a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

correction to strong ferrimagnetic resonance signal and magnetic susceptibility of the nojima pseudotachylyte in japan and their implication for coseismic electromagnetic changes 1 pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement i examine the nojima pseudotachylyte found along the nojima fault which caused the 1995 kobe earthquake m 7 2 in japan using the electron spin resonance technique the nojima pseudotachylyte has a strong ferrimagnetic resonance fmr signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides this fmr signal appears by heating the surrounding fault gouge that is the source material of the nojima pseudotachylyte furthermore the magnetic susceptibilities of the nojima pseudotachylyte are 10 2 10 3 times larger than those of the surrounding fault gouge and are proportional to the fmr signal intensities here i reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the fmr signal the results indicate that the instantaneous magnetization of the nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction

large scale pseudotachylytes and fluidized cataclasites from an ancient subduction thrust fault in the kodiak accretionary complex kodiak island alaska pseudotachylyte occurs in black locally vitreous ultrafine grained fault rock microscopic observations show that the pseudotachylytes are composed of glass with vesicles amygdules microlites and flow structures indicating a frictional melt the pseudotachylyte is gradational to cataclasite and shows outcrop scale injection and ductile deformation structures the cataclasite was ductily mobile i e fluidized simultaneous with the formation and emplacement of pseudotachylyte melt the pseudotachylytic rocks postdate the stratal disruption fabric of associated shear zone melanges and show similar direction of thrust transport and have undergone limited subsequent deformation we interpret the stratal disruption as resulting from underthrusting of the subducting plate and pseudotachylyte development as the final activity of this thrust surface the gradational contacts between pseudotachylyte and cataclasite demonstrate that the cataclasite also formed as a seismic product and may represent paleoseismic rupture zones possibly of very great earthquakes with or without accompanying pseudotachylytes the pseudotachylytes are voluminous and many are spatially disconnected from generation surfaces this style is distinct from pseudotachylytes described in other environments and this may explain the rarity of documented examples of subduction thrust pseudotachylyte

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

selective melting processes as inferred from experimentally generated pseudotachylytes abstract high speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1 0 1 5 mpa and at slip rates up to 2 m s using a rotary shear high speed frictional testing machine the experimentally generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub angular to rounded fragments these locally occur as injected network veins in the fractured rock x ray diffraction analysis has revealed the presence of as much as 40 70 wt glass the mineral contents of clasts in the experimental pseudotachylyte in granite as determined by x ray diffraction analysis indicate that quartz is the most resistant to melting biotite the least and feldspar is intermediate thus the sio2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

fractal dimension of molten surfaces as a possible parameter to infer the slip weakening distance of faults from natural pseudotachylytes high velocity friction experiments on gabbro and monzodiorite using a rotary shear high velocity friction apparatus have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening eventually reaching a nearly steady state the melting surface at the host rock molten layer interface is initially very flat but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the gibbs thomson effect this change in the melting surface topography can be quantitatively expressed by the fractal dimension d as determined by the divider method from about 1 0 near the peak friction to around 1 1 near the steady state friction the ultimate fractal dimension at steady state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting a systematic correlation of d with mechanical behavior of the fault during frictional melting may provide a way of estimating slip weakening distance and heat production rate at steady state friction by measuring d for natural pseudotachylytes on slip surfaces with different displacements the weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength the experimental studies point to ways to estimate these difficult quantities for natural faults

propagation of seismic slip from brittle to ductile crust evidence from pseudotachylyte of the woodroffe thrust central australia abstract the woodroffe thrust central australia is a 1 5 km wide mylonitized shear zone marked by large volumes of mm to cm scale pseudotachylyte veins the pseudotachylytes display typical melt origin features including rounded and embayed clasts spherulitic and dentritic microlites and flow structures within a fine grained matrix three types of pseudotachylyte are identified on the basis of deformation texture vein morphology and host rock lithology cataclasite related c pt mylonite related m pt and ultramylonite related um pt the m pt and um pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization these pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine grained vein matrix and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite these observations constrain the timing and environment of m pt and um pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust the m pt and um pt veins as well as the host mylonite are overprinted by cataclasis and multiple generations of late stage c pt veins that were generated in the brittle dominated regime of the upper crust during uplift and exhumation of the shear zone the coexistence of multiple generations of voluminous c pt m pt and um pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the woodroffe thrust the timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle dominated seismogenic zone and propagated down through the brittle ductile transition into the ductile dominated regime of the crust

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

paleomagnetism and paleothermometry of the sydney basin 1 thermoviscous and chemical overprinting of the milton monzonite the early triassic 245 ma milton monzonite of the sydney basin australia has four distinct components of natural remanent magnetization nrm with only slightly overlapping ranges of unblocking temperatures the low temperature lt component the first to be thermally demagnetized is thought to be a late cretaceous 100 ma thermoviscous overprint acquired in slow cooling during uplift the high temperature ht component the second to be demagnetized is probably the primary thermoremanent magnetization trm of the milton intrusion but could possibly be a jurassic overprint lt and ht are usually carried by magnetite and occasionally by pyrrhotite samples from nine sites have a further nrm component which unblocks at higher temperatures than ht but below the magnetite curie temperature of 580 c this component is argued to be a chemical remanent magnetization crm because of its discrete range of high unblocking temperatures above those of the thermal components ht and lt and is called crm1 crm1 has almost the same direction as lt and is likely carried by authigenic magnetite produced during uplift 100 ma samples from five sites have a fourth nrm component with a direction resembling that of ht but carried by hematite this fourth component could be a primary trm but is more likely a crm and is therefore called crm2 the ht crm2 mean direction is d 50 i 75 5 defining a paleopole at 16 s 172 e the ht crm2 paleopole falls near 150 ma on the australian apparent polar wander path but is a considerable distance from paleopoles of permian and early triassic age there is no known tectonic or other remagnetizing event in the sydney basin around 150 ma for this reason we propose that the ht crm2 paleopole defines a new triassic segment of the australian polar wander path the lt crm1 mean direction is d 348 i 79 with a paleopole falling at 56 s 1580 e near 100 ma on the polar wander path this age is consistent with uplift and cooling related to initial rifting of the tasman sea

fusion by earthquake fault friction stick or slip field observations of pseudotachylites and experimental studies of high speed friction indicate that melting on a slipping interface may sig nificantly affect the magnitude of shear stresses resisting slip we investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer thermodynamics of phase transitions and fluid mechanics results of laboratory experiments conducted at high order of m s slip velocities but low order of mpa normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength presumably due to viscous effects however extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses when transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction resulting in a nearly complete stress drop thermal runaway the amount of energy dissipation associated with the formation of pseudo tachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting clasts from a coarse grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of non equilibrium overheating the maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus via viscous braking and slip arrest or liquidus via thermal runaway and vanishing resistance to sliding our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz rich rocks especially at shallow 5 7 km depths where the driving shear stress is relatively low

a simple probabilistic model to estimate the population of near earth asteroids abstract a simple probabilistic model for estimating the population of near earth asteroids neas by comparing the rate of redetections of previously known asteroids of a given size with new detections during a test interval of time is described we apply this method to the whole sample of nea observations discovery observations and observations from apparitions of previously known asteroids made by the lincoln near earth asteroids research project within the period jan 1 1999 dec 31 2000 and published by the minor planet center the numbers of discoveries and redetections are sufficient to estimate the total population in the absolute magnitude range 13 5 h 20 0 by this method we obtain an estimate of the number of neas of h 18 0 diameter 1 km of 855 101 we note however that the expected biases of this method all work in the direction of underestimating the true population thus our estimate is consistent with other recent estimates for example w f bottke et al 2000 science 288 2190 2194 who obtain 910 110

geochemistry and 40ar 39ar geochronology of pseudotachylyte associated with uhp whiteschists from the dora maira massif italy abstract in situ uv laser ablation 40 ar 39 ar geochronological and geochemical data together with rock and mineral compositional data have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the uhp whiteschists of the dora maira massif italy several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation whole rock xrf data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss the pseudotachylyte contains newly crystallized grains of biotite and k feldspar in a matrix of glass with partially fused grains of quartz zircon apatite and titanite electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer scale changes in mineralogy uv laser ablation icp ms traverses across the mylonitic gneiss pseudotachylyte contact are consistent with cataclastic communition of ree carriers such as epidote monazite allanite zircon and apatite before melting as an efficient mechanism of ree homogenization in the pseudotachylyte the 40 ar 39 ar data from one band of pseudotachylyte indicate formation at 20 1 0 5 ma when the mylonitic gneisses were already in a near surface position the variable effects of top to the west shear deformation within outcrops of the coesite bearing unit are reflected in localized zones of protomylonite cataclasite ultracataclasite and pseudotachylyte preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the uhp rocks it is speculated that deeper crustal seismic events potentially played a role in the unroofing of the uhp rocks at earlier stages in their exhumation history

the physical isolation and identification of carriers of geologically stable remanent magnetization paleomagnetic and rock magnetic microanalysis and electron microscopy paleomagnetic and rock magnetic microanalysis of fully oriented microspecimens provides a more direct understanding of the source and origin of geologically stable remanent magnetizations scanning transmission electron microscopy allows direct observation and characterization of magnetic phases we provide a brief history of micromagnetic research describe a functional microspecimen preparation method evaluate instrumentation and present results of three combined micromagnetic stem studies

chemical and oxygen isotope composition of natural and artificial pseudotachylyte role of water during frictional fusion abstract pseudotachylyte samples from the long ridge fault north carolina the homestake shear zone colorado the fort foster fault maine and experimentally produced frictional melt were analyzed for major elements and o isotopes using an electron microprobe and laser microprobe respectively the pseudotachylytes display a 18o range of 5 9 10 7 standard mean ocean water smow and are both isotopically lighter and heavier than their respective host rocks in general the isotopic composition of bulk pseudotachylyte pt in the absence of water rock interaction can be modeled as 18 o bulk pt x frag 18 o frag x min 18 o min where xfrag refers to the volume fraction of unmelted mineral fragments and xmin refers to the volume fraction of minerals that undergo melting mineral melt isotopic fractionation is considered to be small and less than 1 the majority of pseudotachylyte isotopic compositions can be explained as a disequilibrium partial melt of the protolith rock comprising melted feldspar and micas plus a small proportion 10 of unmelted quartz clasts in contrast the long ridge fault samples are substantially depleted in 18o by up to 4 relative to their host rock which is best explained by closed system water rock interaction of a pore fluid at 100 c with a melt at 1000 c and a water rock weight ratio of 0 2 this fluid is also inferred to have caused oxidation of the pseudotachylyte melts producing the hematite characteristic of this location

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

pressure related feedback processes in the generation of pseudotachylytes pseudotachylyte formation is typically viewed as a self limiting process quantitative models have demonstrated that frictional melting is possible at seismic slip rates but once a thin film of melt forms on a seismic fault plane the dramatic decrease in fault friction is thought to suppress further melting volumes of melt generated pseudotachylyte observed in field studies however are in many cases substantially larger than the amounts of frictional melt predicted by theoretical models this suggests that previously unrecognized physical processes may enhance melting during seismic slip localized decompression at dilational jogs may be one such phenomenon transient unloading could play two important roles in the dynamics of pseudotachylyte generation first by setting up significant fluid pressure gradients it would lead to rapid migration of the melt to sites of low pressure thereby reestablishing frictional contact across the fault surface and favoring further melt generation second at significant depths sudden depressurization might lead to in situ decompression melting

dynamic reorganization of river basins river networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels we demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides assessing this metric throughout a landscape maps the dynamic states of entire river networks revealing diverse conditions drainage divides in the loess plateau of china appear stationary the young topography of taiwan has migrating divides driving adjustment of major basins and rivers draining the ancient landscape of the southeastern united states are reorganizing in response to escarpment retreat and coastal advance the ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape scale interactions among tectonics erosion and ecology

40ar 39ar dating of neogene pseudotachylytes if fresh abundant and formed over a sufficiently long time interval ar ar dating of pseudotachylytes provides a means of assessing the active histories of exhumed faults due to the combination of generally low k content and young formation age the ar ar dating of neogene pseudotachylyte is analytically difficult moreover interpretation of ages is complicated by different extraneous and radiogenic argon isotope source reservoirs and by the intimate coexistence of unmelted clasts neoformed minerals and frictional glass high spatial resolution uv laser ablation ar ar geochronology of pseudotachylytes together with careful textural and chemical characterization provides a means of precisely dating these fossil paleoseismic events we report results of uv laser ablation ar ar geochronology on neogene pseudotachylyte from tectonically active areas in japan and the swiss alps our data yield age probability plots forming near normal distributions but have discernible tails at older ages due to the contribution of partially outgassed clast material therefore we interpret the peak of the age probability function as a maximum age of pseudotachylyte formation pseudotachylytes at the top of the leventina nappe the deepest part of lepontine dome central alps switzerland contain few large clasts and yield an age of 10 ma consistent with prior estimates of orogen parallel extension in alps in contrast two pseudotachylyte samples from the uchinoura shear zone southern kyushu japan yield an age of 11 ma approximately 2 m a younger than previously determined ages fabbri et al 2004 we attribute this age difference to the fact that our high spatial resolution uv laser analyses were focused on areas with fewer clasts thus minimizing the effects of argon incorporation from partially outgassed clast material

anisotropy of diamagnetic susceptibility in thassos marble a comparison between measured and modeled data abstract a study of shear zones within the calcite marble complex of the island of thassos greece shows that the low field anisotropy of magnetic susceptibility ams technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics the strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial neutral to uniaxial prolate shapes of ams ellipsoids the maximum susceptibility is oriented perpendicular to the rock foliation reflecting the preferred orientation of calcite c axes in the protolith as well as in the mylonites for three samples that represent different types of calcite fabrics the ams was recalculated from neutron and electron backscatter diffraction textural data a comparison of the measured and modeled data shows a good coincidence for the orientation of the principal ams axes and for the recalculated anisotropy data both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation

nrm irm s demagnetization plots an aid to the interpretation of natural remanent magnetization af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization irm s are used to establish diagnostic criteria for primary trm and for secondary magnetization in igneous rocks in applying the approach to the interpretation of natural remanent magnetization nrm the demagnetization of irm s documents the nature of the magnetic phases in the rock while the demagnetization of the nrm gives information about the process of magnetization the ratio of nrm irm s measures the efficiency of the magnetization process

correlation of magnetitic susceptibility with18o 16o data in late orogenic granites of the southern appalachian piedmont initial magnetic susceptibility generally indicative of magnetite content has been determined for 445 samples from 17 granites located in the southern appalachian piedmont of georgia and south carolina these values have been correlated with whole rock 18o data from the same plutons yielding a pronounced inverse relationship it has previously been shown for the southern piedmont that low oxygen isotopic 18o enriched values usually occur in s type granites wenner 1 this issue it follows then that i type granites are characterized by high susceptibilities 1 10 4 g oe and s type granites by low susceptibilities 1 10 4 g oe an interesting result of this work has been the observation that some s type granites exhibit good within site clusters of remanent magnetic directions while i type granites generally do not

petrology of pseudotachylytes from the alpine fault of new zealand abstract the petrology of pseudotachylytes derived from contrasting host rocks within the alpine fault of new zealand a well known dextral shear zone is presented the structural observations indicate that parallel synthetic shears formed first in a non dilational system at low angles to the alpine fault then between these shears fracturing and frictional heating occured according to a new dilational riedel system accompanied by listric faults the observations suggest that pseudotachylyte melt is mainly formed in the listric faults and channelled into neighbouring areas of extension pseudotachylyte results from in situ melting and its nature depend strictly on the parent rock mineralogy the microlitic nature of the pseudotachylites is confirmed and the variability of the textures explained in terms of degrees of undercooling the morphology and composition of the minerals in the pseudotachylyte plagioclase biotite and amphibole are documented they crystallized at a higher temperature and pressure than their parent rock minerals the melting process is believed to be governed by the mechanical properties of mafic minerals in a fluid free environment at the beginning of the melting mafic minerals provide water which allows the development of the pseudotachylyte

paleomagnetic evidence for post jurassic stability of southeastern mexico maya terrane the tectonic evolution of southeastern mexico has been a subject of major controversy not only in regard to past geometry but also in the timing of proposed geological events as well for the past 10 years most if not all investigators agree that the gulf of mexico basin was formed by late jurassic time and that the maya terrane was in its current location prior to the cretaceous in order to gain further insight into the drift history of the maya terrane we have undertaken a paleomagnetic study of the uppermost jurassic lower cretaceous tithonian lower neocomian san ricardo formation in southeastern mexico at 93 7 w 16 8 n the sampling site is located east of the isthmus of tehuantepec on the southwest side of the maya block at the base of the yucatan peninsula a suite of 133 samples was collected in stratigraphic succession from a 114 m thick sequence of red shales and sandstones near cintalapa chiapas mexico after progressive thermal demagnetization of all samples at six steps from 350 c to 630 c 89 samples were selected for final paleopole analysis on the basis of their magnetic stability four different polarity intervals were observed the sequence being from bottom to top n r n r which assists in the assessment of the reliability of the observations the mean pole position obtained 160 0 e 69 8 n agrees with the mean pole position of the upper part of the morrison formation of colorado a unit of virtually identical age these results indicate that no discernible rotation or displacement of the maya block has occurred since at least early neocomian times

paleomagnetism of the chiapas massif southern mexico evidence for rotation of the maya block and implications for the opening of the gulf of mexico we have analyzed 180 oriented samples of the batholithic complex and overlying red beds of the chiapas massif in southern mexico for a paleomagnetic study both stepwise alternating field and thermal demagnetization procedures revealed dual polarity characteristic magnetizations above 10 30 mt or 200 400 c in about 60 of the samples demagnetized three distinct and ancient paleomagnetic directions have been identified late permian intrusions 256 10 ma yield nearly equatorial southern hemisphere paleolatitudes and a paleopole 13 4 s 176 1 e 12 sites k 15 4 a95 11 4 rotated 75 counterclockwise with respect to the north american reference pole a selected set of late triassic jurassic red beds of the todos santos formation yields a paleopole at 22 6 n 170 6 e 4 sites k 56 8 a95 12 3 which implies 60 counterclockwise rotation and a near equatorial northern hemisphere paleolatitude middle late jurassic dikes and granites yield a paleopole at 77 4 n 97 4 e 6 sites k 269 2 a95 4 1 near the mid jurassic segment of the north american apparent polar wander path these data are consistent with a pre rift configuration in which the maya block is rotated to a position off the coast of texas and louisiana in the present day area of the gulf of mexico a further implication is that by oxfordian times chiapas and by inference the maya block was essentially in its current position with respect to cratonic north america

plastic behavior of magnetite and high strains obtained from magnetic fabrics in the parry sound shear zone ontario grenville province abstract magnetic anisotropy and quantitative petrology of a transition from relatively undeformed metaanorthosite protolith to highly deformed ultramylonite in the parry sound shear zone provide information on magnetite as a kinematic indicator rock magnetic experiments show that anisotropy of magnetic susceptibility ams is controlled by magnetite in the mylonites and ultramylonites and by paramagnetic minerals hornblende biotite ilmenite in the protolith geothermometry in the ultramylonite indicates a temperature of 630 50 c during deformation a deformation mechanism map calculated from published experimental work for 630 c indicates that magnetite in the parry sound shear zone deformed plastically rather than via rigid body rotation the ams orientations accurately track a foliation trajectory in a portion of the shear zone which allows the utilization of orientation based strain models to determine shear strains shear strains of 1 9 were obtained and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships mmax 0 15 ln x and mmin 0 13 ln z where mi ki kmean describes the principal ams axes or k max k min x z 0 14 these ams strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone predicting shear strains as high as 13 these observations also reveal that ams fabrics do not saturate at high strains as would be expected for rigid body rotation due to the plastic behavior of magnetite during deformation

magnetic properties and paleointensity of pseudotachylytes from the sudbury structure canada petrologic control abstract since a pseudotachylyte is generated through a combination of comminution and frictional melting it may record an ambient earth s magnetic field as a thermal remanence during the cooling from friction melt to test the reliability of determining the paleointensity the coe modified thellier method was carried out on a small suite of pseudotachylytes in large displacement fault zones from the north range sudbury impact structure pseudotachylytes studied here are classified into two types in their protolith lithology gneiss origin and granitoid origin pseudotachylyte petrologic observations confirm the presence of abundant fine grained magnetite inclusions and coarse grained titanomagnetites with exsolved ilmenite lamella in gneiss origin pseudotachylyte in granitoid origin pseudotachylyte we found a scatter distribution of multidomain magnetite although the granitoid origin pseudotachylyte fails to estimate the paleointensity the gneiss origin pseudotachylytes yield weak paleointensity values which are consistent with previous paleointensity data from the norite in sudbury igneous complex these results suggest that the protolith petrology controls a magnetic mineralogy in pseudotachylytes and the reliability as recorder of ancient earth s magnetic field

superheated friction induced melts in zoned pseudotachylytes within the adamello tonalites italian southern alps pseudotachylytes and cataclasites are present along a strike slip fault zone in the tonalites of the adamello intrusion italian southern alps ambient conditions during faulting were 0 25 0 3 gpa and 250 300 c pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain the thickness of the microlitic domains x increases linearly with the half total thickness a of the pseudotachylyte vein commonly 2 cm according to the relation x 0 29 0 12 a the spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts a numerical model explains zoning as the result of the different cooling rates and clast melt interactions at the center and periphery of thick veins zoning is compatible with the injection of a single pulse of superheated friction induced melt tmelt 1450 c melt temperatures estimated by the clast matrix ratio o hara 2001 are considerably lower 316 577 c it is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites pseudotachylytes

earthquake rupture dynamics frozen in exhumed ancient faults just by looking at the traces of solidified friction induced melt produced by ancient earthquakes it is possible to learn about fractures that lasted a few seconds during seismic events that took place millions of years ago in the italian alps large sections of an ancient fault have been uprooted by tectonic movements brought to the surface almost intact and polished by the abrasion of a glacier secondary fractures created during the earthquakes tend to lie to one side of the fault in a particular direction suggesting that rupture propagated in the same direction during repeated earthquakes most of our knowledge about co seismic rupture propagation is derived from inversion and interpretation of strong ground motion seismograms1 2 3 laboratory experiments on rock4 5 and rock analogue material6 or inferred from theoretical and numerical elastodynamic models7 8 9 however additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the earth s surface10 pseudotachylytes solidified friction induced melts11 12 are the most certain fault rock indicator of seismicity on ancient faults13 here we show how the asymmetry in distribution and the orientation of pseudotachylyte filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity rupture velocity and fracture energy by comparison with the theoretical dynamic stress field computed around propagating fractures in particular the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the rayleigh wave velocity

laboratory verification of submicron magnetite production in pseudotachylytes relevance for paleointensity studies abstract pseudotachylytes generally possess stable remanent magnetizations but the processes by which pseudotachylytes are magnetized remain poorly understood magnetic hysteresis and scanning electron microscope studies reveal that experimental frictional melting of granites produces dispersed submicron inclusions of weakly interacting pseudo single domain psd magnetite in artificial pseudotachylyte the magnetite inclusions are absent in the undeformed granite protolith and result from oxidation of fe in melt susceptible mafic minerals during the melt quenched event the pseudotachylytes acquired a stable thermal remanence in fine grained psd magnetites during the rapid cooling of the melt implying that fine grained magnetite has the potential for paleointensity determinations of contemporaneous magnetic fields with co seismic faulting in granitoids

an integrated magnetic and geological study of cataclasite dominated pseudotachylytes in the chiapas massif mexico a snapshot of stress orientation following slip summary the permian age chiapas massif in southeast mexico is locally host to well exposed pseudotachylyte vein networks the veins are black to dark grey and aphanitic in appearance and consist mostly of microbreccia of angular fragments of plagioclase k feldspar biotite and quartz in a cryptocrystalline microscopically irresolvable matrix evidence of melting is present in the form of glass seams dikelets glass clasts included in cataclasite and a distinct chemistry in the pseudotachylite veins pristine glass represents a relatively small volume of the pseudotachylite veins at an exposure along the tablon river valley where the host rock is a medium to coarse grained equigranular quartz diorite individual veins are consistently oriented about 280 are up to 16 mm wide tens of cm apart display a consistent left lateral offset and can be traced for several metres individual pseudotachylyte veins rarely cross each other and they cannot be directly linked to a regional scale fault pseudotachylytes are apparently formed by a combination of crushing comminution and frictional melting but they are cataclasite dominated textures indicate that cataclasis continued after frictional melting had ceased a 40ar 39ar age determination from whole rock chips of one vein shows a climbing ar release spectrum with a date of 114 ma as the most reliable age estimate for ar retention this result is interpreted in the context of pseudotachylyte formation recrystallization and resetting of k bearing minerals for the k ar system in the late early cretaceous ten veins were sampled for palaeomagnetic and magnetic fabric studies with samples collected from both the veins and their host rock remanence data give moderate natural remanent magnetization nrm intensities for both the veins e g nrm mean 6 6 10 3 a m 1 5 5 and host rock mean 7 7 10 3 a m 1 10 8 many samples of host rock yield an ill defined east west directed and shallow magnetization which we interpret as a late permian magnetization based on previous studies of the chiapas massif this magnetization resides in haematite veins as well as immediately adjacent host rock typically have well defined single polarity magnetizations of north northwest declination and moderate positive inclination and these resemble the cretaceous expected field direction the overall mean of the veins is of dec 348 7 and inc 33 6 k 30 5 and 95 12 3 n 6 site means rock magnetic parameters suggest that the remanence in the veins principally resides in low ti pseudo single domain magnetite and maghemite grains but haematite coarse grained magnetite rutile fe cr ni oxides and ilmenite are also present in the veins bulk magnetic susceptibility values range between 0 3 and 1 1 10 3 si volume units and host rock values do not differ significantly from vein values the orientations of the principal susceptibility axes in the host rock and the veins are indistinguishable in both magnetic foliations are near vertical and are essentially parallel to the nearly east west orientation of the veins nonetheless host rock fabrics are predominantly prolate whereas vein fabrics are oblate the mean susceptibility tensors of host rocks and pseudotachylyte are characterized by p t values of 1 041 0 327 and 1 033 0 302 respectively this result suggests that the fabric reflected by magnetic susceptibility anisotropy in the veins was formed under pure shear stress during cooling of the veins the lack of evidence of fabric rotation supports models that associate partial melt with viscous break during fault slip