inversion of rg waveforms recorded in southern spain abstract group velocity dispersion measurements of r g waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of almeria southern spain in principle the usable frequency range of 250 to 2000 mhz allows determination of structures to depths of about 4 km for this purpose the main operations are a detailed dispersion analysis of high frequency rayleigh waves propagating along very short paths and the inversion of r g wave group velocities a total of 21 seismic events were studied these events had small magnitudes 2 0 to 2 5 approximately and very shallow focal depths about 100 m and were taken from a set of 214 events that occurred in 1991 during a spanish italian seismic experiment the events were recorded at seven single component stations belonging to the regional seismic network of andalucia at approximate distances of between 15 and 57 km from the source these events were grouped into six seismic sources according to specific criteria we used digital filtering techniques providing a significant improvement in signal to noise ratio to determine ray path group velocities and we inverted dispersion data via generalized inversion in order to obtain refined dispersion data we have carried out a further regionalization of group velocities and thus six small subregions have been resolved in group velocities the highest group velocity values from 1 93 to 2 25 km sec 1 correspond to the filabres mountain range which is an area containing materials of the nevado filabride complex of paleozoic and triassic age on the other hand low velocity values between 1 39 and 1 56 km sec 1 correspond to the alhamilla mountain range which belongs to the alpujarride complex and contains conglomerates of the cambrian and tortonian periods the velocities obtained for the neogene quaternary basin of the andarax river with materials of the tortonian and pliocene periods are also very low between 1 35 and 1 68 km sec 1 we inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km according to the regional earth models that we obtained we find clear variations in velocity both laterally and vertically for several zones with different composition the filabres mountain range shows high shear velocity values 2 14 to 2 83 km sec 1 in the opposite end we have the andarax basin that presents the lowest shear velocity values consistent with its sedimentary structure 1 56 to 2 55 km sec 1 intermediate shear wave velocities characterize the remaining regions the tabernas sorbas basin the gador mountain range and the volcanic region of nijar cape of gata although the relationship between lateral changes in r g dispersion and geologic structure may not be straightforward in this study we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations

strain partitioning and progressive deformation history in a transpressive belt northern appalachians abstract syntectonic fibres within rocks of the gaspetrough a mid paleozoic slate belt within part of the acadian orogen of the canadian appalachians record the complex structural evolution of a transpressive belt the fibres occur around pyrite framboids and can be divided into five segments which record four distinct structural stages all four structural stages are related to a single progressive deformation since fibre segments can be traced continuously from the first segments to the last fibre growth patterns within the gaspetrough clearly indicate that folding occurred during coaxial deformation during the first three structural stages these structural stages are 1 layer parallel shortening 2 combined flattening and rigid body rotation and 3 cleavage development strain associated with vertical extension during these three stages accounts for approximately 50 shortening within the slate belt during the last structural stage deformation resulting from regional simple shear continued to affect the entire slate belt as local flattening within cleavage planes produced horizontal extension and brought about further tightening of folds strain recorded by the horizontal fibre increments produced during simple shear accounts for 25 shortening in this part of the appalachians oblique convergence during the acadian orogeny was partitioned in time between strike slip and reverse faulting convergence was accommodated by folding flattening and reverse faulting during the first part of the deformation and later on by flattening and dextral transcurrent faulting

curved vein fibres an alternative explanation abstract veins occurring at the edge of the dextral indian islands fault newfoundland are folded in response to a fault parallel ductile shear the veins which locally constitute a penetrative fabric element at outcrop scale are composed mainly of calcite and have a narrow rim of quartz and chlorite both the calcite and quartz are generally fibrous and lattice distortion in the calcite is not commensurate with the observed degree of fibre curvature this observation supports the popular view that curved fibres grow curved as they track the vein opening vector it is demonstrated however that the curvature in the material described here is due to deformation and that the lack of strong lattice distortion is due in part to polygonization and to recrystallization all observations are consistent with both the calcite and the quartz fibres having grown perpendicular to the vein walls the chlorite orientation is controlled by the orientation of mica grains in the country rock in some veins markers make it possible to define the net opening direction and in all examples it is demonstrated that the direction has not been tracked by the fibres since this conclusion may be more generally applicable caution should be exercised in interpreting kinematics on the basis of fibre geometry where vein density is high the veins and country rock screens separating them form a multilayer sequence that is folded into fairly harmonic folds the folds appear to have a cleavage transected relationship to the regional cleavage but the cleavage is shown to predate the folds

progressive localization of deformation during exhumation of a major strike slip shear zone norumbega fault zone south central maine usa detailed structural studies along with 40ar 39ar thermochronology provide important constraints on the temporal spatial and structural evolution of a major orogen parallel strike slip shear zone the norumbega fault zone in maine detailed structural analysis in south central maine reveals two very different styles of dextral noncoaxial deformation a wide zone 25 km of heterogeneously distributed ductile shear structures and a relatively narrow zone 1 km of lower greenschist facies high strain mylonitization 40ar 39ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following middle devonian amphibolite facies metamorphism the wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature post amphibolite facies metamorphism but pre regional cooling below 320 c late devonian to early carboniferous transcurrent tectonism the narrow zone of dextral shear represents both a younger latest carboniferous and significantly cooler 320 c deformational event occurring at much higher structural levels a general model of increasingly narrow but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

a laboratory simulation of fibrous veins some first observations abstract following work by stephen taber 80 years ago we describe vein like arrays of parallel fibrous crystals that grow evaporatively between pairs of brine soaked porous ceramic substrates crystals of solute grow antitaxially from fixed sites on the substrate forcing older parts of the crystals away from the growth site without benefit of any long range cracking parallel to the substrate the nutrients for growth are fed to the growth site advectively or diffusively through the substrate blocks themselves not along the plane of the vein we call such crystallization taber growth and suggest as taber did that it might be an important mechanism for non evaporative fibrous vein development in nature the taber growth model provides a ready explanation for the ability of fibers to track vein opening directions and tracking is indeed the rule in our samples though exceptions are also seen our results lend support to ideas already in the literature that fibrous veins are not necessarily products of a crack seal process and that fibrous veins are not necessarily syntectonic our observations also raise questions about criteria for recognition of syntaxial fibrous veins and underscore the importance of finding new criteria for recognition of the younging direction along fibers

seismotectonics of the southern appalachians abstract the southern appalachians of eastern tennessee and western north carolina is a region for which high quality instrumental seismic data have been very limited in this paper we report more than 100 new or relocated hypocenters obtained from the initial 2 1 2 yr of operation of the southern appalachian regional seismic network sarsn a 16 station four state telemetered network of short period vertical instruments these data provide a first order snapshot of southern appalachian seismicity that confirms some previous observations and adds new results for the region the most important new observations are well constrained focal depths and focal mechanisms for southern appalachian earthquakes the level of activity exceeds expectations and concentrates more heavily in the valley and ridge province of eastern tennessee as opposed to the blue ridge or inner piedmont than is evident in historical listings the large majority of these events lie between the new york alabama lineament and the clingman ocoee lineaments magnetic anomalies produced by deep seated basement structures an analysis of the uniqueness and stability of hypocentral depth yields reliable depths for 58 earthquakes and demonstrates that most southern appalachian seismic activity occurs beneath the decollement thus lending quantitative support to the argument that appalachian seismicity is unrelated to surficial geological or tectonic features several new focal mechanisms of a higher quality than previously available indicate that the basement of the valley and ridge is under ne sw compression and the principal mode of faulting is right lateral strike slip on nearly north south faults in both depth and type of faulting the seismogenic model we propose for the southern appalachians closely emulates the giles county zone to the north of our study area

point stability at shallow depths experience from tilt measurements in the lower rhine embayment germany and implications for high resolution gps and gravity recordings summary from 1996 to 1999 we have studied ground tilts at depths of between 2 m and 5 m at three sites in the lower rhine embayment lre western germany the lre is a tectonically active extensional sedimentary basin roughly 50 kmr100 km the purpose of the tilt measurements was a to provide insight into the magnitude nature and variability of background tilts and b to assess possible limitations of high resolution gps campaigns and microgravity surveys due to natural ground deformation the tilt readings sensed by biaxial borehole tiltmeters of baselength 0 85 m cover a frequency range from 10 x8 hz to 10 x2 hz periods from minutes to years assuming that the tilt signals represent ground displacements on a scale typically not larger than several times the tiltmeters baselength and that tilt signals at shallow depth could in a simple geometric way be related to changes in surface elevation and gravity we try to estimate the magnitude level of point movements and corresponding bouguer gravity effects that is generally not surpassed the largest tilt signals observed were some t50 mrad yr x1 i f they were observable over a ground section of extension e g 10 m the converted rates may correspond to about t0 5 mm per 10 m yr x1 in vertical ground displacement and t0 1 mgal yr x1 in bouguer gravity effect respectively large signals are mostly related to seasonal effects probably linked to thermomechanical strain other causes of ground deformation identified include seepage effects after rainfalls order of t10 mrad and diurnal strains due to thermal heating and or fluctuations in the water consumption of nearby trees order of t1 mrad episodic step like tilt anomalies with amplitudes up to 22 mrad at one of the observation sites might reflect creep events associated to a nearby active fault except for short term ground deformation caused by the passage of seismic waves from distant earthquakes amplitudes of non identified tilt signals in the studied frequency range seem not to exceed t2 mrad as the larger tilt signals are close to the precision achieved with modern gps systems and superconducting gravimeters when converted into height and gravity changes further enhancement in resolution of these techniques may require simultaneous recording of local ground deformation at the observation sites

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

estimation of vorticity from fibrous calcite veins central maine usa an estimate for the vorticity of a mid paleozoic deformation was made using deformed and or rotated calcite veins as natural vorticity and shear strain gauges in a northern part of the u s appalachians the spatial distribution of extended shortened and shortened then extended elongate fibrous calcite veins in limestone layers were used to construct a mohr circle for the position gradient tensor h from which vorticity and flow parameters were calculated the amount of rotation of large axial ratio fibrous calcite veins in quartz phyllite layer boudin necks and linear veins in limestone layers were used to calculate shear strain as a check on flow parameters calculated from the distribution of deformed veins these analyses demonstrate that the d3 phase of deformation was characterized by a mean kinematic vorticity number of 0 67 the existence of a non coaxial fabric defined by reactivated foliation and dextral asymmetric folds is also established age constraints suggest that this non coaxial deformation in eastern central maine began early in the devonian and continued at least through regional peak metamorphism this study represents the first quantitative assessment of the vorticity of devonian deformation in maine and the results suggest that a transpressional tectonic model is appropriate for the area at that time

crustal structure of the appalachian highlands in tennessee abstract crustal structure of the southern appalachians and adjacent interior low plateaus in tennessee is derived from seismic refraction measurements observed by the u s geological survey in 1965 along reversed lines normal nw se and parallel ne sw to the structure of the appalachian highlands major geologic divisions its easternmost part is located approximately 80 km southwest of the westernmost part of the cocorp seismic reflection traverse within the blue ridge province the velocity depth models derived for both observational directions consist of three crustal layers with surprisingly high velocities being about 6 1 6 2 km s in the upper crust down to 7 10 km depth 6 7 6 8 km s for the middle crust between about 17 and 34 km and varying from 7 1 to 7 4 km s for the lower crust at about 40 47 km depth the boundaries between the three crustal layers as well as the crust mantle boundary are transition zones of up to 11 km thickness similar to old orogens in other parts of the earth the main result is a thick crust at places in excess of 50 km with high average velocity and a broad crust mantle transition zone

joint hypocenter velocity inversion for the eastern tennessee seismic zone a joint hypocenter velocity inversion for the eastern tennessee seismic zone etsz has resolved velocity features in basement rock below detached appalachian thrust sheets p and s wave arrival times from 492 earthquakes have been inverted for one 1 d and three dimensional 3 d velocity models to midcrustal depths the 3 d p and s wave velocity solutions are computed independly and are very similar in relation to the 1 d model velocity anomalies range from 8 to 16 in the first layer upper 5 km and between 7 in deeper layers prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer the most persistent anomaly is a low velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity the new york alabama ny al magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the p and s wave velocity images the spatial coincidence between velocity gravity and magnetic gradients suggests that major discontinuities are present in the basement relocation in the 3 d velocity model reduced the number of very deep earthquakes below 20 km and further accentuated differences in seismogenic properties on either side of the ny al lineament after relocation most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth most earthquakes occur in regions characterized by either average velocity or small velocity anomalies

imaging of the near surface shear wave velocity structure of the granada basin southern spain the seismic velocity structure of the granada basin southern spain at depths of 1 4 km is investigated by dispersion analysis of 0 5 to 5 9 sec rayleigh waves we have used records from 12 quarry blasts detonated between 1990 and 2000 and 16 local earthquakes with magnitudes between 2 6 and 3 8 that occurred between 1990 and 1997 all events were recorded on the regional seismic network of andalucia spain and on the national seismic network p s and rg phases were recorded at 13 stations at distances between 10 and 72 km from the source the analysis and inversion of rg waveforms is performed using multiple filtering techniques and a generalized inversion approach we measured ray path group velocities from 0 83 to 2 15 km sec in the 0 6 to 5 8 sec period interval and obtained shear wave velocity profiles for 29 source station paths the shear velocities lie between 0 8 and 3 34 km sec within a standard deviation band of 0 05 0 14 km sec from these models it is possible to infer the most conspicuous features in the upper 4 km of the region and the existence of some degree of lateral variation in velocity after applying an imaging method aimed at volumetric modeling and data visualization we present the first velocity images obtained for the granada basin our results characterize the basin and are correlated with the surface geology manuscript received 7 september 2001

influence of exhumation on the structural evolution of transpressional plate boundaries an example from the southern alps new zealand concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength three dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate bounding structure is stable if asymmetric erosion patterns such as those observed in orographic mountain belts pertain and if earth9s crust has a strong on weak rheology in early stages of oblique convergence of an initially laterally homogeneous material lateral boundary parallel strain is accommodated along a near vertical structure and convergent boundary normal strain is concentrated on structures dipping at moderate angles into the orogen exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure when the upper crust beneath the orogen is significantly weakened by exhumation lateral strain abandons the vertical structure and shifts to the dipping structure combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust as is the case along the alpine fault new zealand the process of thermal thinning is controlled by advection and occurs on time frames of 1 2 m y the two components of strain remain separate in the lower crust during active convergence exhumation of lower crustal material occurs only along those structures accommodating convergent strain consequently material exhumed from lower regions of ductile deformation as is the case along the alpine fault contains lineations that indicate a greater component of convergence than predicted from the total plate motion postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones one dominantly convergent and one dominantly strike slip widely reported orogen parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector but the exhumation of the lateral transport zone

short period surface wave dispersion and shallow crustal structure of central and eastern tennessee group velocity dispersion of explosion generated seismic surface waves with periods ranging from 0 2 to 1 5 sec is used to investigate shallow crustal structure of eastern and central tennessee several modes of both rayleigh and love waves can be identified and separated on the seismograms of seven sarsn regional network stations by zero phase digital filtering dispersion data for sinusoidal wave motion were based on digitized zero crossing times by forward modeling we find that a wave guide of at least two layers over a half space can adequately represent our particular multi mode narrow band observations in a layered section about 3 km thick lower velocities characterize outcropping clastic rocks of the cumberland plateau and higher velocities correspond to shallow carbonate rocks of the nashville dome half space shear velocities of about 3 9 km sec appear to represent lower paleozoic carbonate lithology deeper than 2 to 4 km on most of the observed paths our best data interpreted jointly with earlier data of oliver and ewing 1958 and of chen et al 1989 have a composite period range of 0 2 to 40 sec but they represent different appalachian paths group velocities over this broad spectrum are satisfied by a complex model with two low velocity layers the uniqueness of this model cannot be demonstrated but it represents important hypotheses concerning regional geologic features that can be tested more rigorously by improved surface wave dispersion data

shallow crustal structure from short period rayleigh wave dispersion data in southwestern taiwan strong ground motions in the period range of 1 to 5 sec observed in taiwan in a dense network during the tapu earthquake ml 5 8 of 15 december 1993 were dominated by fundamental mode rayleigh waves the shallow crustal structure beneath this network was determined from the group velocity dispersion data using standard inversion techniques results show that an alluvial layer with a thickness of only about 160 m exists over the sedimentary structure in the western coastal plain a clear lateral variation in the shear wave velocity along a cross section perpendicular to the western structural grain ho 1982 was resolved for the purpose of retrieving the ground motions to confirm the proposed model forward modeling with a two dimensional finite element method was used and agreed well with the observed seismograms when slightly lower velocities than those of the inversions were used it is concluded that the velocity model estimated using group velocity data cannot definitively correspond to the real one at such short dis tances and in such a complex structure on the basis of quantitative simulations a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particle motions the results suggest that these surface waves may be generated by the conversion of body waves at the bound ary of the western coastal plain and foothills

comparing tiltmeters for crustal deformation measurement a preliminary report a collection of high precision tiltmeters is being operated at pinon flat observatory southern california both to compare instruments and to measure tectonic deformation we report on 1 2 years of data from four of these two michelson gale long fluid tiltmeters one long center pressure tiltmeter and a shallow borehole tiltmeter the three long base instruments are all located on the same baseline with a precise leveling line running between their end monuments at nontidal frequencies only the two michelson gale instruments show some coherence 3 for periods of 2 to 4 days while the center pressure instrument is correlated with air temperature at periods from a few days to a few weeks the most stable tilt record shows a secular rate of 0 28 rad a which may be real over much longer times leveling to specially stabilized benchmarks should confirm this comparing instruments has identified more and less successful measurement techniques it appears that low noise data will most probably be produced only by relatively complex and expensive instruments though even for these the operating costs over any reasonable lifetime will exceed the capital cost even the best existing sensors must be improved to measure continuous tectonic motions

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

shallow velocity structure at the shagan river test site in kazakhstan during 1997 and 1998 twelve chemical explosions were detonated in boreholes at the former soviet nuclear test site near the shagan river sts in kazakhstan the depths of these explosions ranged from 2 5 to 550 m while the explosive yield varied from 2 to 25 tons the purpose of these explosions was for closure of the unused boreholes at sts and each explosion was recorded at local distances by a network of seismometers operated by los alamos national laboratory and the institute of geophysics for the national nuclear center nnc short period fundamental mode rayleigh waves rg were generated by these explosions and recorded at the local stations resultingly the waves exhibited normal dispersion between 0 2 and 3 seconds dispersion curves were generated for each propagation path using the multiple filter analysis and phase match filtering techniques tomographic maps of rg group velocity were constructed and show a zone of relatively high velocities for the southwestern sw region of the test site and slow propagation for the northeastern ne region for 0 5 sec rg the regions are separated by the 2 1 km sec contour as propagation in the sw is greater than 2 1 km sec and less in the ne region at 1 0 sec period the 2 3 km sec contour separates the two regions finally for 1 5 and 2 0 sec the separation between the two regions is less distinct as velocities in the ne section begin to approach the sw except for a low velocity region 2 1 km sec near the center of the test site local geologic structure may explain the different regions as the sw region is composed predominantly of crystalline intrusive rocks while the ne region consists of alluvium tuff deposits and paleozoic sedimentary rocks low velocities are also observed along the shagan river as it passes through the sw region of the test site for shorter period rg 0 5 1 0 sec iterative least squares inversions of the rg group velocity dispersion curves show shear wave velocities for the southwestern section that are on average 0 4 km sec higher than the ne region at depths greater than 1 5 km the statistical difference between the models is no longer significant the observed group velocities and different velocity structures correlate with p wave complexity and with spatial patterns of magnitude residuals observed from nuclear explosions at sts and may help to evaluate the mechanisms behind those observations

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

constraints on lateral variation in upper crustal structure beneath southern new england from dispersion of rg waves group velocity dispersion was determined for rayleigh waves between periods of 0 5 and 2 0 sec rg recorded from quarry blasts in southern new england the paths of these surface waves traverse various geologic structures and distinct dispersion regions have been identified normal dispersion was consistently observed in the period range studied indicating a superficial layer of relatively low velocity 4 1 vp 5 5 at least 1 km thick in the upper crust throughout this region seismic velocity in this upper layer varies laterally and correlates to some extent with geologic features the observed dispersion also indicates that seismic velocity at depths of 5 to 10 km beneath southern new england varies laterally

crystallization pressure versus crack seal as the mechanism for banded veins banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain advances in understanding the geochemistry of precipitation data on veins and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins we propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth as materials precipitate the vein walls are pushed apart the resulting veins have shapes typical for mode i cracks except that mechanically crystallization pressure replaces the role of internal fluid pressure in their propagation a nonzero remote differential stress serves to align veins mechanical and geochemical considerations suggest that this process will be most important in fine grained rocks such as greenschist grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass transport process veins may owe their orientation to tectonism but their initiation and growth are due to processes that supersaturate the pore fluid veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation cyclic quartz mica bands may indicate geochemical self organization at the vein wall driven by pressure solution enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall

rg observations from four continents inverse and forward modelling experiments summary using short period recordings of fundamental mode rayleigh waves rg from seven arrays on four continents we have explored structural properties in the respective siting areas through inversion of rg phase velocity dispersion characteristics the arrays studied were noress and arcess in norway geress in germany eskdalemuir in scotland yellowknife in canada gauribidanur in india and alice springs in australia the simplest model used consists of one layer over a half space with shear velocities and layer thickness as unknowns densities were held fixed while p velocities were constrained by a constant poisson ratio of 0 25 with one exception eskdalemuir the estimated s velocities were remarkably consistent between the arrays with an average value of 2 9 0 1 kms 1 in the upper layer and 3 55 0 1 kms 1 in the half space however estimated layer thicknesses varied considerably ranging from 0 12 km yellowknife to 1 6 km alice springs inversions were also performed with a simulated gradient layer near the surface model results and the ability to fit the observed velocity dispersion were very similar with the two types of models thus revealing a resolution problem with the limited frequency range available and the lack of higher modes to examine the effect of lateral inhomogeneities in terms of von karman velocity variations and topography on rg propagation 2 d finite difference synthetics were computed in both cases scattering was most pronounced for frequencies above 1 hz and thus explain the lack of rg phase velocity estimates for periods shorter than 0 6 s

velocity structure and seismicity of southeastern tennessee the seismic zone in southeastern tennessee is at the confluence of major crustal features which have been interpreted largely from potential data and their relation to seismicity could help us understand why major earthquakes sometimes occur in the eastern united states in this paper we solve for the previously unknown velocity structure of the upper crust by an inversion of travel time residuals from relocated earthquakes the gravity anomalies are included by using a linear relation between average anomalous density and average anomalous velocity the velocity model demonstrates that the seismicity is concentrated in areas of average to below average velocity and does not appear to be associated with one of the previously identified major crustal features the high velocity zones mark areas that are generally lacking in seismicity the association of earthquake hypocenters with regions of low velocity crustal rocks is consistent with other intraplate seismic zones and this association supports the conjecture that intraplate earthquakes occur in crust that may have been weakened the velocity anomalies at midcrustal depths do not support the new york alabama ny al lineament as a linear feature extending through southeastern tennessee and parallel to contours in gravity anomalies as originally proposed a continuation of the ny al lineament to the southwest requires either a 15 degree southwestward change in direction or a displacement to be consistent with the velocity anomalies the seismically active areas in southeastern tennessee do not appear to be constrained by the major crustal features but instead the seismicity is characterized by the distribution of hypocenters and their association with low velocity regions at midcrustal depths

reconstruction of deformation and flow parameters from deformed vein sets abstract in any homogeneously deformed material sets of material lines with a similar stretch history occupy sectors with a scale and orientation that depend on finite strain area change rate and vorticity of the parent flow if the size and shape of these material line sectors are known they can be used to determine the sense of shear and to calculate deformation and flow parameters even if actual stretches along material lines are unknown in rocks which have undergone near homogeneous deformation on a large scale the orientation of sets of boudinaged folded and folded boudinaged or boudinaged folded veins can be used for such calculations a general theoretical background to the problem is presented and a mohr circle construction to carry out the deformation and flow analysis is introduced among other statements the theory predicts that shortened or folded boudins must develop in non coaxial monophase deformation with considerable area change in the plane of observation or in polyphase deformation

a comparison of tiltmeters at tidal frequencies different techniques for measuring crustal deformation are now being tested at pinon flat geophysical observatory in southern california as a part of this comparison we have analyzed five months of data from four different tiltmeters three borehole instruments two 4 5 m and one 26 m deep and a long base 535 m instrument at the surface for periods from 1 hour to 10 days the signals from the deep borehole and long base instruments show comparable noise levels the shallow instruments are much noisier tidal analyses give results for the different sensors which agree to within the errors which are several percent

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

tilt observations from shallow and deep borehole arrays measurements of the earth tide using tilt and strainmeter data can be used to infer crustal structure however in almost all measurements of crustal motion using shallow tiltmeters and strainmeters reference is made to the degradation of the data implied to be caused by influence of meteorological parameters on near surface layers for example studies by wood and king and goulty et al indicated that a substantial portion of measured crustal motion by shallow tilt and strain data may be attributed to the weather these studies were limited to data outside the tidal band however it is important to determine the effects of weather on these data within the tidal band if shallow tiltmeters are to be used to determine crustal structure collocated deep and shallow borehole tiltmeter arrays were operated by the air force geophysics laboratory in eastern massachusetts and provided an opportunity to investigate weather effects on shallow tilt the tiltmeters installed in deep boreholes 100 m yielded internally consistent m2 amplitudes 2 and phases 3 cabaniss and mcconnell but the collocated shallow 4 m borehole tilt array did not produce the same results least squares analyses of the shallow tilt data revealed variations of 20 for m2 tidal amplitude compared to the deep data a multichannel wiener filter technique was used to investigate the hypothesis that this discrepancy was due to meteorological influence results indicated that approximately one half of the anomalous m2 power was correlated with air and borehole temperatures it is suggested that the remaining anomalous power may be due to ground deformation caused by the action of groundwater if this is true shallow tiltmeters might not be measuring the same deformation as the deep instruments and therefore not capable of measuring the earth tide with the accuracy required to determine crustal structure

a sensitive vorticity gauge using rotated porphyroblasts and its application to rocks adjacent to the alpine fault new zealand variable aspect ratio porphyroblasts deformed in non coaxial flow and internally containing rotated relicts of an external foliation can be used to characterise plane strain flow regimes the distribution obtained by plotting the orientation of the long axis of such grains classified by aspect ratio against the orientation of the internal foliation is potentially a sensitive gauge of both the bulk shear strain as previously suggested and kinematic vorticity number we illustrate the method using rotated biotite porphyroblasts in the alpine schist a sequence of mid crustal rocks that have been ramped to the surface along the alpine fault a major transpressional plate boundary results indicate that at distances greater than or equal to similar to1 km from the fault the rocks have undergone a combination of irrotational fattening and dextral oblique normal sense shear with a bulk shear strain of similar to0 6 and kinematic vorticity number of similar to0 2 the vorticity analysis is compatible with estimates of strongly oblate bulk strain of similar to 75 maximum shortening dextral reverse transpressional flow characterises higher strain s tectonite mylonite within similar to1 km of the alpine fault these relationships provide insight into the kinematics of flow and distribution of strain in the hangingwall of the alpine fault and place constraints on numerical mechanical models for the exhumation of these mid crustal rocks c 2001 elsevier science ltd all rights reserved

la maree d inclinaison a leduc alberta canada observation analyse et interpretation in geophysics the signal resulting from tidal forcing is the only one precisely known a priori its omnipresence in tilt measurements can be exploited to characterize the spatio temporal properties of rocks in situ but only if experimental uncertainties are less than 2 signal distortions due to unknown material and geometrical discontinuities around sensors limit the observational accuracy tilt tides were measured with a new biaxial tiltmeter installed in a 12 m borehole at leduc alberta where the geology is well defined both locally and regionally at the m frequency the observed parameters error of 2 5 are coherent with results previously obtained with near surface tiltmeters installed 100 m away but 5 times noisier the load tide due to oceans differs according to which configuration is used in its computation and the new measurements favor schwiderski s m model over that of parke and hendershott a topographic perturbation due to a small valley at 500 m from the borehole has been identified by comparing the tilt residuals for m and 0 with the strain tide the results indicate that even a smooth topography can perturb tidal observations to the point of hampering the interpretation of tilt anomalies in terms of regional tectonics

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks central southern alps new zealand the hanging wall of the alpine fault near franz josef glacier has been exhumed during the past similar to2 3 m y providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen presently exposed rocks of the pacific plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid crustal detachment initially they were transpressed across the outboard part of the orogen resulting in oblate fabrics with a down dip stretch later they encountered the alpine fault experiencing an oblique slip backshearing on vertical planes this escalator like deformation tilted and thinned the incoming crust onto that crustal scale oblique ramp this style of hanging wall deformation may affect only the most rapidly uplifting central part of the southern alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp angle at depth a 3d transpressive flow affected mylonites locally near the fault but their shear direction remained parallel to plate motion ruling out ductile extrusion as an important process in this orogen outside the mylonite zone late cenozoic shortening is inferred to be modest 30 40 as measured from deformation of younger biotite grains oblique collision is dominated by translation on the alpine fault and rocks migrate rapidly through the deforming zone preventing the accumulation of large finite strains transpression may play a minor role in oblique collision c 2001 elsevier science ltd all rights reserved

rotation of long tectonic clasts in transpressional shear zones abstract the two dimensional analysis of rotation of rigid ellipsoidal inclusions is not applicable to situations in which either the bulk deformation deviates from plane strain or one of the principal axes of the inclusion is at an angle to the vorticity vector both these situations may occur in certain types of transpressional ductile shear zones the model presented here shows how long cylindrical or ellipsoidal tectonic clasts with the longest axis parallel to the walls of the shear zone but at an angle to the vorticity vector rotate in transpressional shear zones long clasts initially at a low angle to the vorticity vector and at a large angle to the stretching lineation will rotate and tend to become subparallel to the stretching lineation the rotation of the principal axes of the elliptical cross section around the cylinder axis cannot be unlimited for any inclusion including the inclusion with circular cross section r 1 if the deformation is sufficiently large there is always a stable position of orientation the model explains simultaneous occurrence of monoclinic rolling structures in sections parallel and perpendicular to the vorticity vector depending upon the initial orientation the long axes of different clasts in the foliation plane may rotate clockwise and counterclockwise consequently opposite senses of asymmetry of rolling structures may appear in sections parallel to the vorticity vector

silurian deformation and metamorphism of ordovician arc rocks of the casco bay group south central maine the casco bay group in south central maine consists of a sequence of late cambrian to early ordovician interlayered quartzofeldspathic granofels and pelite cape elizabeth formation overlain by early to late ordovician back arc volcanic spring point formation and volcanogenic sedimentary rocks diamond island and scarboro formations these rocks were tightly folded and subjected to low pressure amphibolite facies metamorphism in the late silurian this phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in middle devonian early carboniferous time previously dated plutons within the sequence range in age from 422 389 ma and record a period of prolonged intrusive activity in the region similarities in age volcanic rock geochemistry and lithologic characteristics argue strongly for a correlation between rocks of the casco bay group and those in the miramichi belt of eastern maine and northern new brunswic

the use of mohr circles to describe non coaxial progressive deformation abstract mohr circles are useful in strain analysis but are also potentially powerful tools to translate geological fabric data into tensors describing the deformation and to reconstruct flow parameters this paper shows how mohr circles can be used to visualize the effects of variable flow parameters such as stretching rate volume change rate and vorticity number on parameters of cumulative deformation as described by the position gradient tensor a mean vorticity number is introduced as a deformation parameter besides finite cumulative strain and volume change

a mohr circle construction to plot the stretch history of material lines abstract mohr circles for stretch can be used to find the spatial distribution of material lines which have been shortened extended shortened then extended and extended then shortened in a material plane which underwent area constant homogeneous deformation by steady state flow the distribution is mainly a function of the vorticity of the parent flow and can be used to determine sense of shear finite strain and even vorticity and orientation of finite strain axes the method may be useful in theoretical and experimental studies and can help to understand the complex deformation patterns in sets of cross cutting veins in deformed rocks