isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

shallow virtual source redatuming by multi dimensional deconvolution recently new on shore acquisition designs have been presented with multicomponent sensors deployed in the shallow sub surface 20 m 60 m virtual source redatuming has been proposed for these data to compensate for surface statics and to enhance survey repeatability in this paper we investigate the feasibility of replacing the correlation based formalism that undergirds virtual source redatuming with multi dimensional deconvolution offering various advantages such as the elimination of free surface multiples and the potential to improve virtual source repeatability to allow for data driven calibration of the sensors and to improve robustness in cases with poor sensor spacing in the shallow sub surface resulting in a relatively high wavenumber content we propose a new workflow for this configuration we assume a dense source sampling and target signals that arrive at near vertical propagation angles first the data are preconditioned by applying synthetic aperture source filters in the common receiver domain virtual source redatuming is carried out for the multi component recordings individually followed by an intermediate deconvolution step after this specific pre processing we show that the downgoing and upgoing constituents of the wavefields can be separated without knowledge of the medium parameters the source wavelet or sensor characteristics as a final step free surface multiples can be eliminated by multi dimensional deconvolution of the upgoing fields with the downgoing fields

studies of the rooiberg group bushveld complex south africa no evidence for an impact origin abstract it has been suggested that the bushveld complex of south africa could be the result of multiple large meteorite or comet impacts according to this hypothesis part of the lower rooiberg group which forms the roof of the complex represents a sheet of impact melt breccia and other impact breccias the present study is an attempt to test the viability of the impact hypothesis for the bushveld complex by interpreting newly acquired field geochemical petrographic and textural data for rooiberg group and associated rocks extensive field work throughout the rooiberg group and particularly at the contact between this unit and the underlying pretoria group metasediments has failed to identify any material that could be interpreted as impact related the rooiberg group is predominantly composed of individual volcanic flows and pyroclastic units representing several geochemically distinct magma types these volcanic units are interbedded with thin laterally extensive sedimentary units a few of which are sedimentary breccias the presence of needles of quartz that may represent paramorphs after tridymite in some rooiberg group units has been used as evidence to support the contention that these rocks represent superheated impact melt however quartz paramorphs after tridymite have been recognized in terrestrial volcanic provinces e g the north shore volcanic group in northeastern minnesota structural data including dips of rooiberg group strata suggest that the lobate shape of the complex which resembles several closely spaced ring features is the result of post rooiberg group deformation microdeformation features in quartz from bushveld related rocks do not satisfy the criteria of shock metamorphic planar deformation features pdfs which would be characteristic of impact induced shock pressures between 10 and 30 gpa these data especially the absence of macroscopic and microscopic evidence of shock deformation in pre bushveld rocks are inconsistent with formation of the bushveld complex by impact

the sectional strain ellipse during progressive coaxial deformations the two dimensional strain history on a sheet which is inclined to the principal axes of the strain ellipsoid is considered even if the strain history in three dimensions is coaxial the two dimensional progressive deformation on the surface of the sheet is in general of a non coaxial type it is shown in this paper that the degree and sense of two dimensional non coaxiality is governed by the strain path followed during three dimensional coaxial deformation the general relationship is defined between the gradient of the strain path on the flinn strain ellipsoid diagram and the nature of the two dimensional strain increments for most strain paths an asymmetrical arrangement of structures in the oblique sheet is to be expected hence en echelon folds transected folds and extension veins with curved fibres could be produced during three dimensional coaxial deformation only if the strain path is of a rather special type will the deformation be coaxial in a two dimensional as well as a three dimensional sense

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

petrogenesis of collision related plutonics in central anatolia turkey central anatolia exhibits good examples of calc alkaline and alkaline magmatism of similar age in a collision related tectonic setting continent island arc collision in the central anatolia region late cretaceous post collisional plutonic rocks intrude palaeozoic mesozoic metamorphic rocks overthrust by upper cretaceous ophiolitic units to make up the central anatolian crystalline complex in the complex three different intrusive rock types may be recognised based on their geochemical characteristics i calc alkaline behrekdag cefalikdag and celebi ii subalkaline transitional baranadag and ii alkaline hamit the calc alkaline and subalkaline plutonic rocks are metaluminous i type plutons ranging from monzodiorite to granite the alkaline plutonic rocks are metaluminous to peralkaline plutons predominantly a type ranging from nepheline monzosyenite to quartz syenite all intrusive rocks show enrichment in lile and lree relative to hfse and have high 87sr 86sr and low 143nd 144nd ratios these characteristics indicate an enriched mantle source region s carrying a subduction component inherited from pre collision subduction events the tectonic discrimination diagram of rb vs y nb suggests that the calc alkaline subalkaline and alkaline plutonic rocks have been affected by crustal assimilation combined with fractional crystallisation processes the coexistence of calc alkaline and alkaline magmatism in the central anatolian crystalline complex may be attributed to mantle source heterogeneity before collision the former carries a smaller intraplate component and pre subduction enrichment compared to the latter either thermal perturbation of the metasomatised lithosphere by delamination of the thermal boundary layer tbl or removal of a subducted plate slab breakoff is the likely mechanism for the initiation of the post collisional magmatism in the complex

fast 3d target oriented reverse time datuming imaging of subsalt sediments is a challenge for traditional migration methods such as kirchhoff and one way waveequation migration consequently the more accurate twowaymethodofreverse timemigrationrtmispreferredfor subsalt imaging but its use can be limited by high computation cost to overcome this problem a 3d target oriented reverse time datuming rtd method is presented which can generate redatumed data economically in target areas beneath complex structures such as salt domes the redatumed datainthetargetareathencanbemigratedinexpensivelyusing a traditional migration method if the target area is much smaller than the acquisition area computation costs are reduced significantly by the use of a novel bottom up strategy to calculate the extrapolated green s functions target orientedrtdistestedon2dand3dseg eagesyntheticdata sets and a 3d field data set from the gulf of mexico results show that target oriented rtd combined with standard migration can image sediments beneath complex structures accurately with much less calculation effort than full volume rtm the requirement is that the area over the target zone is smallerthanthatoftheacquisitionsurvey

flooding the topography wave equation datuming of land data with rugged acquisition topography wave equation datuming overcomes some of the problems that seismic data recorded on rugged surface topography present in routine image processing the main problems are that 1 standard optimized migration and processing algorithms assume data are recorded on a flat surface and that 2 the static correction applied routinely to compensate for topography is inaccurate for waves that do not propagate vertically wave based processes such as stacking dip moveout correction normal moveout correction velocity analysis and migration after static shift can be severely affected by the nonhyperbolic character of the reflections to alleviate these problems i apply wave equation datuming early in the processing flow to upward continue the data to a flat datum above the highest topography this is what i refer to as flooding the topography this approach does not require detailed a priori knowledge of the near surface velocity and it streamlines subsequent processing because the data are regridded onto a regularly sampled datum wave equation datuming unravels the distortions caused by rugged topography and unlike the static shift method it does not adversely effect subsequent wave based processing the image obtained after wave equation datuming exhibits better reflector continuity and more accurately represents the true structural image than the image obtained after static shift

variable rates of late quaternary strike slip on the san jacinto fault zone southern california three strike slip displacements of strata with known approximate ages have been measured at two locations on the san jacinto fault zone minimum horizontal offset between 5 7 and 8 6 km in no more than 0 73 m y northeast of anza indicates 8 12 mm yr average slip rate since late pleistocene time two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the coyote creek fault in western imperial valley horizontal slip of 1 7 m has been calculated for the youngest sediment of lake cahuilla since its deposition 275 510 yr b p the corresponding slip rate is 2 8 5 0 mm yr right lateral offset of 10 9 m measured on a buried stream channel older than 5060 yr b p but younger than 6820 yr b p yields average slip rates for the intermediate time periods 400 to 6000 yr b p of 1 2 mm yr the average rates of slip for these three time intervals suggest a relatively quiescent period for the san jacinto fault zone from about 4000 b c to about 1600 a d to the extent that long term variations in seismic activity of major strike slip faults elsewhere are known fluctuations in slip rate for the san jacinto fault would not appear to be abnormal if the san jacinto and adjacent segments of the san andreas fault alternately assume dominant roles in absorbing motion between the pacific and american plates perhaps even more recently than 400 yr ago the san andreas fault south of the transverse ranges expressed most of the motion but has since become relatively inactive

a silicon memory of subduction subduction processes may have operated very early in earth s history according to the heavy silicon isotope compositions of archaean igneous rocks the silicon that precipitated out of the archaean oceans as chert was subducted and melted to yield seawater like heavy isotope signatures in early granitic rocks

source receiver interferometric redatuming using sparse buried receivers to address complex near surface environments a case study of seismic imaging quality and time lapse repeatability

geochemistry geochronology and cathodoluminescence imagery of the salihli and turgutlu granites central menderes massif western turkey implications for aegean tectonics abstract the menderes massif western turkey is an important metamorphic core complex located in the aegean region geochemical and geochronological data from this extensional domain facilitates our understanding of large scale extension of the earth s lithosphere s type peraluminous granites salihli and turgutlu that intrude the alasehir detachment which bounds the northern edge of the central menderes massif may have been generated due to subduction of the eastern mediterranean floor along the hellenic trench in situ th pb ion microprobe monazite ages from the granites range from 21 7 4 5 ma to 9 6 1 6 ma 1 higher uncertainty in the ages is attributed to monazite common pb but the range is consistent with cathodoluminescence cl imagery that document complex textures within the samples salihli and turgutlu granites share many similar characteristics including multiple generations of plagioclase some with shocked cores plagioclase replacing k feldspar and the development of myrmekite evidence for fluid interaction and multiple generations of microcracks and microfaults the granites may have evolved from compositionally distinct magma sources as salihli samples in general contain allanite as the major accessory mineral whereas turgutlu granites contain monazite however the cl imagery document similar alteration textures the youngest turgutlu monazite grain 11 5 0 8 ma is located in the rim of a k feldspar grain close to myrmekite and the youngest salihli monazite grain 9 6 1 6 ma is located in the outer edge of an altered plagioclase crystal these ages likely time episodes of deformation as the granites were subjected to exhumation along the alasehir detachment ages reported here are similar to dates constraining extension reported elsewhere in the aegean but indicate a level of complexity when linking movement within the menderes massif to the large scale geodynamic processes that created other metamorphic core complexes in the region difficulties exist in linking the ages obtained from the granites to specific tectonic events due to the presence of secondary alteration textures generations of mineral growth and multiple episodes of deformation

pleistocene brawley and ocotillo formations evidence for initial strike slip deformation along the san felipe and san jacinto fault zones southern california abstract we examine the pleistocene tectonic reorganization of the pacific north american plate boundary in the salton trough of southern california with an integrated approach that includes basin analysis magnetostratigraphy and geologic mapping of upper pliocene to pleistocene sedimentary rocks in the san felipe hills these deposits preserve the earliest sedimentary record of movement on the san felipe and san jacinto fault zones that replaced and deactivated the late cenozoic west salton detachment fault sandstone and mudstone of the brawley formation accumulated between 1 1 and 0 6 0 5 ma in a delta on the margin of an arid pleistocene lake which received sediment from alluvial fans of the ocotillo formation to the west southwest our analysis indicates that the ocotillo and brawley formations prograded abruptly to the east northeast across a former mud dominated perennial lake borrego formation at 1 1 ma in response to initiation of the dextral oblique san felipe fault zone the 25 km long

earthquake recurrence models and historical seismicity in the mexicali imperial valley abstract we have examined the historical seismicity of the mexicali imperial valley since about 1850 the largest observed earthquake in the area has been the magnitude m w 7 1 december 1934 event the magnitude distribution for large plate boundary rupturing earthquakes m 5 8 is not well fit by the gutenberg richter type b value model models which better fit the observed magnitude distribution are derived from a lognormal distribution of stress drops on faults of constant size and from an exponential distribution of rupture lengths as might be expected from randomly distributed rupture stopping asperities along the length of the fault this may be among the best data in the world for testing earthquake recurrence models e g time predictability slip predictability on a strike slip fault but the uncertainties are still too great to permit a unique interpretation

paleomagnetism of the ocotillo badlands southern california and implications for slip transfer through an antidilational fault jog abstract the right lateral coyote creek fault is the southernmost segment of the san jacinto fault zone in california at the ocotillo badlands an anticlinorium of locally updomed material is exposed at a step over between echelon segments of the fault here uplift and deformation most probably result from multiple slip increments transferred across the antidilational fault jog under the assumption that material away from the jog region deforms by rigid body translation within the 2 km wide fault jog contraction has occurred by folding of the quaternary sedimentary strata about east west trending hinge lines structural analysis of this deformation shows that folding has accommodated 800 m of fault slip transferred through the antidilational jog by comparison total right slip on the coyote creek fault is 2 5 km measured 25 km to the northwest at coyote ridge a magnetostratigraphic study of the deformed strata was undertaken to determine the longevity of this fault discontinuity at the ocotillo badlands the eroded core of the updomed material at the ocotillo badlands exposes a 325 m thick sequence of sedimentary rock made up by the lacustrine borrego formation 200 m and the overlying conglomeratic ocotillo formation 125 m stepwise thermal demagnetization of samples from the exposed strata suggests that the formation boundary also marks a magnetic polarity reversal of reversed field to normal field as one travels upwards through the composite section the inferred primary magnetization is probably produced by a detrital remanent magnetization drm and is often overprinted with a weak viscous present field component comparison with magnetostratigraphy of the borrego badlands 10 km to the northwest indicates that the reversal sampled is the onset of the jaramillo event 0 97 ma the presence of a 20 angular unconformity within the upper portion of the exposed stratigraphy shows that deformation within the ocotillo badlands began during deposition of the ocotillo formation shortly after the time of the field reversal if total slip on the coyote creek fault at the ocotillo badlands is greater than 800 m it appears that the fault jog has been a transitory feature within the fault zone with slip alternately bypassing it or being transferred across it such switchyard behavior of slip transfer through the fault jog has important implications for understanding structural controls on earthquake rupture

the equivalent strength of geometrical barriers to earthquakes we present a quantitative framework for evaluating the influence of non planar fault geometry on repeated seismic ruptures we model quasistatic ruptures on a non planar fault trace imbedded in a two dimensional elastic medium under in plane strain because of the presence of fault segments that are not parallel to the regional shear stress i e bends the apparent strength at a given point on the fault is not fixed but fluctuates with normal stress compressional features behave as increasingly strong barriers to fracture unless the stored normal stress is released in order to unlock the fault since slip on the fault itself cannot get rid of the normal stress this is achieved through the action of off fault morphological features such as secondary faulting folding and vertical motions that we introduce parametrically in the form of an aseismic relaxation the apparent strength of a fault bend will stabilize in a narrow interval of values after repeated ruptures characterized by a non dimensional hardness parameter whereby the relaxation rate is scaled by the tectonic loading rate on a fault structure having several small widely separated bends three families of events can be identified whose frequency and magnitude depend on the hardness relaxation parameter and the geometry small events that cluster in the tension zones of the bends intermediate size ruptures involving a single interbend segment and large ruptures that break through bends and link on or more interbend segments large multi segment events are most likely to occur for low values of the hardness i e fast relaxation and slow loading rate regions with compressional features act as barriers that stop most ruptures stress is stored at these sites until they themselves break and initiate motion on the smoother long reaches of the fault

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

on the relationship between the bushveld complex and its felsic roof rocks part 2 the immediate roof emplacement of large volumes of mafic magma into the crust undoubtedly causes significant thermal perturbation to the overlying crust despite the clear importance of the country rock in modulating the thermal evolution the bushveld complex little is known about the nature and extent of its roof zone this manuscript details the lateral variability of the rocks that make up the immediate roof of the intrusion in the eastern limb in the northern segment of the eastern bushveld the roof is dominated by thermally metamorphosed metapelites in the central segment the roof is dominated by highly metamorphosed meta volcanic rocks and their partially molten equivalents and in the southern segment the roof is likely composed of modestly thermally metamorphosed felsic volcanic rocks the variability of roof lithology is also reflected in the variability of floor rocks to the intrusion a new model for the emplacement of the eastern bushveld complex is proposed in which the mafic magmas intrude at a deeper level in the north and become shallower to the south

magnetic stratigraphy and a test for block rotation of sedimentary rocks within the san andreas fault zone mecca hills southeastern california a 500 m section of the palm spring formation in the southern mecca hills located within the san andreas fault zone in southeastern california has been paleomagnetically sampled to determine possible tectonic rotation in this area and to establish time stratigraphic control this work was partly stimulated by the fact that 80 km farther south previous studies demonstrated 35 of postdepositional rotation in the palm spring formation of the vallecito fish creek basin east of the elsinore fault several lines of evidence suggest that hematite is the main magnetic carrier of the mecca hills samples large anhedral hematite grains observed in magnetic extracts and a positive fold test imply a detrital origin of the remanence the polarity reversal patterns together with earlier vertebrate paleontologic studies restrict the time span for deposition of this unit to the middle late matuyama chron 2 0 0 75 myr ago thus of uppermost pliocene and early pleistocene age characteristic directions of best least squares fit for 73 samples suggest little or no overall rotation despite the severe late quaternary tectonic activity demonstrated by the intense deformation of these strata

linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence summary consider a sequence of say 10 to 20 vector observations in threedimensional space it is suspected that a few subsets of consecutive observations are made up of collinear points the purpose of this paper is to construct a statistically based algorithm to find such linear segments and to assess their accuracy a similar assessment is made for coplanar sets of points this algorithm is applied here to palaeomagnetic data and is claimed to be superior to previous methods of palaeomagnetic analysis in terms of completeness and balance of analysis treatment of measurement errors and other sources of scatter criteria for identification of linear and planar sets of points and statistical rigour stability spectra with statistically based confidence limits are obtained as a by product 1 palaeomagnetic analysis background to the geometrical problem the natural remanent magnetization nrm in rocks is known in general to have originated in one or more relatively short time intervals during their formation and subsequent history i e during rock forming or metamorphic events during which nrm is frozen in stabilized by falling temperature grain growth etc the nrm acquired during each such event is a single vector magnetization parallel to the then prevailing geomagnetic field and is called a component of nrm rocks which have suffered more than one event of this kind are therefore liable to carry multi component nrm in the laboratory these components can be identified by demagnetizing the rock step by step usually by thermal alternating field or chemical demagnetization which involves the analysis of the distribution of blocking rzmperature coercivity and chemical leaching respectively resistance to these treatments is called stability of remanence and the range of treatment over which a particular component is removed is called the stability range of the component the various discrete

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

rates of late cenozoic tectonism in the vallecito fish creek basin western imperial valley california the kinetics of continental rifting are recorded in the late neogene sediments of the western imperial valley california the gulf of california opened in its present form about 4 m y ago capturing the colorado river some 5 km of deltaic and fluvial sediments accumulated in the vallecito fish creek area during the interval 4 3 0 9 m y ago initial sedimentation rates were 5 5 mm yr 1 which diminished in an exponential fashion to 0 5 mm yr 1 when isostatic adjustments are accounted for this implies tectonic subsidence rates of 1 5 mm yr 1 decreasing systematically to 0 1 mm yr 1 after 0 9 m y ago right lateral shearing along the elsinore fault zone cut across the basin causing it to be tilted and uplifted at a mean rate of 5 9 mm yr 1 the basin was concurrently rotated as a unit 35 clockwise the dissected pediments and badlands that now characterize the area were formed and eroded in the past 0 9 m y

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

2 05 ga isotopic ages for transvaal mississippi valley type deposits evidence for large scale hydrothermal circulation around the bushveld igneous complex south africa abstract the known dimensions of hydrothermal fluid circulation associated with emplacement of the 2 055 ga bushveld igneous complex bic are extended with new isotopic ages for paleoproterozoic mineral deposits in wallrocks transvaal supergroup surrounding the bic a fluorite rich mississippi valley type mvt deposit in the zeerust district dated at 2 05 ga sm nd isochron age has nd isotopic compositions that are identical to coeval f rich lebowa granite late in the bic intrusive sequence suggesting transport of magmatic fluids 100 km beyond the present margins of the bic hydrothermal illite from the zn pb mvt bushy park deposit dated at 2 05 ga by k ar geochronology links widespread mvt deposits of the kaapvaal craton to large scale basinal brine circulation associated with emplacement of the bic these data in combination with gold vein phosphate mineral and paleomagnetic resetting ages indicate a regional scale bushveld age hydrothermal system circulating 700 km outward from the prese

focus on land seismic technology the near surface challenge a new era in land seismic is at hand growing energy demand will spur greater efforts in exploration and development into regions of the world where hydrocarbons occur in land environments interpreters will require accurate near surface solutions for mapping low relief structures and stratigraphic traps and to improve data quality for placing horizontal wells this will require solutions to near surface challenges such as energy penetration scattering source generated noise surface generated multiples statics and source and receiver coupling solutions to these problems will become possible due to advances in seismic acquisition including simultaneous source acquisition and wireless seismic driven ultrahigh channel systems additionally advances in seismic processing technology for processing the resulting huge data volumes will be made possible by continual growth in computational capability these advances will for the first time allow acquisition of true 3d seismic data this will not only re

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

age and geochemistry of the paleoproterozoic bhatwari gneiss of garhwal lesser himalaya nw india implications for the pre himalayan magmatic history of the lesser himalayan basement rocks

model based radiation pattern correction for interferometric redatuming in 4d seismic interferometric virtual source vs redatuming converts surface triggered source records into the equivalent records as if they originated from buried receiver locations by crosscorrelating downgoing waves with the corresponding upgoing waves theory suggests that when the receivers are surrounded by an enclosing boundary of sources then the vs has an isotropic radiation pattern and yield accurate response the resultant records should show improving the seismic repeatability and image quality comparing with non vs however in the presence of complex near surface intricate shallow structure and highly variable weathering layers can severely distort the ray path such that it produces the uneven angle coverage to the buried vs additionally near surface reverberations surface multiples and other mode converted waves may leak into the time gated early arrivals and further corrupt the direct wavefields abovementioned issues can result in distorted radiation patterns and contaminated responses of the v

pulsed emplacement of the mount kinabalu granite northern borneo abstract high precision u pb ion microprobe analyses provide new constraints on the emplacement and origin of the kinabalu granite in sabah northern borneo the granite is a sheeted laccolith like body comprising dyke fed granitic units that young downwards each emplaced beneath the previous sheet analyses of concentric growth zones in zircons indicate crystallization between 7 85 0 08 and 7 22 0 07 ma and show that the entire pluton was emplaced and crystallized within less than 800 ka several pulses of magmatism are recognized each lasting for a maximum of 250 ka and possibly as briefly as 30 ka the oldest ages coincide with the highest elevations whereas the youngest ages are found at lower elevations around the edge of the body based on these new age data and field observations we identify the biotite granodiorite hornblende granite and porphyritic facies as the upper middle and lower units respectively inherited zircon ages indicate different protoliths for the upper and middle units the upper unit is derived from attenuated continental crust of the south china margin subducted beneath sabah the middle unit is sourced from melting of the crystalline basement in sabah with little or no contribution from south china crust supplementary material full u pb ion microprobe analytical data and modal and major element composition data are available at http www geolsoc org uk sup18385

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

granitic rocks as a source of granite the gosta and sundsta granites south west sweden granites among them three generations of microline rich granites intruded repeatedly between 1600 and 900 ma in the south western swedish gneiss complex the deformed and metamorphosed gosta and sundsta granites are medium grained pale greyish red rocks belonging to the oldest generation of microclinerich granites

estimating interval shear wave splitting from multicomponent virtual shear check shots measuring shear wave splitting from vertical seismic profilingvspdata can benefit fracture and stress characterization as well as seismic processing and interpretation the classic approach to measuring azimuthal anisotropy at depth involves layer stripping its inherent weakness is the need to measure and undo overburden effects before arriving at an anisotropy estimate at depth that task is challenging when the overburden is complex and varies quickly with depth moreover vsp receivers are rarely present all the way from the surface to the target that necessitates the use of simplisticassumptionsabouttheuninstrumentedpartoftheoverburdenthatlimitthequalityoftheresult weproposeanewtechnique for measuring shear wave splitting at depth that does notrequireanyknowledgeoftheoverburden itisbasedona multicomponent version of the virtual source method in whicheachtwo component2 cvspreceiveristurnedinto a2 cshearsourceandrecordedatdeepergeophones theresultingvirtualdatasetisaffectedonlybythepropertiesofthe medium between the receivers a simple alford rotation transforms the data set into fast and slow shear virtual check shots from which shear wave splitting can be measured easilyandaccuratelyunderarbitrarilycomplexoverburden

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

body wave and surface wave analysis of large and great earthquakes along the eastern aleutian arc 1923 1993 implications for future events the 500 km long alaska peninsula and shumagin islands segments of the aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades to understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface we determine focal mechanisms depths and source time functions from seismic records for the largest events since 1917 the great earthquake of november 10 1938 and seven events with surface wave magnitude ms of 6 9 to 7 5 teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of november 10 1938 along the alaska peninsula body wave inversion of five p and four sh waves gives a duration of about 110 s in which moment was released in two episodes each of about 50 s duration with the second being larger than the first the first source was located in the general vicinity of the epicenter of the mainshock and the second which occurred about 60 s later was centered about 180 km to the northeast the body wave derived seismic moment is 3 7 1021 n m mw 8 3 we corroborate the body wave results by calculating surface waves by normal mode summation and comparing them with data at periods greater than 50 s an adequate fit to observed seismograms is obtained for either a single point source or two point sources with one located about 180 km northeast of the mainshock epicenter rupture in 1938 appears to have been confined to the alaska peninsula segment uniform rupture into the shumagin region is not supported by the data the ms 6 9 earthquake of may 13 1993 ruptured a small portion of the shumagin gap the earthquake of may 14 1948 ms 7 5 occurred on a shallow dipping thrust fault with a depth of about 31 km not 60 km as originally suggested five ms 7 events in the 1938 rupture zone have locations depths and mechanisms that define a shallow dipping 16 19 plate interface which shallows to 8 under kodiak island subduction of seamounts of the gulf of alaska seamount province may explain the location moment focal mechanism and depth of the five ms 7 events the plate interface dips nearly uniformly between the alaska peninsula and shumagin segments indicating that segmentation if any is not controlled by the orientation of the plate interface a slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the kodiak and alaska peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary

hafnium isotopic composition of the bushveld complex requires mantle melt upper crust mixing new evidence from zirconology of mafic felsic and metasedimentary rocks

coda wave interferometry for estimating nonlinear behavior in seismic velocity in coda wave interferometry one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time with this technique we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions this technique can be used in warning mode to detect the presence of temporal changes in the medium or in diagnostic mode where the temporal change in the medium is quantified

geochronology of the deep profile through archean basement at vredefort with implications for early crustal evolution the presence of archean granite forming the core of an updomed and overturned sequence of strata at vredefort south africa has been known for over 70 years recent geophysical geochemical and geological evidence has given rise to the proposal that the basement granite core has also been overturned presenting a section of the earth s granitic crust to view and that radial traverses in the vredefort basement penetrate through a 15 km thick section of the archean crust when moving from the contact of collar strata to the center of the dome the upper levels of the vredefort crust are dominated by a relatively homogeneous outer granite gneiss ogg displaying marked gradients in large ion lithophile elements these rocks grade inward into the deeper levels which occur in granulite facies metamorphism the deeper levels of the vredefort crust exposed in the central parts of the dome consist predominantly of felsic inlandsee leucogranofels ilg which occurs interleaved with inclusions of mafic metavolcanic and metasedimentary rocks termed steynskraal metamorphic zone smz a comprehensive isotopic study using four decay schemes 87rb 232th 238u and 235u on this crustal section is now reported the results permit unique and direct tests on the theories of archean crust forming processes in the upper vredefort crust whole rock isochron ages of 3050 m y are obtained from the four decay schemes and almost all evidence of an extended crustal history prior to 3050 m y has been obliterated establishment of strong vertical gradients in rb sr ratio shortly before setting of rb sr whole rock systems has wide implications for the interpretation of sr initial ratios although the vredefort upper levels of the crust has a low 87sr 86sr initial ratio of 0 7019 0 0002 it probably participated in a crustal prehistory for at least 500 m y in the deeper levels of the vredefort basement the geochronology is much more complex rb sr and th pb isochrons of 3500 m y are recorded in the mafic granulite relicts a companion paper welke and nicolaysen this issue provides evidence for an early crust forming event in this sector 3800 m y ago from 3500 m y onward these deeper crustal levels did not undergo addition of new archean crust forming material on a major scale nevertheless a most important younger archean event affected these levels 2800 m y ago cessation of high grade metamorphism is recorded by setting of the rb sr isotopic system in the felsic rocks and zircon u pb ages an episode of strong uranium depletion affected both the felsic and mafic rocks of this sector 2800 m y ago the inlandsee leucogranofels is evidently a rock unit with a long and complex history beginning with the emplacement as a crustal unit at least 3800 m y ago since then it has been subject to reworking and resetting of isotopic systems which terminated 2800 m y ago in the deeper crust there is scant evidence of a 3050 m y event similarly there is no record of a 2800 m y old imprint in the upper vredefort crust thus the last major events that affected the geochemical constitution of the vredefort upper and deeper crustal sectors differed in time and nature nevertheless they brought about isotopic setting in a downward moving sequence while the overlying crust remained cooler rigid and unaffected these events probably reflect significant discontinuities in the cooling of the archean crust

geochemical characterization and isotopic age of caradocian magmatism in the northeastern iberian peninsula insights into the late ordovician evolution of the northern gondwana margin this paper presents new geochronological geochemical and isotopic data for late ordovician magmatism 455 2 ma from the variscan massifs in the northeastern iberian peninsula this magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc alkaline ignimbrites and epiclastic volcanic rocks which were deformed and metamorphosed during the variscan orogeny the metaplutonic rocks are located in the lower part of a neoproterozoic early cambrian metasedimentary sequence in the canigo massif eastern pyrenees aluminous orthogneisses have isotopic signatures that indicate a crustal origin whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts the metavolcanic rocks define a calc alkaline ignimbritic sequence of crustal origin located in the lower part of the late ordovician sequence in the les gavarres massif catalan coastal ranges tdm ages for all the studied late ordovician rocks range from mesoproterozoic to paleoproterozoic 1 3 ga 1 5 ga 1 7 ga and 2 2 ga suggesting a neoproterozoic crust with paleoproterozoic components and an arc signature as the most probable source for the crustal melts the isotopic similarities between the studied samples and other early ordovician magmatic rocks of the central and western iberian massif suggest repeated melt extractions from a common basement as the source for the early and late ordovician magmatism in the iberian realm however the studied massifs are more closely linked to other massifs from the mediterranean realm the most probable tectonic setting for the emplacement of the late ordovician volcanic and plutonic bodies is that of an extensional regime which postdates a previous ordovician contractional event and the opening of the rheic ocean

collection of microparticles at high balloon altitudes in the stratosphere we report the collection of stratospheric particles at 34 36 km using balloon borne collectors the concentration of particles on the collection surfaces and element concentrations were measured on the majority of the particles using scanning electron microscope sem and proton induced x ray emission pixe analysis respectively particle morphologies elemental composition and electron diffraction data were obtained on a small number of the collected particles using transmission electron microscope tem techniques the concentration of particles between 0 045 lower imaging limit and 1 m in radius is several orders of magnitude above the blank levels on the collection surfaces while the concentration of particles above 1 m is near blank levels more than 106 submicron particles were collected the concentration of submicron particles is between 10 and 50 times the concentration expected at the sampling altitude based on models using the measured flux of extraterrestrial particles to the atmosphere the higher concentration of submicron particles may be due to contributions from volcanic particles inaccuracies in the influx of particles of this size to the earth or breakup of larger particles analysis of the elemental composition using pixe showed cl s ti fe br ni zr zn sr and cu in decreasing order of concentration elements below s could not be detected in the analysis cl s and br are believed to be present due to reaction of the collection surfaces and particles with atmospheric gases while the other elements are present in the particles the elemental composition of the particles does not allow an unequivocal origin to be assigned because particles from several of the possible sources listed above contain the measured elements tem analysis of 23 of the submicron particles showed 16 to be non graphitic the particles ranged from al rich silicates to almost pure fe to one containing almost exclusively ba and s none were definitely chondritic in composition this collection of particles complements existing collections obtained by aircraft they were colleted at much higher altitude and are predominantly submicron in size collection and analysis of particles at high altitudes using balloons presents unique challenges and opportunities in understanding the particle population in the stratosphere

theory of supervirtual refraction interferometry summary inverting for the subsurface velocity distribution by refraction traveltime tomography is a well accepted imaging method by both the exploration and earthquake seismology communities a significant drawback however is that the recorded traces become noisier with increasing offset from the source position and so accurate picking of traveltimes in far offset traces is often prevented to enhance the signal to noise ratio snr of the far offset traces we present the theory of supervirtual refraction interferometry where the snr of far offset head wave arrivals can be theoretically increased by a factor proportional to here n is the number of receiver or source positions associated with the recording and generation of the head wave arrival there are two steps to this methodology correlation and summation of the data to generate traces with virtual head wave arrivals followed by the convolution of the data with the virtual traces to create traces with supervirtual head wave arrivals this method is valid for any medium that generates head wave arrivals recorded by the geophones results with both synthetic traces and field data demonstrate the feasibility of this method there are at least four significant benefits of supervirtual interferometry 1 an enhanced snr of far offset traces so the first arrival traveltimes of the noisy far offset traces can be more reliably picked to extend the useful aperture of the data 2 the snr of head waves in a trace that arrive later than the first arrival can be enhanced for accurate traveltime picking and subsequent inversion by later arrival traveltime tomography 3 common receiver pair gathers can be analysed to detect the presence of diving waves in the first arrivals which can be used to assess the nature of the refracting boundary and 4 the source statics term is eliminated in the correlation operations so that the timing of the virtual traces is independent of the source excitation time this suggests the possibility of applying this method to earthquake data recorded by receivers that are inline with the refraction paths and source locations

near surface fault detection using high resolution shear wave reflection seismics at the co2crc otway project site australia high resolution near surface shear wave reflection seismic measurements were carried out in november 2013 at the co2crc otway project site victoria australia with the aim to determine and if so where deeper faults reach the near subsurface from a previous p wave 3 d reflection seismic data set that was concentrated on a reservoir at 2 km depth we can only interpret faults up to 400 m below sea level for the future monitoring in the overburden of the co2 reservoir it is important to know whether and how the faults continue in the subsurface we prove that two regional fault zones do in fact reach the surface instead of dying out at depth individual first break signatures in the shot gathers along the profiles support this interpretation however this finding does not imply perforce communication between the reservoir and the surface in the framework of co2 injection the shear wave seismic sections image with high resolution better than 3 m vertically and complementary to existing p wave volumes different tectonic structures similar structures also outcrop on the southern coast of the otway basin both the seismic and the outcrops evidence the complex youngest structural history of the area

geochemistry and tectonic implications of two ordovician bimodal igneous complexes southern french massif central two bimodal meta igneous complexes from the southern massif central provide important tectonic constraints on the initial stage of the palaeozoic orogen the metabasites may be subdivided into two groups according to the distribution of the large ion lithophile lil and high field strength hfs elements n type morbs with low lil hfs ratios and subduction type basalts with high lil hfs ratios these associations suggest a complex tectonic setting such as an extensional zone or a back arc basin above a subduction zone the great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust the association of terrigeneous sediments with n type morbs subduction type basalts and high silica peraluminous rhyolites argue against an intra oceanic setting and suggest that the area was one of ensialic crustal tension during middle ordovician the two complexes are identified as remnants of ensialic back arc basins rather than relicts of major oceanic sutures such marginal basins could have been located in a mainly continental domain such as the armorican microplate

on the relationships between the bushveld complex and its felsic roof rocks part 1 petrogenesis of rooiberg and related felsites a major question concerning the bushveld complex is the relationship between the layered mafic rocks and the overlying rooiberg group felsites and related granophyres here we assemble bulk rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks the data indicate that the rooiberg group consists of distinctive magnesian and ferroan lavas the former dominates the basalts to rhyolites of the basal dullstroom formation while nearly all the dacites to rhyolites of the overlying damwal kwaggasnek and schrikkloof formations are ferroan the ferroan rocks also include the stavoren granophyre which exists regionally as a several hundred meter thick concordant sheet between the bushveld complex and rooiberg lavas the compositions of the magnesian lavas are similar to calc alkaline granitoids found in convergent margins suggesting that the lavas could have originated in a mantle affected by previous archean subduction events that are recorded by xenoliths and inclusions in diamonds from most kaapvaal kimberlites in contrast the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings the hypothesis that these rocks are fractional crystallization products of bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations based on compositional characteristics and geologic relations the stavoren granophyre is the most likely candidate for the residual liquid that escaped from the top of the bushveld complex whether the bulk of the bushveld province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper hidden crustal magma reservoir remains unclear

voluminous acid volcanism in the busheveld complex a review of the rooiberg felsite the 2 1 b y old rooiberg felsite roofs and is intruded by the mafic layered rocks and granites of the bushveld complex the felsite unit which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300 000 km3 is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types volcanic rocks of more mafic composition occur towards the base of the sequence the rooiberg episode essentially terminated the volcanic activity in the transvaal basin and heralded the emplacement of the bushveld complex

early pleistocene initiation of the san felipe fault zone sw salton trough during reorganization of the san andreas fault system structural and stratigraphic analyses along the western margin of the salton trough show that the san andreas fault system was reorganized in early pleistocene time from a system dominated by two fault zones the san andreas fault and the west salton detachment fault to a network of dextral faults that include the san andreas and at least four dextral faults to the southwest the san felipe fault zone one of these dextral faults has 5 8 2 8 km of right separation and consists of three principal faults in the peninsular ranges these are the san felipe fault in the wnw sunset fault in the middle and fish creek mountains fault in the ese they form a left stepping array and bound domains in which the sunset conglomerate the older west salton detachment fault and cretaceous mylonitic rocks below the detachment are folded about wnw trending folds a complex flower structure within the left stepovers probably produced this fault parallel folding because all the rocks within stepovers of the san felipe fault zone from cretaceous to pleistocene are deformed about wnw trending folds and record broadly similar shortening strains we infer a quaternary age of deformation parts of the san felipe fault zone cut latest pleistocene to holocene surficial deposits and the fault zone is likely active evidence for early pleistocene initiation of the san felipe fault zone is preserved in conglomerate ne of the sunset fault poorly sorted angular boulder conglomerate and pebbly sandstone of the sunset conglomerate are 600 m thick and lie in angular unconformity on the pliocene palm spring group the conglomerate coarsens upward and toward the fault and is dominated by plutonic clasts derived from sw of it conglomerate beds contain up to 10 sandstone clasts recycled from older basin fill and accumulated in proximal to medial alluvial fans that were shed to the ne from uplifted rocks along the then active sunset fault based on lithologic stratigraphic structural and compositional similarities we correlate the sunset conglomerate to the pleistocene ocotillo formation clasts of recycled sandstone record erosion of detachment related basin fill that predates the san felipe fault and once covered the vallecito and fish creek mountains these crystalline cored mountain ranges first emerged from beneath basin fill during early slip above the nascent san felipe fault ca 1 1 1 3 ma later the san felipe fault zone cut upward folded cut across and deactivated the west salton detachment fault within a 9 km wide contractional bend and pair of left steps areas that accumulated sediment within this step over zone between ca 1 1 and ca 0 6 ma are currently being inverted and folded initiation of the san felipe fault in early pleistocene time was a significant event in the reorganization of the southern san andreas fault system the quaternary dextral faults broadened the plate boundary zone south westward from roughly 25 km during coeval slip on the san andreas fault and west salton detachment fault to 50 70 km and mark a change in the dominant structural style from transtension to distributed dextral faulting south of the big bend

enhanced refractor imaging by supervirtual interferometry refraction surveys are a well established method of imaging subsurface velocities both in terms of the deep crustal structure at global scales and in the shallow near surface these surveys generally involve deploying an array of receivers on the surface or water bottom and recording arrivals from a seismic source initiated at or near the surface

the chronology of magma influxes to the eastern compartment of the bushveld complex as exemplified by its marginal border groups the floor contact of the eastern compartment of the bushveld complex has been mapped in detail for 130 km field relations petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated these represent the 4 main magma influxes which formed the malic phase of the bushveld complex and are in chronological order 1 a contaminated magnesian basalt b1 which gave rise to a saucer shaped laminated marginal zone at the base of the complex with diverse related sills 2 a harrisitic peridotite representing quenched lower zone magma 3 a border group to the more evolved probably basaltic critical zone magma which was intruded in two phases an early quenched norite facies n being followed by a chilled bulk critical zone facies b2 which forms a skin to the whole critical zone finally 4 a border group to the main and upper zone b3 which represents an early magnesian pulse of this last immense volume of tholeiitic liquid there is also evidence for minor influxes of magma within the critical zone and main zone

estimating near surface shear wave velocities in japan by applying seismic interferometry to kik net data 1 we estimate shear wave velocities in the shallow subsurface throughout japan by applying seismic interferometry to the data recorded with kik net a strong motion network in japan each kik net station has two receivers one receiver on the surface and the other in a borehole by using seismic interferometry we extract the shear wave that propagates between these two receivers applying this method to earthquake recorded data at all kik net stations from 2000 to 2010 and measuring the arrival time of these shear waves we analyze monthly and annual averages of the near surface shear wave velocity all over japan shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data the estimated shear wave velocities of each year are stable for the niigata region we observe a velocity reduction caused by major earthquakes for stations on soft rock the measured shear wave velocity varies with the seasons and we show negative correlation between the shear wave velocities and precipitation we also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization this allows us to estimate the polarization with the fast shear wave velocity throughout japan for the data recorded at the stations built on hard rock sites the fast shear wave polarization directions correlate with the direction of the plate motion

leucogranites of the himalaya karakoram implications for magmatic evolution within collisional belts and the study of collision related leucogranite petrogenesis abstract two types of leucogranite have been identified in a collisional setting in the himalaya type a e g manaslu high himalaya are syn tectonic crustal melts related to hot over cold thrusting and subsequent fluid fluxing type b e g baltoro karakoram is related to lower crustal melting during following crustal thickening and thermal re equilibration followed by crystal fractionation for type b melting is triggered promoted in the lower crust by input of heat and material from the sub continental lithospheric mantle which is contemporaneously melting due to removal of the slab input and downward displacement both these granite types can be explained in terms of recent thermal models for continental collision and so might be expected to be found in other orogenic terranes these models also show that extension and thinning will occur as a result of the crust becoming gravitationally unstable due to thickening some time after collision thinning will lead to upwelling and melting of asthenosphere which could then intrude the base of the crust and promote further melting this could explain the common occurrence of a type granitoids as final plutonic events in orogenic terranes following intrusion of the collision related type a and type b granites existing genetic classification schemes only classify granites in terms of source type or broad environmental affinity e g collisional to further subdivide such broad environmental fields it is considered that processes and a general evolutionary model must be used source type is not suitable due to the number of source components that are often involved and the overlap of these components from one environment to another one or two selected features of a granite are not however sufficiently exclusive to identify different petrogenetic histories we propose that an evaluation of field style mineralogy isotope and elemental geochemistry should be used to subdivide the collisional granite field into type a and type b or manaslu and baltoro types respectively

patterns of seismic release in the southern california region southern california experiences earthquakes on the san andreas system of vertical right lateral predominantly strike slip faults and on a second system of faults that includes thrusts oblique slip left lateral and other faults pattern recognition and cluster analysis are used to analyze the catalog of earthquakes with magnitudes 5 5 from 1915 to 1994 we use pattern recognition to find a suite of traits that would characterize each of these two systems and distinguish them from each other both pattern recognition and cluster analysis show that epochs of seismic release occur in which one or the other system is the predominant form of earthquake activity for the past 2 decades the second system has been the active one small changes in the direction of plate movements could account for this phenomenon seismic release on the san andreas system is preceded by episodes of activity in the great basin or in the gulf of california presumably these episodes would represent extension in the former region and spreading and slip on transform faults in the latter

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

virtual source applications to imaging and reservoir monitoring the virtual source method is a breakthrough that allows us to image and monitor the subsurface in cases where surface seismic or vsp fail to deliver in settings where the overburden is complex or changing traditional time lapse signals are weak or nonrepeatable leading to ineffective seismic input to reservoir modeling and little of value generated by repeated seismic surveys on the other hand the virtual source method allows us to image under complex overburden yields repeatable data for reservoir monitoring enables shear seismic and may help us look ahead as we drill

microearthquake s wave observations from 0 to 1 km in the varian well at parkfield california abstract high gain three component seismometers from 0 to 1 km deep along the varian a 1 well at parkfield california were used to record the waveforms of nearby microearthquakes despite being in the thick tertiary sediments of the parkfield syncline the s wave amplification at this site is only about a factor of 3 the spectral content and spectral ratios of s waves along the well show that the average qs in the top 1 km at this site is 37 with the qs in different subintervals varying between 8 and 65 based on initial s wave polarizations a complex s wave velocity structure must exist at and below the varian site this structure appears to include position dependent anisotropy as well as steep lateral velocity gradients at a depth of 1 km s wave splitting parallel and normal to the san andreas fault zone is consistently observed this splitting scales at roughly 0 01 sec km subsequent to the split s waves the particle motion seems to be controlled by event focal mechanism above 1 km the upgoing s waves attenuate and change directions of polarization with a new splitting rate of 0 1 sec km uniquely for some events on the san andreas fault immediately below the varian site large post s wave signals with normal dispersion are present we propose that these phases are fault zone guided waves channeled from the san andreas fault to the varian site along the gold hill fault

predictive painting of 3d seismic volumes predictive painting is a numerical algorithm that spreads information in 3d volumes according to the local structure of seismic events the algorithm consists of two steps first local spatially variable inline and crossline slopes of seismic events are estimated by the plane wave destruction method next a seed trace is inserted in the volume and the information contained in that trace is spread inside the volume thus automatically painting the data space immediate applications of this technique include automatic horizon picking and flattening in applications to both prestack and poststack seismic data analysis synthetic and field data tests demonstrate the effectiveness of predictive painting

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

velocity independent redatuming a new approach to the near surface problem in land seismic data processing it is well known that a complex near surface encountered in areas such as desert mountainous and glacial till environments can significantly distort primary reflections from deeper horizons of interest this has been and remains a major problem in seismic data processing

stratigraphic record of pleistocene faulting and basin evolution in the borrego badlands san jacinto fault zone southern california sedimentary rocks in the borrego badlands southern california contain a record of pleistocene crustal deformation during initiation and evolution of the san jacinto fault zone we used detailed geologic stratigraphic and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault controlled sedimentation in this area the base of the 300 to 500 m thick ocotillo formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the borrego formation at 1 05 0 03 ma this coincides with regional scale progradation of ocotillo formation sand and gravel and appears to record initiation of strike slip faults in the southwestern salton trough at ca 1 1 ma thickness trends clast compositions paleocurrents and distribution of paleosols provide evidence for initiation of the east coyote mountain fault at ca 1 05 ma followed by onset of nne ward basin tilting obliquely toward the santa rosa segment of the clark fault at ca 1 0 ma stratigraphic omission of the ocotillo formation and progressively older units southwest of the coyote creek fault beneath the fonts point sandstone provides evidence that tilting to the northnortheast was related in part to growth of the san felipe anticline during deposition of the ocotillo formation map and stratigraphic data suggest that the coyote creek fault in the western borrego badlands postdates ocotillo deposition and thus appears to have propagated southeast into the study area at ca 0 6 ma the fonts point sandstone is a thin sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the borrego badlands the base of the fonts point sandstone changes from a conformable contact in a narrow belt southeast of the inspiration point fault where it is dated at 0 6 0 02 ma to an angular unconformity on the folded ocotillo formation northwest of the fault the pattern of stratal truncation records initiation of the inspiration point fault at ca 0 6 ma this coincides with a major structural reorganization in the san jacinto fault zone that initiated the modern phase of north south shortening and erosion in the southwestern salton trough

geochemical evolution of the rooiberg silicic lavas in the loskop dam area southeastern bushveld the rooiberg felsite is the voluminous volcanic precursor to the plutonic episodes of bushveld magmatism in the area north of the loskop dam a 3 520 m thickness of felsite is dominated by flows of dacitic and rhyodacitic lava occasional silicic andesites are developed near the base whereas true rhyolites become overwhelmingly dominant toward the top of the sequence nine major stratigraphic units of lava are present these are usually separated by and sometimes include volumetrically minor pyroclastic and sedimentary deposits whole rock chemical data on 165 lava samples embracing the entire stratigraphy suggest that nonuniform alteration processes have been most effective in the upper units nevertheless field petrographic and chemical criteria enable identification of three felsite magma types a high mg type mgo 1 7 containing low zr and y contents is present only within the lowermost 1 400 m of section a low mg type mgo 2 asymp 1 p 2 o 5 asymp 0 35 type which only occurs in the lower part of the succession the high mg lavas exhibit a systematic eruptive progression from more to less evolved compositions apparently reflecting continuous tapping of successively deeper levels of a normally zoned magma chamber during the early volcanism the low mg lavas were erupted intermittently with the high mg types probably from a separate and much larger chamber pronounced differentiation of this magma is only evident in the youngest volcanic units where several element profiles display conspicuous steplike variations across the prominent ash flow horizons the steps reflect upward enrichments in nb zr and y and corresponding incremental depletions in ti p and sc such trends imply the existence of a discontinuous but reversed compositional zonation in the evolved low mg chamber the high fe ti p lavas were also erupted from the low mg chamber this may indicate the development of an early type of chemical stratification that was markedly different from the compositional layering in the evolved system

wavelet crosscorrelation based interferometric redatuming in 4d seismic interferometric virtual source redatuming crosscorrelates downgoing waves with the corresponding upgoing waves to convert records from surface source gathers to virtual gathers at the buried receiver locations the assumption is that the virtual source records show reduced effects of overburden complexity and therefore provide improved seismic repeatability and image quality in practice however the repeatability and quality of the redatumed data can be degraded by one sided illumination and inadequate preprocessing in the presence of complex near surface structure for land redatuming intricate shallow structure and highly variable weathering layers introduce surface multiples ground roll scattering noise interferences among different wave modes which additionally contaminate both down and upgoing wavefields to address these issues we propose a new interferometric redatuming method based on crosscorrelation in the wavelet domain specifically by transforming time offset data into local time sca

influence of near surface volcanic structure on long period seismic signals and on moment tensor inversions simulated examples from mount etna 1 long period lp seismicity on volcanoes is thought to be associated with moving fluids or resonating fluid filled conduits hence lp moment tensor mt source inversions might have a direct bearing on our understanding of the plumbing system using 3 d full wavefield simulations and 2 d sensitivity kernels in a digital elevation model of mount etna we investigate the influence of near surface volcanic structure on lp signals and on moment tensor inversions contrary to common wisdom in crustal seismology we find that despite their relatively long wavelengths lps are severely distorted by near surface structures including layering and topographic features in particular near surface low velocity structures which are commonly observed on volcanoes play a critical role in controlling the nature of lp signals if not accounted for these path effects leak into the source solution leading to the emergence of erroneous source geometries spurious forces and incorrect source time functions this is particularly problematic if one adopts an unconstrained solution space for the source with many free parameters hence there is a fine balance in the trade off between the velocity model and the source in the absence of high resolution near surface velocity control we demonstrate the importance of employing a priori source information from other fields e g structural geology for shallow lps constraining the number of free parameters in the inversion a probabilistic approach should then be taken as the model with the best fit is not necessarily the true solution

a mid crustal contractional stepover zone in a major strike slip system north cascades washington abstract the segmentation of strike slip faults at the earth s surface is well known but the geometry of these faults in the mid crust is poorly understood as are the intervening stepover zones a mid crustal example of a segmented strike slip fault is present within the 500 km long ross lake fault system where the nw striking twisp river fault zone and gabriel peak tectonic belt occur as overlapping left stepping dextral strike slip shear zones the contractional stepover between these structures is occupied by the black peak batholith which is riddled by ductile shear zones that are typically 0 5 1 5 m thick and are restricted to the stepover gently to moderately nw dipping reverse slip ductile shear zones in the center of the stepover strike at a high angle to the twisp river fault zone and gabriel peak tectonic belt and lie in an appropriate position to transfer most of the displacement between the strike slip segments the orientation of the reverse slip ductile shear zones relative to the strike slip segments is broadly similar to that of folds and thrusts in upper crustal stepovers the stepover width is relatively large however in comparison to many active stepovers emplacement of the black peak batholith across an originally narrower stepover or bend in the fault system may have created a rigid buttress and subsequent deformation jumped to the mechanically weak southwest margin of the body because plutons are commonly intruded into strike slip zones such partitioning of later displacements may be widespread

wave equation velocity replacement of the low velocity layer for overthrust belt data seismic land data are commonly plagued by nonhyperbolic distortions induced by a variable near surface low velocity layer lvl first arrival refraction analysis is conventionally employed to estimate the lvl geometry and velocities then vertical static time shifts are used to replace the lvl velocities with the more uniform faster velocities that characterize the underlying refracting layer this methodology has earned a good reputation as a geophysical data processing tool however velocity replacement with static shifts assumes that no ray bending occurred at the lvl base and that waves propagated vertically through the lvl even though conventional refraction analysis methods which are used to derive lvl models from seismic data are less restrictive these assumptions often are inadequate in thick complex lvl situations where resulting errors may considerably hamper a statics based velocity replacement procedure wave equation datuming may be used to perform lvl velocity replacement when statics are inadequate this method extrapolates the seismic data from the surface to the lvl base with the lvl velocities then it extrapolates the data from the lvl base to an arbitrary datum with the replacement velocity field the marine analog of such a procedure has been well documented in the geophysical literature where the object is to remove distortions caused by an irregular water layer application of wave equation datuming to land data is more difficult because of certain common characteristics of land data irregular shooting large data gaps and crooked line geometry combined with lower signal noise and because the lvl estimation procedure is considerably more difficult we demonstrate wave equation velocity replacement on land data from a western u s overthrust belt the lvl in this region was particularly thick and complicated and ideal for a wave theoretical velocity replacement procedure standard refraction analysis techniques were employed to estimate the lvl then wave equation datuming was used to perform the velocity replacement our derived lvl model was not perfect so some imaging errors were expected because wave equation datuming is highly dependent upon the lvl model nevertheless our results show that wave equation datuming generally allowed better shallow reflector imaging than could be achieved with conventional statics processing

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

robust estimates of 3d reflector dip and azimuth much of seismic stratigraphy is based on the morphology of seismic textures the identification of reflector terminations and subtle changes in dip and azimuth allows us to infer coherent progradational and transgressive packages as well as more chaotic slumps fans and braided stream complexes infill of karsted terrains gas seeps and of course faults and angular unconformities a major difficulty in estimating reflector dip and azimuth arises at discrete lateral and vertical discontinuities across which reflector dip and azimuth change the smearing across these boundaries produced by traditional dip and azimuth estimations is avoided by using temporally and spatially shifted multiple windows that contain each analysis point this more robust estimation of dip and azimuth leads to increased resolution of well established algorithms such as coherence coherent amplitude gradients and structurally oriented filtering more promising still is the analysis of high resolution dip and azimuth through vol

fertile granites of precambrian rare element pegmatite fields is geochemistry controlled by tectonic setting or source lithologies abstract a total of twenty fertile granites and their rare element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature namely li rb cs be ga sn nb ta y ree sc ti zr be th u f nyf the range of t time difference between metamorphism and or deformation and granite intrusion is 20 to 465 ma for the examined cases there is some tendency toward syn to late orogenic origin of lct s as opposed to post to anorogenic timing of nyf s but the correlation is poor compared to the previously accepted stereotype similarly both metaluminous and peraluminous fertile granites are about equally spread among syn late and anorogenic environments lct systems are largely associated with peraluminous granites whereas all metaluminous to peralkaline plutons bear the nyf signature tectonic control on geochemistry of granite pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts the lct type suites are derived from undepleted upper crustal lithologies suffering their first anatectic event which mobilizes the most volatile components into low temperature low percentage melts supracrustal sequences as well as ortho and para lithologies of their basement serve as protoliths with variable degrees of involvement the nyf type suites whose plutons have the geochemical signature of a granites may be derived by melting of depleted lower crustal sources residual after preceding anatectic events or possibly by melting of juvenile lithologies with short crustal residence differentiation from mantle derived magmas is highly improbable volatile extraction of nyf elements from the mantle and their pre enrichment in protoliths or melts seems to be required the few mixed systems with variable expressions of a nyf lct signature are derived basically as nyf suites but contaminated by the lct elements from undepleted upper crust via several possible mechanisms

folds and the strain ellipsoid a general model abstract a new general model is proposed to account for non orthogonal relationships of fold axes and axial surfaces to the axes of the finite strain ellipsoid descriptions in the literature of such relationships e g plunge variations and transecting cleavage are reviewed and the development of the model is outlined the model is described in which plane strain k 1 or slight flattening strain k 0 71 or slight constrictional strain k 1 85 are imposed on a suite of layers with oblique orientations to the bulk strain axes x y z folds will initiate perpendicular to the direction of shortening in the layer and develop as non material lines moving through the layer by angular migration at geologically realistic finite strains the folds will be transected by xy cleavage in slight flattening strain folds are likely to be cylindroidal with axes not measurably oblique to xy in slight constrictional strain en echelon periclines are predicted with marked transection by cleavage in some examples the interference of two directions of folding will give rise to severe plunge variations and even downward facing although strain markers frequently indicate finite strains in the flattening field this may result from the superimposition of slightly constrictional tectonic strain on compactional flattening

mineral disequilibrium in the merensky reef western bushveld complex south africa new sm nd isotopic evidence the sm nd isotopic data from the merensky reef and its immediate footwall suggest that orthopyroxene and plagioclase are in isotopic disequilibrium with one another such that plagioclase is enriched in radiogenic nd relative to a bushveld aged reference isochron passing through the pyroxene and whole rock data the whole rocks and orthopyroxenes give end values at 2 06 ga between 7 46 and 8 46 whereas the plagioclases have end values of 1 13 to 3 37 orthopyroxene was derived from a liquid affected by pre emplacement crustal contamination and settled by density driven accumulation into a liquid crystal mush dominated by plagioclase derived from a relatively uncontaminated previous liquid isotopic constraints on the potential contaminant favour pre emplacement contamination by late archaean granitoids and also require that orthopyroxene preceded plagioclase on the liquidus of the incoming magma the presence of isotopic disequilibrium suggests that isotopic compositions of whole rock samples in cumulus rocks must be interpreted with caution

an isotopic study on the volcanics of the rooiberg group age implications and a potential exploration tool many geochronological studies on silicic magmatic rocks associated with the bushveld complex rhyolitic lavas of the rooiberg group and granites of the lebowa granite suite have shown evidence of open system behaviour of the rb sr and pb pb isotopic systems until 1600 1000 ma many hundreds of million years after crystallisation of these rocks this pervasive open system behaviour has been attributed to sustained hydrothermal circulation driven by the high heat productivity of the bushveld granites new sr and pb isotopic data are presented for basaltic to rhyolitic volcanics from the rooiberg group of the transvaal sequence in the dullstroom loskop dam area of the eastern transvaal these data show little evidence of open system behaviour after about 1950 ma and many sample suites retain ages which could reflect the formation of the rooiberg group i e older than 2070 ma it is argued that this preservation is due to the absence of fractionated fluid vapour rich bushveld granites in the immediate vicinity of the volcanic occurrences rooiberg group volcanics with extensively perturbed rb sr and particularly pb pb isotopic systems reflect the action of granite derived hydrothermal fluids as a consequence the isotope systematics in these volcanics could prove a useful exploration tool for sites of granite derived metal deposits

geochemistry of bimodal amphibolitic felsic gneiss complexes from eastern massif central france high grade basic and acidic meta igneous rocks are widespread in the bimodal amphibolitic felsic gneiss complexes which are characteristic formations of the middle allochthonous unit from eastern and southern french massif central the metabasites from the lyonnais and doux complexes are chemically diverse and range from n morb type tholeiitic to transitional types the two populations are not related by fractional crystallization or crustal contamination processes and their chemical characteristics reflect differences in their mantle sources an ensialic setting is supported by the crustally derived character of some of the associated felsic rocks but the presence of n morb type metabasites argues for an extensional environment this bimodal association compares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic crust formation during the cambro ordovician spreading event