on the long distance transport of ferrar magmas the distribution and geochemical relationships of the early jurassic ferrar large igneous province lip are examined and it is concluded that they support the lateral flow model for the emplacement of the province with a source along the strongly magmatic early jurassic antarctica africa rifted margin published data and new analyses from the pensacola range are used to show that the dominant magma type in the ferrar the mount fazio chemical type mfct occurs in the theron mountains shackleton range whichaway nunataks pensacola mountains all antarctica and south africa as well as well known outcrops in victoria land antarctica se australia and new zealand chemical compositions are shown to be somewhat varied but similar enough for them to be considered as representing closely related magmas examination of geochemical trends with distance from the interpreted magma source indicates that mg and mgo abundances decline with distance travelled and it is argued that this is consistent with the lateral flow model the scarab peak chemical type spct which occurs as sills in the theron mountains and whichaway nunataks and as lavas in victoria land is geochemically very homogeneous despite this mg mgo ti y and ti zr all fall with distance from the interpreted source consistent with fractional crystallization occurring during the lateral flow of the magmas flow took place in dykes or more likely sills no feeder dyke swarm has been identified the distances flowed at least 4100 km for mfct and 3700 km for spct are the longest interpreted lateral magma flows on earth

correlations between the classic parana and cape karoo sequences of south america and southern africa and their basin infills flanking the gondwanides du toit revisited

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

african provenance for the metasediments and metaigneous rocks of the cyclades aegean sea greece u pb geochronology on detrital and inherited zircons from metasediments as well as gneissic and metaigneous basement rocks of the cyclades aegean sea greece defines several periods of crustal growth prior to the carboniferous these ages are consistent with cycladic rocks being derived from the northern margin of gondwana there is a distinct mesoproterozoic age gap evident in zircons from the 25 samples considered this is characteristic of the west african craton and we suggest it will also prove characteristic of the aegean region this mesoproterozoic age gap may be used as a tool in terrane reconstruction to distinguish crustal material derived from different parts of gondwana

evidence for cambrian deformation in the ellsworth whitmore mountains terrane antarctica stratigraphic and tectonic implications the ellsworth whitmore mountains terrane is a large geologically and geophysically defined crustal block that lies between the transantarctic mountains and west antarctica the cambrian position of the terrane is controversial with many workers placing it between east antarctica and southern africa and distant from cambrian orogenic belts we present structural and stratigraphic evidence for cambrian deformation in the heritage range ellsworth mountains from our revised stratigraphy and structural history of the heritage range we propose that the ellsworth whitmore mountains block was located within the belt of pan african deformation within the late cambrian continental arc and was part of a collage of allochthonous terranes that included the queen maud terrane and probably the bowers terrane of antarctica these terranes were situated outboard of coats land in the cambrian and were subsequently translated and accreted to east antarctica probably during early paleozoic time

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

coats land crustal block east antarctica a tectonic tracer for laurentia undeformed rhyolite and granophyre in the coats land crustal block of east antarctica dated as 1112 4 ma are identical in age to both the umkondo large igneous province lip of the kalahari craton southern africa and the early keweenawan lip of laurentia north america although marine and satellite data demonstrate that coats land was close to kalahari within the gondwana supercontinent the coats land rocks yield pb isotope compositions strikingly distinct from those of the umkondo province yet indistinguishable from rocks of the keweenawan province the anorogenic red bluff granitic suite along the present day southern laurentian margin in the franklin mountains texas usa is of comparable age general rock type and pb isotope composition to rocks of coats land and may provide a piercing point for a coats land laurentia link paleomagnetic poles permit the coats land block to be close to this part of laurentia ca 1100 ma and allow juxtaposition of kalahari and southern laurentia ca 1000 ma the coats land crustal block may therefore be a critical tectonic tracer for placing laurentia within late mesoproterozoic and neoproterozoic paleogeographic reconstructions if this hypothesis is correct laurentia collided with the kalahari craton along antarctica s maud orogen which would represent a continuation of the ca 1000 ma grenville orogen of eastern and southern laurentia

rapid magma emplacement in the karoo large igneous province abstract understanding the dynamics of continental large igneous provinces lips relies on precise dating of basaltic rocks lip research has traditionally focused on dating lavas often neglecting the volumetrically important sill intrusions in underlying sedimentary basins here we present u pb zircon and baddeleyite ages for fourteen new samples of karoo lip sills and dykes spaced by as much as 1100 km across the half million square kilometer karoo basin the samples yield remarkably coherent ages ranging from 183 0 0 5 to 182 3 0 6 myr probability modeling indicates that basin scale emplacement took place within an interval of about 0 47 myrs less than 0 90 myrs with 95 confidence and could even have represented a single magma emplacement event combining the new ages with the estimated volume of sills in the karoo basin gives an emplacement rate of 0 78 km3 yr which is higher than previous estimates upper crustal magma storage may account for these high rates the results challenge the view that melt emplacement in a sedimentary basin is a prolonged process support a scenario of pulsating catastrophic events within a narrow time frame and strengthens the hypothesis linking lips and sill emplacement to global environmental crises

upper permian magnetic stratigraphy of the lower beaufort group karoo basin abstract we carried out a magnetostratigraphic and geochronological study of late permian sediments in the karoo basin of the western cape province south africa a continuous 700 m thick section of deltaic sediments of the upper waterford formation uppermost ecca group and the fluvial sediments of the abrahamskraal formation lowermost beaufort group were sampled at the meter scale u pb dating of zircons from interbedded volcanic ash beds by ion microprobe shrimp provided absolute age constraints on the age of the sedimentary rocks paleomagnetic analysis reveals a partial overprint of the natural remanent magnetization nrm that is tentatively ascribed to the emplacement of the karoo large igneous province in the western cape region during the middle jurassic a stable component of the nrm was found at temperatures higher than 450 c and was interpreted as a characteristic remanent magnetization chrm acquired during deposition supported by a positive reversals test for this dual polarity chrm the virtual geomagnetic pole position for the waterford and abrahamskraal formations computed from the average chrm direction is in general agreement with the late permian directions for stable gondwana a significantly different average inclination and thus paleomagnetic pole position is obtained by correcting the inclination shallowing error by the elongation inclination method tauxe and kent 2004 the presence of both normal and reversed polarity zones indicate deposition after the end of the kiaman superchron moreover the polarity sequence is in good agreement with the illawarra sequence of steiner 2006 our results indicate a capitanian late guadalupian age for the abrahamskraal fm in agreement with the late permian age based on presence of glossopteris flora and dicynodont fauna traditionally assigned to the fluvial lacustrine sediments of the beaufort group however the u pb zircon ages of ca 264 268 ma suggest an age of 269 ma for the top of the kiaman superchron

stratigraphy and sedimentology of the permian of the falkland islands lithostratigraphic and palaeoenvironmental links with south africa comparison of the facies ichnology and palaeocurrent patterns of the permian of the falkland islands and the dwyka ecca and beaufort part groups in south africa supports the hypothesis that the falkland islands lay east of south africa in a rotated position prior to continental break up key sections in the falklands nw lafonia and the eastern cape ecca pass would have lain about 200 km apart in permian time in the absence of accurate chonostratigraphic and biostratigraphic information from the permian of the falkland islands emphasis is placed on lithostratigraphic comparison the permian successions in both areas pass from diamictite through basin floor sediments including organic rich shale rhythmites and turbidites mud dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments points of close lithostratigraphic comparison are 1 the diamictites have identical appearance facies and compatible palaeocurrents 2 glacial sections are overlain by organic rich black shales 3 k bentonites occur in basin floor successions 4 presence of basin floor turbidite sandstone units with interbedded rhythmite units and a distinctive umfolozia undichna ichnofauna 5 coarsening up delta front sequences are overlain by fining up channel sandstone facies 6 there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc the permian successions of the two areas form parts of the fill of the eastern end of the main karoo basin and were subject to the same influences of tectonic development sediment derivation eustatic variation and climatic change

late palaeozoic to mesozoic structural evolution of the falkland islands a displaced segment of the cape fold belt the falkland islands lie on a displaced crustal block presently forming part of the south american plate the islands possess two roughly orthogonal structural grains the relative chronology of which is here unequivocally established for the first time d1 structures form a southerly verging permo triassic age fold belt striking e w to wnw ese which represents a displaced segment of the gondwanian orogenic belt the strike swing from e w to wnw ese is coincident with a decrease in deformation intensity toward the west on west falkland d1 folds are superimposed by the early mesozoic d2 ne sw trending hornby mountains anticline producing localized kilometre scale type i and iii fold interference patterns the hornby mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre existing ne sw basement fault if the falkland islands are rotated 180 the structure of the d1 fold belt and tectono sedimentary features of the permian age lafonian supergroup display a remarkable correlation with the structural style and tectonic evolution of the eastern cape fold belt of south africa our data therefore support a 180 tectonic rotation of the falkland islands and reaffirm their pre gondwana break up position adjacent to the southeast coast of south africa in such a reconstruction the falkland islands constrain the easterly extension and eventual lateral termination of the cape fold belt

stratigraphic evidence for the ross orogeny in the ellsworth mountains west antarctica implication for the evolution of the paleo pacific margin of gondwana early paleozoic crustal deformation has been recognized along the paleo pacific margin of gondwana including southern africa saldanian tectonism and the transantarctic mountains of east antarctica ross orogeny previously no stratigraphic evidence for the ross orogeny had been recognized within the ellsworth mountains of west antarctica believed to have been contiguous with the transantarctic mountains prior to the mesozoic breakup of gondwana a disconformity between middle to upper cambrian heritage group and upper cambrian to devonian crashsite group strata indicates that the ross orogeny did affect the ellsworth mountains providing additional evidence that these rocks were once part of east antarctica fluvial conglomerates overlying platform carbonates of the upper heritage group were derived in part from plutonic volcanic and metamorphic terranes in east antarctica elsewhere in the ellsworth mountains the disconformity is expressed by an abrupt change from shallow carbonate platform deposits to siliciclastic shelf deposits similarities in lower paleozoic stratigraphy between the ellsworth mountains parts of the pensacola mountains east antarctica and eastern south africa imply that these regions were inboard of the zone of deformation that affected the proto pacific margin of gondwana during the ross orogeny and saldanian tectonism

geochemical evidence for lacustrine microbial blooms in the vast permian main karoo parana falkland islands and huab basins of southwestern gondwana early permian black pyritic shale units i e the whitehill irati and huab formations and black rock member in southwestern gondwana basins have elevated 13c values of total organic carbon up to 17 pdb hydrogen indices between 300 and 1000 mg co2 g c and oxygen indices 30 mg co2 g c these data suggest a manifestation of a microbial bloom event values for 13c between 26 and 22 pdb hydrogen 100 mg co2 g c and oxygen indices of total organic carbon above and below these rock units are typical for palaeozoic terrestrial type iii organic matter carbon 13 isotope stratigraphy concur with palaeontological evidence that these formations are stratigraphical equivalents and therefore that the microbial blooms covered a vast area 5 million km2 possibly earth s most expansive microbial bloom the palaeoenvironment of the rock units preceding and including the black shales was most likely lacustrine with fresh to brackish water as shown by stable isotope c o and s evidence from organic matter carbonates and pyrite the implication is that a broad tectonic belt which formed as a result of compressional tectonics during the late carboniferous to early permian from the palaeo west must have been sufficiently well developed to cut off any contact with seawater earlier than predicted late permian from previous stratigraphic and sedimentological studies stable isotope data supported by mineralogical evidence indicate that conditions particularly in the upper portions of the lacustrine oil shales were brackish and locally may have been evaporitic we envisage that the microbial blooms possibly botryococcus peaked in the upper half of the black shale units conditions rapidly became unfavourable for microbial blooms across the entire basin in the overlying formations manifested by a change from oil shales type i organic matter to organic poor type iii shales this may possibly have been a consequence of tectonic activity and interrelated causes stable isotope data of carbonates in the overlying sediments indicate that in the palaeo north the basin became evaporitic drying up and in the palaeo southern part of the basin high latitude altitude water flooded the basin and so brought to a close the microbial blooms

chapter 8 late dresbachian idamean and other trilobite faunas from the heritage range ellsworth mountains west antarctica

the gondwana plate margin in the weddell sea sector zircon geochronology of upper paleozoic mainly permian strata from the ellsworth mountains and eastern ellsworth land antarctica abstract zircon geochronology has been applied to upper paleozoic mainly permian strata from the ellsworth mountains and from enigmatic outcrops in eastern ellsworth land to aid in constraining their locations in the weddell sea sector of the gondwana plate margin detrital zircons from two polarstar formation ellsworth mountains sandstones yielded maximum depositional ages of 270 ma and 260 ma detrital zircons from permian beds at erehwon nunatak and beds of uncertain age at fitzgerald bluffs both in eastern ellsworth land gave maximum depositional ages of 264 ma and 525 ma respectively two polarstar formation tuffs gave ages of 258 0 2 1 ma and 262 6 2 2 ma the polarstar formation probably spans the entire permian the quartzite at fitzgerald bluffs is comparable in composition and detrital zircon signature to devonian sandstones in both the ellsworth and transantarctic mountains evaluation of the polarstar formation with respect to permian strata in the southern karoo and falkland islands suggests deposition in a separate and distinct basin which was related to that in the transantarctic mountains a separate depositional basin forms an additional constraint on the location of the ellsworth whitmore block adjacent to the coats land pensacola mountains region the eastern ellsworth land outcrops are inferred to document a small continental block located proximal to the ellsworth whitmore mountains block during gondwana time

high precision temporal calibration of late permian vertebrate biostratigraphy u pb zircon constraints from the karoo supergroup south africa therapsid and other tetrapod fossils from the south african karoo supergroup provide the most detailed and best studied terrestrial vertebrate record of the middle and late permian the resulting biostratigraphic scheme has global applicability establishing a temporal framework for these faunas has proven difficult magnetostratigraphy has been hampered by a jurassic overprint and intercorrelation with permian marine sequences has been equivocal here we report u pb zircon isotope dilution thermal ionization mass spectrometry id tims dates for five volcanic ashes interbedded with fossils from the pristerognathus tropidostoma and cistecephalus vertebrate biozones of the beaufort group this temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution and patterns of basin evolution our results identify no correlative vertebrate extinctions in the karoo supergroup to the marine end guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction related to the emeishan large igneous province

cambrian palaeomagnetic data confirm a natal embayment location for the ellsworth whitmore mountains antarctica in gondwana reconstructions summary the ellsworth whitmore mountains ewm are one of five terranes that form west antarctica constraining the positions of these terranes in pre break up gondwana is crucial to understanding the history of the palaeo pacific gondwana margin we report the results of a detailed palaeomagnetic investigation of the ewm which comprises some 150 sites in six formations ranging in age from cambrian to permian five of the studied units yield only viscous remnant magnetizations of recent age or unstable natural remanent magnetizations the remaining unit the mid late cambrian frazier ridge formation yielded stable magnetizations at 16 of 35 sites this component passes a fold test at the 95 per cent confidence level indicating that it pre dates permian deformation and we argue that it is of primary origin the resulting palaeopole 9 n 293 e a95 5 1 is in good agreement with two previously published palaeopoles from similarly aged rocks in the ewm collectively these data indicate that the ewm were located in the natal embayment prior to gondwana break up and underwent 90 of anticlockwise rotation during break up all three studies however yield inclinations

age provinces in the antarctic craton evidence from detrital zircons in permian strata from the beardmore glacier region antarctica abstract the antarctic gondwana sequence crops out along the transantarctic mountains permian strata at mt bowers and mt weeks in the central part of the range were mainly derived from the ice covered east antarctic craton for which the geology is inferred from coastal outcrops and the miller range in the transantarctic mountains analysis of detrital zircons from permian sandstones at those two localities demonstrate a slowly changing zircon age probability distribution for four samples from the lower part of the sequence with an abrupt change for the upper buckley formation sandstone the stratigraphically lowest sample from a unit informally designated the mt bowers sandstone is dominated by zircons in the age range 610 540 ma with minor components at 1 0 ga and 2 7 ga upward the pagoda formation sandstone is likewise dominated by zircons of 600 550 ma age together with a significant grenville age cluster 1 3 1 0 ga and a minor 1 5 1 4 ga cluster the fairchild formation sandstone has a preponderance of grains in the range 600 530 ma with a scatter of older ages zircon grains in the lower buckley formation sandstone are principally in the range 575 520 ma with another cluster at 470 450 ma and a scattering of older ages most zircons in the upper buckley formation sandstone are permian in age but with a very small component in the range 650 475 ma and a trace of older ages archean to early mesoproterozoic zircon grains are few in number and may have been recycled from distal outcrops the 1 5 1 4 ga grains suggest an unexposed mesoproterozoic source within the east antarctic craton which could have been linked to laurentian north america the grenville age source is considered to be similarly a sub glacial belt in the interior of east antarctica zircons with a late neoproterozoic to ordovician signature 600 460 ma were derived from a ross orogen foreland sequence and or an early and inland extension of the orogen itself permian zircons in the upper buckley sandstone reflect the development of a magmatic arc on the gondwana pacific margin

u pb ion microprobe zircon geochronology from the basement inliers of eastern graham land antarctic peninsula new geological mapping combined with u pb ion microprobe zircon geochronology on the isolated but locally extensive exposures of crystalline basement inliers of eastern graham land has greatly improved our understanding of the region s early crustal evolution and has allowed a more thorough evaluation of patagonia antarctic peninsula connections prior to gondwana break up at eden glacier diorite gneisses yield early ordovician protolith ages of 487 3 and 485 3 ma and represent the oldest in situ rocks recorded on the antarctic peninsula and indicate a significant spatial extension of famatinian age magmatism of patagonia zircon overgrowths in the early ordovician protoliths and granitic leucosomes developed within them record two phases of permian metamorphism at c 275 and c 257 ma coincident in part with diorite plutonism of the area at 272 2 ma at adie inlet granitic leucosomes from paragneiss have been dated at 276 3 ma and these are in turn cut by 257 3 ma xenolith rich diorite gneiss the diorite intruded during a second phase of deformation which folded the paragneiss leucosomes into tight folds this whole assembly is cut by intensely brecciated megacrystic granodiorite which yielded a 259 3 ma age south of cabinet inlet a very different sequence of events is evident with triassic magmatism at c 236 ma extensive along the joerg peninsula migmatitic leucosomes are dated at c 224 ma and magmatism and deformation events apparently continued to c 209 ma at cape casey our data indicate that the devonian and carboniferous magmatism at target hill considered to represent the classic basement complex of the antarctic peninsula is not representative regionally the target hill crustal block contains a major break along cabinet inlet to the north ordovician and permian protoliths were variably migmatized during two episodes of permian deformation and metamorphism whereas to the south triassic protoliths and triassic metamorphism are encountered supplementary material u pb isotopic data are available at http www geolsoc org uk sup18526

the gondwana connections of northern patagonia a multidisciplinary study u pb sensitive high resolution ion microprobe geochronology hf and o isotopes in zircon sr and nd isotopes in whole rocks as well as major and trace element geochemistry has been carried out on granitoid samples from the area west of valcheta north patagonian massif argentina these confirm the cambrian age of the tardugno granodiorite 528 4 ma and the late permian age of granites in the central part of the yaminue complex 250 ma together with petrological and structural information for the area we consider a previously suggested idea that the cambrian and ordovician granites of northeastern patagonia represent continuation of the pampean and famatinian orogenic belts of the sierras pampeanas respectively our interpretation does not support the hypothesis that patagonia was accreted in late palaeozoic times as a far travelled terrane originating in the central transantarctic mountains and the arguments for and against this idea are reviewed a parautochthonous origin is preferred with no major ocean closure between the north patagonian massif and the sierra de la ventana fold belt supplementary material u pb shrimp analytical data geochemical analyses and sample global positioning system locations are available at www geolsoc org uk sup18722

west antarctica in gondwanaland crustal blocks reconstruction and breakup processes abstract a combined bas usarp west antarctic tectonic project has provided new constraints on the crustal structure and geological evolution of west antarctica and its relationship to the rest of gondwanaland rb sr age dating has confirmed the presence of 1200 1000 m y old precambrian gneisses within west antarctica and aeromagnetic data have defined the extent of this precambrian crustal block paleomagnetic data have constrained the position of four of the five geologically distinctive west antarctic crustal blocks within gondwanaland they can be restored to their original position prior to breakup by 15 25 counter clockwise rotation with little or no relative displacement between the blocks a major period of within plate middle jurassic magmatism associated with an important crustal melting event that is clearly related to the breakup of the supercontinent has also been identified based on new isotopic and geochemical data

permo carboniferous conglomerates in the trinity peninsula group at view point antarctic peninsula sedimentology geochronology and isotope evidence for provenance and tectonic setting in gondwana field observations from the trinity peninsula group at view point on the antarctic peninsula indicate that thick southward younging and overturned clastic sedimentary rocks comprising unusually coarse conglomeratic lenses within a succession of fine grained sandstone mudstone couplets are the deposits of debris and turbidity flows on or at the foot of a submarine slope three detrital zircons from the sandstone mudstone couplets date deposition at 302 3 ma at or shortly after the carboniferous permian boundary conglomerates predominantly consist of quartzite and granite and contain boulders exceeding 500 mm in diameter zircons from granitoid clasts and a silicic volcanic clast yield u pb ages of 466 3 ma 373 5 ma and 487 4 ma respectively and have corresponding average ehft values between 0 3 and 7 6 a quartzite clast conglomerate matrix and sandstone interbedded with the conglomerate units have broadly similar detrital zircon age distributions and hf isotope compositions the clast and detrital zircon ages match well with sources within patagonia however the age of one granite clast and the ehf characteristics of some detrital zircons point to a lesser south africa or ellsworth mountain like contribution and the quartzite and granite dominated composition of the conglomerates is similar to upper palaeozoic diamictites in the ellsworth mountains unlike detrital zircons large conglomerate clasts limit possible transport distance and suggest sedimentation took place on or near the edge of continental crust comparison with other upper palaeozoic to mesozoic sediments in the antarctic peninsula and patagonia including detrital zircon composition and the style of deformation suggests deposition of the trinity peninsula group in an upper plate basin on an active margin rather than a subduction related accretionary setting with slow extension and rifting punctuated by short periods of compression

the pre cenozoic magmatic history of the thurston island crustal block west antarctica new rb sr and k ar geochronological data are presented for the majority of known pre cenozoic outcrops in thurston island the jones mountains and the western eights coast which collectively represent the basement geology of the thurston island crustal block of west antarctica almost all are of calc alkaline igneous or metaigneous rocks and indicate long standing proximity to a magmatic arc the observable history began with late carboniferous 309 5 ma emplacement of mantle derived orthogneiss precursors in eastern thurston island nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200 1400 ma throughout the area a variety of cumulate gabbros was emplaced soon after gneiss formation followed by crust contaminated diorites that have triassic mineral cooling dates of 240 220 ma in the nearby jones mountains the oldest exposed rock is a muscovite bearing granite with an early jurassic age of 198 2 ma its initial 87sr 86sr ratio of 0 710 and ndt values of 5 to 7 indicate either anatexis or at least a high degree of crustal input during magma genesis this belongs to a suite of such granites known throughout the antarctic peninsula and related to earliest rifting of the gondwana supercontinent the subsequent evolution of the thurston island area was dominated by i type magmatism apparently in two major episodes at 152 142 ma late jurassic granites and 125 110 ma early cretaceous bimodal suite most of these magmas had initial 87sr 86sr ratios of 0 705 0 706 and ndt values of 2 to 4 and were derived from slightly enriched mantle or from juvenile lower crust they are thought to signify subduction of pacific ocean floor as in the adjacent parts of west antarctica although the late jurassic episode was of greater intensity in thurston island than elsewhere the cretaceous magmatism was intense and of andean type between 100 and 90 ma volcanism in the jones mountains became predominantly silicic with increasing incorporation of crustal components initial 87sr 86sr ratios of 0 706 0 709 and ndt values of 3 to 6 as subduction related magmatism ceased in this part of the margin

chapter 4 stratigraphy and sedimentology of the whiteout conglomerate an upper paleozoic glacigenic unit ellsworth mountains west antarctica

aeromagnetic data from the southern weddell sea embayment and adjacent areas synthesis and interpretation a new regional aeromagnetic anomaly map covering the southern weddell sea and surrounding areas was produced by combining russian and british magnetic anomaly data acquired between 1973 and 1989 the map is divided into four zones on the basis of characteristic anomaly wavelength antarctic peninsula weddell sea embayment haag nunataks and the western coast of east antarctica over the southern weddell sea and the ronne and filchner ice shelves the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies a broad positive anomaly is clearly visible running w e across the north of the area which is interpreted as representing the continent ocean boundary the most prominent magnetic feature is the elongate explora anomaly which runs parallel to the coast of east antarctica and continues over berkner island terminating at the dufek massif we interpret the source of this anomaly as rift related volcanic rocks probably emplaced around the same time as the dufek intrusion the rift itself is interpreted as running south between the andenes escarpment and east antarctica and terminating south of berkner island russian gravity profiles show that berkner island does not have an associated bouguer anomaly although a large positive anomaly approx 80 mgal is seen to the east of the island over the filchner ice shelf suggesting the presence of a second rift structure gravity modelling across these features shows thinned crust below the ronne and filchner ice shelves indicating that the whole area has undergone significant extension a gravity model across the flanks of berkner island is used to estimate the ratio of unstretched to stretched crustal thickness the resulting value of approximately 2 7 indicates that enough extension has taken place to induce melting supporting the idea that the source of the magnetic anomaly over berkner island is rift related volcanic rocks

mesozoic evolution of west antarctica and the weddell sea basin new paleomagnetic constraints paleomagnetic data from the antarctic peninsula and our recent results from the ellsworth whitmore mountains block suggest that since the middle jurassic these two west antarctic blocks have undergone little relative movement and together have rotated relative to the east antarctic craton new data from lower cretaceous rocks from the thurston island region of west antarctica suggest that on the basis of paleomagnetic constraints the antarctic peninsula ellsworth whitmore mountains and thurston island blocks define a single entity which we call weddellia some motion between these blocks is possible within the limits of the paleomagnetic data between the middle jurassic and early cretaceous weddellia remained attached to west gondwanaland while east antarctica moved southward dextrally relative to weddellia from the early cretaceous to mid cretaceous weddellia rotated clockwise 30 and moved sinistrally approximately 2500 km relative to east antarctica to its present day position we suggest the early to mid cretaceous to be the time of the main if not initial opening of the weddell sea

tectonic history of the ellsworth mountains west antarctica reconciling a gondwana enigma early paleozoic orogenesis has been recognized along the southern african saldanian orogeny and east antarctic ross orogeny sectors of the gondwana paleo pacific margin however the absence of a contemporaneous orogenic event in the ellsworth mountains of west antarctica once a contiguous part of this margin has resulted in their geology being considered enigmatic in this contribution widespread detailed structural studies from all stratigraphic levels of the ellsworth mountains allow their tectonic evolution to be reassessed geochemical stratigraphic and structural data indicate that the middle to upper cambrian heritage group developed in a continental rift basin multiple observations of the crucial end cambrian contact between this rift sequence and the overlying crashsite group reveal the contact to be regionally conformable there are localized outcrop scale unconformities furthermore structural continuity can be demonstrated across this key boundary thus precluding the possibility of an end cambrian orogenic event the entire stratigraphic succession was affected by two post permian phases of deformation d1 structures are locally developed and superimposed by the main dextral transpressive d2 permian triassic gondwanian deformation event d2 is succeeded by an episode of extension orthogonal to orogenic strike recent geochronological data on the early paleozoic evolution of the cape fold belt reveal a strong tectonostratigraphic correlation with the ellsworth mountains indicating that this sector of the gondwana paleo pacific margin was affected by middle to late cambrian rifting a new tectonic model is proposed for the southwestern paleo pacific margin of gondwana that accounts for the presence of rifting along a margin otherwise dominated by active subduction

inland extent of the weddell sea rift imaged by new aerogeophysical data the weddell sea rift was a major focus for jurassic extension and magmatism during the early stages of gondwana break up and underlies the weddell sea embayment which separates east antarctica from a collage of crustal blocks in west antarctica newly collected aerogeophysical data over the catchments of institute and moller ice streams reveal the inland extent of the weddell sea rift against the ellsworth whitmore block and a hitherto unknown major left lateral strike slip boundary between east and west antarctica aeromagnetic and gravity anomalies define the regional subglacial extent of proterozoic basement middle cambrian rift related volcanic rocks jurassic intrusions and sedimentary rocks of inferred post jurassic age 2d and 3d magnetic depth to source estimates were used to help constrain joint magnetic and gravity models for the region the models reveal that proterozoic crust similar to that exposed at haag nunataks extends southeast of the ellsworth mountains to the margin of the coastal basins thick granitic jurassic intrusions are modelled at the transition between the ellsworth whitmore block and the thinner crust of the weddell sea rift and within the pagano shear zone the crust beneath the inland extension of the weddell sea rift is modelled as being either 4 km thinner compared to the adjacent ellsworth whitmore block or as underlain by an up to 8 km thick mafic underplate

a new stratigraphy for the latady basin antarctic peninsula part 2 latady group and basin evolution recent detailed mapping section logging and an improved understanding of the geological evolution of the antarctic peninsula provide a robust framework for an improved lithostratigraphic subdivision of the latady basin eastern ellsworth land within the latady basin we recognize two main groups ellsworth land volcanic group and latady group the focus of this paper is the latady group which is formally subdivided into five formations anderson formation witte formation hauberg mountains formation cape zumberge formation and nordsim formation middle jurassic shallow marine deposits of the anderson formation are overlain by quiet anoxic deposits assigned to the witte formation the start of the late jurassic is marked by the deposition of higher energy deposits of the hauberg mountains formation subdivided into three members long ridge mount hirman and novocin members that reflect varying lithological and environmental characteristics thermal subsidence during the latest jurassic led to deposition of the basinal cape zumberge formation while uplift of an active continental arc along the antarctic peninsula led to deposition of the terrestrial nordsim formation in the latest jurassic to earliest cretaceous the evolution of the latady basin reflects early extension during gondwana break up from the early jurassic to earliest cretaceous and is consistent with a shift in the underlying forces driving extension in the weddell sea area from intracontinental rifting related to a mantle plume to active margin forces in response to subduction

shrimp u pb zircon dating and palynology of bentonitic layers from the permian irati formation parana basin brazil abstract this paper presents the first shrimp zircon age data from bentonitic ash fall layers intercalated with the irati sedimentary rocks as well as palynofossils retrieved from the petrobras six mine parana state southern brazil the permian irati formation is one the most important oil source horizons within the parana basin and consists mainly of siltstones gray claystones as well as organic rich claystones intercalated with limestones zircon morphology based on cathodoluminescence images includes two different populations a dominant population of euhedral prismatic elongate to acicular grains which are most likely related to an ash fall volcanism and a population of rounded to large prismatic grains that were interpreted as detrital grains shrimp analysis performed on the euhedral and prismatic grains revealed an age of ca 278 4 2 2 ma 7 points with 95 confidence interpreted as the crystallization age of the volcanic eruption based on this new dating the irati formation should be placed on the lower permian cisuralian artinskian in age modifying substantially the traditional ages previously attributed to this unit the palynofossils from this level include key species of the lueckisporites virkkiae zone such as l virkkiae l densicorpus l stenotaeniatus weylandites lucifer alisporites splendens typical species of this zone also occur in african and other south american argentina permian strata allowing stratigraphical correlations the origin of the ash falls is not clearly defined it may be related either to calc alkaline arc magmatism associated to the sanrafaelica orogeny 275 and 250 ma or to bimodal volcanic events associated to an early intracontinental rift developed within southwestern gondwana at ca 278 my the dating of this unit is significative to calibrate biostratigraphic schemes along the parana basin as well as equivalent zones in gondwana areas mainly in its occidental portion

palaeomagnetic evidence for a displaced terrain in western antartica an outstanding problem in antarctic tectonics is the relationship between the east antarctic craton and the late palaeozoic cenozoic orogenic belts that comprise western antarctica1 antarctic peninsula ellsworth land eights coast and marie byrd land see fig 1a related problems are the place of western antarctica in reconstructions of gondwanaland and the evolution of the pacific margin of gondwanaland we present here palaeomagnetic data derived from folded cambrian argillites that crop out in the ellsworth mountains which were collected during the 1979 80 austral field season although the early palaeozoic palaeomagnetic field of gondwanaland is characterized by rapid apparent polar wander at the present level of coverage the data are consistent with rotation of the ellsworth mountain block with respect to the east antarctic craton as postulated by schopf2 this result supports a microplate nature of western antarctica