes camera photos we find that the edz can be naturally divided into two sub zones i e the excavation heavily disturbed sub zone ehdz and excavation slightly disturbed sub zone esdz according to the variation of unloading strain e two inflexions of the accumulative opening displacement curve indicate the lower limits of ehdz and esdz respectively

magnetic stratigraphy of kimmeridgian aptian sections from umbria marche italy new details on the m polarity sequence 1 the magnetic stratigraphy of the maiolica and calcari ad aptici pelagic limestones is documented from four umbro marchean land sections italy the overall biostratigraphic age of the sampled strata encompasses the whole kimmeridgian lower aptian the data were gathered from two new sections arcevia contessa and by resampling more densely than in the past two additional classical sections bosso gorgo a cerbara a detailed record of the polarity chrons m21n m14 and m9 m0 was derived and no difference was observed with respect to the sequence of m polarity chrons inferred from oceanic magnetic anomalies barely measurable remanent magnetization from chron m14 to m10n at contessa concurred with previous evidence from bosso indicating that the maiolica from umbria marche is characterized by a recording gap during chrons m14n to m11 possibly due to diagenetic magnetite dissolution related to the late valanginian carbon isotope event at arcevia and bosso the boundary between maiolica and calcari ad aptici occurs at the top of chron m19n and in the upper part of chron m20n respectively showing that the onset of maiolica sedimentation may be diachronous by 1 5 2 myr along the umbria marche domain arcevia is the most expanded mid upper tithonian land section documented so far here the subchrons m20n 1 and m19n 1 are found to represent 3 2 and 10 1 in time of chrons m20n and m19n respectively evidence is also provided for a reversal excursion occurring within the m20n 1 subchron at bosso an excursion and a reversal excursion were found within chrons m16n and m16 respectively

origin of the smooth zone in early cretaceous north atlantic magnetic anomalies 1 late jurassic early cretaceous marine magnetic anomalies observed in the north atlantic exhibit an abrupt change in character in m5 m15 crust the anomalies are smoother with low amplitudes and are difficult to correlate among nearby profiles the accepted explanation for the origin of this smooth zone is diminished resolution and anomaly interference due to slow spreading rates which narrows the widths of polarity reversals in the crust and causes interference among sea surface anomalies magnetic modeling of these anomalies indicates that neither slow spreading rates alone nor slow spreading rates in combination with a decrease in geomagnetic field intensity can explain the basic character of the smooth zone combined with other geophysical evidence our study suggests that one consequence of slow spreading rates that is responsible for the magnetic smooth zone is a thinned crustal basalt layer or a non basaltic magnetic source layer resulting from low melt supply during a period of ultra slow spreading