hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

paleogene igneous underplating and subsidence anomalies in the rockall faeroe shetland area abstract crustal thickening caused by igneous underplating related to continental break up in the northeast atlantic has been invoked as the cause of the apparent lack of thermal subsidence seen in many mesozoic basins in the northwest european area in this study we examine a set of commercial wells in the rockall faeroe shetland region and calculate tectonic subsidence histories the rate of post rift basement subsidence during the cretaceous cenozoic determined from backstripping calculations always implies a higher degree of extension than the total basement subsidence assuming uniform extension furthermore the size of this discrepancy increases northward along the rockall trough and westwards in the northern north sea towards scotland we interpret this depth anomaly to reflect the effect of igneous underplating rethickening the crust following mesozoic and cenozoic extension late paleocene extension is identified in subsidence reconstructions throughout the west of britain area and is greatest in the rockall trough assuming that the underplating is gabbroic in composition the subsidence depth anomalies indicate a regional pattern of 2 3 km underplated between the erris trough and porcupine basin increasing to 4 5 km in the west lewis basin and hebrides shelf area in the faeroe shetland basin underplating is 1 0 1 5 km in the east and increases to 4 5 km in the west the data support the hypothesis of a major asthenospheric thermal anomaly centered under the faeroes in the paleocene dynamic uplift of the region is often difficult to identify but is locally seen to develop rapidly around 63 ma implying rapid emplacement of the mantle anomaly the intrusion of gabbroic bodies to the base of the crust in the late paleocene may have had an impact on the thermal maturation of petroleum source rocks in the region however thermal maturity indicators show smaller peak heatflows than predicted from simple thermal models suggesting that underplating was a gradual process achieved through the accretion of several small rapidly cooled bodies

chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

continental flood basalts episodic magmatism above long lived hotspots abstract the eruption of continental flood basalt cfb may reflect episodic magmatism above long lived mantle plumes the iceland and yellowstone hotspots have generated successive cfb provinces large intrusive complexes anomalous uplift basin formation and rifting events and linear volcanic chains dating back 120 and 70 ma respectively amagmatic intervals occurred 1 when ascent of plumes to shallow levels was impeded by impenetrable lithosphere resulting in sub lithospheric ponding of plume mantle and 2 in response to dispersion by subducting oceanic lithosphere in convergent margin settings by comparison with the plume eruptive potential of typical oceanic hotspots it is apparent that preservation of only a small portion of plume mantle ponded during an amagmatic interval is necessary to account for large volume of cfbs thermal erosion lithospheric attenuation translation of ponded hotspot mantle to the base of thinner penetrable lithosphere and passage of plume mantle through slab windows in subducting oceanic lithosphere led to subsequent breakthrough and eruption of cfb since both mantle plume and plate tectonic processes have been operating since the archean it seems likely that the migration of continents over hotspots with attendant magmatic and tectonic consequences is a common occurrence in the geological record

glacial lake musselshell late wisconsin slackwater on the laurentide ice margin in central montana usa abstract cosmogenic surface exposure ages of glacial boulders deposited in ice marginal lake musselshell suggest that the lake existed between 20 and 11 5 ka during the late wisconsin glacial stage mis 2 rather than during the late illinoian stage mis 6 as traditionally thought the altitude of the highest ice rafted boulders and the lowest passes on the modern divide indicate that glacial lake water in the musselshell river basin reached at least 920 930 m above sea level and generally remained below 940 m exposures of rhythmically bedded silt and fine sand indicate that lake musselshell is best described as a slackwater system in which the ice dammed missouri and musselshell rivers rose and fell progressively throughout the existence of the lake rather than establishing a lake surface with a stable elevation the absence of varves deltas and shorelines also implies an unstable lake the changing volume of the lake implies that the laurentide ice sheet was not stable at its southernmost position in central montana a continuous sequence of alternating slackwater lake sediment and lacustrine sheetflood deposits indicates that at least three advances of the laurentide ice sheet occurred in central montana between 20 and 11 5 ka between each advance it appears that lake musselshell drained to the north and formed two outlet channels that are now occupied by extremely underfit streams a third outlet formed when the water in lake musselshell fully breached the larb hills resulting in the final drainage of the lake the channel through the larb hills is now occupied by the missouri river implying that the present missouri river channel east of the musselshell river confluence was not created until the late wisconsin possibly as late as 11 5 ka

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

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

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

unroofing of the himalayas a view from apatite fission track analysis of bengal fan sediments apatite fission track analysis of early miocene to quaternary distal bengal fan sediments from odp leg 116 provides a unique perspective on denudation processes associated with the tertiary himalayan orogeny these sediments yield pooled apatite fission track ages that are 0 to 10 m y older than depositional ages mean track lengths range from 13 2 to 14 8 mu m these young ages and long track lengths indicate that rocks in the upper crust of bengal fan source areas cooled rapidly ge 15 degrees c m y implying denudation rates of several hundreds of meters per million years these observations support the hypothesis that nearly continuous unroofing of a large source area similar to the dimensions of the present himalayan and southern tibetan plateau deformation front and an efficient transport system similar to the present himalaya ganges brahmaputra system supplied sediment to the bengal fan since the early miocene

the pattern and variability of antarctic sea ice drift in the indian ocean and western pacific sectors sea ice drift physically redistributes pack ice and changes ice extent concentration and through deformation the ice thickness distribution in this paper data are presented from 39 satellite tracked buoys deployed during various seasons from 1985 to 1996 in the sea ice of the southern ocean off east antarctica between 20 and 160 e longitude the dominant features of the ice motion in the region are a westward drift parallel to the bathymetry near the antarctic continent a cyclonic circulation cell in prydz bay and eastward drift of the ice to the north of the zonal shear zone the oceanic circulation along the coast is generally barotropic and the ice drift is well correlated with bottom topography northward outflows the locations of which are determined by both bottom topography and the seasonally varying position of the zonal shear zone allow the discharge of sea ice from the westward drift in the south into the northerly belt of eastward flow but with considerable variability in the net northward ice transport the ice translation monitored by the buoys is used to derive the spatial pattern of the ice velocity field the daily average ice drift speed in the westward flow is 0 23 m s 1 19 8 km d 1 with considerable spatial and temporal variability and in the eastward flow the average is 0 17 m s 1 15 1 km d 1 seasonal and interannual ice drift variabilities are analyzed the results are compared with satellite data of sea ice extent and concentration over the same time as well as with hydrographic observation of the position of the antarctic divergence

the erosional and uplift history of ne atlantic passive margins constraints on a passing plume new apatite fission track analyses from nw britain indicate that a maximum of 2 5 km of erosion has occurred there during the cenozoic similar to values for se greenland and the east greenland coast north of scoresby sund the erosion may have been facilitated by magmatic underplating during break up however at kangerdlugssuaq east greenland 4 6 km of erosion is measured since 45 ma lower mid eocene marine sedimentary rocks overlying the lavas on the blosseville coast indicate that magmatic underplating on the central greenland coast substantially post dated flood volcanism and break up behaviour not predicted by simple plume rift models subsidence reconstructions of the hebrides shelf and the east and west greenland coasts show that rapid dynamic uplift was effectively synchronous at 63 ma and preceded volcanism by c 400 m is compatible with a mantle temperature anomaly of c 100 c these data suggest very rapid lateral flow of the impacting iceland plume head the predicted crossing of the plume by the east greenland coast in the mid late eocene would account for post rift magmatic underplating and dynamic support on the greenland but not the european side of the north atlantic basin

low temperature thermochronology of the southern east greenland continental margin evidence from apatite u th he and fission track analysis and implications for intermethod calibration abstract the southern coast of east greenland is characterized by high topographic relief and deep fjords but the evolution of the landscape and the low temperature thermochronology of the region is not well understood here we present apatite fission track and u th he ages that suggest several important features of the long term geomorphic history of this region but which also illustrate an important discrepancy in the thermal histories derived from each technique apatite from bedrock of the southern coast of east greenland between 62 n and 67 n has fission track ages ranging from 60 to 840 ma and u th he ages ranging from 21 to 250 ma the ages generally increase with elevation and distance from the coast and fission track analyses show significant differences in thermal histories for the different regions in the kangertittivatsiaq area c 66 67 n apatite fission track data and models suggest two separate periods of slow cooling prior to c 200 ma and between c 160 ma and the late cenozoic more recently than c 20 ma each of which was followed by a period of rapid cooling and inferred exhumation apatite he data in the kangertittivatsiaq region including crystal size age correlations in low elevation samples are most simply interpreted as recording an incision event of at least 1 5 km later than 20 ma near the coast this may have been caused by glacial erosion the u th he data also indicate an earlier phase of rapid exhumation at c 250 ma in the skjoldungen kap mosting area c 62 64 n approximately 200 km south of the kangertittivatsiaq apatite fission track data suggest slow exhumation from c 200 ma into the neogene followed by fast exhumation the similarity of fission track ages 200 ma at sea level in the fjords in the skjoldungen area c 62 64 n do not suggest tilting in the hinterland related to the breakup of the east greenland continental margin furthermore the cenozoic fission track ages and modeling fission track data suggest that pre breakup basins may have covered the outer coast despite the broadly similar topographic implications of the fission track and u th he data the thermal histories derived from these systems are inconsistent fission track data require thermal histories that predict he ages younger than observed and he data require thermal histories that predict fission track ages older than observed similar discrepancies have also been observed in other settings characterized by long term low temperature thermal histories and may reflect changes in annealing or diffusion behavior or both that either develop or become more apparent in such cases

post orogenic tectonic and thermal history of a rifted continental margin the scoresby sund area east greenland abstract fission track ft analysis of zircon sphene and apatite from crystalline basement in the scoresby sund area east greenland and ft modelling reveal information about the cooling and uplift paths for individual tectonic blocks since the caledonian orogeneny up to c 325 ma the area cooled and behaved as a single unit however it subsequently broke into several fault bounded blocks each block moving independently of adjacent blocks net uplift since 325 ma was of a similar magnitude 8 km assuming a geothermal gradient of 30 c km along the crest of the devonian fault zone for the southern blocks but about 9 km for the stauning alper and these blocks were probably tilted away from the rifted continental margin the relief evolution accompanying uplift in the caledonian basement is closely related to mesozoic basin formation subsidence and infilling in the area ft modelling reveals fast cooling from c 70 c to surface temperatures during the last 55 m y probably as a result of tertiary rifting and magmatic activity in the area

thermal history of the eastern gulf of suez ii reconstruction from apatite fission track and k feldspar measurements abstract apatite fission track aft measurements from miocene graben fill sediments of the eastern gulf of suez in three deep boreholes belayim 113 m 2 6 ras garra m 1 and alma 2 yield a wide range of ages from 125 to 320 ma and mean track lengths 10 1 12 7 m this range is similar to that recorded from precambrian crystalline basement flanking the graben which is the major source for the rift fill since the aft ages exceed the age of the host strata which are presently at their highest post depositional temperatures the rift related thermal regime is one of only moderate heating downhole aft data in the belayim 113 m 2 6 borehole attest to a higher rift temperature than in the ras garra m 1 borehole some 85 km to the south where little or no thermal overprinting is evident these findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area despite the higher syn rift thermal regime indicated for belayim 113 m 2 6 apatite provenance ages in ras garra m 1 are considerably younger thus the aft data in the ras garra m 1 area do not record significant rift related thermal effects but rather they mainly retain a pre rift provenance signature which reflects the order and depth of erosion at the uplifted flanks the younger aft ages in ras garra m 1 despite the weaker rift related thermal effect suggest a deeper level proportional to an additional 5 10 c of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by mid miocene time this result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the gulf of suez and earlier exhumation of the rift flank immediately preceding extension and opening of the gulf of suez the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures 110 c total annealing of apatite but 40 ar 39 ar data and non resetting of zircon ft clocks in sinai 40 ar 39 ar data from granite penetrated in graben drilling at 3 89 km further corroborates pre rift palaeotemperatures

constraints on the evolution of the east greenland margin evidence from detrital apatite in offshore sediments we test a new approach to understanding the tectonic evolution of passive margins by using fission track analysis on detrital apatites from sediments deposited offshore east greenland these apatites have not undergone postdepositional track annealing and therefore reflect provenance the apatites preserve a component of the source rocks thermal history that otherwise may not be retainedwithinthepresent dayoutcrop fission track deriveddenudational histories from samples at ocean drilling program drill sites offshore east greenland at lat 63 n are compared with data fromtheonshoresingertatcomplex previousapatitefission track studies and geomorphic mapping of the east greenland coast have shown that locally up to 6 km of denudation may have occurred implying significant tectonic or magmatic activity starting as much as30m y afterbreakupat56ma incontrast apatitefission track datapresentedhererecord 2kmofcenozoicdenudationinsoutheast greenland probably driven by magmatic underplating at the time of breakup large magnitude postrift denudation of east greenland is restricted to the area around kangerdlugssuaq 68 n thetiming 40 50ma andmagnitudeareinaccordwith revised plume track models suggesting that the iceland plume crossed the margin here during the late eocene

the evolution of the east greenland margin as revealed from fission track studies abstract a comprehensive dataset of fission track analyses mainly of apatite covers the east greenland margin between ca 73 n geographical society island and ca 63 n skjoldungen it consists of data from archaean and caledonian basement post caledonian sedimentary rocks and magmatic rocks the studied areas represent different aspects of the east greenland evolution we report new apatite fission track ft analyses from the east greenland gneissic basement south of scoresby sund to skjoldungen and put them into context with existing data a regional pattern along the east greenland continental margin south of 73 n of deeply eroded shorelines and highstanding slowly eroding hinterland with remnants of old peneplain surfaces is overprinted in the central kangerlussuaq area by tertiary magmatic activity to the northeast the thermal evolution is dominated by a domal structure shown by apatite fission track ft ages to have cooled in the neogene this pattern may result from rapid erosion of the locally uplifted basaltic pile in the area and heating caused by late sill intrusions possibly enhanced on a regional scale by climatic deterioration in the hinterland to the south and west of kangerlussuaq the data restrict the basalt cover to

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

detrital geochronology of unroofing magmatic complexes and the slow erosion of oligocene volcanoes in the alps abstract tectonic reconstructions and quantitative models of landscape evolution are increasingly based on detailed analysis of detrital systems since the definition of closure temperature in the 1960s mineral ages of low temperature geochronometers are traditionally interpreted as the result of cooling induced by erosion whose rate is a simple unique function of age patterns such an approach can lead to infer paradoxically high erosion rates that conflict with compelling geological evidence from sediment thickness in basins this indicates that tectonic and landscape models that solely interpret mineral ages as due to cooling during exhumation may not be valid here we propose a new approach that takes into account the effects of both crystallization and exhumational cooling on geochronometers from u pb on zircon to fission tracks on apatite we first model the mechanical erosion of an unroofing magmatic complex and the resulting accumulation and burial of the eroded units in reverse order in the basin detrital mineral ages follow a regular pattern downsection some mineral ages such as e g u pb ages of zircons cluster around the magmatic age i e the crystallization of the magma its value is constant along the stratigraphic column in the sedimentary basin we refer to this behavior as stationary age peak some other mineral ages such as e g apatite fission track ages are often younger than the magmatic age when they vary smoothly with depth they define a moving age peak which is the only possible effect of undisturbed cooling during overburden removal and can therefore be used to calculate an erosion rate the predictions of our model were tested in detail on the extremely well studied bregaglia bergell orogenic pluton in the alps and on the sedimentary succession derived from its erosion the gonfolite group the consistency between predicted and observed age patterns validates the model our results resolve a long standing paradox in quantitative modelling of erosion sedimentation namely the scarcity of sediment during apparently fast erosion starved basins are the observational baseline and modelling must be tuned to include a correct analysis of detrital mineral geochronology in order to reconcile perceived discrepancies between stratigraphical and geochronological information in addition our data demonstrate that volcanoes were active on top of the growing oligocene alps this study illustrates rigorous criteria for detrital mineral geochronology that are applicable to any geological setting including magmatic arcs and collision orogens and provides fundamental interpretive keys to solve complex puzzles and apparent paradoxes in geological reconstructions