paleoceanographic and tectonic implications of a regionally extensive early mississippian hiatus in the ouachita system southern mid continental united states a regionally extensive early mississippian hiatus is recognized at the transition from preflysch to flysch rocks in the ouachita system of the southern mid continental united states it is recognized only in sections where detailed biostratigraphic control exists and it is coeval with a regional unconformity in the ouachita foreland throughout parts of texas new mexico and oklahoma widespread early mississippian erosion and nondeposition is attributed to a reconfiguration in basin geometry caused by the onset of ouachita orogenic activity crustal flexure resulting from the impending collision would have initiated erosional processes throughout the foreland while simultaneously altering bottom current pathways in the basin and allowing for a prolonged period of sedimentary bypass in the late mississippian increased constriction may have eventually cut off transequatorial bottom water circulation allowing deposition to resume on the shelf and in the basin an important tectonic implication of this study is that ouachita deformation started in the late devonian to earliest mississippian at least 10 m y before the onset of flysch deposition

geologic framework of the mississippian barnett shale barnett paleozoic total petroleum system bend arch fort worth basin texas this article describes the primary geologic characteristics and criteria of the barnett shale and barnett paleozoic total petroleum system tps of the fort worth basin used to define two geographic areas of the barnett shale for petroleum resource assessment from these two areas referred to as assessment units the u s geological survey estimated a mean volume of about 26 tcf of undiscovered technically recoverable hydrocarbon gas in the barnett shale the mississippian barnett shale is the primary source rock for oil and gas produced from paleozoic reservoir rocks in the bend arch fort worth basin area and is also one of the most significant gas producing formations in texas subsurface mapping from well logs and commercial databases and petroleum geochemistry demonstrate that the barnett shale is organic rich and thermally mature for hydrocarbon generation over most of the bend arch fort worth basin area in the northeastern and structurally deepest part of the fort worth basin adjacent to the muenster arch the formation is more than 1000 ft 305 m thick and interbedded with thick limestone units westward it thins rapidly over the mississippian chappel shelf to only a few tens of feet the barnett paleozoic tps is identified where thermally mature barnett shale has generated large volumes of hydrocarbons and is 1 contained within the barnett shale unconventional continuous accumulation and 2 expelled and distributed among numerous conventional clastic and carbonate rock reservoirs of paleozoic age vitrinite reflectance ro measurements show little correlation with present day burial depth contours of equal ro values measured from barnett shale and typing of produced hydrocarbons indicate significant uplift and erosion furthermore the thermal history of the formation was enhanced by hydrothermal events along the ouachita thrust front and mineral wells newark east fault system stratigraphy and thermal maturity define two gas producing assessment units for the barnett shale 1 a greater newark east fracture barrier continuous barnett shale gas assessment unit encompassing an area of optimal gas production where dense impermeable limestones enclose thick 300 ft 91 m barnett shale that is within the gas generation window ro 1 1 and 2 an extended continuous barnett shale gas assessment unit covering an area where the barnett shale is within the gas generation window but is less than 300 ft 91 m thick and either one or both of the overlying and underlying limestone barriers are absent

paleozoic plate dynamics current plate motions can be accounted for by a balance of active forces slab pull ridge push and for continental plates trench suction with drag beneath the plate as a resistive force if we assume that the same forces have acted through time we can reconstruct plate motions from the geometry of past plate boundaries paleozoic reconstructions are made with paleomagnetic tectonic climatic and biogeographic data as no ocean floor remains paleomap reconstructions are used to estimate past plate speeds and to test simple dynami cal models in order to determine which ranges of forces best accounts for the observations over the last 600 m y plate speeds averaged over 40 to 100 m y intervals show considerable variation gondwana s speed oscillates from 20 to 60 km m y over a long timescale 200 400 m y with considerable noise superposed over the paleozoic era motions for large continental regions average 28 krn m y force balance models based on present day observations suggest that continental regions without a large attached slab would move 30 mrn yr the opening and closing of the ocean between laurentia and gondwana 560 400 ma is used to test dynamical models and the parameter values assumed in the late precambrian laurentia rifted away from gondwana in the earliest cambrian it was near 40os by late cambrian and ordovician it had moved to the equator during the silurian and devonian laurentia reversed direction and later collided with gondwana at 40os in a model of the forces acting on the plates slab pull ridge push and trench suction are assumed to balance plate drag only certain ranges of ridge push and trench parameters can model both the opening and subsequent closing of the ocean the dynamic models with parameter values inferred from present rates bracket the rates required by the reconstructions

evidence for synchronous thin skinned and basement deformation in the cordilleran fold thrust belt the tendoy mountains southwestern montana the tendoy mountains contain the easternmost thin skinned thrust sheets in the cordilleran fold thrust belt of southwestern montana and are in the zone of tectonic overlap between the rocky mountain foreland and the cordilleran fold thrust belt the three frontal thrust sheets of the tendoy mountains are from north to south the armstead mckenzie and tendoy sheets near the southeastern terminus of the tendoy thrust sheet is a lateral ramp in which the tendoy thrust climbs along strike from the upper mississippian lombard limestone to lower cretaceous rocks this ramp coincides with the southeastern side of the paleozoic snowcrest trough and projection of the range flanking basement thrust of the blacktail snowcrest uplift suggesting either basement or stratigraphic control on location of the lateral ramp axes of major folds on the southern part of the tendoy thrust sheet are parallel to the direction of thrust transport and to the trend of the snowcrest range they are a result of 1 pre thrust folding above basement faults 2 passive transportation of the folds from a down plunge position 3 minor reactivation of basement faults and 4 emplacement of blind sub tendoy thin skinned thrust faults the tendoy sheet also contains a major out of sequence thrust fault that formed in thick upper mississippian shales and created large overturned foreland verging folds in upper mississippian to triassic rocks the out of sequence fault can be identified where stratigraphic section is omitted and by a stratigraphic separation diagram that shows it cutting down section in the direction of transport the prominent lateral ramp at the southern terminus of the tendoy thrust sheet is a result of fault propagation through strata folded over the edge of the blacktail snowcrest uplift

kimberlite u th he dating links surface erosion with lithospheric heating thinning and metasomatism in the southern african plateau documenting the surface response to processes at depth is a central challenge in continental tectonics kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts pipe erosion levels and mantle xenolith suites here we combine new apatite u th he ahe data from four south africa kimberlites ranging in emplacement age from ca 143 ma to ca 74 ma with a wealth of other geologic information from the pipes to resolve the timing patterns and causes of erosion across 200 km of the southern african plateau comparison of the ahe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca 117 90 ma which was most intense from ca 100 to 90 ma this unroofing phase is synchronous with warming thinning and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites these thermochemical changes could be related to still deeper dynamic mantle processes our ahe data appear to directly document the erosional response to surface uplift induced by these various events the ahe results also detect as much as 1 5 km of cenozoic unroofing that is spatially centered on a proposed mid tertiary paleotributary to the orange river suggesting drainage patterns as the cause our results not only tightly refine the surface history of the interior of the southern african plateau but show that kimberlite ahe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links

long term evolution of the west african transform margin estimates of denudation from benin using apatite thermochronology the benin continental margin was formed during the breakup of gondwana through oblique rifting along transform faults the evolution of topography following breakup directly affects the evolution of sedimentary basins which has major implications for hydrocarbon exploration in the region quantitative constraints on erosion across benin are limited to the cenozoic based on analysis of dissected lateritic palaeolandscapes to resolve the mesozoic erosion history we have obtained apatite fission track and single grain u th sm he data from 18 samples collected across a 600 km long transect through benin we invert these data including available geological and geomorphological constraints to obtain time temperature paths which are used to estimate magnitudes of denudation over the last 200 myr our study suggests that denudation was focused over a c 300 km long seaward sloping limb of the marginal upwarp and at the southern margin of the interior iullemmeden basin from 140 to 100 ma with lower magnitudes of denudation characterizing the continental interior and post cretaceous evolution of the margin models are consistent with modest burial c 1 km of the iullemmeden basin between 120 and 85 ma and of the continental margin between 85 and 45 ma by the eocene the first order relief of benin had developed with regional erosion rates supplementary information full details of the analytical data and modelling results including the various constraints and further data on denudation and burial magnitudes and rates are available at https doi org 10 6084 m9 figshare c 4220804

injection induced seismicity and fault slip potential in the fort worth basin texasinjection induced seismicity and fault slip potential in the fort worth basin texas

tectonic setting of kimberlites kimberlites can be viewed as time capsules in a global plate tectonic framework their distribution illustrates clustering in time and space kimberlite ages span the assembly and break up of a number of supercontinents such as rodinia and gondwana these supercontinents show time lines with i broad periods devoid of kimberlite magmatism corresponding to times of continent stability and ii narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations this episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents the onset of kimberlite magmatism is closely associated with thermal perturbations thermal insulation mantle upwelling beneath a stagnant or sluggish supercontinent these perturbations may have caused uplift and the onset of continental break up through fracture zones propagating into the supercontinent subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths resultant strain is accommodated along trans lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration the corridors are manifest as discontinuities in the lithosphere mantle measured as geophysical gradients and as changes in mantle lithosphere composition within the crust these corridors are expressed as a terrane boundaries b incipient continental rifts c fracture zones or d major dyke swarms some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters the association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults such conditions may be attained during different stages in the evolution of continents a supercontinent formation b incipient rifting driven by far field stresses and onset of continental break up and c strain accommodation along the continental continuation of oceanic fracture zones during spreading type c may show concomitant kimberlite magmatism in separate continents after break up

diamonds through time diamonds form in the upper mantle during episodic events and have been transported to the earth s surface from at least the archean to the phanerozoic small diamonds occur as inclusions in robust minerals in tectonically activated ultrahigh pressure metamorphosed crustal rock establishing an association with subduction processes and recycled carbon but providing no economic deposits diamonds in economic deposits are estimated to be mainly 99 derived from subcontinental lithospheric mantle and rarely approx 1 from the asthenosphere harzburgite and eclogite are of roughly equal importance as source rocks followed by lherzolite and websterite diamonds which provide evidence of extensive residence time in the mantle are with minimal exceptions smooth surfaced crystalline diamonds scd with potential commercial value the oldest prolific scd formation event documented on the world s major diamond producing cratons occurs in archean lithospheric mantle harzburgite metasomatized by likely subduction related potassic carbonatitic fluids disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds g10 garnets and diamond inclusion type chromites well used in diamond exploration within the mantle domains of diamond stability there have been repeated episodes of further diamond crystallization and or growth these are associated with old often proterozoic subduction related melt generation metasomatic fluid migration and reaction with preexisting mantle eclogite websterite and peridotite using improved methods of isotope analysis diamond formation ages can be correlated with specific major processes such as craton accretion craton edge subduction and magmatic mantle refertilization fibrous cuboid diamond and fibrous coats on scd are rough surfaced diamonds with abundant fluid inclusions they have low mantle residence time forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain scd the symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption continuing a process of volatile enrichment in the lithospheric mantle already identified in the archean harzburgite diamond event mantle derived scd commonly shows evidence of resorption illustrating that diamond unfriendly processes including lamproite and kimberlite generation are also active and may have a substantial negative effect in extreme cases on scd crystal size exposure of scd to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion free diamonds with our current state of knowledge

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

geology of the gahcho kue kimberlite pipes nwt canada root to diatreme magmatic transition zones abstract the cambrian gahcho kue kimberlite cluster includes four main pipes that have been emplaced into the archaean basement granitoids of the slave craton each of the steep sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite hk to diatreme facies tuffisitic kimberlite breccia tkb the tkb displays many diagnostic features including abundant unaltered country rock xenoliths pelletal lapilli serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate the hk contains common fresh olivine set in a groundmass composed of monticellite phlogopite perovskite serpentine and carbonate a number of separate phases of kimberlite display a magmatic textural gradation from tkb to hk which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity proportion of fresh olivine and the degree of xenolith digestion the pipe shapes and infills of the gahcho kue kimberlites are similar to those of the classic south african pipes particularly those of the kimberley area similar intrusive magmatic emplacement processes are proposed in which the diatreme zone results from the degassing after breakthrough of the intruding magma column the transition zones represent frozen degassing fronts the style of emplacement of the gahcho kue kimberlites is very different from that of many other pipes in canada such as at lac de gras fort a la corne or attawapiskat

evidence of a hydrological connection between the ice divide and ice sheet margin in the aurora subglacial basin east antarctica subglacial hydrology in east antarctica is poorly understood yet may be critical to the manner in which ice flows data from a new regional airborne geophysical survey icecap have transformed our understanding of the topography and glaciology associated with the 287 000 km2 aurora subglacial basin in east antarctica using these data in conjunction with numerical ice sheet modeling we present a suite of analyses that demonstrate the potential of the 1000 km long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin we present results from our analysis of basal topography bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures although no clear cut subglacial lakes are found within the aurora basin itself dozens of lake like reflectors are observed that in conjunction with other results reported here support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the dome c ice divide to reach the coast via the totten glacier

on the geodynamic setting of kimberlite genesis the emplacement of kimberlites in the north american and african continents since the early palaeozoic appears to have occurred during periods of relatively slow motion of these continents the distribution of kimberlites in time may reflect the global pattern of convection which forces individual plates to move faster or slower at different times two dimensional numerical experiments on a convecting layer with a moving upper boundary show two different regimes in the first when the upper boundary velocity is high heat is transferred by the large scale circulation and in the second when the upper boundary velocity is lower heat is predominantly transferred by thermal plumes rising from the lower boundary layer for a reasonable mantle solidus this second regime can give rise to partial melting beneath the moving plate far from the plate boundaries the transition between these modes takes place over a small range of plate velocities for a rayleigh number of 106 it occurs around 20 mm yr 1 we suggest that the generation of kimberlite magmas may result from thermal plumes incident on the base of a slowly moving plate

canadian kimberlites geological characteristics relevant to emplacement in canada more than 770 kimberlites have erupted in diverse tectonic settings over a period of 1000 ma and over an area at least 5000 km across the kimberlites represent at least 30 comagmatic fields one pipe type dominates each of the different kimberlite fields even with the discovery of a significant number of new kimberlite fields this review of recent data substantiates the previous proposal that there are at least three distinct classes of kimberlite pipes which show a correlation with the nature of the country rocks into which they were emplaced new data show that a variation on the prairies pipe type occurs when kimberlites were emplaced into competent paleozoic sediments resulting in steeper sided pipes infilled with similar pyroclastic kimberlite new data for each pipe type are summarised and the contrasting characteristics of the three pipe types are considered to reflect fundamentally different styles of eruption and deposition apparently uniform near surface olivine and volatile rich kimberlite magmas were modified differently during the contrasting emplacement processes resulting in distinct textural rock types pyroclastic kimberlite pk tuffisitic kimberlite tk and associated hypabyssal kimberlite hk or resedimented kimberlite rvk dominating each type of pipe the substantiated correlation of pipe type with country rock geology could indicate variable constraints on volatile exsolution which affected the nature of the magmatic eruptions in different settings

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

thermal maturity of the barnett shale determined from well log analysis intensive development with large scale fracturing treatments has made the barnett shale play newark east field in the fort worth basin the largest shale gas field in the world the mississippian barnett shale is an organic rich self sourced reservoir rock thermal maturity thickness and total organic carbon are the most important geological factors for commercial gas production from this shale formation the log derived thermal maturity index mi has been developed in an effort to better understand and predict hydrocarbon phases across the basin maturity index was calculated using three types of open hole logs neutron porosity deep resistivity and density porosity or bulk density the derivation of mi is based on the hypotheses that shale gas is generated and stored locally without apparent migration from outside sources and that the water saturation and the density of generated hydrocarbons decrease with an increase in thermal maturity maturity index correlates well with initial gas oil ratios gor from well production data based on this correlation an empirical relationship has been demonstrated for the fort worth basin this method is useful in understanding the thermal maturity levels of barnett shale source rock in the gas generation window mapping mi gor and gas heating value from hundreds of wells identifies the various maturity stages and areas of barnett shale that generate oil condensate wet gas or dry gas in the fort worth basin

deep rifts as sources for alkaline intraplate magmatism in eastern north america intracontinental magmatism in eastern north america does not exhibit the regular time progression associated with mantle plume activity instead the magmatism is sporadic but locally persistent suggesting that the magmas rise from a mantle source that drifts with the continental plate a spatial association with ancient rifts further suggests that this source is fertile9 melt generating material emplaced in the mantle lithosphere during the early stages of rifting

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

reevaluation of thermal maturity and stages of petroleum formation of the mississippian barnett shale fort worth basin texas new data including measured reflectance ro programmed open system pyrolysis data and kerogen elemental analyses obtained on the mississippian barnett shale in the fort worth basin texas indicate that secondary gas generation starts at 1 5 ro and not at the previously prescribed 1 1 ro oil cracking kinetic parameters derived from pyrolysis experiments in the presence and absence of water indicate that secondary gas generation will not occur at a thermal maturity as low as 1 1 ro and requires a minimum thermal maturity of 1 5 ro this difference is especially important in using the barnett shale as an analog for evaluating other possible shale gas plays the new reflectance measurements have a good relationship with hydrogen indices his and compare well with other published data sets however the relationship does not compare well with the previously published data used to prescribe 1 1 ro as the start of secondary gas generation in the barnett shale this discrepancy is attributed to differences in measured ro values and not attributed to differences in the hi values lack of publicly available information on the previously reported ro values makes it difficult to ascertain the reason for their lower values these lower ro values also have impact on the previously prescribed relationship for estimating ro from the temperature at maximum yield by programmed open system pyrolysis tmax as a result the new data do not agree with a previously described relationship and the considerable scatter makes the new relationship unreliable however the relationship between the hi and ro has less scatter which indicates that hi offers a better proxy in calculating ro than tmax for the barnett shale comparison of various programmed open system pyrolysis methods i e rock eval ii rock eval 6 source rock analyzer and hawk indicates that variations in hi are within 10 of one another an hi of at least 44 mg g total organic carbon is prescribed as a more certain limit for the start of secondary gas generation and prospective in situ gas shale accumulations

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

emplacement age and thermal footprint of the diamondiferous ellendale e9 lamproite pipe western australia the diamondiferous ellendale 9 e9 pipe is a funnel shaped maar diatreme volcano consisting of inward dipping tuff sequences intruded by lamproite plugs and dykes the host rocks for the e9 pipe are permian sandstones the multiple lithological contacts exposed within the mined maar volcano provide a natural laboratory in which to study the effect of volcanic processes on u th pb he systematics zircon from the regional sandstone and e9 lamproite display a bimodal distribution of ages on u th he u pb plots the zircon u pb ages for the e9 pipe n 52 range from 440 to 2 725 ma while the cluster of u th he ages for the lamproite dyke zircon indicate that dyke emplacement occurred at 20 6 2 8 ma concordant with a maximum emplacement age of about 22 ma from phlogopite 40ar 39ar these ages indicate a xenocrystic origin for the zircon entrained in the e9 dyke the u pb ages of detrital zircon from the regional sandstone host 373 3 248 ma n 41 are indistinguishable from those of the lamproite zircon xenocrysts whereas the detrital zircon in the host sandstone yield u th he ages from 260 to 1 500 ma a thermochronology traverse across the e9 lamproite dyke reveals that the zircon u th he ages in the host sandstone have not been significantly thermally reset during dyke emplacement even at the contact the capability of the zircon u th he method to distinguish deep mantle source lithologies from upper crustal source lithologies could be used in geochemical exploration for diamonds pre screening of detrital samples using etching and helium assay methods will improve the efficiency and decrease the cost of greenfields exploration

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

linking source and sink evaluating the balance between onshore erosion and offshore sediment accumulation since gondwana break up south africa abstract the geomorphic origin and evolution of the tectonically unique interior highland of southern africa the kalahari plateau and its flanking low lying coastal planes remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history here we focus on the southern cape south africa and link onshore denudation based on new apatite fission track thermochronology results to offshore sediment accumulation using abundant well data and a seismic reflection profile we attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern africa the volume of sediment accumulated off south africa s south coast is calculated using 173 wells and a seismic reflection profile a total uncompacted sediment volume of 268 500 km3 accumulated off south africa s south coasts since 136 ma in the outeniqua and southern outeniqua basins accumulation volumes and rates were highest in the early cretaceous 48 800 104 km3 at 8150 km3 ma from 136 to 130 ma and 57 500 104 km3 at 5750 km3 ma from 130 to 120 ma and mid late cretaceous 83 700 104 km3 at 3200 km3 ma from 93 to 67 ma volumes and accumulation rates were lowest for the early mid cretaceous 47 400 104 km3 at 1750 km3 ma from 120 to 93 ma and the cenozoic 31 200 104 km3 at 450 km3 ma from 67 to 0 ma although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion as quantified through apatite fission track thermochronology e g tinker j h de wit m j brown r 2008 mesozoic exhumation of the 439 southern cape south africa quantified using apatite fission track thermochronology tectonophysics doi 10 1016 j tecto 2007 10 009 the timing of increased sediment accumulation closely matches the timing of increased onshore denudation this suggests that the greatest volumes of material were transported from source to sink during two distinct cretaceous episodes and that the processes driving onshore denudation decreased by an order of magnitude during the cenozoic

mantle flow mechanisms for the large scale subsidence of continental interiors evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large scale subsidence we propose that mantle flow associated with the descent of cold plumes and slabs and their interaction with the endothermic phase change at 660 km depth may provide a plausible mechanism for these epeirogenic events simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth and temperature dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary we find that the plume slab flow scenarios are capable of supporting topographic deflections of amplitudes of 1 km and horizontal wavelengths of 1000 km that persist over time scales of 100 150 m y

macrocrystal phlogopite rb sr dates for the ekati property kimberlites slave province canada evidence for multiple intrusive episodes in the paleocene and eocene abstract new rb sr age determinations using macrocrystal phlogopite are presented for 27 kimberlites from the ekati property of the lac de gras region slave province canada these new data show that kimberlite magmatism at ekati ranges in age from at least late paleocene 61 ma to middle eocene time 45 ma older perovskite bearing kimberlites from ekati extend this age range to late cretaceous time 74 ma within this age range emplacement episodes at 48 51 53 55 56 and 59 61 ma can be recognized middle eocene kimberlite magmatism of the previously dated mark kimberlite 47 5 ma is shown to include four other pipes from the east central ekati property a single kimberlite aaron may be younger than the 47 5 ma mark kimberlite the economically important panda kimberlite is precisely dated in this study to be 53 3 0 6 ma using the phlogopite isochron method and up to six additional kimberlites from the central ekati property have early eocene ages indistinguishable from that of panda including the koala and koala north occurrences late paleocene 55 56 ma kimberlite magmatism represented by the diavik kimberlite pipes adjacent to the southeastern ekati property is shown to extend onto the southeastern ekati property and includes three and possibly four kimberlites a precise eight point phlogopite isochron for the cobra south kimberlite yields an emplacement age of 59 7 0 4 ma eight other kimberlites from across the ekati property have similar late paleocene rb sr model ages the addition of 27 new emplacement ages for kimberlites from the ekati property confirms that kimberlite magmatism from the central slave province is geologically young despite ages ranging back to cambrian time from elsewhere in the slave province with the available geochronologic database lac de gras kimberlites with the highest diamond potential are currently restricted to the 51 53 and 55 56 ma periods of kimberlite magmatism

are diamond bearing cretaceous kimberlites related to low angle subduction beneath western north america abstract diamond bearing cretaceous kimberlites of western north america were emplaced 1000 1500 km inboard of the farallon plate subduction margin and overlap with the development of the western interior seaway shut down of the sierra nevada arc and the laramide orogeny these events are consistent with a decrease in subduction angle along much of the margin which placed the subducted farallon plate in close proximity to the continental interior at the time of kimberlite magmatism our numerical models demonstrate that low angle subduction can result from high plate convergence velocities and enhanced westward motion of north america further rapid subduction allows hydrous minerals to remain stable within the cool interior of the subducting plate to more than 1200 km from the trench destabilization of these minerals provides a fluid source that can infiltrate the overlying material potentially triggering partial melting and kimberlite lamproite magmatism

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

kimberlite trends in nw canada kimberlite deposits attract much interest as the host rock for diamonds but their eruption mechanisms and deep root structure remain poorly perceived independent geochronological and seismic shear wave anisotropy studies in nw canada near the world9s newest diamond mines revealed an unexpected correlation in structural trends age of eruption grouped 36 kimberlites into linear clusters forward modelling of shear wave delays indicated two distinct layers of seismic anisotropy with a boundary about 120 km deep the similarity of inferred age and trend direction at surface and at depth indicates that these kimberlites erupted along lithospheric scale fractures oriented by continental stress fields related to north american plate motions

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

post breakup burial and exhumation of passive continental margins seven propositions to inform geodynamic models despite many years of study the processes involved in the post breakup development of passive margins remain poorly understood integration of apatite fission track analysis afta and stratigraph

geochemistry of mississippian tuffs from the ouachita mountains and implications for the tectonics of the ouachita orogen oklahoma and arkansas the ouachita orogeny was the result of plate convergence at the southern margin of the north american continent although the nature of the converging southern plate and the direction of subduction remain uncertain the presence of areally extensive tuff layers interbedded with shale in the mississippian stanley group of the ouachita mountains oklahoma and arkansas provides the potential to define the tectonic environment of volcanism from the geochemistry of the tuffs and thereby delimit the subduction configuration the tuffs contain relic primary magmatic quartz plagioclase and alkali feldspar and range from crystal to vitric rich mineralogical sorting and diagenetic effects have caused chemical variability within individual tuff units but overall the tuffs have retained their primary igneous geochemical characteristics the beavers bend and hatton tuffs are geochemically very similar and are more evolved higher sio2 rb th ree lower sr ba than the mud creek tuff the stratigraphically equivalent sabine rhyolite is geochemically distinct from the tuffs having less fractionated rare earth element ree patterns and different trace element ratios both the ouachita tuffs and sabine rhyolite have the geochemical characteristics of subduction related magmas for example strong depletion of nb and ta relative to other incompatible trace elements consideration of trace element systematics in the tuffs compared to those of modern high silica volcanic rocks from different subduction related tectonic settings suggests a continental arc origin and implies southward subduction beneath a southern continent during carboniferous ocean basin closure

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential discussion montgomery et al 2005 have written a very useful information filled review article on the state of knowledge of the barnett shale play in north texas a topic of great current interest and importance one error exists however the burial history that they present shows no uplift during the early and middle mesozoic and strong uplift after the cretaceous whereas the geologic record indicates major pre cretaceous uplift this error substantially affects the discussion of the maturation history of the barnett and should be corrected in the literature i will also briefly discuss the implications of pre cretaceous erosion and ouachita thrusting to barnett maturity in the deep fort worth basin in montgomery et al s 2005 figure 7 they show a time depth burial history diagram for eastland county that is contrary to what is known about the area in that figure and in the text they indicate that the barnett was rapidly buried during the pennsylvanian and early permian remained at depth with no uplift or subsidence except for minor subsidence in the early cretaceous then was uplifted some 1 8 km 6000 ft between the middle cretaceous and the eocene with very slight subsidence to the present the surface geology of eastland county and surrounding areas barnes 1972 shows that flat lying lower cretaceous strata antlers sand and overlying edwards group marine carbonates lie unconformably above units ranging from the strawn group mingus formation desmoinesian regional age in the southeast to the lower cisco group graham formation virgilian regional age in the northwest the west northwest dipping pennsylvanian strata form part

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential the mississippian barnett shale serves as source seal and reservoir to a world class unconventional natural gas accumulation in the fort worth basin of north central texas the formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce at present production is mainly confined to a limited portion of the northern basin where the barnett shale is relatively thick 300 ft 92 m organic rich presentday total organic carbon 3 0 thermally mature vitrinite reflectance 1 1 and enclosed by dense limestone units able to contain induced fractures the most actively drilled area is newark east field currently the largest gas field in texas newark east is 400 mi 2 1036 km 2 in extent with more than 2340 producing wells and about 2 7 tcf of booked gas reserves cumulative gas production from barnett shale wells through 2003 was about 0 8 tcf wells in newark east field typically produce from depths of 7500 ft 2285 m at rates ranging from 0 5 to more than 4 mmcf day estimated ultimate recoveries per well range from 0 75 to as high as 7 0 bcf efforts to extend the current barnett play beyond the field limits have encountered several challenges including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth patterns of oil and gas occurrence in the barnett in conjunction with maturation and burial history data indicate a complex multiphased thermal evolution with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas

new insights into the lithosphere beneath the superior province from rayleigh wave dispersion and receiver function analysis we study the azimuthally anisotropic upper mantle structure of the superior craton and grenville province in ontario canada using rayleigh wave phase velocity data in the period range 40 160 s 152 two station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least squares technique we perform a series of tests to derive optimal regularization smoothing and damping and to assess the resolution of and trade offs between isotropic and anisotropic anomalies the tomographic inversion is able to resolve isotropic phase velocity anomalies on a scale of 200 300 km and to distinguish between different anisotropic regimes on a 500 km scale across the study region isotropic phase velocity anomalies in the tomographic model span a range of up to 2 per cent around a regional average which is similar to the canadian shield dispersion curve of brune dorman 1963 with phase velocities up to 3 per cent above global reference models the amplitude of azimuthal phase velocity anisotropy reaches a maximum of 1 2 per cent a clear east west division of the study area based on both isotropic phase velocity anomalies and azimuthal anisotropy is apparent in the western superior isotropic phase velocities are generally higher than the regional average anisotropy is observed at all periods with ene wsw to ne sw fast propagation directions at periods 120 s the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries whereas the anisotropy at longer periods is interpreted to arise from present day sublithospheric flow the fast directions from published sks measurements are close to the fast rayleigh wave propagation directions throughout the period range sampled and the large sks splitting times may be accounted for by this near coincidence of fast propagation directions across most of eastern ontario phase velocities are lower than the regional average fast propagation directions rotate from nw se at 40 130 s period to wnw ese at periods 140 160 s the results suggest a difference in fast propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow the superior craton and grenville province are characterized by large scale structural variations that reflect the complex tectonic history of the region this study highlights differences between the characteristics of eastern and western ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle

synchroneity of cratonic burial phases and gaps in the kimberlite record episodic magmatism or preservational bias abstract a variety of models are used to explain an apparent episodicity in kimberlite emplacement implicit in these models is the assumption that the preserved kimberlite record is largely complete however some cratons now mostly devoid of phanerozoic cover underwent substantial phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the slave craton superior craton and cratonic western australia a similar pattern exists in the kaapvaal craton although its magmatic deposition and erosion history differs in key ways from the other localities one explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts even if kimberlites occurred during cratonic burial phases the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced in this model kimberlite magmatism was more continuous than the preserved record suggests implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects either way the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns

denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology 1 the denudational history of a 500 km long transect across the drakensberg escarpment on the high elevation passive margin of se africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples supplemented by an additional 10 outcrop samples a minimum of 4 5 km of denudation since formation of the margin 130 myr ago is estimated for the coastal zone with a marked early cretaceous episode of accelerated denudation broadly coincident with continental breakup samples from the swartberg borehole sw 1 67 located 30 km seaward of the present position of the drakensberg escarpment indicate a total depth of denudation of 3 1 1 2 km since 91 myr with a phase of accelerated denudation of 2 1 0 9 km at a mean rate of 95 43 m myr between 91 and 69 myr samples from the ladybrand borehole la 1 68 west of the lesotho highlands indicate 1 7 0 5 km of denudation since 78 myr with a phase of accelerated denudation at 82 43 m myr from 78 to 64 myr average denudation rates declined to about 10 m myr during much of the tertiary although the apatite fission track data do not provide any direct constraints on the denudational history of the lesotho highlands interpolation between the two boreholes constrained by geological evidence and extrapolated in situ produced cosmogenic 36cl derived denudation rate estimates suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide these patterns of denudation are incompatible with constant retreat of the drakensberg escarpment from an initial position near the present coast we suggest that the drakensberg escarpment formed by rapid post breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100 200 m myr

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling

plate tectonics and basin subsidence history tectonic setting exerts fi rst order control on basin formation as refl ected in basin subsidence history while our approach ignores the effects of fl exural loading and eustatic sea level change consistency of backstripped subsidence histories i e with local loading effects of sediment removed suggests consistent tectonic driving mechanisms in each tectonic setting with the possible exception of forearc basins based on published subsidence curves and open fi le stratigraphic data we show the subsidence characteristics of passive margins strike slip basins intracontinental basins foreland basins and forearc basins passive margin subsidence is characterized by two stages rapid initial synrift subsidence and slow post rift thermal subsidence with increasing subsidence rates toward the adjacent ocean basin subsidence of intracontinental basins is similar in magnitude to that seen in passive margin settings but the former is generally slower longer lived and lacks initial subsidence long lived subsidence for many intracontinental basins is consistent with cooling following thermal perturbation of thick lithosphere found beneath old parts of continents basins associated with strike slip faults are usually short lived with very rapid subsidence changes in local stress regimes as strike slip faults evolve and migrate over time coupled with three dimensional heat loss in these small basins likely explain this subsidence pattern foreland basin subsidence rates refl ect the fl exural response to episodic thrust loading resultant subsidence curves are punctuated by convex up accelerating segments forearc basins have the least consistent subsidence patterns subsidence histories of these basins are complex and may refl ect multiple driving mechanisms of subsidence in forearc settings second order deviations in subsidence suggest reactivation or superimposed tectonic events in many basin settings the effects of eustatic sea level change may also explain some deviations in curves for many of these settings subsidence histories are suffi ciently distinctive to be used to help determine tectonic setting of ancient basin deposits

exploiting radiation damage control on apatite u th he dates in cratonic regions abstract apatites from four pairs of samples of precambrian basement from the western canadian shield were analyzed by u th he thermochronometry to test for the influence of radiation damage on apatite u th he dates in this cratonic region recent studies have demonstrated that the accumulation of radiation damage increases the apatite he retentivity so that apatites with a span of effective u concentrations eu that experienced the same thermal history may be characterized by a range of closure temperatures in this investigation each sample pair consisted of a mafic dike cross cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eu the apatites yielded u th he dates from 846 to 123 ma and were positively correlated with eu within each sample pair these results can be explained using a model that tracks the evolution of he mobility in response to the accumulation of radiation damage when coupled with regional geological constraints the data appear to require partial to complete he loss due to burial and reheating in phanerozoic time new apatite fission track dates and length data were obtained for five of these samples the apatite fission track dates are proterozoic regardless of apatite eu thermal history simulations indicate that the apatite fission track data are compatible with the u th he results although the thermal histories are not identical in detail and the fission track results alone do not require phanerozoic heating together the data are consistent with burial of this region by 1 km of phanerozoic strata that were subsequently denuded thus pointing toward significant phanerozoic deposition in the north american cratonic interior hundreds of kilometers east of where previously documented the results suggest that exploiting radiation damage control on apatite u th he dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible

basal roughness of the east antarctic ice sheet in relation to flow speed and basal thermal state basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water to date basal motion represents one of the largest uncertainties in ice flow models it is that component of the total flow velocity that can change most rapidly and can therefore facilitate rapid variations in dynamic behaviour in this study we investigate the subglacial properties of the east antarctic ice sheet by statistically analysing the roughness of the bed topography inferred from radio echo sounding measurements we analyse two sets of roughness parameters one derived in the spatial and the other in the spectral domain with two roughness parameters each this enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet we further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet we find that one of the roughness parameters the hurst exponent derived in the spatial domain coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

u th he thermochronometry constraints on unroofing of the eastern kaapvaal craton and significance for uplift of the southern african plateau the timing and causes of the 1 0 km elevation gain of the southern african plateau since paleozoic time are widely debated we report the first apatite and titanite u th he thermochronometry data for southern africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau the study area encompasses 1500 m of relief within archean basement of the barberton greenstone belt region of the eastern kaapvaal craton titanite dates are neoproterozoic apatite dates are cretaceous with most results clustering at ca 100 ma thermal history simulations confirm mesozoic heating followed by accelerated cooling in mid to late cretaceous time the lower temperature sensitivity of the apatite u th he method relative to previous thermochronometry in southern africa allows tighter constraints on the cenozoic thermal history than past work the data limit cenozoic temperatures east of the escarpment to 35 c and appear best explained by temperatures within a few degrees of the modern surface temperature these results restrict cenozoic unroofing to less than 850 m and permit negligible erosion since the cretaceous if substantial uplift of the southern african plateau occurred in the cenozoic as advocated by some workers then it was not responsible for the majority of post paleozoic unroofing across the eastern escarpment significant mesozoic unroofing is coincident with large igneous province activity kimberlite magmatism and continental rifting within and along the margins of southern africa compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the effective elastic thickness of the india plate from receiver function imaging gravity anomalies and thermomechanical modelling summary the range and the meaning of the effective elastic thickness eet in continental areas have been subject to controversy over the last two decades here we take advantage of the new data set from the hi climb seismological experiment to re estimate the eet of the india plate along a south north profile extending from the ganges basin to central tibet receiver functions give a high resolution image of the base of the foreland basin at 5 km depth and constrain the crustal thickness which increases northwards from 35 km beneath the indo gangetic plain to 70 km in southern tibet together with available data sets including seismic profiles seismological images from both indepth and himnt experiments deep well measurements and bouguer anomaly profiles we interpret this new image with 2 d thermomechanical modelling solutions using different type of crustal and mantle rheologies we find that 1 the eet of the india plate decreases northwards from 60 80 to 20 30 km as it is flexed down beneath himalaya and tibet due to thermal and flexural weakening 2 the only resistant layer of the india plate beneath southern tibet is the upper mantle which serves as a support for the topographic load and 3 the most abrupt drop in the eet located around 200 km south of the mft is associated with a gradual decoupling between the crust and the mantle we show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region and they are in favour of a rheology in which the mantle is strong

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

drowning of the upper marble falls carbonate platform pennsylvanian central texas a case of conflicting signals abstract marble falls and smithwick formation carbonates and shales were deposited on the western margin of the slowly subsiding fort worth basin of central texas during early pennsylvanian time the upper marble falls member carbonate platform back stepped towards the west southwest as smithwick outer shelf slope black shales drowned platform margin areas to the east the shelf to basin profile observed from subsurface data from the fort worth basin and outcrops from the llano uplift area records platform drowning however the platform began to shoal upwards as it back stepped towards the west southwest due to the presence of the llano uplift paleobathymetric high the geometry of these facies tracts was controlled primarily by a combination of subsidence and second order sea level change while the development of sequences and unconformities was controlled by third order sea level changes and environmental factors and not subsidence the termination of platform deposition in the fort worth basin was caused by the interplay of third order sea level changes environmental deterioration in advance of siliciclastic burial and a lack of accommodation space the depositional history of the upper marble falls carbonate platform does not fit classic examples of drowned platforms described from other areas but may be typical of platforms in slowly subsiding basins

petrology of the renard igneous bodies host rocks for diamond in the northern otish mountains region quebec abstract the renard igneous bodies were discovered in late 2001 as part of a regional diamond exploration program launched by ashton mining of canada and soquem nine bodies have been discovered within a 2 km diameter area and are comprised of root zone to lower diatreme facies rocks including kimberlitic breccia olivine macrocrystic hypabyssal material and brecciated country rock with minor amounts of kimberlitic material many mineralogical and petrographic features are common to both kimberlite and melnoite and strict assignment of the rocks as kimberlite is not possible with these criteria alone whole rock trace element compositions suggest a closer affinity to group i kimberlite with derivation from a garnet bearing mantle exceptions to conventional classification of the rocks along petrographic or mineralogical lines may be due in part to assimilation of felsic country rock into the renard magmas at the time of emplacement the renard magmas were emplaced into northeastern laurentia at 630 ma when the supercontinent was undergoing a change from convergent margin magmatism to rifting the latter being associated ultimately with the opening of the iapetus ocean

thermal maturity of carboniferous strata ouachita mountains the ouachita mountains a relatively untested potential hydrocarbon province contain a thick paleozoic section of apparently favorable source beds reservoir beds and trap configurations to estimate the thermal maturity of these strata vitrinite reflectance was measured on 89 samples collected mostly from carboniferous rocks from throughout the ouachita outcrop area thermal maturity contours generally parallel structural grain in the western two thirds of the ouachitas reflectance values in oil range from 3 0 in the core area in oklahoma and from about 1 0 to 3 0 in analogous positions in arkansas older strata on upthrown sides of thrust faults are more thermally mature than younger strata on downthrown sides in the eastern one third of the outcrop belt thermal maturities are significantly higher 2 0 5 0 ro and thermal maturity contours cut obliquely across structural grain strata on either side of thrust faults display identical levels of maturity in the western two thirds of the ouachitas sedimentary and tectonic burial have been the major influences of thermal maturation although a mesozoic thermal overprint has increased levels of maturity in arkansas throughout this area strata are within the oil and wet gas windows the anomalous levels and patterns of thermal maturity in the eastern third of the ouachitas are probably the result of a thermal overprint related to mesozoic rifting and intrusive activity in the mississippi embayment this thermal overprint has elevated the maturity of strata in most of this area above the level of hydrocarbon preservation although maturities within the dry gas window are retained locally

investigation of links between precambrian basement structure and paleozoic strata in the fort worth basin texas u s a using high resolution aeromagnetic hram data and seismic attributes effective hydraulic fracturing is critical for generating permeability within the barnett shale of the fort worth basin fwb therefore knowledge of the nature of the induced and natural fractures faults and collapse features that may form conduits to the underlying ellenburger aquifer is vital we use coherence and curvature seismic attributes which are sensitive to faults fractures and collapse features to map sedimentary features we then integrate high resolution aeromagnetic hram data with the seismic attributes extracted along the ellenburger formation and the top of basement from the north central portion of the fwb thereby linking features in the precambrian basement to shallower sedimentary structures hram derived maps designed to enhance basement structures confirm that much of the sedimentary faulting is basement controlled specifically attribute lineaments are aligned parallel to hram anomaly lineaments consistent with regional tectonics the northeast southwest and northwest

thermal maturity and organic composition of pennsylvanian coals and carbonaceous shales north central texas implications for coalbed gas potential thermal maturity was determined for about 120 core cuttings and outcrop samples to investigate the potential for coalbed gas resources in pennsylvanian strata of north central texas shallow 600 m 2000 ft coal and carbonaceous shale cuttings samples from the middle upper pennsylvanian strawn canyon and cisco groups in archer and young counties on the eastern shelf of the midland basin northwest and downdip from the outcrop yielded mean random vitrinite reflectance r o values between about 0 4 and 0 8 this range of r o values indicates rank from subbituminous c to high volatile a bituminous in the shallow subsurface which may be sufficient for early thermogenic gas generation near surface 100 m 300 ft core and outcrop samples of coal from areas of historical underground coal mining in the region yielded similar r o values of 0 5 to 0 8 carbonaceous shale core samples of lower pennsylvanian strata lower atoka group from two deeper wells samples from 1650 m 5400 ft in jack and western wise counties in the western part of the fort worth basin yielded higher r o values of about 1 0 pyrolysis and petrographic data for the lower atoka samples indicate mixed type ii type iii organic matter suggesting generated hydrocarbons may be both gas and oil prone in all other samples organic material is dominated by type iii organic matter vitrinite indicating that generated hydrocarbons should be gas prone individual coal beds are thin at outcrop 1 m 3 3 ft laterally discontinuous and moderately high in ash yield and sulfur content a possible analog for coalbed gas potential in the pennsylvanian section of north central texas occurs on the northeast oklahoma shelf and in the cherokee basin of southeastern kansas where contemporaneous gas producing coal beds are similar in thickness quality and rank

interpreting data dispersion and inverted dates in apatite u th he and fission track datasets an example from the us midcontinent abstract new apatite u th he ahe and apatite fission track aft data were acquired for cratonic basement samples from an 80 m span of drillcore in northeastern kansas the short depth interval over which the samples were collected indicates that they should have undergone thermal histories that would be indistinguishable using low temperature thermochronometry techniques individual ahe dates from four samples range from 99 to 464 ma three samples yield dates 300 ma higher eu 39 113 ppm and substantial data dispersion uncorrelated with eu and grain size these apatites have strong and variable eu zonation aft dates for five samples range from 242 to 291 ma the sample with the highest eu apatites and oldest ahe dates yields the youngest aft results these results are inverted with ahe dates distinctly older than the corresponding aft date we explore both the causes of data dispersion and the overall compatibility of this cratonic dataset we find that geologically reasonable thermal histories can 1 explain the distribution of the moderate eu ahe data when accounting for the influence of radiation damage grain size and eu zonation on apatite he diffusivity 2 reproduce the observed dispersion in the high eu ahe data when using a viable range of eu zonation and grain size and 3 explain the aft data for the same samples the ahe and aft data are mutually consistent and viable thermal histories successfully predict the observed pattern of older ahe than aft dates for the high eu apatites together these results suggest that appropriately accounting for the known controls on apatite he diffusivity can explain the observed dispersion and inverted ahe and aft results in some thermochronometry datasets a range of ahe dates should be especially common in cratonic data because small differences in apatite he diffusivity are amplified by the thermal histories that typify cratonic settings we use these results to develop some guidelines for interpreting dispersed ahe datasets first date eu and date grain size correlations should be evaluated and if these patterns occur they can be used to better resolve the thermal history second for samples that yield inexplicably large dispersion of ahe dates uncorrelated with eu and crystal size the appropriate strategy is either to reject these samples from the suite used for thermal history interpretation or to acquire additional data to help decipher the significance of the age distribution

late paleozoic subsidence and burial history of the fort worth basin abstract the fort worth basin in northcentral texas is a major shale gas producer yet its subsidence history and relationship to the ouachita fold thrust belt have not been well understood we studied the depositional patterns of the basin during the late paleozoic by correlating well logs and constructing structure and isopach maps we then modeled the one dimensional 1 d and two dimensional subsidence history of the basin and constrained its relationship to the ouachita orogen because the super middle pennsylvanian strata were largely eroded in the region adding uncertainty to the subsidence reconstruction we used petromod 1 d to conduct thermal maturation modeling to constrain the post middle pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles our results of depositional patterns show that the tectonic uplift of the muenster uplift to the northeast of the basin influenced subsidence as early as the middle mississippian and the ouachita orogen became the primary tectonic load by the late middle pennsylvanian when the depocenter shifted to the east our results show that the basin experienced 3 7 5 2 km 12 100 17 100 ft of burial during the pennsylvanian and the burial depth deepens toward the east we attributed the causes of deep pennsylvanian burial and its spatial variation to flexural subsidence that continued into the late pennsylvanian in response to the growth of the ouachita orogen and southeastward suturing of laurentia and gondwana the modeling results also suggest that the mississippian barnett shale reached the gas maturation window during the middle late pennsylvanian

oil and gas geochemistry and petroleum systems of the fort worth basin detailed biomarker and light hydrocarbon geochemistry confirm that the marine mississippian barnett shale is the primary source rock for petroleum in the fort worth basin north central texas although contributions from other sources are possible biomarker data indicate that the main oil generating barnett shale facies is marine and was deposited under dysoxic strong upwelling normal salinity conditions the analysis of two outcrop samples and cuttings from seven wells indicates variability in the barnett shale organic facies and a possibility of other oil subfamilies being present light hydrocarbon analyses reveal significant terrigenous sourced condensate input to some reservoirs resulting in terrigenous and mixed marine terrigenous light hydrocarbon signatures for many oils the light hydrocarbon data suggest a secondary condensate generating source facies containing terrigenous or mixed terrigenous marine organic matter this indication of a secondary source rock that is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to define petroleum source rocks gases in the fort worth basin are thermogenic in origin and appear to be cogenerated with oil from the barnett shale although some gas may also originate by oil cracking isotope data indicate minor contribution of biogenic gas except for reservoirs in the pennsylvanian bend group which contain gases spanning the complete range of observed maturities the gases appear to be stratigraphically segregated younger reservoirs contain less mature gas and older reservoirs contain more mature gas we cannot rule out the possibility that other source units within the fort worth basin such as the smithwick shale are locally important petroleum sources

what controls the location of ice streams ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood this paper reviews and discusses one very important aspect what controls their location in an ice sheet seven potential controls on ice stream location are identified from the literature topographic focusing topographic steps macro scale bed roughness calving margins subglacial geology geothermal heat flux and subglacial meltwater routing for each control the theoretical basis for its link to rapid ice flow is introduced followed by discussion of the evidence of its influence on the location of both contemporary and palaeo ice streams based on this new synthesis topographic focusing subglacial geology meltwater routing and calving margins appear to be most commonly associated with fast ice flow it is clear that rather than a single control however there exist a number of potential controls of varying influence we propose a hierarchy of factors with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location and where present beneath an ice sheet are very likely to be associated with fast flow those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls in such a hierarchy topographic focusing in the presence of a calving margin is the primary control in the absence of this ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology in the absence of these bed roughness geothermal heat flux and topographic steps may promote ice streaming significantly the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics ice streams governed by the presence of meltwater routing and or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time scales compared to controls that vary over longer time scales e g geothermal heat flux subglacial roughness geology and topography identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity