mechanistic solutions to the opening of the gulf of mexico two mechanistic models which are unlike the traditional plate tectonic landfill models used for most proposed pangea reconstructions of the yucatan block relate the mesozoic opening of the gulf of mexico directly to the movement of the north and south american plates 1 a previous piggyback model in which yucatan moves with south america out of the western gulf and 2 a new edge driven model in which the motion of the yucatan block is caused by forces applied to its margins by the movement of the north and south american plates in the second model yucatan moves out of the northern gulf of mexico as a gear or roller bearing on the basis of magnetic edge anomalies around the gulf this edge driven model predicts that from the bathonian to tithonian 170 to 150 ma yucatan was rotated 60 counterclockwise as a rigid block between north and south america with rift propagation and extension occurring simultaneously in the gulf of mexico and yucatan basin

feedbacks between climate and surface water in northern africa during the middle holocene observations indicate that the area of lakes and wetlands in northern africa was considerably greater during the middle holocene than at present simulations are designed to examine whether expanded surface waters may have had a significant impact on the strength of the summer monsoon of northern africa three experiments with the national center for atmospheric research community climate model ncar ccm3 are analyzed a modern and two middle holocene 6000 years before present simulations one with and one without prescribed expanded surface water there is a significant increase in the strength of the summer monsoon in the middle holocene simulation due to the enhanced seasonal insolation cycle the addition of surface waters result in a june july and august mean increase in the net surface radiation 5 an increase in the latent heat flux 30 a decrease in the sensible heat flux 10 and an increase in the near surface specific humidity 5 compared to the middle holocene simulation without surface water changes the changes in these simulated climate variables are comparable in scale to changes due to orbital forcing alone the expanded surface waters result in a cooling of the atmosphere and anticyclonic flow over the large water bodies in summer relative to the simulation without surface water changes the combination of increased atmospheric moisture and altered circulation results in significant changes to the precipitation distribution in northern africa including a small increase in the zonal mean july precipitation to the north of the lakes and a decrease to the south the geographic distribution of the precipitation with surface water changes is qualitatively in better agreement with observations than the distribution with orbital forcing alone but still does not fully match the expansion implied by observations nor the expansion required to produce the simulated middle holocene surface waters used in this study the results of this study suggest that surface waters were an important factor in the climate of northern africa during the middle and early holocene and that they must be included for accurate simulation of this climate

transformation of a subtropical river into a hyperhaline estuary the casamance river senegal paleogeographical implications abstract located in the south of senegal in the transitional guineo congolian soudanian zone the casamance system is composed of a small river supplied by a narrow drainage basin and followed by a 250 km long estuary the continuous rainfall decrease experienced by the region since the beginning of the century drastically affected the casamance river this climatic change suddenly accelerated during 1968 1970 which led to a four stage succession before 1968 1970 stage 1 rainfall 1300 mm the casamance estuary showed the customary negative upstream salinity gradient between 1968 1970 and 1978 stage 2 rainfall 1000 mm under decreasing rainfall the casamance estuary shifted to a hyperline inverse estuary it showed well defined physical chemical and biological zonations fresh groundwater resources supplying the upper reaches of the river during the dry season were progressively depleted between 1978 and 1986 1987 stage 3 the salinity peak of the end of the dry season progressively rose and shifted upward the ecological zonation showed the increasing marine influence until 1986 between 1986 1987 and 1990 stage 4 the dry season fresh water input had weakened with the depletion of groundwater resources a slight increase in rainfall was insufficient to repulse the hyperhaline waters and the restricted area extended upstream and downstream the physical chemical and biological zonations strongly widened and flattened in the future the return to stage 1 conditions by an increase in average rainfall would take decades inevitably it will be much longer than the transformation into a hyperhaline estuary thus a slight decrease of the average rainfall in the region provoked a drastic change of the hydrological and biological patterns of the ria the result seems to be a major crisis in the region and might be recorded in the fossil records we discuss these observations in terms of possible misreadings of paleoenvironments and in relation with long term climate trends

mid jurassic tectonothermal event superposed on a paleozoic geological record in the acatlan complex of southern mexico hotspot activity during the breakup of pangea u pb isotopic analyses of zircon from the lowest structural units of the acatlan complex of southern mexico indicate that paleozoic tectonothermal events are overprinted by mid jurassic 175 3 to 171 1 ma low pressure migmatization 5 6 kb polyphase deformation and intrusion of felsic and mafic magmas ensuing rapid cooling recorded by 40ar 39ar muscovite biotite and k feldspar ages is estimated to have taken place at 21 3 c my at exhumation rates of 0 6 mm yr such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top to the west kinematic data synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent oaxaca terrane where top to the north extensional shear zones occur in paleozoic strata this pattern of extension suggests tectonic unroofing in response to domal uplift radius 100 km like that associated with core complexes slab windows and hotspots most tectonic analyses for the jurassic place the acatlan complex in the forearc region of an arc in colombia lying 600 800 km inboard of the subduction zone presumably in response to flat slab subduction modern analogues suggest that flat slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid oceanic ridge or a plume since core complexes are typical of arc backarc regions and slab windows generally produce metamorphic belts the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this jurassic tectonothermal pulse in southern mexico this plume activity is synchronous with the opening of the gulf of mexico during the breakup of pangea to which it may have contributed

flow resistance in steep mountain streams resistance to flow at low to moderate stream discharge was examined in five small 12 77 km2 drainage area tributaries of chilliwack river british columbia more than half of which exhibit planar bed morphology the resulting data set is composed of eight to 12 individual estimates of the total resistance to flow at 61 cross sections located in 13 separate reaches of five tributaries to the main river this new data set includes 625 individual estimates of resistance to flow at low to moderate river stage resistance to flow in these conditions is high highly variable and strongly dependent on stage the darcy weisbach resistance factor ff varies over six orders of magnitude 0 29 12 700 and manning s n varies over three orders of magnitude 0 047 7 95 despite this extreme range both power equations at the individual cross sections and keulegan equations for reach averaged values describe the hydraulic relations well roughness is divided into grain and form considered as all non grain sources components form roughness is the dominant component accounting for about 90 of the total roughness of the system i e form roughness is on average 8 6 times as great as grain roughness of the various quantitative and qualitative form roughness indicators observed only the sorting coefficient d84 d50 correlates well with form roughness copyright 2008 john wiley sons ltd

u pb geochronological constraints on the triassic jurassic ayu complex southern mexico derivation from the western margin of pangea a abstract we present la icp ms laser ablation inductively coupled plasma mass spectrometry u pb detrital and igneous zircon data of poly deformed metamorphic and igneous rocks of the ayu area southern mexico these rocks were previously inferred to be part of the late paleozoic acatlan complex but new age data indicate that they formed in the mesozoic and should be placed in the newly designated ayu complex the ayu complex comprises polydeformed metasedimentary rocks chazumba lithodeme of a turbidite like protolith that are intercalated with boudinaged ortho amphibolites with transitional arc to morb tholeiitic geochemistry in the south the metasedimentary sequence is affected by a ca 171 ma partial melting which formed the magdalena migmatite migmatization was accompanied by 171 168 ma intrusions of granodioritic dioritic and granitic dikes and sheets as well as pegmatite bodies which are characterized by inherited zircon populations of ca 260 290 320 360 420 480 880 990 and 1080 1250 ma that are also found in the chazumba lithodeme u pb detrital zircon dating of seven metasedimentary samples from the migmatized and unmigmatized chazumba lithodeme yielded youngest detrital zircons and clusters of 192 198 214 250 266 and 291 ma and are interpreted to reflect the late triassic middle jurassic deposition of turbiditic rocks the transitional arc tholeiitic geochemistry of the chazumba amphibolites is consistent with turbidite sedimentation in a back arc environment along a rifted passive margin close to a contemporaneous magmatic arc inferred flattening of the subduction zone led to subduction erosion during the early middle jurassic and underthrusting of the chazumba lithodeme to depths equivalent to amphibolite facies metamorphism steepening of the subducting slab and diachronous rifting within the gulf of mexico contributed to extensional tectonics recorded on the mexican mainland and facilitated the tectonic exhumation of the chazumba lithodeme by normal faulting along the reactivated providencia shear zone during the middle late jurassic more generally the documentation of arc back arc assemblages in the ayu complex requires deposition adjacent to a subducting ocean and thus supports a pangea a reconstruction that was synchronous with the breakup of pangea

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

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

holocene climatic evolution at 22 23 n from two palaeolakes in the taoudenni area northern mali abstract in the endorheic hydrologically isolated taoudenni depression nowadays receiving 5 mm mean annual rainfall continuous sections in fossiliferous holocene deposits from two palaeolakes with different geological evolutions have been compared the following climatic sequence is determined 9000 8300 yr b p the beginning of a humid phase 8300 6700 yr b p climatic optimum from 6700 yr b p onwards a change in regime 4500 4000 yr b p end of humid phase aridification begins the data imply a 600 km northern shift of the monsoonal belt which confirms models described in the literature

instability in eight sub basins of the chilliwack river valley british columbia canada a comparison of natural and logging related landslides logging causes increased landslide frequency in british columbia in this study the slope type initiation location aspect rate bedrock geology and size of mass movements located in eight logged tributary valleys of the chilliwack river valley are investigated a landslide inventory was created by digitizing landslides identified on aerial photographs dating from 1941 to 2002 this database was analysed using qualitative observations and simple statistical tests and a comparison between natural and logging related landslides was made slope tests displayed a significant difference between natural and logging related events indicating that on average logging related landslides require lower slope gradients to initiate than natural events although aspect initiation location and landslide type did not show a significant relationship between natural and logging related landslides they do suggest subtle differences landslide rate was calculated and compared for natural and logging related landslides it was significantly higher for logging related events attaining a maximum of 31 times the natural rate in 1978 and an overall average increase of nine times the natural rate logging related landslides were on average smaller than natural landslides but this nevertheless resulted in a 3 1 times increase in area affected by logging related landslides compared to natural landslides geology may influence landslide frequency as well it appears to be higher over the less resistive sedimentary rocks of the cultus formation and chilliwack group and lower in granodioritic areas finally roads affected slope stability more than clearcuts in the early decades of the study but this effect decreased over time suggesting a correlation with improved road construction

timing of terminal pleistocene deglaciation at high elevations in southern and central british columbia constrained by 10be exposure dating abstract the cordilleran ice sheet cis covered most of british columbia and southern yukon territory at the local last glacial maximum llgm during marine oxygen isotope stage 2 however its subsequent demise is not well understood particularly at high elevations east of its ocean terminating margin we present 10be exposure ages from two high elevation sites in southern and central british columbia that help constrain the time of initial deglaciation at these sites we sampled granodiorite erratics at elevations of 2126 2230 m a s l in the marble range and 1608 1785 m a s l in the telkwa range at the western margin of the interior plateau the erratics at both sites are near ice marginal meltwater channels that delineate the local ice surface slope and thus the configuration of the ice sheet during deglaciation the locations of the erratics and their relations to meltwater channels ensure that the resulting 10be ages date cis deglaciation and not the retreat of local montane glaciers our sample sites emerged above the surface of the cis as its divide migrated westward from the interior plateau to the axis of the coast mountains two of the four samples from the summit area of the marble range yielded apparent exposure ages of 14 0 0 7 and 15 2 0 8 ka these ages are 1 8 3 0 ka younger than the well established llgm age of ca 17 ka for the puget lobe of the cis in washington state they are 1 7 ka younger than the llgm age for the puget lobe if a snow shielding correction to their uncertainty weighted mean age is applied the other two samples yielded much older apparent exposure ages 20 6 1 4 and 33 0 1 5 ka indicating the presence of inherited isotopes four samples collected from the summit area of the telkwa range in the hazelton mountains yielded well clustered apparent exposure ages of 10 1 0 6 10 2 0 7 10 4 0 5 and 11 5 1 1 ka significant present day snow cover introduces a large uncertainty in the apparent exposure ages from this site a snow shielding correction based on present day snow cover data increases the uncertainty weighted mean exposure age of the telkwa range erratics to 12 4 0 7 ka consistent with deglacial 14c ages from areas near sea level to the west our exposure ages show a thinning of the southern portion of the cis shortly after the llgm and persistence of a remnant mountain ice cap in the central coast mountains into the younger dryas chronozone our data also show that the summit area of the marble range was ice covered during the llgm the presence of an ice body of considerable dimension in north central british columbia until or possibly even after the younger dryas highlights the need for geomorphological and geochronological studies of the ice dispersal centre over the skeena mountains in northwest british columbia and the need for better understanding of the response of the cis to lateglacial climate fluctuations

mid holocene land surface conditions in northern africa and the arabian peninsula a data set for the analysis of biogeophysical feedbacks in the climate system large changes in the extent of northern subtropical arid regions during the holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales models that omit biogeophysical feedback however are unable to account for the full magnitude of african monsoon amplification and extension during the early to middle holocene 9500 5000 years b p a data set describing land surface conditions 6000 years b p on a 1 1 grid across northern africa and the arabian peninsula has been prepared from published maps and other sources of palaeoenvironmental data with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the palaeoclimate modelling intercomparison project the data set includes information on the percentage of each grid cell occupied by specific vegetation types steppe savanna xerophytic woods scrub tropical deciduous forest and tropical montane evergreen forest open water lakes and wetlands plus information on the flow direction of major drainage channels for use in large scale palaeohydrological modeling the data set is available in digital form by anonymous ftp

quaternary environmental change in equatorial regions with particular reference to vegetation history a bibliography abstract it was thought for a long time that equatorial regions had remained virtually unchanged at least since the tertiary period it is only from the late tertiary that we can talk of the present disposition of land and sea about the equator our teaching has supposed that these regions were the hearth from which general spread to the remainder of the planet s surface by gradual adaptation and speciation this warm humid zone has been seen as a refuge for species unable to maintain temperature ranges in the face of stresses and fluctuations to north and south the diversity of species in rain forest environments was assumed to reflect their essential stability so indeed it may but not in quite the manner supposed originally by ecologists the variety of species present in the various regions of rain forest are greatly different with african exhibiting a relatively poor list compared with south america this as it turns out may well be a function of disturbance during the last 25 years and gathering momentum within the last decade a variety of studies have demonstrated that the concept of equatorial biological stasis is no longer tenable evidence for equatorial environmental changes is derived from theoretical as well as empirical studies regarding the former various attempts have been made to model the earth s atmosphere at different periods of the pleistocene and there is now fairly good agreement about the consequences for equatorial regions of a general global cooling namely a critical drying at lower elevations the empirical evidence is derived on the one hand from stratigraphic studies of peats and lake muds and also the analysis of ocean cores it is also derived from studies of the geographical ranges of plant and animal species

deglaciation of chilliwack river valley british columbia quaternary sediments and landforms in chilliwack river valley southwestern british columbia provide a detailed record of deglaciation of this area between 12 000 and 11 000 years bp stratigraphic sedimentological and radiocarbon data show that a large glacier in eastern fraser lowland part of the cordilleran ice sheet blocked the mouth of chilliwack valley at a time when the middle reaches of the valley were ice free a lake existed between the ice dam in the lower part of the valley and a delta sandur complex west of chilliwack lake in the upper part of the valley two relatively minor advances of the fraser lowland ice lobe into lower chilliwack valley occurred about 11 500 and 11 200 years bp these were separated by a brief period of recession during which tephra was deposited and a coniferous forest and soil developed on freshly deglaciated terrain shortly after 11 200 years bp a glacial lake formed in cultus lake basin two sets of terraces in lower chilliwack valley are graded to diffe

episodicity of mesozoic terrane accretion along the pacific margin of gondwana implications for superplume plate interactions a review of evidence for deformation and terrane accretion on the late triassic early jurassic margins of pangea and the mid cretaceous margins of the palaeo pacific ocean shows that deformation was global and synchronous with probable superplume events late triassic early jurassic deformation appears to be concentrated in the period 202 197 ma and was coeval with eruption of the central atlantic magmatic province onset of pangaea break up a period of extended normal magnetic polarity and a major mass extinction event all possible expressions of a superplume event mid cretaceous deformation occurred in two brief periods the first from approximately 116 to 110 ma in the west palaeo pacific and the second from roughly 105 to 99 ma in the east palaeo pacific with both events possibly represented in northeast siberia this deformation was coeval with eruption of major oceanic plateaus core complex formation and rifting of new zealand from gondwana the cretaceous normal polarity epoch and a major radiation of flowering plants and several animal groups all linked with the mid cretaceous superplume event a simple unifying mechanism is presented suggesting that large continental or oceanic plates when impacted by a superplume tend to break up reorganize associated with gravitational spreading away from a broad thermally generated topographic high and with a resulting short lived pulse of plate marginal deformation and terrane accretion

major bounding surfaces in aeolian sandstones a climatic model large areas of fixed vegetated dunes occur in the sahel and southern margin of the sahara these dunes were active during late pleistocene times and have been stabilized as a result of a change to more humid less windy conditions in the holocene the stabilized surfaces which are in part of erosional origin are of regional extent within aeolian sand sequences such surfaces form the highest order bounding surfaces since regional bounding surfaces of this type occur in all the world s major deserts it is suggested that they should also be present in some ancient aeolian sandstone sequences

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

postglacial sediment budget of chilliwack valley british columbia the paraglacial reworking of glacial sediments by rivers and mass wasting is an important conditioning factor for modern sediment yields in mountainous catchments in formerly glaciated regions catchment scale and patterns of sediment storage are important influences in the rate of postglacial adjustment we develop a quantitative framework to estimate the volume sediment type and fractional size distribution of legacy glacial materials in a large 1230 km2 watershed in the north cascade mountains in south western british columbia canada chilliwack valley is exceptional because of the well dated bounds of deglaciation interpolation of paleo surfaces from partially eroded deposits in the valley allows us to estimate the total evacuated sediment volume we present a chronology of sediment evacuation from the valley and deposition in the outlet fan based on infrared stimulated luminescence irsl and 14 c dating of river terraces and fan strata respectively the effects of paraglacial sedimentation in chilliwack valley were intensified through a major fall in valley base level following ice retreat the steepened mainstem valley gradient led to deep incision of valley fills and fan deposits in the lower valley network the results of this integrated study provide a postglacial chronology and detailed sediment budget accounting for long term sorting of the original sediments lag deposit formation in the mainstem deposition in the outlet fan and approximate downstream losses of suspended sediment and wash load the mass balance indicates that a bulk volume of approximately 3 2 km3 of glacial material has been evacuated from the valley copyright 2012 john wiley sons ltd

vashon drift definition of the formation in the georgia depression southwest british columbia vashon drift was deposited during the fraser glaciation late wisconsinan at the time of maximum expansion of the southwestern part of the cordilleran ice sheet when it filled the georgia depression about 14 500 years ago the drift is present throughout the depression and comprises till and glaciofluvial and glaciolacustrine sediments derived from source areas surrounding the coastal trough it is overlain by capilano sediments and underlain by quadra sand also of fraser age drift deposition was diachronous and complex probably caused by alpine glaciers coalescing in the trough with the ice margin repeatedly grounding and floating in seawater studies of bedrock striae till fabrics and clast provenance reveal that vashon ice movement was generally southward although locally controlled by topography a time space diagram is presented that confirms the long held hypothesis that advance and decay of vashon ice were rapid

late paleozoic or early mesozoic magnetizations in remagnetized paleozoic rocks state of oaxaca mexico characteristics of the oaxaca terrane of southern mexico suggest that the record of a complete wilson cycle is present the local basement is composed of high grade rocks of the oaxaca complex which is considered to have north american affinities based on grenvillian lithologies and ages in contrast early paleozoic sedimentary rocks in depositional contact with the oaxaca complex have very close faunal affinities with the olenid ceratopygid trilobite province of argentina late paleozoic faunas indicate that by this time the oaxaca terrane was once again associated with north america in an attempt to gain further insight into the drift history of this area we have undertaken a paleomagnetic study of the paleozoic sedimentary rocks of oaxaca the results of our study indicate that the entire paleozoic section was remagnetized in the same paleomagnetic field at some time following late paleozoic sedimentation emplacement of an igneous complex and the earlier of two folding events fold and conglomerate tests show that the remagnetization occurred prior to deposition and folding of overlying early cretaceous sediments the oaxaca paleozoic rocks were therefore remagnetized sometime between late permian and early cretaceous since the exact age of remagnetization is not known we compare our result with data from cratonic north america for the bracketed range of magnetization ages this analysis indicates that as much as 28 o of net counterclockwise rotation could have occurred between oaxaca and cratonic north america subsequent to the oaxaca remagnetization

timing and extent of allerod and younger dryas age ca 12 500 10 000 14c yr b p oscillations of the cordilleran ice sheet in the fraser lowland western north america the use of shaded digital topographic models to visualizes suites of topographic features stratigraphy and field mapping reveals newly recognized multiple moraines associated with oscillations of the remnants of the cordilleran ice sheet in the fraser lowland along the western washington british columbia border morphologic features show the extent of ice represented by sumas drift ca 11 600 10 000 14c yr b p following ca 3000 yr of retreat from the glacial maximum fraser glaciation positions 80 km south of seattle and in the strait of juan de fuca the paleogeography of the ice margin and timing of ice retreat during the sumas stade is reconstructed and bracketed by 70 radiocarbon dates 24 on marine shells 46 on wood and peat which are secured by morphologic and stratigraphic evidence four topographically distinct phases of the sumas deglaciation model are suggested phase si 11 600 11 400 14c yr b p outermost drift scattered remnants of ice contact deposits phase sii 11 600 11 400 14c yr b p glacier readvance building prominent moraine followed by glacier retreat phase siii 10 980 10 250 14c yr b p glacier readvance building of multiple moraines followed by glacier retreat phase siv 10 250 10 000 14c yr b p glacier readvance building of inner moraine followed by glacier retreat

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

megalakes in the sahara a review the sahara was wetter and greener during multiple interglacial periods of the quaternary when some have suggested it featured very large mega lakes ranging in surface area from 30 000 to 350 000 km2 in this paper we review the physical and biological evidence for these large lakes especially during the african humid period ahp 11 5 ka megalake systems from around the world provide a checklist of diagnostic features such as multiple well defined shoreline benches wave rounded beach gravels where coarse material is present landscape smoothing by lacustrine sediment large scale deltaic deposits and in places tufas encrusting shorelines our survey reveals no clear evidence of these features in the sahara except in the chad basin hydrologic modeling of the proposed megalakes requires mean annual rainfall 1 2 m yr and a northward displacement of tropical rainfall belts by 1000 km such a profound displacement is not supported by other paleo climate proxies and comprehensive climate models challenging the existence of megalakes in the sahara rather than megalakes isolated wetlands and small lakes are more consistent with the sahelo sudanian paleoenvironment that prevailed in the sahara during the ahp a pale green and discontinuously wet sahara is the likelier context for human migrations out of africa during the late quaternary

first evidence for holocene rainfall in the tanezrouft hyperdesert and its margins abstract fresh water lakelets existed between c 10 2 and 5 5 ka b p along the margins of the tanezrouft desert palaeosols have been observed on the plateau itself above eolian formations nearby tl dated at 18 9 ka they give evidence of a steppic vegetal cover of still unknown extension even in this present day rainless desert

geomorphology of a cordilleran ice sheet drainage network through breached divides in the north cascades mountains of washington and british columbia the skagit river valley in northern washington and southern british columbia contains long interconnected valleys barbed tributaries underfit streams and low elevation mid valley divides quantitative measures of connectivity between valleys draining through breached mountain divides aerial photographs and a field verified process form model were used to assess the impact of continental glaciation on the drainage pattern and geomorphology of the watershed structural control on drainage patterns and intense repeated continental glaciation rendered the skagit an extremely interconnected valley with meltwater routes opening it to the fraser and okanagan watersheds divide elimination by drainage of large proglacial lakes led to several hundred meters of erosion at 10 breached divides other geomorphic effects of recurrent continental glaciation include the bisection of a major mountain valley divide migration of as much as 50 km erosion of canyons 15 30 km downstream of breached divides and deposition of exceptionally large alluvial fans at canyon mouths drainage patterns established in the tertiary were reorganized to accommodate southern drainage of glacial meltwater trapped in mountain valleys at the southern margin of the cordilleran ice sheet the skagit basin is now a regional focus for drainage to puget sound from the northernmost north cascades and parts of southern interior british columbia the key to establishing this system was the elimination of a regional divide in what is now skagit gorge which resulted in the capture of the upper skagit valley by the lower skagit river patterns of drainage modification caused by continental glaciation in the skagit basin are similar to those in other mountain areas the skagit s interconnected valleys are similar to the great through valleys of scotland transmountain valleys of british columbia and ice portals of scandinavia

palaeosols in saharan and sahelian dunes of chad archives of holocene north african climate changes limnic sediments and palaeosols are frequently present in dunes of the sahara and sahel deserts of chad this study focused on genesis and degree of development of the palaeosols and the timing of soil formation the soils are cambic arenosols formed by precipitation induced silicate weathering and leaching the upper parts of the cambic arenosols are indurated due to accumulation of salt and amorphous silica during a drier climate following soil formation the optical age data reveal the occurrence of one or two distinct short lived humid climate intervals shortly after 5 ka and before 2 5 ka and the formation of local desert lakes shortly after 5 ka the study further suggests that rapid climate changes occurred mainly in the mid holocene whereas the early holocene humid period lasted longer

amalgamated accumulations resulting from climatic and eustatic changes akchar erg mauritania the akchar erg of the sahara of western mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late pleistocene and holocene deposits that reflect eustatic and climatic events mapping trenching and dating by 14c methods and artefacts show that the prominent complex linear dunes draas of the akchar erg are actually composite features showing at least three constructional and two destructional phases the constructional phases are represented by three convex up layers i a modern veneer moulded into superimposed crescentic dunes which partially mantle the larger linear bedforms ii a middle partly root turbated sand deposited sometime during the last 4000 years and iii a core of linear dune sand formed during the last glacial period 13 000 20 000 yr bp which today shows relict relief intense root turbation and pedogenesis these constructional phases are separated by super bounding surfaces that coincide with erg destructional phases surface 2 bounds the middle aeolian sand and is marked by a lag surface of small granules surface 1 is a very prominent surface with an abundance of neolithic artefacts and represents stabilization of the linear dunes during the humid interglacial period 4000 11 000 yr bp interdraa deposits originated during the interglacial period and consist of continental lacustrine limestones and sandstones humic sands deposited in marshes and sabkhas on the coast the sabkhas originated during interglacial highstand of sea level when interdraa areas were marine embayments and subsequently dried during regression the draa and interdraa sequences therefore in spite of being adjacent facies actually represent different events and were not formed simultaneously the upwind sand sheet margin of the akchar erg shows exposures of the middle and core aeolian sands which were previously protected from deflation by vegetation being progressively cannibilized in the current phase of erg construction and revealing a crystalline basement rock in this proximal area conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record

record of large late pleistocene outburst floods preserved in saanich inlet sediments vancouver island canada abstract two anomalous gray silty clay beds are present in odp cores collected from saanich inlet vancouver island british columbia canada the beds which date to about 10 500 14c yr bp 11 000 calendar years bp contain tertiary pollen derived from sedimentary rocks found only in the fraser lowland on the mainland of british columbia and washington just east of the strait of georgia abundant illite muscovite in the sediments supports a fraser lowland provenance the clay beds are probably distal deposits of huge floods that swept through the fraser lowland at the end of the pleistocene muddy overflow plumes from these floods crossed the strait of georgia and entered saanich inlet where the sediment settled from suspension and blanketed diatom rich mud on the fiord floor the likely source of the floods is late pleistocene ice dammed lakes in the fraser and thompson valleys which are known to have drained at about the time the floods occurred

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

radiocarbon dating and paleoclimatic significance of subfossil limicolaria in northwestern sudan the discovery identification and radiocarbon dating of the large achatinid land snail limicolaria kambeul chudeaui at six places in northwestern sudan north of the 100 mm isohyet provide convincing evidence that at least 300 mm annual rainfall existed there 6000 yr ago since then the rainfall north of ca 20 n lat in the eastern sahara has certainly been 300 mm and probably 200 mm accelerator mass spectrometry has allowed the organic fraction presumably conchiolin to be accurately radiocarbon dated whereas carbonate fractions are ca 600 yr too old comparative studies show the subfossil form l k chudeaui to be the most primitive in its species complex and to occupy a narrow east west belt across africa north of 15 n and west of 35 e the northern limit of living forms lies within the southern part of the sahelian zone in forest or forest savanna they cannot live in open grassland therefore the distribution of l k chudeaui marks the former northern position of this zone during the middle holocene indicating a latitudinal shift of at least 5 500 km

low flow hydraulic geometry of small steep mountain streams in southwest british columbia abstract this investigation explores the at a station hydraulic geometry ahg of small steep mountain streams at low discharge thirteen reaches in five tributaries of chilliwack river british columbia ranging in size from 12 to 77 km 2 are examined the resulting data set is composed of eight to twelve measurements of water surface width mean depth and mean velocity at each of 61 cross sections or 625 unique combinations of the three variables mean velocity in a given cross section responds most rapidly to changing discharge and 31 of the 61 cross sections have velocity exponents that are greater than the water surface width and mean depth exponents combined the velocity exponent m averages 0 51 while the mean water surface width exponent b and mean depth exponent f average 0 20 and 0 29 respectively somewhat surprisingly the ahg of steep mountain streams can be reasonably predicted from just a few measurements of the primary flow variables and stream discharge while conditions at the cross section appear predictable from a few measurements extrapolating the results from one cross section to another in the same reach involves large errors the section to section variability of the exponents and coefficients even when they are located in similar channel units such as riffles prevents accurate extrapolation to unmeasured cross sections

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

scale variation of post glacial sediment yield in chilliwack valley british columbia the holocene volumetric sediment budget is estimated for coarse textured sediments sand and gravel in a large formerly glaciated valley in southwest british columbia erosion is estimated by compiling volumetric loss estimated in digital elevation models dems of gullied topography and by applying a non linear diffusion model on planar undissected hillslopes estimates of steepland yield are based on estimates of post glacial deposition volumes in fans cones and deltas at the outlets of low order tributary catchments erosion of post glacial fans and tributary valley fills is estimated by reconstructing formerly continuous surfaces results are classed by catchment order and compared across scales of contributing area revealing declining specific sediment yield in m3 km 2 a 1 with catchment area for the smaller tributaries 10 km2 and increasing specific sediment yield for larger tributaries and chilliwack valley itself approximately 60 of mobilized sediment is redeposited in first to third order catchments with lesser proportions stored at the outlets of higher order catchments a simple network routing model emphasizes the significant sediment flux contributions from colluvium drift blankets and gullies in steeper terrain as this material is deposited at junctions within the lower drainage network an increasing proportion of material is derived from remnant valley fills and para glacial fans in the major valleys yield from lower order steepland catchments tends to remain in storage indefinitely sequestered on footslopes these observations have implications for modelling the post glacial sediment balance amongst catchments of varying size after 104 years the system remains in disequilibrium the critical linkage lies between low order hillslope catchments fourth order and the larger valleys with more complex fluvial systems copyright 2010 john wiley sons ltd

holocene lake deposits and palaeoenvironments in central sahara northeastern mali abstract extensive and fossiliferous lacustrine deposits have been found in a now hyperarid area south of the tanezrouft 20 30 21 n 1 w between the erg ine sakane and the djebel takabart the assemblage of fish molluscs ostracods and charophytes among which lychnothamnus is observed in africa for the first time testify to fresh water conditions however stable carbon and oxygen isotopic values provide evidence for phases of strong evaporation the presence of limicolaria sp a terrestrial pulmonate first observed in the sahara indicate permanent soil humidity around the palaeolake s the remains of large steppe type mammals elephant antelopes buffalo as well as neolithic sites and burials in brown soils also contribute to evidence a northward shift of the sahel belt 3 4 in latitude this period is radiocarbon dated from 6130 180 b p to 7520 70 b p on molluscan shells the human material could be younger 3750 100 b p on charcoal

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

advance of alpine glaciers during final retreat of the cordilleran ice sheet in the finlay river area northern british columbia canada abstract sharp crested moraines up to 120 m high and 9 km beyond little ice age glacier limits record a late pleistocene advance of alpine glaciers in the finlay river area in northern british columbia the moraines are regional in extent and record climatic deterioration near the end of the last glaciation several lateral moraines are crosscut by meltwater channels that record downwasting of trunk valley ice of the northern cordilleran ice sheet other lateral moraines merge with ice stagnation deposits in trunk valleys these relationships confirm the interaction of advancing alpine glaciers with the regionally decaying cordilleran ice sheet and verify a late glacial age for the moraines sediment cores were collected from eight lakes dammed by the moraines two tephras occur in basal sediments of five lakes demonstrating that the moraines are the same age plant macrofossils from sediment cores provide a minimum limiting age of 10 550 10 250 cal yr bp 9230 50 14c yr bp for abandonment of the moraines the advance that left the moraines may date to the younger dryas period the finlay moraines demonstrate that the timing and style of regional deglaciation was important in determining the magnitude of late glacial glacier advances

mesozoic tectono magmatic evolution of mexico an overview abstract the geological framework of mexico is formed by a mosaic of tectono stratigraphic terranes that were assembled during the paleozoic and mesozoic as the result of the complex interaction between laurentia gondwana and the paleo pacific plate even though the mexican tectono magmatic evolution is still object of study some main representative stages can be recognized determining the time span and regional distribution of magmatism throughout the geological history of mexico is crucial for the exploration of mineral resources this paper contains a brief description of the terranes and is focused on summarizing the main mesozoic tectono magmatic events from the assembling of pangea in late paleozoic time to the end of the mesozoic these main events are 1 permo triassic continental submarine arc that developed along eastern oaxaquia and was related to a subduction zone developed along the paleo pacific margin 2 magmatic activity ceased in central and southern mexico during middle to late triassic time and a wide submarine fan was formed along the western margin of pangea which apparently acted like a passive margin evidence of a contemporaneous intra oceanic arc is found at baja california 3 inland early jurassic volcanism is found in northeast and central mexico whether this magmatic event evolved in a rift or arc nazas tectonic setting is still undetermined 4 by late jurassic magmatism was widespread it is the time of the breakup of pangea leading to the opening of the gulf of mexico and the arperos basin therefore volcanism was mostly related to subduction and supra subduction rifting 5 the gulf of mexico and arperos basins were formed by early cretaceous time igneous rocks of that age have a wide range of composition arc continental rift strike slip and oceanic rift within plate and are geographically widespread 6 at the end of early cretaceous volcanism moved to a more localized belt along the paleo pacific margin it is also the time of formation of major carbonate platforms along east mexico 7 coeval to sevier laramide orogeny of north america a large fold and thrust belt developed throughout mexico during late cretaceous time it was accompanied by arc magmatism along the pacific margin of mexico tarahumara arc it is the first continental arc that evolved after accretion and consolidation of guerrero and alisitos terranes to continental mexico these mesozoic tectonic processes originated westward eastward migrations of arc related and rift related volcanism which in turn lead to the present distribution of mesozoic mineral deposits

subsurface combustion in mali refutation of the active volcanism hypothesis in west africa surface heat anomalies have been known in the timbuktu region in northern mali for more than a century since about 1960 several authors have argued that these heat anomalies are caused by incipient volcanic and hydrothermal activity surface temperatures as high as 765 c were measured locally in january 2002 and smoke emanated from holes and fractures in the ground we demonstrate that subsurface combustion of organic material is the source of the heat and the gases several square kilometers are currently active or have been affected by subsurface fires since 2001 self ignition during biological degradation of organic rich layers in the lacustrine deposits is the most likely mechanism that started the subsurface combustion that caused the heat anomalies in the area an important consequence of this conclusion is that west africa should still be regarded as volcanologically inactive and that possible reactivations of the major ew trending guinean nubian lineament are not associated with volcanism we suggest that the subsurface combustion in the timbuktu region today represents a phenomenon with a very long record in the trans saharan region

paleomagnetism of the acatlan terrane southern mexico evidence for terrane rotation 232 drill samples were collected from 33 sites from the acatlan terrane southern mexico the medium grade metamorphosed basement rocks schists granitoid greenstones showed magnetic directions that are randomly distributed and no meaningful results could be obtained however late paleozoic red beds the ordovician totoltepec granite and the early paleozoic tecomate limestone metamorphosed in acadian times give coherent paleomagnetic directions the samples responded well to thermal demagnetization but not to alternating field demagnetization the ages of the magnetizations are reasonably bracketed between carboniferous and jurassic compared with data from the north american craton and from the oaxaca terrane conclusive evidence for major north south displacements of the acatlan terrane is not present but significant clockwise rotations of the terrane with respect to the craton and the adjacent oaxaca terrane are quite evident these rotations occurred during the jurassic or early cretaceous

a glacial readvance during retreat of the cordilleran ice sheet british columbia central coast descriptions of the cordilleran ice sheet retreat after the last glacial maximum have included short lived readvances occurring during the older and younger dryas stadial periods and into the holocene but identification of these events has been largely limited to southwest and central british columbia and northwest washington state we present evidence of a late pleistocene readvance of cordilleran ice occurring on the central coast of british columbia on calvert island between northern vancouver island and haida gwaii evidence is provided by sedimentological and paleoecological information contained in a sedimentary sequence combined with geomorphic mapping of glacial features in the region results indicate that a cold climate existed between 15 1 and 14 3 cal ka bp and that ice advanced to and then retreated from the western edge of the island between 14 2 and 13 8 cal ka bp these data provide the first evidence of a major fluctuation in the retreating ice sheet margin in this region and suggest that a cold climate was a major factor in ice readvance these data contribute to the understanding of past temperature ice loading and crustal response the nature of ice margin retreat and the paleoenvironment of an understudied area of the pacific northwest

late pleistocene stratigraphy and chronology of lower chehalis river valley southwestern british columbia evidence for a restricted coquitlam stade sediments in lower chehalis valley span middle wisconsin olympia nonglacial interval to holocene time sediments are divided into six units with chronological control provided by 14 new radiocarbon ages fluvial gravel spans the transition from the late olympia nonglacial interval to the early fraser glaciation glaciolacustrine sedimentation represents the first definitive glacial activity in the valley and indicates that vashon ice in the fraser lowland blocked the mouth of the chehalis valley at ca 18 17 ka bp ice then flowed down the chehalis valley the chehalis valley deglaciated while ice persisted in the fraser lowland forming another lake after this lake drained terraces and fans formed this style of glaciation deglaciation is typical of many watersheds peripheral to the fraser lowland in that local valley ice was slightly out of phase with ice in the lowland this resulted in glacial lakes forming during both advance and retreat phases however in contrast to watersheds in the northwestern fraser lowland no definitive evidence of a coquitlam ice advance was found within the chehalis valley although glaciers in the area were likely active and advancing data from the chehalis valley indicates that they were not as extensive as previously thought since ice source areas in the northeastern fraser lowland are in the leeward area of the coast mountains it is suggested that lower precipitation resulted in limited glacier activity there during the coquitlam stade

radiocarbon chronology of holocene hydrologic changes in northeastern mali abstract a detailed chronology of hydrological changes that occurred in northeastern mali during the holocene is based on approximately 30 14 c dates of molluscs which were collected from the lacustrine and paludal deposits of the ine sakane sand sea the presence of cemented paleodunes indicates that the end of the pleistocene was and arid period this was followed by an early holocene lacustrine episode ca 9500 6500 yr b p during which numerous lakes occupied depressions formed in the cretaceous limestones and between the sand ridges these lakes reflect a significant rise in the water table between ca 6500 and 5500 yr b p the lakes dried up although the water table remained close to the ground surface in several areas calcareous concretions formed at the water table fringe during this time thereby fossilizing some of the middle holocene dunes a second lacustrine episode is dated between ca 5500 and 4500 yr b p it corresponds to the establishment of numerous neolithic settlements in the area arid conditions have developed since about 4500 yr b p to attain the conditions of the present day the isotopic composition 18 o and 13 c of the fauna reflects highly variable hydrological conditions groundwaters were recharged by precipitation which was occasionally very depleted in heavy oxygen 18 o 13 evaporation induced an enrichment in 18 o high concentrations characterize some lakes the 13 c content of fossil shells reflect 1 species ecological requirements 2 isotopic composition of the total inorganic dissolved carbon tidc in groundwaters and 3 the lake metabolism

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

readvance of glaciers in the british columbia coast mountains at the end of the last glaciation abstract late quaternary sediments and landforms at the head of howe sound a fjord in the british columbia coast mountains provide a detailed record of final decay of the southwestern sector of the cordilleran ice sheet at the end of the pleistocene the upper reaches of mamquam river valley a mountain valley at the head of the fjord east of the town of squamish were ice free and locally treed as early as 14 000 years ago 11 900 14 c yr bp folding and faulting of late glacial lacustrine sediments in mamquam valley may record a glacier readvance correlative with the sumas event in the fraser lowland to the southeast by 12 900 years ago 10 690 14 c yr bp the trunk glacier in howe sound had retreated from a sumas age moraine at porteau cove 15 km south of squamish by 12 800 years ago 10 650 14 c yr bp the glacier terminus stood near the present fjord head with the sea at an elevation of about 40 50 m a s l shortly after 12 800 years ago during the younger dryas chronozone the trunk glacier in squamish valley readvanced a short distance and then retreated for a final time tributary valleys were probably completely deglaciated by 11 300 years ago 9920 14 c yr bp and certainly by 10 600 years ago 9360 14 c yr bp relative sea level fell as deglaciation progressed however it did not fall below its present datum in the squamish area until after 11 200 years ago at least 1300 years later than in the fraser lowland

the calcareous tufa in the tadrart acacus mt sw fezzan libya an early holocene palaeoclimate archive in the central sahara article i nfo the palaeoclimatic signi cance of calcareous tufa deposits found in the presently hyperarid tadrart acacus massif central sahara sw libya is discussed on the basis of u th datings stable isotope analyses and the regional hydrogeological context these deposits formed mostly between c 9600 and 8100 yr bp the isotopic signature 18 o and 13 c of this tufa indicates that the water forming these carbonates originated as isotopically depleted rainwater as a result of the northward expansion of the sw african monsoon system during the early holocene carbonate sedimentation in the tadrart acacus decreased shortly before c 8000 yr bp attributed to a drop in precipitation in the course of the well known 8 2 kyr event a correlation between tufa development and climate change in n and e africa and in the mediterranean basin during the early holocene is discussed the effect of early holocene climate change on both the landscape and the dwellers living in the tadrart acacus region is also evaluated

caltepec fault zone an early permian dextral transpressional boundary between the proterozoic oaxacan and paleozoic acatlan complexes southern mexico and regional tectonic implications 1 the tectonic boundary between the grenville age oaxacan and paleozoic acatlan crystalline complexes in southern mexico named the caltepec fault zone cfz is characterized for the first time as a dextral transpressional nnw trending and ene dipping ductile fault zone of early permian age the complexes are welded by a syntectonic magmatic epidote bearing granite along the entire length of the cfz from east to west the 2 6 km wide cfz consists of disrupted and retrograded banded gneisses of the oaxacan complex quartz feldspar mylonite and the syntectonic magmatic epidote bearing cozahuico granite czg with huge xenoliths up to several kilometers long and up to 600 m wide of the proterozoic gneisses thrust westward over metasedimentary tectonites of the acatlan complex the czg shows magmatic fabrics that represent a transition to solid state deformation characterized by subvertical foliation subhorizontal nne and sse dipping mineral stretching lineation and dextral kinematics the megaxenoliths underwent partial melting developing banded migmatites with layers of epidote bearing granitic neosome the parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation migmatization of xenolithic gneisses and granite emplacement along the cfz were coeval the neosome yielded a u pb zircon concordant age of 275 6 1 ma probably dating the peak of the tectonothermal event we interpret the cfz as a major terrane boundary accommodating transpressional interaction between the acatlan and oaxaquia blocks which were amalgamated in an oblique convergent setting by early permian time as the leading edge of gondwana impinged onto the southern margin of laurentia along the marathon ouachita suture to form pangea

holocene palaeoecology of the eastern sahara selima oasis buried lake sediments and tufa strandlines of holocene age provide evidence of a former lake in the selima oasis depression northwest sudan near the hyperarid core of the eastern sahara desert a 1 6 m section through laminated lake muds and precipitates has been analysed in detail and 30 radiocarbon dates geochemical sedimentological diatom and pollen analyses indicate that the unit dated from approximately 8 4 ka bp to 6 ka bp and was deposited in an environment that passed through one arid to humid to arid cycle the arid episodes from prior to 8 4 ka bp and subsequent to 7 ka bp are characterized by high caco3 deposition abundant diatom indicators of relatively high salinity and well developed periphytic communities and pollen spectra dominated by desert grassland indicators a humid period centered on 8 ka bp is indicated by low caco3 deposition high organic rich mud and al and si oxide deposition diatoms indicating a fresh deep 20 m lake and a pollen assemblage with maximum values for sahelo sudanian tree taxa suggesting a thorn tree and shrub savanna vegetational mosaic this reconstruction concordant with higher groundwater table and precipitation between 9 3 and 7 ka bp agrees with palaeoecological inferences from other sites in north africa

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

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

cordilleran ice sheet mass loss preceded climate reversals near the pleistocene termination the cordilleran ice sheet cis once covered an area comparable to that of greenland previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost canada as late as 12 5 thousand years ago ka new data indicate that substantial areas throughout westernmost canada were ice free prior to 12 5 ka and some as early as 14 0 ka with implications for climate dynamics and the timing of meltwater discharge to the pacific and arctic oceans early bolling allerod warmth halved the mass of the cis in as little as 500 years causing 2 5 to 3 0 meters of sea level rise dozens of cirque and valley glaciers along with the southern margin of the cis advanced into recently deglaciated regions during the bolling allerod and younger dryas

isotopic studies of the acatlan complex southern mexico implications for paleozoic north american tectonics the paleozoic acatlan complex and grenville age oaxaca terrane of southern mexico have been suggested to be the southern continuation of north american orogenic systems the oaxaca terrane yields interpreted ages of 1 0 ga and has many of the characteristics typical of grenville belt rocks the paleozoic acatlan complex consists of multiply deformed metasedimentary rocks schists granitoids and eclogites that have been compared to rocks of the appalachian belt the northward extension of both the oaxaca terrane and acatlan complex how ever are obscured by younger rock cover as they enter central mexico furthermore the configuration of these orogenic systems in southern mexico is the reverse of that of the rest of north america with the grenville oaxaca terrane to the east of the paleozoic acatlan complex isotopic studies show that the acatlan complex records three tectonothermal events the sm nd whole rock mineral isochrons from schists as well as eclogites yield metamorphic ages of 410 380 ma this age of metamorphism is supported by u pb zircon data from a granitoid which yields an age of 370 34 ma a later intrusion of a large stock in the late pennsylvanian 287 2 ma was probably closely followed by a less significant deformational event small granitic intrusions and migmatites were later emplaced at 205 170 ma rb sr and sm nd mineral whole rock the metasedimentary rocks and paleozoic granitoids of the acatlan complex have present day e nd of 9 to 11 with crustal residence ages t dm of 1 3 1 6 ga these rocks must have been derived at least in part from a proterozoic source area and it is significant that crustal residence ages are identical to those of the oaxaca terrane the oaxaca terrane along with some south american precambrian complexes of which the oaxaca terrane was probably once a part may be considered the most likely source areas for the acatlan complex the less extensive eclogite trondjhemite and amphibolite bodies in the acatlan complex yield model ages that show them to be juvenile additions to the crust neodymium model ages of the acatlan complex are unlike those of some accreted crustal blocks of the pacific margin such as the alexander stikine and wrangellia terranes which have little signs of crustal recycling similarities between the acatlan complex and the acadian belt as well as current paleozoic paleogeographic and paleomagnetic reconstructions suggest that the devonian metamorphic event that affected these two areas was the result of a laurentia gondwana collision a later collision in late carboniferous time caused deformation in the acatlan complex plutonic activity in southern mexico and deformation in the ouachita marathon and appalachian belts both the acatlan and oaxaca terranes would have been continuous with south america until the break up of pangea in the mesozoic era

lateral spreading of the middle to lower crust inferred from paleocene migmatites in the xolapa complex puerto escondido mexico gravitational collapse of a laramide orogen abstract field and laboratory studies including textural petrological and geochronological analysis of migmatites of the xolapa complex affected by the same deformational event suggests that the deformation occurred during crystallization of neosome the deformation represents sub horizontal spreading of middle to lower crust according to structural data and occurred during the paleocene as inferred from u pb laser ablation dating of zircon according to a compilation of stratigraphic magmatic and deformational data reported in southern mexico the sub horizontal spreading occurred after the culmination of the upper cretaceous laramide orogeny compiled regional data indicate a coast to coast mass transfer in the upper crust from zihuatanejo to veracruz from the cretaceous tertiary boundary until the mid eocene b contemporaneous adakite like magmatism and c extension in the mixteco and oaxaca terranes of southern mexico coeval to compression in the surrounding crust because all these observations are spatially associated to areas of the mixteco and oaxaca terranes we propose that these areas experienced a post orogenic gravitational collapse since the cretaceous tertiary boundary until the mid eocene this proposal implies that the gravitational collapse was recorded in the xolapa complex by migmatite formation in response to ascent of the brittle ductile transition decompression of the orogenic root and or shear driven asthenospheric upwelling gravitational collapse deformation was accommodated by the subsequent flow of the migmatites towards the foreland preceding the eastward escape of the chortis block