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

paleomagnetic results from honduras and their bearing on caribbean tectonics paleomagnetic results have been obtained from 37 sites in central and western honduras and one site in north western nicaragua the rocks are limestones sandstones and granites and range in age from 140 to 60 m y regional fold tests demonstrate that the rocks carry a prefolding pre laramide magnetization the data indicate a very large clockwise rotation of the sampling area during the early cretaceous followed by a counter clockwise rotation in the late cretaceous the changes in latitude are minor during this time period it is highly unlikely that these large rotations apply to the entire caribbean plate rather they reflect the motion of a smaller area known as the chortis block this interpretation is supported by paleomagnetic data from neighboring areas the results also suggest that the chortis block sutured onto southern mexico guatemala in latest cretaceous time in agreement with geochemical data and several plate tectonic models these new data together with published paleomagnetic data from around the caribbean imply that a rigid caribbean plate did not exist until well into the tertiary

neogene caribbean plate rotation and associated central american tectonic evolution during the neogene the caribbean plate moved eastward with respect to the north and south american plates reconstructing this motion by closing the cayman trough along its bounding transform faults creates a gore of ocean between northern central america and southern mexico we present instead a reconstruction which juxtaposes these two continental areas by strike slip motion using geological evidence from around the cayman trough the strike slip fault zones of northern central america and the continental margin of southern mexico as our constraints we propose that seafloor spreading began in the cayman trough at 30 5 ma and that subsequently there was 1100 km of relative plate displacement at an average rate of 3 7 0 6 cm yr this neogene plate motion cannot be achieved by rigid plate rotation about a single pole which implies that central america must have rotated anticlockwise relative to the rest of the plate there is evidence for internal plate deformation in central america consistent with such a sense of rotation our suggestion of an originally easterly trending arc in southern central america is more consistent with the timing and geometry of the collision of panama with northern colombia and the resultant deformation than were earlier models that depicted collision of a northerly trending arc

venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

imaging surface contacts power law contact distributions and contact stresses in quartz calcite glass and acrylic plastic a procedure has been developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials while the surfaces are subjected to static loads or undergoing frictional slip static loading experiments with quartz calcite soda lime glass and acrylic plastic at normal stresses to 30 mpa yield power law distributions of contact areas from the smallest contacts that can be resolved 3 5 m2 up to a limiting size that correlates with the grain size of the abrasive grit used to roughen the surfaces in each material increasing normal stress results in a roughly linear increase of the real area of contact mechanisms of contact area increase are by growth of existing contacts coalescence of contacts and appearance of new contacts mean contacts stresses are consistent with the indentation strength of each material contact size distributions are insensitive to normal stress indicating that the increase of contact area is approximately self similar the contact images and contact distributions are modeled using simulations of surfaces with random fractal topographies the contact process for model fractal surfaces is represented by the simple expedient of removing material at regions where surface irregularities overlap synthetic contact images created by this approach reproduce observed characteristics of the contacts and demonstrate that the exponent in the power law distributions depends on the scaling exponent used to generate the surface topography

linking hematite u th he dating with the microtextural record of seismicity in the wasatch fault damage zone utah usa techniques directly dating fault slip are few limiting the ability to interpret the rock record of seismicity hematite is commonly found in fault zones amenable to u th he dating and slip surface hematite may be reset by shear heating events and or recrystallization glossy hematite coated fault surfaces in the wasatch fault footwall damage zone utah usa exhibit evidence of hematite cataclasis and preserve pliocene hematite u th he dates apatite u th he and fission track data from the host gneiss indicate footwall unroofing through 2 km by ca 4 5 ma internally reproducible but disparate hematite u th he dates 4 5 ma and younger from isolated locations on a single fault surface do not reflect ambient cooling we hypothesize that these dates and associated iridescence and annealed crystal texture document rapid cooling from friction generated heat during small seismic slip events between ca 4 5 and 2 5 ma thus hematite u th he dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 10 6 yr time scales

seismic stratigraphy of western colombian basin caribbean sea multichannel seismic reflection profiles disclose the regional stratigraphy of the western colombian basin the basement complex is the seismic unit below the deepest continuous reflection horizon that can be traced throughout the basin the basement complex reflection signature on the flanks of the mono rise and adjacent areas is smooth continuous and characterized by local occurrences of internal reflectors and is equivalent to the late cretaceous horizon b in the venezuelan basin in the central basin the reflection signature is rough with abundant diffractions typical of normal oceanic crust the sediment overlying the basement complex is subdivided into five mapping units unit cb5 which directly overlies the basement complex is thickest on the mono rise and thins down the flanks of the rise this unit is equivalent to the upper cretaceous to the middle eocene pelagic unit bounded by seismic horizons a and b in the venezuelan basin unit cb4 characterized by pervasive small offset faulting is restricted to the crest of the mono rise units cb3 and cb2 contain subparallel variable amplitude continuous reflectors that fill the regional basement complex relief they are middle tertiary terrigenous distal turbidites and hemipelagic deposits unit cb1 thickens toward southern central america andmore shows complicated reflection patterns typical of a deep sea fan complex a jump correlation to deep sea drilling project site 154 is used to assign a late miocene to quaternary age to unit cb1 development of unit cb1 was concurrent with the uplift of and magmatic activity in southern central america less

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

the influence of ambient fault temperature on flash heating phenomena 1 recent high velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface i e flash heating here we explore the effects of elevated ambient fault temperature up to that at the base of the earth s seismogenic zone on the flash heating behavior of quartzite westerly granite and india gabbro the experiments demonstrate that elevated ambient fault temperature causes 1 an increase in the characteristic weakening velocity for flash heating and 2 an increase in the friction coefficient at a given velocity above this characteristic weakening velocity these unexpected results are reconciled with flash heating theory when temperature specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

evaluation of asperity scale temperature effects during seismic slip abstract for the common rock forming minerals the joule thompson effect produces cooling on compression and heating on decompression experimental rock friction studies and seismological evidence suggest that the strength of faults is controlled by high stress asperities that represent a small fraction of the total fault area during seismic rupture the area immediately beneath asperities is inferred to undergo adiabatic compressions and decompressions on the order of 1 2 8 gpa joule thompson coefficients are in the range 261 372 c gpa resulting in temperature changes of 336 880 c quartz asperities with a compressive strength of 2 8 gpa can produce a heating or cooling of 880 c immediately beneath an asperity by comparison frictional heating flash temperatures are less than 1000 c for asperity contact areas 2 the joule thompson effect is independent of asperity size and may augment or counteract frictional heating the paucity of pseudotachylytes in the geologic record and the absence of localized heat flow anomalies on active faults might be explained by the joule thompson cooling effect

slickenline orientations as a record of fault rock rheology abstract contact of geometric asperities across rough faults causes perturbations to the shear traction resolved on a fault surface that could potentially deflect the local slip direction slickenlines which record the relative displacement of opposing sides of a fault may therefore be sensitive to fault surface geometry to investigate the relationship between fault geometry shear traction and slip we use ground based lidar to measure the orientations of slickenlines defined by centimeter amplitude corrugations on three fault surfaces slickenline rakes measured in the mean plane of each fault rarely vary by more than a few degrees deviations from the mean rake do not correlate with the fault surface topography at scales up to a few meters but a weak correlation may be present at larger scales slip directions are therefore insensitive to any shear traction perturbations from the contact of geometric asperities at small scales this observation is consistent with scale dependent deformation we show that the roughness of fault surfaces implies that short wavelength asperities fail inelastically because of flattening during fault slip a crossover to elastic deformation occurs at lengths of 10 2 to 10 0 m which defines the minimum dimension of a strength asperity that influences the displacement field the roughness at the crossover length scale corresponds to the typical thickness of fault rocks in the fault core suggesting that fault rock thickness is related to the crossover length scale the data requires that multiple processes combine to produce the fault surface geometry we reconcile the variety of processes with the consistent surface roughness scaling by noting that all of the processes are governed in different ways by the elastic limit of rock

cenozoic rift formation in the northern caribbean rifts form in many different tectonic environments where the lithosphere is put into extension we outline the distribution orientation and relative ages of 16 cenozoic rifts along the northern edge of the caribbean plate and suggest that these structures formed successively by localized extension as the caribbean plate moved eastward past a continental promontory of north america evidence leading to this conclusion includes 1 recognition that the rifts become progressively younger westward 2 a two phase subsidence history in a rift exposed by upthrusting in jamaica 3 the absence of rifts east of jamaica and 4 the observation that removal of 1400 km of strike slip displacement on the cayman trough fault system places the paleogene rifts of jamaica in an active area of extension south of yucatan where the rifts of honduras and guatemala are forming today

hot faults iridescent slip surfaces with metallic luster document high temperature ancient seismicity in the wasatch fault zone utah usa and localized iridescence in the footwall dam age zone of the wasatch fault utah usa and document elevated temperatures on these faults we propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates wasatch fault zone abstract we document new geological indicators of ancient seismicity in the form of highly reflec tive iridescent hematite coated fault surfaces small faults that cut the paleoproterozoic farmington canyon complex in the footwall damage zone of the brigham city segment of the wasatch fault utah usa are smooth to striated surfaces tens of square centimeters to 30 m 2 in area the dull rusty to high metallic luster and moderate to high gloss surfaces exhibit multicolored elliptical iridescent patches 0 5 3 cm across preexisting hematite crystals were deformed during slip on 1 200 mm thick slip surfaces textural observations x ray diffraction x ray photoelectron spectroscopy electron backscattered diffraction analysis surface metrology and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high temperature 300 c reduction of iron fe 3 to fe 2 and associated conversion of hematite to magnetite we pro pose that the iridescent slip surfaces in the wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults the thousands of these sur faces represent coseismic or aftershock deformation down to magnitude 3 in the exhumed footwall damage zone of the wasatch fault

dynamical weakening of faults by acoustic fluidization when a fault slips seismically some of the energy released may excite strong short wavelength vibrations near the fault core such vibrations can temporarily reduce the normal stress on the fault allowing it to slip at lower shear stresses than predicted by laboratory coefficients of friction this phenomenon of acoustic fluidization provides an alternative to theories that invoke pressurized fluids as an explanation for why some faults appear to be so weak

pseudotachylite from the main zone of the hidaka metamorphic belt hokkaido northern japan pseudotachylite veins have been found in the mylonite zone of the hidaka metamorphic belt hokkaido northern japan they are associated with faults with wnw ese to ene wsw or ne sw trends which make a conjugate set cutting foliations of the host mylonitic rocks with high obliquity the mylonitic rocks comprise greenschist facies to prehnite pumpellyite facies mineral assemblages the mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near surface environments an analysis of the deformational and metamorphic history of the hidaka main zone suggests that the ambient rock temperature was 200 300 c immediately before the formation of the hidaka pseudotachylite three textural types of veins are distinguished cryptocrystalline microcrystalline and glassy the cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline type veins the microcrystalline type is largely made up of quench microlites of orthopyroxene clinopyroxene biotite plagioclase and opaque minerals with small amounts of amphibole microlites the interstices of these microlites are occupied by glassy and or cryptocrystalline materials the presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km the bulk chemical composition of the pseudotachylite veins is characterized by low sio2 and a high water content and is very close to that of the host mylonitic rocks this indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high al contents the presence of pigeonite and high al pyroxene microlites of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins the melt temperatures were estimated to be up to 1100 c using a two pyroxene geothermometer using published data relating water solubilities in high temperature andesitic magmas to pressure a depth estimate of about 4 km is inferred for the hidaka pseudotachylites evidence derived from pseudotachylites in the hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals

a pseudotachylyte geothermometer abstract pseudotachylytes invariably contain a conspicuous concentration of clasts and lithic fragments in their matrices which reflect brittle wear processes during frictional melting the ratio of clasts to matrix is interpreted as the ratio of wear to melt w m and measurements of this ratio show a range of 0 1 0 7 based on models of melting and wear processes it is shown that the ratio w m is independent of fault displacement stress fault area and mineralogy thermodynamic considerations indicate that w m corresponds to the thermodynamic efficiency of the conversion of work to heat and is defined as w q thigh tlow thigh where thigh refers to the melt temperature and tlow is the ambient country rock temperature this provides a simple new technique for estimating the ambient crustal temperature tcrust in degrees kelvin t crust 1 w m t melt assuming a reasonable value for the temperature of the melt estimates of country rock temperatures tcrust for pseudotachylytes from four different localities in the usa indicate a range of 123 387 c eclogite facies pseudotachylytes from western norway yield a mean country rock temperature of 658 c in good agreement with independent fe mg exchange geothermometry this new geothermometer appears to be applicable throughout the entire crust and may provide a better understanding of melting and wear processes during seismic faulting

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

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

mesozoic stratigraphy of central honduras geologic mapping near lake yojoa in central honduras has shown the need to modify the published mesozoic stratigraphic column the ilama formation is not a useful unit for regional mapping inasmuch as ilama type strata are discontinuous and in various stratigraphic positions the formal name ilama formation should be abandoned a new formal name the jaitique formation is introduced to designate a major cenomanian limestone unit that is stratigraphically about midway in the valle de angeles group in the santa barbara lake yojoa region the jaitique formation consists of up to 140 m of shallow marine strata that can be divided into two members 1 a lower member of thick bedded shelf carbonate strata and 2 a distinctive upper member of thin bedded laminated nonfossiliferous limestone which may represent a shallow possibly hypersaline lagoon or intertidal mud flat environment because the lower member of this cenomanian limestone was formerly misidentified as the atima formation of the yojoa group the upper thin bedded member was originally defined as the guare formation of the yojoa group the name guare is retained but the unit is downgraded to the guare member of the newly defined jaitique formation within the valle de angeles group although the jaitique formation is similar in stratigraphic position and age to the esquias formation in the minas de oro area on the east it is lithologically distinct and probably slightly older the two units are not thought to be lithostratigraphically contiguous recognition of major cenomanian limestone units within the valle de angeles group has led to reappraisal of the hydrocarbon potential of certain structural zones in honduras

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

late cretaceous early tertiary tectonic history of southern central america the paleomagnetic analysis of samples from three late cretaceous through eocene sites in southwesternmost nicaragua yields pole positions close to the present pole the results from five equivalent sites in the pacific coastal zone of costa rica on the other hand suggest a 14 northward translation in post eocene time this movement cannot have been initiated by the development of the cocos nazca spreading center and it seems likely that the extensive late miocene plutonism in costa rica defines the time of collision of the nicoya complex sensu largo and the costa rican island arc the nicaraguan as well as the costa rican data are very different from paleomagnetic results from honduras and the tectonic boundary between northern and southern central america therefore must lie to the north and east of the nicaraguan sites

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

dynamic weakening of smectite bearing faults at intermediate velocities implications for subduction zone earthquakes the hydrous clay mineral smectite which is pervasive in sediments on subducting oceanic plates is thought to weaken and stabilize subduction thrust faults however these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 m s to 1 3 m s for water saturated smectite quartz mixtures with various smectite contents so as to shed light on the frictional response of smectite bearing faults at intermediate to high slip rates at slip rates of 30 to 150 m s the friction coefficients decreased gradually from 0 5 0 6 to 0 1 with an increase in smectite content from 20 to 50 wt in contrast at slip rates higher than 1 3 mm s friction exhibited marked slip weakening resulting in low friction coefficients of 0 1 0 05 even for low smectite contents roughly 30 wt drastic slip weakening occurred at smectite contents of 10 30 wt at slip rates of 10 mm s which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types the intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear enhanced compaction and microscopic thermal pressurization of pore fluids this process could weaken the fault even below seismic slip rates leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench

the subduction of the farallon plate beneath north america as derived from relative plate motions high rates of subduction and near normal convergence of the farallon plate correlate with the duration of the laramide orogeny in the western cordillera the motion of the farallon plate relative to north america is reconstructed back to the late mesozoic using relative plate motion data this reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates two models for relative motions are considered the preferred model predicts an increase in the subduction rate of the farallon plate exceeding 150 mm yr with a direction almost due east and perpendicular to the continental margin the duration of this nonoblique normal high rate subduction correlates well with the duration of the laramide orogeny this correlation must be regarded as only suggestive the probable errors when combined and propagated through the relative motion circuit may be of the order of the effect observed however it is encouraging that these reconstructed farallon north american relative motions are similar to the published predictions of others using the alternative more direct approach of hot spot reconstructions

the effect of normal stress on the real area of contact during frictional sliding in rocks the real area of contact during frictional sliding has been determined as a function of changing normal stress in triaxial experiments through the use of thermodyes utilizing the technique described by teufel and logan in 1978 with saw cut surfaces inclined 35 to the load axis determinations were made for monolithologic sliding of tennessee sandstone and indiana limestone and dilithologic sliding of the same rocks confining pressures to 200 mpa were investigated at a constant shortening rate of 10 2 mm sec and at room temperature direct measurements were made of single asperity areas and the asperity density the product of these measurements gives the percent area of real contact across the sliding surface single asperity area and density are found to remain relatively constant during the displacement single asperity areas are in the ranges of 0 4 to 6 10 2 mm2 for sandstone 0 8 to 2 10 2 mm2 for limestone and 0 2 to 24 10 2 mm2 for sandstone sliding against limestone these values are smaller than the grain size of either rock the values increase with increasing normal stress for both monolithologic and dilithologic sliding in sandstone the asperity density increases from about 0 8 to 2 75 contacts per square millimeter in a logarithmic fashion monolithologic limestone has values of about 0 9 contacts per square millimeter and does not show significant change with increasing normal stress the percent area of real contact increases in all cases with average maximum values of 16 of the apparent area at a normal stress of 374 mpa in sandstone 18 at 25 mpa in limestone and 22 at 123 mpa in the dilithologic specimens the normal stress recalculated for the real area of contact approaches the unconfined compressive strength for sandstone and limestone

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

mineralogy and petrology of a mullite bearing pseudotachylyte constraints on the temperature of coseismic frictional fusion pseudotachylyte from the homestake shear zone central colorado contains acicular 5 10 x 100 m mullite crystals 1 5 m octahedra of magnetite and variably melted clasts of quartz in an originally molten matrix several lines of evidence support the interpretation that the pseudotachylyte represents the quenched equivalent of a molten phase derived from preferential melting of primarily biotite microcline and plagioclase and to a lesser extent quartz from the gneissic protolith the morphology of the mullite crystals which nucleate on and radiate from relic sillimanite clasts abundant magnetite octahedra scattered throughout the pseudotachylyte matrix and intergrown with mullite forming plumose quench structures partially melted relict quartz clasts the composition of the isotropic pseudotachylyte matrix and chemical gradients surrounding quartz clasts in pseudotachylyte matrix although the sample had been nearly completely molten except for 10 100 m relict quartz clasts hrtem imaging of the pseudotachylyte matrix reveals that it has devitrified the slightly birefringent matrix consists of intergrowths of submicrometer flakes of white mica microprobe analysis shows that the mullite is highly enriched in fe 8 10 wt fe 2 o 3 compared with other natural mullites hrtem and sem images reveal the presence of oriented platelets of an fe oxide phase dominantly hematite that appears to have exsolved from the mullite during cooling indicating that the limit of solid solution of fe 3 in mullite decreases with temperature the estimated melt temperature ranges from 1150 to 1200 c this interpretation is based on the fe 2 o 3 content of mullite the liquidus temperature for melts corresponding to the matrix compositions and liquidus temperatures for gneissic wall rocks which underwent complete to very high degrees of melting

the evolution of middle america and the gulf of mexico caribbean sea region during mesozoic time a plate tectonic model for the evolution of middle america and the gulf of mexico caribbean sea region is presented the model which is based upon the existence of the mojave sonora megashear incorporates into the triassic pangea reconstruction three microplates between north and south america thus avoiding the overlap of the bullard fit these plates are the yaqui bounded on the north by the mojave sonora megashear the east and west maya plates bounded on the north by the mexican volcanic zone and on the south by a predecessor of the motagua fault zone and the chortis plate parts of guatemala and honduras during late jurassic time as north america split away from europe africa and south america shear with left lateral sense of displacement occurred along the transform faults that bounded the micro plates if 800 km of left lateral displacement along the mojave sonora megashear 300 km along the mexican volcanic belt and 1 300 km along a proto motagua megashear are restored and if yucatan and cuba are rotated to fit against northern south america then 1 a curvilinear belt of late paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in texas eastern mexico nuclear central america and colombia 2 a mediterranean like sea is delineated that was a precursor of most of the present gulf of mexico 3 correlation is implied between the distinctive quartzose san cayetano formation of cuba and the caracas and juan griego groups of venezuela geometric constraints suggest that probably shear initially occurred along the mexican volcanic zone near the end of the middle jurassic subsequently probably about 160 m y ago displacements that total 800 km began along the mojave sonora megashear contemporaneously yucatan and fragments of pre cretaceous rocks that compose parts of central and western cuba migrated northward toward their present positions rotation of yucatan was facilitated by considerable displacement along the proto motagua zone and along a zone that is probably coincident with the modern salina cruz fault accumulation of widespread major salt units of late jurassic callovian to early oxfordian age in the gulf basin probably occurred contemporaneously with the arrival of these blocks at their present positions clastic units that interfinger with some of the youngest salt units and rim the gulf of mexico have not recorded major recognized translations since their accumulation clockwise rotation of south america and the chortis plate occurred during early cretaceous time this movement which was manifested by subduction of jurassic ocean floor against the previously rifted precursor of the island of cuba and under parts of hispaniola and puerto rico is recorded by circum caribbean orogeny abrupt changes in the relative motions between north and south america during late cretaceous time may have resulted in extension and outpourings of basalt upon the jurassic rocks of the ocean floor of the venezuelan basin west of beata ridge sea floor spreading formed the colombian basin related subduction occurred as the chortis plate including part of central america the nicaraguan rise and southeastern cuba was sutured against the maya east plate along the present motagua fault and cayman trench our model is constrained by published geologic data the relative positions of north and south america from atlantic sea floor magnetic anomalies and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between north and south america paleomagnetic data from middle america are sparse but do not conflict with the predicted motions of some of the microplates especially chortis

fault rock injections record paleo earthquakes fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface the width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone we measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections they have aspect ratios of 0 2 0 177 0 025 and 0 227 0 077 respectively within 99 confidence these are significantly proportionally wider than the aspect ratios of ordinary dikes from 10 4 to 10 3 the injection aspect ratios exceed the usual limit of elastic strain in the wallrock 1 so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock which is difficult to detect observationally the fully elastic model therefore offers an upper bound on the stress required to open these injection veins 10 8 210 10 pa with higher pressures in stiffer host rock lithologies these overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms as the pressure along the fault is limited to 10 7

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records

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

sedimentary environments and chemical composition of pacific pelagic sediments abstract the chemical compositions of 140 deep sea sediment samples collected in the pacific ocean were determined by inaa and icp oes methods in order to study the relationship between elemental abundances and the environment of their sedimentation in particular we observed horizontal changes along the longitude of 170 w across the pacific in calcareous ooze there is a positive relationship between ca content and sr content however the mg content is influenced more by basic rock detritus than by biogenic carbonate our observations of red clays suggest 1 the ti al ratio is a good indicator of the contribution of mafic rock detritus 2 the hydrogenous component of pelagic sediments is characterized by high co and mn content but not high v content fe content is influenced primary by basic rock detritus 3 there is a good correlation between p content and cu content high p and cu contents are characteristic of the high primary production zones in pelagic areas for example the equatorial zone the mn content is also considered to be influenced by the primary productivity of overlying surface water

vitrinite reflectance and raman spectra of carbonaceous material as indicators of frictional heating on faults constraints from friction experiments abstract vitrinite reflectance r o and raman spectra of carbonaceous material rscm are both widely used as indicators of the maximum attained temperatures in sedimentary and metamorphic rocks however the potential of these methods to estimate temperature increases associated with fault slip has not been closely studied to examine this issue friction experiments were conducted on a mixture of powdered clay rich fault material and carbonaceous material cm at slip rates of 0 15 mm s and 1 3 m s in nitrogen n 2 gas with or without distilled water after the experiments we measured r o and rscm and compared to those in starting material the results indicate that when fault material suffers rapid heating at 500 c in 9 s at 1 3 m s r o and the intensity ratio of d1 and d2 raman bands of cm i d2 i d1 markedly increase comminution with very small temperature rise in 32 min at 0 15 mm s is responsible for very limited changes in r o and i d2 i d1 our results demonstrate that r o and rscm could be useful for the detection of frictional heating on faults when the power density is 0 52 mw m 2 however the conventionally used r o and rscm geothermometers are inadequate for the estimation of peak temperature during seismic fault slip the reaction kinetics incorporating the effects of rapid heating at high slip rates and studies of the original microtexture and composition of cm are required to establish a reliable thermometer for frictional heating on faults

coseismic dehydration of serpentinite evidence from high velocity friction experiments abstract high velocity friction hvf experiments on serpentinite under conditions equivalent to large amounts of earthquake slip produced large volumes of water vapor derived from the frictional heating induced dehydration of serpentinite fourier transform infrared ftir and thermogravimetry tg differential scanning calorimetry dsc microspectroscopic analyses show that the water in the slip zone and its bounding zones was released due to dehydration during the hvf experiments our experimental results demonstrate that i the run product consists of 10 wt molten materials with abundant vesicles and 90 wt fine grained clasts ii both serpentine and olivine minerals were melted during high velocity slip 1 1 1 3 m s iii rapid serpentine dehydration occurred in a zone of up to 3 mm wide including the slip zone and its bounding zones accompanying frictional melting during high velocity slip and iv no distinct dehydration or frictional melting occurred during low velocity slip