distinguishing between archean and paleoproterozoic tectonism and evolution of the isortoq fault zone eqe bay area north central baffin island canada structural metamorphic and u pb geochronological data bear on the distinction between archean and paleoproterozoic tectonism along the southeastern margin of the rae province on baffin island archean rocks include ca 3 0 2 8 ga gneiss two greenstone belts of the mary river group and various younger granitoid intrusions in the greenstone belts intermediate felsic volcanism 2 74 2 725 ga was accompanied and outlasted by calc alkaline plutonism 2 73 2 715 ga deformation low to medium pressure metamorphism and peraluminous plutonism followed at ca 2 7 ga archean rocks and the locally overlying piling group ca 2 2 1 9 ga were deformed and metamorphosed together during development of the paleoproterozoic foxe fold belt tectonism is linked to the isortoq fault zone a major southeast dipping structure marking an abrupt northwestward transition to granulite facies within a 5 km wide zone tight folds of the archean mary river group give way down section to moderately southeast dipping high

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

miocene extension and extensional folding in an anticlinal segment of the black mountains accommodation zone colorado river extensional corridor southwestern united states 1 recent studies demonstrate that rifts are characterized by linked tilt domains each containing a consistent polarity of normal faults and stratal tilt directions and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults the mid miocene black mountains accommodation zone of southern nevada and western arizona is a well exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains in the southeastern part of this zone near kingman arizona east dipping normal faults of the whipple tilt domain and west dipping normal faults of the lake mead domain coalesce across a relatively narrow region characterized by a series of linked extensional folds the geometry of these folds in this strike parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions whereas large fault overlaps produce elongate folds parallel to faults analysis of faults shows that the folds are purely extensional and result from east northeast stretching and fault related tilting the structural geometry of this portion of the accommodation zone mirrors that of the black mountains accommodation zone more regionally with both transverse and strike parallel antithetic segments normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of kingman arizona however isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the lake mead domain began to form between 1 m y to 0 2 m y prior to east dipping faults of the whipple domain the accommodation zone formed above an active and evolving magmatic center that prior to rifting produced intermediate composition volcanic rocks and that during rifting produced voluminous rhyolite and basalt magmas

connection between igneous activity and extension in the central mojave metamorphic core complex california the development of metamorphic core complexes and associated low angle detachment faults commonly is intimately associated with synextensional igneous activity in most areas studied to date the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events we present data from the early miocene central mojave metamorphic core complex cmmcc which indicate that extension was accompanied by igneous activity as reflected by prekinematic synkinematic and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins the principal intrusion is an early miocene granite pluton exposed in outcrops across an area greater than 400 km2 dikes adjacent to the pluton are common in the mitchel range at the buttes and at fremont peak the overall orientation of the pluton and associated dikes is west northwest roughly perpendicular to the extension direction results of u pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 ma two other dike samples yield inconclusive results synextensional basins formed by detachment faulting during the core complex development rocks in these basins compose the jackhammer and pickhandle formations and filled an elongate nw trending trough more than 50 km long the 4oar 39ar ages for tuff beds are as old as 23 8 0 3 ma near the base of the lower pickhandle formation and as young as 21 3 0 5 ma in the uppermost lower pickhandle hence volcanism and plutonism are coeval the diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the cmmcc precludes any simple cause and effect relationship between magmatism and extensional deformation rather magmatism and extension may have been localized at a releasing bend in a transfer fault system which links extension in the cmmcc with extension in the colorado river area to the east

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

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

paleomagnetic quantification of upper plate deformation during miocene detachment faulting in the mohave mountains arizona 1 paleomagnetic data from miocene 20 ma volcanic rocks and dikes of west central arizona reveal the tilt history of proterozoic crystalline rocks in the hanging wall of the chemehuevi whipple mountains detachment fault we obtained magnetization data from dikes and flows in two structural blocks encompassing crossman peak and standard wash in the mohave mountains in the crossman block the dike swarm records two components of primary magnetization 1 cnh a normal polarity high unblocking temperature or high coercivity component inclination i 48 5 declination d 6 4 and 2 crhm a reversed polarity high temperature or high coercivity component i 33 6 d 197 5 argon age spectra imply that the dikes have not been reheated above 300 c since their emplacement and a baked contact test suggests that the magnetization is likely to be miocene in age crhm deviates from the expected direction of the miocene axial dipole field and is best explained as a result of progressive tilting about the strike of the overlying andesite flows these data suggest that the crossman block was tilted 60 to the southwest prior to intrusion of the vertical dike swarm and the block continued to tilt during a magnetic field reversal to normal polarity cnh miocene dikes in the crossman block are roughly coplanar so the younger dikes with normal polarity magnetization intruded along planes of weakness parallel to the earlier reversed polarity swarm an alternative explanation involves cnh magnetization being acquired later during hydrothermal alteration associated with the final stages of dike emplacement in the standard wash block the primary component of magnetization is a dual polarity high temperature or high coercivity component swhl i 7 2 d 0 7 to produce agreement between the expected miocene magnetic direction and the swh component requires 1 correcting for a 56 tilt about the strike of flow bedding and 2 removing a counterclockwise vertical axis rotation of 20 the two rotations restore the standard wash dikes to vertical make parallel the dike layering in the crossman and standard wash blocks and align the strikes of bedding in both blocks geologic mapping geochemical evidence and paleomagnetic data are consistent with the upper plate of the mohave mountains having tilted in response to formation of the underlying detachment fault

tertiary basin development and tectonic implications whipple detachment system colorado river extensional corridor california and arizona a stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly miocene deposits of the whipple detachment system colorado river extensional corridor in the mohave mountains and aubrey hills of arizona and the eastern whipple mountains of california near parker dam these deposits comprise four unconformity bounded sequences composed of locally derived epiclastic and volcanic rocks and the peach springs tuff the three older sequences represent syntectonic units that were deposited coeval with detachment faulting and the fourth is interpreted to be postextensional the sequences are correlated between four fault bounded regions which are the remnants of four different depositional basins similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units the basins developed in about the same positions relative to each other and to volcanic sources as they occupy at present this is shown by gradational changes of pre tertiary rock types between regions systematic variations in the abundance of magmatic units and correlative volcanic units that occur in two adjacent regions the basins formed in the early miocene from segmentation of the upper crust into blocks bounded by high angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low angle detachment fault extreme rotation of one large crustal block which constitutes the central mohave mountains is recorded by a major unconformity in the lower miocene section of one basin because coeval sections that formed in adjoining basins do not record this rotation the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently these proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the whipple detachment system

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

stratigraphy structure and potassic alteration of miocene volcanic rocks in the sleeping beauty area central mojave desert california the sleeping beauty area of the southeastern cady mountains central mojave desert contains a sequence of volcanic rocks 3 km thick which was erupted approximately 20 m y ago plate reconstructions indicate that the change from subduction to transform fault tectonics off the coast of california occurred about 20 m y ago as well and so these rocks contain a record of volcanism extensional faulting and potassic metasomatism during this time of tectonic transition the volcanic sequence comprises basalt to rhyolite flows and tuffs which are overlain by the widespread peach springs tuff and by terrestrial sediments bedding below the peach springs tuff generally dips 10 50 to the southwest and is cut by numerous steep northwest to north trending faults tilting predated deposition of the peach springs tuff bedding fault relationships indicate that deformation was not by tilted book geometry in its simplest form no significant low angle faults are exposed but such faults occur to the west and may underlie the area a major open southeast trending anticline in the area may be a drag fold related to the post late miocene right lateral north trending ludlow fault all units below the peach springs tuff were locally affected by severe potassic metasomatism which raised measured k2o contents to as high as 13 3 wt metasomatized rocks occur in irregular zones that follow northwest trending faults and are best developed around northwest trending breccia zones which have jasper matrices ba and mn prospects are invariably found in metasomatized rocks geologic and geochemical constraints indicate that metsomatism occurred at shallow depth 1 or 2 km and low temperature 150 c metasomatism occurred in at least two distinct pulses and apparently predated deposition of the peach springs tuff the k may have been derived from percolating closed basin brines or through hydrogen metasomatism of rocks deeper in the complex

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

correlation of the peach springs tuff a large volume miocene ignimbrite sheet in california and arizona the peach springs tuff is a distinctive early miocene ignimbrite deposit that was first recognized in western arizona recent field studies and phenocryst analyses indicate that adjacent outcrops of similar tuff in the central and easten mojave desert may be correlative this proposed correlation implies that outcrops of the tuff are scattered over an area of at least 35 000 km2 from the western colorado plateau to barstow california and that the erupted volume allowing for posteruption crustal extension was at least several hundred cubic kilometres thus the peach springs tuff may be a regional stratigraphic marker useful for determining regional paleogeography and the time and extent of tertiary crustal extension

multi mineral geochronology insights into crustal behaviour during exhumation of an orogenic root under rare conditions reworked cratons and their margins preserve the orogenic roots of ancient mountain building events however based on the preservation of high temperature 800 c middle and lower crustal metamorphic assemblages present day exposure of these terrains is not simply a result of protracted denudation but also must reflect a multifaceted exhumational history in situ analysis within thin section preserves the textural setting of target minerals that can be used as thermochronometers such as u pb of zircon monazite titanite and apatite and sm nd of apatite in situ analyses of these chronometers has the potential to provide critical timing constraints on exhumation processes related to decompression melting and cooling across large metamorphic terrains the repulse bay block of the rae craton preserves a large composite amphibolite granulite area 50 000 km2 of archean orthogneiss migmatite and slivers of proterozoic metasediments that underwent high grade metamorphism partial melting ductile flow and finally exhumation during the paleoproterozoic trans hudson orogeny the granulite domain preserves dry granitoid assemblages whereas the amphibolite domain is dominated by hydrated migmatites and orthogneiss metasediments occur in both domains and preserve mineral assemblages that are consistent with having undergone tectonometamorphic conditions of 9 kbar 800 c during burial u pb thermochronometers document identical cooling histories of the granulite and amphibolite domains through the u pb closure temperatures of titanite 650 c and apatite 450 c this suggests that melt loss from the underlying granulite domain and melt gain to the amphibolite domain prior to cooling through 650 c are a controlling factor of the metamorphic assemblages across the composite granulite amphibolite terrains such as the repulse bay block rather than significant differences in burial history cooling history and or reorganization of the crust

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

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

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

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

mid crustal cretaceous roots of cordilleran metamorphic core complexes thermobarometry for cretaceous to mid tertiary plutonism and deformation in the lower plate of whipple and santa catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust moreover they are characterized by a striking acceleration of tectonic decompression coincident with middle tertiary low angle detachment faulting leading to erosional and tectonic unroofing by mid miocene time depth estimates for emplacement of five intrusive suites within the whipple complex include 1 33 4 km for the peraluminous 89 ma whipple wash plutonic suite 2 29 1 km for the 73 ma axtel quartz diorite 3 16 5 km for mylonitization and synkinematic plutonism at 26 ma 4 6 2 1 9 km for the postkinematic 19 ma war eagle gabbro quartz diorite complex and 5 5 2 2 3 km for a 17 ma postkinematic granodiorite decompression initially occurred at a low rate of 0 3 mm yr from 89 to 26 ma and increased to approximately 2 mm yr during the late oligocene to middle miocene estimated depths for four pluton emplacement or deformational events in the santa catalina mountains include 1 21 1 km for the magmatic epidote bearing 68 ma leatherwood quartz diorite 2 15 3 km for the garnet two mica 47 ma wilderness granite 3 9 3 1 9 km for post wilderness mylonitzation and 4 6 3 2 6 km for the 27 ma catalina monzogranite post laramide decompression estimated at 0 3 mm yr accelerated to 1 3 mm yr prior to the cessation of detachment faulting whereas most batholithic terranes of the north american cordillera are representative of an upper crustal setting core complexes provide as a consequence of their tectonic evolution a petrological and structural view into middle crustal processes

subglacially precipitated carbonates record geochemical interactions and pollen preservation at the base of the laurentide ice sheet on central baffin island eastern canadian arctic the mineralogy and isotopic compositions of subglacially precipitated carbonate crusts spccs provide information on conditions and processes beneath former glaciers and ice sheets here we describe spccs formed on gneissic bedrock at the bed of the laurentide ice sheet lis during the last glacial maximum on central baffin island geochemical data indicate that the ca in the crusts was likely derived from the subglacial chemical weathering ca bearing minerals in the local bedrock c and sr isotopic analyses reveal that the c in the calcite was derived predominantly from older plant debris the 18o values of the spccs suggest that these crusts formed in isotopic equilibrium with basal ice lis preserved in the barnes ice cap bic columnar crystal fabric and the predominance of sparite over micrite in the spccs are indicative of carbonate precipitation under open system conditions however the mean 18o value of the calcite crusts is 10 higher than those of primary lis ice preserved in the bic demonstrating that spccs record the isotopic composition of only basal ice palynomorph assemblages preserved within the calcite and basal bic ice include species last endemic to the arctic in the early tertiary the source of these palynomorphs remains enigmatic

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

origin of bimodal volcanism southern basin and range province west central arizona miocene volcanic rocks in the castaneda hills area west central arizona are interbedded with continental clastic sedimentary rocks minor limestone gravity glide blocks of precambrian and paleozoic rocks and monolithologic megabreccia the sedimentary and volcanic units dip to the southwest and are offset by northwest trending listric and high angle normal faults the listric faults coalesce at the rawhide detachment fault which overlies mylonitic gneiss the volcanic suite is strongly bimodal rocks with 55 to 71 wt sio 2 are rare on the basis of age geomorphic position and petrography five volcanic units can be distinguished older basalts 18 7 and 16 5 m y old quartz bearing basalts 13 7 and 12 4 m y old rhyolite lavas and tuffs 15 1 to 10 3 m y old mesa forming basalts 13 1 to 9 2 m y old and megacryst bearing basalts 8 6 to 6 8 m y old most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali olivine basalts many rhyolites contain more than 75 wt sio 2 the initial whole rock sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle the chemical composition of some of the megacrysts in megacryst bearing basalts with 87 sr 86 sr i equal to 7035 and 7038 supports a high pressure mantle origin the low 7034 sr ratio and lack of evidence for mixing with young rocks indicate that the quartz bearing basalts were also derived from the mantle other basalts with 87 sr 86 sr i 0 705 probably were derived from old lithospheric mantle with a high rb sr ratio and do not appear to be contaminated with old upper crustal material the rhyolites have initial sr isotopic ratios of 0 7093 and 0 7141 these ratios indicate that the rhyolites were not differentiated from the basalts partial melting of 1 3 b y old lower crustal material with rb sr 0 10 to 0 19 satisfactorily explains the isotopic ratios of the rhyolites granulite which may constitute the lower crust in this part of arizona has rb sr ratios similar to those required to produce the rhyolites k substituted for na during cooling and devitrification in some of the rhyolites partial melting of upper mantle peridotite and old lower crustal granulite from 19 to 7 m y ago in the castaneda hills area produced the bimodal volcanic suite the nearly contemporaneous production of basaltic and rhyolitic magma from the earth9s crust and mantle requires extremely heterogenous source regions asthenospheric upwelling associated with basin range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma the basaltic and rhyolitic magmas formed in separate source regions rose independently and erupted at the same time and place

use of landsat thematic mapper images in regional correlation of syntectonic strata colorado river extensional corridor california and arizona this study utilizes enhanced landsat thematic mapper tm images in concert with field work to solve problems in regional correlation of miocene rocks in the colorado river extensional corridor of california and arizona these syntectonic volcanic and sedimentary deposits preserve a dateable record of crustal extension although regional correlation and analysis of the deposits are difficult because of the small sizes of the sedimentary basins in which they accumulated and disruption by postdepositional faulting based on field investigations four sequences of sedimentary and volcanic strata distinguished by stratigraphic and structural relations can be recognized in the mohave mountains arizona and the eastern whipple mountains california although the sequences are found in each of four structural regions the relative volumes and organization of lithologies within the sequences differ considerably between regions these sequences have distinctive appearances on the tm image the volcanic component is particularly important in distinguishing between regions and sequences because differences in color and textural patterns in the image are usually related to variations in the proportion and composition of the volcanic material and in the geometry of the volcanic bodies the recognition criteria derived from field mapping and image interpretation in the mohave and whipple mountains weremore applied to an adjacent area in which stratigraphic affinities were less well known stratigraphic and structural relations suggested by the tm image were confirmed by subsequent field work less

sedimentation and extensional basin evolution in a miocene metamorphic core complex setting alvord mountain central mojave desert california usa the lower miocene clews formation at alvord mountain mojave desert california comprises an upward coarsening sequence of synextensional continental deposits of variable thickness this sequence pinches out to the west against the alvord mountain pre tertiary basement complex and thickens eastward to c 300 m over a distance of 7 km two stages of sedimentation are recognized in the formation during the initial stage of sedimentation the depocentre was delineated by a lacustrine system that was bounded to the west by small sheetflood dominated alluvial fans and to the north by a southerly flowing fluvial braidplain lacustrine mudstone siltstone and carbonate suggest an initially closed basin conglomerate to the west represents small sheetflood dominated fans igneous clasts and a south east palaeoflow direction suggest a proximal low relief source in the western alvord mountain area pebbly sandstone of the southward prograding braidplain has a metasedimentary provenance in the paradise range to the north the second stage of deposition was dominated by coarse conglomerate and breccia west to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the cronese hills 8 km to the east these strata reflect rapid westward progradation of sheetflood and debris flow dominated alluvial fans that advanced across the axial braidplain and lacustrine system basin development is interpreted to have been controlled by regional ne directed extension on a detachment fault associated with the central mojave metamorphic core complex 30 km to the west the alvord mountain cronese hills region was initially transported as a single hangingwall block on the e dipping detachment initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall subsequent westward progradation of alluvial fans out of the cronese hills is believed to record the propagation of a nw striking sw dipping normal fault antithetic to the low angle e dipping detachment fault

lithology and evolution of the crust mantle boundary region in the southwestern basin and range province mantle and crustal xenoliths from volcanic rocks in the southwestern basin and range province and colorado plateau transition zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region seismic transects in this area show a strongly reflective moho of generally low relief which in the area of modern transects consists of a thin zone 2 km thick of short reflectors the upper mantle is transparent and has a pn of 7 8 8 0 km s similar to much of the western united states a lower crustal zone 2 13 km thick has variable internal reflectivity and a relatively low velocity of 6 6 6 8 km s upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths whereas melt infiltration preceded and followed ductile deformation brittle deformation represented by closely spaced joint systems and faults followed ductile deformation and is related to the youngest magmatic episodes these structural characteristics and high uppermost mantle temperature 1000 c may combine to explain the relatively low pn alternating layers of ductily deformed and undeformed peridotites with or without igneous intrusions may contribute to the reflectivity of the moho lower crustal xenoliths are dominantly igneous textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths the crustal xenoliths also commonly are jointed and in addition many show partial melting and have abundant cavities that probably were filled with co2 rich fluids these rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records the mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems partial melts and high temperatures garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late miocene extension felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust the preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes the earliest magmatic events preceded extension and later events accompanied and followed extension

miocene synextension sedimentation patterns whipple mountains southeastern california implications for the geometry of the whipple detachment system a sedimentary basin formed in the southern and eastern whipple mountains during early to middle miocene detachment faulting stratigraphic relations and facies patterns indicate that the shape and size of the sedimentary basin was controlled both by fault activity and by sedimentary processes high angle normal faults formed the southwestern and perhaps also the northeastern boundary of the half graben whereas a transfer fault probably formed the southeastern boundary the transfer fault influenced the earliest synextension sedimentation in the basin the location of this fault and similar faults in the mohave mountains and aubrey hills of arizona is inferred to have been controlled by the position of primary irregularities or corrugations of the detachment fault shifts in facies patterns particularly a shift in the position of the basin depocenter reflect changes in basin geometry comparison with adjacent areas is necessary to distinguish between tectonic and nontectonic causes a shift in depocentral position parallel to regional transport direction in the whipple mountains basin is interpreted as resulting from a change in the location of the high angle normal fault forming the southwestern margin of the basin whereas a shift perpendicular to transport direction probably resulted from overtopping of topographic barriers within a fault block the most active phase of faulting occurred prior to deposition of stratigraphic units younger than the 18 5 ma peach springs tuff relative uplift of the core of the whipple mountains probably as a result of isostatic rebound following tectonic denudation influenced sedimentation patterns after deposition of the peach springs tuff

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

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

crustal growth through successive arc magmatism reconnaissance u pb shrimp data from the northeastern superior province canada abstract north of the la grande subprovince in northern quebec the superior province is dominated by plutonic rocks of the bienville subprovince and minto block which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology and projected along strike using aeromagnetic anomalies to test the validity of the extrapolations u pb ages were obtained through shrimp analyses of zircon from 18 small archival samples selected from a suite of 2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s the results reveal a history of successive arc magmatism between 3 07 and 2 70 ga followed by metamorphism emplacement of crustally derived granites 2 702 2 685 ga and late hydrothermal events ca 2 65 ga in the northeastern minto block the douglas harbour domain previously extrapolated from dated localities to the northeast is substantiated through an age of 2 87 ga the older tonalitic crust is punctured by 2 734 2 725 ga granodioritic plutons of the leaf river suite that locally carry 3 01 ga inherited zircon the northern utsalik domain is made up of pyroxene and hornblende bearing leaf river plutons with consistent 2 725 2 723 ga ages and local inheritance of 2 82 and 2 77 ga however the southern extension of utsalik domain contains pyroxene bearing granodiorites both older 2 762 ga and younger 2 692 ga than the leaf river suite a tonalitic rock from the southern goudalie domain with an age of 2 833 ga supports a linkage to 2 84 2 83 ga units of the qalluviartuuq domain along strike to the north tikkerutuk domain represents a north trending 50 km wide calc alkaline magmatic arc with ages between 2 71 and 2 70 ga that extends southward into the bienville subprovince on the basis of aeromagnetic anomalies and geochronology zircon inheritance ages indicate that the tikkerutuk arc was built upon antecedent arcs of 2 84 2 77 and 2 725 ga the inukjuak domain in the west consists mainly of granite dated in one locality at 2 72 ga

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

structural reconstruction and zonation of a tilted mid crustal magma chamber the felsic chemehuevi mountains plutonic suite structural relief resulting from middle tertiary extensional deformation in the chemehuevi mountains of california exposes a unique cross section through an extensive 280 km 2 calc alkalic compositionally zoned sill like granitic intrusion of late cretaceous age minimum estimates for emplacement pressure 4 to 6 kbar imply that the chemehuevi mountains plutonic suite was initially intruded at mid crustal depths and has undergone 10 to 15 of postemplacement tilting tectonic denudation and erosion reconstruction of the pre tertiary pre tilt configuration suggests that this metaluminous to peraluminous granitic suite exhibits crude normal vertical and temporal zonation from granodiorite to granite the zonation involves a decrease in age and an increase in silica away from the walls and roof the youngest and most evolved members being concentrated toward the center and floor of the intrusion the lower part of the intrusion had a flat floor which was penetrated by at least three feeder dikes providing magma to the chamber the chamber grew laterally in stages each pulse being more evolved than the preceding one structural reconstruction indicates that the roof is less than 1 km above the exposed top of the intrusion suggesting a tabular geometry with an aspect ratio of about 5 1 the magma apparently ponded along the contact between undeformed proterozoic basement above and subhorizontally foliated mylonitic gneisses below and was therefore structurally controlled this reconstruction provides information on the geometry of mesozonal intermediate to silicic magma chambers in addition the restoration provides a rare opportunity to observe crosscutting relations between different types of mid crustal structures thick mylonitic shear zones granitic intrusions and temporally unrelated detachment faults the geometry of which emphasizes the need for careful evaluation of seismic reflection profiles across complexly deformed and intruded continental crust

the tiger formation paleogene fluvial sediments deposited adjacent to a deforming cordilleran metamorphic core complex northeastern washington abstract the eocene to oligocene tiger formation and associated upper plate rocks of northeastern washington are juxtaposed along low angle fault zones with structurally lower or lower plate rocks of the spokane dome a metamorphic core complex strata of the tiger formation that were derived solely from upper plate rock are termed intraplate strata and those that include clasts of lower plate rock are termed extraplate strata the cliff ridge and ione regions are situated on opposite sides of an arch of typical lower plate rock the tiger formation here consists of predominantly conglomeratic intraplate strata that were deposited by braided streams on both valley fills and alluvial fans these strata were derived from a tectonically active source terrane where lower plate rock now exists intraplate strata of the cusick region consist of conglomeratic sandstone and siltstone facies alluvial fans and a sandy braided alluvial plain graded downstream to a floodplain dominated environment intraplate strata exposed in the cliff ridge ione and cusick regions record deposition in response to differential uplift of the core complex in the subsurface extraplate conglomeratic strata deposited in alluvial fan and braided alluvial plain environments conformably and unconformably overlie the intraplate sandstone and siltstone facies of the cusick region these extraplate conglomerates were derived from the area now occupied solely by the core complex and record the erosion of intraplate tiger strata and other upper plate rocks exposure of the fault zone at the paleosurface and erosion of the core complex the tiger formation provides valuable insight to the spatial and temporal distribution of deformation that accompanies the development of a cordilleran metamorphic core complex

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

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

introduction to special section on the california arizona crustal transect cactis part 3 the cactis california arizona crustal transect interim synthesis workshop in may 1988 brought researchers together in flagstaff arizona to discuss the geologic evolution and crustal structure of the southern cordillera between the san andreas fault in southeastern california and the colorado plateau in arizona sass et al 1988 the first set of papers resulting from the workshop appeared in the journal of geophysical research jgr as the special cactis 1 section in january 1990 howard et al 1990 and a second as cactis 2 in november 1990 haxel et al 1990 twelve papers in this issue form part 3 of the cactis series a final collection of papers will appear in the future as a joint special section with the california consortium for crustal studies calcrust

surface expression of an accommodation zone within the gulf of suez rift egypt the gebel gharamul region in the western gulf of suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination the surface expression of the accommodation zone is dominated by a basement promontory which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half grabens both of which face the gulf of suez the southern half graben represents the breakaway fault of the southwest dipping tilt block domain and the northern half graben represents the collapse of the upper plate above the northeast dipping tilt block domain basement and nubian rocks in the hanging walls above the opposing low angle detachments on either side of the accommodation zone are broken by gulf parallel synthetic normal faults and orthogonal transfer faults which act as block terminations and incrementally accommodate the large scale rotation and displacement between adjacent blocks the prerift and synrift stratigraphic succession is draped over this fault template resulting in a complex and variable distribution of structures the product of an inhomogeneous stratigraphic succession within an accommodation zone

detached strata in a tertiary low angle normal fault terrane southeastern california a sedimentary record of unroofing breaching and continued slip miocene sedimentary strata exposed in the eastern chemehuevi mountains southeastern california record development of an evolving low angle normal fault system the sequence includes more than 1 km of conglomerate and sandstone with rare interbedded monolithologic breccia and volcanic flows clasts of peach springs tuff in basal units indicate that this succession is younger than 18 ma these rocks have been displaced by a regionally extensive low angle normal fault the chemehuevi detachment and are folded and faulted structural reconstructions and the character of associated fault rocks indicate that the chemeheuvi fault was initiated at a low angle and that the footwall was progressively unloaded through thinning and displacement of its cover during extensional deformation the syntectonic sedimentary rocks described here provide evidence that movement continued on the gently dipping 15 sup 0 fault even after part of the fault was breached and the footwall eroded the conglomerates and sandstones were deposited by stream flow and debris flow on alluvial fans synsedimentary faulting is suggested by angular discordance below one monolithologic breccia bed and by local coarsening upward sequences clast types reveal progressive unroofing of hanging wall rocks to exposer the chemehuevi fault zone from which chloritic brecciated granite clasts were derived clasts weremore then derived from both the hanging wall and the footwall footwall debris being dominant high in the section distinctive clasts show that late displacement on this evolving fault system was on the order of 1 to 5 km less

mechanisms for the formation of domal and basinal detachment faults a three dimensional analysis detachment faults i e regional low angle normal faults in metamorphic core complexes of the north american cordillera commonly exhibit doubly plunging antiformal domal and synformal basinal geometries these geometries have been previously attributed to the superposition of two processes 1 the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip and 2 the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation however the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the u s cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process a three dimensional elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults this model explores the interactions among the vertical horizontal and basal shearing forces during the formation of warped detachment faults the model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction the results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults 1 an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt and 2 buckling caused by a compression perpendicular to the extension direction both mechanisms are consistent with the geologic constraints during the development of cordilleran core complexes in addition to isostatic uplift due to tectonic denudation upward warping of detachment faults can also be caused by buckling basal shearing and buoyant forces

late cenozoic history and styles of deformation along the southern death valley fault zone california late cenozoic deposits in the southern death valley region have been offset 35 km by right lateral strike slip faulting on the southern death valley fault zone since miocene time virtually all slip took place prior to 1 m y ago along western traces of the fault zone during the past 1 m y the eastern traces of the fault zone have been active and characterized by oblique slip with a lateral component of only a few hundred metres movement along these eastern traces has formed normal faults and gentle to isoclinal folds that have uplifted fan gravel and lacustrine sediments as much as 200 m above the modern alluvial fan surface surveying of the longitudinal profile of the amargosa river which flows within the eastern traces of the fault zone suggests that vertical deformation continues today the 35 km of right lateral offset which is based on matching offset alluvial fan gravel with its source area refines earlier estimates of 8 to 80 km of movement for the southern death valley fault zone and it is consistent with the geometry of a pull apart basin model for central death valley causes for the observed differences in the style and timing of movement of the eastern and western subzones are not well understood the study area however is located a few kilometres north of the intersection of the southern death valley and garlock fault zones the garlock fault zone changes its sense of movement from left lateral strike slip to east vergent thrusting against the southern death valley fault zone the resulting compression may have caused the shift in activity and the change in style of deformation along branches of the southern death valley fault zone

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

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

an axial view of a metamorphic core complex crustal structure of the whipple and chemehuevi mountains southeastern california a 135 km long nw se trending seismic refraction wide angle reflection profile provides a unique along strike view of the crustal structure of a belt of metamorphic core complexes in southeastern california the whipple chemehuevi and sacramento mountains metamorphic core complexes interpretation of the seismic data was done by two dimensional forward modeling of travel times and amplitudes the final model consists of 1 a thin 1 5 km veneer of upper plate and fractured lower plate rocks velocities of 1 5 5 3 km s 1 overlying a fairly homogeneous basement with velocities of 6 0 km s 1 2 a localized high velocity 6 4 km s 1 body situated directly beneath the whipple mountains 3 a 6 3 6 4 km s 1 middle crust that is thickest beneath the core complexes 4 a 6 65 0 15 km s 1 lower crust 5 crustal thickness of 27 km with a deeper crustal root 3 km beneath the whipple mountains metamorphic core complex and 6 a pn velocity of 8 0 0 10 km s 1 the crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust upper crustal velocities are higher beneath the whipple mountains where velocities increase to 6 4 km s 1 at 5 km depth than beneath the chemehuevi and sacramento mountains in addition midcrustal discontinuities rise 2 5 km beneath the whipple complex compared to the other complexes these observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the whipple mountains compared to rocks in the chemehuevi and sacramento mountains despite the enhanced uplift and extension in the whipple mountains the crust is thicker here 30 km than anywhere else along the colorado river extensional corridor this may be in part a relic of compressional and magmatic thickening during the mesozoic however we suggest that inflation of the crust during tertiary extension was the dominant mechanism both mantle derived magmatism and lateral ductile inflow in the crust are proposed

structural and stratigraphic development of the whipple chemehuevi detachment fault system southeastern california implications for the geometrical evolution of domal and basinal low angle normal faults recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults in addition intense investigations have concentrated on how the detachment fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates the superb exposure of the mid tertiary whipple mountains detachment fault system in southeastern california provides an unusual opportunity to address these questions analysis of the stratigraphy and structures of upper plate rock units constrains the three dimensional kinematic evolution of the mid tertiary whipple chemehuevi detachment fault system in southeastern california tertiary volcanic strata in the northwestern whipple mountains and central chemehuevi valley consist of similar rock types that have undergone about 15 extensional strain in contrast a different succession of tertiary volcanic and sedimentary rocks underlies the northeastern whipple mountains along the structural trend of central chemehuevi valley these strata have been extended by 40 differences in magnitudes of extension and mid tertiary stratigraphies between these two areas argue for the presence of a synformal gently east dipping normal fault above the whipple fault in the central chemehuevi valley this fault accommodates 15 km of the total 40 km extension along the whipple chemehuevi detachment fault system in addition tilting patterns of upperplate strata and the presence of an areally extensive syn extension landslide 70 km2 show that 1 the whipple detachment fault was active at low angles 70 km2 from the north side of the whipple mountains its emplacement occurred before the end of detachment faulting

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

u pb lu hf and ree in zircon from 3 2 to 2 6 ga archean gneisses of the repulse bay block melville peninsula nunavut abstract combined in situ u pb and lu hf isotope and ree analyses of zircons from the tonalite trondhjemite granodiorite ttg type gneisses and granitoids within the repulse bay block rbb of melville peninsula nunavut reveal that the block is composed of ca 2 86 ga and 2 73 2 71 ga granitoids that formed by reworking of a 3 25 3 10 ga basaltic crust the interpretation is supported by initial hf values for ca 2 86 ga and ca 2 73 2 71 ga gneisses of 2 7 to 0 7 and 4 8 to 2 3 respectively and the preservation of high uranium xenocrystic cores at 3 15 0 05 ga neoarchean plutonism also occurred at ca 2 69 2 66 ga and is characterized by two pyroxene bearing charnockite enderbite and hornblende biotite granitoid intrusions that yield initial hf values that range from 7 1 to 1 0 demonstrating that they were derived from juvenile magmas with significant assimilation of the existing crust the pyroxene and non pyroxene bearing granitoids also contain elevated lree in zircon a feature that is interpreted to reflect metasomatism of the source this study also demonstrates that in the rbb the results of lu hf isotopes in zircon combined with in situ u pb geochronology from inherited zircon cores differ from the results of whole rock sm nd isotopes

regional gravity studies in southeastern california western arizona and southern nevada bouguer gravity anomaly low pass and band pass maps of southeastern california western arizona and southern nevada are analyzed to determine regional crustal structure based on 100 250 km band pass wavelength filtering of the bouguer anomaly map a low amplitude gravity maximum is associated with the highly extended terrane along the colorado river region of california arizona and nevada two crustal models constructed using bouguer gravity anomaly data and constrained by geological mapping aeromagnetic data and seismic refraction data are consistent with the dome shaped midcrustal lens of probable dioritic composition centered under the whipple mountains proposed by mccarthy et al 1989 although other models are not precluded this midcrustal lens extends from the western edge of the colorado river extensional corridor to east of the buckskin and hualapai mountains in arizona dioritic material is exposed in various mountain ranges in the colorado river extensional corridor and may be part of the source of the gravity maximum that is associated with each range

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

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

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

three dimensional strain produced by 50 my of episodic extension horse prairie basin area sw montana u s a abstract the horse prairie basin of southwestern montana is a complex east dipping half graben that contains three angular unconformity bounded sequences of tertiary sedimentary rocks overlying middle eocene volcanic rocks new mapping of the basin and its hanging wall indicate that five temporally and geometrically distinct phases of normal faulting and at least three generations of fault related extensional folding affected the area during the late mesozoic to cenozoic all of these phases of extension are evident over regional or cordilleran scale domains the extension direction has rotated 90 four times in the horse prairie area resulting in a complex three dimensional strain field with 60 east west and 25 north south bulk extension extensional folds with axes at high angles to the associated normal fault record most of the three dimensional strain during individual phases of extension phases 3a 3b and 4 cross cutting relationships between normal faults and tertiary volcanic and sedimentary rocks constrain the ages of each distinct phase of deformation and show that extension continued episodically for more than 50 my gravitational collapse of the sevier fold and thrust belt was the ultimate cause of most of the extension

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

allochthonous 2 78 ga oceanic plateau slivers in a 2 72 ga continental arc sequence vizien greenstone belt northeastern superior province canada embedded within the vast granitoid terrane of the minto block of northeastern superior province are late archean greenstone belts of the goudalie domain that preserve a long lived record of continent ocean interaction the vizien greenstone belt is one such belt and it contains four fault bounded structural panels the 2786 ma mafic ultramafic sequence is an allochthonous package of pillowed basaltic andesite komatiite and volcaniclastic rocks cut by peridotite and gabbro sills the mafic rocks are lree depleted tholeiites which have primitive mantle prim normalized abundances of th la nb and a range of nd values from 0 1 to 1 7 the 2722 ma lac serindac bimodal subaerial tholeiitic volcanic sequence contains andesite locally with tonalite xenoliths basalt gabbro sills lenses of quartz rich sedimentary rocks and a thick upper rhyolite sequence the lac serindac tholeiites are lree enriched have prim normalized th la nb high zr to 300 ppm and ti contents and low nd values from 0 8 in basalt to 1 4 in rhyolite the 2718 ma basement cover sequence comprises 2 94 ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 ma gabbro siliceous high mg basalt shmb and andesite the shmb are characterised by lree depletion and nd values of 2 6 whereas the andesite is lree enriched and has nd values of 0 3 the 2786 ma mafic ultramafic sequence is interpreted as a sliver of plume related oceanic plateau crust the 2724 lac lintelle sequence represents a continental arc formed on the eastern protocraton the 2722 ma lac serindac volcanic sequence represents late continental rift deposits the various 2 8 2 7 ga supracrustal sequences were accreted deformed and metamorphosed to mid amphibolite facies during late stage assembly of the minto block between 2 718 and 2 693 ga

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

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

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

geology geochronology and geochemistry of archean rocks in the eqe bay area north central baffin island canada constraints on the depositional and tectonic history of the mary river group of northeastern rae province results of stratigraphic u pb geochronological and geochemical study are reported for rocks in a 2800 km2 area along the southeastern margin of the archean rae province on north central baffin island archean rocks include a gneiss complex two greenstone belts of the mary river group and various younger plutonic rocks the 3000 2800 ma gneiss complex contains intrusions of orthogneiss dated at 2780 2770 ma intermediate felsic volcanism in overlying greenstone belts occurred at 2740 2725 ma and was accompanied and outlasted by calc alkaline plutonism 2730 2715 ma peraluminous plutonism at ca 2700 ma possibly associated with low to medium pressure metamorphism represents the culmination of the archean tectonic cycle dating of metamorphic zircon and titanite in archean gneissic rocks indicates that overprinting high grade metamorphism in the northwest part of the area footwall of the isortoq fault zone is paleoproterozoic ca 1820 ma a weaker somewhat older thermal disturbance ca 1850

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

neoarchean high potassium granites of the boothia mainland area rae domain churchill province u pb zircon and sm nd whole rock isotopic constraintsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the boothia mainland region of the north central rae domain is underlain by remnants of a neoarchean volcano sedimentary sequence dismembered by two regionally extensive neoarchean high potassium granitoid suites with rare occurrences of a structurally interleaved paleoproterozoic sedimentary cover sequence the granitoids and their gneissic equivalents are dominated by variably deformed and metamorphosed i type metaluminous polyphase commonly porphyritic to augen biotite hornblende monzogranite and subordinate granodiorite with rare tonalite new geochronological results the first for this area demonstrate that the widespread neoarchean granitoid plutonism is dominantly 2 61 2 59 ga with a less prominent 2 66 ga plutonic event the age of zircon recrystallization suggests that ca 2 60 ga archean metamorphism and fabric development s1 affected the 2 66 ga plutons prior to or contemporaneously with intrusion of the voluminous ca 2 6 ga suite end t for the ca 2 61 2 59 ga suite range from

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