hydrocarbon generation and expulsion characteristics of eocene source rocks in the huilu area northern pearl river mouth basin south china sea implications for tight oil potential abstract in recent years new oil reservoirs have been discovered in the eocene tight sandstone of the huilu area northern part of the pearl river mouth basin south china sea indicating good prospects for tight oil exploration in the area exploration has shown that the huilu area contains two main sets of source rocks the eocene wenchang e2w and enping e2e formations to satisfy the requirements for further exploration in the huilu area particularly for tight oil in eocene sand reservoirs it is necessary to re examine and analyze the hydrocarbon generation and expulsion characteristics based on mass balance this study investigated the hydrocarbon generation and expulsion characteristics as well as the tight oil resource potential using geological and geochemical data and a modified conceptual model for generation and expulsion the results show that the threshold and peak expulsion of the e2w source rocks are at 0 6 vitrinite reflectance and 0 9 vitrinite reflectance respectively there were five hydrocarbon expulsion centers located in the western eastern and northern huizhou sag and the southern and northern lufeng sag the hydrocarbon yields attributed to e2w source rocks are 2 4 1011 tons and 1 6 1011 tons respectively with an expulsion efficiency of 65 the e2e source rock threshold and peak expulsion are at 0 65 vitrinite reflectance and 0 93 vitrinite reflectance respectively with hydrocarbon expulsion centers located in the centers of the huizhou and lufeng sags the yields attributed to e2e source rocks are 1 1 1011 tons and 0 2 1011 tons respectively with an expulsion efficiency of 20 using an accumulation coefficient of 7 13 the eocene tight reservoirs could contain approximately 1 3 1010 tons to 2 3 1010 tons with an average of 1 8 1010 tons of in place tight oil resources highest recoverable coefficient can reach 17 18 indicating that there is significant tight oil potential in the eocene strata of the huilu area

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

the crustal structure from the altai mountains to the altyn tagh fault northwest china 1 we present a new crustal section across northwest china based on a seismic refraction profile and geologic mapping the 1100 km long section crosses the southern margin of the chinese altai mountains junggar accretional belt and eastern junggar basin easternmost tianshan mountains and easternmost tarim basin the crustal velocity structure and poisson s ratio which provide a constraint on crustal composition were determined from p and s wave data despite the complex geology the crustal thickness along the entire profile is nearly uniform at 50 km the thickest crust 56 km occurs at the northern end of the profile beneath the altai mountains and the thinnest 46 km crust is beneath the junggar basin beneath surficial sediments the crust is found to have three layers with p wave velocities vp of 6 0 6 3 6 3 6 6 and 6 9 7 0 km s respectively the southern half of the profile including the eastern tianshan mountains and eastern margin of the tarim basin shows low p wave velocities and 0 25 to a depth of 30 km which suggests a quartz rich granitic upper crustal composition the northern half of the profile below the altai mountains and junggar accretional belt has a higher poisson s ratio of 0 26 0 27 to a depth of 30 km indicative of an intermediate crustal composition the entire 1100 km long profile is underlain by a 15 30 km thick high velocity 6 9 7 0 km s 0 26 0 28 lower crustal layer that we interpret to have a bulk composition of mafic granulite at the southern end of the profile a 5 km thick midcrustal low velocity layer vp 5 9 km s 0 25 underlies the tianshan and the region to the south and may be indicative of a near horizontal detachment interface pn velocities are 7 7 7 8 km s between the tianshan and the junggar basin and 7 9 8 0 km s below the altai mountains and eastern margin of the tarim basin we interpret the consistent three layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after paleozoic terrane accretion the thickness 50 km of the crust appears to be related to compression resulting from the indo asian collision

geochemistry origin and accumulation of petroleum in the eocene wenchang formation reservoirs in pearl river mouth basin south china sea a case study of hz25 7 oil field abstract the pearl river mouth basin in the south china sea has accumulated 2 km of eocene sediments in its deep basin and has become the exploration focus due to the recent discoveries of the hz25 7 oil field in the eocene wenchang e 2 w formation in this study the geochemical characteristics of potential source rocks and petroleum in the hz25 7 oil field are investigated and the possible origins and accumulation models developed the analytical results reveal two sets of potential source rocks e 2 w and enping e 2 e formations developed in the study area the semi deep to deep lacustrine e 2 w source rocks are characterized by relatively low c 29 steranes low c 19 c 23 tricyclic terpane 24 tetracyclic terpane c 30 hopane trans trans trans bicadinane t c 30 hopane most 30 4 methyl sterane c 29 sterane 0 2 ratios in contrast the shallow lacustrine and deltaic swamp plain e 2 e source rocks are characterized by relatively high c 29 steranes high c 19 c 23 tricyclic terpane 0 6 high c 24 tetracyclic terpane c 30 hopane 0 1 variable yet overall high t c 30 hopane and low c 30 4 methyl sterane c 29 sterane 19 c 23 tricyclic terpane ratios mean value 0 39 low c 24 tetracyclic terpane c 30 hopane ratios mean value 0 07 high c 30 4 methyl sterane c 29 sterane ratios mean value 1 14 and relatively high c 27 regular sterane content of petroleum in the hz25 7 oil field indicate that the petroleum most likely originated from the e 2 w formation mudstone in the huizhou depression one stage of continuous charging is identified in the hz25 7 oil field oil injection is from 16 ma to present and peak filling occurs after 12 ma thin sandstone beds with relatively good connectivity and physical properties porosity and permeability in the e 2 w formation are favorable conduits for the lateral migration of petroleum this petroleum accumulation pattern implies that the e 2 w formation on the western and southern margins of the huizhou depression are favorable for petroleum accumulation because they are located in a migration pathway thus exploration should focus in these areas in the future

global positioning system measurements of present day crustal movements in the arabia africa eurasia plate collision zone we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1994 at 54 sites extending east west from the caucasus mountains of southern russia georgia and armenia to the aegean coast of turkey and north south from the southern edge of the eurasian plate pontus block to the northern edge of the arabian platform viewed from a eurasia fixed reference frame sites on the northern arabian platform move n38 13 w at 20 3 mm yr roughly consistent with the velocity implied by nuvel 1a circuit closure n23 7 w at 24 2 mm yr the motion of arabia appears to be transferred directly to the region of turkey north of the suture however eastern turkey is characterized by distributed deformation while central western turkey is characterized by coherent plate motion involving westward displacement and counterclockwise rotation of the anatolian plate internal deformation within the central part of the anatolian plate is less than 2 mm yr the anatolian plate is decoupled from eurasia along the right lateral strike slip north anatolian fault naf this different response in eastern and western turkey to the collision of arabia may result from the different boundary conditions the hellenic arc forming a free boundary to the west and the asian continent and oceanic lithosphere of the black and caspian seas forming a resistant boundary to the north and east we derive a best fitting euler vector for anatolia eurasia motion of 29 2 0 8 n 32 9 0 4 1 3 0 1 m y the mapped surface trace of the naf corresponds well to a small circle about this pole the new euler vector implies an upper bound for naf slip rate of 30 2 mm yr i e assuming all relative motion is accommodated along the naf using the nuvel 1a euler vector for arabia eurasia and the gps euler vector for anatolia eurasia we determine an arabia anatolia euler vector of 31 2 n 45 2 e 0 9 0 1 m y and an upper bound on the east anatolian fault slip rate of 15 3 mm yr the aegean trough region of western turkey deviates significantly from coherent plate rotation in addition to rotating with anatolia this region shows roughly n s extension at a rate of 14 5 mm yr taken together with satellite laser ranging results along the hellenic arc the contemporary pattern of deformation indicates increasing motions toward the arc suggesting that the westward displacement and counterclockwise rotation of anatolia is driven both by pushing from the arabian plate and by pulling or basal drag associated with the foundering african plate along the hellenic subduction zone

shallow velocity structure and poisson s ratio at the tarzana california strong motion accelerometer site the 17 january 1994 northridge california earthquake produced strong ground shaking at the cedar hills nursery referred to here as the tarzana site within the city of tarzana california approximately 6 km from the epicenter of the mainshock although the tarzana site is on a hill and is a rock site accelerations of approximately 1 78 g horizontally and 1 2 g vertically at the tarzana site are among the highest ever instrumentally recorded for an earthquake to investigate possible site effects at the tarzana site we used explosive source seismic refraction data to determine the shallow 70 m p and s wave velocity structure our seismic velocity models for the tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking p wave velocities range from 0 9 to 1 65 km sec and s wave velocities range from 0 20 and 0 6 km sec for the upper 70 m we also found evidence for a local s wave low velocity zone lvz beneath the top of the hill the lvz underlies a cdmg strong motion recording site at depths between 25 and 60 m below ground surface bgs our velocity model is consistent with the near surface 30 m p and s wave velocities and poisson s ratios measured in a nearby 30 m borehole high poisson s ratios 0 477 to 0 494 and s wave attenuation within the lvz suggest that the lvz may be composed of highly saturated shales of the modelo formation because the lateral dimensions of the lvz approximately correspond to the areas of strongest shaking we suggest that the highly saturated zone may have contributed to localized strong shaking rock sites are generally considered to be ideal locations for site response in urban areas however localized highly saturated rock sites may be a hazard in urban areas that requires further investigation

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

faulting apparently related to the 1994 northridge california earthquake and possible co seismic origin of surface cracks in potrero canyon los angeles county california apparent southward dipping reverse fault zones are imaged to depths of about 1 5 km beneath potrero canyon los angeles county california based on their orientation and projection to the surface we suggest that the imaged fault zones are extensions of the oak ridge fault geologic mapping by others and correlations with seismicity studies suggest that the oak ridge fault is the causative fault of the 17 january 1994 northridge earthquake northridge fault our seismically imaged faults may be among several faults that collectively comprise the northridge thrust fault system unusually strong shaking in potrero canyon during the northridge earthquake may have resulted from focusing of seismic energy or co seismic move ment along existing related shallow depth faults the strong shaking produced ground surface cracks and sand blows distributed along the length of the canyon seismic reflection and refraction images show that shallow depth faults may underlie some of the observed surface cracks the relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have devel oped from nontectonic alluvial movement but others may be fault related imme diately beneath the surface cracks p wave velocities are unusually low 400 m sec and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m on the basis of velocity data we suggest that unconsolidated soils 800 rrdsec extend to depths of about 15 to 20 m beneath our datum 25 m below ground surface the underlying rocks range in velocity from about 1000 to 5000 m sec in the upper 100 m this study illustrates the utility of high resolution seismic imaging in assessing local and regional seismic hazards

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

a complex seismogenic shear zone the duzce segment of north anatolian fault turkey abstract this work highlights how surface geological data can integrate and complete a fault characterization including its seismogenic depth we investigated the area struck by the november 1999 mw 7 1 earthquake that ruptured the duzce segment of the north anatolian fault zone nafz after the august 1999 mw 7 4 izmit earthquake to explore the persistency or evolution of the active fault setting at the surface we compared detailed coseismic surface expressions with the long term morphology and structural architecture of the duzce zone we identified a simple 1999 coseismic fault trace cft and an older complex fault system rfs involving a wider zone of deformation two different parts of the duzce segment were recognized a western section presents a staircase trajectory of the coseismic fault trace that reactivated part of the older fault system an eastern section presents a straight coseismic fault trace that cross cuts the older and complex fault system the western duzce fault segment splays out from the izmit counterpart of the nafz forming a releasing fault junction by comparing a model of its coseismic surface deformation field with the long term morphology we observed that the present topography of this western section results from the nafz activity by repeating coseismic deformations like those in 1999 the mechanical interaction in the release fault junction produces local transtension across the western duzce fault and justifies its stable complexity it does not evolve into a straight trace the surface boundary between the duzce fault sections also separates at depth two fault sections one with low slip to the west and another with a big asperity to the east thus the izmit duzce interaction 1 may produce an unfavorable setting for a build up of asperities in the western duzce segment and 2 could delay the propagation of the izmit rupture along the duzce segment that nucleates mainshocks only on its eastern section in other words the arrangement of the fault junction may permanently control how the two sections of the duzce fault behave and rupture

first successful 40ar 39ar dating of glauconies argon recoil in single grains of cryptocrystalline material potassium argon dating of glaucony one of the few widely occurring authigenic materials in the sedimentary record has provided a large percentage of the dates used for the calibration of the geologic time scale this material has been considered unsuitable for 40 ar 39 ar dating because of the large losses of neutron induced 39 ar during or subsequent to irradiation of samples we have measured recoiled 39 ar and 37 ar from single grains of glaucony 100 350 m in diameter irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation the observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains when the microampoule technique is used the integrated 40 ar 39 ar ages and their associated precision compare very favorably with conventional k ar results because glauconies are inherently heterogeneous populations owing to their complex evolution it is anticipated that single grain 40 ar 39 ar dating may lead to significant refinements in the geologic time scale especially for the cenozoic

the surface rupture and slip distribution of the 17 august 1999 izmit earthquake m 7 4 north anatolian fault the 17 august 1999 izmit earthquake occurred on the northern strand of the north anatolian fault zone the earthquake is associated with a 145 km long surface rupture that extends from southwest of duzce in the east to west of hersek delta in the west detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step overs herein named the hersek karamursel golcuk izmit sapanca lake sapanca akyazi and karadere segments from west to east respectively the hersek segment which cuts the tip of a large delta plain in the western end of the rupture zone has an orientation of n80 the n70 80 e trending karamursel golcuk segment extends along the linear southern coasts of the izmit gulf between karamursel and golcuk and produced the 470 cm maximum displacement in golcuk the northwest southeast striking golcuk normal fault between the karamursel golcuk and izmit sapanca segments has 2 3 m maximum vertical displacement the maximum dextral offset along the izmit sapanca lake segment was measured to be about 3 5 m and its trend varies between n80 e and east west the sapanca akyazi segment trends n75 85 w and expresses a maximum displacement of 5 2 m the karadere segment trends n65 e and produced up to 1 5 m maximum displacement the karadere and sapanca akyazi segments form fan shape or splaying ruptures near their eastern ends where the displacement also diminished

morphology displacement and slip rates along the north anatolian fault turkey 1 geological and geomorphological offsets at different scales are used to constrain the localization of deformation total displacement and slip rates over various timescales along the central and eastern north anatolian fault naf in turkey the naf total displacement is reevaluated using large rivers valleys 80 15 km and structural markers pontide suture 85 25 km tosya vezirkopru basins 80 10 km these suggest a neogene slip rate of 6 5 mm yr over 13 myr the river network morphology shows offsets at a range of scales 20 m to 14 km across the main fault trace and is also used to estimate the degree to which deformation is localized at a smaller scale the morphology associated with small rivers is offset by 200 m along the naf the age of these features can be correlated with the holocene deglaciation and a slip rate of 18 3 5 mm yr is determined this is consistent with a rate of 18 5 mm yr deduced independently from the 14c dating of stream terrace offsets over the short term gps data gives a similar rate of 22 3 mm yr all our results tend to show that most of the deformation between the anatolian and eurasian lithospheric plates has been accommodated along or very close to the active trace of the naf the difference between the neogene and the holocene slip rate may be due to the recent establishment of the current plate geometry after the creation of the naf

rapid determination of near fault earthquake deformation using differential lidar improved near field measurements of earthquake slip and deformation patterns have the potential for expanding our understanding of fault behavior and the relationship of active faulting to topography current techniques for obtaining these measurements including field observation global navigation satellite systems displacement estimation and optical or radar remote sensing have limitations that can be mitigated by the inclusion of results from differential airborne light detection and ranging lidar analysis of the rupture zone the 2005 airborne lidar survey of the southern san andreas san jacinto and banning faults the b4 survey mapped 1100 km of the most seismically active fault systems in southern california for the purpose of providing a baseline for determining slip from a future earthquake we used the b4 survey to develop a processing algorithm that yields rapid estimates of near fault ground deformation using simultaneous cross correlation of both topography and backscatter intensity from pre earthquake and simulated postearthquake lidar datasets we show robust recovery of the direction and magnitude of an applied synthetic slip of 5 m in the horizontal and 0 5 m in the vertical within our area of study with clear discrimination between areas with and without applied slip we also successfully recovered more complex deformation from a modeled fault stepover in the same study area our results indicate that we should be able to recover slip to accuracies of better than 20 cm in the horizontal and 1 cm in the vertical at a spatial resolution of 15 m for lidar datasets with sample densities as low as 0 5 points m2

the temporal and spatial distribution of volcanism in the south china sea region cenozoic magmatism in the south china sea the associated continental margins coastal south china and indochina is reviewed volcanism contemporaneous with rifting and sea floor spreading in the south china sea was very weak in the margins and adjacent areas there was only very limited magmatic activity most probably after the cessation of sea floor spreading the south china sea has non volcanic margins large basement relief caused by extension formed major basins and a continental oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting the paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the south china sea which was supposedly linked to the escape of indochina induced by indo eurasia collision c 2005 published by elsevier ltd

surface rupture and slip distribution along the karadere segment of the 17 august 1999 izmit and the western section of the 12 november 1999 duzce turkey earthquakes on 17 august 1999 the mw 7 5 i zmit turkey earthquake produced surface rupture in excess of 120 km and perhaps as much as 200 km with up to 5 m of dextral slip along a western portion of the north anatolian fault zone the 12 november 1999 mw 7 1 duzce turkey earthquake produced a 40 km long sur face rupture including 9 km of rupture overlap with the eastern end of the august event our mapping focused on the 40 km long karadere rupture segment the east ernmost segment of the august event as well as on the western 20 km of the no vember rupture maximum dextral slip along the karadere segment is approximately 1 5 m and the average slip on this segment is close to 1 m although slip along the karadere segment is considerably less than that on segments to the west this segment is of particular interest for three reasons 1 the western boundary of the karadere segment is defined by the most striking structural discontinuity along the entire au gust surface rupture i e a 5 km wide zone of no surface rupture as well as a 25 change in trend from e w to ene and such a discontinuity may have important implications for rupture dynamics 2 surface rupture terminates at the east end of the karadere rupture segment at a 1 5 to 3 km wide extensional step over at eften lake and 3 the 12 november 1999 duzce earthquake reruptured the easternmost 9 km of the karadere segment raising interesting questions about rupture dynamics and interactions between events on adjacent fault segments the details of the 17 august 1999 and l2 november 1999 surface rupture traces suggest that rupture may have partially propagated across the eften lake extensional step over although this step over seems to have acted as an effective barrier to rupture propagation

surface breakage of the 1992 landers earthquake and its effects on structures abstract the landers california earthquake m w 7 3 provides an exceptional opportunity to study surface rupture of an earthquake fault detailed maps of the lateral distribution of fracturing adjacent to main traces show that rupture patterns are much more complex than documented in past studies of surface ruptures the rupture occurs in tabular zones up to hundreds of meters wide a main trace within each rupture zone accommodates much of the shear deformation but considerable fracturing occurs throughout the tabular zone the en echelon pattern of fracturing in step over zones between main traces is typically even more complex than those along major fault zones inspection of several on grade concrete slabs indicates that unreinforced concrete foundations generally crack when subjected to distinct ground ruptures beneath them or when they are twisted because of differential ground movements across broad zones methods of mitigating the potential hazards associated with earthquake fault rupture are presented

the age of illite cement growth village fields area southern north sea evidence from k ar ages and 18o 16o ratios in this paper we describe k ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the permian lower leman sandstone formation rotliegendes group the samples come from a large about 100 100 km part of the southern north sea basin village fields area and from a range of burial depths 2 8 3 4 km subsea bed mean illite ages in the gas fields of ravenspurn north 164 ma 20 1 2 sgr n 11 hyde 161 ma 12 3 n 7 cleeton 155ma 20 2 n 3 and hoton 152 ma 14 0 n 14 fields and in two wells located in block 47 5a 162 ma 16 1 n 4 are all in the middle to late jurassic apart from two early cretaceous ages considered to represent unreliable analyses the m an for all ages is 158 ma 18 6 2 sgr n 39 we largely avoided contamination with old detrital k bearing minerals by careful sample selection and monitoring to a first approximation the assumptions of k ar dating appear to be fulfilled so the measured ages are likely to date illite cement precipitation oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the middle jurassic when illite grew both k ar dating and oxygen isotope ratios are compatible with a regional middle to late jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time the age of illite cementa ion coincides with rifting which ultimately may have caused illite growth by driving pore water flow

direct dating of carbonate remagnetization by 40ar 39ar analysis of the smectite illite transformation abstract the occurrence of remagnetizations in carbonate rocks is a longstanding issue the result of a cryptic process that affects rocks in both fold thrust belts and undisturbed continental interiors this paper examines the clay mineralogy of insoluble material from remagnetized carbonates taken from the cantabrian asturian belt northern spain detailed x ray diffraction patterns indicate that mixed layer clays from these rocks are mostly illitic with an increasing proportion of authigenic illite polytype 1m d in finer grain size fractions the 40 ar 39 ar age data from the different grain size fractions coupled with quantitative polytype modeling indicates an authigenic age that is coeval with the established late paleozoic remagnetization age of these rocks we propose that the transformation of fe rich smectite to fe poor illite created a population of authigenic magnetite that is responsible for the remagnetization of these rocks the irreversible kinetic dependence of clay mineral transformations appears to account for the irreproducibility and unpredictability of carbonate remagnetization and may explain such vexing phenomena as synfolding magnetizations this technique provides an independent assessment of the presence and timing of rock remagnetization with numerous potential applications in future studies of carbonate sequences

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

slip distribution and stress changes associated with the 1999 november 12 duzce turkey earthquake mw 7 1 the 1999 november 12 duzce earthquake m w 7 1 was apparently the eastward extension of the august 17 izmit earthquake m w 7 4 the duzce event caused heavy damage and fatalities in the cities of duzce and bolu here a finite fault inversion method with five discrete time windows is applied to derive the co seismic slip distribution of the duzce earthquake the fault plane is best modelled as a 40 x 20 km 2 plane with a strike of 262degrees and a dip of 65degrees to the north and that the majority of slip occurred in two distinct patches on either side of the hypocentre implying bilateral rupture the possible triggering of this event by the izmit earthquake is investigated using coulomb stress modelling of all large events since 1943 with the inclusion of secular loading the results show that although the duzce rupture plane was in a stress shadow prior to the izmit earthquake that event caused a significant coulomb stress load taking the duzce fault out of the stress shadow which probably precipitated failure a comparison of the mapped coulomb stress change with the inferred slip shows no correlation between the two finally the stress modelling indicates that the northern branch of the north anatolian fault zone beneath the sea of marmara towards the city of istanbul is presently the most highly loaded segment of the north anatolian fault zone

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

crustal structure of the central qaidam basin imaged by seismic wide angle reflection refraction profiling abstract we present the results of a seismic wide angle reflection refraction profile across the central qaidam basin the largest basin within the qinghai tibetan plateau the 350 km long profile extends from the northern margin of the east kunlun shan to the southern margin of the qilian shan the p and s wave velocity structure and poisson s ratio data provide constraints on composition the crust here consists of a near surface sedimentary layer and a four layered crystalline crust having several significant features 1 the sedimentary fill of the qaidam basin reaches a maximum thickness of 8 km and the basin shape mirrors the uplifted moho 2 within the four layers of the crystalline crust p s wave velocities increase with depth and fall within the following velocity ranges 5 9 6 3 km s 3 45 3 65 km s 6 45 6 55 km s 3 7 km s 6 65 km s 3 8 km s and 6 7 6 9 km s 3 8 3 9 km s respectively 3 low velocity zones with a 3 5 reduction in seismic velocity are detected in the lower half of the crust beneath the qaidam basin and its transition to the qilian shan 4 the crystalline crust is thickest beneath the northern margin of the basin towards the qilian shan 58 62 km and thinnest beneath the center of the basin 52 km variations in crustal thickness are caused most pronouncedly by thickness variations in the lowermost layer of the crust 5 poisson s ratio and p wave velocity values suggest that the qaidam crust has an essentially felsic composition with an intermediate layer at its base based on the crustal structure reported here we suggest that late cenozoic convergence is accommodated by thick skinned tectonic deformation with thickening involving the entire crust across the kunlun qaidam qilian system

present day kinematics of the middle east and eastern mediterranean the middle east and eastern mediterranean regions contain three major strike slip fault zones the dead sea fault zone dsfz and the north and east anatolian fault zones nafz and eafz these accommodate northward motion of the african and arabian plates relative to eurasia and westward motion of the small turkish plate this study deduces an internally consistent solution for the present day kinematics of these plates which is accurate to better than 20 and applicable throughout time since 5 ma by constraining the slip senses and rates on these strike slip fault zones the 7 0 5 mm yr 1 left lateral slip rate on the dsfz is associated with counterclockwise rotation of arabia relative to africa at 0 33 m y 1 around an eulcr pole near 33 n 23 e the nafz takes up counterclockwise rotation of the turkish plate relative to eurasia at 0 83 m y 1 13 around a pole at 31 n 35 5 e with right lateral slip rate 15 mm yr 1 the resulting prediction for the eafz involves 14 mm yr 1 left lateral motion and 2 mm yr 1 of shortening between arabia and turkey the triple junction of africa arabia and turkey is near the city of kahramanmaras this locality contains a narrow promontory bounded by ssw trending left lateral faults at the edges of the arabian and turkish plates which is loosely attached to the african plate moving westward relative to its interior at 4 mm yr 1

k ar dating of time of gas emplacement in rotliegendes sandstone netherlands the time of emplacement of gas into each of two reservoirs in the rotliegendes sandstone one in the groningen field and one in the broad fourteens basin was estimated from potassium argon k ar ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred in each reservoir the intensity of diagenetic alteration increased downward within a relatively thin 15 to 175 m or 50 to 580 ft interval apparently reflecting a longer period of alteration for deeper samples this is expected in cases where a gradually accumulating gas column expels pore water from progressively deeper parts of the reservoir rock illite is a common diagenetic phase in the rotliegendes because detrital contamination in these rocks has been infer ed to be minimal k ar ages of very fine grained illite should indicate the time the illite forming process ceases in each case reported here the age of the deeper sample is younger than that of the shallower we interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock our results indicate that gas emplacement in the groningen field commenced prior to 150 ma and that the growth of diagenetic illite continued in the present day water zone from that time until at least 120 ma

gps derived strain rate field within the boundary zones of the eurasian african and arabian plates we use the gps velocity field 1988 1998 for eastern mediterranean and asia minor to determine the crustal deformation strain rate field in an area bounded by 35 n and 43 n and 20 e and 48 e we calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data uncertainties in the calculation of the strain rates reach 50 nstrain yr 1 in sparsely observed parts of anatolia whereas we estimate errors 20 nstrain yr 1 in the aegean and marmara regions the largest compressional strain rate components in the eastern part of the study area occur along the greater caucasus mountain front reaching 70 nstrain yr 1 1 nstrain yr 1 0 0317 10 15 s 1 the north anatolian fault zone is the clearest feature in the shear strain rate field it is expressed as a pronounced dextral strike slip fault zone reaching rates of up to 170 nstrain yr 1 this holds true also for the izmit area where the august 17 1999 earthquake occurred central anatolia is almost strain free whereas extension prevails in western anatolia the principal axes of extension vary around the n s direction with strain rates of up to 85 nstrain yr 1 these extensional areas coincide with graben features and normal faulting earthquakes the central and southwestern aegean sea is strain free with values far below 40 nstrain yr 1 the seismic cluster around the dodekanissa islands southeastern aegean sea coincides with nw se oriented extension attaining strain rates of up to 90 nstrain yr 1 this area of extension also exhibits recent active volcanism the entire hellenic arc shows compressional strain rates perpendicular to the arc the pliny strabo troughs along the eastern segment of the arc show left lateral shear strain rates reaching 80 nstrain yr 1 significant extension is found in central greece with a nne ssw oriented maximum of 120 nstrain yr 1 centered around the gulf of corinthos the kephalonia fault zone in nw greece is a distinct dextral fault zone separating apulia from the rapidly moving aegean microplate right lateral shear strain rates reach 150 nstrain yr 1

topographic site effects a comparison of two nonreference methods abstract we present an experimental study of topographic site effects the data we use come from an experiment carried out during the summer of 1989 in epire northern greece ten digital stations recorded a total of 68 small earthquakes a recent article chavez garcia et al 1996 presented a comparison between site effects determined using horizontal to vertical spectral ratios hvsr for this data set and theoretical modeling in this note we compare the topographic site effects determined using hvsr with another independent experimental estimate a generalized inversion scheme gis neither hvsr nor gis depend on the availability of a reference site we obtain a very good agreement between both estimates of topographic site effects for both horizontal components our results support the use of hvsr to determine topographic site effects

the anomalous seismic response of the ground at the tarzana hill site during the northridge 1994 southern california earthquake a resonant sliding block the northridge california earthquake of 17 january 1994 mw 6 7 caused widespread damage in the san fernando valley and adjacent areas in many places the earthquake motion was amplified or strongly modified by the local ground response but the most localized and extreme site effect occurred at tarzana a small hill of gentle topography at the northern edge of the santa monica mountains where strong amplification horizontal peak acceleration of 1 82g and other unusual site effects were recorded at a cdmg accelerograph the anomalous site effect was con fined to a small area 50 m in radius around the station beyond which the ground shaking was down to its theoretically expected level if strong ground motion can vary so much in such a small scale microzonation of urban areas may be an impos sible goal at worst or at best only reliable at those sites where ground response has previously been measured hence to clearly understand the dynamic response of the ground at the tarzana site has become an important problem in earthquake hazard assessment and microzonation this article reports results on the characteristics of the seismic response of the tarzana site based on the investigation of aftershock data recorded on a local array and recorded accelerograms preliminary results strongly suggest that the tarzana hill is formed by an old landslide of the modelo formation such a structure is consistent with the anomalous resonant behavior at coo the observed resonant fre quencies and the complex body wave recordings observed throughout the local array the results also suggest that the 1 82g peak acceleration at co0 could have been caused by a small mw 1 3 earthquake located some 60 m from the station generated by a downslope sliding of the hill with a maximum displacement of 4 cm over a circular area of nearly 50 m in radius the slide was triggered by the north ridge mainshock

age of metamorphism in the lesser himalaya and the main central thrust zone garhwal india results of illite crystallinity 40 ar 39 ar fusion and k ar studies

comparative 40ar 39ar and k ar dating of illite type clay minerals a tentative explanation for age identities and differences abstract the 40 k 40 ar k ar and 40 ar 39 ar dating methods are applied here to the same very small micrometric illite type particles that crystallized under low temperature 5 9 ma the same size fractions dated by the 40 ar 39 ar method give total gas ages ranging from 173 3 2 0 to 228 8 1 6 ma nearly all 40 ar 39 ar total gas ages are slightly older which cannot be explained by the recoil effect only the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics with varied crystallinity indices and a particle width about 10 times large than thickness the 40 ar 39 ar data outline some advantages such as the plateaus obtained by incremental step heating of the various size fractions even if not translatable straight as ages of the illite populations they allow identification of two generations of authigenic illite that formed at about 200 and 175 ma and one detrital generation however 40 ar 39 ar dating of clay minerals remains challenging as technical factors such as the non standardized encapsulation may have potential unexpected effects both dating methods have their limitations 1 k ar dating requires relatively large samples ca 10 20 mg incurring potential sample homogeneity problems with two aliquots required for k and ar analysis for an age determination also inducing a higher analytical uncertainty 2 an identified drawback of 40 ar 39 ar dating is ar recoil and therefore potential loss that occurs during neutronic creation of 39 ar from 39 k mostly in the finer mineral particles if all the recoiled 39 ar is redistributed into adjacent grains minerals the final 40 ar 39 ar age of the analyzed size fraction remains theoretically identical but it is not systematic in clay type material the finest grain sizes e g 39 ar recoil and are therefore only datable by the 40 ar 39 ar method using an encapsulation technique that still needs to be technically evaluated in this study 39 ar recoil during irradiation was quantified by encapsulation it ranged from about 20 of the total 39 ar for the 39 ar recoil confirms that grain size increasing minimizes the recoil effect but how it proceeds has still to be explained despite recent developments in 40 ar 39 ar dating the conventional k ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well controlled technical approach the present comparison of the two ar dating methods as applied to clay material shows that neither method is presently outdated and that they are even of reciprocal use both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

deformation during the 12 november 1999 duzce turkey earthquake from gps and insar data only 87 days after the mw 7 5 17 august 1999 izmit earthquake the duzce earthquake ruptured a ca 40 km long adjoining strand of the north anatolian fault naf system to the east we used displacements of 50 global positioning system gps sites together with interferometric synthetic aperture radar insar range change data spanning the event to estimate the geometry and slip distribution of the coseismic rupture postseismic deformation transients from the duzce earth quake and the preceding i zmit event that are included in some of the measurements are corrected for using dislocation models fit to gps data spanning the various time periods nonlinear inversions for fault geometry indicate that the rupture occurred on a ca 54 north dipping oblique normal right lateral fault distributed slip inver sions indicate maximum strike slip near the center of the duzce fault close to the earthquake hypocenter slip magnitude and depth of faulting decrease to the west and east of the hypocenter both gps and insar data suggest that normal slip is restricted to the shallow portion of the rupture the duzce earthquake had the highest slip to rupture length ratio of any historic earthquake along the naf

sphalerite 40ar 39ar progressive crushing and stepwise heating techniques abstract it is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods the 40ar 39ar method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation this preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40ar 39ar using crushing and heating techniques the 40ar 39ar crushing experiments of two sphalerite samples from the fankou lead zinc deposit yield well defined and concordant isochron ages of 265 8 1 0 1 ma and 269 7 6 2 ma corresponding to initial 40ar 36ar ratios of 290 7 2 4 and 294 6 8 1 respectively the crushed powders of the later sphalerite yields with stepwise heating a flat age spectrum with 40ar 39ar plateau age of 267 2 2 5 ma which define a good isochron with an age of 271 3 5 4 ma and initial 40ar 36ar ratio of 294 0 1 7 this is the first report to distinguish secondary and primary fluid inclusions within sphalerite to obtain their ages using the crushing technique and to obtain good concordant ages both of fluid inclusions by crushing and heating of k containing minerals we expect progressive crushing and stepwise heating techniques will become an effective approach to 40ar 39ar dating of the mineralization ages of hydrothermal sulfide deposits

seismotectonic setting of the karadere duzce branch of the north anatolian fault zone between the 1999 izmit and duzce ruptures from analysis of izmit aftershock focal mechanisms 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 duzce area da based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors 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 duzce 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 duzce basin db and a first order strike slip regime along the western duzce fault df we conclude that the duzce 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 duzce fault that bounds the duzce basin to the south reflects mostly strike slip events representing a major asperity along the north anatolian fault zone nafz before initiating the duzce rupture 87 days after the izmit event

crustal structure of the northeastern tibetan plateau from the southern tarim basin to the sichuan basin china abstract we present a new crustal cross section of the northeastern tibetan plateau based on active source seismic data recorded along a 1600 km long profile crossing the southern tarim basin the western flank of the south qilian shan the northeastern margin of qaidam basin east kunlun shan songpan ganzi terrane and sichuan basin the crustal p and s wave velocity structure and poisson s ratio outline the seismic characteristics of the crust and provide constraints on the crustal composition the derived crustal cross section shows several significant features 1 the crustal thickness varies considerably along this profile from 48 km to 70 km 2 north of the kunlun fault variations in total crustal thickness are mainly caused by variations in lower crustal thickness whereas south of the kunlun fault they are caused by thickness variations throughout the crust 3 north of the kunlun fault we detect a mid crustal low velocity zone that is not apparent south of the fault 4 the kunlun fault seems to act as a compositional boundary for the lower crust with a poisson s ratio of 0 29 north of the fault kunlun qaidam terrane and 0 26 south of the fault songpan ganzi terrane measured poisson s ratio and p wave velocity values suggest that the lower crust throughout the tibetan plateau south qilian shan margins of the qaidam basin east kunlun shan songpan ganzi terrane is of intermediate composition thus the ne tibetan plateau along our profile is missing a mafic lower crustal layer

coseismic ruptures and tectonic landforms along the duzce segment of the north anatolian fault zone ms 7 1 november 1999 1 this paper presents a comparison between the pattern of surface ruptures produced by a single earthquake and patterns of cumulative deformation we performed a detailed study of the 1999 earthquake coseismic ruptures and of the long term tectonic landforms in a key area of the duzce fault segment of the north anatolian fault we observed a scale independent en echelon arrangement of the coseismic surface ruptures as a whole the long term geomorphic expression of the duzce fault near the 1999 ruptures is evidence of the principal slip zone at depth that accommodates the bulk of the displacement during an individual rupture event this may stay localized through many rupture episodes with persistent geometry and kinematics the long term tectonic and geomorphic expression of the fault in a broader area around the 1999 ruptures defines a wider deformation zone in fact an old and complex fault arrangement has been mapped partially coinciding with the 1999 rupturing fault suggesting that the 1999 ruptures are an incomplete expression of the long term duzce fault system the relationships between the coseismic and the old fault systems suggest an evolution of the fault pattern trough time with a tendency to simplify a geometric complexity into a straighter mature trace the integrated investigation of long term tectonic morphologies and structural pattern offers a noteworthy frame to interpret the coseismic rupture kinematics and clarifies their complexities moreover to fully understand the principal slip zone at depth this work shows the importance of the study of strain distribution pattern and evolution of surface rupturing faults

laser 40ar39ar dating of microgram size illite samples and implications for thin section dating laser 40ar 39ar ages were obtained from a series of replicate illite samples ranging in mass from 14 to 77 pg using vacuum encapsulated irradiation and the results were compared with earlier analyses of the same material doug et al 1995 this test confirmed that measured argon ages in particular argon retention ages are independent of sample size since the argon retention age does not require the 39ar recoil gas fraction it is clear that retention ages can be measured on illite within thin sections laser spot ages were obtained from thin sections of the same welsh basin shale analyzed as illite separates early results on untreated thin sections yielded a wide range of ages some much younger than those measured previously however apparent age was negatively correlated with measured clk ratios suggesting the presence of a young water soluble component which had been removed during mineral separation prolonged water rinsing of the thin sections reduced or virtually eliminated the young phase yielding age results consistent with earlier work on illite separates this indicates that dating of clays in thin section has the potential of being a powerful tool in unraveling complex histories of diagenesis and metamorphism

westward propagation of the north anatolian fault into the northern aegean timing and kinematics we present new evidence for the propagation processes of the north anatolian fault folding in the dardanelles straits region allows us to document the timing of the deformation preceding and the finite displacement after the passage of the propagating tip of the fault the accuracy of the observations is due to interplay between deformation and the sea level changes in the mediterranean the well known messinian regression followed by the pliocene transgression the long term kinematics around the sea of marmara pull apart total displacement of about 85 km over the past 5 m y is similar to the present day kinematics deduced from space geodesy at a larger scale westward propagation of the north anatolian fault over nearly 2000 km in the past 10 m y appears to be associated with strain recovery suggesting that the continental lithosphere retains long term elasticity

high precision 40ar 39ar age of the gas emplacement into the songliao basin the problem of determining an exact isotopic age of hydrocarbon emplacement is complex because minerals suitable for dating with common isotopic methods are often lacking in the sedimentary domain however the igneous quartz from the cretaceous volcanic rocks that host the gas reservoir in the songliao basin northeastern china contains abundant secondary fluid inclusions with high concentrations of k and high partial pressures of methane trapped during gas emplacement quartz with abundant k rich fluid inclusions provides an excellent closed system well suited for 40 ar 39 ar dating three igneous quartz samples were measured by stepwise crushing to release the inclusion based argon gas all three samples yielded well defined isochrons with ages in close agreement precisely constraining the gas emplacement at 42 4 0 5 ma 2 below the daqing oil field in the songliao basin extending possible gas reservoirs from the upper cretaceous to the middle eocene

direct radiometric dating of hydrocarbon deposits using rhenium osmium isotopes rhenium osmium re os data from migrated hydrocarbons establish the timing of petroleum emplacement for the giant oil sand deposits of alberta canada at 112 5 3 million years ago this date does not support models that invoke oil generation and migration for these deposits in the late cretaceous most re os data from a variety of deposits within the giant hydrocarbon system show similar characteristics supporting the notion of a single source for these hydrocarbons the re os data disqualify cretaceous rocks as the primary hydrocarbon source but suggest an origin from older source rocks this approach should be applicable to dating oil deposits worldwide

evaluation of bitumen as a 187re 187os geochronometer for hydrocarbon maturation and migration a test case from the polaris mvt deposit canada abstract re os analyses of bitumen from the polaris mississippi valley type zn pb deposit demonstrate the potential utility of the re os system for establishing the absolute timing of hydrocarbon migration and generation the polaris deposit is hosted by carbonate rocks of the upper ordovician thumb mountain formation and sulphide mineralization during late devonian time is constrained by a rb sr sphalerite date of 366 15 ma and a paleomagnetic date bitumen occurs in open space fillings and coatings and is interpreted to have formed during the main and late stages of sulphide mineralization samples of bitumen were taken from a variety of locations from the main deposit at polaris re and os contents are 3 78 ppb and 55 1155 ppt respectively the 187re 188os ratios are very high ranging from 300 to 1800 and the os isotopic composition is moderately to highly radiogenic with 187os 188os ratios ranging from 3 3 to 13 7 calculated 187os 188os ratios at 366 ma are variable but an obvious grouping is defined by 11 of 16 analyses on a re os isochron diagram and these data yield a re os date of 374 9 ma mswd 12 model 3 in agreement with rb sr sphalerite and paleomagnetic dates for mineralization at polaris the combined dates suggest that the migration of fluids responsible for both pb zn mineralization and bitumen formation at polaris was broadly contemporaneous and was coeval with the late devonian ellesmerian orogeny though considerable further research is required to understand the re os isotopic systematics of migrated hydrocarbon systems bitumen re os geochronology may provide the possibility of establishing a more comprehensive understanding of mvt deposits sedimentary basin and petroleum systems

the gps strain rate field in the aegean sea and western anatolia the gps velocity field of the aegean sea and western anatolia is used to determine crustal deformation strain rates for the period 1988 to 1996 the zone from the marmara sea to the north aegean trough is associated with strong right lateral shear motion with maximum strain rates of 170 nstrain a in the northern aegean sea the extensional deviatoric axes are oriented nne ssw associated with strain rates reaching 150 nstrain a and in western anatolia varying around n s with rates of up to 95 nstrain a the extensional areas mostly coincide with active graben features the south western aegean sea is almost strain free in general the areas of high geodetic strain rates are accompanied by distinct seismic clusters whereas the strain free regions are nearly aseismic

surface rupture and slip distribution of the 12 november 1999 duzce earthquake m 7 1 north anatolian fault bolu turkey the 12 november 1999 earthquake m 7 1 occurred on the duzce fault a splay of the north anatolian fault in the bolu basin approximately three months after the 17 august 1999 m 7 4 earthquake the surface rupture was 40 km long and the maximum right lateral offset was 500 5 cm averaging 300 cm the 9 km of the westernmost part of the rupture along the southern margin of the eften lake had a 350 cm maximum vertical displacement normal faulting some of which was already ruptured during the 17 august 1999 event with few tens of centimeters the surface rupture has a generally simple narrow deformation zone of 0 5 5 m however in some places it widens to 50 m transtensional and transpressional structures were observed within releasing and restraining step over areas respectively the larger dextral offsets on some streams indicate previous events the dextral slip measurements along the rupture reflect a symmetric distribution the eastern connection of this rupture zone with the main trace of north anatolian fault remains unclear because the bakacak and elmalik fault which are connecting faults had no surface rupture manuscript received 14 november 2000

illite k ar dating and crystal growth processes in diagenetic environments a critical review k ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting in spite of a great number of specialized papers some problems remain unresolved these are mostly due to a misunderstanding of the argon accumulation process during illitization criteria for identifying detrital authigenic mineral mixtures crystal ripening fast precipitation or continuous nucleation growth processes are discussed using k ar data available in the literature using different parameters such as age agek ar agestrati cryst diagenetic agek ar agestrati or frac agek arfraction agek arfinest it is shown that the k ar age significance depends on the illite nucleation growth processes a diagenetic age is obtained when these processes are rapid the k2o accumulation period is shorter than 2 if lower than this value the k ar ratio depends on the proportions of new and old particles respectively which are controlled by the relative rates of nucleation crystal growth and ripening

paleoseismological evidence of recent earthquakes on the 1967 mudurnu valley earthquake segment of the north anatolian fault zone this study contributes paleoseismological data on the recent history of surface rupturing earthquakes on the southern main branch of the north anatolian fault zone nafz turkey west of the bolu basin we focused on the fault segment that ruptured during the 1967 mudurnu valley earthquake m s 7 1 which preceded the devastating earthquakes of 1999 in the sequence of westward migrating nafz earthquakes since 1939 geomorphological mapping was carried out in a search for trenching sites on the central part of the 22 july 1967 earthquake segment in the absence of locations in sedimentary environments best suited for paleoseismological interpretation of faulted holocene deposits we trenched fluvial channel deposits at a terrace of the gravel dominated mudurnu river the most conservative interpretation of the trench stratigraphy and faulting evidence suggests that at least one paleoearthquake most probably two occurred after a d 1693 the 1967 earthquake segment has ruptured at least once since the historical earthquake of a d 1668 which was previously considered to be a likely candidate for the penultimate event and before 1967 it is not possible to confidently assign the penultimate event to one of the post a d 1693 historical earthquakes in the broader area around the mudurnu valley e g the earthquake of a d 1719 because the historical data published so far do not provide conclusive information about when past ruptures of the mudurnu valley branch of the nafz a secluded area did or did not take place

shear zones formed along long straight traces of fault zones during the 28 june 1992 landers california earthquake surface rupturing during the 28 june 1992 landers california earthquake east of los angeles accommodated right lateral offsets up to about 6 m along segments of distinct en echelon fault zones with a total length of 80 km the offsets were accommodated generally not by faults distinct slip sur faces but rather by shear zones tabular bands of localized shearing along simple stretches of fault zones at landers the rupture is characterized by telescoping of shear zones and intensification of shearing broad shear zones of mild sheafing containing narrow shear zones of more intense sheafing containing even nar rower shear zones of very intense sheafing which may contain a fault thus the ground ruptured across broad belts of shearing with clearly defined subparallel walls oriented nw each broad belt consists of a broad zone of mild sheafing extending across its entire width 50 to 200 m and much nan ower a few meters wide shear zones that accommodate most of the offset of the belt and are por trayed by en echelon tension cracks in response to right lateral shearing the slices of ground bounded by the tension cracks rotated in a clockwise sense producing left lateral sheafing and the slices were forced against the walls of the shear zone producing thrusting even narrower shear zones formed within the narrow shear zones although these probably are guides to right lateral fault segments below the surface rupturing during the earthquake is characterized not by faulting but by the formation of shear zones at various scales

40ar 39ar dating of oil generation and migration at complex continental margins oil bearing fluid inclusions trapped within authigenic k feldspar in late jurassic reservoirs throughout the uk atlantic margin are evidence for an early oil migration event from late jurassic source rocks in the faeroe shetland basin previous compound specific isotope studies confirm the hydrocarbon provenance temperature composition time data show that the late jurassic source rocks were oil mature 113 3 5 ma 2 this new 40ar 39ar authigenic k feldspar age shows that the late jurassic source rocks were oil mature 30 40 m y prior to current estimates

evaluation and models of cenozoic sedimentation in the south china sea abstract the south china sea is a marginal basin with the most complex geological structures in the west pacific region and is characterized by the extensive distribution large thickness diversity and complicated evolution of the cenozoic sediments the author believes that studies of the sedimentary features and evolution of the cenozoic are fundamental to understanding the cenozoic evolution of the south china sea and that the definition of sedimentation models will be helpful in inferring the basin development at early stages thus defining the basin patterns and guiding hydrocarbon exploration this article based on data obtained from hydrocarbon exploration in the south china sea together with related references analyses the characteristics of cenozoic sediments in the south china sea and infers their evolution and models of formation the cenozoic in the south china sea is divided into eight sedimentary provinces a brief introduction is given to changes in sedimentary environments of the south china sea during the eocene late oligocene early miocene middle late miocene and pliocene quaternary the line linking taiwan and natuna as this article points out was the sea land boundary of the south china sea in the early cenozoic the transgression cycle is dominant west of this line while regression prevails to the east during the cenozoic five different patterns of sedimentation existed in the south china sea offshore lake shallow sea deep sea transgression regression and platform the former two belong to the lower cycle whilst the latter three to the upper cycle the time and spatial variations in the sedimentary sequence of these patterns is also discussed

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

pb pb dating of hydrocarbon migration into a bitumen bearing ore deposit north wales previous attempts at u pb dating of uraniferous bitumens have had limited significance because of radioelement migration pb pb dating which can be undertaken regardless of recent lead migration has been successfully applied to uraniferous solidified bitumen from the ty gwyn copper deposit north wales analyses of five bitumen samples with variable mixtures of radiogenic and common lead yield a sup 207 pb sup 206 pb age of 248 plus minus 21 ma early triassic this age is interpreted as the date of hydrocarbon migration into the deposit and is reasonably consistent with the timing of hydrocarbon generation calculated from the regional burial history the pb pb dating method could be applied to date uraniferous bitumens representing hydrocarbon migration in diverse geologic environments

rupture processes of the 1999 august 17 izmit and november 12 duzce turkey earthquakes summary we derive the rupture history of the 1999 august 17 izmit mw 7 4 and 1999 november 12 duzce mw 7 1 earthquakes in turkey from teleseismic body waves using broad band data of the global seismograph network aftershock locations and mapped surface breaks the centroid solutions indicate strike slip mechanisms for both events the izmit earthquake was characterized by rupture propagating predominantly eastwards it consisted of a main rupture lasting about 25 s followed within 1 min by two more events of mw 6 9 and mw 7 0 with the teleseismic data we could not resolve the westward extent of rupture into the marmara sea however an upper bound of the seismic moment release west of the epicentre of the izmit event is estimated to be 1 9 1019 n m the duzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture in excellent agreement with mapped surface breaks and aftershock locations

crustal structure of the northeastern margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin abstract the 1000 km long darlag lanzhou jingbian seismic refraction profile is located in the ne margin of the tibetan plateau this profile crosses the northern songpan ganzi terrane the qinling qilian fold system the haiyuan arcuate tectonic region and the stable ordos basin the p wave and s wave velocity structure and poisson s ratios reveal many significant characteristics in the profile the crustal thickness increases from northeast to southwest the average crustal thickness observed increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane the crust becomes obviously thicker south of the haiyuan fault and beneath the west qinlin shan the crustal velocities have significant variations along the profile the average p wave velocities for the crystalline crust vary between 6 3 and 6 4 km s beneath the songpan ganzi terrane west qinling shan and haiyuan arcuate tectonic region p wave velocities of 6 3 km s are 0 15 km s lower than the worldwide average of 6 45 km s north of the kunlun fault with exclusion of the haiyuan arcuate tectonic region the average p wave velocity is 6 4 km s and only 0 5 km s lower than the worldwide average a combination of the p wave velocity and poisson s ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base a mafic lower crust is absent in the ne margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin there are low velocity zones in the west qinling shan and the haiyuan arcuate tectonic region the low velocity zones have low s wave velocities and high poisson s ratios so it is possible these zones are due to partial melting the crust is divided into two layers the upper and the lower crust with crustal thickening mainly in the lower crust as the ne tibetan plateau is approached the results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane south of the kunlun fault both the conrad discontinuity and moho in the west qinling shan and in the haiyuan arcuate tectonic region are laminated interfaces implying intense tectonic activity the arcuate faults and large earthquakes in the haiyuan arcuate tectonic region are the result of interaction between the tibetan plateau and the sino korean and gobi ala shan platforms

first generation site response maps for the los angeles region based on earthquake ground motions abstract ground motion records from aftershocks of the 1994 northridge earthquake and mainshock records from the 1971 san fernando 1987 whittier narrows 1991 sierra madre and 1994 northridge earthquakes are used to estimate site response relative to a rock site for the urban los angeles area site response is estimated at 232 mainshock and 201 aftershock sites relative to a low amplitude site in the santa monica mountains average amplification values are calculated for the frequency bands 1 to 3 3 to 5 and 5 to 7 hz these bands are chosen based on limitations in aftershock recording equipment at lower frequencies and reduced significance to the building inventory at higher frequencies site amplification factors determined at the instrumented locations are grouped by the surficial geology and contoured to produce a continuous spatial estimation of amplification the maps in this article represent the first attempt to produce estimates of site amplification based on observations of ground motion for such a large areal extent of the los angeles region these maps are expected to evolve as more data become available and more analysis is done

thermal 40ar 39ar separation of diagenetic from detrital illitic clays in gulf coast shales the 40ar 39ar age spectra for illite rich mixed layered illite smectite i s from texas gulf coast bentonites contain only a diagenetic component and are characteristically staircase shaped this spectrum shape contrasts with patterns seen for anchizonal and epizonal grade bentonites that are flatter this may be due to the structural and compositional heterogeneity mixed layering typical of low grade clays especially with respect to k on the other hand shales from depths exceeding those of the smectite to illite transformation have both detrital and diagenetic components although diagenetic illite rich i s dominates shale fine fraction detrital mica is observed even in the 0 2 m size fraction age spectra of shales consist of two portions 1 a staircase shaped low temperature section corresponding to degassing of the diagenetic illite rich i s component and 2 a peak in the high temperature section corresponding to degassing of well crystallized detrital mica the data obtained from 40ar 39ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages we have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture giving rise to an age corrected for the detrital contamination the resulting corrected 40ar 39ar ages are closer to the diagenetic ages this is in sharp contrast to the old k ar ages of the same samples although still in the developmental state these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

xrd based 40ar 39ar age correction for fine grained illite with application to folded carbonates in the monterrey salient northern mexico abstract due to their minute size 40ar 39ar analysis of illite faces significant analytical challenges including mineral characterization and especially effects of grain size and crystallography on 39ar recoil quantifying the effects of 39ar recoil requires the use of sample vacuum encapsulation during irradiation which permits the measurement of the fraction of recoiled 39ar as well as the 39ar and 40ar retained within illite crystals that are released during step heating total gas ages tga are calculated by using both recoiled and retained argon which is functionally equivalent to k ar ages while retention ages ra only involve retained ar in the crystal natural applications have shown that tga fits stratigraphic constraints of geological processes when the average illite crystallite thickness ict is smaller than 10 nm and that ra matches these constraints for icts larger than 50 nm we propose a new age correction method that takes into account the average ict and corresponding recoiled 39ar for a sample with x ray corrected ages xca lying between total gas and retention ages depending on ict this correction is particularly useful in samples containing authigenic illite formed in the anchizone with typical ict values between 10 and 50 nm in three samples containing authigenic illite from cretaceous carbonates in the monterrey salient in northern mexico there is a range in tgas among the different size fractions of 46 49 36 43 and 40 52 ma while ras range from 54 64 47 52 and 53 54 ma respectively xca calculations produce tighter age ranges for these samples of 52 5 56 45 5 48 5 and 49 52 5 ma respectively in an apparent age vs ict or 2m 1illite plot authigenic illite grains show a slope that is in general slightly positive for tga slightly negative for ra but close to zero for xca with thinner crystallites showing more dispersion than thicker ones in order to test if dispersion is due to a different formation history or the result of retention capability degassing spectra were modeled for site xca averages and overall xca average modeling shows that local site age average best match the measured spectra instead of a global average age indicating that illite growth reflects local deformation and is not the result of regional metamorphism modeling also shows that ar degassing spectra are very sensitive to grain size such that age interpretation based on ar plateaus is meaningless for most fine grained clays

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating the timing of petroleum emplacement is crucial in understanding petroleum forming processes and predicting new prospecting regions this study investigates the possibility of determining the age of petroleum emplacement by an illite 40ar 39ar laser stepwise heating technique a specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields resulting in credible 40ar 39ar data the sandstone samples are from the zhuhai and zhujiang formations in the huizhou sag in the pearl river mouth basin south china sea illite 40ar 39ar laser stepwise heating consistently yields gradually rising age spectra at the low temperature steps until reaching age plateaus at high temperature steps the first step apparent ages of the six samples from the zhujiang formation give a weighted mean age of 11 4 0 5 ma 1 in agreement with an age of 12 1 1 1 ma in the first three steps of the sample from the zhuhai formation these dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation the plateau ages ranging from 58 to 276 ma in high laser output steps are interpreted as being caused by variable contributions from detrital k feldspar in the sandstones the older total ages based on the illite 40ar 39ar stepwise heating results are equal to the k ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital k feldspar the 40ar 39ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional k ar methods

the 17 august 1999 izmit earthquake on 17 august 1999 an earthquake of magnitude 7 4 shook northwestern turkey details on the earthquake9s characteristics and effects across northwestern turkey are provided and are put in the historical context of previous earthquakes in the region the earthquake was the seventh in a series of earthquakes migrating westwards along the north anatolian fault since 1939 information on historical earthquakes modeling and estimates of the slip rate along the fault indicate that the location and severity of the earthquake should not have come as a surprise

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating discussion isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry yun et al recently published in the aapg bulletin v 94 no 6 p 759 771 on that topic a study entitled dating petroleum emplacement by illite 40ar 39ar laser stepwise heating technique this study raises questions about basic aspects of 40ar 39ar dating fine grained authigenic clay minerals describes flawed methodology and ignores important previous work as isotopic dating of diagenetic processes may interest readers of the aapg bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study we discuss especially the fact that the authors discarded 39ar recoil followed by an evaluation of its impact on the results and the interpretations

trinet strong motion data from the m 7 1 hector mine california earthquake of 16 october 1999 the m w 7 1 hector mine earthquake of october 16 1999 was recorded by more than 300 stations of trinet which is administered cooperatively by the california division of mines and geology s california strong motion instrumentation program cdmg csmip california institute of technology and the u s geological survey usgs the earthquake occurred in a remote part of the mojave desert approximately 190 km northeast of downtown los angeles and there were no strong motion stations close to the surface rupture the nearest station hector is about 27 km north of the epicenter it recorded a peak horizontal ground acceleration of 0 33g the two next closest stations amboy and joshua tree are to the east and south both at epicentral distances of about 50 km each recorded peak ground accelerations of about 0 2g the new digital instruments installed for the trinet project recorded a large set of reliable data at epicentral distances up to 275 km these data can significantly improve empirical peak ground motion attenuation relationships which are usually developed for distances only up to 100 km boore et al 1993 1997 because adequate data have not been available at greater distances hector mine peak ground motions demonstrate reasonable agreement with empirical attenuation relationships for acceleration in contrast higher than expected ground velocities and displacements were recorded at epicentral distances of about 150 to 220 km especially in the los angeles sedimentary basin where anomalously high amplitude displacements with periods of 6 to 7 sec were recorded in los angeles long beach and other areas these long period surface or basin generated waves can have significant effects on large structures the m w 7 3 landers earthquake of 1992 similarly produced strong long period waves in the basin the peak ground motions produced by the landers earthquake were on average 1 6 times higher than for the hector mine earthquake in the los angeles area ground motion data recorded by digital instruments were uniformly processed in the frequency band 0 067 to 46 hz 0 022 15 sec the processed data set includes records from 213 ground response stations in an effort to make strong motion data available quickly to the engineering and scientific communities important records from this event were made available by file transfer protocol ftp beginning the day of the earthquake

strike slip fault geometry in turkey and its influence on earthquake activity the geometry of turkish strike slip faults has been reviewed and described from this data set it appears that fault geometry the distribution of discontinuities such as bends and stepovers along the main fault trace plays an important role in controlling the location of large earthquake rupture segments along the fault zones large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 more important than the size of each discontinuity however is the total geometric pattern i e the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size certain geometric patterns restraining single or double bends are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior for example large earthquake epicenters often occur near restraining bends or double bends furthermore aftershocks and swarm activity can often be associated with releasing areas areas along the strike slip fault that are undergoing a relatively large amount of extension

analysis of izmit aftershocks 25 days before the november 12th 1999 duzce earthquake turkey abstract we investigate spatial clustering of 2414 aftershocks along the izmit mw 7 4 august 17 1999 earthquake rupture zone 25 days prior to the duzce earthquake mw 7 2 november 12 1999 we analyze two spatial clusters namely sakarya sc and karadere duzce kdc we determine the earthquake frequency magnitude distribution b value for both clusters we find two high b value zones in sc and one high b value zone in kdc which are in agreement with large coseismic surface displacements along the izmit rupture the b values are significantly lower at the eastern end of the izmit rupture where the duzce mainshock occurred these low b values at depth are correlated with low postseismic slip rate and positive coloumb stress change along kdc since low b values are hypothesized with high stress levels we propose that at the depth of the duzce hypocenter 12 5 km earthquakes are triggered at higher stresses compared to shallower crustal earthquake the decrease in b value from the karadere segment towards the duzce basin supports this low b value high stress hypothesis at the eastern end of the izmit rupture consequently we detect three asperity regions which are correlated with high b value zones along the izmit rupture according to aftershock distribution the half of the duzce fault segment was active before the 12 november 1999 duzce mainshock this part is correlated with low b values which mean high stress concentration in the duzce basin this high density aftershock activity presumably helped to trigger the duzce event mw 7 2 after the izmit mw 7 4 mainshock

the seismic activity of the marmara sea region over the last 2000 years we examined the seismicity of the region of the sea of marmara in northwest turkey over the last 2000 yr using the historical record of the region we found no evidence for truly large earthquakes of a size comparable to those in the north anatolian fault zone in the marmara basin events are smaller in keeping with the known fault segmentation of the basin over the last 2000 yr seismic mo ment release accounted for the known right lateral shear velocity across the marmara region observed by global positioning system measurements its average rate is relatively constant and varies between 1 6 and 2 4 cm yr the long term seismicity in the marmara sea region shows that large earthquakes are less frequent than pre dicted from the 100 yr long instrumental period

gps and geologic estimates of the tectonic activity in the marmara sea region nw anatolia a dense network of 52 global positioning system gps sites is used to determine the velocity field and strain rate pattern at the western end of the north anatolian fault zone nafz in the marmara sea region nw anatolia based on four gps campaigns carried out biannually between 1990 and 1996 the detailed kinematic field of crustal motion is calculated the southernmost station shows an average rate of 22 3 mm yr oriented westward relative to istanbul eurasia this value is indicative of the dextral strike slip motion of nw anatolia relative to the black sea the gps velocity field reveals that the sites located on the thrace black sea block show negligible motion relative to istanbul the gps based deformation pattern of three sections crossing the westward prolongation of the nafz is compared with neotectonic data both the gps and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow e w oriented zone this belt extends from the single fault trace of the nafz through the gulf of izmit the marmara sea the arkoy region and the gulf of saros into the north aegean trough nat in addition the cross sections show the increasing influence of the n s oriented extension in the aegean sea region from the comparison of the directions and rates of shear and normal strain deduced from gps data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along mudurnu valley the northern marmara sea and the saros trough where it joins the nat

directional topographic site response at tarzana observed in aftershocks of the 1994 northridge california earthquake implications for mainshock motions abstract the northridge earthquake caused 1 78 g acceleration in the east west direction at a site in tarzana california located about 6 km south of the mainshock epicenter the accelerograph was located atop a hill about 15 m high 500 m long and 130 m wide striking about n78 e during the aftershock sequence a temporary array of 21 three component geophones was deployed in six radial lines centered on the accelerograph with an average sensor spacing of 35 m station c00 was located about 2 m from the accelerograph we inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion we identified a 3 2 hz resonance that is a transverse oscillation of the hill a directional topographic effect the top base amplification ratio at 3 2 hz is about 4 5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill this resonance is seen most strongly within 50 m of c00 other resonant frequencies were also observed a strong lateral variation in attenuation probably associated with a fault caused substantially lower motion at frequencies above 6 hz at the east end of the hill there may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill causing particle motions not spectral ratios at the top of the hill to be rotated about 20 away from the direction transverse to the hill the resonant frequency but not the amplitude of our observed topographic resonance agrees well with theory even for such a low hill comparisons of our observations with theoretical results indicate that the 3d shape of the hill and its internal structure are important factors affecting its response the strong transverse resonance of the hill does not account for the large east west mainshock motions assuming linear soil response mainshock east west motions at the tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill probable variations in surficial shear wave velocity do not account for the observed differences among mainshock acceleration observed at tarzana and at two different sites within 2 km of tarzana

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites

san andreas fault geometry at desert hot springs california and its effects on earthquake hazards and groundwater abstract the mission creek and banning faults are two of the principal strands of the san andreas fault zone in the northern coachella valley of southern california structural characteristics of the faults affect both regional earthquake hazards and local groundwater resources we use seismic gravity and geological data to characterize the san andreas fault zone in the vicinity of desert hot springs seismic images of the upper 500 m of the mission creek fault at desert hot springs show multiple fault strands distributed over a 500 m wide zone with concentrated faulting within a central 200 m wide area of the fault zone high velocity up to 5000 m sec rocks on the northeast side of the fault are juxtaposed against a low velocity m 6 0 earthquakes in the area in 1948 and 1986 occurred at or near the depths 10 to 12 km of the merged san andreas fault large magnitude earthquakes that nucleate at or below the merged fault will likely generate strong shaking from guided waves along both fault zones and from amplified seismic waves in the low velocity basin between the two fault zones the mission creek fault zone is a groundwater barrier with the top of the water table varying by 60 m in depth and the aquifer varying by about 50 m in thickness across a 200 m wide zone of concentrated faulting

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults