gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

reverse polarity magnetized melt rocks from the cretaceous tertiary chicxulub structure yucatan peninsula mexico abstract we report paleomagnetic results for core samples of the breccia and andesitic rocks recovered from the yucatan 6 petroleos mexicanos exploratory well within the chicxulub structure about 60 km ssw from its center northern yucatan mexico a previous study has shown that the rocks studied contain high iridium levels and shocked breccia clasts and an ar ar date of 65 2 0 4 ma sharpton et al 1992 andesitic rocks are characterized by stable single component magnetizations with a mean inclination of 42 6 2 4 breccias present a complex paleomagnetic record characterized by multivectorial magnetizations with widely different initial nrm inclinations however after alternating field demagnetization well defined characteristic components with upward inclinations are defined irm acquisition experiments comparison of irm and nrm coercivity spectra and the single component magnetization of the andesitic rocks indicate the occurrence of iron rich titanomagnetites of single or pseudo single domain states as the dominant magnetic carriers mean inclinations from the andesitic rocks and most of the breccia samples give a mean inclination of about 40 to 45 indicating a reverse polarity for the characteristic magnetization that is consistent with geomagnetic chron 29r which spans the cretaceous tertiary k t boundary the inclination is also consistent with the expected value and corresponding paleolatitude for the site estimated from the reference polar wander curve for north america we suggest that the characteristic magnetizations for the andesitic and breccia rocks are the result of shock heating at the time of formation of the impact structure and that the age polarity and paleolatitude are consistent with a time at the k t boundary

radiometric and kinematic evidence for middle jurassic strike slip faulting in southern mexico related to the opening of the gulf of mexico one of the least known aspects of the evolution of the gulf of mexico is the nature and location of shear zones along which the relevant continental fragments were displaced the sierra de juarez mylonitic complex located in southern mexico is a polyorogenic north northwest trending structure here we report u pb mylonitization dates of 165 20 ma for igneous zircon from the syntectonic san felipe granite and an integrated 40 ar 39 ar age of 169 3 1 7 ma from synkinematic muscovite both of which indicate a middle jurassic age for the strike slip event along the sierra de juarez mylonitic complex this event therefore occurred during the opening of the gulf of mexico and we propose that the shear zone was kinematically related to the southeast displacement of the yucatan block

constraints on the tectonic development of the eastern gulf of mexico provided by magnetic anomaly data boundaries between oceanic and transitional crust in the eastern gulf of mexico have been delineated using detailed accurately navigated high quality aeromagnetic data these data indicate that considerably more oceanic crust may be present than previously suggested by other geophysical data especially seismic reflection data the n s extent of oceanic crust substantially increases westward from approximately 280 km near 87 w to roughly 390 km near 90 w indicating the proximity 10 of the pole of rotation variations in the amount of oceanic crust together with discontinuities in the magnetic anomaly patterns interpreted as the traces of fossil transform faults or fracture zones have been used to obtain a rough estimate of the location of the pole of rotation 24 n 81 5 w for the seafloor spreading phase of opening the magnetic data have been used to develop a model for the opening of the gulf and to identify some geometrical relationships between its conjugate continental margins based upon the estimated pole position the eastern gulf is interpreted to have opened in an approximately nne ssw direction with the counterclockwise rotation of yucatan away from north america the seafloor spreading phase of movement accounts for about 25 of the counterclockwise motion of yucatan rotating the yucatan block clockwise by this amount to restore it to its predrift position places the campeche bank adjacent to the mississippi trough louisiana shelf further clockwise rotation of yucatan by an additional 30 35 about this pole restores extended crust to its original thickness and is consistent with rotations of yucatan predicted from paleomagnetic data from chiapas

lithospheric structure of the texas gulf of mexico passive margin from surface wave dispersion and migrated ps receiver functions the seismic velocity structure beneath texas gulf coastal plain gcp is imaged by migrating ps receiver functions with a seismic velocity model found by fitting surface wave dispersion we use seismic data from a linear array of 22 broadband stations spaced 16 20 km apart a common conversion point ccp stacking technique is applied to earthquake data to improve the s n ratios of receiver functions using an incorrect velocity model for depth migration of a stacked ccp image may produce an inaccurate image of the subsurface to find sufficiently accurate p and s velocity models we first apply a nonlinear modeling technique to fit rayleigh wave group velocity dispersion via very fast simulated annealing vs ranges from 1 5 km s in shallow layers of the gcp to 4 5 km s beneath the llano uplift and just outboard of the balcones fault zone bfz the bfz is characterized by slow velocities that persist to nearly 100 km depth in the stacked image the largest amplitude positive polarity event ranges from the surface at the llano uplift to a maximum depth of 16 km beneath matagorda island we attribute this event to the sediment basement contact which is expected to produce a large impedance contrast another large amplitude and positive polarity event at 35 km depth which likely marks the moho disappears outboard of the luling fault zone the disappearance of the moho beneath the gcp may be due to serpentinization of the upper mantle which would reduce the impedance contrast between the lower crust and upper mantle dramatically

morphological modelling of rivers with erodible banks a bank erosion mechanism and provisions to account for the associated planform changes and input of bank erosion products are added to a two dimensional depth averaged model of river morphology the model is applied to a reach of the meandering gravel bed river oh e eger in the former state of czechoslovakia the agreement with observations is poor but this can be ascribed to shortcomings in the flow and bed topography submodels rather than to shortcomings in the bank erosion submodel better results are expected when a three dimensional flow model equations for sediment mixtures and a bank accretion mechanism are included this inclusion will have to be preceded by fundamental research on bank accretion mechanisms and on hiding and exposure effects in the relationship for the influence of gravity pull on sediment transport direction 1998 john wiley sons ltd

modelling free forming meander evolution in a laboratory channel using three dimensional computational fluid dynamics abstract the present study investigated the use of computational fluid dynamics cfd in predicting the formation development and migration of free forming meander bends the three dimensional cfd model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes different algorithms and parameters were tested to provide an insight into the application range of cfd when modelling free forming meander formation the computational domain was discretized by an unstructured grid a control volume method was used for the discretization of the navier stokes equations for the flow calculation and of the convection diffusion equation for the sediment transport calculation turbulence was modelled by the k e turbulence model the simulation was started from an initially straight grid with neither sediment feed nor any perturbation at the inflow boundary the model computed the river bed evolution over a real time period of 3 d results were compared with laboratory experiments and showed that the cfd model can predict many of the characteristics of the alluvial meander formation and migration however some limitations and uncertainties exist that have to be clarified in future investigations

pressure temperature time evolution of high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans comment vega granillo et al 2007 1 are to be congratulated for publishing high quality pressure temperature time p t t data on the high pressure hp rocks within the acatlan complex fig 1 2 these data expand on previously published p t data of meza figueroa et al 2005 3 the

latest paleozoic early mesozoic structures in the central oaxaca terrane of southern mexico deformation near a triple junction abstract paleozoic rocks in the oaxaca terrane of southern mexico occur as two outliers rio salinas and santiago ixtaltepec unconformably overlying the 1 ga oaxaca complex they consist of the upper cambrian lower ordovician tinu formation mississippian santiago formation lower middle pennsylvanian ixtaltepec formation and the unfossiliferous yododene formation of presumed permian or younger age these paleozoic rocks have been deformed by several sets of structures three moderately westerly dipping bedding parallel shear zones displaying dextral kinematic indicators c s fabrics curvilinear isoclinal folds en echelon boudinage en echelon veins and strained amygdales occur along the boundaries between the tinu and santiago formations between the santiago and ixtaltepec formations and within the ixtaltepec formation in the santiago ixtaltepec outlier the bedding and these bedding parallel shear zones are deformed by n s upright asymmetric subhorizontal folds accompanied by a slaty or spaced cleavage that are in turn deformed by several sets of kink bands subhorizontal and steeply dipping e w nw se and ne sw in the northern part of the santiago ixtaltepec outlier the stratigraphy bedding parallel shear zones and slaty cleavage are displaced by nw se normal faults all of which are truncated by the angular unconformity at the base of the cretaceous which brackets their age as post early permian and pre cretaceous geometric correlation of the major n s folds with e vergent thrusting of the oaxacan complex over the juarez terrane suggests that they are older than middle jurassic no age constraints are available for the kink and chevron folds however most may be related to laramide structures in the overlying cretaceous rocks unfolding the major structures in the paleozoic rocks reorients the bedding parallel shear zones to subhorizontal detachment shear zones or faults with a top to the north sense of displacement they may be related to either gravity sliding following amalgamation of pangea extensional rifting of the yucatan peninsula from southern laurentia or intra arc rifting associated with a permo triassic arc within southern mexico the major n s fold structures and e vergent thrusting of the oaxacan complex over the juarez terrane may represent a response to 1 convergent subduction of the kula and or farallon plates beneath western mexico 2 migration of a gulf of mexico yucatan south america triple junction

2 8 1 7 ga history of the jiao liao ji belt of the north china craton from the geochronology and geochemistry of mafic liaohe meta igneous rocks abstract the assembly and long term evolution of the eastern block of the north china craton are poorly constrained here we use bulk rock geochronological and geochemical data from mafic meta igneous rocks hornblendites amphibolites and a metagabbro of the liaohe group to reconstruct the neoarchean to paleoproterozoic history of the jiao liao ji belt located between the longgang and nangrim blocks that together form the eastern block of the north china craton the mafic ultramafic meta igneous rocks have intrusive or tectonic contacts with the liaoji granitic rocks 2 2 2 0 ga which form the basement of the jiao liao ji belt the major and trace element data indicate that the protoliths had calc alkaline composition and formed along an active continental margin subduction zone the mafic rocks form a whole rock 176lu 177hf isochron with an age of 2 25 0 31 ga overlapping with u pb zircon ages for mafic and granitic rocks from the jiao liao ji belt and consistent with being the emplacement age of the mafic protoliths along the active continental margin in contrast the whole rock 147sm 144nd isochron age of 2 83 0 18 ga is likely to reflect the average age of the lithospheric mantle source from which the mafic ultramafic protoliths were extracted together with geological evidence we propose that the southwestern portion of the longgang block was an active continental margin since at least the early paleoproteorozic literature age data from metamorphic zircons show that peak granulite metamorphism took place at 1 96 1 88 ga resulting from the collisional event that fused the longgang and nangrim blocks into the eastern block of the north china craton our bulk rock 207pb 206pb age of 1824 19 ma and our 87rb 86sr age of 1671 58 ma reflect retrograde cooling stages during the exhumation of the jiao liao ji belt after the orogenesis

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

location of the tectonic boundary between the paleoproterozoic jiao liao ji belt and the archean longgang block in the lianshanguan area northeastern china abstract the tectonic evolution of the paleoproterozoic jiao liao ji belt which is located in the north china craton in northeastern china is the subject of ongoing debate most previous studies on this belt have been based on traditional geological methods with most of the data being obtained from the surface leading to uncertainties regarding the deep geological structure of the belt this has restricted our understanding of the tectonic character and development of the belt however this limitation could be overcome by geophysical studies here we analyze and interpret newly collected rock physical property data density magnetic susceptibility and resistivity and non seismic geophysical data gravity magnetic and magnetotelluric sounding data for the lianshanguan area to infer the location configuration and nature of the tectonic boundary between the paleoproterozoic jiao liao ji belt and the archean longgang block the lianshanguan rock mass which belongs to the archean longgang block shows a weak gravity anomaly 0 6 mgal and high resistivity 103 6 to 104 m the area south of the lianshanguan rock mass consists of the paleoproterozoic liaohe group and displays an intermediate magnetic anomaly 0 200 nt and low resistivity 10 102 5 m we therefore infer that the tectonic boundary of the jiao liao ji belt is located along the southern margin of the lianshanguan intrusion we also show that the tectonic boundary in the eastern liaoning southern jilin area trends e w and extends from shuangtaling through lianshanguan caohecheng balidianzi and to guaimozi the boundary between the longgang block and the jiao liao ji belt dips predominantly to the south suggesting an extensional tectonic regime during the paleoproterozoic time

paleoproterozoic structural evolution of the southern segment of the jiao liao ji belt north china craton abstract the paleoproterozoic jiao liao ji belt separates the eastern block of the north china craton into two small sub blocks the northern longgang and the southern rangrim blocks however it still remains unknown or controversial about the subduction polarity collisional deformation and kinematics between two sub blocks the southern segment of the belt consists of the paleoproterozoic fenzishan and jingshan groups and paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the jiaodong complex all of which are separated from the jiaodong complex of neoarchean ttg gneisses by stz1 ductile shear zones structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the jiao liao ji belt underwent three distinct episodes of folding d1 to d3 and two stage of ductile thrust shearing stz1 coeval to d1 and d2 stz2 between d2 and d3 the d1 deformation formed penetrative axial planar foliations s1 bedding parallel ductile shear zone mineral stretching lineations l1 and rarely preserved small isoclinal d1 folds in the jingshan and fenzishan groups in the jingshan group however penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding parallel ductile shear zones stz1 at a period of about 1956 ma to 1914 ma the kinematic indicators of stz1 in the jingshan group with resultant prograde peak metamorphism up to granulite facies grade and the fenzishan group with peak metamorphism up to amphibolite facies grade indicate nw directed compression d2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914 1893 ma the d2 deformation produced nw verging asymmetric and recumbent folds interpreted to have resulted from basement involved thicken skin structures the jiaodong complex was also involved into the development of wnw verging asymmetric tight folds associated with d2 in the jingshan and the fenzishan groups ongoing collision led to the development of transpressional ductile shearing stz2 forming the transpressional taipingzhuang dextral ductile shear zone between the jingshan group and the southern archean complex and the transpressional tading xiadian sinistral ductile shear zone between the jingshan group and the northern archean complex all three lithotectonic units were superposed during the late d3 deformation with amphibolite facies metamorphism the d3 deformation developed wnw trending open to tight upright folds at about 1893 1875 ma the structural pattern resulting from superimposition of d2 and d3 is a composite synform in the fenzishan and jingshan groups the structural events of d1 and stz1 and d2 and stz2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites the structural patterns and deformational history of the fenzishan and jingshan groups suggest a southeastward directed oblique subduction beneath the northwestern margin of the rangrim block and that the final scissor shaped closure of the rift led to collision between the two blocks to form the coherent basement of the eastern block of the north china craton

a review of batholiths and other plutonic intrusions of mexico abstract granitic plutons constitute a major portion of the phanerozoic continental crust of mexico with the great majority ca 90 associated to the laramide late cretaceous eocene orogeny and the eastward subduction of the pacific ocean plates as well as to magmatic arcs essentially built since the early mesozoic at the western margin of north america exposed mainly as a wide up to 300 km and over 3000 km long batholithic belt at the mexican pacific margin from baja california to chiapas granitoids conform large intrusive complexes and hundreds of smaller plutons the age of which vary from ca 1400 ma mesoproterozoic to ca 10 ma late miocene in many cases uplift and erosion have revealed the deep roots of the batholiths whereas in other places many intrusions were emplaced in upper crustal environments as suggested by the extremely variable cooling rates of 200 c ma very shallow to 1 10 c ma very deep lithologies and isotopic data indicate unambiguously the central participation of the local lower crust in the genesis of the batholiths and plutons imprinting on them marked petrologic geochemical and structural zoning across the paleozoic paleomargins and through the present nw trending mexican continental edge according to the lithospheric component involved laurentia in the northern and northwestern regions of mexico accreted mesozoic terranes in western mexico and oaxaquia gondwana in eastern and southern mexico major problems related to the evolution of the mexican main granitoids are outlined in time slices for the proterozoic paleozoic permo triassic jurassic early cretaceous late cretaceous paleogene and neogene which represent in mexico major epochs of crustal growth and continental recycling mainly associated not only with pacific north america convergence but also with extensional and transpressional events that altogether marked not only the lithological and structural evolution of most of the country but also its extraordinary mineral wealth finally some preliminary comparisons differences and similarities are made between the mexican batholiths and other plutonic complexes in central asia lhasa terrane and japan

the location and evolution of the tectonic boundary between the paleoproterozoic jiao liao ji belt and the longgang block northeast china abstract in this paper we discuss the location nature and evolution of the tectonic boundary between the jiao liao ji belt and the longgang block northeast china the boundary in terms of paleogeography which involves a study of the liaohe group or tectonics employing non seismic geophysical data e g gravity data and magnetotelluric soundings existing geological data and the differences in physical properties between the jiao liao ji belt and the longgang block we clarified the spatial distribution of the tectonic boundary and characterized the geological bodies at different depths on either side of the boundary we analyzed the most recently updated bouguer gravity anomaly gravity magnetic and electrical profile data as well as comprehensive data on the physical properties of the rocks in the research area thus providing information on the geological nature of the boundary at deep levels the results indicate that during the early period of formation of the jiao liao ji belt a basin controlling boundary fault developed that constituted the original tectonic boundary between the jiao liao ji belt and the longgang block this fault extended from shuangtaling to sandaoling lian shanguan dawanggou balidianzi and erhulai to guaimozi following a roughly east west trend most previous studies have considered that the liaohe group which extends from benxi city to benxi county and huanren county is the paleo geographic boundary of the jiao liao ji belt

concordant paleomagnetic directions from the tehuantepec isthmus constraints on the neogene evolution of the north american caribbean plate boundary we report paleomagnetic data for miocene igneous rocks of the southeastern tehuantepec isthmus region mexico thermal and alternating field demagnetization reveal predominantly norma polarity magnetization interpreted as primary trm the overall mean direction dee 355 6 inc 3 y k 36 4 ag5 9 3 n 8 sites is statistically indistinguishable from the expected direction calculated using the north american reference pole this result indicates that significant rotation or displacement of the tehuantepec region relative to the craton has not occurred since 13 ma

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

tectono stratigraphic evolution of eastern mexico during the break up of pangea a review abstract the lower jurassic cenomanian successions of mexico can be grouped into two different tectono stratigraphic systems that were developed under the influence of two distinct geodynamic processes 1 the mesozoic atlantic system of mexico which occupies the eastern two thirds of the country and was dominantly influenced by the break up of pangea and consequent opening of the atlantic ocean and 2 the mesozoic pacific system of mexico which is exposed along its western margin and was controlled by the subduction of the farallon plate beneath the pacific margin of north america the mesozoic atlantic system of mexico can be subdivided into two tectono stratigraphic subsystems representing the two main tectonic stages of the break up of pangea the early middle jurassic rift subsystem was developed during the initial stage of lithospheric attenuation of western equatorial pangea which is evidenced by the opening of a number of right lateral transtensional troughs that were opened according to the nw se regional extension imposed by north america south america divergence rift related troughs were progressively infilled by continental to marginal marine deposits and locally by volcanic lava flows that display a geochemical signature typical of supra subduction settings based on these characteristics the rift subsystem is interpreted in this work as a hybrid tectono stratigraphic domain that displays characteristics that are typical both of the mesozoic atlantic system of mexico and mesozoic pacific system of mexico reflecting the superposition of the atlantic and pacific tectonic processes during the initial phase of north america south america divergence by late callovian time most of intra continental extension in western equatorial pangea had occurred and was followed by anticlockwise rotation of the yucatan block to its present position and initial seafloor spreading in the gulf of mexico the opening of the gulf of mexico promoted the incursion of marine waters in eastern mexico and the onset of carbonate basins and platforms that make up the late jurassic cenomanian drift and cooling subsystem the lack of arc like igneous rocks in the drift and cooling subsystem suggests that by the late callovian the mesozoic atlantic system of mexico was developed independently of the tectonic processes related to subduction along the pacific margin according to our synthesis the mesozoic atlantic system of mexico represents a unique stratigraphic record that was developed at least in part under the combined influence of two main tectonic processes that are 1 the divergence between north america and south america and 2 the subduction of the farallon plate in light of this the mesozoic atlantic system of mexico can be considered a natural laboratory to explore the evolution of complex geodynamic settings related to the interaction of plate tectonic boundaries

nonlinear modeling of mean flow redistribution in curved open channels 1 phenomena concerning flow morphology and water quality in rivers are often investigated by means of a depth integrated flow model coupled to a sediment transport model and a water quality model in such depth integrated models the vertical structure of the flow is represented by a closure submodel which mainly has to account for the secondary circulation which 1 redistributes by advection the flow the boundary shear stresses the sediment transport and dissolved and suspended matter 2 causes the direction of the bed shear stress to deviate from the direction of the depth averaged velocity and thereby influences the bed topography and 3 gives rise to additional friction losses as compared with straight uniform flow the commonly used linear closure submodels are shown to fail in reproducing essential features in moderately to strongly curved flow because they neglect the feedback between the downstream velocity and the secondary circulation a nonlinear closure submodel taking this feedback into account is proposed and shown to yield results that compare well with experimental data even for very strongly curved flow the feedback effects turn out to be controlled almost exclusively by a single parameter which enables their parameterization in a relatively simple way this control parameter also helps to objectively distinguish weak moderate and strongly curved flows the proposed closure submodel has the potential of improving the performance of depth integrated flow sediment water quality models without much extra computational effort

paleomagnetism of the chiapas massif southern mexico evidence for rotation of the maya block and implications for the opening of the gulf of mexico we have analyzed 180 oriented samples of the batholithic complex and overlying red beds of the chiapas massif in southern mexico for a paleomagnetic study both stepwise alternating field and thermal demagnetization procedures revealed dual polarity characteristic magnetizations above 10 30 mt or 200 400 c in about 60 of the samples demagnetized three distinct and ancient paleomagnetic directions have been identified late permian intrusions 256 10 ma yield nearly equatorial southern hemisphere paleolatitudes and a paleopole 13 4 s 176 1 e 12 sites k 15 4 a95 11 4 rotated 75 counterclockwise with respect to the north american reference pole a selected set of late triassic jurassic red beds of the todos santos formation yields a paleopole at 22 6 n 170 6 e 4 sites k 56 8 a95 12 3 which implies 60 counterclockwise rotation and a near equatorial northern hemisphere paleolatitude middle late jurassic dikes and granites yield a paleopole at 77 4 n 97 4 e 6 sites k 269 2 a95 4 1 near the mid jurassic segment of the north american apparent polar wander path these data are consistent with a pre rift configuration in which the maya block is rotated to a position off the coast of texas and louisiana in the present day area of the gulf of mexico a further implication is that by oxfordian times chiapas and by inference the maya block was essentially in its current position with respect to cratonic north america

microbial remains and other carbonaceous forms from the 3 24 ga sulphur springs black smoker deposit western australia well preserved carbonaceous materials including filamentous microbial remains have been isolated from an archaean sedimentary package overlying the sulphur springs volcanic hosted massive sulphide vhms deposit in the pilbara of western australia one of the oldest best preserved vhms deposits on earth the isotopically light carbonaceous material 13 c values of 26 8 to 34 0 v pdb occurs as finely striated lenticular to banded organic matter om within fine grained epiclastic sediments and silicified materials that overlie the deposit the predominant banded lenticular carbonaceous material occurs parallel to original sedimentary bedding planes and is distinctly visible in light microscopy transmission electron microscopy tem reveals this om as bundles of filamentous and tubular structure closely resembling both modern day and more ancient microbial forms documented from sea floor hydrothermal environments total organic carbon toc has a range of 1 0 to 2 3 while the thermal maturity of the filamentous microbial bundles indicated by reflectance ro points to maximum temperatures since deposition ca 3 24 ga of around 90 100 c a factor that has enabled the preservation of their morphology these results and the intimate association of sulphides with the om are suggestive of a well developed archaean sediment hosted microbial community situated within a basinal environment associated with an active centre of seafloor hydrothermal activity 2007 elsevier b v all rights reserved

new insights into the history and origin of the southern maya block se mexico u pb shrimp zircon geochronology from metamorphic rocks of the chiapas massif the histories of the pre mesozoic landmasses in southern mexico and their connections with laurentia gondwana and among themselves are crucial for the understanding of the late paleozoic assembly of pangea the permian igneous and metamorphic rocks from the chiapas massif as part of the southern maya block mexico were dated by u pb zircon geochronology employing the shrimp sensitive high resolution ion microprobe facility at stanford university the chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks shrimp data from an anatectic orthogneiss demonstrate that the chiapas massif was part of a permian 272 ma active continental margin established on the pacific margin of gondwana after the ouachita orogeny latest permian 252 254 ma medium to high grade metamorphism and deformation affected the entire chiapas massif resulting in anatexis and intrusion of syntectonic granitoids this unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics shrimp data of zircon cores from a metapelite from the ne chiapas massif yielded a single grenvillian source for sediments the majority of the zircon cores from a para amphibolite from the se part of the massif yielded either 1 0 1 2 or 1 4 1 5 ga sources indicating provenance from south american sunsas and rondonian san ignacio provinces

structural and tectonic evolution of the acatlan complex southern mexico its role in the collisional history of laurentia and gondwana 1 correlation of deformational phases and thermal events in the acatlan complex permits definition of nine major tectonic events seven events are related to the evolution of the iapetus and rheic oceans by the early ordovician the xayacatlan suite of laurentian affinity was metamorphosed to eclogite facies and exhumed before colliding with the el rodeo suite from the late ordovician to silurian the upper ordovician ixcamilpa suite with peri gondwanan affinity was metamorphosed to blueschist facies exhumed and subsequently was overthrust by the xayacatlan el rodeo block part of the esperanza suite is an early silurian intra iapetian continental arc that collided with and subducted beneath continental crust generating a silurian eclogitic event by the late devonian collision of the three high p t suites closed the iapetus ocean the resulting composite terrain was thrust over the gondwana bordering cosoltepec suite to close the rheic ocean during the earliest mississippian the collision of the acatlan complex and the oaxaca terrane uplifted and deformed the composite terrane by the early permian deposition of the chazumba suite occurred in a pull apart basin a latest early permian event produced thrust shear zones with n s directed shortening during the middle jurassic plume rise caused regional doming magmatism and amphibolite facies metamorphism during the late cretaceous paleogene folds and thrust faults overprint the acatlan complex most events in the acatlan complex correlate with events in the appalachian chain suggesting that the acatlan complex was located near the appalachians until the jurassic when it was displaced by the gulf of mexico opening

an integrated magnetic and geological study of cataclasite dominated pseudotachylytes in the chiapas massif mexico a snapshot of stress orientation following slip summary the permian age chiapas massif in southeast mexico is locally host to well exposed pseudotachylyte vein networks the veins are black to dark grey and aphanitic in appearance and consist mostly of microbreccia of angular fragments of plagioclase k feldspar biotite and quartz in a cryptocrystalline microscopically irresolvable matrix evidence of melting is present in the form of glass seams dikelets glass clasts included in cataclasite and a distinct chemistry in the pseudotachylite veins pristine glass represents a relatively small volume of the pseudotachylite veins at an exposure along the tablon river valley where the host rock is a medium to coarse grained equigranular quartz diorite individual veins are consistently oriented about 280 are up to 16 mm wide tens of cm apart display a consistent left lateral offset and can be traced for several metres individual pseudotachylyte veins rarely cross each other and they cannot be directly linked to a regional scale fault pseudotachylytes are apparently formed by a combination of crushing comminution and frictional melting but they are cataclasite dominated textures indicate that cataclasis continued after frictional melting had ceased a 40ar 39ar age determination from whole rock chips of one vein shows a climbing ar release spectrum with a date of 114 ma as the most reliable age estimate for ar retention this result is interpreted in the context of pseudotachylyte formation recrystallization and resetting of k bearing minerals for the k ar system in the late early cretaceous ten veins were sampled for palaeomagnetic and magnetic fabric studies with samples collected from both the veins and their host rock remanence data give moderate natural remanent magnetization nrm intensities for both the veins e g nrm mean 6 6 10 3 a m 1 5 5 and host rock mean 7 7 10 3 a m 1 10 8 many samples of host rock yield an ill defined east west directed and shallow magnetization which we interpret as a late permian magnetization based on previous studies of the chiapas massif this magnetization resides in haematite veins as well as immediately adjacent host rock typically have well defined single polarity magnetizations of north northwest declination and moderate positive inclination and these resemble the cretaceous expected field direction the overall mean of the veins is of dec 348 7 and inc 33 6 k 30 5 and 95 12 3 n 6 site means rock magnetic parameters suggest that the remanence in the veins principally resides in low ti pseudo single domain magnetite and maghemite grains but haematite coarse grained magnetite rutile fe cr ni oxides and ilmenite are also present in the veins bulk magnetic susceptibility values range between 0 3 and 1 1 10 3 si volume units and host rock values do not differ significantly from vein values the orientations of the principal susceptibility axes in the host rock and the veins are indistinguishable in both magnetic foliations are near vertical and are essentially parallel to the nearly east west orientation of the veins nonetheless host rock fabrics are predominantly prolate whereas vein fabrics are oblate the mean susceptibility tensors of host rocks and pseudotachylyte are characterized by p t values of 1 041 0 327 and 1 033 0 302 respectively this result suggests that the fabric reflected by magnetic susceptibility anisotropy in the veins was formed under pure shear stress during cooling of the veins the lack of evidence of fabric rotation supports models that associate partial melt with viscous break during fault slip