new insights into the geology and tectonics of the san dimas mining district sierra madre occidental mexico abstract the san dimas district is a world class silver gold low sulfidation epithermal deposit located in the central part of the sierra madre occidental of mexico within the eastern part of the gulf of california extensional province previous works assumed a single period of mineralization between 38 5 and 31 9 ma which is at odds with the existence of vein systems with two different orientations and ag au ratios we present a re evaluation of this district based on new zircon u pb and apatite fission track ages as well as petrographic and field observations of mineralization styles our study also includes two new prospective areas of causita and mala noche located to the south of the main district within the lower volcanic complex we identify a late cretaceous volcanic succession 77 to 69 ma correlative with the tarahumara formation from southern sonora and coeval with the san ignacio batholith exposed to the west this volcanic succession hosts au rich mineralized sub volcanic felsic bodies yielding slightly younger ages and is covered by paleocene intermediate lavas with hypabyssal intrusions with ages around 48 ma a voluminous intrusive suite piaxtla batholith and associated dike swarms was emplaced in the region between 49 and 44 ma early extensional basins were filled by a continental sedimentary sequence palmas formation which yielded u pb age peaks at 66 and 56 ma from detrital zircons and a maximum depositional age of 43 ma the last magmatic activity as in the rest of the sierra madre occidental consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of 31 5 29 ma and 24 20 ma nnw sse extensional fault systems expose the mineralization and tilted all the succession prior to the emplacement of a 24 ma ignimbrite package this late oligocene extension associated to the early stage of the gulf of california rift is confirmed by apatite fission track dating of samples from the piaxtla batholith which consistently indicate an episode of cooling at 25 23 ma followed by a second episode at 12 5 ma our absolute ages and geologic mapping allow to infer that an older wsw ene trending normal fault system with up to 1 km of displacement must exist between the san dimas district and the causita area to the south this fault system currently buried beneath oligocene miocene ignimbrites may have controlled the intrusion of the piaxtla batholith and played a crucial role in the preservation of large vein systems of san dimas in a tectonic depression setting the main epithermal mineralization is associated with two kinds of structures ag au veins with wsw ene to e w orientation and au ag veins with nnw sse to n s orientation the dominant ag au veins slightly post date the piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization the nnw sse to n s au ag veins are most probably associated with the oligocene silicic volcanism

tectonic implications of early miocene extensional unroofing of the sierra mazatan metamorphic core complex sonora mexico sierra mazatan in northwest mexico is the southernmost metamorphic core complex in the north american cordillera new geologic structural and 4 0 ar 3 9 ar thermochronologic data demonstrate that the core complex detachment fault accommodated 15 35 km of slip at a rate of 3 3 7 7 mm yr between 20 5 and 16 ma these data also suggest that the lower plate was tilted eastward 20 50 since 20 ma indicating that the detachment fault initially dipped 30 60 rapid slip on the fault occurred concurrently with subduction in sonora and thus was not related to the ca 12 ma plate boundary change at this latitude or the late miocene opening of the gulf of california rapid extension at sierra mazatan was synchronous with core complexes along the length of the cordillera suggesting a distinct 20 15 ma core complex event the fundamental causes of which remain a mystery

depth extent of the fault zone seismic waveguide effects of increasing velocity with depth summary the damage zone of a major fault can act as a low velocity seismic waveguide the fault zone guided waves provide a potential method to constrain the in situ physical properties of the fault zone fz at depth recently there has been debate over the depth extent of observed fault waveguides and whether fault properties at seismogenic depth can be constrained by guided waves gws to address these questions elastic finite difference synthetic seismograms were generated for fault zone models that include an increase in seismic velocity with depth both inside and outside the fz previous synthetic studies for a homogeneous fault showed that earthquakes off of the fault do not generate gws unless the waveguide is restricted to a few kilometres depth in contrast earthquakes both inside and outside of a depth varying fault waveguide generate strong gws within the near surface portion of the fz this is because the frequency dependent trapping efficiency of the waveguide changes with depth the nearsurface fault structure efficiently guides waves at lower frequencies than the deeper fz the low frequency waves that are guided at the surface are not efficiently guided at greater depth and therefore travel as body waves fault structure at seismogenic depth requires the analysis of data at higher frequencies than the gws that dominate at the surface and have been the subject of most previous investigations

discretizing a discrete fracture model for simulation of radial transport 1 in this study we examine the required spatiotemporal discretization for numerical simulation of field scale radial solute transport using a discrete fracture model point to point and borehole to point scenarios considering a single fracture hydrodynamic dispersion and matrix diffusion were modeled in two dimensional 2 d and three dimensional 3 d domains and were verified using existing semianalytical solutions the results clearly show the need for accurate spatiotemporal discretization in numerical simulations of this type spatial discretization away from the injection point well in the horizontal plane was found to be more demanding than away from the fracture in the vertical direction concentration control which was used in the time step discretization of the transport solution varied over two orders of magnitude depending on the scenario being modeled the fine spatiotemporal discretization required may significantly limit the ability to simulate radial transport in more complex hydrogeological conditions i e multiple fractures and injection points wells particularly in 3 d because of long computational runtimes

cross hole fracture connectivity assessed using hydraulic responses during liner installations in crystalline bedrock boreholes abstract in order to continually improve the current understanding of flow and transport in crystalline bedrock environments developing and improving fracture system characterization techniques is an important area of study the presented research examines the installation of flexible impermeable flute liners as a means for assessing cross hole fracture connectivity flute liners are used to generate a new style of hydraulic pulse with pressure response monitored in a nearby network of open boreholes drilled in gneissic rock of the canadian shield in eastern ontario canada borehole liners were installed in six existing 10 15 cm diameter boreholes located 10 35 m apart and drilled to depths ranging between 25 45 m liner installation tests were completed consecutively with the number of observation wells available for each test ranging between one and six the collected pressure response data have been analyzed to identify significant groundwater flow paths between source and observation boreholes as well as to estimate inter well transmissivity and storativity using a conventional type curve analysis while the applied solution relies on a number of general assumptions it has been found that reasonable comparison can be made to previously completed pulse interference and pumping tests results of this research indicate areas where method refinement is necessary but nonetheless highlight the potential for use in crystalline bedrock environments this method may provide value to future site characterization efforts given that it is complementary to and can be used in conjunction with other currently employed borehole liner applications such as the removal of cross connection at contaminated sites and the assessment of discrete fracture distributions when boreholes are sealed recreating natural hydraulic gradient conditions

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

extremely rapid and localized recharge to a fractured rock aquifer the factors that govern recharge to fractured rock aquifers with overlying soil are poorly understood the objective of this study is to determine if recharge to a fractured crystalline aquifer in a humid climate is governed by heterogeneous fracture networks and or overlying soil characteristics a 10 km2 study area is instrumented with a network of 15 bedrock wells completed to observe conditions in the shallow bedrock aquifer hydrogeological characterization detailed observation of the 2007 snowmelt freshet and numerical simulations are used snowmelt is a valuable natural tracer because it has a distinct thermal and isotopic signature and is applied simultaneously and evenly to the entire ground surface results indicate that soil thickness and bedrock transmissivity are both highly heterogeneous at the site scale but that much of the study area is underlain by silty sand with a thickness of 1 m cold 2h depleted snowmelt locally recharged the bedrock aquifer to depths of at least 20 m within two days since recharge is typically quantified and discussed as an annual flux the snowmelt event is extremely rapid however hydraulic isotopic and thermal data also indicate that most wells did not rapidly recharge numerical simulations indicate that soil thickness and hydraulic conductivity are critical parameters that controls whether the underlying bedrock aquifer rapidly recharges the vertical fracture aperture number of vertical fractures the amount of snowmelt water table gradient depth to water table and pressure saturation relations are all less significant controls on the rate and amount of rapid recharge both field results and numerical simulations indicate that the rapid recharge process is localized to areas where the soil is very thin such as the fringes of outcrops outcrops are exposed in 0 1 of the study area indicating that the process of rapid recharge is also extremely localized this poorly documented hydrogeologic phenomenon has broad implications for groundwater management and protection as well as our understanding of recharge processes in fractured rock aquifers

seismic guided waves trapped in the fault zone of the landers california earthquake of 1992 a mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the m7 4 landers earthquake of june 28 1992 with a maximum offset of 1 km from the trace we found a distinct wave train with a relatively long period following the s waves that shows up only when both the stations and the events are close to the fault trace this wave train is interpreted as a seismic guided wave trapped in a low velocity fault zone to study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone we eliminated source and recording site effects by the coda normalization method the normalized amplitudes of guided waves show a spectral peak at 3 4 hz which decays sharply with distance from the fault trace spectral amplitudes at high frequencies 8 15 hz show an opposite trend increasing with distance from the fault trace the normalized amplitudes of guided waves at 3 4 hz also show a systematic decrease with hypocentral distance along the fault zone from which we infer an apparent q of 50 in order to confirm the existence of the guided waves a dense array of 31 stations was deployed at one of the 11 sites the resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone by modeling the waveforms as s waves trapped in a low velocity waveguide sandwiched between two homogeneous half spaces with velocity vs 3 0 km s we infer a waveguide width of about 180 m a shear velocity of 2 0 2 2 km s and a q of 50 hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low velocity fault zone in three dimensions we find that the zone extends to a depth of at least 10 km this zone apparently continues to the south across the pinto mountain fault because guided waves are observed at stations north of the pinto mountain fault for earthquakes with epicenters south of it on the other hand the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides

analysis of hydraulic and tracer response tests within moderately fractured rock based on a transition probability geostatistical approach 1 a transition probability and markov chain geostatistical approach is applied to synthesize the discrete permeability structure of moderately fractured rock the approach can infuse either hard or subjective categorical information that is consistent with geological interpretations the methodology is tested using data collected from the moderately fractured rock mfr experiment area of the underground research laboratory url in southeastern manitoba canada attributes pertaining to fracture location frequency and orientation along an array of boreholes intersecting the mfr experiment area taken together with results from hydraulic response tests within packed off intervals along the boreholes are used to produce conditional stochastic realizations of hydraulic conductivity and effective porosity using the generated hydraulic conductivity and porosity realizations we compare predicted tracer concentrations to the results of measured breakthrough data in a stochastic framework the results show that solute migration behavior in moderately fractured rock can be successfully characterized and reasonably predicted upon careful error analysis of the results obtained from the various medium realizations synthesized from the conditional categorical descriptions of the fractured crystalline rock

modeling of seismic guided waves at the dead sea transform 1 on several recordings of linear seismometer arrays crossing the arava fault af in the middle east we see prominent wave trains emerging from in fault explosions which we interpret as waves being guided by a fault zone related low velocity layer the af is located in the arava valley and is considered the principal active fault of the mainly n s striking dead sea transform system in this section observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations they exhibit large amplitudes and are almost monochromatic we model them by a two dimensional 2 d analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces a hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space resolution is investigated by synthetic tests the observations are adequately fit by models with a narrow only 3 12 m wide waveguide with s wave velocity reduced by 10 60 of the surrounding rock we relate this vertical low velocity layer with the damage zone of the af since the location of receivers observing and of shots generating the guided waves respectively match with the surface trace of the fault the thickness of the damage zone of the af at least at shallow depths seems to be much smaller than in other major fault zones this could be due to less total slip on this fault

fine structure of the landers fault zone segmentation and the rupture process observations and modeling of 3 to 6 hertz seismic shear waves trapped within the fault zone of the 1992 landers earthquake series allow the fine structure and continuity of the zone to be evaluated the fault to a depth of at least 12 kilometers is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent this zone forms a seismic waveguide that extends along the southern 30 kilometers of the landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter another fault plane waveguide disconnected from the first exists along the northern rupture surface these observations in conjunction with surface slip detailed seismicity patterns and the progression of rupture along the fault suggest that several simple rupture planes were involved in the landers earthquake and that the inferred rupture front hesitated or slowed at the location where the rupture jumped from one to the next plane reduction in rupture velocity can tentatively be attributed to fault plane complexity and variations in moment release can be attributed to variations in available energy

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

structural and thermal constraints on the initiation angle of detachment faulting in the southern basin and range the chemehuevi mountains case study the cenozoic normal fault system exposed in the chemehuevi mountains of the southern cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching there three stacked brittle low angle normal faults that formed at depths as great as 10 12 km cut gently down section northeastward through deformed proterozoic and mesozoic crystalline basement hanging wall blocks are displaced relatively northeastward the upper crust above the chemehuevi detachment fault was pulled apart along high angle normal faults that rotated to more gentle dips through time in contrast rocks of originally mid crustal affinity in the footwall were only gently rotated and accommodated minor extension application of 40 ar 39 ar and fission track thermochronology to rocks in the footwall of the chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting at the onset of extension between 22 and 24 ma granitic rocks exposed in the southwestern and northeastern portions of the footwall were at 200 c and 350 400 c respectively separated by a distance of some 23 km down the known slip direction this gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre extension isothermal surfaces and constrains the exposed part of the chemehuevi detachment fault to have had a regional dip initially of about 15 to 30 the fault system apparently cut gently down through the upper crust to a minimum depth of 10 12 km the deepest exposed parts of the system today and was domed from midcrustal depths and locally denuded during continued slip together the structural and thermochronologic data confirm the suggestion that faults accommodating large magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles that is 30

latest cretaceous early paleogene boom of porphyry cu mineralization associated with the laramide magmatic arc of mexico abstract porphyry copper and associated deposits are widespread through most northwestern mexico particularly in sonora and sinaloa in the northwestern part of the country these deposits belong to a nw se trending copper belt that extends from southwestern usa to southern mexico the copper mineralization is clearly linked to subduction related calc alkaline plutons formed during the eastward migration of the cordilleran magmatic arc largely coincident in time and space with the laramide orogeny 80 40 ma the economically most important porphyry copper deposits were emplaced in the southwest margin of the north american craton the porphyry copper mineralization was formed during the latest cretaceous to early paleogene between 75 and 52 4 ma but it was particularly fertile between 69 and 54 ma when the larger copper concentrations were deposited to form the famous copper cluster of arizona new mexico and sonora during this time more than half of the total porphyry copper related metals of the entire cordillera was deposited this large copper accumulation developed south of the classical flat slab laramide province characterized by uplift of large crustal blocks and mostly amagmatic conditions an inferred slab tear zone separated this zone from a slab segment above which asthenospheric magma flux favored copper concentration the mexican porphyry copper belt accounts for 30 deposits out of which the buenavista del cobre 30 mt cu and the la caridad 8 mt cu are the most relevant more to the south close to the boundary between sonora and sinaloa the porphyry copper deposits are smaller

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

numerical simulation of fault zone guided waves accuracy and 3 d effects fault zones are thought to consist of regions with reduced seismic velocity when sources are located in or close to these low velocity zones guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth observations above several fault zones suggest that they are common features of near fault radiation yet their interpretation may be highly ambiguous analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities these solutions can be used for accurate modeling of wave propagation in plane parallel layered fault zone structures however at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion as more complicated models cannot be solved by analytical means numerical methods must be employed in this paper we discuss 3 d finite difference calculations of waves in modestly irregular fault zone structures we investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3 d structures we also show that simple mathematical operations on 2 d solutions generated with line sources allow accurate modeling of 3 d wave propagation produced by point sources the discussed simulations indicate that structural discontinuities of the fault zone e g fault offsets larger than the fault zone width affect significantly the trapping efficiency while vertical properly gradients fault zone narrowing with depth small scale structures and moderate geometrical variations do not the results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer

tertiary metamorphic core complexes in sonora northwestern mexico several ranges encompassing more than 35 000 km 2 of sonora mexico contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed prominent northwest trending fractures dikes and normal faults are orthogonal to the lineation most kinematic indicators in lower plate mylonitic rocks record top to the southwest sense of shear upper plate stratigraphic sequences include mesozoic supracrustal rocks tertiary volcanic and sedimentary rocks and allochthonous precambrian basement tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults previously published u pb and k at ages from lower plate granitic orthogneisses upper plate volcanic sequences and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late oligocene early miocene partitioning of extensional strain in sonora was influenced by pre tertiary crustal structure the belt of core complexes developed across two contrasting blocks of continental crust separated by the n60ow striking mojave sonora megashear portions of the southern papago block northeast of the megashear consisting of jurassic magmatic arc rocks and upper jurassic cretaceous siliciclastic and carbonate strata resting upon a concealed tectonically fragmented precambrian basement were especially susceptible to crustal attenuation some core complexes of the southern papago block occur within zones trending northwest that may coincide with late jurassic lineaments in the caborca block southwest of the megashear core complex related rocks and structures have not been identified where surface exposures of middle proterozoic basement and overlying upper proterozoic paleozoic platform strata are common however extensional mylonitic fabrics are locally developed along the margins of a tertiary two mica granite batholith core complexes on both sides of the megashear appear to be preferentially developed where tertiary granites have intruded regions of crust with basement disrupted by pre tertiary structures sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the united states and canadian cordillera the timing of mid crustal extension in sonora 25 18 ma is contemporaneous with the timing of core complex development in arizona nevada and utah extension occurred later in these areas than in the pacific northwest british columbia region but earlier than in the

pulling apart the mid to late cenozoic magmatic record of the gulf of california is there a comondu arc the composition of the lithosphere can be fundamentally altered by long lived subduction processes such that subduction modified lithosphere can survive for 100 s myrs incorrect petrotectonic interpretations result when spatial temporal compositional trends of and source contributions to magmatism are not properly considered western mexico has had protracted cenozoic magmatism developed mostly in board of active oceanic plate subduction beneath western north america a broad range of igneous compositions from basalt to high silica rhyolite were erupted with intermediate to silicic compositions in particular showing calc alkaline and other typical subduction related geochemical signatures a major oligocene rhyolitic ignimbrite flare up 300 000 km3 switched to a bimodal volcanic phase in the early miocene 100 000 km3 associated with distributed extension and opening of numerous grabens extension became more focussed 18 ma resulting in localised volcanic activity along the future site of the gulf of california this localised volcanism known as the comondu arc was dominantly effusive and andesite dacite in composition past tectonic interpretations of comondu age volcanism may have been incorrect as these regional temporal compositional changes are alternatively interpreted as a result of increased mixing of mantle derived basaltic and crust derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting guadalupe plate

deep structure of the mozumi sukenobu fault central japan estimated from the subsurface array observation of fault zone trapped waves summary in order to estimate the averaged fault zone structure i e the width shear wave velocity and qs of the mozumi sukenobu fault central japan we analysed the love wave type fault zone trapped waves recorded by a seismic array deployed in a subsurface survey gallery we selected the typical love wave type trapped waves based on the characteristics of dispersion amplitude apparent velocity and the polarization of wave trains following the direct shear wave we selected nine events showing typical love wave type trapped waves from 154 earthquakes epicentres of the events were located along the surface trace of the mozumi sukenobu fault the nine events showing trapped waves were found to be located in a plane dipping to the south at an angle of approximately 75 to 85 the duration of the fault zone trapped waves increase non linearly with the increase of hypocentral distance this result implies an increase in shear wave velocity of the fault zone with depth and or horizontal distance from the array in order to obtain averaged fault zone structure from hypocentre to receiver we assumed fault zone structure to be a 2 d uniform low velocity waveguide and modelled the dispersion curve and the waveform of trapped waves using an analytical solution the width shear wave velocity and qs of the fault zone were estimated to be 160 to 400 m 2 9 to 3 1 km s 1 and 60 to 90 respectively the width of the fault zone estimated in this analysis is consistent with the width of the extent of fault zone at the survey gallery we also conducted 3 d finite difference simulation 3 d fdm on fault zone trapped waves using depth dependent structure models referred to the 2 d averaged structure indicating the possible depth dependent structures the 2 d modelling is useful as an initial guide to the 3 d imaging of a fault zone

diffusion vs fluid alteration in alkali feldspar 40ar 39ar thermochronology does cross correlation of log r r0 and age spectra validate thermal histories abstract for six decades geoscientists have been trying to quantitatively understand the nature of radiogenic ar loss from alkali feldspar some researchers suggest that volume diffusion is the dominant mechanism and they use conventional step heating 40ar 39ar data from alkali feldspar to recover the thermal histories of rocks they argue that a high degree of correlation between log r r0 and 40ar 39ar age spectra which is observed in a number of natural examples justifies this hypothesis in contrast other investigators suggest that fluid mediated recrystallisation and alteration control the radiogenic ar redistribution hence rendering alkali feldspar useless as a thermochronometer by means of numerical modelling we found that the latter mechanism as well is able to produce samples with highly correlated log r r0 and 40ar 39ar age spectra in addition we show that apparent thermal histories recovered for altered alkali feldspar crystals by interpreting step heating 40ar 39ar data may be grossly inaccurate and yet seemingly fit the prevailing understanding of regional geology such inaccurate apparent thermal histories can be obtained even from alkali feldspar crystals that underwent volumetrically low degrees of alteration therefore we conclude that conventional step heating 40ar 39ar data are insufficient to support the assumption that radiogenic ar loss from alkali feldspar occurred solely by volume diffusion and validate the constrained thermal histories even if upheld by a priori knowledge of regional tectonics we further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of alkali feldspar

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

calculation of strong ground motion due to an extended earthquake source in a laterally varying structure abstract lateral heterogeneity of the crust combined with the complexity of an extended earthquake source may account for observations of strong ground motion not easily explained by simplified source models in a laterally homogeneous medium both the effects of an extended source and lateral heterogeneity can be examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane with dynamic ray tracing in a three dimensional medium using this technique velocity seismograms are calculated at ranges less than 10 km for a finite source which occurs in a wedge shaped zone of low velocity this structure is chosen to approximate the structure of fault zones found in reflection and refraction surveys across active faults in california iterative paraxial ray tracing is used to solve the two point ray tracing problem the low velocity zone surrounding the fault causes arrivals in the laterally varying structure to be delayed and rays to leave the fault more closely to the fault normal at most locations examined the effect of the three dimensional structure on isochrone velocity is small compared to the effects on geometric spreading and radiation pattern because rays are bent substantially in the near source region the seismograms calculated for the laterally varying structure vary substantially in amplitude and polarity from seismograms calculated for the laterally homogeneous structure

fault damage zones of the m7 1 darfield and m6 3 christchurch earthquakes characterized by fault zone trapped waves abstract to characterize the subsurface structure of the damage zones caused by the 2010 2011 canterbury earthquake sequence in new zealand s south island we installed two short linear seismic arrays array 1 across the greendale fault gf surface rupture and array 2 over the surface projection of the blind port hills fault phf that ruptured in the 2010 m7 1 darfield and 2011 m6 3 christchurch earthquakes respectively we recorded 853 aftershocks for 4 months after the christchurch earthquake fault zone trapped waves fztws identified at array 1 for aftershocks occurring on both the gf and the phf show that the post s durations of these fztws increase as focal depths and epicentral distances from the array increase suggesting an effective low velocity waveguide formed by severely damaged rocks existing along the gf and phf at seismogenic depths locations of aftershocks generating prominent fztws delineates the subsurface gf rupture extending eastward as bifurcating blind fault segments an additional 5 8 km beyond the mapped 30 km surface rupture into a zone with comparably lower seismic moment release west of the phf rupture which extends westward to within 5 3 1 km of the subsurface gf the propagation of fztw through the intervening gap indicates moderate gf phf structural connectivity we interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress aligned microcracks that enable the propagation of phf sourced fztws into the gf damage zone simulations of observed fztws suggest that the gf rupture zone is 200 250 m wide consistent with the surface deformation width velocities within the zone are reduced by 35 55 with the maximum reduction in the 100 m wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing the damage zone extends down to depths of 8 km or deeper consistent with hypocentral locations and geodetically derived fault models

assessing the flow regime in a contaminated fractured and karstic dolostone aquifer supplying municipal water the silurian dolostone bedrock in ontario canada is a broad 400 km long swath northward from niagara falls through the bruce peninsula that represents an important water source for municipal industrial and agricultural uses where the quaternary overburden is thin or absent karst is common this study concerns an urban area where the dolostone aquifer is 100 m thick beneath up to 50 m thick quaternary deposits and where karst features identified by borehole information are common hydraulic tests show moderate to large bulk rock hydraulic conductivity and rock core tests indicate much smaller matrix hydraulic conductivity than the bulk rock values therefore the aquifer is essentially a dual permeability fully saturated system in which conduits occur within a network of ubiquitous extensive horizontally and vertically interconnected fractures karst features are concentrated in a thin zone at the top of rock likely representing former epikarst and also in a thicker zone in the middle of the aquifer some pumping test results and large yields of some municipal wells are consistent with conduit occurrences however atmospheric tritium distributed source contamination cl view the mathml source and a point source pesticide plume metolachlor show detailed concentration distributions lacking influence of flow in conduits detailed hydraulic head profiles also show no influence of conduit flow this study shows that when designing monitoring networks for groundwater quality and source water protection in similar contexts locating conduits is not necessary because contaminant distributions are governed by the combined influences of the rock matrix fractures and conduits the hydraulic boundary conditions and the interconnected fracture network with only minimal conduit effects prior to glaciations an integrated karstic aquifer could develop with flow controlled by conduits however this original converging flow system became non functional when the quaternary sediments drastically modified the boundary hydrologic conditions and the head distribution

cordilleran deformation along the eastern edge of the valles san luis potosi carbonate platform sierra madre oriental fold thrust belt east central mexico the eastern margin of the cretaceous valles san luis potosi carbonate platform hidalgo queretaro and san luis potosi states became activated during the formation of the sierra madre oriental fold thrust belt a series of thrust sheets with a minimum linear shortening of 3 to 4 km each and 10 to 12 km in total formed where the platform edge was approximately normal to the greatest principal stress trajectories as deduced from fold axes a tear fault developed where the platform edge formed an acute angle with the same stress trajectories the overthrusts cut across the competent layers of the lower cretaceous on tectonic ramps that dip 10 and became partly steepened by subsequent imbrication or folding in the mechanically weak upper cretaceous rocks the thrust faults are nearly parallel to bedding for several kilometres the location of the thrusts is controlled mainly by the lithological and thickness change of the middle cretaceous rocks from an orthotropically layered basin facies into a two to five times thicker homogeneous platform edge assemblage the bank margin was a zone of stress concentration the magnitude of the horizontal tectonic components which caused the deformation may have changed by a factor of two to five across the platform margin due to the change in cross sectional area oblique and layer parallel discrete shears and subordinate tectonic stylolites appear to have been the dominant deformational mechanisms the rocks did not suffer any measurable penetrative ductile deformation ooids present at the base of the xilitla thrust are unflattened but are marked by a closely spaced stylolitic cleavage perpendicular to bedding the deformations are bracketed by the paleontological age of the youngest strata affected by the overthrusts globotruncana contusa planktonic foraminiferal zone and by the isotopic age of a post tectonic pluton 62 2 ma and are thus of late maastrichtian paleocene age

fault zone trapped seismic waves two instances of fault zone trapped seismic waves have been observed at an active normal fault in crystalline rock near oroville in northern california trapped waves were excited with a surface source and recorded at five near fault borehole depths with an oriented three component borehole seismic sonde at parkfield california a borehole seismometer at middle mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the san andreas fault zone at oroville recorded particle motions indicate the presence of both love and rayleigh normal modes the love wave dispersion relation derived for an idealized wave guide with velocity structure determined by body wave travel time inversion yields seismograms of the fundamental mode that are consistent with the observed love wave amplitude and frequency applying a similar velocity model to the parkfield observations we obtain a good fit to the trapped wavefield amplitude frequency dispersion and mode time separation for an asymmetric san andreas fault zone structure a high velocity half space to the southwest a low velocity fault zone a transition zone containing the borehole seismometer and an intermediate velocity half space to the northeast in the parkfield borehole seismic data set the locations depth and offset normal to fault plane of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone we conclude that it is feasible to obtain near surface borehole records of fault zone trapped waves because trapped modes are excited only by events close to the fault zone proper thereby fixing these events in space relative to the fault a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body wave tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane it is an open question whether near surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device

seismic radiation from an sh line source in a laterally heterogeneous planar fault zone a solution to the class of problems in which an sh line source is operating in a medium consisting of two quarter spaces with an arbitrary number of semi infinite vertical layers in between is given in a frequency wavenumber domain the general result is applied to a model with a single vertical layer and synthetic seismograms are calculated for a source specified as an sh line dislocation at the interface between the vertical layer and the faster quarter space the numerical examples demonstrate that moderate fault normal material heterogeneity can have significant effects on the seismic response of a medium the overall distribution of travel times amplitudes and motion polarities in the presence of fault zone material heterogeneity is different from a corresponding distribution in a homogeneous half space the effects are most prominent near the fault zone where head waves surface waves and trapped modes result in waveform complexities large amplifications over an order of magnitude in the fault zone itself and first motion polarities which are reversed from homogeneous medium predictions the results imply that material heterogeneity should be included in fault zone models which are used for the interpretation of observed seismic data and theoretical calculations of near fault responses such models can be utilized to obtain important fault zone and earthquake source parameters

numerical simulations of large earthquakes dynamic rupture propagation on heterogeneous faults our current conceptions of earthquake rupture dynamics especially for large earthquakes require knowledge of the geometry of the faults involved in the rupture the material properties of the rocks surrounding the faults the initial state of stress on the faults and a constitutive formulation that determines when the faults can slip in numerical simulations each of these factors appears to play a significant role in rupture propagation at the kilometer length scale observational evidence of the earth indicates that at least the first three of the elements geometry material and stress can vary over many scale dimensions future research on earthquake rupture dynamics needs to consider at which length scales these features are significant in affecting rupture propagation

cretaceous and tertiary sedimentary magmatic and tectonic evolution of north central sonora arizpe and bacanuchi quadrangles northwest mexico the arizpe and bacanuchi quadrangles provide a geologic history representative of the north central part of sonora where lithologies are dominated by late mesozoic and cenozoic igneous rocks in this study new geologic mapping 40ar 39ar dating and geochemical analyses have been combined to provide a stratigraphic framework for this area ten lithostratigraphic units and several igneous and tectonic events can be recognized the oldest outcropping rocks are lower cretaceous strata of the bisbee group which along with the picacho conglomerate record a middle cretaceous compressive tectonic event and associated sedimentation laramide igneous activity is widespread and represented by 1 highly altered andesitic flows and volcaniclastic rocks arroyo alcaparros andesitic rocks of late campanian to maastrichtian age 2 less altered andesitic and dacitic flows cerro las jarillas volcanic rocks of late paleocene age and the intrusive bodies of 3 sierra el manzanal granodiorite and 4 rancho vaqueria quartz monzonite the sierra el manzanal granodiorite was emplaced at ca 68 ma on the basis of a 40ar 39ar biotite age 67 97 0 19 ma and cooled relatively rapidly according to less precise 40ar 39ar hornblende and k feldspar ages from the same sample 64 8 1 0 ma and 62 8 0 3 ma respectively the cerro las jarillas volcanic rocks are slightly younger 40ar 39ar biotite age of 58 67 0 17 ma the rancho vaqueria quartz monzonite was emplaced at ca 57 ma 40ar 39ar biotite age of 56 73 0 14 ma and a less precise 40ar 39ar hornblende age of 55 0 0 7 ma a protracted cooling history of this pluton is indicated by the age spectrum of k feldspar from the same sample a probable magmatic lull and denudation seem to have occurred between middle and late eocene time and probably until the early oligocene subsequently rhyolitic to mafic volcanism began close to late oligocene time and lasted until the early miocene felsic volcanism is represented by the cerro cebadehuachi volcanic rocks from which 40ar 39ar hornblende ages of 27 25 0 09 and 27 32 0 06 ma and a biotite age of 26 97 0 06 ma were obtained at three different localities the mesa pedregosa volcanic rocks represent the transition to younger mafic volcanic activity that occurred during the late oligocene as indicated by a sanidine 40ar 39ar age of 25 48 0 05 ma this late oligocene and early miocene magmatism was paired by two episodes of extensional deformation the first phase is characterized by northwest striking normal faults and folds which expose the deepest structural levels of the area and by the related basin fill the bacanuchi conglomerate the second phase is represented by north striking normal faults and by the syntectonic basin fill the arizpe conglomerate basaltic andesite volcanic flows at the base of the arizpe conglomerate yielded 40ar 39ar whole rock ages of 23 52 0 17 and 21 0 20 ma the extensional deformation 27 to 23 ma in the study area is coeval with the development of metamorphic core complexes in neighboring areas of sonora and with the onset of extension in southern sonora the mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of sonora are assigned to the baucarit formation geochemical information from samples representing each of the igneous events displayed high k calc alkalic and mostly metaluminous compositions the older units including the arroyo alcaparros andesitic rocks the cerro las jarillas volcanic rocks the sierra el manzanal granodiorite and the rancho vaqueria quartz monzonite are characterized by steep chondrite normalized ree rare earth element slopes and generally well developed negative eu anomalies suggesting garnet and plagioclase removal in the source the younger igneous events including the cerro cebadehuachi and mesa pedregosa volcanic rocks and the basaltic flows associated with the arizpe conglomerate showed basin shaped ree slopes with no eu anomalies suggesting clinopyroxene or amphibole fractionation

inferring flow pathways between bedrock boreholes using the hydraulic response to borehole liner installation abstract fractured bedrock aquifers provide an important source of drinking water in many regions around the world in settings with low matrix porosity and a small number of highly transmissive features such as crystalline bedrock high groundwater velocities through connected fracture pathways can increase aquifer vulnerability to contamination multiple fracture network characterization techniques e g geological geophysical and hydraulic are available to better understand this vulnerability conceptualizations of fracture network connectivity may be examined through the use of numerical models recently data gathered during the installation of flute blank liners have been used to assess cross hole transmissivity and fracture interconnections modelling and interpreting this type of hydraulic test presents a challenge since the open borehole length decreases during a liner installation this study provides a method to simulate a flute liner installation using hydrogeosphere a finite element code capable of simulating 3d transient flow conditions in discrete fracture networks results from previously published studies are used to inform the initial model configuration from which the sensitivity to various inputs including fracture aperture are then assessed good correlation between simulated and observed hydraulic response profiles was obtained however modelled fracture apertures yield an estimated bulk borehole transmissivity of 1 3 10 3 m2 s which was found to exceed a previous estimate of 2 1 10 4 m2 s obtained from constant head injection testing this may suggest overestimation of fracture apertures in the presented approach nonetheless this study supports numerical modelling as a viable method to simulate borehole liner installations

the 40ar 39ar thermochronology of core complexes and other basement rocks in sonora mexico implications for cenozoic tectonic evolution of northwestern mexico 1 northwestern mexico remains an important region for understanding the tectonic evolution of the north american cordillera however many fundamental aspects of the geologic history of the region remain poorly known we present new 40ar 39ar thermochronology of exposed basement rock from throughout sonora mexico including from metamorphic core complexes these results provide new constraints on the late cretaceous to cenozoic tectonic evolution of northwestern mexico following laramide ca 80 50 ma emplacement most sonoran plutons experienced rapid post magmatic cooling to 400 200 c which we attribute to shallow level emplacement following this rapid cooling plutons cooled slowly 300 c until the late oligocene rapid footwall cooling documents that extension at sonoran core complexes began synchronously at ca 25 ma despite the fact that these regions are separated by 200 km extension at most sonoran core complexes continued until ca 15 ma although extension at the magdalena core complex ceased earlier ca 21 20 ma slip at each core complex was 10 30 km and occurred at average rates of 1 9 mm yr this major period of extension primarily occurred during subduction indicating that relative plate motions between the pacific and north american plates cannot be invoked as a driving force for this phase of extension

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

an experimental study of the flow of gas along synthetic faults of varying orientation to the stress field implications for performance assessment of radioactive waste disposal critical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor this paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite filled synthetic fault as a function of fault dip a total of 22 gas injection experiments were conducted on faults oriented 0 15 30 and 45 to horizontal both with and without active shear gas flow was seen to be complex repeat gas injection testing showed a consistent gas entry pressure but considerably different nonrepeatable gas peak or breakthrough pressure gas flow occurred along discrete dilatant pathways the physics governing the pressure at which these features formed was repeatable however permeability was dependent on the number distribution and geometry of the resultant pathways the nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties no fault orientations were seen to exhibit nonflow characteristics although critical stress theory predicted that two of the investigated fault angles should be effective seals however a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile therefore in kaolinite gouge filled faults shear is not an effective self sealing mechanism to gas flow

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

the response of two joined quarter spaces to sh line sources located at the material discontinuity interface summary exact analytical solutions to problems of sh line force and line dislocation operating at the interface between two quarter spaces are derived and discussed it is shown that the resulting fields differ from that of a homogeneous half space in the distribution of travel times wave amplitudes and polarity of first motions these effects are documented in the literature it is therefore suggested that when constructing theoretical seismic displacement fields from source path and site effects it is important to use response functions that account for material heterogeneity in the source region an extension of the formulation used in the case of sh line dislocation leads to a 3 d representation theorem which gives the displacement field in terms of the displacement discontinuities along the rupture area the resulting overall sue scaling parameter is the potency which does not suffer from an ambiguity associated with seismic moment in the case of heterogeneous source region

numerical modeling of fault zone trapped waves acoustic case abstract we have obtained numerical results for an acoustic wave propagating in a 3d medium with mixed boundary conditions the specific model involves a vertical low velocity fault zone of varying thickness embedded in an otherwise homogeneous half space at the fault zone boundary the stress and displacement are continuous on the free surface the stress is zero a pseudo spectrum method is employed to achieve sufficient resolution with reasonable computation time on a cray results show the development of guided waves trapped in the fault zone these guided waves display large amplitudes and lengthening waveforms they propagate at lower velocity with amplitudes that drop off mainly due to energy leakage out of the fault zone as the width of the fault zone varies the wave energy tends to funnel into the new low velocity wave guide at large distance these guided waves become the dominant arrivals on seismograms the waveforms are useful to recover the geometry and properties of the fault zone

finite difference seismograms for sh waves the accuracy of the finite difference method for generating synthetic seismograms of sh wave propagation in cylindrically symmetric media is discussed the finite difference method has the advantage that arbitrary density and velocity fields in the medium may be specified a point source is generated by a simple transformation of a line source the accuracy of the finite difference seismograms in flat and dipping layered media is confirmed by comparison with the generalized ray method a source radiation pattern is inserted by introducing a near field which has permanent displacement near the source strong motion synthetics are constructed with this new method for the 1968 borrego mountain earthquake as recorded at el centro good fits to the data are achieved using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling shallow faulting is no longer necessary to explain the long period surface wave development

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

numerical modelling of seismic wave propagation along the plate contact of the hellenic subduction zone the influence of a deep subduction channel summary we model seismic wave propagation from intermediate depth earthquakes in a subduction zone using a 2 d chebyshev pseudospectral method particular attention is directed to the influence of a deep low viscosity subduction channel on top of the plate contact where metamorphic rocks may be exhumed by forced return flow the study is motivated by observations of complicated dispersive and high amplitude p and s wave trains in the forearc of the hellenic subduction zone the basic model is a subducted slab with a thin oceanic crust forming a lowvelocity layer our model setup closely follows recent results on the structure of the hellenic subduction zone obtained from receiver functions and surface wave studies they exhibit an abrupt change of the dip of the downgoing slab at about 70 km depth the subduction channel is modelled as a thin wedge shaped layer of intermediate seismic velocity above a slower oceanic crust and below a faster overlying mantle wedge we also look into the effects of a continuous phase transition from basalt to eclogite in the subducted oceanic crust and near surface crustal structures in all models wave propagation is characterized by dispersive guided channel waves trapped in the low velocity subducted crust they produce high amplitude arrivals in the forearc a fast guided wave train gp originates from the direct p wave and a slower one gs from the direct s wave guided waves are radiated into the overlying mantle where the dip of the slab is abruptly changing seismogram sections for models without a subduction channel typically show two spatially separated guided wave trains one following the oceanic crust and one travelling more steeply towards the forearc high a subduction channel above the plate contact enhances the radiation effect of gp waves at the slab bend due to the weaker velocity contrast and inhibits the separation of the wave trains in models with additional near surface crustal structures the wave field is dominated by reverberations however guided waves remain discernible in seismogram sections because of their high amplitudes when a basalt to eclogite phase transition is considered guided waves are preferentially generated in the presence of a subduction channel a pronounced gp wave train develops particularly for sources inside this channel on the contrary guided waves are hardly distinct for sources below the subducted crust

san andreas fault zone head waves near parkfield california microearthquake seismograms from the borehole seismic network on the san andreas fault near parkfield california provide three lines of evidence that first p arrivals are head waves refracted along the cross fault material contrast first the travel time difference between these arrivals and secondary phases identified as direct p waves scales linearly with the source receiver distance second these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct p arrivals third the first motion polarities of the emergent arrivals are reversed from those of the direct p waves as predicted by the theory of fault zone head waves for slip on the san andreas fault the presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms the fault zone head waves indicate that the velocity contrast across the san andreas fault near parkfield is approximately 4 percent further studies of these waves may provide a way of assessing changes in the physical state of the fault system

a continuum model of continental deformation above subduction zones application to the andes and the aegean large scale continental tectonics of back arc extensional and andean type compressional environments are investigated by using the thin viscous sheet model to calculate the deformation within continental lithosphere that is subjected to horizontal forces on its plate boundaries and to basal drag from the asthenospheric flow beneath the shear tractions acting on the base of a deformable lithosphere are derived from a corner flow model that assumes a rigid subducting plate and a deformable overlying plate because the calculated shear tractions and the deformation within the overlying plate are interdependent the corner flow and the thin viscous sheet models are solved simultaneously we use a perturbation method to obtain analytical solutions for the velocity and strain rate fields within the overlying continental lithosphere the solutions depend on the angle of subduction the dimensionless thickness of the lithosphere and the ratio of asthenospheric to lithospheric viscosities which governs the viscous coupling between the asthenosphere and the lithosphere calculations are compared with observations from the andes and the aegean our results explain some of the features of the deformation in these regions that have heretofore not been explained by other models our model predicts that in a compressional environment a broad region of uplifted topography will tend to develop above a more steeply dipping slab 30o rather than above a shallower slab 10o 15o this is in accord with observations in the various segments of the central andes for an extensional environment the model predicts that a zone of compression can develop near the trench and that extensional strain rate can increase with distance from the trench as is observed in the aegean we also estimate the effective viscosities of 102o pa s for the asthenosphere 2x 1021 pa s for the aegean lithosphere and 1022 pa s for the andean lithosphere

timing of rifting in the southern gulf of california and its conjugate margins insights from the plutonic record the gulf of california is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin previous studies focused along the southwestern rifted margin in baja california indicated rifting was initiated after subduction and related magmatism ceased at ca 14 12 5 ma however the geologic record on the mexico mainland sinaloa and nayarit states indicates crustal stretching in the region began as early as late oligocene the timing of cooling and exhumation of pre and synrift plutonic rocks can provide constraints on the timing and rate of rifting here we present results of a regional study on intrusive rocks in the southern gulf of california sampled along the conjugate baja california and nayarit sinaloa rift margins as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf forty one samples were dated via u pb zircon and 40ar 39ar mineral ages providing emplacement age and thermochronological constraints on timing and rate of cooling we found an extensive suite of early and middle miocene plutons emplaced at shallow depths within the basement cretaceous paleocene peninsular range and sinaloa jalisco batholiths early miocene granitoids occur in an elongated wnw ese belt crossing the entire southern gulf from southern baja california to nayarit and sinaloa most have an intermediate composition 75 sio2 wt was emplaced 20 1 18 3 ma near the end of the early miocene age span and chemical composition of the early miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern sierra madre occidental and in southern baja california suggesting that eruptive sources for the early miocene ignimbrite flare up may also have been located within the southern gulf of california early miocene plutons cooled below the 40ar 39ar biotite closure temperature 350 400 c in less than 2 5 m y which we interpret as evidence of a regional extensional event leading to the opening of the gulf of california a less widely distributed suite of intermediate composition middle miocene granitoids 15 13 ma was sampled from the central western part of the gulf west of the pescadero basin and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore comondu group in baja california our widely spaced sampling of the generally sediment covered igneous crust suggests that middle miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust comondu arc thermobarometry data also indicate a very shallow depth 5 km of emplacement for the middle miocene plutonic rocks some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure early and middle miocene granitoids away from the gulf axis yielded 40ar 39ar cooling ages very close to u pb zircon ages demonstrating rapid cooling to 350 c which we attribute to their shallow emplacement and possibly to exhumation soon after intrusion since comondu age and middle miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction we suggest that intense crustal stretching controlled the pattern and timing of comondu age magmatism rather than the middle miocene magmatism controlling the locus of 12 ma extension

miocene detachment faulting predating epr propagation southern baja california at the southern tip of the baja california peninsula we characterize the onshore structures and kinematics associated with crustal necking leading up to the pliocene breakup and early east pacific rise seafloor spreading from a combination of tectonic field investigations k ar and cosmogenic isotope dating and geomorphology we propose that the los cabos block represents the exhumed footwall of a major detachment fault this north trending detachment fault is marked by a conspicuous low dipping brittle ductile shear zone showing a finite displacement with top to the se ending to the ese this major feature is associated with fluid circulations which led to rejuvenation of the deformed cretaceous magmatic rocks at a maximum of 17 5 ma the detachment footwall displays kilometer scale corrugations controlling the present day drainage pattern this major detachment is synchronous with the development of the san jose del cabo basin where syntectonic sedimentation took place from the middle miocene to probably the early pliocene we propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the baja california passive margin which predates the onset of formation of the east pacific rise spreading axis in the cabo puerto vallarta segment our data illustrate an apparent anticlockwise rotation of the stretching direction in baja california sur from 17 ma to the pliocene

a chemical and isotopic study of the laramide granitic belt of northwestern mexico identification of the southern edge of the north american precambrian basement along the laramide belt of northwestern mexico granitic rocks of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of the north american precambrian basement in the northern part the laramide plutons the northern granites intruded proterozoic crystalline rocks and a thick late proterozoic through paleozoic miogeoclinal cover of north american affinity in the central part the granitic bodies the central granites were emplaced into a sequence of paleozoic eugeoclinal rocks overlain by late triassic clastic units the southern part of the belt the southern granites intruded a less known crust characterized by middle to late mesozoic island arc related volcanic and sedimentary rocks of the guerrero terrane data from a suite of metaluminous to slightly peraluminous calc alkalic granitic rocks along the belt display north to south geochemical and isotopic variations which could correlate with the type of intruded basement the northern and central granites are characterized by strongly fractionated light rare earth element ree enriched patterns which display generally pronounced negative europium anomalies whereas the southern granites have lower total ree enrichments and much flatter chondrite normalized slopes displaying almost no europium anomalies isotopic results also suggest regional variations as shown by the following initial sr and end ranges 0 7070 to 0 7089 and24 2 to25 4 respectively for the northern granites 0 7060 to 0 7079 and23 4 to25 1 for the central granites and 0 7026 to 0 7062 and20 9 to 14 2 for the southern granites on the basis of their isotopic similarities the proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic flows in southern arizona and northern mexico is interpreted as a reasonable parental source for the northern and central granites however mantle derived melts are not excluded the more primitive southern granites are interpreted to come from a source that lacked proterozoic basement instead they were probably derived by mixing of juvenile mantle melts with partial melts of the lower parts of the guerrero terrane in general the north to south compositional variations of the laramide granitic rocks of northwestern mexico reflect the crustal structure underneath the batholiths the sr and nd data indicate that the edge of the north american precambrian basement extends approximately southeastward from the coastal batholith of central sonora then about 200 km south of hermosillo in southern sonora the edge bends eastward and continues to the east beneath the sierra madre occidental volcanic province

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

new geologic mapping and shrimp u pb zircon data in the peninsular ranges batholith baja california mexico evidence for a suture geologic and shrimp u pb zircon studies in the sierra san pedro martir area of northern baja california mexico suggest that the western and eastern parts of the peninsular ranges batholith originated as separate arcs they are now juxtaposed along a well exposed ductile thrust main martir thrust marked by an age discontinuity of at least 10 to 15 m y and sharp changes in rock type metamorphic grade and deformation history the east dipping thrust and adjacent highly deformed rocks were stitched by 108 97 ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs lack of chemical isotopic and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin or development of the arc on a prism of continentally derived sediments adjacent to the continental margin instead the western arc may have originated as an island arc above a subduction zone outboard of north america if so a second concurrent subduction zone along the north american margin must have driven convergence and suturing of the two arcs and the main martir thrust may mark a nonterminal suture within a wider convergence related deformation zone that formed ca 115 108 ma

onshore and offshore apatite fission track dating from the southern gulf of california insights into the time space evolution of the rifting abstract we present the results of a apatite fission track aft study on intrusive rocks in the southern gulf of california sampled along the eastern margin of baja california sur western rift margin as well as from islands and submerged rifted blocks within the gulf of california and from the conjugate mexican margin nayarit state for most of the samples u pb zircon and 40ar 39ar mineral ages were already available duque trujillo et al 2015 coupled with the new aft data these ages provide a more complete information on cooling after emplacement our samples span a wide range of ages between 5 5 1 1 and 73 7 5 8 ma and show a general spatial distribution with late miocene aft ages about 6 ma aligned roughly nw se along a narrow offshore belt parallel to baja california peninsula separating older ages on both sides this pattern suggests that in late miocene deformation due to plate transtension focused at the eastern rheological boundary of the baja california block some early miocene aft ages onshore baja california could be related to plutons emplaced at shallow depths and thermal resetting associated with the onset of volcanism at 19 ma in this part of the peninsula on the other hand an early extensional event similar to that documented in the eastern gulf cannot be ruled out in the westernmost baja california

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

timing and origin of silicic volcanism in northwestern mexico insights from zircon u pb geochronology hf isotopes and geochemistry of rhyolite ignimbrites from palmarejo and guazapares in southwest chihuahua abstract precise timing and constraints for magma source s are critical to understand the post subduction tectonomagmatic evolution of the world s largest cenozoic silicic large igneous province in western mexico we provide in situ zircon u pb ages and hf isotope compositions of rhyolitic ignimbrites exposed in the palmarejo and guazapares at the northwestern extremity of sierra madre occidental four rhyolite ignimbrite samples from palmarejo yielded zircon weighted mean u pb ages between 23 and 24 ma however five individual analyses from one sample yielded the oldest age of 25 09 0 41 ma mswd 1 7 n 5 a rhyolitic tuff from the guazapares district yielded older individual dates varying from 25 7 to 28 6 ma with a main population yielding a weighted mean age of 27 31 0 36 ma zircon u pb ages suggest that the ignimbrite flare up in southwestern chihuahua remained restricted to the early middle oligocene to early miocene we suggest that the zircon u pb dates 24 ma particularly from guazapares are the antecrystic zircons included from the underlying older ignimbrites the zircon u pb dates of 25 28 ma are related to the culmination of syn extensional first ignimbrite flare up and bimodal volcanism started as early as 35 ma the first ignimbrite flare up is possibly resulted from the detachment of the leading edge of the farallon slab around 35 28 ma the younger age of 24 23 ma from palmarejo can be correlated to the second episode of ignimbrite flare up erupted during the pervasive regional extension normal faulting and onset of formation of metamorphic core complexes this second flare up is resulted by the subsequent events of break off rollback of the farallon plate the positive hf isotope composition ehf t 1 0 to 5 8 and concomitant mafic volcanism suggest partial melting of subcontinental lithospheric mantle and the overlying laramide crust by upwelling of decompressing asthenosphere during post laramide regional extension this interpretation is consistent with the successive detachments fragmental removal and or rollback of the leading edges of the farallon slab beneath the western continental margin of north america we did not find any xenocrystic zircon inheritance neither from the laramide plutonic basement roots of the jurassic guerrero terrane nor from the proterozoic north american crust however zircon inheritance of laramide age has been reported from the guazapares district partial melting of laramide basement was possibly facilitated by the heat advection and ponding of subcontinental lithospheric mantle related magmas in the crust during the cenozoic regional extension

joint inversion of fault zone head waves and direct p arrivals for crustal structure near major faults a newly recognized class of seismic phases namely fault zone head waves arising from refraction at a transfault velocity contrast provides additional constraints to direct p waves in inversions of near fault arrival time data for earthquake location and velocity structure incorporation of fault zone head waves in the inversion process increases the available data and broadens the spatial coverage of ray paths between sources and receivers equally important the explicit recognition of fault zone head waves eliminates a source of error resulting from misidentifying head wave first arrivals as direct p waves a simple algorithm for the joint inversion of fault zone head waves and direct p phases is illustrated with observed and synthetic data application to a small data set recorded by the borehole seismic network at parkfield california provides separate seismic velocity depth profiles for the crustal blocks on the two sides of the san andreas fault for depths less than 3 km the obtained velocity contrast is 10 20 for greater depths it decreases to 3 7 numerical tests with synthetic data show that for sources with known locations and for well located local earthquakes randomized mislocation errors of up to 600 m proper use of fault zone head waves can significantly improve the accuracy of the fitted velocity structure the results of this work motivate the inclusion of fault zone head waves in state of the art joint structure hypocenter inversions using high quality near fault data

fault zone waves observed at the southern joshua tree earthquake rupture zone waveform and spectral characteristics of several aftershocks of the m 6 1 22 april 1992 joshua tree earthquake recorded at stations just north of the indio hills in the coachella valley can be interpreted in terms of waves propagating within narrow low velocity high attenuation vertical zones evidence for our interpretation consists of 1 emergent p arrivals prior to and opposite in polarity to the impulsive direct phase these arrivals can be modeled as headwaves indicative of a transfault velocity contrast 2 spectral peaks in the s wave train that can be interpreted as internally reflected low velocity fault zone wave energy and 3 spatial selectivity of event station pairs at which these data are observed suggesting a long narrow geologic structure the observed waveforms are modeled using the analytical solution of ben zion and aki 1990 for a plane parallel layered fault zone structure synthetic waveform fits to the observed data indicate the presence of ns trending vertical fault zone layers characterized by a thickness of 50 to 100 m a velocity decrease of 10 to 15 relative to the surrounding rock and a p wave quality factor in the range 25 to 50

middle miocene extension in the gulf extensional province baja california evidence from the southern sierra juarez new geologic mapping structural studies and geochronology of miocene volcanic and sedimentary rocks in the southern sierra juarez baja california shed light on the extensional history of the gulf extensional province prior to sea floor spreading in the gulf of california the southern sierra juarez is underlain by lower middle miocene rocks including fluvial strata intermediate composition volcanic deposits basalt lava flows and cinder cones and dacite pyroclastic deposits and lavas that nonconformably overlie the cretaceous peninsular ranges batholith the 40 ar 39 ar geochronology indicates that basaltic rocks are 16 90 0 05 ma and dacite pyroclastic deposits are between 16 69 0 11 ma and 15 98 0 13 ma these strata were subsequently cut by two generations of faults first generation faults comprise a dominant set of north south striking west dipping normal faults a secondary set of north south striking east dipping normal faults and a lesser set of variably oriented strike slip faults all three fault sets are temporally and spatially related and were produced by east west extension the dominant west dipping faults which are antithetic to and oblique to the east dipping main gulf escarpment may have been a precursor or an early phase accommodation zone along the escarpment west dipping normal faults are cut by a 10 96 0 05 ma dacite hypabyssal intrusion thus bracketing the age of east west extension between 15 98 0 13 ma and 10 96 0 05 ma hence this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated miocene extension within baja california second generation faults which are comprised of east west striking strike slip faults that cut first generation faults and associated northwest striking northeast dipping normal faults may be related to early development of the transpeninsular strike slip province global plate reconstructions suggest that transtensional motion between the north american and pacific plates along the western margin of baja california began during middle miocene time coeval with east west extension in the southern sierra juarez this observation supports a hypothesis that middle miocene transtensional plate motion was partitioned into two components a strike slip component parallel to active faults along the western offshore margin of baja california and an extensional component normal to the margin but located in what is now the gulf extensional province hence the onset of extension within the circum gulf region was in response to plate boundary processes

amplification of ground motion and waveform complexity in fault zones examples from the san andreas and calaveras faults summary p wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of gaussian beams for point sources located in a low velocity wedge surrounding a fault the computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the san andreas and haywardcalaveras faults calculations in these models show that the 10 20hz vertical displacements of earthquakes located at 3 10 km depth are amplified by up to an order of magnitude in a 1 2 km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust this amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike slip earthquake depending on the source depth of the earthquake and the structure and velocity contrast of the wedge multiple triplications in the travel time curve of direct p and s waves will occur at stations in the fault zone a wedge model successfully predicts the triplications observed in the p waveforms of aftershocks of the coyote lake earthquake recorded in the fault zone showing that body waves from microearthquakes can be used to determine the three dimensional velocity structure of the fault zone the amplification waveform complexity and distortion of ray paths introduced by the lowvelocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone for realistic models of the wedge asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20hz numerical methods may be necessary to calculate accurately the wavefield at lower frequencies

jurassic to holocene tectonics magmatism and metallogeny of northwestern mexico the present metallic distribution in northwestern mexico is the culmination of superposed magmatism tectonism erosion and burial over more than 150 m y detailed palinspastic reconstructions of preextensional configurations the first study of its kind for this region clarify the interplay among these features on the present distribution and character of mineralized geologic systems this new synthesis goes beyond previous metallogenic investigations of northwestern mexico by separating events into specific timing and structural relationships and by restoring the geology to its preextensional configuration metallogenic factors such as enrichment preservation and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern mexico the analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history rather than simply metallic belts or interpreted angle of a subducting slab three main extensional events in the oligocene holocene have been restored and the palinspastic distributions have been analyzed reconstructions reveal the following 1 mineralization events igneous centers and sedimentary sequences are continuous across the gulf of california and other areas with large amounts of extension 2 middle tertiary gold silver mineralization in baja california may be the western part of the sierra madre oc present address bhp billiton avenida america vespucio sur 100 8th floor las condes santiago chile e mail john mark g staude bhpbilliton com cidental metallogenic province thus expanding the previously recognized extent of this province s mineralization 3 late cretaceous early tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km with parts of the belt buried beneath the younger sierra madre occidental volcanic fields 4 interpreted alignments of older geologic features including lineaments of ore deposits are displaced in the reconstructions 5 sedimentary rock hosted gold deposits and low angle detachment gold systems are closely related and occur around core complexes by using structurally restored time slices it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic rock hosted these characteristics express themselves in the regional distribution of deposit types second mineralization of widely differing ages is spatially superposed commonly associated with coeval magmatic and tectonic events the structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern mexico

cenozoic mineral deposits and subduction related magmatic arcs in mexico spatial distribution of isotopic ages of igneous rocks in northern mexico shows that a magmatic arc commenced at 140 m y b p near a paleotrench progressed 1 000 km eastward by 40 m y b p and then regressed to within 200 km of the continental margin by 18 m y b p during progression a calcic and calc alkaline continental margin volcano plutonic regime developed but farther east high k calc alkaline and alkaline facies formed in northeastern mexico and west texas following regression calc alkaline volcanism was renewed on the pacific margin at 18 m y b p and continued until cessation of subduction by initiation of transform movements between pacific and american plates benioff zone depth estimates from k 57 5 relationships suggest decreasing dip during progression and a variable dip variable depth regime from 140 to 16 m y b p both the dip angle convergence rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m y b p followed by decreasing convergence and sinking rates from 42 to 16 m y b p changes in subduction dynamics are attributed to plate reorganization at 44 m y b p and the approach of the east pacific rise differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time cu mo w ores 106 to 40 m y old in calc alkaline magmas formed during progression whereas mo and caf 2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths and pb zn ag deposits 49 26 m y b p were formed during late arc progression and early regression these geodynamic variations plus sr rb and s isotopic characteristics of the arc terrane suggest that processes associated with subduction and some crustal contamination rather than local compositional anomalies in crust or mantle account for different magma and mineral deposit suites southern mexico exhibits fe and cu zones whereas the overlapping calc alkaline trans mexican chiapanecan arc of late miocene to holocene age contains several base and precious metal deposits

three dimensional calculations of fault zone guided waves in various irregular structures summary the detailed structure of fault zones fzs plays an important role in problems related to fault mechanics earthquake rupture wave propagation and seismic hazard fzs are thought to consist of an o 10 100 m wide region of decreased seismic velocity but structural details such as their depth extent lateral and vertical variations etc are elusive the small spatial scales involved make such structures difficult to image with ray theoretical methods such as tomography however seismic energy trapped inside fz layers can provide dispersive wave trains that carry information on the fz structure these waves can travel many kilometres inside the fz before reaching the surface and are therefore strongly altered by its properties candidate trapped waves have been observed above several active faults inversion algorithms exist that can model these observations in terms of planar fault zone structures however at present it is not clear how reliable these estimates are as the effects of even small 3 d variations on trapped waves are not well understood the goal of this study is to distinguish 3 d structures that do and do not significantly affect fz waves to achieve this we perform numerical calculations of wave propagation in various fz geometries and analyse the waveforms spectra and envelopes of the synthetic seismograms the main results are that 1 moderate changes of the shape of fz or 2 small scale heterogeneities or 3 depth dependent properties do not strongly affect the observed fz waves in contrast strong effects are to be expected from 4 breaks in the continuity of fz structure e g offsets which may at some point allow imaging of such features at depth

shallow structure of the landers fault zone from explosion generated trapped waves with fault zone trapped waves generated by near surface explosions within the fault zone of the 1992 landers california earthquake and recorded by linear seismic arrays deployed along and across the fault we have delineated the shallow seismic structure near the 30 km long southern rupture on the johnson valley fault the explosion excited trapped waves with relatively large amplitude and long duration wave train after the s waves are similar to those generated by aftershocks li et al 1994a b but have lower frequencies and travel more slowly coda normalized amplitude spectra of explosion excited trapped waves show a maximum at 2 hz which decreases rapidly with the station offset from the fault trace on the cross fault profiles normalized amplitudes of trapped waves on the along fault profile also decreased with distance between the explosion and station giving an apparent q of 18 at 1 2 hz in the fault zone for the shot near the profile the dispersion of trapped waves from 0 6 to 2 5 hz recorded on this along fault profile implies a shear velocity of 1 0 km s for the fault zone and 1 8 km s for the wall rock while the data from the farther shot show an increase in velocity and q with depth measured group velocities and q values were used as constraints in the numerical modeling of trapped waves on cross fault and along fault profiles results reveal that the shallow johnson valley fault is marked by a zone 250 m wide where the shear velocity is 1 km s and q is 20 calculation of finite difference synthetics for a depth varying fault structure show that these model parameters apply to the depth of 1 km below which the fault zone shear velocity increases to 1 9 km s and q increases to 30

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

deformation of the central andes 15 27 s derived from a flow model of subduction zones a simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab our formulation accounts for two forces that dominate the deformation in the overriding lithosphere tectonic forces and buoyancy forces the tectonic forces arise from the subduction of oceanic lithosphere and produce minor extension in the lithosphere near the trench and compression farther inland buoyancy forces can induce horizontal pressure gradients due to variations in crustal thickness which cause lithospheric extension where the crust is thick numerical solutions obtained by using a finite element technique are compared with observations from the central andes 15 27 s the model predicts the observed deformation pattern of extension in the forearc compression in the western monocline corresponding to magmatic activity extension in the altiplano compression in the eastern monocline and subandes and no deformation in the brazilian shield by comparing the calculated solutions with the large scale tectonic observations we are able to evaluate the forces that govern the deformation in the central andes the approximately constant subduction velocity in the past 26 m y suggests that the rate of crustal shortening in the andes has decreased with time due to the thickening of the crust

a comparative study of discrete fracture network and equivalent continuum models for simulating flow and transport in the far field of a hypothetical nuclear waste repository in crystalline host rock abstract one of the major challenges of simulating flow and transport in the far field of a geologic repository in crystalline host rock is related to reproducing the properties of the fracture network over the large volume of rock with sparse fracture characterization data various approaches have been developed to simulate flow and transport through the fractured rock the approaches can be broadly divided into discrete fracture network dfn and equivalent continuum model ecm the dfn explicitly represents individual fractures while the ecm uses fracture properties to determine equivalent continuum parameters we compare dfn and ecm in terms of upscaled observed transport properties through generic fracture networks the major effort was directed on making the dfn and ecm approaches similar in their conceptual representations this allows for separating differences related to the interpretation of the test conditions and parameters from the differences between the dfn and ecm approaches the two models are compared using a benchmark test problem that is constructed to represent the far field 1 1 1 km 3 of a hypothetical repository in fractured crystalline rock the test problem setting uses generic fracture properties that can be expected in crystalline rocks the models are compared in terms of the 1 effective permeability of the domain and 2 nonreactive solute breakthrough curves through the domain the principal differences between the models are mesh size network connectivity matrix diffusion and anisotropy we demonstrate how these differences affect the flow and transport we identify the factors that should be taken in consideration when selecting an approach most suitable for the site specific conditions

internal structure of the san jacinto fault zone at blackburn saddle from seismic data of a linear array local and teleseismic earthquake waveforms recorded by a 180 m long linear array bb with seven seismometers crossing the clark fault of the san jacinto fault zone northwest of anza are used to image a deep bimaterial interface and core damage structure of the fault delay times of p waves across the array indicate an increase in slowness from the southwest most bb01 to the northeast most bb07 station automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves the observed fault zone head waves imply that the clark fault in the area is a sharp bimaterial interface with lower seismic velocity on the southwest side the moveout between the head and direct p arrivals for events within 40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3 2 per cent a constant moveout for events beyond 40 km to the southeast is due to off fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance the lack of head waves from events beyond 20 km to the northwest is associated with structural complexity near the hemet stepover events located in a broad region generate fault zone trapped waves at stations bb04 bb07 waveform inversions indicate that the most likely parameters of the trapping structure are width of 200 m s velocity reduction of 30 40 per cent with respect to the bounding blocks q value of 10 20 and depth of 3 5 km the trapping structure and zone with largest slowness are on the northeast side of the fault the observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest this inference is consistent with results of other geological and seismological studies

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california