a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

geochemistry of thermal fluids in nw honduras new perspectives for exploitation of geothermal areas in the southern sula graben abstract the results of a geochemical survey on thermal waters and for the first time for this site gas discharges in five geothermal sites azacualpa la cueva rio ulua rio gualcarque el olivar and laguna de agua caliente in nw honduras are here presented and discussed el olivar and laguna de agua caliente in the southern part of the sula graben are very close to a quaternary basaltic field whereas azacualpa la cueva rio ulua and rio gualcarque located to the southwest of the yojoa lake direcly emerge from the cretaceous limestone deposits the measured temperatures range between 37 5 and 104 8 c mature alkaline na so 4 thermal waters discharge from azacualpa la cueva while those from el olivar and laguna de agua caliente are immature and show a na hco 3 composition chemical equilibria of waters and gases from the azacualpa la cueva thermal springs indicate temperatures ranging from 150 to 200 c conversely gas discharges from el olivar and laguna de agua caliente have attained a partial chemical equilibrium in the liquid phase at slightly higher temperatures 200 250 c although gas gas faster reactions involving co seem to be adjusted in an isothermally separated vapor phase unlike azacualpa sio 2 geothermometer at el olivar and laguna de agua caliente indicates equilibrium temperatures for the liquid phase much lower than those calculated for the gas phase 120 c we conclude that thermal waters from the azacualpa area likely represent the direct emergence of a water dominated reservoir having temperatures 150 200 c by contrast at el olivar and laguna de agua caliente hot springs are supplied by a boiling shallow aquifer fed by a vapor phase rising from a steam dominated zone the above geochemical model is consistent with a geothermal reservoir hosted within the cretaceous carbonate sequences of the yojoa group in the whole investigated sites the reservoir extensively crops out in the azacualpa area whereas the geothermal sites of the southern sula graben laguna de agua caliente and el olivar are covered by the oligocene miocene volcano sedimentary sequences of the matagalpa formation possibly acting as efficient impermeable cap rocks these results significantly differ from those reported by previous studies and emphasize that the southern sula graben in particular the el olivar area can represent among the investigated thermal springs the most promising site for the exploitation of a high enthalpy geothermal field

bomb produced radiocarbon in the shell of the chambered nautilus rate of growth and age at maturity abstract the temporal pattern of bomb produced radiocarbon in the oceans provides a chronometer to estimate the rate of growth of marine organisms radiocarbon measurements by tandem accelerator mass spectrometry of samples from the septa and shell wall of a mature specimen of nautilus macromphalus reveal progressively increasing values in more recently formed shell material this pattern is similar to that recorded in a banded coral from surface waters of the great barrier reef australia however the pre bomb values of 14 c are lower in the nautilus than in the coral this offset is probably due to the difference in the respective habitat depths of the two animals taking this difference into account we have used the temporal pattern of 14 c in the coral as a point of reference to help interpret the pattern of 14 c in the nautilus according to our calculations the age of the specimen of n macromphalus is 10 12 years and the timing of septal formation ranges from 80 to 240 days septum

compositions sources and depositional environments of organic matter from the middle jurassic clays of poland abstract the comprehensive biomarker characteristics from previously undescribed middle jurassic clays of poland are presented the molecular composition of the organic matter om derived from clays of aalenian to callovian age has not changed significantly through time high relative concentrations of many biomarkers typical for terrestrial material suggest a distinct dominance of om derived from land plants increasing concentrations of c29 diaster 13 17 enes towards the northern part of the basin indicate an increase in terrestrial input this terrestrial material would have originated from the enhanced transport of organic matter from land situated at the northern bank of the basin i e the fennoscandian shield the organic matter was deposited in an oxic to suboxic environment as indicated by relatively low concentrations of c33 c35 homohopanes moderate to high pr ph ratio values an absence of compounds characteristic for anoxia and water column stratification such as isorenieratane aryl isoprenoids and gammacerane as well as common benthic fauna and burrows 18o measurements from calcitic rostra of belemnites suggest that the mean value of the middle jurassic sea water temperature of the polish basin was 13 1 c it is suggested that this mirrored the temperature of the lower water column because belemnites are considered here to be necto benthic the organic matter from the middle jurassic basin of poland is immature this is clearly indicated by a large concentration of biomarkers with the biogenic configurations such as hopanes hop 13 18 enes hop 17 21 enes diasterenes and sterenes the identification of preserved unaltered biomolecules like ferruginol 6 7 dehydroferruginol and sugiol in protopodocarpoxylon wood samples from these sediments present particularly strong evidence for the presence of immature om in the middle jurassic sediments moreover the occurrence of these polar diterpenoids is important due to the fact that they are definitely the oldest known natural products detected in geological samples

kinematics of the north american caribbean cocos plates in central america from new gps measurements across the polochic motagua fault system 1 the polochic motagua strike slip fault system in guatemala marks the on land plate boundary between the north american na and the caribbean ca plates gps observations in 1999 and 2003 show that the far field velocity across the system na ca relative velocity is 20 mm yr this is significantly higher than the nuvel 1a velocity but is consistent with the gps based ca na velocity proposed by demets et al 2000 the observations are modeled by a fault centered on the motagua fault locked at a depth of 20 km with a slip rate decreasing from eastern to central guatemala from 20 to 12 mm yr towards the na ca cocos triple junction this decrease is accommodated by 8 mm yr of e w extension in the westernmost part of ca south of the motagua fault about 10 mm yr of dextral slip is observed across the mid american volcanic arc the na ca cocos triple junction is thus a complex 400 km wide wedge shaped area

tectonic reconstruction of the northern andean blocks oblique convergence and rotations derived from the kinematics of the piedras girardot area colombia abstract a detailed kinematic study in the piedras girardot area reveals that approximately 32 km of ene wsw oblique convergence is accommodated within a northeast trending transpressional shear zone with a shear strain of 0 8 and a convergence factor of 2 early campanian deformation is marked by the incipient propagation of northeast trending faults that uplifted gentle domes where the accumulation of sandy units did not take place maastrichtian unroofing of a metamorphic terrane to the west is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodates less than 5 extension this extensional fabric distortion of fossil molds and a moderate cleavage accommodating less than 5 contraction developed concurrently but before large scale faulting and folding paleogene folding and southwestward thrust sheet propagation are recorded by syntectonic strata neogene deformation took place only in the western flank of this foldbelt the amount direction and timing of deformation documented here contradict current tectonic models for the cordillera oriental and demand a new tectonic framework to approach the study of the structure of the northern andes thus an alternative model was constructed by defining three continental blocks the maracaibo cordillera central and cordillera oriental blocks oblique deformation imposed by the relative eastward and northeastward motion of the caribbean plate was modeled as rigid body rotation and translation for rigid blocks derived from published paleomagnetic and kinematic data and as internal distortion and dilation for weak blocks derived from the piedras girardot area this model explains not only coeval dextral and sinistral transpression and transtension but also large clockwise rotation documented by paleomagnetic studies in the caribbean northern andean region

the transpressive left lateral sierra madre de chiapas and its buried front in the tabasco plain southern mexico the sierra madre de chiapas evolved in the vicinity of the triple junction between the cocos north america and caribbean plates the sierra madre de chiapas tectonics reflects positive topographic growth along its main core and a northwards directed collapse through a free border related to the gulf of mexico major exhumation and topographic growth occurred during the middle late miocene 16 10 ma evidence for this deformational event is provided by fault activity major stratigraphic unconformities along the sierra madre de chiapas and the tabasco coastal plain i e southern gulf of mexico major salt related motion northward progradation of the sedimentation and northward migration of the buried deformational front during the neogene strike slip deformation and its related exhumation migrated landwards from the western edge of the chiapas massif complex to the chiapas sierra horizontal displacement along the main strike slip faults in the chiapas sierra has been indirectly estimated to be between 30 and 43 km during the last 6 5 ma implying 0 5 0 8 cm a 1 of lateral accommodation these values suggest that a significant amount of the motion transferred by the caribbean and north american plates is currently being accommodated along the chiapas area supplementary material a geological map of the sierra madre de chiapas is available at www geolsoc org uk sup18507

amatitlan an actively resurging cauldron 10 km south of guatemala city a 14 16 km diameter collapse caldera has been recognized 10 km south of guatemala city guatemala the caldera is north of the presently active volcano pacaya and west of agua a large stratovolcano the caldera was not previously recognized because its eastern and western margins coincide with faults that outline the guatemala city graben and because the northern margin of the caldera is buried by pyroclastic rocks the existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults hot springs well log data and lithological changes in sedimentary rocks a sequence of nine silicic pyroclastic deposits totaling a volume of more than 70 km 3 dense rock were erupted from the caldera the ages of these eruptions are mainly between about 300 000 years b p to less than 23 000 years b p the rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow surge and fall deposits mainly of rhyolitic to dacitic composition successive pyroclastic eruptions have generally become smaller in volume and more silicic with time major and minor element chemistry distinguish amatitlan pyroclastics from those of other nearby calderas the caldera lies at the intersection of an offset of the volcanic chain the palin shear and the faults along the volcanic front jalpatagua fault zone the caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben the graben s alignment with the jalpatagua fault zone suggests a genetic relationship the longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome near a young silicic intrusion the caldera is active and will probably erupt again over 1 million people live within 20 km and would be threatened in the event of a moderate eruption suggestions for future research focus on hazard assessment and forecasting

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

palaeostress determinations from fault kinematics application to the neotectonics of the himalayas tibet and the central andes this paper concerns the determination of the stress deviator from a set of striated faults or of focal mechanisms of earthquakes southern tibet and the cuzco region of the high andes illustrate the applied methodology then the spatial and temporal changes in the tectonic regimes of tibet and central andes are briefly examined the quaternary present day stress patterns in these two high plateaus favour models explaining the spatial changes in the state of stress at a large wavelength as a result of a compensated elevated topography major temporal changes in the tectonic regimes are attributed tentatively to uplift for the tibetan plateau and for the already elevated andean plateau to changes in the magnitude of the boundary forces due to subduction processes

changes in vegetation and marine environments in the eastern mediterranean rhodes greece during the early and middle pleistocene pollen records marine faunal associations and stable isotope compositions of sediments from rhodes greece have been determined to track environmental changes in the eastern mediterranean during the early and middle pleistocene a detailed chronostratigraphic curve based on magnetostratigraphic data was obtained by correlating pollen spectra with the mediterranean oxygen isotopic curve of ocean drilling program site 975 five sedimentary sequences that correspond to marine oxygen isotope stage mis 31 27 and to mis 20 17 have been investigated in the confined tsampika microbasin high amplitude pinus variations confirm glacio eustatic changes deduced from changes in marine faunal associations and sedimentary depositional environments reflecting climatic cycles identified in the marine carbonate oxygen isotope record eight vegetation successions characterized by the dominance first of mesothermic elements then of mid and high altitude elements with pinus and ending with maxima in herb and steppe elements have been documented most of them were probably driven by changes in insolation occurring in high northern latitudes obliquity impact during the late early pleistocene and early middle pleistocene

neotectonic development of the el salvador fault zone and implications for deformation in the central america volcanic arc insights from 4 d analog modeling experiments the el salvador fault zone esfz is an active approximately 150 km long and 20 km wide segmented dextral strike slip fault zone within the central american volcanic arc striking n100 e although several studies have investigated the surface expression of the esfz little is known about its structure at depth and its kinematic evolution structural field data and mapping suggest a phase of extension at some stage during the evolution of the esfz this phase would explain dip slip movements on structures that are currently associated with the active dominantly strike slip and that do not fit with the current tectonic regime field observations suggest trenchward migration of the arc such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the cocos plate beneath the chortis block during the miocene pliocene we carried out 4 d analog model experiments to test whether an early phase of extension is required to form the present day fault pattern in the esfz our experiments suggest that a two phase tectonic evolution best explains the esfz an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures this extensional phase is followed by a strike slip dominated regime which results in intersegment areas with local transtension and segments with almost pure strike slip motion the results of our experiments combined with field data along the central american volcanic arc indicate that the slab rollback intensity beneath the chortis block is greater in nicaragua and decreases westward to guatemala

carbon isotope δ13c differences between late cretaceous ammonites and benthic mollusks from antarctica abstract ammonites are one of the most common fossils of the mesozoic but there is little consensus on their mode of life isotopic studies have been used to reconstruct their preferred water temperature from 18 o measurements but 13 c values have more ambiguous interpretations previous studies have recorded population differences in 13 c values between ammonite and benthic organisms without explaining the cause in detail here we examine a molluscan community from seymour island antarctica and find a 4 carbon isotope offset between ammonites and other benthic mollusks the most likely cause of this anomaly is an increased metabolic carbon contribution to ammonite shell material when compared with other mollusks but we cannot rule out the possibility of a methane seep contribution increased respired co 2 production could be generated by a more active lifestyle and increased energy demands which may have increased the susceptibility of ammonites to the end cretaceous mass extinction

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

beyond the motagua and polochic faults active strike slip faulting along the western north america caribbean plate boundary zone abstract i investigate the role of two strike slip faults in the tectonics of the western north america caribbean plate margin the ixcan fault located in guatemala north of the polochic fault is seismically active with earthquakes of magnitude up to 5 7 reported recently fault plane solutions along this curvilinear but generally e w trending fault indicate left lateral strike slip displacement several historic earthquakes appear to have taken place along the ixcan fault since 1728 the largest one being the 1816 event m 7 5 the nw se trending concordia fault in southeastern mexico appears to be the site of the great m 7 6 earthquake of 1902 isoseismals for this event suggest shallow left lateral strike slip faulting i propose a seismotectonic model in which both faults are part of the deformation associated to the north america caribbean plate boundary zone transpressive structures are found in the fault steps between strike slip fault systems

timing and sources of neogene and quaternary volcanism in south central guatemala abstract five new and six existing radiometric age dates place constraints on the timing of volcanic episodes in a 1400 km2 area east of guatemala city the source of the voluminous miocene rhyolitic welded tuffs was the newly discovered santa rosa de lima caldera in the northern part of the area not fissure eruptions as was previously believed resurgence during the pliocene included the eruption of more silicic tuffs followed by post collapse volcanism around the perimeter volcanism in the southern part of the area occurred along the neogene volcanic front the sources for these late miocene and pliocene andesitic lavas were not fissure eruptions as was once believed but were four large volcanic centers cerro pinula ixhuatan teanzul and cerro la gabia the santa rosa de lima caldera structure deflects the jalpatagua fault forming tensional fractures along which eruptions in the quaternary cuilapa barbarena cinder cone field took place pleistocene ash flows were erupted from ixhuatan and tecuamburro volcanoes in the southern part of the area tephras from ayarza amatitlan and atitlan blanket the northern and central portions present day activity is restricted to hot springs around the northern and eastern base of tecuamburro volcano based on the work in this area it is proposed that rocks of the miocene chalatenango formation throughout northern central america were erupted from calderas behind the neogene volcanic front rocks of the mio pliocene balsamo formation in guatemala and el salvador were erupted from discrete volcanic centers along the neogene volcanic front pliocene rocks of the cuscatlan formation probably represent post collapse volcanism around earlier caldera structures

seismic strain rates and distributed continental deformation in the northern andes and three dimensional seismotectonics of northwestern south america abstract the seismic strain rates and configuration in the northern andes are estimated by summing the components of the centroid moment tensor cmt solutions for earthquakes with mw 5 0 and are compared with those predicted by various models of relative plate motions and gps data moment tensor components are summed linearly in four different regions with similar seismotectonics to infer changes in the regional strain patterns along the northern andes and to retrieve strain rates and relative plate velocities between this region and the stable south american plate the andean seismic strain rates are the result of east southeast directed contraction due to the collision of the nazca and caribbean plates with the south american plate and the collision of the panama block against central colombia with a component of northeast directed motion due to the obliquity of the eastern boundary of the northern andes with respect to the boundary of the nazca and caribbean plates with the south american plate along the colombia trench and the southern caribbean deformation zone respectively where the convergence is normal the present motion of the central andes towards the northeast does not require the plate boundary between the caribbean and south american plates to be along the eastern cordillera of colombia and the bocono fault in venezuela the seismic moment rates for the northern andes are small compared to the total expected seismic strain rates calculated from plate motion models and gps data suggesting that most of the strain is released aseismically and that is accommodated by folding fault creeping rigid block translation or other undetermined processes finally a three dimensional model of the cmt solutions and other seismicity in northwestern south america is presented where major subducting slabs including the peru flat slab and the ecuador and central colombia segments of the nazca and caribbean plates become more evident and easier to interpret the caribbean plate is interpreted to culminate at the location of the bucaramanga nest offering an explanation to this swarm of seismicity in the northern andes

guayape papalutla fault system a continuous cretaceous structure from southern mexico to the chortis block tectonic implications recent papers have opened the debate over whether the chortis block was located off the coast of southern mexico or in a more outboard position and this led me to explore whether correlations with older structures could be established to determine the evolution of the southwest corner of the north america plate in this paper i hypothesize that the papalutla fault of mexico and the guayape fault system of honduras both considered to be terrane boundaries were roughly continuous in the cretaceous extending from southern mexico to the chortis block they influenced early cretaceous clastic sedimentation of the zicapa formation on the guerrero morelos platform southern mexico and the tepemechin formation of the central chortis terrane honduras the units accumulated in fault bounded basins associated with left lateral slip the basins developed in the central chortis terrane and guerrero morelos platform both geological entities appear to represent a continuous cretaceous geologic province characterized by a trangression that occurred earlier in honduras than in southern mexico this nonrotational hypothesis for the location of the chortis block adjacent to southern mexico during the cretaceous is consistent with northeastward displacement of the caribbean plate during cretaceous time and implies 1500 km of chortis block displacement during the cenozoic

tectonic wedges and offset laramide structures along the polochic fault of guatemala and chiapas mexico reaffirmation of large neogene displacement fault wedges along the polochic fault of northern guatemala and chiapas mexico record left lateral slip of from 20 to 65 km that accompanied documented neogene slip of 130 km across this north american caribbean plate boundary fault wedges can be correlated to source areas consistent with left lateral displacement along the polochic fault they occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped these wedges which have been previously interpreted as allochthons emplaced along low angle reverse faults are bounded by high angle faults folds that terminate against the polochic fault are part of a regional fold belt that originated during the campanian through paleocene laramide orogenic event a previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect the 130 km model of left lateral offset across the polochic fault is reviewed along with evidence for and against a neogene time of major displacement additional evidence for the model is presented 1 late cretaceous granitoid rocks from near motozintla chiapas mexico northern fault block are correlated with granitic rocks of the santa maria batholith in the region of huehuetenango guatemala southern block 2 mineralization at the contacts of the granitoid intrusives of the two areas is correlated 3 points of deflection of laramide structural axes are correlated across the fault 4 a major thrust sheet similar to that of the sierra de santa cruz in eastern guatemala is reassembled by reversing 130 km of left lateral slip which has offset serpentinites north of the fault in western guatemala and those south of the fault in central guatemala

a gps and modelling study of deformation in northern central america summary we use gps measurements at 37 stations in honduras and el salvador to describe active deformation of the western end of the caribbean plate between the motagua fault and central american volcanic arc all gps sites located in eastern honduras move with the caribbean plate in accord with geologic evidence for an absence of neotectonic deformation in this region relative to the caribbean plate the other stations in the study area move west to west northwest at rates that increase gradually from 3 3 0 6 mm yr 1 in central honduras to 4 1 0 6 mm yr 1 in western honduras to as high as 11 12 mm yr 1 in southern guatemala the site motions are consistent with slow westward extension that has been inferred by previous authors from the north striking grabens and earthquake focal mechanisms in this region we examine the factors that influence the regional deformation by comparing the new gps velocity field to velocity fields predicted by finite element models fems that incorporate the regional plate boundary faults and known plate motions our modelling suggests that the obliquely convergent 20 direction of caribbean north american plate motion relative to the motagua fault west of 90 w impedes the ene directed motion of the caribbean plate in southern guatemala giving rise to extension in southern guatemala and western honduras the fem predictions agree even better with the measured velocities if the plate motion west of the central american volcanic arc is forced to occur over a broad zone rather than along a single throughgoing plate boundary fault our analysis confirms key predictions of a previous numerical model for deformation in this region and also indicates that the curvature of the motagua fault causes significant along strike changes in the orientations of the principal strainrate axes in the fault borderlands in accord with earthquake focal mechanisms and conclusions reached in a recent synthesis of the structural and morphologic data from honduras poor fits of our preferred models to the velocities of gps sites near the gulf of fonseca may be an artefact of the still short gps time series in this region or the simplifying assumptions of our fems

neogene north american caribbean plate boundary across northern central america offset along the polochic fault abstract the polochic fault was a segment of the north american caribbean plate boundary across central america in the neogene its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central guatemala across the fault additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault a reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the polochic became active with the reconstruction carried across the isthmus on an east west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the motagua suture zone confined within this belt narrowed to some 60 km by the reconstruction lie the major laramide thrusts folds and tectonically emplaced serpentinites of guatemala crystalline rocks of guatemala re join the chiapas massif and paleozoic sedimentary rocks exposed in the core of an almost continuous anticlinorium extend from southern chiapas to lake izabal the polochic does not bend in eastern guatemala but continues eastward to the motagua fault where it dies westward drift of the northern block resulted in rifting which extended from eastern guatemala into the caribbean along the cayman trough the honduras depression may represent an element of a triple junction along with the polochic and izabal cayman rift the polochic continues westward into the pacific ocean and offsets the middle america trench the polochic has offset the miocene volcanic belt of northern central america confirming the previous estimate of a neogene time of movement about 300 km of relative east west neogene displacement has been recorded on the mid cayman rise only 130 km of which can be accounted for across the polochic it is suggested that cumulative extension on north south faults south of the motagua fault zone between the trench and the honduras depression might make up that difference

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

stable isotopic studies of nautilus macromphalus sowerby new caledonia and nautilus pompilius l fiji abstract two specimens of nautilus macromphalus sowerby from new caledonia and two specimens of nautilus pompilius l from fiji have been selected for stable isotope analysis of all septa these animals were captured at depths of 80 m and 200 m respectively the early formed septa were also analyzed from three additional specimens of n macromphalus captured in new caledonia at 300 400 m oxygen isotope compositions of septa from both populations are characterized by 1 an abrupt increase in su18o of 1 3 5 from septa 6 to 8 and 2 septum to septum variations in 13 and 18o which are roughly sympathetic and larger for carbon than oxygen septa from the fiji specimens are slightly depleted in 18o relative to those from new caledonia the isotopic step between septa 7 and 8 is interpreted to record the hatching of the embryo after development of the seventh septum marked 18o depletion of septa 1 7 evidently reflect non equilibrium metabolic isotope fractionation during embryonic groth stages or perhaps more likely a difference in the isotopic compositions of nepionic fluid and sea water the oxygen isotope compositions of septa 10 and higher and shell walls appear to closely approach isotopic equilibrium with sea water as demonstrated by a comparison of calculated isotopic and known temperatures for aragonite precipitation by an aquarium grown specimen a slightly warmer average environment of growth is indicated for the fiji samples

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

paleosurfaces paleoelevation and the mechanisms for the late miocene topographic development of the altiplano plateau abstract we construct a topographic datum across the central andes between 18 s and 22 s using extensive well preserved paleosurfaces that span the entire range this datum combined with estimates of topographic change from paleoaltimetry in the altiplano and geomorphic relief generation estimates from the plateau flanks allow us to reconstruct the paleo topography of the range at 10 ma using the 10 ma topography we explore which geophysical processes can create the observed topographic change over the

recent state of stress change in the walker lane zone western basin and range province united states the nw to north trending walker lane zone wlz is located along the western boundary of the northern basin and range province with the sierra nevada this zone is distinguished from the surrounding basin and range province on the basis of irregular topography and evidence for both normal and strike slip holocene faulting inversion of slip vectors from active faults historic fault offsets and earthquake focal mechanisms indicate two distinct quaternary stress regimes within the wlz both of which are characterized by a consistent wnw 3 axis these are a normal faulting regime with a mean 3 axis of n85 9 w and a mean stress ratio r value r 2 1 3 1 of 0 63 0 74 and a younger strike slip faulting regime with a similar mean 3 axis n65 70 w and r values ranging between 0 1 and 0 2 this younger regime is compatible with historic fault offsets and earthquake focal mechanisms both the extensional and strike slip stress regimes reactivated inherited mesozoic and cenozoic structures and also produced new faults the present day strike slip stress regime has produced strike slip normal oblique slip and normal dip slip historic faulting previous workers have explained the complex interaction of active strike slip oblique and normal faulting in the wlz as a simple consequence of a single stress state with a consistent wnw 3 axis and transitional between strike slip and normal faulting maximum horizontal stress approximately equal to vertical stress or r 0 in both regimes with minor local fluctuations the slip data reported here support previous results from owens valley that suggest deformation within temporally distinct normal and strike slip faulting stress regimes with a roughly constant wnw trending 3 axis zoback 1989 a recent change from a normal faulting to a strike slip faulting stress regime is indicated by the crosscutting striae on faults in basalts 300 000 years old and is consistent with the dominantly strike slip earthquake focal mechanisms and the youngest striae observed on faults in plio quaternary deposits geologic control on the timing of the change is poor it is impossible to determine if there has been a single recent absolute change or if there is rather an alternating or cyclical variation in stress magnitudes our slip data in particular the cross cutting normal and strike slip striae on the same fault plane are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the wlz by wesnousky and jones 1994 the location of the wlz between the deep seated regional extension of the basin and range and the right lateral strike slip regional tectonics of the san andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone in early to mid tertiary time the wlz appears to have had a similarly complex deformational history in this case as a back arc or intra arc region accommodating at least part of the right lateral component of oblique convergence as well as a component of extension

stress transmission across an active plate boundary an example from southern mexico abstract paleostress tensors calculated from inversion of fault slip data at 60 stations between zihuatanejo and tehuantepec in southern mexico were separated into four distinct groups and assigned to time intervals between 150 ma and recent the change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the north american and the farallon cocos plates for the same time interval depending on the angle between the convergence direction and the plate margin both thrust fault and strike slip stress regimes were active based on this comparison we infer that the stress distribution in the continental crust of southern mexico has been controlled by interplate stress transmission latest cretaceous early tertiary mylonites at the northern edge of the xolapa complex portray left lateral oblique crustal extension and oceanward migration of the upper plate the orientation of extension at mid crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level during the last 20 ma a tensional stress regime as a response to surface uplift has been dominant in southern mexico

the stable isotopic records of fossils from the peterborough member oxford clay formation jurassic uk palaeoenvironmental implications carbon and oxygen isotopic compositions of calcareous nekton aragonite kosmoceras and calcite belemnites and benthos aragonite nuculacean bivalves and calcite gryphaea from the peterborough member of the oxford clay formation vary considerably at any one horizon but show no obvious trend with time this suggests that the temperature and isotopic composition of ambient seawater may have varied considerably over short time intervals but did not change systematically during deposition of the unit thus short term fluctuations were superimposed on longer term 9stability9 in this shallow shelf environment the overlap in carbon isotopic compositions between calcareous nekton and benthos suggest a well mixed water column oxygen isotopic palaeotemperatures suggest thermal stratification provided that kosmoceras 16 28 c inhabited near surface waters and belemnites 12 19 c indistinguishable from the range for benthic bivalves were nektobenthic oxygen isotopic compositions of phosphate from both shallow and possible deeper dwelling vertebrates reptiles bony fish and sharks yield the same range of palaeotemperatures 20 29 c suggesting that all vertebrate taxa may have lived predominantly in warm shallow waters

rate and processes of river network rearrangement during incipient faulting the case of the cahabon river guatemala deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics although the reorganization of deeply incised drainage systems has been documented the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood to address this problem we have explored the rapid drainage reorganization that affected the cahabon river in guatemala during the quaternary sediment provenance analysis field mapping and electrical resistivity tomography ert imaging are used to reconstruct the geometry of the valley before the river was captured dating of the abandoned valley sediments by the be 10 al 26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes such as the rate of tectonic deformation of the paleovalley the rate of propagation of post capture drainage reversal and the rate at which canyons that formed at the capture sites have propagated along the paleovalley transtensional faulting started 1 to 3 million years ago produced ground tilting and ground faulting along the cahabon river and thus generated differential uplift rate of 0 3 0 1 up to 0 7 0 4 mm y 1 along the river s course the river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence then the river experienced two captures and one avulsion between 700 ky and 100 ky the captures breached high standing ridges that separate the cahabon river from its captors captures occurred at specific points where ridges are made permeable by fault damage zones and or soluble rocks groundwater flow from the cahabon river down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges valley fill formation and capture occurred in close temporal succession suggesting a genetic link between the two we suggest that the aquifers accumulated within the valley fills increased the head along the subterraneous system connecting the cahabon river to its captors and promoted their development upon capture the breached valley experienced widespread drainage reversal toward the capture sites we attribute the generalized reversal to combined effects of groundwater sapping in the valley fill axial drainage obstruction by lateral fans and tectonic tilting drainage reversal increased the size of the captured areas by a factor of 4 to 6 at the capture sites 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm y 1 deepening at a rate of 0 7 to 1 5 mm y 1 at this rate 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley it is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field which involved transient sedimentation along the river s course if the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

gps constraints on deformation in northern central america from 1999 to 2017 part 1 time dependent modelling of large regional earthquakes and their post seismic effects we use continuous and campaign measurements from 215 gps sites in northern central america and southern mexico to estimate coseismic and afterslip solutions for the 2009 m w 7 3 swan islands fault strike slip earthquake and the 2012 m w 7 3 el salvador and m w 7 4 guatemala thrust faulting earthquakes on the middle america trench our simultaneous time dependent inversion of more than 350 000 daily gps site positions gives the first jointly consistent estimates of the coseismic slips for all three earthquakes their combined time dependent post seismic effects and secular station velocities corrected for both the co seismic and post seismic deformation our geodetic slip solutions for all three earthquakes agree with previous estimates that were derived via static coseismic offset modelling our time dependent model which attributes all transient post seismic deformation to earthquake afterslip fits nearly all of the continuous gps site position time series within their several millimetre position noise afterslip moments for the three earthquakes range from 35 to 140 per cent of the geodetic coseismic moments with the largest afterslip estimated for the 2012 el salvador earthquake along the weakly coupled el salvador trench segment forward modelling of viscoelastic deformation triggered by all three earthquakes for a range of assumed mantle and lower crustal viscosities suggests that it accounts for under 20 per cent of the observed post seismic deformation and possibly under 10 per cent our results thus point to afterslip as the primary and perhaps dominant mode of post seismic deformation for these c the author s

the oxfordian stable isotope record δ18o δ13c of belemnites brachiopods and oysters from the kachchh basin western india and its potential for palaeoecologic palaeoclimatic and palaeogeographic reconstructions abstract the study combines the results of stable isotope 13c 18o analyses of 187 belemnites brachiopods and oysters from the middle to upper jurassic upper callovian to kimmeridgian of the kachchh basin in western india generally belemnites show lower 13c and higher 18o ratios than the benthic fauna it is proposed that this discrepancy is caused by different life habitats of both groups with belemnites spending major parts of their life in deeper water outside the kachchh basin and entering shallow areas only occasionally e g during seasonal migrations to spawning grounds a mode of life as fast swimming nektonic predators similar to many modern squid species seems plausible the study emphasises that results of stable isotope analyses should be evaluated separately for each fossil group in addition to the influence of diagenetic alteration weathering salinity changes and sea level fluctuations the ecology of the analysed fossil taxa has to be considered temperatures calculated from 18o ratios point to relatively warm conditions around the middle to late jurassic transition with no evidence for a glacial phase which had been proposed for this boundary a distinct fall in temperatures is observed during the middle oxfordian reaching a minimum in the early late oxfordian it is tempting to connect this event with the break up of gondwana causing stronger upwelling in the northwestern tethys or alternatively the influx of polar water by the opening of the transgondwanan seaway however such connections cannot be verified without further data preferably from localities in the southern malagasy gulf e g madagascar and from younger horizons kimmeridgian tithonian

crustal deformation in northern central america abstract evaluation of the seismic moment tensor for earthquakes on plate boundary is a standard procedure to determine the relative velocity of plates which controls the seismic deformation rate predicted from the slip on a single fault the moment tensor is also decomposed into an isotropic and a deviatoric part to discover the relationship between the average strain rate and the relative velocity between two plates we utilize this procedure to estimate the rates of deformation in northern central america where plate boundaries are seismically well defined four different tectonic environments are considered for modelling of the plate motions the deformation rates obtained here compare well with those predicted from the plate motions models and are in good agreement with actual observations deformation rates on faults are increasingly being used to estimate earthquake recurrence from information on fault slip rate and more on how we can incorporate our current understanding into seismic hazard analyses

metamorphic reworking of a high pressure low temperature melange along the motagua fault guatemala a record of neocomian and maastrichtian transpressional tectonics abstract the guatemala suture zone is a major east west left lateral strike slip boundary that separates the north american and caribbean plates in guatemala the motagua fault the central active strand of the suture zone underwent two major collisional events within a system otherwise dominated by strike slip motion the first event is recorded by high pressure low temperature hp lt eclogites and related rocks that occur within serpentinites both north and south of the motagua fault lawsonite eclogites south of the fault are not significantly retrograded and give 40ar 39ar ages of 125 116 ma and sm nd mineral isochrons of 144 132 ma eclogites north of the fault give similar sm nd isochron ages 131 126 ma but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and along with associated hp lt rocks give much younger 40ar 39ar ages of 88 55 ma indicating a later amphibolite facies metamorphic event we propose therefore that all serpentinite hosted eclogites along the motagua fault formed at essentially the same time in different parts of a laterally extensive lower cretaceous forearc subduction system but subsequently underwent different histories the southern assemblages were thrust southwards present coordinates immediately after hp metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca 70 ma they were subsequently juxtaposed opposite each other by major strike slip motion this model implies that the hp rocks on opposing sides of the motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the late cretaceous as a result of oblique convergence the juxtaposition of a convergent and strike slip system means that hp lt rocks within serpentinites can be found at depth along much of the modern guatemala suture zone and its eastward extension into the northern caribbean both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

destructive upper crustal earthquakes of central america since 1900 we have compiled a catalog of 51 destructive upper crustal earthquakes in central america since 1900 an event is included if it caused casualties or heavy damage of modified mercalli mm intensity vii all events larger than magnitude ms 5 7 are found to be destructive we believe the catalog is complete for events of ms 6 along the volcanic front the catalog includes estimates of epicentral coordinates depth magnitude and casualties for all events and presents mm intensity vii contours for most events data in this catalog place severe constraints on the spatial temporal and magnitude distribution of destructive upper crustal earthquakes in central america the catalog contains 30 events of ms 6 depths of well constrained mainshock hypocenters range from 5 to 15 km the most striking feature of the catalog is the spatial alignment along the volcanic front of 23 of the 30 events volcanic front earthquake source mechanisms for such events show east west tension and are compatible with a model of the volcanic front as a zone of right lateral strike slip faulting volcanic front earthquake magnitudes do not exceed ms 6 5 except in southeastern guatemala where they reach ms 6 9 maximum earthquake magnitudes are apparently related to the distance between adjacent holocene volcanic centers destructive events occur at offsets in the volcanic chain as isolated mainshock aftershock sequences however within linear segments destructive earthquakes have a strong tendency to cluster and are often preceded by significant foreshock activity although the volcanic front is the locus of both destructive earthquakes and volcanism we find no temporal correlation between the two of the seven events of ms 6 that did not originate near the volcanic front six originated near the transcurrent caribbean north american plate boundary including the ms 7 5 guatemala earthquake of 1976 along the volcanic front destructive upper crustal earthquakes have occurred at a rate of about one event every 2 1 2 years during this century much more frequently than along either the caribbean north american plate boundary or the subduction zone in central america since 1900 about 17 000 people have died from volcanic front earthquakes but only about 2000 people are known to have died from subduction zone earthquakes although subduction zone earthquakes can have larger magnitudes and produce more widespread damage destructive volcanic front earthquakes pose the greater hazard because they are more frequent and originate much closer to the bulk of central america s population which is concentrated along the volcanic front

quaternary silicic pyroclastic deposits of atitlan caldera guatemala abstract atitlan caldera has been the site of several silicic eruptions within the last 150 000 years following a period of basalt andesite volcanism the silicic volcanism began with 5 10 km3 of rhyodacites erupted as plinian fall and pyroclastic flows about 126 000 yr b p at 85 000 yr b p 270 280 km3 of compositionally distinct rhyolite was erupted in the los chocoyos event which produced widely dispersed plinian fall deposits and widespread mobile pyroclastic flows in the latter parts of this eruption rhyodacite and minor dacite were erupted which compositionally resembled the earliest silicic magmas of the atitlan center as a result of this major eruption the modern atitlan iii caldera formed following this event rhyodacites were again erupted in smaller 5 13 km3 volumes partly through the lake and mafic volcanism resumed forming three composite volcanoes within the caldera the bimodal mafic silicic atitlan volcanism is similar to that which has occurred elsewhere in the guatemalan highlands but is significantly more voluminous mafic lavas are thought to originate in the mantle but rise intrude and underplate the lower crust and partly escape to the surface eventually silicic melts form in the crust possibly partly derived from underplated basaltic material rise crystallize and erupt the renewed mafic volcanism could reflect either regional magmato tectonic adjustment after the large silicic eruption or the onset of a new cycle

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

stable isotope fractionation in live benthic foraminifera from the southern california borderland live specimens of benthic foraminifera have been analyzed from surface sediment samples collected from 108 to 1134 m depth in the southern california borderland the temperature and isotopic composition of the ambient water and dissolved inorganic carbon have also been measured to permit comparison of foraminiferal compositions to expected equilibrium values the species analyzed break down into four groups which reflect the influence of mineralogy aragonite vs calcite biological fractionation vital effect and microhabitat effects on isotopic composition the aragonitic hoeglundina elegans show an enrichment in 18o relative to equilibrium calcite of 0 41 0 in shallower water increasing to at least 1 06 0 in deeper water the 13c enrichment of h elegans relative to the dissolved inorganic carbon dic is also depth dependent increasing from 1 27 0 at 108 m to 2 13 0 at 940 m this depth dependence results from the change in composition of the dic the 13c composition of h elegans remains constant with depth for some unknown reason one possible explanation is that the change in composition of the dic is balanced by an increase in hoeglundina hco 3 fractionation with decreased temperature the 13c values of the cassidulina species are almost identical to those of the ambient dissolved inorganic carbon dic and reflect the decrease in the 13c of the dic with depth the 13c values of species not in 18o equilibrium vary independently of the changing composition of the dic there is a correlation between 13c and 18o among coeval species consistent with the model which explains vital effect as the result of incorporation of isotopically light metabolic co2 into the shell the magnitude of disequilibrium is relatively constant for 18o but not for 13c suggesting that in isotopic stratigraphy studies it may be difficult to calibrate the 13c of different species as has been done with 18o there appears to be a relationship between taxonomic group and vital effect the foraminifera in approximate 18o equilibrium with the water including species of cassidulina bolivina uvigerina and globobulimina are members of the family cassidulinidae or superfamily buliminacea those not in 18o equilibrium including species of pyrgo quinqueloculina triloculina and lenticulina belong to the families miliolidae and vaginulinidae no disequilibrium species have been found as yet in the low oxygen environments of the borderland this observation and published findings on the ultrastructure of benthic foraminifera suggest that isotopic behavior may be related to a foraminifer s ability to exchange gases rapidly with its ambient water

carbon and oxygen isotope composition of nautilus macromphalus a record of thermocline waters off new caledonia abstract this study investigates the controls on the stable isotope composition of shell nacreous layer of a modern nautilus macromphalus from new caledonia closely spaced samples were taken both along growth lines isochrons and growth axis time series carbon isotope compositions 13 c 0 3 to 1 pdb of the shell decrease in the domains of maximal growth 0 3 and muscle insertion 0 4 maximal 13 c variations of about 1 are recorded in time series a global ontogenetic decrease of 13 c values is observed for both dorsal and ventral areas 0 4 to 0 8 and can be ascribed to a metabolic effect with age or to changes in the food source by contrast oxygen isotope compositions are quite constant 1 1 18 o

two high pressure low temperature serpentinite matrix melange belts motagua fault zone guatemala a record of aptian and maastrichtian collisions left lateral motion along the north american caribbean plate boundary has juxta posed two high pressure low temperature hp lt belts from separate cretaceous colli sions these two belts have quite different ages and different suites of high pressure as semblages yet they both contain jadeitite a relatively rare rock type this part of the plate boundary zone follows the motagua river valley in guatemala where it separates the maya block north american plate from the chortis block caribbean plate on both sides of the bounding motagua fault tectonic slices of serpentinite matrix melange host the hp lt rocks south of the fault the melange slices contain eclogite lawsonite eclogite glaucophane eclogite and blueschist blocks north of the fault the melange slices contain omphacite metabasite albitite and garnet amphibolite blocks but lack intact eclogite in addition to the dissimilar rock assemblages 40 ar 39 ar geochronology of phen gitic micas yields 77 65 ma for northern and 125 113 ma for southern blocks these data suggest that the southern belt formed during early cretaceous aptian northeastward dipping subduction of the farallon plate and collision of the chortis block with western mexico the block was then displaced southeastward along this suture in contrast the northern belt records subduction related to the maastrichtian collision of an extension of the chortis block perhaps the nicaraguan rise with the maya block

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

ontogeny and habitat change in mesozoic cephalopods revealed by stable isotopes δ18o δ13c abstract stable isotope 18 o and 13 c ratios were measured in successive aragonitic shell sequences of ammonoids class cephalopoda to determine whether their depth distributions changed within ontogeny and whether stable isotope values differ in various morphological groups e g leiostraca vs trachyostraca we concentrate mainly on 18 o for temperature results and added 13 c data to obtain information on the ontogenetic history for which full spiral measurements were undertaken for the first time to obtain valid stable isotope data from ammonoid shells we measured ontogenetic sequences full shell within different genera data sets from the jurassic cadoceras and cretaceous hypacanthoplite s nowakites were chosen due to the pure primary aragonitic shell preservation the study was designed to extract better information on the habitat and life cycle of fossil cephalopods e g ammonoids in comparison with recent cephalopods e g nautilus spirula sepia possessing equivalent or comparable hard parts the data from three genera suggest different modes of life in at least two morphological groups we detected and established two main groups with different ontogenetic strategies based on the 18 o data the wcw type warm cool warm type of cadoceras resembles strategies in nautilus and sepia which migrate from shallow into deeper environments and back in ontogeny wc type warm cool type and the cw type cool warm type of hypacanthoplites resembling the first two migration phases of spirula cwc type which migrates from deeper into shallower and back again into deeper habitats the main three phases revealed by both 18 o and 13 c data sets most probably reflect diet changes in juvenile to mid aged individuals followed by a habitat change for spawning adults in cadoceras the temperatures range from 21 2 c for juveniles down to 12 1 c for mid aged individuals and back up 16 9 c in adults the cw type strategy of hypacanthoplites involves a temperature range of 22 8 c to 28 9 c the respective mean values are 24 2 c juveniles 25 8 c middle phase and 27 8 c adults the 13 c values also revealed three ontogenetic stages in cadoceras and hypacanthoplites including two major shifts from positive to negative and from negative to positive values which probably correspond to sexual maturation the initiation of reproduction and concomitant changes in diet the presented data combined with previous ontogenetic studies e g stable isotopes on spirula nautilus and sepia can be used as proxies to directly correlate the habitats and ontogeny of recent and fossil cephalopods

sedimentary architecture structural setting and late cenozoic depocentre migration of an asymmetric transtensional basin lake izabal eastern guatemala abstract lake izabal located in eastern guatemala lies in an ew trending basin located along the transform margin between the north american and caribbean plates this plate boundary consists of two main left lateral strike slip faults known as the polochic and motagua fault system pmfs the basin is a 100 20 km lens shaped depression in which the lake occupies its eastern half to the north the basin is confined by a principal deformation zone pdz which is recognized in this area as the most important upper crustal branch of the polochic fault the eastern part of the basin has been uplifted and dissected exposing the basin infill along the slopes of the montana del mico analysis of a set of multichannel seismic reflection profiles most of them acquired within the lake combined with well and outcrop information allowed to outline the structure and evolution of the basin and its stratigraphic architecture the basin is strongly asymmetrical both in cross and along strike directions and is filled up by neogene to quaternary sediments with the deepest side of the basin being to the north close and parallel to the pdz the base of the infill is a mid tertiary unconformity whose differential subsidence through time brought the basin to its present structural setting five main evolutionary stages are recognized based on seismo stratigraphic and structural features which are related to the development and activity of the bordering faults development of the basin began in its easternmost sector a small half graben tilted to the south developed in response to the activity of an oligocene or early middle miocene growth fault in the second stage late miocene the basin tilts to the north causing the sedimentary sequences to thicken towards the northern pdz this stage is dominated by oblique extension and by progressive migration of the basin depocenter towards the west the third stage early pliocene records a strong acceleration of the strike slip activity which causes a rapid westward migration of the depocenter the fourth stage late pliocene although still dominated by regional transtension is related to the development of a transpressional event along the eastern side of the basin as evidenced by the occurrence of folds and reverse faults parallel to the northern bend of the polochic fault during the fifth stage quaternary the eastern part of the basin remains relatively quiescent while the western part undergoes subsidence with localized transtension still related to the activity in this sector of the polochic fault left lateral movement along the polochic fault caused the depocenter to migrate 75 80 km at different velocities with slip rates ranging between 2 1 and 16 3 mm yr this study suggests that the polochic fault developed in the early middle miocene in connection to the deposition of the early sediments of the izabal basin even though an older age oligocene could also be possible to our knowledge this is one of the best examples of sedimentary architecture documenting depocentre migration due to fault movement along a transform system

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

influences of soil water characteristic curve on rainfall induced shallow landslides the analysis of slope instability induced by rainfall was usually performed using the main drying curve as the measurement of the main wetting curve is a more time consuming and costly task in this study the influences of the main drying and wetting curves on rainfall induced shallow landslides are examined three designed scenarios and a real case scenario are used to conduct this examination the prediction of shallow landslide occurrence is related to the main drying and wetting curves due to the strong relation between groundwater pressure head and hysteresis effect the main wetting curve may have a less minimum landslide triggering rainfall amount and a less rainfall duration threshold for landslide occurrence than the drying wetting curve for safety s sake an underestimation of shallow landslide occurrence may be produced by the commonly used main drying curve in addition besides the shallow landslide occurrence the failure depth and the time to failure are also influenced by the main drying and wetting curves the hysteresis effect should be taken into account for assessing rainfall induced shallow landslides

isotope paleontology growth and composition of extant calcareous species abstract isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms especially temperature and to obtain clues about life history and modes of growth in calcareous fossils oxygen isotopes are mainly used in the former and carbon isotopes in the latter however since physical surroundings and organism response are intimately associated both types of information are contained in each of the isotopic signals to explore the potential of isotope paleontology and to provide a basis for reconstruction a broad range of extant organisms has been studied taking the pioneering work of epstein and associates as a starting point results are summarized for a representative sampling of these studies with emphasis on work at the laboratories of the authors from the mid seventies to the present the organisms considered are nannoplankton benthic algae planktonic and benthic deep sea foraminifera larger foraminifera sponges corals bryozoans polychaetes arthropods bivalves gastropods cephalopods and vertebrates fish otoliths the survey broadly suggests that regarding oxygen isotopes materials tend to be precipitated close to equilibrium with the surrounding seawater as postulated by urey and that for carbon isotopes disequilibrium is the rule life spans growth rates differential seasonal growth and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts in detail the interpretation of isotope records of individual shells is quite complicated and simple models will fail to give satisfactory results in many or most cases

isotopic signals from callovian kimmeridgian middle upper jurassic belemnites and bulk organic carbon staffin bay isle of skye scotland abstract the stable isotope data presented here significantly extend and expand upon previous isotopic investigations of the middle to late jurassic interval the belemnite samples collected from the staffin bay and staffin shale formations from the isle of skye scotland yielded oxygen isotope values consistent with callovian kimmeridgian palaeotemperatures of 6 7 20 6 c the carbon isotope data comprise one of the first moderately high resolution investigations of the relationship between terrestrial 13 c org predominantly fossil wood debris and marine 13 c carb belemnites as derived from a geologically coeval record the staffin bay data reveal a broad early to mid oxfordian positive carbon isotope excursion the excursion maximum occurs in the cordatum zone british boreal ammonite zonation although high values persist into the tenuiserratum zone the correspondence between the marine and terrestrial records indicates a strong coupling of the ocean atmosphere system and suggests that the total exchangeable carbon reservoir would have been affected at this time the mid oxfordian negative carbon isotope excursions identified in published tethyan records and commonly attributed to methane release are not recorded in the staffin bay data which may suggest that the tethyan excursions do not represent fluctuations in the global carbon reservoir and that the fidelity of the methane hypothesis should be re evaluated supplementary material isotopic and geochemical data are available at http www geolsoc org uk sup18339

stable carbon and oxygen isotope compositions of mollusc shells from britain and new zealand abstract stable carbon and oxygen isotope analysis of the carbonate shells of two fossil mollusc species from the plio pleistocene of new zealand and two recent species from the west coast of scotland has revealed an unexpectedly high degree of variability for each species and a positive correlation between 13 c and 18 o in each case this phenomenon may be due to kinetic isotope effects which are inherent in fastgrowing shells or areas of shell if this is the case then equilibrium isotope partitioning may not have had time to occur before the completion of calcite precipitation kinetic effects would tend to favour the lighter isotopes of both carbon and oxygen it is therefore the highest 13 c and 18 o values that are most likely to reflect equilibrium with the environment very small carbonate particles often give particularly low 13 c and 18 o values these results have implications for the use of isotopic data in environmental reconstruction based on fast growing shells and may also be applicable to other carbonate precipitating organisms

warm palaeotemperatures from high late jurassic palaeolatitudes falkland plateau ecological environmental or diagenetic controls abstract oxygen and carbon isotopes have been determined from late jurassic oxfordian tithonian belemnites and inoceramid bivalves from two deep sea drilling project dsdp sites located on the falkland plateau mean belemnite 18o values derived from well preserved skeletal material were 1 29 from dsdp site 330 and 1 45 from dsdp site 511 assuming a seawater smow value of 1 0 mean palaeotemperatures calculated from the oxygen isotopic composition are 17 2 c and 17 9 c respectively the inoceramid bivalves yielded much lighter 18o values mean 3 58 petrographic and geochemical evidence points to the inoceramid bivalves being altered by diagenesis which accordingly accounts for the observed differences in isotopic values vital effects or the importation of belemnites or inoceramids from another area are considered not to account for the observed isotopic trends the palaeotemperatures interpreted from the belemnites are significantly warmer than other recent estimates of late jurassic temperature from oxygen isotope studies and climate model predictions from similar southern palaeolatitudes we suspect our apparent warmer temperatures are because of a combination of increased freshwater runoff depleting surface waters with respect to 18o and related to the semi enclosed nature of the depositional basin retaining warmth relative to the open ocean of similar latitudes

reconstruction of late bajocian bathonian marine palaeoenvironments using carbon and oxygen isotope ratios of calcareous fossils from the polish jura chain central poland abstract oxygen and carbon isotope analyses were carried out on upper bajocian bathonian calcitic belemnite rostra and oyster shells as well as aragonitic ammonite nautiloid and trigoniid shells from the ore bearing czestochowa clay formation in south central poland all samples were studied by means of cathodoluminscence microscopy trace element x ray and scanning electron microscope sem analysis in order to select best preserved material sem studies allowed classifying some aragonitic ammonite and nautiloid shells 99 aragonite as diagenetically altered average 18o values of oysters belemnites and trigoniids translate into relatively low palaeotemperatures of 10 1 9 2 and 7 4 c respectively the comparable 18o values of bivalves and belemnites indicate that belemnites had a nectobenthic lifestyle and did not record surface water temperatures the ammonite fauna is interpreted to have been nektonic however palaeotemperatures derived from ammonite 18o around 18 6 c may be overestimated due to a lower than normal marine salinity of surface waters the reduced salinity is inferred as well from sedimentological observations and faunal composition the obtained 18o values indicate cool climatic conditions during the late bajocian bathonian in south central poland which may have been linked with the occurrence of a cold climatic episode despite the fact that all studied shells were not precipitated in carbon isotope equilibrium with ambient waters the 13c shell values enabled us to roughly estimate isotope composition of dissolved inorganic carbon dic in the late bajocian bathonian sea of south central poland water 13cdic values calculated from ammonite 13c are lower in comparison to 13cdic values calculated from oyster and belemnite carbon isotope ratios which in conjunction with ammonite 18o values indicates lower salinity of surface or shallow near coastal waters

oxygen and stable carbon isotopes from a nautiloid from the middle pennsylvanian late carboniferous impregnation lagerstatte buckhorn asphalt quarry primary paleo environmental signals versus diagenesis abstract two shell fragments one of an undetermined cephalopod and one of an orthoconic nautiloid pseudorthoceratidae gen et sp indet from the pennsylvanian buckhorn asphalt lagerstatte oklahoma were studied to reconstruct paleotemperatures and to explore possible diagenetic alterations for the first time not only parts of the shell of an orthoconic nautiloid but also its cameral deposits were sampled in high resolution the mineralogy geochemistry and microstructure of both shell remains the pseudorthoceratid and the undetermined cephalopod were investigated in detail before high resolution transects for oxygen and stable carbon isotopes were sampled the shell fragment of the undetermined cephalopod showed no or very little diagenetic alteration therefore the measured 18o variation 18o range of 0 68 and the inferred temperature range of 28 to 31 c are interpreted to represent an original signal the temperatures indicate that the specimen lived in shallow warm tropical waters the 13c signal with a 13c range of 3 43 is interpreted to be the consequence of a strong metabolic effect a septum of the phragmocone of the pseudorthoceratid nautiloid was only slightly altered by diagenesis and a largely original 18o signal could be obtained the 18o values exhibit an inferred temperature range of 14 to 15 c indicating that this species lived in deeper and cooler waters the cameral deposits were investigated in detail by energy dispersive x ray edx and by xrd gadds x ray diffraction general area detector diffraction system analyses part of the cameral deposits in the orthoconic nautiloid turned out to originally consist of high mg calcite hmc instead of the estimated aragonite anyhow the massive cameral deposits show signs of strong small scale alteration resulting in a distinct spreading of oxygen values the oxygen and carbon isotope plots show a linear trend for the oxygen carbon correlation this is interpreted as the mixing of two end members with the lightest end members suggesting a meteoric diagenesis with a diagenetic fractionation of the aragonite to low mg calcite lmc respectively the hmc to lmc and dolomite the calculated temperatures of the pseudorthoceratid specimen and the undetermined cephalopod indicate two different paleoenvironments additionally the pseudorthoceratid nautiloid displays the narrow range of temperatures in the deeper parts of the sea and the undetermined cephalopod the more fluctuating temperatures in the shallow areas it is also shown how important a detailed investigation of microstructures and mineralogy of samples with an according cautious interpretation of the achieved dataset is the importance of an intensive examination of samples is pointed out to avoid small scale alterations to be overlooked since this would lead to a misinterpretation of the results

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

latest cretaceous to miocene deformation events in the eastern sierra madre del sur mexico inferred from the geometry and age of major structures the cretaceous cenozoic major lithologic units and structures of the sierra madre del sur are well known the laramide orogeny is generally considered as the cause of the contractile structures but the details about the migration kinematics and intensity of deformation are poorly known furthermore the deformation events responsible for the post laramide strike slip and normal faults have not been identified in this paper we document the migration of the deformation events that occurred in southern mexico from maastrichtian to miocene time we identify different groups of structures representing three successive deformation events based on the geometry age and kinematics of tectonic structures deformation migrated from west to east the first event corresponding to the laramide orogeny occurred during late cretaceous time in the guerrero morelos platform and ended in the middle eocene in the east within the veracruz basin the oaxacan fault system which bounds the acatlan oaxacan block to the east records laramide shortening from six structural sections we interpret the juxtaposition of the oaxacan complex against the mylonite belt of the sierra de juarez with subsequent uplift of the eastern border of the oaxacan complex and finally the gravitational overriding of the sedimentary cover in a radial centripetal arrangement the second event produced strike slip faulting during ne sw horizontal shortening from eocene to oligocene time the third event produced normal and strike slip faults indicating ne sw horizontal extension during oligocene miocene time major structures produced during these three deformation events are roughly distributed in an arcuate pattern bounding the block formed by the acatlan and oaxacan complexes based on this pattern and the relatively less deformed mesozoic rocks within the acatlan oaxacan block we interpret that most of the deformation resulted from the impingement of this block on thinner crustal domains adjoining the block

quaternary seismo tectonic activity of the polochic fault guatemala 1 the polochic motagua fault system is part of the sinistral transform boundary between the north american and caribbean plates in guatemala and the associated seismic activity poses a threat to 70 of the country s population the aim of this study is to constrain the late quaternary activity of the polochic fault by determining the active structure geometry and quantifying recent displacement rates as well as paleo seismic events slip rates have been estimated from offsets of quaternary volcanic markers and alluvial fan using in situ cosmogenic 36cl exposure dating holocene left lateral slip rate and mid pleistocene vertical slip rate have been estimated to 4 8 2 3 mm y and 0 3 0 06 mm y respectively on the central part of the polochic fault the horizontal slip rate is within the range of longer term geological slip rates and short term gps based estimates in addition the non negligible vertical motion participates in the uplift of the block north of the fault and seems to be a manifestation of the regional far field stress regime we excavated the first trench for paleo seismological study on the polochic fault in which we distinguish four large paleo seismic events since 17 ky during which the polochic fault ruptured the ground surface

localization and characterization of an active fault in an urbanized area in central guatemala by means of geoelectrical imaging article i nfo article history the polochic and motagua faults define the active plate boundary between the north american and caribbean plates in central guatemala a splay of the polochic fault traverses the rapidly growing city of san miguel uspantan that is periodically affected by destructive earthquakes this fault splay was located using a 2d electrical resistivity tomography ert survey that also characterized the fault damage zone and evaluated the thickness and nature of recent deposits upon which most of the city is built ert images show the fault as a 50 m wide near vertical low resistivity anomaly bounded within a few meters by high resistivity anomalies forward modeling reproduces the key aspects of the observed electrical resistivity data with remarkable fidelity thus defining the overall location geometry and internal structure of the fault zone as well as the affected lithologies our results indicate that the city is constructed on a 20 m thick surficial layer consisting of poorly consolidated highly porous water logged pumice this soft layer is likely to amplify seismic waves and to liquefy upon moderate to strong ground shaking the electrical conductivity as well as the major element chemistry of the groundwater provides evidence to suggest that the local aquifer might at least in part be fed by water rising along the fault therefore the potential threat posed by this fault splay may not be limited to its seismic activity per se but could be compounded its potential propensity to enhance seismic site effects by injecting water into the soft surficial sediments the results of this study provide the basis for a rigorous analysis of seismic hazard and sustainable development of san miguel uspantan and illustrate the potential of ert surveying for paleoseismic studies

the caribbean north america cocos triple junction and the dynamics of the polochic motagua fault systems pull up and zipper models the polochic motagua fault systems pmfs are part of the sinistral transform boundary between the north american and caribbean plates to the west these systems interact with the subduction zone of the cocos plate forming a subduction subduction transform triple junction the north american plate moves westward relative to the caribbean plate this movement does not affect the geometry of the subducted cocos plate which implies that deformation is accommodated entirely in the two overriding plates structural data fault kinematic analysis and geomorphic observations provide new elements that help to understand the late cenozoic evolution of this triple junction in the miocene extension and shortening occurred south and north of the motagua fault respectively this strain regime migrated northward to the polochic fault after the late miocene this shift is interpreted as a pull up of north american blocks into the caribbean realm to the west the pmfs interact with a trench parallel fault zone that links the tonala fault to the jalpatagua fault these faults bound a fore arc sliver that is shared by the two overriding plates we propose that the dextral jalpatagua fault merges with the sinistral pmfs leaving behind a suturing structure the tonala fault this tectonic zipper allows the migration of the triple junction as a result the fore arc sliver comes into contact with the north american plate and helps to maintain a linear subduction zone along the trailing edge of the caribbean plate all these processes currently make the triple junction increasingly diffuse as it propagates eastward and inland within both overriding plates

reconstructing the palaeoenvironment of the middle russian sea during the middle late jurassic transition using stable isotope ratios of cephalopod shells and variations in faunal assemblages abstract oxygen and carbon isotope data of well preserved belemnite rostra and ammonite shells are presented from the callovian oxfordian boundary uppermost lamberti to lowermost cordatum zones of the dubki section near saratov in the russian platform palaeotemperatures calculated for nektobenthic belemnites averages of 5 c and 8 c for cylindroteuthids and belemnopseids respectively show the presence of cold bottom waters in the central part of the middle russian sea during the studied interval palaeotemperatures calculated for ammonites which are assumed to have lived in near surface waters are considerably higher average 13 c the presented data show a vertical thermal gradient in the middle russian sea the belemnite oxygen isotope record and the relative abundances of ammonite families in the dubki section do not correlate with each other probably as a result of different depth habitats of ammonites and belemnites a review of literature isotope data shows the climatic zonation in european seas at the middle late jurassic transition despite the flux of cold polar waters to the middle russian sea and the area of scotland there is no evidence for glaciation at the middle late jurassic transition changes in water circulation during a sea level highstand were likely a source of spreads of cold bottom waters and cardioceratid ammonite fauna in this time period the belemnite isotope record of the callovian oxfordian boundary in the russian platform is characterized by significant scatter of 13c values no temporal carbon isotope trend is observed the 13c values of russian belemnite rostra average 2 6 vpdb being 1 to 2 higher than the values of coeval lower oxfordian belemnites from the area of the submediterranean ammonite province higher than submediterranean 13c values of russian belemnite rostra are likely related to high biologic productivity and or high organic matter burial in semi isolated boreal subboreal marine basins

primary and diagenetic isotopic signals in fossils and hemipelagic carbonates the lower jurassic of northern spain stable isotope and trace element analyses of 230 jurassic pliensbachian toarcian samples from northern spain have been performed to test the use of geochemical variations in fossils belemnites and brachiopods and whole rock hemipelagic carbonates as palaeoceanographic indicators although the succession analysed reinosa area westernmost basque cantabrian basin has been subject to severe thermal alteration during burial diagenesis the samples appear to be well preserved the degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic chemical and cathodoluminescence analyses it is demonstrated that brachiopods and whole rock carbonates although widely used for palaeoceanic studies do not retain their primary marine geochemical composition after burial diagenesis in contrast there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock well preserved belemnite shells non luminescent to slightly luminescent typically show stable isotope values of 4ae3 to 0ae7 d 13 c 0ae7 to 3ae2 d 18 o and trace element contents of 950 l gg 1 sr and sr mn ratios 80 this study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low mg calcite fossil shells and hemipelagic bulk carbonates behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated multiple non luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders yet their d 13 c and d 18 o values exhibited moderate scatter such variability is likely to be related to the palaeoecological behaviour of belemnites and or highfrequency secular variations in sea water chemistry superimposed on the longterm isotopic trend a pronounced positive carbon isotope excursion up to 4ae3 is documented in the early toarcian serpentinus biozone which correlates with the toarcian d 13 c maximum reported in other european and tethyan regions

constraints from finite element modeling on the active tectonics of northern central america and the middle america trench 1 we have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the central american volcanic arc and the chortis block we present synthetic focal mechanisms maps of tectonic regime and strain crosses to analyze the results the models show that to achieve the observed state of stress on the volcanic arc the arc must be modeled as a lithospheric weak zone also the forces related to the eastward drift of the caribbean plate must be higher than those related to the subduction of the cocos plate the coupling on the subduction interface must be low with or without slip partitioning due to the obliquity of the subduction at the trench at guatemala the western edge of the chortis block is pinned against north america even with low trench normal forces making the triple junction between the cocos north american and caribbean plates a zone of diffuse deformation the extension in the western part of the chortis block from guatemala to the honduras depression is explained by the geometry of the north american caribbean plate boundary and the direction of motion of the caribbean plate with respect to north america the direction of extension in the chortis block is always e w regardless of the magnitude of the applied forces and the main part of the deformation is absorbed between the ipala graben and the honduras depression both features being consistent with our models

intra arc extension in central america links between plate motions tectonics volcanism and geochemistry this study revisits the kinematics and tectonics of central america subduction synthesizing observations of marine bathymetry high resolution land topography current plate motions and the recent seismotectonic and magmatic history in this region the inferred tectonic history implies that the guatemala el salvador and nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension extension arising from the interplay between subduction roll back of the cocos plate and the 10 15mm yr slower westward drift of the caribbean plate relative to the north american plate the ages of belts of magmatic rocks paralleling both sides of the current nicaraguan arc are consistent with long term arc normal extension in nicaragua at the rate of 5 10mm yr in agreement with rates predicted by plate kinematics significant arc normal extension can hide a very large intrusive arc magma flux we suggest that nicaragua is in fact the most magmatically robust section of the central american arc and that the volume of intrusive volcanism here has been previously greatly underestimated yet this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits observed geochemical differences between the nicaraguan arc and its neighbors which suggest that nicaragua has a higher rate of arc magmatism are consistent with this interpretation smaller amplitude but similar systematic geochemical correlations between arc chemistry and arc extension in guatemala show the same pattern as the even larger variations between the nicaragua arc and its neighbors

geochemical signatures of the early campanian belemnite belemnellocamax mammillatus from the kristianstad basin in scania sweden article macrofossil calcite plays a central role for palaeoenvironmental reconstructions because its preservation state and geochemical signatures can be checked using a variety of screening techniques reconstructions of the ecology and seawater composition from such data require knowledge about species specific compositions of well preserved calcitic fossils comprehensive datasets recording averages intraspecimen and intraspecific variability of geochemical proxies in fossil calcite however are still largely lacking here we report 13 c and 18 o values as well as mg ca sr ca and mn ca ratios of 204 aliquots sampled along transectsthroughthree rostraof thelate earlycampanianbelemnite speciesbelemnellocamax mammillatus geo chemical signatures of partially altered samples are consistent to those reported for early marine calcite cemen tationandshowtrendstowardsmoderatelyhighermn ca heavier 13 candlowersr ca incontrast mg caratios and 18 o values remain similar to projected original values median 18 ov alues of 0 3 to 0 0 are comparable to the heaviest values of coeval oysters from the same localities and internal variability of the well preserved parts of the rostra is low these findings suggest that b mammillatus lived mostly in the deep part of the basin where seasonal changes were low and water temperatures cool the 13 c is offset from oysters and most coeval

constraints for the recent tectonics of the el salvador fault zone central america volcanic arc from morphotectonic analysis abstract we have used hypsometric analysis to improve our understanding of the current tectonic deformation and structure of el salvador fault zone a n90 e oriented strike slip fault zone that extends 150 km through el salvador central america our results indicate an important amount of transtensive strain along this fault zone providing new data to understand the tectonic evolution of the salvadorian volcanic arc we have defined kilometric scale tectonic blocks and its relative vertical movements length of segments with homogeneous vertical motions and lateral relay of active structures we have identified and quantified slip rate variations along strike of the el triunfo fault within el salvador fault zone ranging from 4 6 mm year in its central parts to 1 mm year towards the tips of the fault this study supports the hypothesis of a recent rotation in the maximum shortening direction and the accommodation of the current deformation through the reactivation of pre existing structures inherited from a previous tectonic regime

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

bivalve molluscs and cenozoic paleoclimatic events in the northwestern pacific ocean abstract a new method for the calculation of cenozoic paleotemperatures is proposed which is based on the statistical relation between the august ta and annual ty temperature of surface water and the coefficient of thermotropy ct the coefficient presents the percentage ratio of the number of species of bivalve molluscs from temperate and warm water genera to the total number of species in a definite region for the northwestern pacific at 1 1 ct 92 1 the most adequate temperature estimations gave the equations t y 3 836 1 014 ct and t a 10 998 1 009 ct paleotemperatures in the vicinity of western tochilinsky key section and eastern korfa gulf key section kamchatka south sakhalin makarovsky key section and a number of areas of hokkaido and honshu japan have been reconstructed using the proposed procedure an early middle miocene around 16 ma temperature maximum which is of high climatic stratigraphic value has been found to be present in all areas studied and to correspond to the level of the kakertian horizon of western kamchatka the takinoue lower togeshita horizon of hokkaido and the nishikurosawa formation of north honshu the average annual surface water temperatures at that time were 7 1 c up to 9 6 c and 11 5 c respectively in close similarity to the northern areas of the atlantic haq 1982 a four fold advancement of temperate faunistic complexes toward higher latitudes has been observed near western kamchatka specified as the kuluvenian kakertian ilynian early and late etolonian temperature maxima the paleoceanological situation near western kamchatka during these maxima is assumed to be sufficiently close to that presently existing in peter the great bay and in more southerly areas of the sea of japan the basic paleotemperature trends which turn out to be similar for all regions studied and for the main neogene north pacific paleoclimatic events tsuchi 1989 1990 1992 suggest the validity and suitability of the proposed method for the determination of paleotemperatures using the coefficient of thermotropy this is also evidenced by the agreement between the paleotemperature estimates obtained here and the oxygen isotope values

the origin of a terrane u pb zircon geochronology and tectonic evolution of the xolapa complex southern mexico terrane analysis has established differences in lithology style of deformation metamorphism and radiometric ages among several basement complexes in southern mexico in this paper we identify similarities in the geologic history of the adjacent acatlan oaxaca and xolapa complexes through a u pb zircon study on crustal sources and timing of magmatic metamorphic and tectonic activity in the xolapa complex our regional tectonic scenario is based on both new chronologic and previously established structural data inherited zircon in the xolapa complex indicates the presence of a proterozoic 1 0 1 3 ga crustal component with an age range that overlaps grenville crystallization dates 1 0 1 2 ga from the oaxaca basement and an inherited grenville crustal component 1 0 1 1 ga identified in zircons from the acatlan complex the proterozoic component in the xolapa complex indicates either that it received sediments from a continental region of grenvillean age for example the oaxaca basement which is the closest exposed grenville crust in southern mexico or that the xolapa complex has a grenville basement in the latter case the xolapa complex has been modified by widespread high grade metamorphism and large scale migmatization metamorphism and migmatization occurred from 66 to 46 ma and locally continued into oligocene time magmatism in the xolapa complex terminated with crustal growth by plutonism which is characterized by a systematic pattern of eastward younging crystallization ages from 35 ma in the west west of acapulco to 27 ma in the east east of puerto angel metamorphism and migmatization in the xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the farallon and north american plates e g rate and direction of subduction and the early evolution of the caribbean this plate reorganization triggered sinistral transtension distributed across the southern mexican continental margin and the eastward translation of the chortis block the eastward younging of the xolapa plutons also is consistent with the motion of the chortis block during eastward displacement of the caribbean plate this induced a shift of the magmatic arc from its cretaceous to early tertiary position along the xolapa and chortis blocks to its present mid mexican position we conclude that a single tectonic framework accounts for the mesozoic and cenozoic geologic history of the southern mexican terranes

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

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

paleoenvironment of the western interior seaway inferred from δ18o and δ13c values of molluscs from the cretaceous bearpaw marine cyclothem abstract 18 o and 13 c values of ammonites and inoceramids provide information on the paleoenvironmental conditions in the western interior seaway wis during the campanian maastrichtian bearpaw marine cycle the last major transgression regression t r cycle to affect the seaway during the cretaceous period ammonites and inoceramids exhibit distinct stable isotope values inoceramids have higher 13 c values 0 6 to 5 0 and lower 18 o values 4 0 to 2 8 whereas coeval baculitid ammonites have lower 13 c values 4 8 to 0 3 and higher 18 o values 2 3 to 0 0 with other ammonites having stable isotope values ranging between the baculitids and inoceramids the isotopic composition of the inoceramids and ammonites are unlikely due to kinetic or vital effects because species from open ocean environments have values expected from marine water and there is no correlation between the 13 c and 18 o values of the inceramids as has been reported for modern foraminifers and calcareous algae suspected of exerting vital effects the heterogeneity in stable isotope values of coeval molluscs implies that the bearpaw sea was isotopically stratified isotopic exchange between the water and sediments and the formation of 18 o depleted deepwater mass by mixing of tethyan and boreal waters in the wis may have caused such isotopic stratification a clear relationship exists between the 18 o values of bearpaw zonal baculitids and their biostratigraphic sequence baculitids from zones during peak transgression have the lowest average 18 o values 2 3 to 0 7 whereas those from the underlying and overlying zones have higher 18 o values 0 8 to 0 2 this pattern of 18 o values can be explained by fluctuations in temperature rather than variations in freshwater influx this influx probably having been reduced by lower precipitation and run off under drier warmer climatic conditions the bearpaw sea was not brackish and other paleoenvironmental factors likely account for the character of the bearpaw fauna that is dominated by ammonites and inoceramids and lacks many of the taxa prevalent in the contemporaneous open oceans and seas

elemental and oxygen isotope composition of early jurassic belemnites salinity vs temperature signals well preserved calcitic belemnite samples from the basque cantabrian basin of northern spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the early jurassic a period punctuated by times of oceanic anoxia and global mass extinctions because the oxygen isotope composition of calcite depends not only on temperature but also on salinity we present here a combination of sr ca mg ca and 18o analyses that helped us to evaluate both effects separately comparison between the temporal records and cross plots of these geochemical parameters shows a good correlation of mg ca with 18o and a weak correlation of sr ca with 18o comparison of our belemnite 18o record with coeval o isotope profiles from other paleogeographic domains suggests the existence despite differences in the absolute isotopic values of a major negative shift during the serpentinus zone that is regionally reproducible this event is linked to the early toarcian ocean anoxic event oae and suggests the existence of global changes in paleoclimatic and paleoceanographic conditions that are reflected largely in the geochemical composition of seawater compared to our results however other early toarcian 18o data reported from northern european basins are generally depleted 1 3 1 6 in contrast mg ca ratios display similar values these differences in 18o data suggest a salinity component for the northern european data and provide evidence for a north to south salinity gradient within the north european interior boreal seaway during the early jurassic this study suggests that the secular variation of mg ca is a useful proxy for discriminating between temperature and salinity effects on the oxygen isotope composition of fossil shells and highlights the prospect of using the mg ca ratios of belemnite calcite to calculate paleotemperatures independently of salinity the findings of our study may help to calibrate 18o curves in other basins by removing the salinity component of their 18o records which may help to calculate water density gradients between the jurassic seas of northern and southern europe

tectonic significance of cretaceous tertiary magmatic and structural evolution of the northern margin of the xolapa complex tierra colorada area southern mexico the tierra colorada area sits along the northern limit of the xolapa complex where it is juxtaposed against the mixteco paleozoic and guerrero mesozoic terranes of southern mexico just north of acapulco this paper presents combined structural and geochronological data from tierra colorada area that show evidence of four deformational events and several episodes of arc magmatism during mesozoic and cenozoic time the oldest magmatism is represented by ca 165 ma granitoids and was followed by intrusion of the foliated el pozuelo granite 129 0 5 ma concordant u pb zircon analysis this intrusion postdates d 1 metamorphism and migmatization in the xolapa complex the next magmatic episode is represented by the peraluminous foliated el salitre granite 55 3 3 3 ma mineral whole rock rb sr isochron and the protomylonitic las pinas i type granite 54 2 5 8 ma lower intercept u pb zircon las pinas granite is characterized by d 2 ductile fabric with normal top tothe north northwest sense of shear deformed at 45 50 ma rb sr and k ar ages the ca 34 ma undeformed granites correspond to the last intrusive pulse in the area postdating both d 3 south southwest verging thrusting of the cretaceous morelos formation over sheared granites and lower cretaceous volcanic rocks and open folding during d 4 these four pulses of subduction related magmatism in the tierra colorada area indicate a regular northeastward subduction at the mesoamerican trench since jurassic time and alternate with contractile and or extensional tectonic events the gap in magmatic activity ca 90 100 ma roughly coincides with deposition of platformal limestones of the morelos formation during the middle cretaceous the stable conditions during deposition of the morelos formation may have resulted from a combination of backarc extension and development of a passive margin during the early middle cretaceous which postdated the accretion of an exotic block either the guerrero terrane or the chortis block following the laramide orogeny in southern mexico roughly during the late cretaceous the paleocene miocene tectonic evolution in the tierra colorada area involved an alternation of magmatic pulses with extensional and contractile events this was the result of a combination of several factors including the geometry of the subducted slab convergence rate stress transmission between the subducting and overlying plates and the rate of subduction erosion

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

late cretaceous allochthons and post cretaceous strike slip displacement along the cuilco chixoy polochic fault guatemala the disposition of allochthonous masses emplaced in late cretaceous to early tertiary time along the cuilco chixoy polochic strike slip fault zone in western guatemala constrains the amount of tertiary displacement the absence of major disruption of the allochthons argues against lateral displacements of hundreds of kilometers during the tertiary in westernmost guatemala along the flank of the cuilco chixoy polochic fault granite volcaniclastic and volcanic rocks minor carbonate and phyllite comprise a sequence of south dipping allochthons these tilted slabs lie against almost horizontal jurassic and cretaceous beds to the north field relations indicate that the slides were emplaced across the cuilco chixoy polochic fault from south to north probably during late cretaceous or early tertiary time other fault bounded masses which crop out tens of kilometers eastward near san sebastian huehuetenango probably were emplaced contemporaneously with those to the west locally these allochthons lie astride the active trace of the cuilco chixoy polochic fault along which offset streams record about 1 km of left lateral motion total displacement recorded by the apparent offset of allochthonous sheets is no more than several kilometers farther east along much of the cuilco chixoy polochic fault zone a throughgoing strike slip trace has not been identified we postulate that the pre tertiary tectonic record is obscured by an extensive series of allochthons composed of regional stratigraphic suites emplaced during late cretaceous time in western guatemala stratigraphic relationships between serpentinized peridotite and carbonate beds of late cretaceous age indicate turonian as a maximum age for initial emplacement of allochthonous material regional relationships indicate that deformation culminated during latest cretaceous earliest tertiary time the emplacement of thrust sheets along the cuilco chixoy polochic fault is a likely product of contemporaneous cretaceous convergence along the motagua zone to the south collision of the chortis terrane against southern yucatan resulted in crustal shortening uplift and consequent northward directed emplacement of slides allochthons composed of ultramafic rocks sedimentary cover and less commonly granitic rocks were probably derived from the suture zone and the uplifted region between the cuilco chixoy polochic and motagua faults

paleotemperature variations of early jurassic seawater recorded in geochemical trends of belemnites from the basque cantabrian basin northern spain abstract here we present new o isotope mg ca and sr ca data of early jurassic belemnites from the basque cantabrian basin of northern spain a succession deposited in a periodically anoxic restricted hemipelagic basin nearly 200 pliensbachian lower toarcian belemnites were collected and examined using optical cathodoluminescence and chemical techniques to constrain the effects of potential diagenetic alteration after a precise diagenetic screening of samples 176 belemnites were selected to provide records of changes in seawater geochemistry and temperature the records show a fairly good correlation of 18o with mg ca ratios r 0 65 and a weak correlation of 18o with sr ca ratios r 0 34 the oxygen isotope data if interpreted in terms of paleotemperature have revealed a period of relatively uniform and warm seawater temperatures during the early pliensbachian followed by a sharp temperature drop during the late pliensbachian earliest toarcian the onset of this cooler period occurred within the protogrammoceras celebratum subzone and the coolest seawater temperatures were reached at the end pliensbachian pleuroceras spinatum zone this was followed by a rapid seawater warming during the harpoceras serpentinus zone of the early toarcian in addition the data set provides new information regarding paleoceanographic conditions for anoxic events in a critical region of connection between the boreal and tethys realms it is the background to relatively high 18o values that prevailed during the anoxic episodes concerning the impact of the early toarcian anoxic event in northern spain the detected 18o and mg ca excursions reached their minimum and maximum respectively during the lower h serpentinus zone while the anoxic black shale interval developed earlier during the upper dactylioceras tenuicostatum lowermost h serpentinus zones

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin