the dependency of sand transport rate by wind on the atmospheric stability a numerical simulation abstract a theoretical model of the process for wind sand flow is developed through consideration of the coupling between wind flow and the motion of sand particles under different atmospheric stability conditions using this model we studied the effects of atmospheric stability on the sand transport rate the number of sand particles per unit area and time and the duration before a steady state is achieved in detail the results show that atmospheric stability has a strong effect on the movement of the wind sand flow and produces results with different characteristics from those previously reported in the literature which apply only to conditions of neutral stability under unstable conditions the wind sand flow reached equilibrium more quickly with a higher total sand flux and sand flux at all heights than under neutral or stable conditions

shear and normal load perturbations on a two dimensional continuous fault 2 dynamic triggering 1 the influence of normal and shear stress static perturbations on a strike slip fault is addressed on the basis of a two dimensional continuous and quasi dynamic model friction along the fault plane is described using a rate and state friction law with depth variable properties normal and shear stress perturbations result in similar effects in terms of earthquake triggering iftm s is constant t ands being the amplitude of the shear and normal stress fluctuations respectively and m being a constant which can be interpreted as the static friction coefficient on the fault in a coulomb failure model therefore the coulomb stress changecff tm s is a useful tool to account simultaneously for normal and shear stress variations in our model we also show that when estimating the clock advance or clock delay of an earthquake the simple coulomb failure model is at first order in good agreement with our results during the first 90 of the earthquake cycle however it differs significantly during the last 10 due to the sharp velocity increase predicted by the rate and state friction law before rupture this suggests that as long as static variations of stress are concerned realistic fault models using rich laboratory based friction laws like rate and state friction laws may lead topredictions fairly close to the ones made using one of the simplest failure model i e the coulomb failure model this may explain why coulomb stress change computations although often based on drastic approximations have been able in many occasions to explain earthquake triggering sequences index terms 7209 seismology earthquake dynamics and mechanics 7215 seismology earthquake parameters 7260 seismology theory and modeling keywords earthquake triggering static triggering coulomb stress change rate and state friction laws clock advance delay

triggered deformation and seismic activity under mammoth mountain in long valley caldera by the 3 november 2002 mw 7 9 denali fault earthquake the 3 november 2002 mw 7 9 denali fault earthquake triggered defor mational offsets and microseismicity under mammoth mountain mm on the rim of long valley caldera california some 3460 km from the earthquake such strain offsets and microseismicity were not recorded at other borehole strain sites along the san andreas fault system in california the long valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes mammoth mountain a young volcano on the southwestern rim of the caldera the largest recorded strain offsets were 0 1 microstrain at po on the west side of mm 0 05 microstrain at mx to the southeast of mm and 0 025 microstrain at bs to the northeast of mm with negative strain extensional high sample rate strain data show initial triggering of the offsets began at 22 30 utc during the arrival of the first rayleigh waves from the alaskan earthquake with peak to peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0 06 mpa the strain offsets grew to their final values in the next 10 min the associated triggered seismicity occurred beneath the south flank of mm and also began at 22 30 utc and died away over the next 15 min this relatively weak seismicity burst included some 60 small events with magnitude all less than m 1 while poorly constrained these strain observations are consistent with triggered slip and intrusive opening on a north striking normal fault centered at a depth of 8 km with a moment of 10 16 n m or the equivalent of a m 4 3 earthquake the cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller these observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of mm following the 16 october 1999 m 7 1 hector mine california earthquake however in this case we see little post event slip decay reflected in the strain data after the rayleigh wave arrivals from the denali fault earthquake and onset of triggered seismicity did not lag the triggered defor mation by 20 min these observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 m 7 3 landers earthquake in the long valley caldera thus each of the three instances of remotely triggered unrest in long valley caldera recorded to date differ in each case however the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

the 2002 denali fault earthquake alaska a large magnitude slip partitioned event the mw moment magnitude 7 9 denali fault earthquake on 3 november 2002 was associated with 340 kilometers of surface rupture and was the largest strike slip earthquake in north america in almost 150 years it illuminates earthquake mechanics and hazards of large strike slip faults it began with thrusting on the previously unrecognized susitna glacier fault continued with right slip on the denali fault then took a right step and continued with right slip on the totschunda fault there is good correlation between geologically observed and geophysically inferred moment release the earthquake produced unusually strong distal effects in the rupture propagation direction including triggered seismicity

the 9 december 2003 central virginia earthquake sequence a compound earthquake in the central virginia seismic zone the 9 december 2003 central virginia earthquake sequence was a compound earthquake consisting of two nearly identical events occurring about 12 sec apart the second event is separated by about 300 m from the first event along the azimuth of approximately 195 10 the source mechanism determined from regional waveform inversion indicates predominantly thrust faulting at a depth of approximately 10 2 km the sequence with two events attained the combined seismic moment of m 0 2 64 1 01 10 15 n m m w 4 3 the focal mechanism indicates a subhorizontal p axis trending 301 and plunging 19 a regional stress model for the central virginia seismic zone cvsz derived from the 9 december 2003 events and 11 previous earthquakes indicates a thrust faulting stress regime with 1 trending 133 12 and plunging 14 3 the least principal stress axis 3 trends 25 10 and plunges 52 3 the 1 axis is rotated approximately 68 clockwise relative to the average maximum horizontal compressional stress s hmax direction for eastern north america approximately 65 the 9 december 2003 earthquake sequence occurred among the systems of paleozoic and mesozoic faults above the southern appalachian decollement which is at depths from 12 to 19 km in the piedmont geologic province of central virginia

the status of research on the development and characteristics of mass flux density profiles above wind eroded sediments a literature review the mass flux density profile above wind eroded sediments mfdp plays an important role in defining the characteristics and impacts of wind sand flows and therefore has great theoretical and practical significance as a result it has been extensively studied from the perspectives of geography mechanics physics and engineering this paper reviews the research work that has been conducted since bagnold s pioneering efforts summarizes the research achievements discusses the problems that have not yet been solved and proposes future research directions this field of research has progressed from field observations to theoretical analyses and numerical simulations accompanied by increasing sophistication in the research and the research scope includes land with mobile sands wind eroded farmland gravel gobis and degraded grasslands many mathematical functions have been proposed to describe mfdp including exponential power modified exponential modified power and polynomial functions this variation results from differences in the study conditions and in the parameters included in a given model and on differences between wind tunnel experiments and field observations however there are the limitations of both wind tunnel experiments and theoretical derivations and the key parameters of the profile lack a clear physical meaning which limits the practical application of these functions future research must aim to solve these problems

a recrystallized grain size piezometer for experimentally deformed feldspar aggregates a recrystallized grain size piezometer for low temperature migration recrystallization in feldspar has been experimentally calibrated hot pressed samples of fine grained albitic feldspar grain size 1 10 m and a natural albitic feldspar aggregate 150 m were deformed in simple shear and axial compression at a temperature of 900 c and a confining pressure of 1500 mpa transmission electron microscopy tem verified that deformation occurred by recrystallization accommodated dislocation creep grain size distributions were measured from scanning electron microscopy sem photographs of polished and etched samples the results yield a recrystallized grain size piezometer relationship of d 55 5 0 66 0 07 where d is the geometric mean grain size m and the differential stress mpa the exponent is higher than that determined in previous studies of other materials for rotation recrystallization and high temperature migration recrystallization indicating that different recrystallization mechanisms have distinct recrystallized grain size piezometer relationships

jointing within the outer arc of a forebulge at the onset of the alleghanian orogeny the oldest joint set in devonian rocks of western new york state has an atypical ns strike and predates regionally more abundant nw striking and ene striking joints driven by hydrocarbon related fluid decompression the ns joints originated in higher modulus diagenetic carbonate and were driven initially by a different mechanism either joint normal stretching and or thermoelastic contraction the origin of these joints in higher modulus carbonate concretions indicates the presence of a tensile stress produced by uniform regional extensional strain upper devonian shale hosting the ns joints crops out in that area of the appalachian basin where a morrowan erosional unconformity marks the region of maximum upward lithospheric flexure of an early alleghanian forebulge the ns strike of these early joints points to a forebulge stretching axis oriented approximately east west and associated in time and space with crustal loading that drove both the northfieldian orogeny and the underplating of the bronson hill anticlinorium in new england ultimately subsidence of the morrowan forebulge buried the upper devonian shale succession to the oil window during the latter part of the alleghanian tectonic cycle resulting in the propagation of fluid driven nw and ene trending joints in black shale

geochronology of the mesoproterozoic state farm gneiss and associated neoproterozoic granitoids goochland terrane virginia the goochland terrane is an isolated block of mesoproterozoic crust in the piedmont province of central virginia we report u pb zircon dates and whole rock major and trace element data for the state farm gneiss one of the main units in the terrane and additional results for a newly recognized suite of neoproterozoic granitoids that intrude the gneiss the state farm gneiss ranges in bulk composition from quartz monzodiorite to granite and three samples yield u pb zircon dates of 1046 7 6 ma 1039 18 11 ma and 1023 10 ma we interpret these dates as igneous crystallization ages which indicate a maximum emplacement interval of ca 1057 1013 ma for these samples neoproterozoic granitoids including the fine creek mills and flat rock granites are more alkaline than the state farm gneiss and display all compositional characteristics of a type granites e g high fe mg ga ga al nb zn y zr u pb zircon analyses from five separate bodies indicate crystallization ages from ca 654 to 588 ma but all results are complicated by mesoproterozoic ca 1 ga inheritance coupled in some cases with secondary pb loss we interpret the ages and compositions of the younger rocks to reflect neoproterozoic rifting of the goochland terrane but its location during rifting is uncertain the terrane may have been separated from laurentia during the neoproterozoic breakup of rodinia and later reattached

testing the utility of the porphyroclast hyperbolic distribution method of kinematic vorticity analysis kinematic vorticity wk is a dimensionless measure of rotation relative to finite stretching and is essential for complete understanding of flow in ductile shear zones the porphyroclast hyperbolic distribution phd method is a widely used technique for estimating wk based on the acute angle between the flow eigenvectors determined from the orientations of back rotated s porphyroclasts the utility of the phd was tested with tertiary ultramylonites from the santa catalina mountains arizona proterozoic ultramylonites from the virgin mountains nevada and a paleozoic ultramylonite from the hylas zone in the virginia piedmont bootstrapping statistics and a computational sieving process were used to analyze the phd data set average standard deviations of the bootstrapped data sets yield a 1s standard error of 9 for a wk value measured as a simple shear sieving results imply that back rotated porphyroclasts may not orient parallel to the extensional eigenvector the phd method is useful for discerning between deformations that are pure shear dominated general shear or simple shear dominated but is not accurate enough to report precise wk values when performed on multiple sections the phd method can identify zones of monoclinic versus triclinic shear 2007 elsevier ltd all rights reserved

alleghanian tectono thermal evolution of the dextral transcurrent hylas zone virginia piedmont u s a abstract the hylas zone is the northernmost segment of the eastern piedmont fault system and lies in the grenville goochland terrane virginia like the other major segments the nutbush creek zone the hollister zone and the modoc irmo zone the hylas zone experienced late paleozoic dextral transcurrent shearing early deformation in the zone was ductile and produced type i and type ii s c mylonites under amphibolite grade metamorphic conditions later deformation was at the brittle ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons the feldspar exhibits a strain dependent sequence of microstructures including kink bands antithetic extensional microfaults pull apart bends in the microfaults by development of transverse fractures and microboudinage correlation of mineral mechanical response with a thermal decay curve based on isotopic mineral ages yields a temporally constrained deformation history ductile dextral shearing occurred subsequent to the intrusion of the 330 ma petersburg granite and passed into the brittle ductile transition by approximately 260 ma dextral faulting terminated before 240 ma the appalachian dextral transcurrent faulting event therefore continued through permian in some areas

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

conditions for veining in the barrandian basin lower palaeozoic czech republic evidence from fluid inclusion and apatite fission track analysis abstract the interplay between fracture propagation and fluid composition and circulation has been examined by deciphering vein sequences in silurian and devonian limestones and shales at kosov quarry in the barrandian basin three successive vein generations were recognised that can be attributed to different stages of a basinal cycle almost all generations of fracture cements host abundant liquid hydrocarbon inclusions that indicate repeated episodes of petroleum migration through the strata during burial tectonic compression and uplift the earliest veins that propagated prior to folding were displacive fibrous beef calcite veins occurring parallel to the bedding of some shale beds hydrocarbon inclusions within calcite possess homogenisation temperatures between 58 and 68 c and show that the beef calcites originated in the deeper burial environment during early petroleum migration from overpressured shales e w striking extension veins that postdate beef calcite formed in response to variscan orogenic deformations based on apatite fission track analysis afta data and other geological evidence the veins probably formed 380 315 ma ago roughly coinciding with peak burial heating of the strata folding and the intrusion of variscan synorogenic granites the veins that crosscut diagenetic cements and low amplitude stylolites in host limestones are oriented semi vertically to the bedding plane and are filled with cloudy twinned calcite idiomorphic smoky quartz and residues of hardened bitumen calcite and quartz cements contain abundant blue and blue green fluorescing primary inclusions of liquid hydrocarbons that homogenise between 50 and 110 c geochemical characteristics of the fluids as revealed by gas chromatography mass spectrometry particularly the presence of olefins and parent aromatic hydrocarbons phenonthrene suggest that the oil entrapped in the inclusions experienced intense but geologically fast heating that resulted in thermal pyrolysis of its hydrocarbons this implies that the organic fluids in the fractures may have been partly influenced by heating associated with igneous intrusions that are hidden below the surface subvertical n s striking veins represent the most recent fracturing event s some of these veins are only a few millimeters thick and sparsely mineralised with thin leaf like quartz crystals that contain tiny blue and yellow orange fluorescing hydrocarbon inclusions most of the n s veins however occur as thick calcite veins that generally crystallised at 70 c or less from h2o nacl solutions of variable salinity with admixture of petroleum the origin of these fluids is interpreted in terms of deeply circulating meteoric waters that partially mixed with deep basinal fluids wider structural considerations combined with fission track analysis of adjacent host sediments suggest that n s veins formed during post mesozoic uplift of the area probably in response to major tertiary alpine deformations transmitted far into the bohemian massif

remotely triggered seismicity in continental china following the 2008 mw 7 9 wenchuan earthquake we perform a systematic search of remotely triggered seismicity in con tinental china following the 2008 mw 7 9 wenchuan earthquake we visually identify earthquakes as impulsive seismic energy with clear p and s arrivals on 5 hz high pass filtered three component velocity seismograms 1 hr before and after the wenchuan earthquake out of the 271 stations in the updated chinese digital seismic network csn 17 stations show statistically significant seismicity increase with statistic values larger than 2 following the wenchuan earthquake these include 11 stations in the north china block which is seismically active and is in the rupture propagation direction of the wenchuan earthquake three stations along the coastal lines in the relatively stable south china block one station near the haiyuan fault zone in north west china and two stations near the tanlu fault zone and the longgang volcano in northeast china these observations suggest that dynamic triggering in intraplate regions tends to occur near active faults that have ruptured in historic times and in the rupture propagation directions of the mainshock however it is worth noting that many sites that satisfy the criteria are not triggered suggesting that these conditions would help but are not sufficient enough to guarantee remote triggering in intraplate regions the tectonic environments near the sites with clear triggered activity range from transpressional to tranextentional and most regions are not associated with active geothermal or volcanic activity indicating that dynamic triggering could occur in a wide range of tectonic environments online material tables of station information and measured parameters and two supplementary figures showing locally triggered earthquakes

ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for alleghanian tectonics monazite ce th po4 from a pegmatite in the morefield mine of the eastern piedmont of central virginia has unusually high and variable amounts of common pb leading to problematic interpretations of its u th pb ages and how the monazite relates to nearby granite intrusions and faults to address these issues we analyze a single large monazite grain from the pegmatite using electron microprobe analysis empa n 64 laser ablation inductively coupled plasma mass spectrometry la icp ms n 58 and secondary ion mass spectrometry sims n 59 the monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization although the compositions of these regions fit the ideal stoichiometry of monazite they have lower si th u and y and higher p rare earth element ree and ca concentrations compared to visibly unaltered portions of the grain la icp ms and sims analyses demonstrate that common pb as proxied by 204pb is enriched in proximity to microcrack regions and correlates with 137ba sims 232th 208pb analysis from grain regions with lowest contents of common pb 208pb comprises 99 of pb isotopes yields two sets of ages 263 5 3 0 ma 1 mswd 1 7 n 11 and 234 1 3 3 ma 1 mswd 0 4 n 13 regionally the ages are similar to the youngest appalachian pegmatite bodies emplaced during the terminal alleghanian laurentia africa collision however the monazite ages are younger than locally surrounding intrusions the closest intrusive in distance 30 km and age is the petersburg granite 296 33 0 11 ma zircon 238u 206pb the emplacement of which coincided with activity along the hylas fault because the fault experienced a brittle ductile transition in the late permian 262 ma coeval with the older 232th 208pb monazite age group we postulate that the hylas fault may have operated as a fluid migration system between the petersburg granite and morefield mine pegmatite the younger monazite age coincides with triassic normal and or sinistral faulting linked to the development and deformation of local rift basins

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

a geophysical study of the earth s crust in central virginia implications for appalachian crustal structure a regional seismic reflection line i 64 across the virginia piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region a highly reflective upper crust an allochthonous blue ridge province underlying thrust sheets including the blue ridge master decollement and a basal decollement at a depth of about 9 km 3 s are confirmed on the seismic data immediately east of the blue ridge province appalachian structures plunge to as much as 12 km 4 s depth the evington group hardware terrane and chopawamsic metavolcanic rocks carolina terrane crop out in the piedmont province and numerous eastward dipping reflections originate from these rocks in the subsurface these eastward dipping reflectors overlie a gently west dipping 10 15 highly reflective zone that varies in depth from 1 5 s 4 5 km beneath the goochland terrane to 4 s 12 km beneath the rocks of the evington group some of the overlying eastward dipping reflections apparently root in this zone the zone may include decollement surfaces along which the overlying rocks were transported relatively few reflections originate from within autochthonous grenville basement at the western end of the profile the goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the chopawamsic metavolcanic rocks a broad zone of east dipping 20 45 reflections bounds the goochland terrane on the east these reflections may originate from deformation zones and continue to moho depths they appear to be correlative with similar events seen on other appalachian lines the pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent piedmont gravity high suggest continuity of crustal features along the length of the appalachians a major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in virginia that is the appalachian gravity gradient and the piedmont gravity high the crust thins from about 52 km beneath the appalachian mountains to about 35 km beneath richmond virginia and then rethickens by up to 10 km beneath the zone of east dipping reflections mylonites east of richmond the i 64 seismic data also contain a sequence of reflections at about 9 12 s indicative of lower crustal layering the base of this zone of reflections coincides almost exactly with the mohorovicic discontinuity interpreted from earlier refraction work the layering extends about 70 km west from richmond virginia and is interpreted as a lower crustal transition zone that is believed to persist across most of virginia

synkinematic magmatism heterogeneous deformation and progressive strain localization in a strike slip shear zone the case of the right lateral karakorum fault 1 new structural observations coupled with 15 u pb and 24 ar ar new ages from the karakorum shear zone ksz constrain the timing and slip rate of the right lateral karakorum fault zone kfz one of the great continental asian strike slip faults in the tangtse darbuk area the tangtse sw and muglib ne mylonitic strands of the ksz frame the less deformed pangong range inherited u pb ages show that granitic protoliths are mostly from the karakorum and ladakh batoliths with a major miocene melting event lasting from 21 5 to 13 5 ma some of the miocene granitic bodies show structural evidence for intrusion synkinematic to the ksz the oldest of these granitoids is 18 8 0 4 ma old implying that deformation started prior to 19 ma microstructural data show that right lateral deformation pursued during cooling ar ar data show that ductile deformation stopped earlier in the tangtse 11 ma than in the muglib strand 7 ma deformation ended at 11 ma in the tangtse strand while it is still active in the muglib strand suggesting a progressive localization of deformation when merged with published observations along the kfz these data suggest that the kfz nucleated in the north ayilari range at least 22 ma ago the long term fault rate is 0 84 to 1 3 cm yr considering a total offset of 200 to 240 km the ksz collected magma produced within the shear zone and or deeper in crust for which the producing mechanism stays unclear but was not the lower crustal channel flow

diachronous rifting drifting and inversion on the passive margin of central eastern north america an analog for other passive margins integration of new data with existing information indicates that the tectonic development of the passive margin of eastern north america between the carolina trough and scotian basin was considerably more complex than the classic two stage rift drift model first the transition from rifting to drifting was diachronous in the southeastern united states the rift drift transition occurred after the late triassic synrift deposition and before eastern north america magmatism in the earliest jurassic 200 ma in maritime canada the rift drift transition occurred after eastern north america magmatic activity and synrift deposition in the early jurassic and before postrift deposition in the early middle jurassic 185 ma second the deformational regime changed substantially after rifting on both the southern and northern segments of the margin generally northwest southeast postrift shortening replaced northwest southeast synrift extension northeast striking reverse faults formed and many of the rift basin boundary faults had reverse displacements in the southeastern united states the change in the deformational regime occurred in the late triassic early jurassic during the rift drift transition simultaneously diabase sills and dikes many striking nearly perpendicular to the trend of the rift basins intruded the continental crust and a massive wedge of volcanic or volcaniclastic rocks developed near the continent ocean boundary in maritime canada the change in the deformational regime occurred during or after the early jurassic and before or during the early cretaceous that is during the rift drift transition or early stages of sea floor spreading

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

stress interactions of three moderate size earthquakes in afyon southwestern turkey abstract we analyze three moderate size earthquakes occurred in the afyon region of southwestern turkey on december 15 2000 m w 6 0 and february 3 2002 m w 6 5 and m w 5 8 this earthquake sequence took place on the sultandag fault having dominantly normal faulting mechanisms with different orientations in an extensional tectonic regime we relocate these mainshocks and their large aftershocks and use the waveform data to determine the rupture dimensions and directivities we carry out both static and dynamic stress analysis to investigate stress interactions and possible earthquake triggering mechanisms in this sequence we show that the 2002 m w 6 5 rupture started at the termination of the 2000 m w 6 0 earthquake where the calculated coulomb stress changes increased more than 0 5 bar thus we conclude that the coseismic stress increase by the 2000 event favored the nucleation and promoted the timing of the 2002 event an intense seismic activity identified as the kizildag aftershock zone developed following both of the large mainshocks where the third largest earthquake m w 5 8 of the sequence occurred 2 h after the 2002 m w 6 5 mainshock the coulomb stress calculations of the 2000 earthquake shows no influence in this zone while the static coulomb stress changes due to the 2002 m w 6 5 mainshock raised the coulomb stresses an additional 0 3 0 7 bar in the kizildag aftershock zone thus we infer that such changes in the coulomb stresses positively influence the occurrence of the aftershocks in this zone since the kizildag aftershock zone takes place in a geothermal area we compute dynamic stress changes imposed by the 2000 and 2002 mainshocks the computed peak dynamic stress levels for both mainshocks are found much higher than the static stress changes calculated within the kizildag aftershock zone thus we propose that transient stress triggering most likely involving elevated fluid pressures in the hydrothermal systems present in this region may have played a major role in the initiation of increased seismicity followed by local multiple elastic stress triggering leading to the further development of aftershocks and the occurrence of the m w 5 8 mainshock

clay mineral authigenesis in coal and shale from the anthracite region pennsylvania textural chemical and mineralogical analyses of authigenic clays in anthracite rank coal and associated shale from eastern pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region minerals in anthracite occur in distinct assemblages associated with the following microenvironments coal matrix two orthogonal joint sets termed systematic and nonsystematic cleat and a third joint set mineralogical differences among microenvironments allow inferences about clay mineral origins and the parameters that controlled mineral authigenesis kaolinite occurs in the shale and all microenvironments within the coal seam authigenic minerals that replaced preexisting kaolinite during the latest stage ofcoalification anthracitization at t 200 c include nhu rich illite pyrophyllite and the following minerals which are primarily restricted to the systematic cleat set tosudite rl ordered dioctahedral mixed layer chlorite smectite sudoite di trioctahedral chlorite and rectorite rlordered mixedjayer paragonite smectite alteration of smectite to illite may be responsible for formation of authigenic illite and na bearing illite which are present only in the shale and coal matrix and fe al rich chlorite and quartz in the third joint set the chemical components for authigenesis appear to have multiple sources ai and si from preexisting kaolinite and quartz n from local organic matter mg and na for tosudite sudoite and rectorite largely from metasomatic hydrothermal fluids fe and mg for fe al rich chlorite from smectite illitization minerals in the shale the coal matrix and the nonsystematic cleat set are interpreted to represent authigenesis in a low permeability environment closed system alteration however the assemblage sudoite tosudite rectorite in the systematic cleat set is interpreted to be the result ofone or two stages ofhydrothermal alteration open system alteration we suggest hat differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the alleghanian orogeny hydrothermal alteration may be related to large scale basinal flow induced by alleghanianage uplift such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank ofcoalification in this region

formation of single domain magnetite by a thermophilic bacterium magnetite is a common product of bacterial iron reduction and may serve as a potential physical indicator of biological activity in geological settings here the authors report the formation of single domain magnetite under laboratory conditions by a thermophilic fermentative bacterial strain tor 39 that was isolated from the deep subsurface time course analyses were performed at 65 c to study the effect of bacterial activity on solution chemistry and magnetite formation during the growth of tor 39 run products were examined by transmission electron microscopy magnetite particles formed exclusively outside of bacterial cells and exhibited octahedral shapes having relatively equal length and width 15 difference tiny magnetite particles 12 nm nucleated between 10 and 11 h of incubation and increased to average lengths of 55 4 26 8 nm after 24 h of incubation between 24 h and 22 d of incubation magnetite particles maintained average lengths of 56 2 24 8 nm based on size constraints greater than 85 of the particles observed fell within the magnetic single domain little to no magnetite was detected in abiotic controls at 65 or 95 c or in tor 39 cultures whose activity was suppressed unlike mesophilic iron reducing bacteria e g gs 15 tor 39 produced crystals having shapes and sizesmore similar to some particles produced intracellularly by magnetotactic bacteria thus the single domain magnetite produced by thermophiles such as tor 39 may represent a heretofore unrecognized biological contribution to natural remanent magnetization in sedimentary basins and other geothermal environments less

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

constraints on the thermal history of taylorsville basin virginia u s a from fluid inclusion and fission track analyses implications for subsurface geomicrobiology experiments abstract microbial populations have been found at the depth of 2621 2804 m in a borehole near the center of triassic taylorsville basin virginia to constrain possible scenarios for long term survival in or introduction of these microbial populations to the deep subsurface we attempted to refine models of thermal and burial history of the basin by analyzing aqueous and gaseous fluid inclusions in calcite quartz veins or cements in cuttings from the same borehole these results are complemented by fission track data from the adjacent boreholes homogenization temperatures of secondary aqueous fluid inclusions range from 120 to 210 c between 2027 and 3069 m depth with highest temperatures in the deepest samples the salinities of these aqueous inclusions range from 0 to 4 3 eq wt nacl four samples from the depth between 2413 and 2931 m contain both two phase aqueous and one phase methane rich inclusions in healed microcracks the relative ch 4 and co 2 contents of these gaseous inclusions was estimated by microthermometry and laser raman spectroscopy if both types of inclusions in sample 2931 m were trapped simultaneously the density of the methane rich inclusions calculated from the peng robinson equation of state implies an entrapment pressure of 360 20 bar at the homogenization temperature 162 5 12 5 c of the aqueous inclusions this pressure falls between the hydrostatic and lithostatic pressures at the present depth 2931 m of burial if we assume that the pressure regime was hydrostatic at the time of trapping then the inclusions were trapped at 3 6 km in a thermal gradient of 40 c km the high temperatures recorded by the secondary aqueous inclusions are consistent with the pervasive resetting of zircon and apatite fission track dates in order to fit the fission track length distributions of the apatite data however a cooling rate of 1 2 c ma following the thermal maximum is required to match the integrated dates the thermal maximum would have occurred at 200 ma the timing of the maximum temperature is consistent with rapid burial of the taylorsville basin to twice its present day depth and thermal re equilibration with a 40 c km geothermal gradient followed by slow exhumation the results may imply that the microorganisms did not survive in situ but were transported from the cooler portions of the basin sometime after maximum burial and heating

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

application of rheology gauge to determine rock competency in taili western liaoning ne china abstract the intensive deformation along with the rheological behavior of the tectonic units in taili eastern north china craton ncc are recognized as the manifestation of the presence of the ne sw striking ductile shear zone in the region the presence of two types of granitic veins aplitic and pegmatitic in taili exhibit obvious strain refraction and boudinage structures ten samples from different rock types are collected to analyze their deformation characteristics and the rheological properties the analysis is performed by measuring the rock competency following the methods of mainly two types of rheology gauges 1 pinch and swell structures and 2 refraction of the veins we conclude that two main factors control the deformation patterns in taili i e grain size and proportion of the mineral component grain size is found to be one of the dominant factors controlling the deformation pattern i e the larger the grain size the higher the rock viscosity is the proportion of the mineral content is another important parameter which is critical to infer the rheological contrast along with the deformational style of the studied region the rock viscosity of the samples is inversely proportional to the mica and feldspar content of the same micro structural observation suggests that these rocks experienced strong extensional deformation at a low to middle temperature condition around 450 550 c during the mesozoic this event transformed the granitic veins into boudins associated with intensive strain refraction which are also responsible for the formation of voluminous felsic rocks with high viscosity contrast in this region

understanding basement tectonics of an interior cratonic basin southern illinois basin usa abstract although the illinois basin is one of the world s most studied interior cratonic basins little is known of its deep structure or seismotectonic framework in this study seismic reflection profiles over the major structures of the basin reveal reverse faults that penetrate the upper precambrian crust disrupt the surface of precambrian basement and accommodate folding of paleozoic sediments observations of deformation indicate concordant folding of the basement surface and overlying strata and or possible laramide style basement controlled folding most profiles show evidence of dual fold vergence and some profiles suggest transpression involving lower paleozoic strata and the basement surface the moderate seismicity in the southern illinois basin just north of the new madrid seismic zone nmsz defines an active deformation regime of interpreted nne striking dextral strike slip and reverse faulting upper crustal fault structure such as shown in this study could provide a fabric capable of reactivation by stress manifested as contemporary seismicity a tectonic framework for the possible kinematic linkage of deformation in the nmsz and the southern illinois basin is proposed on the basis of a hypothetical distribution of branched fault and or fold patterns in southern illinois that broaden northward beyond the nmsz such a dispersive deformation pattern is thought to be associated with the gradual cessation in displacement north of the nmsz i e displacement is dissipated over many faults the abrupt change in seismotectonic regime from a relatively discrete highly seismic nmsz with infrequent great earthquakes to a multi stranded fault pattern with more frequent moderate earthquakes in southern illinois is typical of active strike slip systems involving a bend or shift in direction

a slow slip event triggered by teleseismic surface waves 1 in recent years slow slip events sse have been observed to occur at regular intervals on the deep portions of subduction zone interfaces these are accompanied by seismic tremor that occurs over their duration it has been observed that tremor activity shows transient modulations in response to earth tides and the passage of seismic waves from distant earthquakes here we show for the first time geodetic evidence for the triggering of an interplate sse itself by teleseismic surface waves this sse in southwest japan which had an equivalent magnitude mw 5 3 and duration of 1 5 days was triggered by the surface waves of a mw 7 6 earthquake in tonga this evidence was captured by a newly deployed sensitive strainmeter network the triggered sse occurred on a place on the plate interface where the recurrence time for such events had almost expired whereas other regions at up to 90 of the recurrence time were not triggered this provides information for the conditions for triggering and generation of sses and perhaps for regular earthquakes

a model for the development of crenulations in shear zones with applications from the southern appalachian piedmont abstract two sets of crenulations are associated with a major alleghanian dextral shear zone which deformed stratigraphic and structural boundaries in the eastern piedmont of south carolina both sets of crenulation planes are oblique to the boundaries of the shear zone the morphologies and orientations of the crenulation sets and their spatial distributions indicate that they are related to slip along foliation planes and that they serve to compensate for displacement components of foliation slip normal to the overall movement direction in the shear zone the crenulations act to maintain the initial thickness of the shear zone our evaluation of the recent literature on shear zones suggests that crenulations related to foliation slip are abundant and constitute a reliable sense of shear indicator

mixing of thermogenic natural gases in northern appalachian basin stable isotopic studies were undertaken to investigate the origin of 20 oil associated and non associated gases from western and central new york state in the northern appalachian basin gas wetness c2 c5 c1 c5 methane dgr13c and methane dgrd values range from 0 8 to 26 9 47 9 to 30 7 pmil and 305 to 143 pmil respectively corresponding ethane and propane dgr13c values range from 40 9 to 32 0 pmil and 34 9 to 27 9 pmil these data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature equivalent vitrinite reflectance ro approximately 0 5 to post mature equivalent ro 2 0 inferred gas maturities generally in rease with increasing reservoir age from late to middle devonian marcellus and overlying rocks middle devonian to latest silurian oriskany onondaga akron silurian lockport herkimer medina to ordovician queenston and black river oil and gas in many late to middle devonian reservoirs may have migrated from interbedded organic rich shales in other reservoirs particularly those of ordovician and silurian age unusual isotopic reversals methane dgr13c ethane dgr13c suggest that gas generated locally from equivalent age rocks within the oil window mixed with post mature gas migrated from deeply buried sources to the south and southeast the post mature gas was probably sourced from middle to late ordovician shales regional migration of middle ordovician gas may have occurred along the middle ordovician knox unconformity gas was presumably channeled into overlying silurian and devonian reservoirs along faults and fractures

organic inclusions and their relations to oil and gas occurrences abstract 154 core and outcrop samples of proterozoic and palaeozoic reservoir and non reservoir strata were collected from renqui subei guangxi sichuan etc organic inclusions contained in the samples were studied by means of polarized light microscope fluorescence microscope micro cooling heating stage and compositional analysis according to hc phase colour refractive index and fluorescent properties organic inclusions can be classified as two braod types hc inclusions and hc containing inclusions which can be further subdivided into eight subtypes also discussed in this paper are the types and peculiarities of organic inclusions as well as the composition of gases parameters for organic molecules and paleotemperature which are considered to be useful indicators for oil and gas migration evolution and prospective evaluation some typical examples are also given in this study

triggered seismicity in utah from the 3 november 2002 denali fault earthquake immediately after the arrival of the surface waves from the m w 7 9 denali fault earthquake on 3 november 2002 the university of utah regional seismic network recorded an abrupt increase in local microseismicity throughout most of utah s main seismic belt we examined this increase in the context of the regional background seismicity using a catalog of 2651 earthquakes from 1 january 2000 to 30 june 2003 statistical analyses of this catalog above spatially varying magnitudes of completeness ranging from 1 2 to 1 7 allow us to reject with 95 confidence the null hypothesis that the observed increases were due to random occurrence the elevated seismicity was most intense during the first 24 hr 10 times the average prior rate but continued above background level for 25 days at the 95 confidence level in most areas we conclude that the increased seismicity was triggered by the denali fault earthquake which occurred more than 3000 km from the study region high peak dynamic stresses of 0 12 to 0 35 mpa that occurred during the passage of the love waves are consistent with the interpretation of triggering the peak dynamic stresses were estimated by measuring peak vector velocities at 43 recording sites 37 of which were relatively new strong motion stations of the advanced national seismic system the triggered events ranged in magnitude m c and or m l from less than 0 to 3 2 and were widely distributed across the state primarily in seismically active regions in contrast to many previously published observations of remotely triggered seismicity the majority of the triggered earthquakes did not occur near quaternary volcanic vents or in areas of magma related geothermal activity in several areas the triggered seismicity was spatially clustered more than five earthquakes each separated by 5 km double difference relative relocations for the earthquakes in three of these clusters indicate that most but not all of the triggered events were spatially separated from source zones of prior seismicity during 2000 2002 focal mechanisms for the two largest triggered events have northeast to northwest trending tension axes which are unusual for the region where they occurred the temporal decay of the triggered activity was similar to that of utah aftershock sequences and can be described by the modified omori s law with a p value of 0 6 to 0 7 the frequency magnitude distribution of the triggered earthquakes is also similar to that of utah aftershocks and for the study area as a whole can be described by the gutenberg richter relation with a b value of 0 81 0 16 these similarities between the triggered seismicity and utah aftershock sequences suggest the possibility that the initiation and development of both could result from the same causative mechanisms online material catalog of earthquakes used in this study

alleghanian deformation and metamorphism in the eastern north carolina piedmont rb sr isotopic data document alleghanian ages for the metamorphic event which formed the upper greenschist to amphibolite grade raleigh belt and for the d 3 deformational event which formed regional scale folds and mylonite zones in the eastern north carolina piedmont the butterwood creek granite which straddles the eastern boundary between the raleigh belt and the eastern slate belt has a whole rock isochron age of 292 31 ma biotite ages indicate that the eastern undeformed part of the pluton cooled rapidly after intrusion whereas a younger biotite age of 251 ma indicates slow cooling for the western foliated portion of the granite which lies in the raleigh belt other biotite ages within the raleigh belt range from 247 238 ma the thermal maximum for the raleigh belt probably accompanied the intrusion of numerous granitic plutons during the time 315 to 285 ma the biotite ages indicate that the raleigh belt remained at elevated temperatures and did not cool to the blocking temperature of biotite until 250 240 ma rapid cooling of the eastern part of the butterwood creek helps define the eastern boundary of the alleghanian thermal event which corresponds to the eastern limit of the high grade rocks of the raleigh belt the s 3 tectonic biotite foliation in the western part of the butterwood creek and in other alleghanian plutons of the raleigh belt associated regional f 3 folds and the d 3 hollister mylonite zone which cuts the butterwood creek pluton formed between the time of intrusion of the butterwood creek at 292 ma and regional cooling by 250 240 ma this study further defines the narrow alleghanian thermotectonic belt along the eastern piedmont which is known as the raleigh belt in virginia and north carolina and reappears as the kiokee belt in south carolina and georgia

regional fractures ii fracturing of mesaverde reservoirs in the piceance basin colorado 1 cretaceous strata in the east central piceance basin of northwestern colorado were subjected to several phases of stress due to horizontal tectonic compression burial and uplift these rocks contain a regional set of west northwest extension fractures observed in abundant vertical and deviated core from the u s department of energy s multiwell experiment and slant hole completion test wells time depth relationships fracture orientation and fluid inclusion analyses indicate that the fractures formed at about 36 40 ma during a phase of increased laramide west northwest compression and in a pressure temperature regime compatible with the geologically reconstructed maximum burial depths this regional fracturing is an example of load parallel extension fracturing and basinwide dilatancy at depth under conditions of high pore pressure and anisotropic tectonically created horizontal stress

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

strain partitioning in the mid crust of a transpressional shear zone system insights from the homestake and slide lake shear zones central colorado abstract kinematic analysis and field mapping of the homestake shear zone hsz and slide lake shear zone slsz in central colorado may provide insight into the interaction between subvertical and low angle shear zones in the middle crust the northeast striking steeply dipping hsz comprises a 10 km wide set of anastomosing ductile shear zones and pseudotachylyte bearing faults approximately 4 km south of the hsz north northeast striking shallowly dipping mylonites of the slsz form three 1 10 m thick splays oblique stretching lineations and shear sense in both shear zones record components of dip slip top up to the northwest and top down to the southeast and dextral strike slip movement during mylonite development quartz and feldspar deformation mechanisms and quartz c axis lattice preferred orientation lpo patterns suggest deformation temperatures ranging from 280 500 c in the hsz to 280 600 c in the slsz quartz c axis lpos suggest plane strain general shear across the shear system based on the relative timing of fabric development compatible kinematics and similar deformation temperatures in the slsz and the hsz we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid crust at 1 4 ga

the 1989 earthquake swarm beneath mammoth mountain california an initial look at the 4 may through 30 september activity abstract mammoth mountain is a 50 000 to 200 000 year old cumulovolcano standing on the southwestern rim of long valley in eastern california on 4 may 1989 two m 1 earthquakes beneath the south flank of the mountain marked the onset of a swarm that has continued for more than 6 months in addition to its longevity noteworthy aspects of this persistent swarm include 1 an exponential like increase in the rate of activity through the first month 2 a vertically oriented planar distribution of hypocenters at depths between 6 and 9 km with a north northeast strike roughly perpendicular to the average t axis orientation for the swarm earthquakes 3 recurring spasmodic bursts rapid fire sequences of similar sized earthquakes with overlapping coda and occasional earthquakes with enhanced low frequency energy 4 a uniform temporal distribution of the four largest m 3 events over the first 4 months of the swarm with a cumulative seismic moment for the entire sequence through 30 september corresponding to a single m 4 earthquake 5 a b value of 1 2 and 6 submicrostrain perturbations on the nearby borehole dilatometer the first of which led the onset of swarm activity by more than 2 weeks these aspects of the swarm together with its location along the southern extension of the youthful mono inyo volcanic chain which last erupted 500 to 600 years ago point to a magmatic source for the modest but persistent influx of strain energy into the crust beneath mammoth mountain

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

a satellite based digital data system for low frequency geophysical data a reliable method for collection display and analysis of low frequency geophysical data from isolated sites which can be throughout north and south america and the pacific rim has been developed for use with the geostationary operational environmental satellite goes system geophysical data primarily intended for earthquake hazard and crustal deformation monitoring are digitized with either 12 bit or 16 bit resolution and transmitted every 10 min through a satellite link to a bank of unix based computers in menlo park california there the data are available for analysis and display within a few seconds of their transmit time this system provides real time monitoring of crustal deformation parameters such as tilt strain fault displacement local magnetic field crustal geochemistry and water levels as well as meteorological and other parameters along faults in california and alaska and in volcanic regions in the western united states rabaul and other locations in the new britain region of the south pacific various mathematical statistical and graphical algorithms process the incoming data to detect changes in crustal deformation and fault slip that may indicate the first stages of catastrophic fault failure alert trigger levels based on physical models signal resolution and previous history have been defined for particular instrument types computer driven remote paging and mail systems are used to notify appropriate personnel when alarm status is reached the system supports continuous historical records of low frequency geophysical data software for extensive analysis of these data and programs for modeling fault rupture with and without seismic radiation as well as providing an environment for real time attempts at earthquake prediction

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

deciphering fracturing and fluid migration histories in northern appalachian basin an integrated structural and geochemical analysis of mineralized faults and fractures revealed that the most widespread fracture systems in the northern appalachian basin resulted from two periods of post alleghenian deformation fluid inclusion homogenization measurements on calcite samples from pre syn and post kinematic structural settings bracketed the temperature ranges predominantly a function of depth of burial for each period of fracturing the first period of deformation produced northwest west northwest and east northeast fractures and related faults that experienced recurrent strike slip movements during a cooling event from 176 to 120 c 349 to 248 f this cooling episode was associated with the waning of an elevated thermal grad ent attained during dewatering caused by late paleozoic burial metamorphism at depths of 3 to 3 5 km 9 800 to 11 500 ft later normal faulting reopened the earlier strike slip fault zones during subsequent cooling of the system from 112 c 234 f to below 76 c 169 f this second cooling stage resulted from the 1 to 2 km 3 300 to 6 600 ft of uplift and erosion that occurred throughout the eastern united states during the cretaceous the ubiquitous occurrence of carbonate silicate and sulfide epigenetic mineralization in many environments allows for widespread application of the dating techniques demonstrated in this study basic to this approach is the analysis of mineral assemblages deposited during increases in hydrothermal circulation resulting from enhanc d permeability associated with periods of faulting and fracturing the presence of different mineral assemblages representing a range of ages and crystallization environments on various generations of brittle structures provides a means for developing detailed faulting and circulation histories and the determination of approximate as well as precise relative ages for each event

hydrogeologic constraint on the origin of deep subsurface microorganisms within a triassic basin the thermal history of the taylorsville basin indicates that the thermophilic anaerobic bacteria extracted from triassic strata at 2800 m below the land surface within the basin probably migrated to their current location in order to ascertain the most probable scenario for microbial transport a two dimensional transient fluid flow and heat transport model of the taylorsville basin was developed using the finite element method the numerical model was constrained by tectonic and thermal histories of the basin coupled with the permeability and porosity data extracted from core samples the model demonstrates that a topographically driven groundwater flow system caused by the tectonic uplift and erosion during the jurassic can explain the observed differential cooling of the basin the computed groundwater flow rates during this period were on the order of 1 to 100 mm yr combined with the cooling history of the microbially sampled strata such rates provided a possible mechanism for the introduction of ancient surface or subsurface microorganisms to the deep subsurface this tectogenetic mechanism may explain the origins of deep subsurface microorganisms for other regions of the earth

deep burial diagenesis its implications for vertical movements of the crust uplift of the lithosphere and isostatic unroofing a review abstract various techniques of study of strata exposed at the surface in undeformed areas of the appalachian basin and ozark done imply that these strata have been heated to temperatures that suggest a former great depth of burial the data reveal that the strata have been much more deeply buried than previously thought if such former deep burial has taken place then subsequent uplift and erosion must also have taken place to bring these formerly deeply buried strata to the present land surface unexpectedly large amounts of uplift and erosion ranging from 4 3 to 7 km have re exposed these formerly deeply buried rocks this leads to the surprising conclusion that isostatic unroofing has stripped off thick sections of strata whose presence was previously unsuspected therefore the lithosphere in its isostatic unroofing of thick sequences of sedimentary strata has undergone much larger vertical motions than many geologists had previously estimated case histories in this review include strata of the silurian of the northern appalachian basin and of the ordovician of the ozark dome which reached interpreted maximum burial depths of 5 and 4 3 km respectively devonian strata in the catskill mountains of new york imply a former depth of burial of 6 5 km lower ordovician carbonate sequences of the northern appalachian basin imply a depth of burial in excess of 7 km middle ordovician strata from the same basin signify a depth of burial of approximately 5 km and devonian strata a paleodepth of 4 5 5 km such former great depths of burial of undeformed strata which are now exposed at the surface reflect large scale vertical movements of the crust and uplift of the lithosphere these drastic changes represent isostatic unroofing with widespread implications for paleogeography of a kind unrecognized at present

dynamic triggering of shallow earthquakes near beijing china summary we perform a comprehensive analysis of dynamic triggering around the babaoshan and huangzhuang gaoliying faults near beijing china the triggered earthquakes are identified as impulsive seismic arrivals with clear p and s waves in 5 hz high pass filtered three component velocity seismograms during the passage of large amplitude body and surface waves of large teleseismic earthquakes we find that this region was repeatedly triggered by four earthquakes in east asia including the 2001 mw 7 8 kunlun 2003 mw 8 3 tokachi oki 2004 mw 9 2 sumatra and 2008 mw 7 9 wenchuan earthquakes in most instances the microearthquakes coincide with the first few cycles of the love waves and more are triggered during the large amplitude rayleigh waves such an instantaneous triggering by both the love and rayleigh waves is similar to recent observations of remotely triggered non volcanic tremor along major plate boundary faults and can be explained by a simple coulomb failure criterion we are able to locate five of the earthquakes triggered by the kunlun and tokachi oki earthquakes these events occurred at shallow depth 5 km above the background seismicity near the boundary between nw striking babaoshan and huangzhuang gaoliying faults and the fangshan pluton we suggest that these triggered earthquakes occur near the transition between the velocity strengthening and weakening zones in the top few kilometres of the crust and are likely driven by relatively large dynamic stresses on the order of few tens of kpa

seismicity in idaho and montana triggered by the denali fault earthquake a window into the geologic context for seismic triggering we present a case study of dynamically triggered seismicity in idaho and western montana from the 2002 mw 7 9 denali fault earthquake to investigate the relationship between measured geological discriminants and propensity for trig gering we first establish triggering we find events that are not reported in the advanced national seismic system catalog in idaho and yellowstone following the denali fault earthquake by filtering broadband waveforms an ml 4 6 earthquake is discovered near new meadows idaho during the passage of the rayleigh waves and another earthquake probably located near yellowstone we find that central western idaho and yellowstone have statistically significant seismicity increases by applying a b test to the cataloged events in the 48 hr after the denali fault earthquake in idaho and surrounding regions we also find that pine idaho may have triggered events but the measurement is not robust because of uncertainty in the background seismicity rate both central western idaho and yellowstone triggered previously during the 1992 landers earthquake we then try to determine the local geological conditions necessary for triggered seismicity to occur geothermal regions with high total dissolved solids and background seismicity appear to be favorable conditions to trigger earthquakes in the study

character of the alleghanian orogeny in the southern appalachians part ii geochronological constraints on the tectonothermal evolution of the eastern piedmont in south carolina a nearly concordant u pb zircon age of 550 4 ma is interpreted to closely date crystallization of the epizonal little mountain metatonalite in the southeastern part of the charlotte belt in south carolina this confirms field studies which indicate that the charlotte belt contains a plutonic metaigneous complex that developed as a sub volcanic arc infrastructure contemporaneous with vulcanism manifested in the carolina slate belt both paleontological and geochronological controls indicate that the south carolina slate belt is mostly younger than 570 ma cambrian whereas the slate belt in north carolina and virginia is mostly of late proterozoic age a regionally significant mid paleozoic ca 340 360 ma thermal event is suggested by discordant 40ar 39ar whole rock age spectra of slate phyllite in the northwestern carolina slate belt and from hornblende in the southeastern charlotte belt it is uncertain if this event was associated with deformation in the eastern piedmont however mid paleozoic deformation has been previously documented elsewhere in the western piedmont a slightly disconcordant u pb zircon age of 317 4 ma is interpreted to closely date initial crystallization of the deformed edge field granite and confirms a record of late paleozoic penetrative deformation in the kiokee belt of south carolina u pb isotope data for the lake murray orthogneiss and clouds creek granite are discordant and suggest that the magmas of these plutons were derived by partial melting of a sialic precam brian source and then emplaced in the eastern piedmont at ca 315 ma prior to or during the early stages of the alleghanian orogeny the thermal maximum of alleghanian regional metamorphism amphibolite facies in the kiokee belt greenschist facies in the southeastern part of the carolina slate belt occurred during ca 295 315 ma during the late carboniferous and early permian the eastern piedmont experienced differential uplift erosion and relatively rapid postmeta morphic cooling isothermal surfaces were folded into an antiform synform antiform configuration corresponding to the kiokee carolina slate and charlotte belts respectively the geochronological data provide the following calibration for the late paleozoic deformational chronology recorded in the kiokee and carolina slate belts d2 lake murray deformation ca 295 315 ma d3 clarks hill deformation ca 285 295 ma and d4 irmo deformation ca 268 290 ma

comparison of radon in soil over faulted crystalline terranes glaciated versus unglaciated radon in soil correlates directly with the bedrock geology in unglaciated terranes in the hylas shear zone virginia the rock units exhibit a marked contrast in uranium concentration which is reflected in soil radon the soils are clay rich and are derived directly from the underlying bedrock even small geologic features such as pegmatite veins and thin shear zones are discernable by differences in soil radon concentrations this bedrock soil radon correlation is obscured in areas with thick glacial covers the reservoir fault area new jersey is proximal to a terminal moraine although the bedrock units exhibit a strong contrast in radioactivity and uranium concentration soil radon concentrations in the overlying tills reflect the local chemical and physical properties of the till rather than the bedrock

character of the alleghanian orogeny in the southern appalachians part i alleghanian deformation in the eastern piedmont of south carolina the eastern piedmont province in south carolina contains a sequence of cambrian volcanic and sedimentary rocks that was penetratively deformed d1 and regionally metamorphosed m1 to the greenschist facies during the early and or middle paleozoic the eastern piedmont was subsequently affected by late paleozoic alleghanian polyphase deformation d2 d4 and regional metamorphism the earliest alleghanian event d2 is associated with amphibolite facies regional metamorphism and felsic plutonism in a mid crustal infrastructure at ca 295 315 ma the gradational interface between infrastructure and overlying suprastructure contained a steep m2 metamorphic gradient between amphibolite facies below and greenschist facies above numerous sheets of felsic orthogneiss and a deformation front marking the upper limit of intense d2 penetrative deformation d3 is represented by northwestward vergent folding of s1 and s2 foliations and m2 isothermal surfaces the kiokee belt in south carolina is interpreted as a d2 infrastructure exposed within the core of a d3 antiform this interpretation suggests that an alleghanian infrastructure may be present in the subsurface beneath much of the piedmont and coastal plain the polygenetic modoc zone forms the northwest boundary of the kiokee belt it is interpreted to represent an interface between d2 infrastructure and suprastructure which was rotated into a steep northwest dipping attitude during development of the kiokee belt antiform between ca 267 and 290 ma portions of the kiokee belt modoz zone and carolina slate belt were overprinted by ductile deformation d4 along steeply dipping northeast trending dextral shear zones

northern ancestry for the goochland terrane as a displaced fragment of laurentia the ancestral position of the goochland terrane an isolated block containing mesoproterozoic crust in the appalachian piedmont of eastern virginia during grenvillian orogenesis has direct bearing on the breakup of rodinia ages lithology geochemistry and pb and nd isotope compositions of the montpelier anorthosite and state farm gneiss of the goochland terrane allow correlation with the grenvillian roseland anorthosite and associated igneous suites of the blue ridge farther west in virginia in contrast extensive metapelites which underwent devonian metamorphism and intrusion and amphibolites of the goochland terrane lack equivalents in the blue ridge dextral slip faults which bound the goochland terrane as well as dextral faults farther north suggest large scale translation prior to late paleozoic reaccretion ages of extension related a type granitoids within the virginia blue ridge 735 680 ma suggest physical separation from compositionally similar granitoids in the goochland terrane 630 588 ma reading prong 602 ma and manhattan prong 563 ma pre iapetan restoration of the goochland terrane northeastward of the blue ridge and outboard of the reading prong accounts for its affinities to the blue ridge unique anorthosites manhattan prong abundant amphibolites and reading prong ca 600 ma magmatism thus similar to the translation of madagascar out of africa the goochland terrane was rifted from laurentia marooned in oceanic crust and then dextrally translated southward 500 km prior to late paleozoic reaccretion documentation of large scale dextral translation of eastern laurentia relative to its fragments and other cratons during the neoproterozoic and paleozoic may assist in locating far traveled pieces of the grenville orogen for reconstruction of rodinia

twenty million years of continuous deformation along the karakorum fault western tibet a thermochronological analysis the role of the karakorum fault zone kfz is debated south of 33 n ongoing dextral oblique slip along the sw edge of the gar basin exhumes metamorphic and magmatic rocks of the ayilari range minerals have recorded a continuum of deformation from temperatures 600 400 c down to 20 ma of deformation along the fault greenschist facies deformation superimposed upon the medium to high grade deformation marks a kinematic change from pure dextral to dextral normal motion associated with the onset of rapid cooling at the regional scale the coexistence of transtension in the gar basin with transpression documented along the pangong range farther north suggests another example of the zipper tectonics model developed along the red river fault the kinematic shift induced the rise of the ayilari range starting at 16 12 ma and the incision of major river courses the indus river might have become captive of the relief at this time the river s 120 km of apparent offset implies dextral motion at a long term rate of ca 8 5 1 5 mm yr

the eastern piedmont fault system and its relationship to alleghanian tectonics in the southern appalachians alleghanian continental collision across the southern appalachians resulted in northwestward over thrusting in the blue ridge and valley and ridge and emplacement of a large crystalline nappe in the piedmont however fault patterns in the eastern piedmont fault system are consistent with the geometry expected in a strike slip fault network formation of the eastern piedmont fault system mainly postdates the last regional metamorphism in the piedmont and is interpreted to be an alleghanian structure penecontemporaneous with thrusting in the western part of the appalachian orogen this discrepancy in tectonic style across the orogen is attributed to lateral movement of parts of the raleigh and kiokee belts and of the precambrian paleozoic basement of northern florida out of the principal collision zone during latter stages of the alleghanian collision a modern analogy for such lateral movement is the present strike slip regime in turkey which represents post collisional lateral escape of the anatolia

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

seismicity characteristics of a potentially active quaternary volcano the tatun volcano group northern taiwan the tatun volcano group tvg is located at the northern tip of taiwan near the capital taipei and close to two nuclear power plants because of lack of any activity in historical times it has been classified as an extinct volcano even though more recent studies suggestthattvgmighthavebeenactiveduringthelast20ka inmay2003aseismicmonitoringprojectatthetvgareawasinitiated by deploying eight three component seismic stations some of them equipped with both short period and broadband sensors during the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events or as a continuous sequence in the form of spasmodic bursts mixed and low frequency events were also present during the same period eventhoughtheyoccurredonlyrarely arrivaltimesfromeventswithclearp s wavephaseswereinvertedinordertoobtaina minimum 1dvelocity modelwithstation corrections probabilistic nonlinear earthquakelocationswere calculated for allthese events using the newly derived velocity model most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity forming tight clusters at depths shallower than 4 km relative locations calculated using the double difference method and utilising catalogue and cross correlation differential traveltimes showed insignificant differences when compared to the nonlinear probabilistic locations in general seismicity in the tvg area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts mixed low frequency events and a b value 1 17 0 1 higher than in any other part of taiwan these observations that are similar to those reported in other dormant quaternary volcanoes indicate that a magma chamber may still exist beneath tvg and that a future eruption or period of unrest should not be considered unlikely

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

evolution of stretching lineations in granulite hosted ductile shear zones eastern ghats province india role of temperature strain rate and pre existing stretching lineations abstract shear zone kinematics are interpreted from sections x z parallel to the stretching lineation x direction and perpendicular to the mylonitic foliation x y the stretching lineation is a product of finite strain defined by minerals or mineral aggregates that deform plastically in quartzofeldspathic rocks deformed below 600 c rheological contrasts between quartz and feldspar are amplified and the mesoscopic stretching lineation in the shears may be indistinct in outcrop as quartz ribbons anastomose around relatively competent feldspar clasts in high strain rate zones rheological contrasts between these minerals are reduced and a stretching lineation suitable for kinematic interpretation develops in late amphibolite and greenschist facies shear zones transecting pre lineated quartzofeldspathic granulites of the eastern ghats province india the stretching lineation represents the resultant of pre existing stretch and superposed increments of later shearing until the stretch accumulated in the x direction of the later deformation exceeds the earlier finite stretch the pre existing stretching lineation progressively rotates towards the later x direction with decreasing aspect ratio these intermediate lineation orientations have no kinematic relevance to either earlier or later deformations this example highlights the importance of temperature strain rate and orientation of pre existing stretching lineations in the appearance and kinematic interpretation of stretching lineations in ductile shear zones

shearing during progressive retrogression in granitoids abrupt grain size reduction of quartz at the plastic brittle transition for feldspar the kashio shear zone chubu district japan affects the southeast margins of the ryoke granitic plutons as well as the ryoke metasedimentary rocks and is truncated by the median tectonic line near the shear zone centre coarse quartz grains within the granitic rocks have been dynamically recrystallized to quartz aggregates towards the pluton margins the grain size data show an abrupt reduction from various sizes 59 148 m to a steady state size around 35 m at approximately 400 m from the boundary of the granitic plutons without any tectonic boundary such as a fault being present this abrupt grain size reduction accompanies brittle deformation in feldspar that overprints earlier plastic deformation and develops strong mylonitic fabrics in contrast feldspar away from the pluton margins contains only weak microstructural evidence for plastic deformation and still locally preserves magmatic polycrystal structures amongst the inhomogeneously recrystallized quartz grains these features combined with an isochronological study suggest that the kashio shear zone began at high temperature 450 c as a ductile event that affected relatively wide regions in the ryoke granitic plutons before rapid cooling by ca 70 ma at this latter stage strong mylonitization at lower temperatures 300 c in a semiductile regime occurred in a narrow zone towards the shear zone centre here recrystallized quartz aggregates reached a steady state grain size by ca 60 ma that is cooling within the granitic plutons resulted in strain localization with the abrupt grain size reduction of quartz aggregates correlating with the plastic brittle transition of feldspar