constraining the late mesozoic and early tertiary tectonic evolution of southern mexico structure and deformation history of the tierra caliente region abstract we analyze the structure and assess the deformation history of the tierra caliente metamorphic complex tcmc of southern mexico where laramide accretion of exotic terranes is in debate the tcmc consists of a south plunging antiform fault that is bounded on both its eastern and western flanks tierra caliente metamorphic complex rocks show at least two phases of compressional deformation the first and most prominent records a mean tectonic transport direction of 068 this phase is responsible for east verging asymmetrical folding and thrusting of both metamorphic and superjacent sedimentary rocks the second phase has an average transport direction of 232 and is restricted to the western portion of the tcmc a third phase is responsible for normal faulting lack of discernible deformation before late cretaceous time indicates that the main deformation phase is coincident with laramide orogenesis elsewhere in the north american cordillera the stratigraphy structure and deformational histor

age limits on the late quaternary evolution of the upper loire river abstract the loire river is one of the largest rivers in europe draining a basin area of southern france which exceeds 100 000 km 2 despite its significance the loire remains poorly studied and has a very limited amount of chronological control on the deposited sedimentary record of fluvial activity these factors coupled with the availability of a set of 10 radiocarbon dates on younger c

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

fluvial changes of the guadalquivir river during the holocene in cordoba southern spain this research was funded by a grant from proyectos de cordoba siglo xxi coordinated by prof r jose roldan canas universidad de cordoba

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

palaeogene neogene present day growth folding in the bahamian foreland of the cuban fold and thrust belt the santaren anticline constitutes the frontal termination of the cuban fold and thrust belt within the bahamian foreland new well and seismic data allow us to constrain in detail the evolution of this anticline pre growth and syntectonic partly post tectonic units separated by a major unconformity are associated with the santaren anticline their geometrical features are consistent with a detachment fold the precise age of the beginning of fold growth remains unknown however the complete record of well dated syntectonic sediments documents its kinematic evolution from mid eocene to pliocene present day and reveals an approximately constant and very slow growth rate from early miocene to pliocene present day the timing of evolution of the santaren anticline is not consistent with previous models that postulate that deformation associated with the cuban fold and thrust belt ended in the eocene our data suggest that the most external part of the cuban fold and thrust belt was still being deformed under a compressional regime during the late palaeogene neogene and probably during the quaternary we propose that this folding may result from compressive stresses transmitted approximately 400 km northwards from the actual plate boundary as a result of slow convergence between the n and s american plates

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

periglacial fluvial systems in northwest europe during marine isotope stages 4 and 3 comparison of fluvial successions in river valleys dating from marine isotope mi stages 4 and 3 in a west east transect from britain to poland shows the spatial and temporal variation in palaeohydrological characteristics of northwest european river valleys during these stages mi stage 4 has been a period of deep fluvial incision following this erosion most valleys were filled during mi stage 3 with successions of gravelly river deposits britain or sand silt peat successions northwest european lowlands the sedimentology of the deposits suggests strong discharge variations caused by a nival discharge regime with pronounced spring snowmelt discharges the mi stage 3 climate oscillations generally cannot be traced in the fluvial record an incision phase of hengelo interstadial age has been found in the netherlands but similar incision phases related to other warming events have not been proved convincingly the restricted reaction of the river systems is caused by the absence of geomorphologically effective vegetation changes and relatively minor variations in precipitation during mi stage 3 over longer time scales silt and peat beds gradually decrease in the younger post hengelo upton warren part of mi stage 3 corresponding with evidence of aeolian activity and permafrost this is related to the longer duration of the stadials in this part of mi stage 3

tectonics along the trans mexican volcanic belt according to palaeomagnetic data abstract subduction of the cocos plate beneath the southern mexico continental margin takes place at the middle america trench volcanism of calc alkaline character occurs along the e w trending trans mexican volcanic belt tmvb which includes several active stratovolcanoes monogenetic cinder cone fields and silicic centres however a direct association between the volcanism and plate subduction has been questioned for a number of reasons including the oblique orientation of the tmvb with respect to the trench most palaeomagnetic directions for the cretaceous to quaternary within or immediately to the south of the tmvb appear discordant the declinations are rotated anticlockwise relative to expected declinations this angular divergence may be explained by rotations of crustal blocks by different degrees in response to large scale left lateral shear across central mexico the fault system may have acted as an efficient structural control for the rise of magmas which may explain some of the peculiarities of the tmvb

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

geomorphological evolution of a lowland valley system during the weichselian weichselian geomorphological and fluvial evolution has been recorded in the valley fill of the reusel a small tributary of the lower maas river it has been studied by means of borehole sections a first deep incision into the substratum occurred during the early weichselian lower pleniglacial this was followed by aggradation at first by a braided river at a later date by a meandering river dating respectively from the lower and middle pleniglacial the upper pleniglacial aggradation again points to a braided river system the end of the pleniglacial is characterized by aeolian infilling of the valley and decreasing carrying capacity of the river

tectonic interpretation of the trans mexican volcanic belt discussion discussion sur l article de shurbet d h et cebull s e paru dans tectonophysics nld 1984 101 159 165

climate related river activity at the weichselian holocene transition a comparative study of the warta and maas rivers huvial systems from the dutch and polish lowlands are compared for their evolution during the weichselian deglaciation period in both regions the pleniglacial braided river pattern changed to a meandering pattern in the beginning of the lateglacial several transitional stages between braided and high sinuosity meandering systems have been recognized a difference in fluvial activity is found during the younger dryas when the maas transformed to a braided pattern while the warta maintained its meandering pattern phases of erosion and aggradation are more or less synchronous in the different river basins but their intensity may differ greatly the major characteristics of the fluvial systems are obviously a response to the general climatic evolution and related to changes in vegetation the regional differences are of minor importance and can be explained by site specific factors threshold values within the latter factors define the changes in river dynamics

spatio temporal evolution of the choisille river southern parisian basin france during the weichselian and the holocene as a record of climate trend and human activity in north western europe abstract the morpho sedimentary evolution of the choisille floodplain lowland river catchment 288 km2 a tributary of the river loire in the south western parisian basin was studied through 61 core drillings along eight transects and a geophysical survey located in four stretches of the river stretches a and b correspond to two sub catchments and stretches c and d are in the main valley sixty 14c and four osl datings were obtained and sediments were analysed on seven reference cores eight phases of evolution differing markedly from the evolution of more northern areas in the parisian basin and north western europe were identified from spatio temporal distribution of nine lithological facies the deepest incision phase 1 occurred during the first part of the weichselian followed by the deposition of a gravelly sandy unit phase 2 during the middle pleniglacial which was deeply incised phase 3 probably during the bolling from the allerod up to the last third of the boreal phase 4 sedimentation was continuously dominated by peaty deposits with no evidence of either increased hydraulic energy during the younger dryas or of incision during the lateglacial holocene transition this trend seems to reflect the specificity of the south western parisian basin climate from the late weichselian up to the end of the boreal due to the influence of the atlantic ocean compared to more northern areas where the climate was more continental the downstream incision trend during the last third of the boreal up to the subatlantic phases 5 and 6 indicates a sharp increase in precipitation and vegetation cover the lack of peaty sediments widespread in north western europe and also of precipitated carbonates frequent in the parisian basin seems to be due to local physiographic characteristics the main part of the sediment filling which is principally silty and retrograde began during the subatlantic phase 7 and 8 as a result of deforestation of the plateaux for crop farming high human induced sediment yield and storage concealed the possible impacts of climate change on fluvial dynamics lithological facies change from phase 7 to 8 can only indicate the autogenic morphological evolution of the floodplain in accretion the non univocal upstream downstream variation in the start of phase 7 shows that sediment yield varied in space and time in the catchment particularly in relation to the agricultural potential of the different areas this observation could be used to testify human induced sedimentation in other catchments

river response to variations of periglacial climate in mid latitude europe abstract the last glaciation was characterised by considerable changes in climate many european river basins reacted to these changes by initial incision and subsequent pattern change earlier research explained this by the time lag of vegetation development after a climatic change which considerably affected the sediment load however since some river basins react differently or do not react at all this model needs to be refined this paper deals with the fluvial evolution of several rivers in northern and central europe during the last glaciation and two of them the weise elster river in the leipzig area and the spree river in the niederlausitz germany are discussed in more detail the vegetation cover on the floodplain in combination with the presence of frozen ground which affects the discharge characteristics largely determines the river type nevertheless when the sequences are compared not only synchronous changes in fluvial development may be observed but also distinct differences in fluvial development by analysing the different rivers in their varying geomorphological settings it appears that grain size basin configuration and catchment size are important parameters also which can determine whether a threshold is exceeded or not

morphological and sedimentary evolution of an alluvial floodplain in an urban area geoarchaeological approaches and applications tours france urban zones on alluvial plains offer considerable geoarchaeological potential for understanding river dynamics over large time spans and their relationship with land use because of the extensive subsoil data that is available in the alluvial plain of tours lying between the loire and the cher multidisciplinary studies of the relationships between societies and environment have been conducted as part of an archaeological research programme launched in the 1960s a sedimentary database containing data for 1309 surveys has been compiled and assembled in a geographic information system to produce geostatistical models of valley bottom geomorphology the stratigraphy and chronology of alluvial filling have also been studied with information and c14 and osl dating obtained during archaeological operations taken together the results offer a new interpretation of the morphological evolution of the alluvial plain of tours from the weichselian to the present day by providing new information that either validates or invalidates previous hypotheses bedrock incision prior to the end of the weichselian upper pleniglacial coarse sedimentation during the lateglacial relative morphological stability up to the late holocene the morphological context of the first human settlements which are not concentrated on the low alluvial reliefs and the role of urbanization conditions on the present morphology of the valley floor are also presented this applied study shows the relevance of combining different methodological approaches

quantification and regulation of organic and mineral sedimentation in a late holocene floodplain as a result of climatic and human impacts taligny marsh parisian basin france quantification in grams per metres squared per year of the sediment accumulation in a flood plain marsh located in the southwestern parisian basin showed that there is no close relationship between the accumulation of organic matter om and mineral matter mm during the late holocene and provided an accurate view of the distinct yield and storage conditions of both sediment components endogenic om accumulation in peaty sediments is not related to the climate but to felling of the alder forest and its substitution by cyperaceae and paludal taxa in the marsh iron age and middle ages mm accumulation expresses mainly the sediment yield on the slopes determined by landuse during an initial phase from the late neolithic to the early middle ages land use change from crop cultivation to pastureland possibly related to climate deterioration led to a decrease in the sediment yield during a second phase since the early middle ages the considerable development of crop cultivation over pasturing even during periods of climate deterioration such as the little ice age led to a sharp increase in sediment yield however although sediment yield was high the hydrodynamics in the fen did not favour particle retention thus since the neolithic yield and storage of om and mm sediment have been marked by human activities initially with high climatic stress but since the middle ages without significant climatic stress

high resolution record of geomagnetic secular variation from late pleistocene lake lisan sediments paleo dead sea we measured geomagnetic secular variation in lake lisan sediments paleo dead sea more than 1500 oriented samples were collected from a 27 3 m section of alternating aragonite and detritus laminae in the dead sea basin ranging in age from 67 to 32 ka the natural remanent magnetization nrm is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic the nrm is very stable and was acquired several hundred years after deposition the mean direction of 878 horizons is dd 005o id 45o 95d 1o d 22 we observed three modes of directional geomagnetic variation as a function of and by inference time very rapid inter sample changes slow variation in mean direction and inclination shallowing of about 1o m the overall rate of change in direction is 0 57 0 57o year not significantly different from zero for about 83 of the record the rate of change is less than 1o year and comparable to historical values high rates of change are observed more frequently in the lisan than in historical records and peak rates are up to ten times faster a smoothed curve resulting in a maximum rate of change of 0 66o year and a mean 0 10 0 10o year may be a more realistic representation of the field behavior no reverse nrms were observed but geomagnetic field excursions may be present where the vgps deviate by more than 40o from the geographic north at about 52 and 41 ka the latter may represent the laschamp event 1998 elsevier science b v all rights reserved

isotopic geochemical and temporal characterization of proterozoic basement rocks in the quitovac region northwestern sonora mexico implications for the reconstruction of the southwestern margin of laurentia a detailed geochemical characterization of 19 representative proterozoic basement rocks in the quitovac region in northwestern sonora mexico has identified two distinct paleoproterozoic basement blocks that coincide spatially with the previously proposed caborca and north america blocks new u pb zircon geochronology revises their age ranges the caborca 1 78 1 69 ga and north america 1 71 1 66 ga blocks at quitovac and precludes a simple age differentiation between them in addition grenvillian age granitoids ca 1 1 ga spatially associated with the caborca block have been identified at quitovac nd isotopes and major and trace element geochemistry support the distinction of these paleoproterozoic blocks granitoids of the north america block are characterized by depleted end values 3 4 3 9 and younger nd model ages 1800 1740 ma and have lower k 2 o y rb ba th ree and fe mg values than coeval rocks of the caborca block the caborca block granitoids are likewise characterized by slightly less depleted end 0 6 2 6 and older nd model ages 2070 1880 ma despite the subtle differences granitoids from both the caborca and north america blocks exhibit island arc like affinities we propose that the proterozoic basement rocks from the quitovac region are an extension of the proterozoic crustal provinces in the southwestern united states specifically rocks of the caborca block exhibit an affinity to rocks of either the yavapai province or the mojave yavapai transition zone whereas rocks of the north america block have signatures similar to those of the mazatzal province or possibly the yavapai province of arizona the new isotopic ages and geochemical data do not support the existence of the late jurassic mojave sonora megashear at quitovac as originally proposed however the quitovac region accounts only for a small fraction of the proterozoic basement in sonora so these findings do not eliminate the possibility of a megashear elsewhere in northern sonora our new data create the possibility of alternative hypotheses for the distribution of paleoproterozoic crustal provinces in southwestern north america that affect reconstructions of the original southwestern margin of laurentia and reduce uncertainties in the configuration timing and existence of the proterozoic supercontinent rodinia

fluvial incision and channel downcutting as a response to late glacial and early holocene climate change the lower reach of the river meuse maas the netherlands detailed fieldwork and new extensive 14c dating of residual channel infillings provide data for the reconstruction of the late glacial channel downcutting and incision history of the venlo boxmeer lower reach of the river meuse maas in the southern netherlands within a period of 500 1300 yr after late glacial climatic amelioration the meuse responded to increased discharges and decreased sediment supply by adjusting the width depth ratio of its channels two main phases of channel downcutting are followed by two main phases of floodplain lowering and narrowing indicating net floodplain degradation by the fluvial system as a non linear response to late glacial and early holocene climate change some 1300 yr after initial late glacial warming channels downcut rapidly during the early bolling 13 3 12 5 kyr bp and adopted a high sinuosity meandering style channel downcutting paused around 11 9 kyr bp possibly in response to rising groundwater levels and or the older dryas cooling event between 11 9 and 11 3 kyr bp a new floodplain was formed then lateral erosion took place and initiated a first phase of 2 6 m floodplain lowering during the late allerod gradual climate deterioration during the allerod progressively broke up soils and vegetation cover from 11 3 to 10 9 kyr bp the meuse gradually adjusted to an increased ratio of sediment supply over transport capacity through higher width depth ratios main channels became shallower and adopted a low sinuosity pattern finally culminating in a braided river system during the younger dryas the final holocene warming resulted within 500 yr in renewed rapid channel downcutting by a single low sinuosity channel during the early preboreal followed by a second phase of 1 8 2 8 m floodplain lowering copyright 1999 john wiley sons ltd

late miocene volcanism and intra arc tectonics during the early development of the trans mexican volcanic belt abstract the early stage of the trans mexican volcanic belt hereafter tmvb is marked by widespread mafic to intermediate volcanism emplaced between 11 and 7 ma from the pacific coast to the longitude of mexico city to the north of the modern volcanic arc petrological and geochronological data support the hypothesis that this volcanism made up a unique late miocenic central mexican comagmatic province mafic lavas at the mouth of the gulf of california and along the northwestern sector of the tmvb made up the nayarit district which includes calc alkaline to transitional varieties the central sector of the tmvb is characterized by two basaltic districts the jalisco guanajuato and the queretaro hidalgo which are distinguished from the westernmost ones by their lower nb la and generally lower hfse lile values as well as by spider diagrams characterized by larger negative spikes at th ta nb and ti the surface occurrence of the late miocene basalts appears to be controlled by pre existing zones of crustal weakness that channeled the mafic magmas field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the tmvb starting from 12 ma the tmvb separates a northern tectonic domain subject to the developing divergent pacific north america plate boundary from a southern tectonic domain characterized by oblique subduction of the rivera and cocos plates apparently far field stresses related to these complex plate boundaries reactivated older suture zones allowing rapid uprise of mantle derived magmas the subduction related signature shown by miocene mafic lavas of the jalisco guanajuato district argues against the existence of mantle plumes beneath this sector of the north america plate on the other hand the occurrence in the western tmvb and in the guadalajara region of a large volume of mafic magmas which sometimes show characteristics transitional to ocean island basalts could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction related magmas and continental lithosphere to produce the observed basaltic varieties subslab magmas may have flowed through slab free areas along the northern and eastern edges of the subducting rivera plate which in the late miocene was already detached from the farallon plate remnants and diverging from the cocos plate

pleistocene fluvial terraces from northern france seine yonne somme synthesis and new results from interglacial deposits the seine yonne and somme are the main rivers draining northwestern france and flowing into the channel during pleistocene cold stages they were tributaries of the fleuve manche they are characterised by well developed stepped terrace systems showing up to 10 incision steps for the last 1 ma after 15 years of research and the synthesis of stratigraphical sedimentological bioclimatical data and numerical dating these terrace systems are interpreted as the response of the fluvial environments to climatic cyclicity superimposed on a background of slow tectonic uplift the comparison of these three terrace systems shows that the incision budget is similar within the studied area about 55 65 m 1 ma implying relative homogeneity of uplift in the main part of the area beyond marine influence the analysis of each stepped alluvial formation shows regular sediment balance generally composed of coarse periglacial gravels pleniglacial covered by fine grained sediments locally overlain by calcareous tufa interglacial however in the lower seine valley close to the present coast the low terrace includes two estuarine interglacial units interbedded within periglacial gravel units nevertheless until recently evidence of full interglacial conditions remained elusive in fluvial contexts especially in the somme to address these issues a new research programme has been launched on tufa deposits that are the best candidate to register the climatic optimum initial results from la celle seine saint acheul and caours somme allow palaeoenvironmental reconstructions of mis 11 and 5e and thus confirm the importance of calcareous tufa to define a reference record of pleistocene interglacials for northern france moreover it has been previously demonstrated that the main incision leading to terrace formation can be attributed to the transition between interglacial and glacial during early glacial phases the new results from interglacial records have important implications concerning the precise occurrence of the major incision phases within the climatic cycle as they provide the starting point immediately preceding downcutting

uplift and subduction erosion in southwestern mexico since the oligocene pluton geobarometry constraints details of the late oligocene to middle miocene uplift and tectonic erosion episodes of the southwestern continental margin of mexico can be inferred using altot geobarometry of igneous hornblendes geochronology and field relations on the basis of such analyses carried out between acapulco and huatulco we find the following 1 calc alkaline batholiths exposed along the coast from acapulco to huatulco mostly in the 35 25 ma age range were emplaced at depths between 13 and 20 km 2 the contact relationships between these plutons and their host rocks and the exposure of volcanic counterparts 70 km from the coastline indicate a landward decrease in the amount of uplift 3 a comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were in general higher between acapulco and huatulco than those along the margin between puerto vallarta and manzanillo 700 km northwest of acapulco 4 the uplift of this coastal belt occurred during the late stages of magmatism and after its cessation triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the xolapa complex these findings in conjunction with the geometry of the present continental margin as well as the offshore tectonic and stratigraphic features support previous interpretations of very active late oligocene to middle miocene subduction erosion after the onset of strike slip tectonics related to the detachment and subsequent eastward displacement of the chortis block subduction erosion involved both trench sediments and crystalline continental framework rocks different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework comparing the calculated depths of pluton crystallization with the present depth of the continental crust subducted slab boundary interpreted using previously published seismic refraction and gravity models we conclude that onshore basal erosion played a subordinate role during miocene episodes of subduction erosion major removal of lower crustal sections was probably restricted to offshore regions plate reconstructions of the cocos plate and its predecessors with respect to north america indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the guadalupe plate beneath southwestern mexico

u pb geochronology of the type nazas formation and superjacent strata northeastern durango mexico implications of a jurassic age for continental arc magmatism in north central mexico u pb ages of ignimbrites and detrital zircon analyses from middle jurassic through lowermost cretaceous strata in northeastern durango mexico indicate the age of local volcanism and the age range respectively of the jurassic continental margin arc in north central mexico where it is termed the nazas arc the type nazas formation along the rio nazas consists of red continental sandstone and siltstone ignimbrites and intermediate flows that provide an important record of the extensional jurassic nazas magmatic arc overlying shallow marine sandstone and shale of late jurassic to earliest cretaceous age represent deposits of younger extensional basins that are part of the border rift system which extended from southern arizona to the gulf of mexico the younger strata constitute volcanic litharenite and lithic arkose derived from the jurassic volcanic rocks and from basement rocks of the coahuila block to the north two ignimbrites in the upper part of the nazas formation have statistically indistinguishable ages of 170 2 and 169 2 ma the ages are consistent with their stratigraphic order and position above volcanic lithic sandstone with a maximum depositional age of ca 180 178 ma calculated from detrital zircons six detrital zircon age populations were defined from four sandstone samples n 377 zircon grains of the nazas formation and overlying la casita and carbonera formations the most prominent age clusters 10 of the entire grain population have ranges ca 1277 890 ma grenville 22 of total ca 486 310 ma early paleozoic 11 and ca 202 159 ma jurassic two populations constituting 54 of total grains the jurassic age cluster indicates that nazas arc magmatism in the region took place during the early and middle jurassic and ended by early late jurassic time the nazas formation correlates with arc related middle jurassic strata in sonora southern arizona and southeastern california that contain interbedded eolian sandstone derived from the jurassic ergs of the colorado plateau absence of interbedded eolianite in the nazas formation thus indicates that the outcrops of north central mexico were likely not translated to their present locations along a regional late jurassic transcurrent fault system as postulated by some workers detrital zircon data from the upper jurassic and lowermost cretaceous strata indicate that jurassic volcanic rocks covered the coahuila block directly north of the study area at the end of middle jurassic time sandstone composition and detrital zircon ages record erosional unroofing of the coahuila block whereby removal of lower and middle jurassic volcanic and volcaniclastic strata exposed grenville pan african and peri gondwanan basement of the block during late jurassic time

a northwest trending jurassic fold nappe northernmost zacatecas mexico the caopas rodeo and nazas formations exposed in the san julian uplift of northern zacatecas are distinguished principally on the basis of style and intensity of deformation they are parts of the same early mesozoic volcanogenic suite this suite was the source for overlying volcaniclastic conglomerate and sandstone la joya formation that appears transitional into succeeding late jurassic oxfordian zuloaga limestone deformation that was contemporaneous with the deposition of the lower part of the zuloaga produced an asymmetrical northwest trending fold nappe that was driven southwestward the massive quartz porphyry core of the structure caopas formation moved somewhat independently of the encasing more ductile rocks rodeo and nazas formations phyllonite developed in the nose of the nappe the upright limb was attenuated and ruptured in zones where deformation due to the nappe forming process was weaker a gently dipping foliation possibly related to an older episode of recumbent folding is preserved orientation age and location of the nappe suggest that it is a manifestation of transpressional stress developed along the mojave sonora megashear as left lateral movement carried the jurassic arc toward the southeast

climatic reconstruction of the weichselian pleniglacial in northwestern and central europe a multiproxy approach is applied to reconstructing accurately the weichselian pleniglacial climate 72 13 ka in northwestern and central europe standardised translations are used to transform proxy data into climate parameter values for six characteristic time windows quantitative reconstructions of the temperature regime are derived from periglacial coleoptera and botanical evidence while aeolian and fluvial evidence provide qualitative information on wind activity and precipitation respectively the early pleniglacial 74 59 ka the cold period between 41 38 ka and the late pleniglacial 27 13 ka are characterised by a strong north to south climatic gradient over northwestern europe during the last glacial maximum discontinuous permafrost was established in northern france whereas the continuous permafrost zone extended from the uk belgium the netherlands germany and poland to the nordic ice sheets prominent wind activity and a relatively low precipitation typify these periods in contrast an indistinct west to east climate gradient was present in the relatively more temperate intervals e g 50 41 ka seasonally frozen ground conditions prevailed in northwestern europe whereas discontinuous permafrost may be suggested for central germany it appears that the climate conditions in northwest and central europe were controlled by three major factors the scandinavian ice sheet the north atlantic surface water circulation and the russian continent 1998 john wiley sons ltd

geodynamic evolution of the northern and central andes during early to middle mesozoic times a tethyan model the early to middle mesozoic sedimentary and magmatic history of the northern and central andes i e the nne trending colombian ecuadorian segment and the nw trending peruvian segment exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings the late triassic to late liassic period began with a widespread marine transgression from the latest triassic the colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in peru during this time no significant magmatic activity is recorded except in the emerging colombian area this period is interpreted as the result of rifting of a tethyan oceanic arm which separated the colombian and palaeo mexican margins from latest liassic an important calc alkaline magmatic arc developed along the emergent colombian segment further south the north peruvian shelf probably emerged but marine sedimentation continued in southern peru during middle and early late jurassic times the colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation along the peruvian segment a turbiditic trough emergent areas and continental basins were created and the scarcity of calc alkaline magmatism suggests that only very local subduction took place this period is regarded as one of the southeastward subduction beneath the colombian segment and of tethyan oceanic crust originating in the newly formed colombian oceanic arm this pattern would have induced a chiefly left lateral transform motion along the peruvian segment by kimmeridgian tithonian times the palaeogeographic framework had drastically changed along the colombian segment magmatic activity ceased and continental accretions occurred along dextral strike slip sutures in peru tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation in the coastal area arc related volcanism indicated that subduction took place beneath this segment this geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the tethyan ridges and replacement by pacific spreading centres inducing a roughly northeastward convergence direction

facies sequence of triassic jurassic red beds in the sierra madre oriental ne mexico and its relation to the early opening of the gulf of mexico abstract the configuration of upper triassic grabens in the gulf of mexico rift system is pointed out by the suture zones of late palaeozoic plate convergence rifting particularly started on both sides of the ancient magmatic arc the taphrogenic red beds of the huizachal formation in the sierra madre oriental represent the western parts of this system the early syn rift sediments are characterized by fining upward cycles of a fluvial environment with a low sinuosity channel pattern later on decreasing of relief intensity caused the change to a rather meandering system climate was evidently semiarid during that time the petrographic composition of the clastic rocks suggests a nearby sedimentary metamorphic basement the transition to early post rift sediments is marked by an angular unconformity its formation approximately coincides with the first emplacement of oceanic crust in the central parts of the gulf basin the early post rift sediments of the la joya formation show within a single fining upward megacycle the transition from terrestrial to marine conditions that persisted up to the beginning of cenozoic time

cenozoic magmatism of the sierra madre del sur and tectonic truncation of the pacific margin of southern mexico abstract in southern mexico the sierra madre del sur sms preserves the record of a long lived magmatic arc that vanished diachronically during paleogene early miocene time decreasing in age to the southeast the magmatic record consists of a coastal plutonic belt dominated by granites and granodiorites with minor mafic and intermediate intrusions and an inland volcanic belt dominated by intermediate and silicic volcanic centers the juxtaposition of the plutonic belt with the trench indicates the removal of most of the paleogene forearc during the oligocene abundant evidence indicates that subduction erosion played a significant role in the reconfiguration of the continental margin of southern mexico following cessation of magmatism this is evident in the subsidence of continental segments contiguous to the present trench and the loss of lower continental crust as inferred from the present shape of the wedge as well as in the estimations of the former crustal thickness based on geobarometry of the exposed levels of plutonic complexes the late cretaceous cenozoic decreasing age trend in the sms shows three time intervals in which magmatism ceased at different rates the general southeastward younging trend of magmatism as well as the left lateral shear zones of the central and coastal zones of the sierra madre del sur in combination with constraints imposed by the prevalent model of the caribbean plate evolution have been invoked to consider the chortis block as the removed segment during margin truncation the main drawbacks of this model are the mismatch in the chronology of migmatitic complexes of northern chortis and the xolapa terrane in the continental margin of southern mexico and the presence of the late cretaceous to present unperturbed marine sequence in the gulf of tehuantepec in the assumed trajectory of the chortis block these considerations and the subduction erosion evidence suggest a more complex scenario than having chortis juxtaposed immediately adjacent to the present day margin there are alternative possibilities to explain removal of the forearc ranging from a western provenance for the chortis block as has been suggested in other models to intermediate positions for chortis with a more southern trajectory with respect to the present day trench such a trajectory would leave a broad zone with shear strain structures more susceptible to removal by subduction erosion after the passage of the trench trench transform triple junction this would reconcile the evidence of subduction erosion and the indications of the northwestern provenance of the chortis block

changing coastal levels of south america and the caribbean region from tide gauge records abstract tide gauge records from southern mexico the caribbean islands and central and south america that span the interval 1940 1970 reveal long term changes of relative sea level according to regression analysis and eigenanalysis the results indicate such large variations in both direction and rate of secular movement as to rule out changes in volume of ocean water as being more than a subordinate factor the only satisfactory explanation is that the land level beneath the tide gauges is rising in some places and sinking in others complex spatial patterns of relative seal level change in southern mexico and the caribbean mirror the tectonic regime of these regions exhibiting both submergence and emergence of the land central american tide gauge records similarly show considerable complexity responding to relative movement along plate boundaries the pacific coast of south america appears to correlate with the depth of the benioff zone subduction of aseismic ridges produces local highs in the benioff zone flanked by troughs at either side near the benioff highs relative land level is rising between these ridges relative land level is falling sea level trends in southern and atlantic coasts of south america are closely linked with continental crustal rifting and subsidence data do not allow unambiguous separation of changes in ocean level from changes in land level and no simple eustatic ocean level change can be estimated accurately from these data

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

long lasting interactions between tectonic loading unroofing post rift thermal subsidence and sedimentary transfers along the western margin of the gulf of mexico some insights from integrated quantitative studies abstract after jurassic rifting numerous carbonate platforms i e the orizaba cordoba and golden lane tuxpan platforms developed during the lower and middle cretaceous episode of thermal subsidence along the western passive margin of the gulf of mexico with intervening basinal domains i e the tampico misantla zongolica veracruz and deep gulf of mexico dgm basins during the late cretaceous paleocene the east verging sierra madre oriental thrust belt developed resulting in tectonic uplift and unroofing of the allochthonous units i e tectonic units made up of former orizaba and cordoba platforms and zongolica basin series this new topography provided also an important source of clastics to feed the adjacent foredeep where coeval tectonic loading accounted for the bending of the foreland lithosphere however shallow water facies or even emersion persisted until the eocene in the forebulge area at the present location of the golden lane preventing locally the clastics to reach the dgm this topographic barrier was ultimately bypassed by the clastics only during the oligocene and neogene once 1 the prograding clastic wedge had exceeded accommodation and 2 the long lasting thermal subsidence of the passive margin could overpass the effect of the bending and force the former bulge to sink numerous paleo thermo meters tmax ro paleo thermo barometers fluid inclusions pvt and coupled forward kinematic and thermal modeling have been used to calibrate and date the progressive unroofing of the thrust belt coupled tectonic and sedimentologic modeling was applied in the foreland to predict the distribution of sand versus shale ratios in the oligocene to plio quaternary clastic sedimentary wedge of the passive margin where gravitational gliding of post eocene series occurred during the neogene along major listric faults mantle dynamics are advocated as the main process accounting for post orogenic uplift and regional tilting of the basement which initiated a massive transfer of sediments from the cordillera towards the gulf of mexico from oligocene onward resulting in a destabilization and gravitational collapse of the western slope of the gulf of mexico in neogene times

late maastrichtian paleoenvironments and dinosaur biogeography in the western interior of north america during late maastrichtian time three major sedimentary depositional provinces existed in continental environments of the western interior region of north america the sediments accumulated within these provinces are herein termed the piedmont alluvial plain and coastal lowland lithosomes these sedimentary lithosomes correspond to broad paleoenvironmental and paleophysiographic provinces that resulted from the westward shift of the pacific coast the bisection and retreat of the interior epeiric sea and the development of an intermontane basin system during maastrichtian time these dramatic physiographic changes created environments markedly different from those that had prevailed during most of the late cretaceous when western north america consisted of a narrow peninsula flanked on the east by a remarkably stable and monotonous coastal plain in conjunction with these physiographic and environmental changes three diverse dinosaur dominated continental vertebrate faunas arose herein termed the leptoceratops triceratops and alamosaurus faunas each was peculiar to a sedimentary environmental province and among them there was little interchange the separation of these faunas indicates that the dinosaur dominated communities had adapted to changing or deteriorating environmental conditions at the close of cretaceous time the alamosaurus fauna characterized markedly seasonal semi arid environments of the intermontane basins south of about 35 n latitude the triceratops fauna inhabited humid coastal floodplains and swamps bordering the retreating northern embayment of the epeiric sea north of latitude 35 n the leptoceratops fauna inhabited cool piedmont environments flanking the mountainous cordilleran overthrust belt north of 35 n latitude recognition of this biogeographic zonation indicates that the triceratops biozone is only of local utility and that the biostratigraphy of upper cretaceous continental deposits in north america must reflect the provincialism now recognized moreover the evident adaptability of dinosaur faunas to varied and changing environmental conditions suggests that climatic change alone may not have been responsible for their demise

palaeomagnetic results from the southern sierra madre oriental mexico evidence for early cretaceous or laramide remagnetization abstract a large suite of samples from the latest triassic huizachal and early jurassic huayacocotla groups and the latest jurassic taman and earliest cretaceous pimienta formations was collected in the southern sierra madre oriental for a palaeomagnetic study only the samples from three sites belonging to the huizachal group and the las juntas formation possibly have retained their primary magnetization if so their pole position does not reveal any palaeomagnetically discernable motion relative to cratonic north america all remaining sites were remagnetized as evidenced by a negative fold test at four sites and the fact that the pole positions cluster better at the 95 significance level if no structural corrections are applied the tightness of the cluster 95 4 6 and the same polarity suggest that the samples were remagnetized at some common time these results permit two interpretations 1 if the sampling region has not suffered any significant tectonic rotation then the remagnetization can be dated by comparison with the polar wander path for north america as early cretaceous 130 ma this early cretaceous phase of deformation is not recognized in the northern sierra madre oriental and clearly pre dates the early tertiary laramide orogeny 2 if the southern sierra madre oriental did rotate counterclockwise by 20 then the remagnetization could have originated in any cretaceous or early tertiary time and may indeed be related to the laramide deformation in either case the data imply that the southern sierra madre oriental constitutes an independent tectonic domain

a chemical and isotopic study of the laramide granitic belt of northwestern mexico identification of the southern edge of the north american precambrian basement along the laramide belt of northwestern mexico granitic rocks of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of the north american precambrian basement in the northern part the laramide plutons the northern granites intruded proterozoic crystalline rocks and a thick late proterozoic through paleozoic miogeoclinal cover of north american affinity in the central part the granitic bodies the central granites were emplaced into a sequence of paleozoic eugeoclinal rocks overlain by late triassic clastic units the southern part of the belt the southern granites intruded a less known crust characterized by middle to late mesozoic island arc related volcanic and sedimentary rocks of the guerrero terrane data from a suite of metaluminous to slightly peraluminous calc alkalic granitic rocks along the belt display north to south geochemical and isotopic variations which could correlate with the type of intruded basement the northern and central granites are characterized by strongly fractionated light rare earth element ree enriched patterns which display generally pronounced negative europium anomalies whereas the southern granites have lower total ree enrichments and much flatter chondrite normalized slopes displaying almost no europium anomalies isotopic results also suggest regional variations as shown by the following initial sr and end ranges 0 7070 to 0 7089 and24 2 to25 4 respectively for the northern granites 0 7060 to 0 7079 and23 4 to25 1 for the central granites and 0 7026 to 0 7062 and20 9 to 14 2 for the southern granites on the basis of their isotopic similarities the proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic flows in southern arizona and northern mexico is interpreted as a reasonable parental source for the northern and central granites however mantle derived melts are not excluded the more primitive southern granites are interpreted to come from a source that lacked proterozoic basement instead they were probably derived by mixing of juvenile mantle melts with partial melts of the lower parts of the guerrero terrane in general the north to south compositional variations of the laramide granitic rocks of northwestern mexico reflect the crustal structure underneath the batholiths the sr and nd data indicate that the edge of the north american precambrian basement extends approximately southeastward from the coastal batholith of central sonora then about 200 km south of hermosillo in southern sonora the edge bends eastward and continues to the east beneath the sierra madre occidental volcanic province

a speculative reconstruction of the middle paleozoic continental margin of southwestern north america geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern north america with respect to the middle paleozoic continental margin the southwestern margin of the middle paleozoic continent based on these data is quite irregular in its present configuration but most of the irregularities can be accounted for by strike slip displacement of continental fragments during several post middle paleozoic tectonic events we conclude that large scale late paleozoic sinistral displacements and mesozoic and cenozoic dextral displacements have been the major events affecting the configuration of this margin our preferred model includes the following elements 1 500 km of left lateral displacement of a large part of the margin during a late paleozoic truncation 2 slivering of the margin by right lateral faulting during the mesozoic amounting to about 275 km of total displacement and 3 further deformation of the margin during the cenozoic primarily by right lateral faulting oroclinal bending and crustal extension

lead isotope signatures of holocene fluvial sediments from the loire river valley abstract the distribution of mn v th pb and isotopes of pb in the labile fraction of sediments from a channel infill in the middle loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10 ka bp the acid extractable matter aem in the sediment samples is variable in amount and in trace element contents iron mn oxyhydroxides are the principle trace element carrying phase in the labile fraction and carbonates are a secondary carrier vanadium and pb originate from the weathering of silicates and are used as a silicate erosion rate index in the fluviatile record most of the aem data plot along a general trend between 3 endmembers basalts cretaceous carbonate rocks and granites in the relationship between 207pb 206pb and 206pb 206pb these endmembers have been mixed in various proportions depending on natural holocene inputs erosion volcanic events or human influences mining and smelting of ore

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

holocene floodplain metamorphosis in the midlands united kingdom abstract many lowland rivers in the united kingdom including the nene soar and severn have layered floodplains with a basal gravel of pleistocene or late glacial age and a structureless silty clay superficial unit burying the entire former floodplain this burial is illustrated by the existence of variable mixed and pedologically disturbed sediments and palaeo landsurfaces between the basal gravels and superficial silty clay this paper presents a comparison of the pre and post late holocene palaeo landsurfaces and palaeochannels using data from the nene soar and severn valleys from this comparison it is argued that during the mid to late holocene ca 4500 yr bp to 2500 yr bp floodplains and river channels underwent a metamorphosis this is indicated by accelerated vertical accretion a reduction in floodplain relative relief changed floodplain soil conditions a reduction in channel w d ratios and a resultant increase in the silty clay proportion of channel perimeter sediments there are indications that hydrological change preceded this metamorphosis but the primary cause was an increase in fine sediment supply during the later holocene and a disequilibrium between channel bed and floodplain aggradation rates resulting in relative incision this metamorphosis which is explained in this paper by the proposed stable bed aggrading banks model sbab is the key factor in the holocene evolution of low energy floodplain systems in the united kingdom upon which more subtle short term fluctuations are superimposed

late quaternary climatic change in france estimated from multivariate pollen time series abstract in regions like southern france where usual analytical techniques are unsatisfactory due to the heavy influence of human activity and the existence of a complex climate the quantitative reconstruction of climate from pollen is particularly difficult that is why an original method has been performed the first step of the method based on best analogs estimation and multiple regression is to calculate a relationship between climate monthly temperature and precipitation and modern pollen spectra for data from 182 sites the result is called the analog climate the second step of the method an original combination of canonical correlation and principal components analyses extracts the common information from several fossil pollen sequences to produce a so called paleobioclimate this step removes the effects of local differences in the vegetation among nearby sites and also reduces the effects of human disturbance in the third step the signals obtained from the first two steps are merged by kalman filter into a final reconstruction where the noise is reduced by around 37 the results presented with a 95 confidence level suggest that the pleni wurm was 7 to 13 c colder than present and had 20 to 60 less precipitation than today this large confidence takes into account the climatic diversity of the region at 13 000 yr b p the climate reached the modern level of precipitation but the temperature was just less than present the subsequent decrease of precipitation is less important than that of temperature this fact may explain the advance of alpine glaciers during the younger dryas preboreal warming was abrupt 3 to 4 c per 500 yr and precipitation increased more slowly

paleomagnetic evidence for post jurassic stability of southeastern mexico maya terrane the tectonic evolution of southeastern mexico has been a subject of major controversy not only in regard to past geometry but also in the timing of proposed geological events as well for the past 10 years most if not all investigators agree that the gulf of mexico basin was formed by late jurassic time and that the maya terrane was in its current location prior to the cretaceous in order to gain further insight into the drift history of the maya terrane we have undertaken a paleomagnetic study of the uppermost jurassic lower cretaceous tithonian lower neocomian san ricardo formation in southeastern mexico at 93 7 w 16 8 n the sampling site is located east of the isthmus of tehuantepec on the southwest side of the maya block at the base of the yucatan peninsula a suite of 133 samples was collected in stratigraphic succession from a 114 m thick sequence of red shales and sandstones near cintalapa chiapas mexico after progressive thermal demagnetization of all samples at six steps from 350 c to 630 c 89 samples were selected for final paleopole analysis on the basis of their magnetic stability four different polarity intervals were observed the sequence being from bottom to top n r n r which assists in the assessment of the reliability of the observations the mean pole position obtained 160 0 e 69 8 n agrees with the mean pole position of the upper part of the morrison formation of colorado a unit of virtually identical age these results indicate that no discernible rotation or displacement of the maya block has occurred since at least early neocomian times

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

quantitative analysis of climate versus human impact on sediment yield since the lateglacial the sarlieve palaeolake catchment france minimum rates of solid ssy and dissolved dsy sediment yield sy were evaluated in t km2 per yr from sediments stored in the sarlieve palaeolake french massif central for seven phases of the lateglacial and holocene up to the seventeenth century the catchment 29 km2 mainly formed of limestones and marls is located in an area rich in archaeological sites in the massif central the respective impacts of human activities and climate on sy were compared by quantification of human settlements through archaeological survey and palynological data during the lateglacial and early holocene up to about 7500 yr cal bp variations in ssy and dsy rates were mainly related to climate change with higher rates during colder periods younger dryas and preboreal and lower rates during warmer periods bolling allerod and boreal however cf1 tephra fallout induced a sharp increase in sy during the allerod during the middle and late holocene after 7500 yr cal bp ssy and dsy greatly increased by factors of 6 5 and 2 8 respectively particularly during the final neolithic at about 5300 yr cal bp when the climate became cooler and more humid after this date at least 75 of the ssy increase and more than 90 of the dsy increase resulted from human activities but ssy rates showed little variation during protohistoric and historic times up to the seventeenth century ssy and dsy rates and dsy ssy ratio indicate that catchment soils began to form during the lateglacial and preboreal thickened considerably during the boreal and atlantic finally thinning rejuvenation mainly as the result of human induced erosion during the sub boreal and sub atlantic increased mechanical erosion during the late holocene also induced an increase in chemical erosion

urban geo archaeology in medieval alzira prov valencia spain abstract alzira is situated in a cut off meander loop of the rio jucar geo archaeological study shows that the islamic city wall was built during the second quarter of the 11th century ad at a time when the river had a low amplitude and low energy flood regime after mid century overbank silts began to invade the city as peak flood discharge increased with a spate of destructive floods reflected by high energy deposits during the late 11th century urban expansion within the city wall is dated to the later 11th or early 12th century interrupted by local abandonment and moderate energy flood silts major construction during the mid 12th century was followed by protracted abandonment after a siege in ad 1171 and further moderate energy flood silts that redistributed occupation and collapse debris reoccupation during the late 14th century was terminated by catastrophic floods in 1517 and 1571 after which much of the western end of alzira was not rebuilt following further flood catastrophes during the 17th century parts of the central walled city remained abandoned until the 19th century with new occupation atop flood sands of yet another disaster in 1864 the change in flood regime after 1150 was a result of deforestation of the watershed but progradation of a downstream tributary fan after 1517 created an unstable aggrading floodplain increasingly prone to severe flooding climatic anomalies were responsible for periods of recurrent severe floods archaeologically verified during the 11th and 12th centuries and historically documented beginning in 1318 the geo archaeological methodology illuminates the constructional and settlement history of alzira allows distinction of fluvial and cultural components and provides the necessary microstratigraphic detail and dating control to document the true complexity of alluvial processes during the last 1000 years

sedimentary and crustal structure of the northwestern gulf of mexico we conducted a large offset seismic experiment in the northwestern gulf of mexico using largecapacity air guns and digital ocean bottom seismographs to determine the velocity structure of the sediments crust and upper mantle five multiple detector split profile lines were shot over an area extending from the shallow midshelf south of galveston to the continental rise just south of the sigsbee escarpment the data were of adequate spatial density and quality to allow combined use of near vertical reflections wide angle reflections and refractions for interpretation several techniques were used to obtain velocity depth functions 1 conventional constant velocity layer analysis 2 interval velocities from moveouts of the precritical reflections 3 analysis in the tau p domain to determine extremal depth bounds 4 estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions and 5 forward modeling using two dimensional 2 d ray tracing a sedimentary sequence of nearly constant thickness 13 to 15 km covers the study area various mobilized salt features exist within the sediments in the slope area ranging from deeply buried layers and diapirs under the inner slope to shallow thin allochthonous bodies under the outer slope in contrast to the sediments the crust shows considerable variation in thickness from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf the transition under the slope however is not monotonie but includes thinning to nearly oceanic thickness under the midslope possibly suggesting an incipient rift that failed to materialize during the opening of the gulf

river system evolution and environmental changes during the lateglacial in the paris basin france abstract lateglacial sequences from the paris basin floodplains provide a general overview of the valley s evolution related to climatic changes from the weichselian upper pleniglacial to the beginning of the holocene comparison between morphostratigraphical and palaeobiological data allows discussion of fluvial dynamics and related palaeoenvironmental conditions the upper pleniglacial oldest dryas transition remains poorly documented it is represented by sands and silts deposited by a braided channel network the bolling begins with a significant incision related to a meandering system which is followed by local organic and minerogenic deposition contemporaneous with vegetation development the older dryas is evident in several sequences the allerod appears more stable and the forest extension is accompanied by a thin soil formation on the banks the younger dryas is characterised by a strong infilling of the valleys by calcareous silts this last sharp event precedes a second major downcutting phase which takes place in the early postglacial and is followed by a decline in river flow and sediment supply this evolution reflects a broad regional pattern of fluvial response which illustrates its sensitivity to climatic change the study of various morphosedimentary contexts shows their influence on the fluvio dynamic record and bio proxies which complicates the detection of minor phases

organic walled microfossils of possible dinoflagellate affinity from the lower permian of sonora northwest mexico abstract shallow water carbonates from the lower permian of sonora northwest mexico have yielded small organic walled microfossils that show several features characteristic of dinoflagellates the oldest unequivocal dinoflagellate fossils however presently known come from the middle triassic earlier reports of microfossils with possible dinoflagellate affinity from the paleozoic and neoproterozoic together with dinoflagellate specific biomarkers found in early cambrian rocks and molecular phylogenetic analyses suggest the presence of an intermediate stage of evolution or protodinoflagellate group in pre mesozoic times

geochemistry of the coastal sonora batholith northwestern mexico new major and trace element and radiogenic sr and nd data from granitic rocks help to characterize the geochemical composition of the coastal sonora batholith which represents the westernmost portion of the laramide magmatic arc 80 40 ma in mexico compared with more inland located regions of the arc in central and eastern sonora inner granites the plutons within the coastal batholith have higher mgo feo and cao and lower k2o contents and tend to be more depleted in nb u and to a lesser extent rb but relatively enriched in sr these rocks display flatter slopes of rare earth elements with la abundances around 80 to 200 times chondrite values and smaller but well developed negative eu anomalies initial 87sr 86sr ratios and end values from a few studied samples of the coastal sonora batholith range from 0 70586 to 0 70679 and from 2 3 to 4 7 respectively these isotopic signatures allow recognition of a more mantle related magma source for this batholith as compared with the inner granite

pangea in permian to jurassic time the apparent polar wander paths apwps for the northern and southern continents have been redetermined using revised time scales and data when plotted on the conventional pangea pangea a the paths are distinct but essentially linear and parallel from late carboniferous to late permian time both paths then undergo a sharp change in direction and converge so as to overlap in early to middle jurassic time if the permo triassic magnetic field was an axial dipole then paleomagnetic data show that pangea was mobile during most and possibly all of the mid triassic to mid jurassic interval because of the uncertainties in pole ages and positions and errors in measurements the new apwps are consistent with a range of pangeas in particular the data are compatible with two paleomagnetic pangeas a2 and a new class of pangea here named the d pangeas these resemble morel and irving s 1981 pangea b and the c pangeas of smith et al 1981 unlike the a2 pangeas the b c and d pangeas all require large 2000 km relative motion between the northern and southern continents geological data require the transition from an earlier pangea to pangea a to take place along a transform zone here named the pangea transform zone or ptz the ptz is considered to be technically analogous to the great glen fault of scotland four types of possible ptz are examined passing through either the western or eastern end of the brunswick magnetic anomaly of eastern north america or north and south of iberia the geological data suggest that those pangeas that require large displacements on ptzs that pass east of the brunswick magnetic anomaly are improbable b and c pangeas are snapshots giving precise paleomagnetic reconstructions d pangeas fit long stretches of the apwps as well as possible a2 pangeas fit the apwps moderately well but also require the geological displacement to be smaller the differences in displacement between the d and a2 pangeas reflect geometrical relationships between euler poles ptz curvature and the apwps the general geological evidence suggests that d pangeas are less probable than a2 pangeas but that they cannot be rejected transition from a2 to a is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation in detail the conventional pangea requires modification africa must be broken into at least two fragments along the benue trough and its northern continuation northwest africa then fits more tightly into northern south america which modifies the apwps and reduces the gap between them by including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the a63 circles touch or overlap non dipole fields or an expanding earth need not be invoked to explain the mismatch of the apwps on the conventional pangea the finite rotation required for the older pangea to pangea a is very close to van der voo and french s 1974 single rotation for a2 to a but is actually the sum of three smaller rotations that reflect the modification to pangea a continental extension and the postulated ptz displacement of the possible ptzs a ptz parallel to the west african continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the east coast magnetic anomaly of north america further refinements to pangea a2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified pangea a and incorporation of rotations due to major faults such as the south atlas fault in morocco

regional stratigraphy and distribution of epigenetic stratabound celestine fluorite barite and pb zn deposits in the mvt province of northeastern mexico northeastern mexico hosts numerous epigenetic stratabound carbonate hosted low temperature hydrothermal deposits of celestine fluorite barite and zinc lead which formed by replacement of mesozoic evaporites or carbonate rocks such deposits can be permissively catalogued as mississippi valley type mvt deposits the deposits studied in the state of coahuila are associated with granitic and metasedimentary basement highs horsts marginal or central to the mesozoic sabinas basin these horsts controlled the stratigraphy of the mesozoic basins and subsequently influenced the laramide structural pattern the sabinas basin consists of 6 000 m thick jurassic to cretaceous siliciclastic carbonate and evaporitic series the mvt deposits are mostly in barremian and in aptian albian to cenomanian formations and likely formed from basinal brines that were mobilized during the laramide orogeny although earlier diagenetic replacement of evaporite layers barite and celestine deposits and lining of paleokarstic cavities in reef carbonates zn pb deposits is observed fluid inclusion microthermometry and isotopic studies suggest ore formation due to mixing of basinal brines and meteoric water homogenization temperatures of fluid inclusions range from 45 c to 210 c salinities range from 0 to 26 wt nacl equiv and some inclusions contain hydrocarbons or bitumen sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from barremian to cenomanian evaporites regional geology and a compilation of metallogenic features define the new mvt province of northeastern mexico which comprises most of the state of coahuila and portions of the neighboring states of nuevo leon durango and perhaps extends into zacatecas and southern texas this province exhibits a regional metal zonation with celestine deposits to the south fluorite deposits to the north and barite and zn pb deposits mostly in the central part