petrogenesis of volcanic rocks that host the world class ag pb navidad district north patagonian massif comparison with the jurassic chon aike volcanic province of patagonia argentina abstract we present the first study of the volcanic rocks of the canadon asfalto formation that host the navidad world class ag pb epithermal district located in the north patagonian massif patagonia argentina these volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of gondwana and represent the mafic to intermediate counterparts of the mainly silicic jurassic chon aike volcanic province lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation la icpms u pb ages of zircon crystals from the volcanic units yielded middle jurassic ages of 173 9 1 9 ma and 170 8 3 ma in the navidad district volcanic rocks of the canadon asfalto formation show arc like signatures including high k basaltic andesite to high k dacite compositions rb ba and th enrichment relative to the less mobile hfs elements nb ta enrichment in light rare earth elements lree y ti depletion and high zr contents these characteristics could be explained by assimilation of crustal rocks in the jurassic magmas which is also supported by the presence of zircon xenocrysts with permian and middle upper triassic ages 281 3 ma 246 5 218 1 and 201 3 ma and quartz xenocrysts recognized in these volcanic units furthermore sr and nd isotope compositions suggest a contribution of crustal components in these middle jurassic magmas high k basaltic andesite has initial 87sr 86sr ratios of 0 70416 0 70658 and nd t values of 5 3 and 4 high k dacite and andesite have initial 87sr 86sr compositions of 0 70584 0 70601 and nd t values of 4 1 and 3 2 the range of pb isotope values 206pb 204pb 18 28 18 37 207pb 204pb 15 61 15 62 and 208pb 204pb 38 26 38 43 of navidad volcanic rocks and ore minerals suggest mixing pb sources with contributions of mantle and crust 206pb 204pb isotopic ratios of jurassic volcanic rocks of the chon aike volcanic province and sulfides of associated epithermal deposits increase with time from the volcanic event v1 188 178 ma to volcanic events v2 172 162 ma and v3 157 153 ma reflecting variations in the radiogenic pb source as volcanism was migrating towards the proto pacific margin of gondwana

palaeomagnetism of the ferrar dolerite in the northern prince albert mountains victoria land antarctica summary during the sixth italian expedition to victoria land antarctica jurassic ferrar dolerite sills were sampled at 23 sites in the northern prince albert mountains the rock magnetic properties were similar in all sites saturation remanence remanent coercive force and curie temperature values pointed to titanomagnetite as the main carrier of magnetization thermal and af demagnetizations were both effective in removing weak secondary magnetization components below 250 c and 20 mt peak field further demagnetization resulted in single component stable chrm palaeomagnetic directions were derived after af demagnetization at 20 mt and site mean vgps computed for all sites but one thern a where systematic changes of the chrm inclination have resulted in single sample vgps dispersed astride the equator the site mean vgps distribution is clearly bimodal 15 vgps cluster around a palaeopole lat 48 s long 226 e in good agreement with the literature data whereas seven vgps are displaced toward a palaeopole at higher latitude lat 79 s long 181 e all vgps can be regarded as jurassic because no evidence of substantial secondary components was found and experimental data showed that the ptvrm partial thermal viscous remanent magnetization acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous vgps the thern a and the southward displaced vgps plot along a great circle through the east antarctica jurassic palaeopole as a high reversal rate in the jurassic is well documented by magnetostratigraphy and the reversal vgp path is often confined to a narrow longitudinal band we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous vgps were intruded during transitional phases of the geomagnetic field similar anomalous vgps previously reported from the ferrar supergroup rocks of northern victoria land also plot along the same palaeomeridian which is only 30 away from the palaeomeridian derived from the jurassic intrusives of the dufek massif in the transantarctic mountains burmester sheriff 1980 the jurassic palaeopole of east antarctica has been recalculated from 103 vgps available in the literature the position is 57 s 221 e when all vgps are taken into account and 51 s 223 e when the high latitude vgps from victoria land are discarded as anomalous

flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

tectonic rotations in the deseado massif southern patagonia during the breakup of western gondwana abstract paleomagnetic investigation in the deseado massif southern patagonia suggests that triassic sedimentary rocks carry a latest triassic to jurassic remagnetization and that earliest jurassic plutonic complexes carry a reversed polarity magnetization of thermoremanent origin despite uncertainties in the timing of the observed remanence in the triassic rocks and the lack of paleohorizontal control on the plutonic complexes comparison of the derived pole positions with the most reliable late triassic jurassic apparent polar wander paths indicates that the study areas underwent significant clockwise vertical axis rotation in contrast paleomagnetic results from mid cretaceous rocks in the region indicate no rotation the observed crustal rotations in the deseado massif are thus bracketed to have occurred between jurassic and early cretaceous times documenting southern patagonian deformation during the breakup of western gondwana and then enlarging the regional record of clockwise rotations associated with this event these results suggest a more complex than previously supposed tectonic evolution of this part of south america

early jurassic magmatism on the antarctic peninsula and potential correlation with the subcordilleran plutonic belt of patagonia early jurassic silicic volcanic rocks of the chon aike province v1 187 182 ma are recognized from many localities in the southern antarctic peninsula and ne patagonia and are essentially coeval with the extensive karoo 182 ma and ferrar 183 ma large igneous provinces of pre breakup gondwana until recently plutonic rocks of this age were considered either rare in or absent from the antarctic peninsula batholith which was thought to have been mainly constructed during the middle jurassic and the mid cretaceous new u pb zircon geochronology from the antarctic peninsula and recently published u pb ages from elsewhere in the peninsula and patagonia are used to demonstrate the more widespread nature of early jurassic plutonism eight samples are dated here from the central and southern antarctic peninsula they are all moderately to strongly foliated granitoids tonalite granite granodiorite and locally represent the crystalline basement they yield ages in the range 188 181 ma which overlap with published ages of 185 180 ma from granitoids from elsewhere on the antarctic peninsula and from the subcordilleran plutonic belt of patagonia 185 181 ma whereas early jurassic plutons of the subcordilleran plutonic belt of patagonia are directly related to subduction processes along the proto pacific margin of gondwana coeval volcanic rocks of the chon aike province are interpreted to be directly associated with extension and plume activity during the initial stages of gondwana breakup this indicates that subduction was continuing when chon aike province volcanism started the early jurassic plutonism on the antarctic peninsula is transitional between subduction related and breakup related magmatism supplementary material cathodoluminescence images of analysed zircon grains from sites on the antarctic peninsula are available at https doi org 10 6084 m9 figshare c 3521973

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

deep schlumberger sounding and the crustal resistivity structure of central australia summary three 200 km schluniberger resistivity soundings have been conducted over the central australian shield using telephone lines to obtain the large electrode spacings these represent the first crustal scale controlled source electrical study to be carried out in this continent a computer controlled data acquisition system was used which allowed precise measurements to be made with only modest emission currents 0 1 0 5 a the three soundings centred on the towns of renner springs wauchope and aileron showed the southern part of the study area the arunta block to be an order of magnitude more resistive than the more northerly section the tennant creek block this difference correlates with the higher heat flow of the tennant creek block a lowering of apparent resistivity at large electrode spacings for one sounding wauchope is taken to indicate the presence of a low resistivity layer in the middle crust at a depth less than 20 km however the effect of the highly conductive overburden characteristic of inland australia combined with the large transverse resistance of the crust prevented the other two soundings from detecting such a layer without support from these two soundings it is impossible to be sure that the lowered resistivity at wauchope is not caused merely by lateral variations in near surface resistivity the data also show that crustal resistivities are much lower than the expected values for dry rock whether or not a low resistivity layer is included in the model this implies a widespread occurrence of free water in the crust with greater amounts occurring at depth if the low resistivity zone exists

ascent of a granitoid diapir in a temperature varying medium a relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth dependent temperature to predict emplacement histories ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it ascent velocity is described by the expression u 2 gd3z 0r where is the density contrast between diapir and country rock g is gravitational acceleration d is the deformation zone width z is a parameter which describes the increase in viscosity across the zone 0 is the country rock viscosity at the contact and r is the diapir radius the width of the deformation zone as dictated by the temperature distribution and rheological function is the point at which the viscosity increases by a factor f from the contact value although ascent rates vary with choice of this parameter ascent distances are not strongly dependent on f results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock but only weakly dependent on the initial diapir temperature ascent velocity varies with the diapir size with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons as a result of the proportionately larger amount of thermal energy available for ascent larger bodies will rise greater distances than smaller diapirs however the additional ascent appears nominal relative to the contrast in available energy modeling diapirs using a no slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress free boundary condition the time scale of granitoid diapirism over crustal distances cannot be less than 104 years due to the rate limiting effect of thermal conduction above the body and not greater than 105 years reflecting solidification times of kilometer sized plutons a consequence of ascent by a deformation zone mechanism is the generation following emplacement of a mechanically weakened crust which extends from beneath the diapir to near the source perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust

modeling of surface deformation in volcanic areas the 1970 1972 and 1982 1984 crises of campi flegrei italy campi flegrei is a densely inhabited volcanic area within a caldera 12 14 km in diameter immediately west of naples italy in 1970 1972 and in 1982 1984 campi flegrei experienced considerable ground surface deformation producing a bulge about 3 5 m high at its peak in the two inflation episodes the point of maximum uplift did not change coinciding with the caldera center about which the displacements exhibited radial symmetry the 1982 1984 uplift was accompanied by considerable seismic activity gravity changes have been interpreted as reflecting the occurrence of a subvertical magmatic intrusion volcanological petrological and geophysical data point to the existence of a shallow magma chamber just beneath the center of the caldera we assume that the observed surface displacements are the response of an essentially elastic medium to a pressure increase in the magma chamber static deformation is calculated by using a finite element method assuming axial symmetry around a vertical axis through the caldera center the role played by various components of the model geometry boundary conditions elastic properties is analyzed it is shown that models assuming uniform elasticity are inadequate therefore the major volcanotectonic features as well as the influence of temperature and pressure on the elastic properties are taken into account the results of various more complex models are presented it is suggested that a progressively fractured zone whose size and shape depend also on the possible occurrence of magmatic intrusions may explain the observed surface displacements and the other data collected at campi flegrei it is suggested that the 1970 1972 uplift was possibly accompanied by larger scale changes in the stress field

papoose flat pluton forceful expansion or postemplacement deformation intense solid state deformation in the margin of the papoose flat pluton white mountians california and severe attenuation of nearby wall rocks encouraged earlier workers to suggest that this pluton was emplaced by blistering or in situ radial expansion reexamination of the pluton and wall rocks indicates that many features present in and around the pluton are not compatible with this model these features include the following 1 lack of compositional zoning or magmatic foliations in the pluton 2 lack of high temperature solid state deformation or a transition from magmatic to high temperature solid state flow 3 the presence of widespread moderate to low temperature deformation in the pluton 4 a northwest southeast trending stretching lineation and associated kinematic indicators of sinistral shear everywhere within and adjacent to the western half of the pluton 5 significant cleavage transposition after the growth of porphyroblasts in the contact metamorphic aureole and 6 upright to inclined folds with a steeply dipping axial planar cleavage in wall rocks that postdate aureole cleavages and porphyroblasts many other plutons in the white and inyo mountains also have deformed western or northwestern margins and weakly deformed eastern margins instead of interpreting all of these plutons as post tectonic and inferring that they all expanded in a western direction more the authors suggest that significant cretaceous regional deformation occurred during and after emplacement of these plutons less

cretaceous dykes discovered in the falkland islands implications for regional tectonics in the south atlantic new aeromagnetic data resolve the dykes of the falkland islands into three swarms a hitherto unrecognized suite of north south dykes is established as early cretaceous by an ar ar date of about 121 ma swarms of ne sw and east west dykes are both early jurassic the former gives an ar ar age of about 178 ma whereas the latter has been previously dated to about 190 ma the intrusion of the cretaceous dykes marks the onset of oceanic crust generation in the south atlantic and so restricts to mid jurassic the microplate rotation envisaged in most models for the falklands break out from gondwana

structural pattern and ascent model in the central extremadura batholith hercynian belt spain abstract the central extremadura batholith constitutes an important alignment of plutonic rocks occupying part of the internal zones of the iberian hercynian belt it comprises 13 plutons and numerous minor intrusions and covers a wide compositional range from quartz diorites to alkali feldspar granites structural study of the individual plutons reveals that their internal structural patterns may be correlated correlation between the different structural patterns in the plutons and interpretation of the superimposed deformation in the metasedimentary host rocks indicate that the plutons were emplaced and deformed in an e w dextral intracontinental shear zone developed during the second deformation phase 302 320 ma a qualitative ascent model has been established as follows 1 development of extensional fractures at 45 to the e w dextral shear zone 2 intrusion of granitic magmas along these fractures and 3 rotation of the earlier intrusions in a dextral sense inducing antithetic shear zones n s sinistral and a transverse shortening perpendicular to the major axis of the earlier deformed plutons as well as the forced emplacement of the later plutons

do smoothly curving spiral shaped inclusion trails signify porphyroblast rotation smoothly curving spiral shaped inclusion trails may form in porphyroblasts during the overprinting of near orthogonal foliations in schists that are relatively quartz rich and in general graphite poor the smooth curvature results from the combined effects of 1 the protection from progressive shearing provided by projecting crystal faces and 2 greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic therefore lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving spiral shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew this has considerable significance for shear sense determinations in deformed and metamorphosed rocks

deformation of incompletely crystallized magma systems granitic gneisses and their tectonic implications certain textures in two feldspar calc alkaline granitic gneisses have been identified as indicators of the presence of intercrystalline melt during deformation of crystal melt mushes fluid relocation or fracting is to be expected in an incompletely crystallized magmatic system as deviatoric stresses are applied to it relocation of late stage fluids capable of crystallizing k feldspar quartz microaplite and myrmekite to low pressure zones such as the pressure shadow region around earlier crystals occurs within some developing mylonitic gneiss fabrics and results in a distinctive assemblage of textures strain in these rocks is expressed by various brittle and ductile features and by fluid relocation textures the petrogenesis of these particular gneisses is therefore both magmatic and metamorphic contrasting with gneiss forming processes that had no participating magmatic component migmatitic complexes evolving by partial anatexis of non magmatic rocks in a tectonically active environment are not

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

gondwana breakup and plate kinematics business as usual 1 a tectonic model of the weddell sea is built by composing a simple circuit with optimized rotations describing the growth of the south atlantic and sw indian oceans the model independently and accurately reproduces the consensus elements of the weddell sea s spreading record and continental margins and offers solutions to remaining controversies there at their present resolutions plate kinematic data from the south atlantic and sw indian oceans and weddell sea rule against the proposed but controversial independent movements of small plates during gondwana breakup that have been attributed to the presence or impact of a mantle plume hence although supercontinent breakup here was accompanied by extraordinary excess volcanism there is no indication from plate kinematics that the causes of that volcanism provided a unique driving mechanism for it

electrical conductivity anomalies and their relationship with the tectonics of south australia summary geomagnetic variation studies have been conducted in the gawler craton and adelaide geosyncline of south australia the magnetometer stations extend from the coast up to the southern edge of the 1970 array of gough mcelhinny lilley the coast effect is the dominant feature of the data but use is made of the hypothetical event technique to identify two zones of telluric current concentration both of them appear to be associated with linear zones of enhanced electrical conductivity within the crust the southern eyre peninsula anomaly lies within the gawler craton and may identify a major fracture or shear in the upper crust the conductivity anomaly within the adelaide geosyncline appears to be the continuation of the flinders anomaly discovered by the 1970 array study it correlates well with the arcuate fold pattern of the southern flinders zone of the

progressive crystallization of a granitoid diapir and its relationship to stages of emplacement the cannibal creek granite in northeastern australia consists of two adamellite plutons and several dykes each group representing two very similar batches of peraluminous magma the magma ascended through the crust and fractionated plagioclase biotite and zircon producing a zonation within the larger of the two plutons in these minerals and their constituent elements fe mg ca zr sr v ti and ba minor amounts of quartz and k feldspar were probably also fractionated during this period but not sufficiently to result in a detectable zonation across the main pluton at this stage the magma probably contained less than 50 crystals and hence was theologically weak which enabled it to ascend via fractures and by kinking and stoping the country rock slates the later stages of crystallization proceeded when the crystals formed a continuous framework they could not segregate from the melt and hence no zonation was developed in alkali feldspars or quartz or in si al na or k with falling temperat

paleomagnetism of jurassic and cretaceous rocks in central patagonia a key to constrain the timing of rotations during the breakup of southwestern gondwana abstract a paleomagnetic study in jurassic and cretaceous rocks from the canadon asfalto basin central patagonia indicates the occurrence of about 25 30 clockwise rotation in upper jurassic lowermost cretaceous rocks whereas the overlying mid cretaceous rocks do not show evidence of rotation this constrains the tectonic rotation to be related to a major regional unconformity in patagonia which in turn seems to be close in time with the early opening of the south atlantic ocean the sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern gondwana suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent on the other hand the mid cretaceous rocks in the region yield a paleopole located at lat 87 south long 159 east a95 3 8 this pole position is consistent with coeval high quality paleopoles of other plates when transferred to south american coordinates implying it is an accurate determination of the aptian circa 116 ma geomagnetic field in south america

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

deformation mechanisms in granitic rocks at shallow crustal levels abstract microstructural observations of naturally faulted granitic gneisses show that feldspar grains are weaker than quartz grains at temperatures below 325 c and at depths of less than 8 10 km feldspar grains sustained most of the deformation by grain scale faulting slip along cleavage controlled fractures and cataclasis fracture of feldspar grains within fault zones promoted their alteration to kaolinite quartz grains also deformed by fracturing and often healed to form quartz porphyroclasts and mosaics in a comminuted matrix of feldspar and kaolinite syntectonic alteration of the feldspar grains may have weakened the fault zones over time and resulted in foliated textures within the fault zones this study of naturally deformed rocks confirms published experimental results on the behavior of granitic rocks at low temperatures and pressures and taken together these data show that the theology of the upper 10 km of the crust is greatly influenced by cataclastic processes in feldspar

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

large volume loss during cleavage formation hamburg sequence pennsylvania green reduction spots in red slate of the hamburg sequence exposed near shartlesville pennsylvania have axial ratios of 1 42 1 0 0 28 on the limbs of near isoclinal folds and 1 0 0 79 0 41 in fold hinge zones conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44 preceded the tectonic strain associated with cleavage formation this strain having identical maximum extensions but greater shortening in fold hinges as compared to limbs was characterized by 41 extension in x no change in y 50 to 59 shortening in z and 29 to 42 tectonic volume loss the general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates where deformation was an almost constant volume process and extension in x compensated for shortening in z

late gondwanide tectonic rotations within gondwanaland geologic and paleomagnetic evidence from the ellsworth whitmore mountains crustal block of west antarctica and from the falkland islands of south america indicates that tectonic displacement of major portions of the samfrau geosyncline occurred after the early mesozoic gondwanide folding but prior to seafloor spreading comparison with continental deformation in the collisional alpine himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the gondwanian orogeny resulted from collision of a fore arc and magmatic arc terrane with the panthalassic margin of the gondwana craton during closure of a marginal basin the displacements had major consequences for the ensuing evolution of the south atlantic weddell sea ross sea region

a single late orogenic permian episode of gold mineralization in the hodgkinson province north queensland australia quartz hosted gold deposits in the hodgkinson province north queensland are widely distributed with the most heavily endowed areas comprising three separate spatially distant goldfields the west normanby goldfield the palmer river goldfield and the hodgkinson goldfield relationships among quartz vein and shear hosted deposits in each of the three goldfields indicate that the introduction of gold bearing quartz displays consistent structural timing relative to a regionally recognized deformation history gold was emplaced during the waning stages of d4 the main contractional phase of the permian triassic hunter bowen orogeny mineralization and structures have been structurally timed relative to isotopically dated plutons of the spatially associated syn d4 whypalla supersuite indicating emplacement of the mineralization in the early permian or later emplacement of gold bearing quartz veins in the fossiliferous permian normanby formation in the north of the province supports this contention the permian age is younger than the originally published carboniferous 300 ma age for gold mineralization in the hodgkinson province fluid generation and gold transport were related to regional scale devolatilization during province wide metamorphism the final distribution of fluids and contained gold was controlled by faults produced during the hunter bowen orogeny and by major late devonian shear zones that were reactivated during this orogeny heat from synchronously emplaced d4 granites focused enhanced fluid flow into spatially associated dilatant structures in these faults

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

deformation around a rising diapir modeled by creeping flow past a sphere a new experimental method for determining the pattern of flow and three dimensional progressive finite deformation of fluid elements in low reynolds number re viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder this is intended as a model analogue of natural rock deformation associated with magmatic and solid state diapirs rising through ductile crust the kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures the experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent newtonian test fluid used sgm36 are discussed in detail the progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis the progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere the results compare favorably with the external strain pattern associated with post tectonic mesozonal diapiric intrusions

questions about myrmekite in deformed rocks abstract several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered although a general relationship between deformation and myrmekite appears to be well established the fact that myrmekite grows into relatively non deformed k feldspar while being simultaneously deformed and recrystallized at the rear of the growing lobes suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony furthermore because myrmekite typically nucleates on existing plagioclase rather than in k feldspar strain energy may not contribute directly to nucleation of myrmekite either at least in many instances however strain energy probably contributes indirectly by facilitating access of fluids to growth sites thereby altering the local chemical environment and so promoting development of myrmekite because myrmekite recrystallizes readily in deformed felsic rocks it is one of the main contributors to the development of folia

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

aureole deformation by flattening around a diapir during in situ ballooning the cannibal creek granite the cannibal creek granite a foliated pluton within a strongly deformed aureole shows several diagnostic features of a diapir the intensity of the aureole deformation diminished away from the contact the aureole foliation is continuous with that of the granite and forms closed elliptical trend lines and the contact metamorphic minerals are synkinematic with regard to the aureole foliation the pluton deformed its aureole in three stages involving initially brittle then ductile and finally brittle deformation styles initial intrusion of a low strength magma stoped kinked and flexed aside its overburden crystallization of the magma prevented further rise and it ballooned and deformed its aureole evidently entirely by ductile bulk shortening perpendicular to the contact this the major stage of aureole deformation records no evidence of vertical movement of the pluton or of progressive simple shear along its margin during this stage the magma may have been theologically a high strength bing

the effects of retrograde reactions and of diffusion on 40ar 39ar ages of micas the effects of metamorphic reactions occurring during decompression were explored to understand their influence on the ar ar ages of micas monometamorphic metasediments from the lepontine alps switzerland reached lower amphibolite facies during the barrovian metamorphism related to the collision between european and african adria continental plates mineral assemblages typically composed of garnet plagioclase biotite muscovite and paragonite or margarite were screened for petrological equilibrium to focus on samples that record a minimum degree of retrogression x ray diffraction data indicate that some mineral separates prepared for ar ar stepwise heating analysis are monomineralic whereas others are composed of two white micas muscovite with paragonite or margarite or biotite and chlorite in monomineralic samples ar ar and ar ar proportional to ca k and cl k ratios did not change and the resulting ages can be interpreted unambiguously in mineral separates containing two white micas ca k and cl k ratios were variable reflecting non simultaneous laboratory degassing of the two heterochemical ar reservoirs these ratios were used to identify each ar reservoir and to unravel the age in a chlorite margarite biotite calcschist equilibrated near 5608c and 0 65 gpa biotite margarite and muscovite all yield ages around 18ma at slightly higher grade 560 5808c 0 8 0 9 gpa the assemblage muscovite paragonite plagioclase is in equilibrium and remains stable during retrogression in this case muscovite and paragonite yield indistinguishable ages around 16 5ma above 5908c paragonite was mostly consumed to form plagioclase 45908c whereby the relict mica yields an age up to 5 6 ma younger than muscovite this partial or total resetting of the ar clock in paragonite is interpreted to reflect plagioclase growth during decompression where biotite is present within this same assemblage it systematically yields a younger age than muscovite by 0 5 2 ma however these biotites all show small amounts of retrograde chlorite formation we conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the ar clock as is the formation of plagioclase from paragonite decomposition multi equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved and this helps to obtain geologically meaningful metamorphic ages however even samples passing such equilibrium tests may still show retrograde effects that affect the ar retention of micas a more robust interpretation of such ar ar results may require use of a second geochronometer such as u pb on monazite

sedimentological and petrographic evolution of a fluvio lacustrine environment during the onset of volcanism volcanically induced forcing of sedimentation and environmental responses

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

palaeomagnetic study of the el quemado complex and marifil formation patagonian jurassic igneous province argentina summary the upper jurassic el quemado complex was sampled at 36 sites from five localities in the cordilleran foothills of southern patagonia between lago argentino and lago posadas sierra colorada and the middle jurassic marifil formation at 12 sites in the somuncura massif near camarones the main lithology was ignimbrite with minor tuff and lava petrographical and sem observation show that the el quemado rocks suffered an intense high temperature alteration which resulted in transformation of most primary ti magnetite in pseudobrookite rutile and minor ti haematite and fe hydrated oxides a similar less pronounced alteration occurred in the marifil rocks 40ar 39ar dating of el quemado was possible for one sample from sierra colorada and yielded an age of 156 5 1 9 ma magnetic mineralogy measurements isothermal remanence hysteresis loop curie balance show that the remanent magnetization is dominated by psd low ti magnetite often associated to a minor high coercive mineral haematite secondary magnetization components are usually absent or weak at el quemado sites strong at marifil they were completely erased by thermal and af demagnetization and a characteristic remanence chrm stable up to temperatures higher than 550 c or peak field values of 100 mt was successfully isolated the virtual geomagnetic pole vgp from the marifil formation 83 s 138 e is in agreement with the literature data for jurassic rocks from stable south america the el quemado vgps fall in two groups the localities to the north of latitude 48 s lago posadas sierra colorada yield a vgp 81 s 172 e close to that of marifil whereas those south of latitude 49 s lago san martin lago argentino show a highly elongated vgp distribution consistent with counter clockwise block rotation about vertical axes these rotations were likely caused by thrust sheets which were rotating counter clockwise at the same time they were advancing towards the foreland the amount of rotation varies according to the location of the sampling sites in the thrust and fold belt

the late cretaceous paleomagnetic field in north america a south american perspective determining the late cretaceous paleomagnetic pole for north america has been difficult because of the lack of suitable rocks of that age in cratonic areas to provide the necessary data as an alternative different studies have appealed to paleomagnetic data from rocks in western north america using paleopoles from stable areas in neighboring continents it is suggested that the available late cretaceous paleomagnetic record in western north america should be analyzed in terms of rigid body deformations rather than be used to represent the cratonic reference field

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

syn orogenic extensional and contractional deformation related to granite emplacement in the northern tasman orogenic zone australia abstract the hodgkinson province of the northern tasman orogenic zone comprises uniform low grade metasediments of mid palaeozoic age that host numerous permian granite intrusions three widely developed sequential deformations d2 d4 are recognized on the basis of penetrative fabrics and folds which show clear regionally consistent overprinting relationships d3 and d4 were contemporary with phases of granite emplacement as shown by relationships of s3 and s4 microstructures to porphyroblasts which grew in thermal aureoles adjacent to intrusive contacts granitic dyke relationships with s3 and the deflection of s3 and s4 foliation trajectories around plutons support this view granites of d3 and d4 affiliation have a close spatial relationship and strong geochemical similarities suggesting that they were closely associated in time isotopic dating of granites of d4 association show that this deformational phase represents the hunter bowen orogeny which has been recognized as the terminal permian triassic episode of tectonism for the eastern sector of the tasman orogenic zone d3 fabrics are best developed in zones of enhanced metamorphic grade and are typically subhorizontal or of gentle inclination and associated with recumbent folds when not affected by subsequent deformation d3 is attributed to a phase of tectonism linked to the hunter bowen orogeny but preceding the main episode of compression uplift above tracts of rising magma combined with the thermal weakening of crustal material and the reduction of compressive stress due to plate convergence sponsored widespread ductile extension at a high crustal level the zone of failure was a discontinuous carapace above the magmatic front d4 is attributed to the main episode of the hunter bowen orogenic compression and involved crustal shortening with the imposition of upright structures and the widespread emplacement of granite s3 foliation surfaces have been reoriented in the vicinity of d4 granite bodies which behaved as rigid bodies during d4 compression the preparation of crustal pathways by d3 granite was utilized by granites of the d4 generation with the hodgkinson province displaying an intimate interplay of magmatism stress regime and structural evolution over a time period of perhaps 15 20 ma

emplacement kinematics of a granite diapir the chindamora batholith zimbabwe the geometric features of a large 50 40 km granitic batholith and its surrounding greenstone belt envelope are described the batholith is made up of several intrusive components and an intrusion sequence based on contact relations and xenoliths has been established the igneous rocks show a local foliation s fabric and preferred orientation of minerals the intensity of this fabric accords with the finite strain determined from xenolith shape analysis when the strain variations are analysed in detail it appears that the batholith was built up by a succession of magmatic pulses each of which led to a stretching of the previously consolidated igneous material so called balloon tectonics evidence is presented that xenolith deformation commences only when the surrounding igneous material has crystallized and a method is suggested whereby the position of the surface separating crystal supported phase plastic behaviour from melt supported phase liquid behaviour at different stages in the plutonic history can be determined ductile shear zones are present and their geometry accords well with the shape changes necessary to allow the outer parts of the pluton to stretch

deformation within the cannibal creek pluton and its aureole queensland australia a re evaluation of ballooning as an emplacement mechanism abstract structures within the foliated cannibal creek granite and its deformed aureole indicate that the pluton appears to have been emplaced after a minimum of two strong regional deformation events d1 d2 that contact metamorphic porphyroblast growth began during the early stages of a third aureole centred event da and that a fourth weak deformation overprints all earlier structures at least within the strain softened aureole the aureole deformation da is most strongly developed at the pluton contact decreasing in intensity both inwards to the core and outwards into the wall rock earlier interpretations linking da strain to expansion ballooning of the pluton during emplacement are no longer favoured previously discussed diagnostic criteria of ballooning such as closed elliptical trend lines for the aureole foliation and new field evidence such as radial magmatic lineations and moderate to high temperature solid state deformation of external sheath dykes are either ambiguous or point strongly toward syn to pre tectonic emplacement of a piercement diapir less than 30 of the pluton volume appears to have been accommodated by wall rock shortening as the result of ballooning or diapiric ascent the rest was likely accommodated by a combination of broad wavelength regional doming 1 5 mm year 1 above the pluton far field redistribution of mass into the source area plus or minus some component of diapiric backflow stoping assimilation and dilation in fault bends or fold hinges

palaeomagnetism and 40ar 39ar dating from lower jurassic rocks in gastre central patagonia further data to explore tectonomagmatic events associated with the break up of gondwana new 40 ar 39 ar data indicate ages of c 185 ma for the lonco trapial volcanic field in gastre north central patagonia implying that this andesitic unit is roughly coeval with the marifil silicic province that crops out in the eastern part of northern patagonia these volcanic fields are therefore roughly coeval with the karoo ferrar large igneous province further contributing to the huge 185 180 ma magmatic outpouring in southern gondwana the comparison of palaeomagnetic results from the unit with early jurassic reference palaeopoles suggests that gastre records a small anticlockwise finite rotation with respect to stable south america this contrasts sharply with the large clockwise rotation recorded by roughly coeval dolerites from the falkland malvinas islands and argues against a direct relationship between the rotation of the islands and the tectonic activity in gastre the results support previous suggestions that the gastre fault system is not the locus of major strike slip displacement activity in the jurassic as some workers have suggested taken as a whole palaeomagnetic data from patagonia suggest a more complex than previously thought pattern of distributed and variable deformation during the early stages of gondwana breakup the crustal block boundaries in the region remain cryptic supplementary material 40 ar 39 ar analytical procedures and expanded data are available at http www geolsoc org uk sup18522

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

resistivity studies over the flinders conductivity anomaly south australia summary seven schlumberger resistivity soundings with maximum current electrode spacings of 20 km have been conducted south of lake frome in south australia these experiments were done partly to test new electrical sounding equipment and partly to investigate a large conductivity anomaly previously delineated by other workers using magnetometer array and mt methods the flinders anomaly these previous studies left some doubt as to the depth to the conductive region responsible for the anomaly the electrical soundings did not detect a buried conductive zone which constrains it to lie deeper than 5 7 km however the study did show the surface sediments of the region to be very conductive resistivities of 2 9 m were measured over thicknesses of 50 400 m with sediment thickness inferred to be up to 2 km to the north of the studied area this raises the question of whether current channelling in the surface sediments could have been responsible for the earlier results simple modelling and application of the criteria given by jones suggest this may be so the equipment used for this study is a low power 200 w computer controlled system which employs synchronous stacking and other signal processing to achieve signal to noise improvement ratios of up to 1000

the falkland islands a key element in gondwana paleogeography stratigraphic and sedimentological fieldwork on the falkland islands confirms that they were originally part of south africa east and west falkland have contrasting geological and deformation histories and are separated by the falkland sound fault which marks a significant deformation edge to the cape fold belt the degree of structural deformation and metamorphism in west falkland is substantially less than that of east falkland adjustments to differential shortening between the two islands being reflected by structures developed along the falkland sound fault prior to fragmentation of gondwana the falkland platform and maurice ewing bank were positioned in the natal valley adjacent to the east coast of south africa this predrift reconstruction shows that the falkland islands have subsequently been rotated by 180 relative to south africa and complements recent paleomagnetic data there was a complete overlap of the maurice ewing bank and falkland platform to the exclusion of the falkland plateau basin which is a small oceanic basin formed during the jurassic fragmentation of gondwana extension in this basin was accommodated by movements along the aghulas falkland fracture zone and the gastre fault system predrift restoration of these faults shows that patagonia was positioned closer to both africa and antarctica the falkland plateau basin was an integral component of the west gondwana jurassic microplate system and provides an invaluable insight into the evolution of its western margin at some time prior to the main opening of the south atlantic the falkland platform was rotated clockwise by 120 against the agulhas falkland fracture zone such that the falkland islands were translated some 300 km to the south the driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the falkland plateau basin as the gastre fault system within continental crust became unable to accommodate the continuous dextral shear in the agulhas falkland fracture zone the falkland islands represent an outboard extension of the eastern cape and are significant for understanding the paleogeography of both the cape and karoo supergroups east falkland is closely similar to south africa in its geology while west falkland shows significant contrasts with both the devonian and permian successions being more proximal in character but still demonstrate the eastward continuation of the cape and karoo basins

structural characteristics of granitic plutons emplaced during weak regional deformation examples from late carboniferous plutons morocco abstract strain patterns have been investigated around shallow crustal granitic plutons emplaced during weak regional deformation in the western moroccan hercynian belt the plutons are distributed along reactivated deep faults and display elliptical shapes which are consistently oriented with respect to the regional strain field pluton age orientation and ellipticity can be correlated with strain history ductile deformation and contact metamorphism are mainly located in the thermally softened aureoles finite strain gradients towards plutons are accompanied by the development of superimposed microstructures finite strain trajectories and variations of strain ellipsoid shapes exhibit perturbations of the regional strain field around plutons structures internal to the plutons are continuous with regional structures foliation development in granitoids records a decrease in temperature during syntectonic pluton cooling geometry and kinematics of deformation reflect the influence of both regional deformation and pluton intrusion

paleomagnetism and magnetic fabric of middle jurassic dykes from western patagonia argentina abstract jurassic volcanism in patagonia is widespread although associated dyke swarms are conspicuous they have been almost neglected in previous geologic studies of this region radiometric paleomagnetic and magnetic anisotropy studies are reported from a middle jurassic basic to intermediate dyke swarm exposed in the sierra de mamil choique western patagonia in an area of over 350 km2 two whole rock k ar determinations indicate that these dykes were intruded at around 170 ma an anisotropy of magnetic susceptibility ams study on 15 dykes 74 block samples shows that they carry different kinds of magnetic fabric both normal and inverse apparently governed by compositional differences a paleomagnetic study of these samples suggests that ti poor titanomagnetite is the probable carrier of the characteristic remanence mean site characteristic directions pass a reversal test grade c a paleomagnetic pole computed by averaging vgps from each individual dyke is situated at 70 2 s 190 4 e n 13 95 9 7 not substantially apart from other middle jurassic poles from south america further refinement of the jurassic south american apparent polar wander path is needed to establish whether or not the mamil choique dykes were affected by a small tectonic rotation

forceful emplacement of the eureka valley joshua flat beer creek composite pluton into a structural basin in eastern california internal structure and wall rock deformation abstract anisotropy of magnetic susceptibility parameters have been analyzed at 311 locations in the eureka valley joshua flat beer creek ejb pluton of eastern california the large amount of data has allowed for the ams parameters to be contoured using techniques that both reveal map scale trends and emphasize small scale differences the contour maps suggest that magnetic susceptibility is dominantly controlled by composition of the magma but may also be affected by emplacement related strain as the magma chamber inflated and forced the wall rocks outward pluton construction involved two major pulses of different composition magmas that were emplaced sequentially but with overlapping periods of crystallization the magmas initially intruded as sill like bodies into a structural basin the magnetic foliation of the pluton cuts across internal magmatic contacts on the map scale and is parallel to local contacts between the pluton and surrounding metasedimentary wall rocks the magnetic fabric is similar in orientation and symmetry to intense flattening strains recorded in the aureole rocks the metasedimentary wall rocks have been shortened between 60 and 70 and this strain magnitude is approximately equal on the west south and east margins of the pluton strain in the wall rocks is dominantly flattening and concentrated into a narrow 1 km wide inner aureole mapping of bedding cleavage intersection lineations south of the pluton indicates that the magma made room for itself by translating the wall rocks outward and rotating the already inward dipping wall rocks of the structural basin to sub vertical stretching of the inner aureole around an expanding magma chamber was responsible for the intense shortening limited data on the marble canyon pluton to the south of the ejb pluton indicates a very similar emplacement process

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

gondwana breakup via double saloon door rifting and seafloor spreading in a backarc basin during subduction rollback abstract a model has been developed where two arc parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback the resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double saloon doors about their hinges as movement proceeds and the two terranes rotate a gap begins to extend between them where a third rift initiates and propagates in the opposite direction to subduction rollback observations from the oligocene to recent western mediterranean the miocene to recent carpathians the miocene to recent aegean and the oligocene to recent caribbean point to a two stage process initially pairs of terranes comprising a pre existing retro arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents terrane docking reduces the width of the subduction zone leading to a second phase during which subduction to strike slip transitions initiate the clockwise rotated terrane is caught up in a dextral strike slip zone whereas the counterclockwise rotated terrane is entrained in a sinistral strike slip fault system the likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline by analogy with the above model a revised five stage early jurassic to early cretaceous gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the weddell sea area seven features are considered diagnostic of double saloon door rifting and seafloor spreading i earliest movement involves clockwise and counterclockwise rotations of the falkland islands block and the ellsworth whitmore terrane respectively ii terranes comprise areas of a pre existing retro arc fold thrust belt the permo triassic gondwanide orogeny attached to an accretionary wedge magmatic arc the falklands islands block is initially attached to southern patagonia west antarctic peninsula while the ellsworth whitmore terrane is combined with the thurston island block iii paleogeographies demonstrate rifting and extension in a backarc environment relative to a pacific margin subduction zone accretionary wedge where simultaneous crustal shortening occurs iv a ridge jump towards the subduction zone from east of the falkland islands to the rocas verdes basin evinces subduction rollback v this ridge jump combined with backarc extension isolated an area of thicker continental crust the falkland islands block vi well documented ew oriented seafloor spreading anomalies in the weddell sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback vii the dextral strike slip gastre and sub parallel faults form one boundary of the gondwana subduction rollback whereas the other boundary may be formed by inferred sinistral strike slip motion between a combined thurston island ellsworth whitmore terrane and marie byrd land east antarctica

multiscale magmatic cyclicity duration of pluton construction and the paradoxical relationship between tectonism and plutonism in continental arcs abstract the close relationship between crustal magmatism an expression of heat dissipation and tectonics an expression of stress dissipation leads to the question of their mutual relationships indeed the low viscosity of magmas and the large viscosity contrast between magmas and surrounding rocks favor strain localization in magmas and then possible magmatic initiation of structures at a wide range of scales however new data about 3 d pluton shape and duration of pluton construction perturb this simple geological image and indicate some independence between magmatism and tectonics in some cases we observe a direct genetic link and strong arguments for physical interactions between magmas and tectonics in other cases we observe an absence of these interactions and it is unclear how magma transfer and emplacement are related to lithospheric plate dynamics a simple explanation of this complexity follows directly from the pulsed incremental assembly of plutons and its spatial and temporal characteristics the size of each pluton is related to a magmatic pulsation at a particular time scale and each of these coupled time space scales is related to a specific process in small plutons we can observe the incremental process the building block of plutons in larger plutons the incremental process is lost and the pulsation which consists of a cycle of injections at different timescales must be related to the composition and thermal regime of the source region itself driving magmatic processes melting segregation and transfer that interact with tectonic boundary conditions the dynamics of pulsed magmatism observed in plutonic systems is then a proxy for deep lithospheric and magmatic processes from our data and a review of published work we find a positive corelation between volume and duration of pluton construction the larger a pluton the longer its construction time large fast or small slow plutons have not been identified to date one consequence of this observation is that plutonic magmatic fluxes seem to be comparable from one geodynamic setting to another and also over various geologic time spans a second consequence of this correlation is that small plutons which are constructed in a geologically short length of time commonly record little about tectonic conditions and result only from the interference between magma dynamics and the local geologic setting the fast rate of magma transfer in the crust on the order of cm s relative to tectonic rates on the order of cm yr explain why the incremental process of pluton construction is independent of but not insensitive to the tectonic setting however in large plutonic bodies which correspond to longer duration magmatic events regional deformation has time to interact with the growing pluton and can be recorded within the pluton wall rock structure magma transfer operates at a very short timescale comparable to volcanic timescales which can be sustained over variable periods depending on the fertility of the magma source region and its ability to feed the system the fast operation of magmatic processes relative to crustal tectonic processes ensures that the former control the system from below

cannibal creek granite post tectonic ballooning pluton or pre tectonic piercement diapir a discussion groves d i eds recent advances in understanding precambrian gold deposits univ west australia geology dept and extension pub 11 and 1989 paleosols and their relevance to the precambrian atmospheric composition jour geology v 97 p 77 92 phillips g n 1988 widespread fluid infiltration during metamorphism of the witwatersrand goldfields generation of chloritoid and pyrophyllite jour meta geol v 6 p 311 332 meyers r e et al 1988 recent advances in the geology of the witwatersrand goldfields including the importance of postdepositional processes ext abs bicentennial gold 88 melbourne p 319 324

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

on the role of diapirism in the segregation ascent and final emplacement of granitoid magmas only bodies of magma with a high crystal content and partially molten crustal country rocks can ascend as diapirs once such an envelope is pierced the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content which may then continue ascent via fractures and dykes the details of the mechanisms of granitoid magma segregation are still unknown but it appears that many magmas hegin their ascent through the crust as mushes with at least 50 melt and that such magmas are rheologically able to ascend through a thickness of crust this ascent mechanism explains the dearth of structures attributable to the ascent of granitoids in contrast to the abundance of structures that developed during their final emplacement when a magma becomes too crystalline melt 25 to continue its ascent via dykes it is immobilised at approximately this stage a hydrous magma may become saturated with water and release fluids into the aureole making it particularly susceptible to deformation magma that continues to arrive at this level is also immobilised and the pluton grows as a ballooning diapir these characteristically deform themselves and their aureoles by bulk shortening magmas that are able to ascend to shallow depths largely by virtue of lower water contents and higher initial temperatures tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence high level intrusions lend to be tabular are also fed by dykes or conduits and assemble in tabular batholiths

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

evaluation of multiple emplacement mechanisms the huichizi granite pluton qinling orogenic belt central china three end member granite emplacement mechanisms host rock ductile shortening hrds host rock rigid displacement hrrd and magmatic absorption ma are identified in this study of the huichizi pluton the internal fabrics are mainly flattening typesona 0 4 0 9u and strain is highoesa 1 0 0 6u near the boundaries and lowoesa 0 7 0 30u toward the central parts of the pluton in the 1 5 km wide contact aureole extension e is measured in syn emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1 14 km and 0 74 km respectively from this we conclude that the hrds produced only about 20 space for the present volume of the pluton the regional hrrd derived from the uplift by about 8 km of the qinling group in a flower structure contributed 19 032 space ms including stoping and material transfer processes yielded another 37 space of this stoping contributed a minimum of 1 space estimated from the surface area of 2 8 km 2 covered by xenoliths transfer processes provided about 36 of the needed space deduced from the average 64 36 ratio of hybridization of mantle derived to crustal qinling group derived magma the three mechanisms may act nearly simultaneously as a multiple emplacement mechanism this situation may apply to many other plutons as well 2000 elsevier science ltd all rights reserved

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

new paleomagnetic pole for the upper jurassic chon aike formation of southern argentina south america testing the tectonic stability of patagonia with respect to south america and implications to middle jurassic early cretaceous true polar wander abstract a paleomagnetic pole and a new ar ar date of 156 08 0 05 ma are reported for the chon aike formation an extensive plateau of ignimbrites outcropping in the deseado massif southern patagonia argentina the geographic coordinates and statistical parameters for the paleomagnetic pole validated by a reversal test are lat 84 3 s long 191 3 e a95 8 6 k 13 3 n 23 this pole shows a complete coherence with late jurassic and early to mid cretaceous poles of south america obtained exclusively from igneous rocks avoiding any potential inclination shallowing it is suggested that a southern patagonia was an integral part of south america in the late jurassic b any reported vertical axis tectonic rotations must pre date the late jurassic in this region c according to updated paleomagnetic data from igneous rocks south america had very limited latitudinal drift between about 160 and 140 ma and therefore d its apparent polar wander apw path when compared with global apparent polar wander paths does not support a large global shift or true polar wander tpw between 160 and 140 ma

magmatic strains and foliation triple points of the merrimac plutons northern sierra nevada california implications for pluton emplacement and timing of subduction abstract cross cutting relationships between regional structures and post tectonic plutons of known age are commonly used to date regional deformation in orogenic belts in order to show that a pluton is post tectonic one must demonstrate that the strains within the pluton are not due to regional tectonic processes this paper reports strain measurements associated with pluton emplacement in northern sierra nevada these strain data and a microstructural analysis indicate that strains within the merrimac plutons are due to igneous not tectonic processes which strengthens the hypothesis that the plutons were emplaced after the formation of regional structures and melanges radiometric ages of the merrimac pluton in concert with structural data place a more reliable upper jurassic limit on the age of the subduction in the western belt of northern sierra nevada this paper also reports constrictional strains around plutons and suggests that the merrimac plutons contributed significantly to the ductile strains observed in the wall rocks

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

falkland plateau evolution and a mobile southernmost south america assessment of southwest gondwana break up and its implications for regional hydrocarbon prospectivity must now take into account the origin of the southeast margin of the falkland islands as a volcanic rifted continental margin and of the floor of the major part of the falkland plateau basin as elevated oceanic crust a reconstruction of the falkland plateau against southern africa shows a southward extensional widening of the outeniqua basin across the line of the falkland agulhas fracture zone changing from stretched continental to oceanic crustal structure such a model for outeniqua basin opening and the independent westward and clockwise rotation of the falkland islands block suggests that southernmost south america was also a collection of microplates moving independently within a generally extensional environment in the late triassic early jurassic this is incompatible with assumptions of a rigid southernmost south america over this time and a dominant role for a continuous dextral strike slip gastre fault

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

geomagnetic deep sounding experiment in the central flinders ranges of south australia abstract a closely spaced geomagnetic deep sounding array study was conducted in the central flinders ranges of south australia the results indicate an anomaly which follows the main fold trend to the east rather than the north south trend suggested by earlier studies although the coast effect obscures some of the detailed characteristics of the anomaly the data particularly at high frequencies favour an interpretation in terms of a crustal anomaly probably associated with the lower members of the adelaide geosyncline the torrens hinge zone which is a major tectonic boundary at the western margin of the adelaide geosyncline has no associated induction anomaly

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

cannibal creek granite post tectonic ballooning pluton or pre tectonic piercement diapir the cannibal creek granite in northern queensland australia previously has been interpreted to be a post tectonic pluton which underwent solid state deformation during in situ ballooning a reinterpretation of structures in and around the pluton indicates that this pluton is more appropriately viewed as a pierce ment diapir emplaced by magmatic flow significant deformation took place after pluton emplacement in fact the cannibal creek granite has some characteristics similar to pre tectonic granites examined elsewhere therefore it is somewhat misleading to view the granite as a post tectonic pluton

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma

miocene block uplift and basin formation in the patagonian foreland the gastre basin argentina abstract the intraplate fault block mountains and intermontane deposits of the gastre basin which are recorded more than 550 km east of the andean trench in central patagonia argentina are analyzed the gastre basin is one of the largest patagonian intermontane basins limited by uplifted blocks strongly oblique to the andean chain it was originated by reverse faulting and inversion of pre existing normal faults associated with a mesozoic rift basin and defined by older crustal heterogeneities the deformational event occurred during the middle miocene related to a short contractional episode 16 1 14 86 ma probably in response to an eastward migration of the andean fold and thrust belt during pliocene to quaternary times neither younger fault block uplifts nor reconfigurations of the basin occurred similarities between the study area and other parts of the patagonian foreland such as the presence of miocene reverse or inversion tectonics as well as the accommodation of the miocene sedimentary successions suggest that the gastre basin is part of a major late early to middle miocene broken foreland system i e the patagonian broken foreland that exhumed discrete fault block mountains and generated contemporary basins along more than 950 km parallel to the andean trench i e between 40 00 and 48 00 south latitude based on recent studies on the southern andean margin this continental scale contractional episode may be the result of a flat slab subduction segment nevertheless such a hypothesis is very difficult to support when analyzing such a large flat subduction segment along the entire patagonian trench this suggests the need to consider alternative flat slab trigger mechanisms or other factors in the generation of broken foreland systems