coastwise transport reconsidered lateral displacements in oblique subduction zones and tectonic consequences coastwise transport margin parallel displacement of arc and forearc slivers is a common feature of zones of oblique subduction today but at rates that rarely exceed a few tens of kilometers per million year paleomagnetic studies strongly indicate that coastwise transport in the north american cordillera went on during late cretaceous early tertiary time at rates several times higher than this other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over riding crust the analysis developed here indicates that in the absence of a buttress a physical impediment to sliver motion at its leading edge sliver motion should obey the relationship tan p r tan 1 1 r2 sin2 12 where is the angle of obliquity is the angle of subduction and r rfrs is the ratio of resistance to slip on strike slip faults cutting the over riding plate to resistance to slip on the subduction zone in this equation p is a partitioning factor 0 p 1 where p 1 implies oblique subduction and no sliver motion and p 0 is the case in which the sliver moves at velocity v sin equivalent to the transverse component of relative motion rates of coastwise transport calculated from this relationship are systematically too high presumably indicating that most modern subduction zones are buttressed key factors in the buttress effect are 1 the greater strength of oceanic vs continental lithosphere and 2 the energy required to thicken continental crust large scale tectonic transport in the north american cordillera probably was the result of a combination of favorable factors long continued highly oblique convergence and a plate margin geometry that avoided the buttress problem

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

one way one return boundary element method and salt internal multiples surface related multiples or internal multiples can produce artifacts in migrated images using one way wave equation methods salt body related internal multiple removal remains a challenging problem for this approach to investigate the effects of salt body related internal multiples on subsalt imaging the boundary element method is implemented in a one way one return scheme to separate the primary transmission reflection arrivals at the salt boundaries from the internal multiples numerical results demonstrate that the proposed method is effective for modeling primary transmission reflection arrivals through from irregular interfaces between strong velocity contrast media internal multiples are obtained through straight subtraction of primary reflection arrivals from full reflected wave data the image from prestack depth migration of these pure internal multiples using one way propagators is compatible with some strong artifacts in the subsalt region from migration of full wave data using the same one way propagators results suggest that a significant part of artifacts in the subsalt region in the prestack image for the 2d seg eage salt model using one way propagators might be caused by salt internal multiples

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

fracture network fluid pathways and paleostress at the tolhuaca geothermal field abstract in this study we examine the fracture network of the tolhuaca geothermal system located in the southern andean volcanic zone that may have acted as a pathway for migration and ascent of deep seated fluids under the far local stress field conditions of the area we collected the orientation slip data and mineralogical content of faults and veins recovered on a ca 1000 m deep borehole tol 1 located in the nw flank of the tolhuaca volcano tol 1 is a non oriented vertical borehole that recovered relatively young 1 axis 083 74 and a subhorizontal ns trending 3 axis 184 03 within the topmost 400 m of the borehole faults and veins are randomly oriented whereas below 400 m depth faults and veins show preferential ne to ew strikes and steep 50 dips the ew striking veins are compatible with the calculated local stress field whereas ne striking veins are compatible with the regional stress field the morphological elongation of volcanic centers alignments of flank vents and dikes orientation our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re orient vertical boreholes such as tol 1 furthermore our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the ne striking fault zone belonging to the liquine ofqui fault system favoring the activation of both nw and ne striking pre existent discontinuities especially the latter which are favorably oriented to open under the prevailing stress field the vertical 1 and ns trending subhorizontal 3 calculated in the tgs promote the activation of ew striking extensional veins and both ne and nw striking hybrid faults constituting a complex fluid pathway geometry of at least one kilometer depth

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

calculation of synthetic seismograms in a laterally varying medium by the boundary element discrete wavenumber method abstract we investigate the propagation of lg waves in laterally varying crustal structures by numerical simulation the method of calculation is formulated in terms of boundary integral equations where the green9s functions are evaluated by wavenumber summation the approach is well suited to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others we investigate the effect of a crustal fault with vertical offset and study the case of a lateral change in crustal thickness the results show that the lg wave amplitude is only slightly affected by the presence of these heterogeneities they confirm the robustness of lg wave propagation in presence of lateral heterogeneities observed in other numerical simulations they show that large scale geometric features of the crust cannot account alone for the strong attenuation of lg waves observed in many regions the results also suggest a possible relation between the level of lg wave coda and the degree of roughness of the moho they further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal heterogeneities

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

paleomagnetism of lower cretaceous rocks from east of the liquine ofqui fault zone southern chile evidence of small in situ clockwise rotations abstract one of the most conspicuous tectonic features of southern chile is a 1000 km long n s trending lineament known as the liquin e ofqui fault although generally regarded as a strike slip fault the amount and timing of strike slip motion are unknown and even the full extent and exact geometry of the feature remain in doubt as part of ongoing investigations by chilean and u s scientists we undertook a paleomagnetic study of neocomian volcanosedimentary alto palena formation and intrusive north patagonian batholith rocks located just east of the main trace of the fault all sites have normal polarity and form a common circular cluster indirect evidence strongly suggests extensive remagnetization of the alto palena rocks by intrusion of the batholith during the mid cretaceous long normal period a paleomagnetic pole calculated from combined alto palena and north patagonian batholith sites lies at 84 2 s 159 9 e 95 5 5 k 50 when compared with cretaceous reference poles for the south american craton this suggests a small but significant amount of clockwise rotation but little or no poleward transport these results are consistent with other paleomagnetic data for the chilean andes and point to widespread rotational shear associated with the liquin e ofqui fault

orogen parallel transition from a decoupled fore arc sliver to andean type mountain chain paleomagnetic and geologic evidence from southern chile 37 39 s

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

2d full waveform modeling of seismic waves in layered karstic media abstractwe have developed a new propagator matrix scheme to simulate seismic wave propagation and scattering in a multilayered medium containing karstic voids the propagator matrices can be found using the boundary element method the model can have irregular boundaries including arbitrary free surface topography any number of karsts can be included in the model and each karst can be of arbitrary geometric shape we have used the burton miller formulation to tackle the numerical instability caused by the fictitious resonance due to the finite size of a karstic void our method was implemented in the frequency space domain so frequency dependent q can be readily incorporated we have validated our calculation by comparing it with the analytical solution for a cylindrical void and to the spectral element method for a more complex model this new modeling capability is useful in many important applications in seismic inverse theory such as imaging karsts caves sinkholes and clandestine tunnels

paleomagnetic rotation pattern of the southern chile fore arc sliver 38 s 42 s a new tool to evaluate plate locking along subduction zones the chile fore arc at 37 s 47 s represents the coseismic deformation zone of the 1960 mw 9 5 valdivia earthquake here we report on the paleomagnetism of 43 oligocene pleistocene volcanic sites from the fore arc sliver between 38 s and 42 s sites were gathered west of the 1000 km long liquine ofqui dextral fault zone lofz that represents the eastern fore arc sliver boundary nineteen reliable sites reveal that the fore arc is characterized by counterclockwise ccw rotations of variable magnitude except at 40 s 41 s where ultrafast 50 myr clockwise cw rotations occur within a 30 km wide zone adjacent to the lofz ccw rotation variability even at close sites and rapidity 10 myr suggest that the observed block rotation pattern is related to nw se seismically active sinistral faults crosscutting the whole fore arc according to previously published data cw rotations up to 170 also occur east of the lofz and have been related to ongoing lofz shear we suggest that the occurrence and width of the eastern fore arc sliver undergoing cw rotations is a function of plate coupling along the subduction zone interface zones of high coupling enhance stress normal to the lofz induce high lofz strength and yield a wide deformation zone characterized by cw rotations conversely low coupling imply a weak lofz a lack of cw rotations and a fore arc entirely dominated by ccw rotations related to sinistral fault kinematics our locking inferences are in good agreement with those recently derived by gps analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 ma

numerical simulation of high and low frequency lg wave propagation summary lg waves are sometimes extinguished when traversing geological structures such as mountain ranges or grabens previous investigations have applied numerical full waveform simulations at relatively low frequencies on the order of 1 hz to investigate the relationship of different structural models and the amplitude anomalies of the lg waves these studies often fail to link specific known structures to the amplitude anomalies however the observed waveforms also often contain data of significantly higher frequency than the full waveform simulations we compare such low frequency simulations computed with the boundary integral equation bie technique with synthetic seismograms generated by dynamic ray tracing drt beginning with a plane layered model of the crust in central france we show that the general characteristics of the lg wave train over distances ranging from 100 km to around 500 km can be modelled by computing a relatively small number of rays reflected between the free surface and the moho matching the details of the waveforms is more difficult but can be improved by the addition of reflections between other interfaces in the model considering a one layer model of the crust with a step like change in moho depth that leads to a rapid decrease in crustal thickness we show again that the bie and drt solutions are very similar in character significantly both solutions predict that the lg wave will not be extinguished on traversing the moho step finally we consider a more complete and detailed model of the pyrenees though it is more difficult to match exactly the ray and integral equation solutions the general trends continue to be the same and the lg waves are still predicted to cross the tectonic structures associated with the mountains therefore it appears that observed amplitude changes must be caused by something other than the general structure of the pyrenees such as scattering by smaller scale features within the lower crust

depositional environments of the lower permian dwyka diamictite and prince albert shale inferred from the geochemistry of early diagenetic concretions southwest karoo basin south africa abstract the upper dwyka and lower ecca groups in the karoo basin of south africa document the climatic and palaeoenvironmental changes associated with the final permo carboniferous deglaciation of the gondwana supercontinent the depositional environments of these groups have until recently been interpreted on the basis of sedimentological and palaeontological evidence here we use the geochemistry of early diagenetic concretions septarian calcite concretions from the upper dwyka group and phosphatic chert concretions and beds from the lower ecca group to infer the depositional environment of these rocks in the southwestern karoo basin 18o values 7 8 to 8 9 smow suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than permian seawater 13c values 15 to 3 pdb indicate that the carbon was derived from a mixture of craton derived calcareous material and organic matter bacterially degraded in the lower sulphate reduction to upper methanogenesis zones during early burial diagenesis the rare earth element ree patterns sr concentrations and 87sr 86sr ratios 0 716 0 737 significantly greater than permian seawater 0 708 together also support the interpretation that calcite and phosphatic concretions formed in glacial fresh water sediments

co2 levels in the late palaeozoic and mesozoic atmosphere from soil carbonate and organic matter satpura basin central india abstract a number of calcic palaeosols have been identified within the fluvial deposits of the motur permian the denwa triassic the bagra jurassic and the lameta cretaceous formations of the satpura sedimentary succession central india these palaeosols show accumulation of pedogenic carbonates in rhizocretions and glaebules the carbon isotopic compositions of these carbonates and the coexisting soil organic matters are used to determine the isotopic composition and the partial pressure of atmospheric co 2 using the co 2 palaeobarometer developed by cerling am j sci 291 1991 377 it is seen that the atmospheric co 2 level increased by a factor of 8 from the permian to the jurassic and declined again during the cretaceous the nature of the changes agrees with the result of the co 2 evolution model of berner geocarb ii but the magnitude of the co 2 increase in the middle jurassic and the late cretaceous was higher than the predicted value degassing of earth s interior due to rapid break up of the gondwana landmass during the triassic and jurassic period could have caused the rapid co 2 increase

glaciogenic coarse clastics in a permo carboniferous bedrock trough in india a sedimentary model abstract physical aspects of sedimentation particularly of coarse clastics within a bedrock trough in the basal 28 5 m of the permo carboniferous talchir formation in west bokaro india have been addressed herein the trough fill facies assemblage bearing abundant signs of glacial activity was eventually buried under wave agitated open nearshore and shelf sediments analysis of several sedimentary characteristics and their interrelationship help to recognize various types of glacial sediments and their frequent resedimentation through various mass flow processes it brings to light plausible transportational and depositional mechanism facies patterns in relation to the various morphometric parts of the trough reveal a changing pattern of sedimentation whereas available palaeocurrent data indicate palaeoflows converging from the flanks and turning axis parallel on the floor of the trough trough filling through fan progradation initially from single but subsequently from multipoint sources failed to impart systematic changes in bed thickness clast size and composition in the sequence this continued till the major coastal onlapping attributed to rapid deglaciation a model with predictive generalization has been proposed for coarse clastic sedimentation in fjord like coastal glacier fed troughs on the basis of observations made herein

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

concretion morphology classification and genesis abstract a discussion of the most relevant morphological features of concretionary bodies and the different classifications and the criteria involved in these classifications is presented together with suggestions for improvements to the various classification schemes the meaning of syngenetic diagenetic and epigenetic related to the relative timing and environment of growth of concretionary bodies is also reviewed and discussed the replacement of mixed morphogenetic classifications which lead to conflicting results by classification categories based on textural features is proposed because the identification of the genetic environment of a concretionary body tells little about its history i recommend defining growth patterns in which the successive steps associated with changes in composition and or texture and the development of new structures are recorded a new path is presented that accounts for the contradiction between textural and isotope features which suggests respectively syngenetic and diagenetic or even epigenetic signatures this new path is characterized by the preservation of large porosities over very long time spans and down to depths that are somewhat greater than expected due to the inhibition of normal compaction caused by early overpressuring this overpressuring is the result of the development of hydraulic seals since approximately the syngenetic phase times the concepts of force of crystallization and displacive growth are also reviewed it is being suggested to discard some controversial interpretations of their actual importance for true concretionary displacive growth under epigenetic conditions in accordance with other authors the conclusion is reached that displacive growth is only possible if the shear strength of the host is very low and the stress field within the host is almost hydrostatic a new model the symcompactional concretionary growth model is proposed which explains how a concretionary body generally a nodule can grow while its host thins down by compaction this overpressured undercompacted model will be useful for the interpretation of an assemblage of features such as injection dikes hydraulic breccias cone in cone structures among others that are thought to be representative of the former presence of overpressured horizons these overpressured horizons could serve as detachment planes in paleotectonically active basins seals that could have controlled the movement of brines and hydrocarbons during the diagenetic evolution of the basin can also be assessed this is relevant for the identification of maturation conditions the potential development of secondary fracture permeability due to hydraulic fracturing in buried areas of the basin can also be evaluated based on the identification of formerly overpressured horizons in outcrops the origin of concretions is generally a geological puzzle clifton 1957

magmatism and tectonics in continental chiloe chile 42 42 30 s the chiloe chonos region seems to preserve the oldest depositional events in the fore arc accretionary complex of the southeast pacific margin there are isolated occurrences of low grade metamorphic rocks including slates with a devonian trilobite fauna and schists that give rb sr evidence of a ca 290 ma metamorphism pillow basalts and ultramafic rocks may represent parts of the pan thalassic ocean floor on which the palaeozoic sediments were laid down emergence of a magmatic arc is indicated by jurassic to early cretaceous volcanogenic and marine deposits during the mid cretaceous climax of plutonic activity these were intruded by monzogranites which here constitute the eastern portion of the north patagonian batholith they give rb sr isochron ages of 120 100 ma barremian albian initial 87sr 86sr ratios of 0 7040 0 7045 and ndt values of 0 5 to 1 5 indicate a simple petrogenesis with a mantle source the western part of the batholith is petrologically more primitive being composed predominantly of tonalite diorite and gabbro and initial 87sr 86sr ratios are more variable late cenozoic movement of the liquine ofqui fault zone lofz generated deep pull apart basins to the west of the uplifted batholith basement complex these were filled by thick marine sequences of volcanogenic debris indicating the wide extent of a mainly rhyolitic volcanic field during miocene times pliocene tonalite and granodiorite plutons dated by a rb sr whole rock isochron at 4 7 0 5 ma and holocene andesite basalt stratovolcanoes are located along the lofz the latter feature has thus been a major influence on the tectonic evolution of the area there is no evidence for major post palaeozoic compression or crustal shortening

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

attenuation of crustal waves across the alpine range we present observational evidence of an anomalous propagation of lg waves across the south western part of the alpine range and we use numerical simulations to model these observations we consider a set of 48 earthquakes which occurred in switzerland northern italy and southeastern france and were recorded by the french laboratoire de detection et de geophysique and northwest italian istituto geofisico di genova seismic networks while the amplitude of the pn phase is stable throughout the region studied lg wave amplitude undergoes strong variations we map this anomaly in lg wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the lg pn amplitude ratios computed for all the paths which cross this cell the image obtained shows that the extinction of lg waves occurs in a limited region of the western alps which corresponds to the zone of highest positive bouguer anomaly this zone located to the east of the high peaks of the massifs cristallins externes does not correspond either to the region of the highest topographies or to the one of the deepest moho at a frequency of 2 hz the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths in order to investigate the cause of the anomaly we perform numerical simulations of sh wave propagation through a model of the western alps which includes the main characteristics of the geological structure our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves the simulation predicts a decay of the amplitude by only a factor between 2 and 3 the introduction of corrugation in the interface shapes further adds to the decay however other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations

isotopic analysis of permo carboniferous talchir sediments from east central india signature of glacial melt water lakes abstract presence of calcareous nodules in the upper part of the talchir sedimentary sequence permo carboniferous in gondwana sediments of east central india offers a chance to delineate the environment of deposition and the regional palaeo climate carbon and oxygen isotopic compositions of the carbonate cement in these early diagenetic nodules indicate that this phase of talchir sedimentation took place in lakes formed by glacial melt water the estimated mean 18o of the meteoric water is about 22 5 close to the expected composition at the 70 s palaeo latitude of this area majority of the nodules have highly depleted carbon isotope ratios indicating significant contribution from biogenic carbon dioxide since the lower part of the talchir formation is characterized by glacier derived materials conglomerates and diamictites the presence of biotic signature in the upper part indicates a climatic change from the cold glacier regime to a warm period

87sr 86sr ratios in permo carboniferous sea water from the analyses of well preserved brachiopod shells abstract sr isotopic analyses of well preserved portions of permo carboniferous brachiopods distributed globally confirm the general shape of the sr isotope age curve established by previous workers for this time interval there is little variation between the sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered based on cathodoluminescence elemental and stable isotopic data however the sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic the sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool

p sv wave field connection technique for regional wave propagation simulation a boundary element be method is developed to calculate the two dimensional p sv elastic response for crustal wave guides with irregular topographic features to simulate long range propagation of regional waves a connection technique is proposed to avoid large matrix inversions that become formidable for long range high frequency problems by using this technique a long crustal wave guide can be divided into relatively shorter sections and the be method can be used section by section to model the effects of rough topography on wave propagation at extended regional distances the validity of the technique is tested by comparison with a direct calculation numerical simulations with this scheme show that rough topography can scatter the p and rayleigh waves and attenuate the energy propagating in the wave guide this method can be used in computing the site effects on sites such as canyons mountains and valleys the connection technique expands this method to deal with large earth models with irregular topography

evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones a new model incorporating shear deformation within a subducting slab is proposed to explain slip partitioning for oblique plate motion at subduction zones on the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is largely correlated with the calculated slab pull force such correlation suggests that other than the upper plate deformation the slab pull force plays an important role in controlling oblique subduction our model proposes that the force balance between the slab pull force the interplate coupling resistance and the viscous mantle drag the latter two are passive forces to the former one produces a lateral shear within the slab which causes the slab to deform and change its motion direction gradually toward trench normal as it subducts the amount of direction change which would be observed as the slip partitioning during earthquakes therefore is closely related to the major plate driving force at the subduction zone which is the slab pull force in our model

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

ray theoretical analysis of lg ray theory is used to investigate the effect that irregular crustal interfaces have on the different phases that comprise lg we concentrate on the effects of a variable moho in this initial study while keeping the surface flat the moho models considered are a cosinusoid a model of the moho below the german part of the european geo traverse egt and a model representing an ocean continent transition it is found that even slight undulations of the moho give rise to extensive multipathing this affects the amplitudes of the supercritically reflected crustal multiples comprising lg in a dramatic way two different types of ray behavior are distinguished the first type corresponding to regular or weakly chaotic ray behavior occurs at smaller values of the ray parameter and gives rise to large amplitudes at distinct epicentral distances due to focusing accompanied by gaps containing very little energy due to defocusing in between the second type corresponding to strongly chaotic ray behavior occurs at larger values of the ray parameter small changes in the take off angle in this chaotic regime produce large fluctuations in the epicentral distance of arrivals the moho below germany for example produces 140 supercritically reflected arrivals at an epicentral distance of 400 km since the amplitudes of arrivals in the chaotic region are smeared out over a large epicentral distance range they are much less prominent than the arrivals from the regular region even though they are supercritically reflected travel time fluctuations are less sensitive to the undulations in the moho than amplitude variations they are more affected by the magnitude than by the wavelength of the undulations amplitude undulations of more than 3 km produce multiple arrivals that have differences in travel time of up to 10 sec

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

triggered seismic activity in the liquine ofqui fault zone southern chile during the 2007 aysen seismic swarm summary we relocated the six large magnitude 5 2 mw 6 2 earthquakes of the destructive tsunamigenic aysen seismic swarm which occurred from 2007 january october in patagonian chile we used p and sh arrival times from near source stations of a temporary seismic network fortuitously deployed in the area when the swarm began and also traveltimes to stations of the permanent global networks to locate the 2007 january 23 mw 5 2 earthquake the first of the six large magnitude events this earthquake s hypocentre lies at shallow depth 10 km on the eastern strand of a major intraarc shear zone the dextral liquine ofqui fault zone using the hypocentre of the january 23 earthquake as a fixed location we relocated the five other large magnitude aysen earthquakes by joint hypocentral determination four of these five events also occurred at shallow depth on the eastern strand liquine ofqui fault whereas the 2007 april 2 earthquake occurred some 45 km to the west on the aysen fault a strike slip duplex fault that segments the area between the eastern and western liquine ofqui fault strands the five earthquakes on the liquine ofqui fault were all produced by dextral slip on n s nodal planes approximately parallel to the mapped trace of the fault the april 2 earthquake resulted from normal slip on the aysen fault modelling of coulomb stress changes on the nodal planes of the april 2 earthquake shows that the cumulative slip on the liquine ofqui fault strand could have triggered the april 2 earthquake similarly the april 2 earthquake may have triggered the mw 6 2 april 21 earthquake which caused mass wasting into aysen fjord generating a destructive tsunami the system of channels and fjords in the study region is a major shipping route around south america and therefore careful evaluation of the seismic hazard is warranted

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

understanding kinematics of intra arc transcurrent deformation paleomagnetic evidence from the liquine ofqui fault zone chile 38 41 s the liquine ofqui fault zone lofz is a major 1000 km long dextral shear zone of southern chile likely related to strain partitioning of nazca plate oblique convergence with south america to understand block rotation pattern along the lofz we paleomagnetically sampled 55 sites 553 samples between 38 s and 41 s we gathered oligocene to pleistocene volcanics and miocene granites at a maximum distance of 20 km from the lofz and at both sides of it rotations with respect to south america evaluated for 36 successful sites show that crust around the lofz is fragmented in small blocks 1 to 10 km in size while some blocks at both fault edges undergo very large 150 170 rotations others do not rotate even adjacent to fault walls we infer that rotations affected equidimensional blocks while elongated crust slivers were translated subparallel to the lofz without rotating rotation pattern across the lofz is markedly asymmetric east of the fault and adjacent to it rotations are up to 150 170 clockwise and fade out 10 km east of fault these data support a quasi continuous crust kinematics characterized by small rigid blocks drag by the underlying ductile crust flow and imply 120 km of total fault offset conversely crust west of the lofz is cut by seismically active nw se sinistral antithetic faults and yields counterclockwise rotations up to 170 at 8 10 km from lofz besides the unrotated blocks further data from the chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the lofz

new early cretaceous palaeomagnetic pole from cordoba province argentina revision of previous studies and implications for the south american database summary a continental sequence of red beds and interbedded basaltic layers crops out in the sierra chica of cordoba province argentina 31 5 s 64 4 w this succession was deposited in a half graben basin during the early cretaceous we have carried out a palaeomagnetic survey on outcrops of this basin 147 sites in seven localities from an analysis of irm acquisition curves and detailed demagnetization behaviour three diverent magnetic components are identified in the volcanic rocks components a b and x are carried by single or pseudo single domain titano magnetite haematite and multidomain magnetite respectively component a is interpreted as a primary compo nent of magnetization because it passes conglomerate contact tilt and reversal tests the carrier of the primary magnetization fine grained titano magnetite is present in basalts with a high degree of deuteric oxidation this kind of oxidation is interpreted to have occurred during cooling components b and x are discarded because they are interpreted as recent magnetizations in the sedimentary rocks haematite and magnetite are identified as the carriers of remanence both minerals carry the same component which passes a reversal test the calculated palaeomagnetic pole based on 55 sites is lat 86 0 s long 75 9 e a 95 3 3 k 35 this palaeomagnetic pole supersedes four with anomalous positions reported in previous papers

high fluoride in groundwater of an industrialized area of eastern india odisha inferences from geochemical and isotopic investigation an assessment of fluoride distribution in groundwater of an industrialized area talchir odisha india was carried out to understand the potential sources and processes this study is significant as the water resources are being endangered by the mining and other industrial activities in this region high concentration of fluoride 4 4 mg l was observed in groundwater of this area which was attributed mainly to mining activities additional source for high fluoride is the basement granitic rock containing fluoride bearing minerals like amphibole biotite and fluorite the dominant chemical types are found to be na ca hco3 na mg hco3 ca mg hco3 and ca na hco3 the groundwater facies variation was found in accordance with the local lithology there was no marked difference in the fluoride concentration of groundwaters between near and far off mining zone geochemical modeling indicated precipitation of calcite and dolomite as the driving force for the release of fluoride from the formation in addition to the precipitation of calcite and dolomite evapotranspiration also contributes to high fluoride in groundwater environmental stable isotopes inferred that there was no clear isotopic distinction between high and low fluoride groundwater and the groundwater was found to be evaporated environmental tritium data suggested that the groundwater was modern and fluoride enrichment was facilitated by long residence time of groundwater statistical methods also corroborate the geochemical and isotopic inferences the study highlights that natural geogenic source is the main cause for high fluoride in groundwater refuting earlier theories that attribute mining activities as the only source for high fluoride in this region

structural controls on fluid circulation at the caviahue copahue volcanic complex ccvc geothermal area chile argentina revealed by soil co 2 and temperature self potential and helium isotopes abstract natural geothermal systems are limited areas characterized by anomalously high heat flow caused by recent tectonic or magmatic activity the heat source at depth is the result of the emplacement of magma bodies controlled by the regional volcano tectonic setting in contrast at a local scale a well developed fault fracture network favors the development of hydrothermal cells and promotes the vertical advection of fluids and heat the southern volcanic zone svz straddling chile and argentina has an important yet unexplored and undeveloped geothermal potential studies on the lithological and tectonic controls of the hydrothermal circulation are therefore important for a correct assessment of the geothermal potential of the region here new and dense self potential sp soil co 2 and temperature t measurements and helium isotope data measured in fumaroles and thermal springs from the geothermal area located in the north eastern flank of the copahue volcanic edifice within the caviahue caldera the caviahue copahue volcanic complex ccvc are presented our results allowed to the constraint of the structural origin of the active thermal areas and the understanding of the evolution of the geothermal system ne striking faults in the area characterized by a combination of sp co 2 and t maxima and high 3 he 4 he ratios up to 8 16 0 21ra whereas atmospheric ra is 1 382 10 6 promote the formation of vertical permeability preferential pathways for fluid circulation wnw striking faults represent low permeability pathways for hydrothermal fluid ascent but promote infiltration of meteoric water at shallow depths which dilute the hydrothermal input the region is scattered with sp co 2 and t minima representing self sealed zones characterized by impermeable altered rocks at depth which create local barriers for fluid ascent the ne striking faults seem to be associated with the upflowing zones of the geothermal system where the boiling process produces a high vapor dominated zone close to the surface whereas the wnw striking faults could act as a boundary of the copahue geothermal area to the south

a tale of two continents some tectonic contrasts between the central andes and the north american cordillera as illustrated by their paleomagnetic signatures comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large scale tectonic processes comparison of the paleomagnetic signatures of the western cordillera of north america and the central andes shows that the western edges of these belts have had profoundly different mesozoic and cenozoic histories specifically the north american cordilleran pattern shows strong evidence of post middle cretaceous relative northward displacement of outboard crustal blocks but there is almost no comparable evidence of margin parallel displacement in the andes we speculate that this may largely be a consequence of a simple difference in shape the convex westward western margin of north america facilitates margin parallel displacement as a response to oblique subduction whereas the concave westward margin of south america inhibits it the patterns of block rotations found along the western edge of the two orogens also are quite different nearly everywhere within the western north american cordillera crustal blocks have rotated clockwise since mid cretaceous time reflecting a pervading state of dextral shear within the western andes of peru and northern chile however mesozoic and cenozoic rocks in peru and northernmost chile are rotated strongly counterclockwise whereas rocks of the same age in the remainder of chile to about latitude 48 s are rotated clockwise a model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

numerical modelling of sh lg waves in and near continental margins summary the effect of transition regions between continental and oceanic structures on the propagation of l waves from continental sources is examined in particular the attenuation due to variations in layer thickness in such transition regions is calculated and explained for a suite of simple models the measured attenuation due tq the geometry of the transition regions between the oceanic and continental structures within a partially oceanic path with source and receiver in a continental structure is at most a factor of four for frequencies from 0 01 to 1 hz this is inadequate to explain the observed extinction of l along such paths this extinction has previously been attributed to the effects of the transition region geometry the method used to calculate the results presented in this study is developed and its validity and accuracy are demonstrated propagator matrix seismograms are coupled into a finite element calculation to produce hybrid teleseismic sh mode sum seismograms these hybrid synthetics can be determined for paths including any regional transition zone or other heterogeneity that exists as part of a longer mostly plane layered path numerical results presented for a suite of transition models show distinct trends in each of the regions through which the wavefield passes the wavefield passes through a continent ocean transition region then a region of oceanic structure and finally through an ocean continent transition region when an l wavefront passes through a continent ocean transition the amplitude and coda duration of the l wave at the surface both increase at the same time much of the modal l energy previously trapped in the continental crust is able to escape from the lower crust into the subcrustal layers as body waves the magnitude of both these effects increases as the length of the transition region increases when the wavefront passes through the region of oceanic structure further energy escapes from the crustal layer and produces a decrease in l amplitude at the surface the rate of amplitude decrease is maximum near the transition region and decreases with distance from it when the wavefield passes through the oceancontinent transition region a rapid decrease in the l amplitude at the surface of the crust results the energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer some of the body wave phases produced when the wavefield passes through the continent ocean transition region are incident on the continental crust in the ocean continent transition region these waves are predominantly transmitted back into the crust the other body wave phases reach depths below the depth of the base of the continental crust before reaching the ocean continent transition and thus escape from the system

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

propagation efficiency of long period lg waves in the south american continent summary we investigate the propagation efficiency characteristics of long period 3 12 s lg waves in continental south america as recorded in the last few years on stations of the gtsn iris usgs irisiida and geoscope networks this paper reports the results of a first look at the available lg data for events with m 5 0 from 1990 to 1995 in this period seven main seismic events and aftershocks along the length of the andes produced a total of 29 usable paths from which preliminary results indicate that 1 long period lg waves are unusually prominent in seismograms of continental paths covering the shield areas of south america and are clearly observed on broad band instruments at epicentral distances exceeding 3500 km 2 lg propagation is substantially attenuated weak or absent for paths along the strike of the andes and along the three active volcanic chains of chile peru and colombia 3 away from the active volcanoes the data show lg propagating more efficiently across than along the strike of the andes and 4 in general the ratio of the transverse to the vertical components of the lg phase decreases with decreasing propagation efficiency these first results also raise a number of exciting questions regarding the exploration of the crustal waveguide in south america such as the potential for lgbased tomography to detect a postulated fossil plume in southeastern brazil or how the propagation efficiency of high frequency lg in volcanically active areas may be used to detect magma chambers consequently we intend to carry out a systematic study of lg in the continent as part of the ongoing surface wave inversion for the south american lithosphere sisal project a collaborative effort vdovin et al 1996 to determine the crustal and lithospheric structure of the south american plate from measurements of surface wave dispersion in the period range 2 200 s

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

analysis of short period regional phase path effects associated with topography in eurasia propagation of regional seismic phases is highly dependent on path effects but we have a limited understanding of these effects and no general procedure for accounting for path variation influence on regional waveforms thus there is strong regional variability in the effectiveness of regional wave discriminants used to identify small earthquakes and nuclear explosions motivated by many observations of correlation between surface geology and regional phase behavior we empirically explore the relationship between short period regional and upper mantle distance signal energy and statistics of topography along different travel paths using data for underground nuclear explosions at semipalatinsk kazakhstan we find strong linear correlations of the logarithmic rms amplitude ratio sn lg and to a lesser extent p lg with mean altitude rms roughness rms slope and skewness of topography along the paths to receivers in eurasia this indicates that energy partitioning in the regional wave field is controlled by wave guide structure and attenuation variations that are manifested in surface topography this suggests that it is feasible in many cases to directly calibrate regional discriminants for path effects in terms of observable surface topography as a surrogate for overall path properties the relationships also help to understand the nature of regional phase propagation

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

the effect of 3 d structure on lg propagation patterns the method of ray diagrams based on the constructive interference of multiple s reflections gives a visual measure of the interaction of guided wavetrains with variations in crustal structure the technique is extended to 3 d heterogeneity and by representing the s ray systems in plan view through their basement reflection points a semi quantitative measure of the disruption of lg at different phase velocities can be made in addition the tilt of the ray systems can be used as an indication of the degree of interconversion between rayleigh and love wave energy two simple crustal models are examined to investigate the effects of oblique incidence and transverse topographic gradients on multiple s reflections using ray diagrams the method is then applied to several real earth models of the crust in central asia using topographic data and assuming isostasy interpretations of the ray diagrams for paths across the major tectonic features compare well with many of the observations from previous studies ruzaikan et al 1977 ni barazangi 1983 and suggest that the gross characteristics of lg propagation can be attributed to the shape of the crustal waveguide

guided wave tracking in 3 d a tool for interpreting complex regional seismograms abstract the tracking of guided wave trains by raytracing in 3 d structures can provide a means of interpreting complex seismograms at regional distances the method relies on the interpretation of lg as the constructive interference of multiple s reflections within the crust and strong scattering can be simulated by the inclusion of secondary sources this approach has been applied to californian events observed in the southwestern united states whose records at regional distances frequently exhibit an extended and complex lg coda with significant late energy arriving with group velocities of 2 5 km sec or less the general character of this energy precludes an interpretation as the result of stochastic scattering processes or dispersion in low velocity surface sediments therefore some alternative explanation needs to be sought the observed wavetrains of events from two sites in southwestern california coalinga and north baja at stations lac mnv and elk have been compared with the predictions from tracking guided wave patterns multipathing of energy by major changes in the thickness of the crustal wave guide is consistent with the extended and often pulse like nature of the lg coda examples of plausible multipathing mechanisms include reflection from the ocean continent transition scattering associated with topographic features such as the sierra nevada and resonance within the narrow corridor of baja california

a global oxygen isotope model semi empirical zonally averaged a simple model which is zonally averaged for the transport of atmospheric water vapour is presented which uses as input the zonally averaged evaporation field and the mean meridional travel distance of tropospheric water vapour as functions of latitude the model demonstrates that for polar regions each of the 10 latitude strips poleward of 25 is of equal importance as a moisture source the model is used to predict zonal averages of 6 1s 0 for the present day and 18 ka bp both annual average values and seasonal amplitudes are presented and compared to observations sea ice cover is an important factor in determining both annual averages and seasonal amplitudes today and at 18 ka bp an earlier model jouzel and others 1983 linking 6eso the deuterium excess and sea salt content in an antarctic ice core to the relative humidity of the source region is based on a single source atmospheric water vapour cycle type model and is re ev aluated using the present model

chapter 22 paleomagnetism of continental north america implications for displacement of crustal blocks within the western cordillera baja california to british columbia

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

palaeolimnology of palaeozoic lakes focussing on a single lake cycle in the middle devonian of the orcadian basin scotland there are few palaeolimnological studies of palaeozoic lake deposits using methodologies developed for the study of quaternary lakes new research described here indicates the benefits of applying multidisciplinary high resolution sampling and analysis on the middle devonian lake deposits of the orcadian basin in order to understand lake palaeoecology and response to water chemistry change and possible climate change in a continuously sampled 4 m thick lake cycle in two correlated boreholes in caithness there are two distinct features there is a broad trend in 13c of sedimentary organic matter from high values in the shallow water deposits to lower values in the deeper water deposits attributed to changing contributions from terrestrial plant material 13corg c 28 and algal material 13corg c 33 this variation is consistent with palynological data and is best explained by proximal distal variation as the lake filled a smaller but well correlated up section decreasing 13corg trend occurs over approximately 1 m starting at the cycle centre where the lake was at its deepest since the same feature is seen in two boreholes we suggest that local effects of migrated bitumen diagenesis or facies changes are unlikely to be the cause and that changes in the carbon cycle of the lake are responsible explanations for this trend may relate to the source of carbon that algae utilised as the lake changed from a fresher to a more saline environment and to changes in the carbon isotope composition of the lake water during periods of enhanced productivity

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

efficiency of lg propagation from sms dynamic ray tracing in three dimensionally varying crustal waveguides the effects of variations in crustal and basin thicknesses on the efficiency of lg propagation in central asia are investigated by comparing the predictions of ray theoretical modeling of lg as multiple critically reflected sms rays with observations at the knet cnet and ilpa arrays ray modeling predicts that strong spatial gradients in crustal thickness within 10 to 50 km of either the source or receiver can affect lg efficiency over paths exceeding 500 km variations in the efficiency of lg propagation to knet including instances of strong variations across the aperture of the array are consistent with predicted blockage of sms by high slopes of moho topography associated with the hindu kush and pamir mountains at cnet and ilpa the blocking effects of moho slopes associated with the caucausus and zagros mountain belts and black and caspian seas are accentuated by the effects of thick basins thick basins greater than 2000 m are found to have large effects on the efficiency of lg propagation from shallow 1 km crustal sources blocking either a portion or all of the lg phase along paths crossing these basins the basin effects are strongly sensitive to source depth nearly vanishing for sources 15 km and deeper

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

paleozoic lake faunas establishing aquatic life on land abstract lakes are important archives for continental records of paleoenvironmental as well as paleoclimatic change they also record a unique macroevolutionary pattern that occurred when faunas invaded the continental realm in order to document that pattern we compiled a database of over one hundred and six lake basins from the neoproterozoic to permian looking at this critical time in lacustrine evolution several patterns emerge for the neoproterozoic through silurian time there is a paucity of documented lake deposits and lake faunal records however the records that are available indicate that lakes were oligotrophic and their nutrient cycling regimes primitive it is not until the establishment of land plants in the silurian that lakes began to respond with higher diversities and more complex physical and chemical conditions during the devonian carboniferous periods diversity was on the rise as trophic structure became more complex most lakes of the devonian and carboniferous formed along continental margins or in tectonic basins with occasional connection to the marine realm the faunas from these types of lakes were commonly comprised of mixed marine and freshwater elements and were far more diverse than other more inland lakes this estuary effect created a gateway or filter through which faunas invaded the continental realm the early history of lake faunas is one of opportunity and amelioration the feedback loops created by the establishment of vascular plants altered the nutrient cycle on land and in lakes all trophic levels were established early but became increasingly complex throughout the paleozoic as roles changed and faunal elements became established groups invading the continents via the estuary effect did so numerous times before establishing themselves permanently this was linked with the episodic re establishment in marine freshwater connections along these continental margins