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

the karoo igneous province a problem area for inferring tectonic setting from basalt geochemistry tholeiitic basalts and associated intrusives are the major component of the karoo igneous province they are of mesozoic age and constitute one of the world s classic continental flood basalt cfb provinces it has been argued that most karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the proterozoic the only exceptions to this are late stage morb like dolerites near the present day continental margins which are considered to be of asthenospheric origin when data for the southern karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting essentially all of them would be classified as calc alkali basalts cab s or low k tholeiites virtually none of them plot in the compositional fields designated as characteristic of within plate basalts there is little likelihood that the compositions of the karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a subduction component has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out it must be concluded that the mantle source areas for cab s and the southern karoo basalts have marked similarities in contrast the data for northern karoo basalts largely plot in the within plate field on geochemical discriminant diagrams available data suggest that the source composition and or the restite mineralogy and degree of partial melting are different for southern and northern karoo basalts there is no evidence for any difference in tectonic setting between the southern and northern karoo basalts at the time they were erupted this appears to be clear evidence that specific mantle source characteristics and or magmatic processes can vary within a single cfb province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks

the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

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

proterozoic early paleozoic evolution in western south america a discussion in tectonophysics 354 121 137 2002

pacific subduction coeval with the karoo mantle plume the early jurasssic subcordilleran belt of northwestern patagonia the early mesozoic magmatism of southwestern gondwana is reviewed in the light of new u pb shrimp zircon ages 181 2 ma 181 3 ma 185 2 ma and 182 2 ma that establish an early jurassic age for the granites of the subcordilleran plutonic belt in northwestern argentine patagonia new geochemical and isotopic data confirm that this belt represents an early subduction related magmatic arc along the proto pacific margin of gondwana thus subduction was synchronous with the initial phase of chon aike rhyolite volcanism ascribed to the thermal effects of the karoo mantle plume and heralding rifting of this part of the supercontinent overall there is clear evidence that successive episodes of calc alkaline arc magmatism from late triassic times until establishment of the andean patagonian batholith in the late jurassic involved westerly migration and clockwise rotation of the arc this indicates a changing geodynamic regime during gondwana break up and suggests differential rollback of the subducted slab with accretion of new crustal material and or asymmetrical scissor like opening of back arc basins this almost certainly entailed dextral displacement of continental domains in patagonia

the evolution of the patagonian ice sheet from 35 ka to the present day patice abstract we present patice a gis database of patagonian glacial geomorphology and recalibrated chronological data patice includes 58 823 landforms and 1 669 geochronological ages and extends from 38 s to 55 s in southern south america we use these data to generate new empirical reconstructions of the patagonian ice sheet pis and subsequent ice masses and ice dammed palaeolakes at 35 ka 30 ka 25 ka 20 ka 15 ka 13 ka synchronous with the antarctic cold reversal 10 ka 5 ka 0 2 ka and 2011 ad at 35 ka the pis covered of 492 6 x103 km2 had a sea level equivalent of 1 496 mm was 350 km wide and 2090 km long and was grounded on the pacific continental shelf edge outlet glacier lobes remained topographically confined and the largest generated the suites of subglacial streamlined bedforms characteristic of ice streams the pis reached its maximum extent by 33 28 ka from 38 s to 48 s and earlier around 47 ka from 48 s southwards net retreat from maximum positions began by 25 ka with ice marginal stabilisation then at 21 18 ka which was then followed by rapid irreversible deglaciation by 15 ka the pis had separated into disparate ice masses draining into large ice dammed lakes along the eastern margin which strongly influenced rates of recession glacial readvances or stabilisations occurred at least at 14 13 ka 11 ka 6 5 ka 2 1 ka and 0 5 0 2 ka we suggest that 20th century glacial recession a 1 is occurring faster than at any time documented during the holocene

geology of the argentine continental shelf and margin from aeromagnetic survey new aeromagnetics show a complex of magnetic patterns and source depths of the north and central argentine continental and adjacent oceanic crust the 40 km wide positive magnetic g anomaly reflects an end rift associated volcanic edifice at the continental margin m4 mo and cretaceous quiet zone oceanic crust are identified the colorado magnetic discontinuity forms the southern boundary of the precambrian la plata craton to the south the continental crust of the patagonia platform is composed of amalgamated late precambrian through end paleozoic weakly magnetic sedimentary prisms the sw patagonia platform crust shows a strong high frequency magnetic pattern associated with mesozoic volcanics while zones of mid crustal level intrusives and some volcanics on the outer shelf are expressed as a broad higher amplitude magnetic pattern

discussion on early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate by z ben avraham c j h hartnady and j a malan

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

orogen parallel transition from a decoupled fore arc sliver to andean type mountain chain paleomagnetic and geologic evidence from southern chile 37 39 s

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

middle jurassic within plate granites in west antarctica and their bearing on the break up of gondwanaland five post tectonic granitic plutons isolated within the central ellsworth whitmore mountains crustal block in west antarctica form a distinctive geochemical suite all have some characteristics of s type granites and are atypical of active continental margins they range in composition from a within plate granite wpg end member with the lowest 87 sr 86 sr initial ratio 0 707 to granites with a much more marked crustal signature and high initial ratios 0 722 the granitic suite was emplaced over a restricted middle jurassic time interval at the same time as the extensive ferrar karoo tasman mafic suite and just prior to the disintegration of the supercontinent gondwanaland petrogenetic modelling suggests that the wpg end member could have been derived entirely by differentiation of the enriched mantle derived ferrar magma and the end member with the highest initial ratio by partial melting of a crustal source low initial 143 nd 144 nd ratios and proterozoic model ages are compatible with a precambrian crustal component but may alternatively as in the case of ferrar supergroup magmas reflect partial inheritance from enriched lithospheric mantle geochemically coupled to the lower crust since precambrian differentiation data from these granites are consistent with large scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the gondwanaland lithosphere related in some way to break up of the supercontinent

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

geometry of eclogite facies structural features implications for production and exhumation of ultrahigh pressure and high pressure rocks western gneiss region norway 1 well preserved eclogite facies structural features in metamorphosed gabbro and diorite gneiss in baltica crust formed at a minimum depth of 60 70 km during the scandian orogeny allow direct inferences to be made regarding the geometry of folds kinematics and original structural orientations related to production and exhumation of high pressure rocks folds associated with eclogite facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation strain estimates and l s or l s fabrics indicate that these structures were formed in a constrictional strain field locally they are well preserved in eclogite facies mylonite zones at least 40 m thick that cut proterozoic gabbro and adjacent gneiss where steeply dipping they show a north side up shear sense across a younger northeast trending shallowly plunging amphibolite facies anticlinorium formed in a constrictional noncoaxial strain field where original structural facing direction can be inferred small circle rotation of the eclogite facies lineation about the anticline axis indicates that the relative motion vector of baltica with respect to overlying imbricated crust and laurentia was oriented 320 in a present day reference frame this result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with late silurian to early devonian oblique sinistral convergence between baltica and laurentia results of this study indicate that coherent baltica crust at 60 70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops southeast thrusting parallel to plate motion high pressure rocks were progressively imbricated and stacked along eclogite facies mylonite zones with the deforming zone separating coherent baltica crust from imbricated basement above the constrictional strains under eclogite facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

basement evolution of the sierra de la ventana fold belt new evidence for cambrian continental rifting along the southern margin of gondwana u pb sensitive high resolution ion microprobe data together with geochemical and nd isotope analyses obtained in the basement complex of the sierra de la ventana fold belt indicate that the early palaeozoic passive margin history of the basin followed cambrian magmatism related to rifting in a 600 ma neoproterozoic crust the cambrian episode started with intrusion of 531 4 and 524 5 ma a and i type granites derived from a dehydrated infracrustal source e nd 530 3 1 to 5 9 and culminated with eruption of high zr peralkaline spherulitic rhyolites derived from an undepleted lithospheric mantle 509 5 ma e nd 509 0 5 to 1 0 these rift related magmatic rocks were covered by shelf sediments deposited along a once continuous passive margin encompassing the sierra de la ventana fold belt the cape fold belt the falkland malvinas microplate and the ellsworth mountains block in antarctica the cambrian rifting event defined the outline shape of the southern part of gondwana and can be regarded as the initiation of the supercontinent stage which lasted until jurassic break up the conjugate continental fragments separated from gondwana during the cambrian rifting could be the source for microcontinents with c 1000 ma basement rocks that collided with the proto andean margin during ordovician silurian times

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

reply to discussion on early tectonic extension between the agulhas bank and the falkland plateau due to rotation of the lafonia microplate

when did the falklands rotate abstract similarities in the styles and relative timings of tectonic events in the outeniqua basin south africa and the north falkland basin suggest that basin formation in both regions may have preceded rotation of the falklands microplate contrary to previous models for the break up of gondwana which suggest jurassic rotation the data implies valanginian rotation contemporaneous with the first recorded motion on the agulhas falkland fracture zone and south atlantic rifting the data also suggests that the formation of the falkland plateau basin may also be a cretaceous event as opposed to the previously assumed jurassic age such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for jurassic rotation does not exclude later rotation as a possibility

the auriga nunataks shear zone mesozoic transfer faulting and arc deformation in northwest palmer land antarctica the auriga nunataks shear zone places new tectonic and temporal constraints on the mesozoic evolution of west antarctica the shear zone is a long lived arc orthogonal ductile transfer fault that preserves a history of regional mesozoic compressional basement deformation and extensional arc pluton emplacement in the antarctic peninsula magmatic arc it forms an east west trending positive flower structure 2 4 km wide exposed along 5 km of strike marble graphite bearing pyroxene granulite granodiorite diorite amphibolite and gabbro are deformed to mylonite and marble hosted tectonic breccia the mesozoic history of the shear zone is interpreted as follows emplacement of granodiorite diorite at circa 206 ma during early jurassic dextral transtension and metamorphism that peaked at circa 188 ma d1 brecciation of marble and mylonitization of gneiss by late jurassic to early cretaceous sinistral shear during the peninsula wide palmer land orogeny d2 early cretaceous dextral transtension with emplacement of gabbro and garnet leucogranite between 140 and 135 ma d3 and mid cretaceous ocean vergent thrusting and sinistral transpression between 125 ma and 80 ma with a peak at circa 110 ma that folded mylonitized and brecciated preexisting plutonic and metamorphic rocks d4 this is responsible for the current geometry of the shear zone the auriga nunataks shear zone transferred motion between arc parallel compressional and extensional structural elements and by hosting plutons appears to have acted like a leaky transform fault during episodes of regional extension

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

a tectonostratigraphic synthesis of the sub andean basins inferences on the position of south american intraplate accommodation zones and their control on south atlantic opening it has been shown in the accompanying paper that the sub andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains to test the hypothesis that changes in palaeo depositional setting rely on the presence of a series of transverse zones of structural accommodation data have been digitally compiled from across the south american continent spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries structural accommodation zones and patterns of subsidence and intraplate deformation the results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which on a regional scale interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization their strong spatial relationship with mesozoic intraplate alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during gondwana breakup these localized zones of high heat flow have important implications for source rock maturity in the interior phanerozoic intracratonic basins of south america on the south atlantic margin the majority of these crustal lineaments correlate with failed arms of triple junction rifts and define the boundaries to tectonostratigraphic domains recognized along the south atlantic rift system

structure and tectonics of the agulhas falkland fracture zone abstract the 1200 km long agulhas falkland transform developed during the early cretaceous break up of west gondwanaland on the african plate the continent bordering the agulhas fracture zone extends between the agulhas bank and the south tugela reentrant it is divided into four distinct parts mallory trough segment i diaz ridge segment ii east london segment iii and durban segment iv from southwest to northeast each segment differs from the ohters in its physiography and in the nature of the continent ocean crustal boundary in segment i a wedge like eastward narrowing deep water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace along segment ii the south african continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture zone trace above a probable fragment of oceanic crust representing a northerly extension of the jurassic falkland plateau basin in segment iii the continental margin is steep especially along the middle slope it lies directly along the transform trace but conspicuously rugged along the continental slope due to the effects of submarine canyoning and sediment slumping in segment iv the continent ocean boundary also lies directly along the transform fault trace and the margin shows evidence of seaward tilting above landward dipping faults along the middle or lower slope the segmentation is inherited from jurassic structural fabrics formed prior to early cretaceous transform motion when tectonic rotation of microplates caused rifting and partial oceanization along the future agulhas falkland megafault trajectory

basalt geochemistry and tectonic discrimination within continental flood basalt provinces abstract continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings in one well studied classic flood basalt province the mesozoic karoo province of southern africa magmatism occurred in the following tectonic settings a continental rifting leading to ocean floor spreading in the south atlantic ocean etendeka suite of namibia b stretched continental lithosphere and rifting not leading directly to ocean floor formation lebombo suite of southeastern africa and c an a tectonic within plate continental setting characterized by an absence of faulting or warping lesotho highlands and karoo dolerites of south africa by means of spidergrams of the elements rb ba th nb k la ce sr nd p hf zr sm ti tb y v ni and cr uncontaminated tholeiites from c above i e the lesotho type continental flood basalts ltcfb are compared with mid ocean ridge basalts morb ocean island tholeiites oit and tholeiites and calc alkali basalts from subduction environments the comparison reveals the ltcfbs are geochemically distinct the differences are reflected in relative enrichments or depletions of the more incompatible elements rb ce to less incompatible elements ce y i e the overall slope of the spidergrams and in anomalous enrichments or depletions of one or more of the elements th k nb sr ti hf and zr the distinctive geochemical character of the lesotho ltcfbs is interpreted in terms of a lithospheric mantle source for the basalts this is supported by isotopic data there are no major geochemical differences between lesotho cfbs and basalts of the rift related etendeka and lebombo suites although the latter are somewhat enriched in rb ba and k however unlike the lesotho basalts the lebombo and etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late stage dolerites having morb oit i e asthenospheric geochemical characteristics the flood basalt silicic magmatism late stage dyke swarm association is characteristic of several rift or thinned lithosphere environments e g ethiopia skye eastern greenland but in many of these the flood basalts have ocean island basalt oit geochemical characteristics the lesotho type cfb geochemistry is exhibited by the grande ronde basalt of the columbia river group a possible subduction related flood basalt province and the basic rocks associated with mesozoic rifting in the north and south atlantic basalt geochemistry alone is unhelpful in determining the tectonic setting of cfbs although the rift related environments may be identified by the petrology and geochemistry of the whole igneous suite a two source model is proposed for the mantle derived basic rocks in rift related cfb provinces early enriched basalts are derived from the lithosphere and following pronounced lithospheric attenuation or rifting later morb like melts are emplaced from the rising asthenosphere the presence of both lesotho and oit type geochemical patterns in rift related cfbs suggests that the lithosphere exhibits different styles of enrichment

are the oroclines of the variscan belt related to late variscan strike slip tectonics terra nova 23 241 247 2011 abstract early variscan structures and magnetic anomalies delineate four oroclines along the variscan belt the arcs bend the lithostratigraphic zones of the autochthon and the allochthonous terranes which include ophiolites of the rheic suture and units of gondwanan derivation subducted on both sides of the rheic ocean a transcurrent model dominated by right lateral motion explains the genesis and evolution of the oroclines and a simple plate tectonics scenario links late variscan dextral strike slip movements in the variscides to a broad intracontinental shear zone kinematically equivalent to a transform fault terra nova 00 1 7 2011

evolution of a kyanite bearing belt within a ht lp orogen the case of nw variscan iberia kyanite replaces andalusite in a belt of ordovician and silurian pelitic rocks that form a narrow synform pinched between high grade antiforms in nw variscan iberia kyanite occurs across the belt in al rich black pelites in assemblages i kyanite chloritoid chlorite muscovite and ii kyanite staurolite chlorite muscovite in i kyanite occurs in the matrix and in kyanite muscovite aggregates that pseudomorph earlier andalusite porphyroblasts the aggregates are found across the belt and can still be recognized in assemblage ii and even in iii andalusite staurolite biotite muscovite this latter being a hornfelsic silurian schist where kyanite is relic and staurolite occurs in the matrix and is resorbed inside new massive pleochroic andalusite kfmash and mnkfmash pseudosections have been constructed using thermocalc for al rich and al poorer compositions from the belt chloritoid zoning in al rich rocks containing assemblage i plus chloritoid chlorite thermometry complemented with garnet chlorite thermometry in al poorer lithologies mean that the path is one of increasing pressure and temperature conditions prior to assemblage i with earlier andalusite stable are those of the andalusite chloritoid chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims the passage from assemblage i to ii implies a prograde path within the kyanite field assemblage iii represents peak conditions indicating a prograde staurolite consuming reaction across a kfmash field leading eventually to a locally found andalusite biotite muscovite hornfels the lowest pressure stages are recorded by cordierite biotite in al poor pelites garnet bearing mnkfmash assemblages in al poorer pelites record conditions similar to assemblages ii and iii the replacement of andalusite by kyanite in assemblage i is attributed to downdragging of andalusite bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the vivero fault

the falkland plateau in the context of gondwana breakup abstract during the jurassic the falkland plateau was part of gondwana and occupied a position between the african and antarctic plates several contrasting models exist for the breakup of gondwana that depend on assumptions about the currently unknown crustal structure of the falkland plateau here we present the results of recently acquired wide angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions in contrast to published crustal models the new data show that the falkland plateau basin consists of up to 20 km thick oceanic crust which is bounded to the east by a continental fragment the maurice ewing bank in a refined geodynamic model rifting started between the falkland islands and the maurice ewing bank at 178 ma and ceased at around 154 ma the plateau s exceptionally thick oceanic crust likely results from its position in an extensional back arc regime situated over a mantle thermal anomaly that was also responsible for the extensive onshore karoo ferrar and chon aike volcanic provinces

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

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

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

mesozoic break up of sw gondwana implications for regional hydrocarbon potential of the southern south atlantic abstract this work provides new palinspastic palaeofacies reconstructions of sw gondwana incorporating rotation of a falkland malvinas microplate we discuss the implications of this for the tectonic evolution of the southern south atlantic and hence for the regional hydrocarbon potential existing gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents in most continental reconstructions the falkland malvinas plateau was assumed to be a rigid fragment of pre permian south american crust however it has been suggested on the basis of palaeomagnetic data that the falkland malvinas islands were rotated by 180 after 190 ma this rotation hypothesis has been successfully tested on the basis of devonian stratigraphy and palaeontology permian stratigraphy and sedimentology and late palaeozoic and early mesozoic structure making it unlikely that the plateau behaved as a rigid structure during breakup we have explored the consequences of accepting this hypothesis for the tectonic evolution of sw gondwana by compiling new palaeogeographic maps for the permian cretaceous of the southern atlantic area to achieve a realistic close fit we have devised a pre rift proxy for the ocean continent boundary for the south atlantic in order to produce the best fit it is necessary to subdivide south america into four plates the consequences of this are far reaching our work suggests that although sedimentary basins were initiated at different times three major tectonic phases can be recognised in regional terms these can be thought of as pre syn and post rift during the pre rift time until the late triassic the area was dominated by compressional tectonism and formed part of the gondwana foreland the falkland malvinas islands lay east of africa the falkland malvinas plateau was 33 shorter and patagonia was displaced east with respect to the rest of south america in part along the line of the gastre fault system potential source facies are dominantly post glacial black shales of late permian age deposited in lacustrine or hyposaline marine environments these rocks would also be an effective regional seal sandstones deposited in the late permian would be dominantly volcaniclastic with poor reservoir qualities triassic sandstones tend to be more mature there was significant extension from about 210 ma end triassic until the south atlantic opened at about 130 ma early cretaceous in the early syn rift phase extension was accompanied by strike slip faulting and block rotation later extension was accompanied by extrusion of large volumes of lava early opening of the south atlantic was oblique which created basins at high angle to the trend of the ocean on the argentine margin and resulted in microplate rotation in ne brazil intermittent physical barriers controlled deposition of upper jurassic cretaceous anoxic sediments during breakup some of these mudrock units are effective seals with likely regional extent during crustal reorganisation clastic sediments changed from a uniform volcaniclastic provenance to local derivation with variable reservoir quality in the late rift and early post rift phase continental extension changed from oblique to normal and basins developed parallel to the continental margins of the south atlantic this change coincides with the main rifting in the equatorial basins of brazil and the early impact of the santa helena plume it resulted in widespread development of unconformities the abandonment of the reconcavo tucano jatoba rift and the end of ne brazil plate rotation which remained attached to south america there was extensive deposition of evaporites concentrated in but not restricted to the area north of the rio grande rise walvis ridge widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements our main conclusion is that the regional hydrocarbon potential of the southern south atlantic has been constrained by the tectonic evolution

microseismicity and tectonics in the granada basin spain a microseismic experiment carried out in 1994 in the granada basin spain permitted the precise recording of more than 80 local earthquakes the dense distribution of the local network with 40 to 50 instrumental records for each event enabled us to have well controlled hypocenters and also 10 reliable focal mechanisms the above observations are interpreted together with topographic data neotectonics and sub surface information microtectonic observations in sierra elvira padul and zafarraya gave a set of fault planes and striae which were interpreted in terms of the recent regional stress tensor the actual stress tensor obtained from the microseismic campaign data gives a regime in radial extension with r1 vertical and r3 oriented ns to nne microtectonic information is coherent with these orientations but closer to 3 axial extension a set of 64 mechanisms obtained from the permanent andalusian network favors a ns orientation for r3 this results are interpreted in terms of the general model implying the lateral ejection of the betic ranges towards the atlantic d 2002 elsevier science b v all rights reserved

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

40ar 39ar ages for dykes from the falkland islands with implications for the break up of southern gondwanaland 40 ar 39 incremental heating spectra analyses were made of five microgabbro dykes from the falkland islands two of which yielded an acceptable plateau and a third a close maximum estimate all three indicate an age of dyke intrusion of about 190 ma supporting a single published k ar date this provides further support for the geological continuity of the falkland islands and the eastern karoo basin where dyke swarms of similar petrological affinities were intruded at about the same time the age constrains the previously published palaeoreconstruction of the falkland islands based on palaeomagnetic data which places the falklands immediately to the east of southern africa to be valid until at least the late early jurassic

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

the age of the weddell basin the deep oceanic basin occupied by the weddell sea separates the east antarctic craton from the antarctic peninsula a largely post palaeozoic orogenic belt an understanding of the tectonic evolution of the basin would resolve most of the remaining problems of gondwanaland reconstruction which revolve around the relative positions of these two fragments and south america norton and sclater1 conclude that indian ocean marine geophysical data essentially support the smith and hallam fit2 but the unacceptable overlap ofundoubtedly old continental crust3 5 on the antarctic peninsula and falkland plateau implies relative motion between the peninsula and east antarctica such motion is not required by the reconstruction of barron and others7 8 which however can be criticized on other grounds9 10 the marine geophysical observations within the intervening ocean basins have proved useful in formulating plate tectonic reconstructions fracture zone trends seafloor spreading type magnetic anomalies crustal heat flux and crustal depth all provide clues to the timing and direction of the continental motions the acquisition of geophysical data from the weddell sea region has been slow11 13 however the combined interpretation of the two largest data sets extant12 13 now being undertaken seems likely to provide a more tightly constrained gondwanaland solution as a first step we report here an estimated age for the weddell sea floor in the area bounded by 60 and 73 s 5 and 35 w shown in fig 1

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

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

modified gulf of california model for south georgia north scotia ridge and implications for the rocas verdes back arc basin southern andes new field mapping and an integrated trace element and nd isotopic study have been undertaken on the jurassic cretaceous larsen harbour complex on the island of south georgia recognized as a displaced part of the southern andes ensialic autochthonous rocas verdes marginal basin five basalt groups groups i v are identified from trace element data initial nd isotopic compositions suggest that they are derived from only two sources magmas produced during early stages of continental lithospheric attenuation groups i iii were derived by varying degrees of partial melting and fractional crystallization from a large ion lithophile element lile enriched low epsilon sub nd mantle source relative to normal n type mid ocean ridge basalt morb magmas produced during later stages of rifting groups iv and v were however derived from a high epsilon sub nd asthenospheric mantle source similar to n type morb unaffected by earlier lile enrichment our data do not occur with previous suggestions that the larsen harbour complex and by implication the rocas verdes formed in a supra subduction zone setting on the basis of geologic and geochemical evidence we favor basin formation along an oblique slip margin akin to that of the gulf of california

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

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

the crustal structure and sedimentation of the weddell sea embayment implications for gondwana reconstructions abstract the weddell sea embayment is central to reconstructions of the west antarctic region of gondwana some reconstructions represent this area as a pre break up component the filchner block defined by the present 2000 m isobath and coastline but take no account of crustal extension and sedimentary progradation of the continental margin since break up seismic refraction shows a wide deep sedimentary basin beneath the central part of the embayment indicating extensive rifting and stretching of continental crust during the break up of gondwana this may have increased the width of the block by about 300 km potential field data suggest extensive progradation of sediments on the weddell sea continental margin this may have moved the margin seaward between 200 and 400 km taken together these modifications in the dimensions of the pre break up filchner block represent between one half and one third the area of the present weddell sea embayment the presence of extensive rifting is indicative that hot thin crust was present during break up providing further evidence that several closely spaced mantle plumes or a single megaplume impinged on the weddell sea sector of gondwana in the early jurassic the lithospheric stretching in the embayment provided a mechanism for the rotation of the ellsworth mountains region during the early stages of gondwana break up commensurate with the paleomagnetic data

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

the falkland islands and the early fragmentation of gondwana implications for hydrocarbon exploration in the falkland plateau abstract during the early breakup of gondwana the falkland platform became detached from southern africa as a separate microplate at this time the falkland plateau basin developed between the falkland platform and the maurice ewing bank significant extension occurred within this basin with accompanying high heat flow this heat flow will have thermally overmatured any late jurassic to mid cretaceous source rocks present within the basin while still beneath a thin sedimentary cover this early overmaturation is confirmed by anomalously high thermal maturity levels and vitrinite reflectivity gradients from dsdp sites on the maurice ewing bank it is envisaged that only restricted hydrocarbon source potential remained for subsequent generation during later cretaceous and tertiary burial source rock analysis of the permian black rock member port sussex formation in east falkland shows it to have once had significant hydrocarbon generating potential although now destroyed by metamorphism some potential may still remain in southernmost east falkland in contrast thermal maturities in west falkland are still within the oil window such that these permian shales remain a viable source rock

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

the late variscan ht lp metamorphic event in nw and central iberia relationships to crustal thickening extension orocline development and crustal evolution

the triassic cobquecura pluton central chile an example of a fayalite bearing a type intrusive massif at a continental margin abstract the late triassic cobquecura pluton is a small epizonal intrusive body that was emplaced into the paleozoic basement at the western side of the cordillera de la costa near to the town of cobquecura 35 57 36 12 s central chile the pluton displays a wide spectrum of compositions that include gabbro and fayalite granite and less abundant hybrid rocks sio2 content ranges from 48 to 76 wt with a compositional gap between 57 and 63 wt sio2 sio2 poor rocks have mg fe minerals olivine fa33 34 diopside augite mg 56 91 mg mg mg fe2 100 and pargasite mg 4 8 which with progressive differentiation become fe rich olivine fa89 98 hedenbergite ferroaugite mg 10 43 and ferroedenite mg 8 29 similar k ar isotopic ages mingling structures and coherent arrays in harker diagrams as well as mineral compositions show that the sio2 poor and sio2 rich rocks are comagmatic the fe rich mineralogy and ga content of the cobquecura pluton point to an anorogenic a type magmatism the compositional similarities as well as the bimodal character of the magmatism of cobquecura and other upper triassic granitoids and lavas north of 30 s at the chilean continental margin indicate a common origin of the magmatism with anorogenic signature and coeval processes along the southwestern margin of gondwana in the triassic

metallogenic and tectonic characteristics of the paleozoic ophiolitic belt of the southern chile coastal cordillera the comparison of the tectonic geochemical and metallogenic information of the crystalline paleozoic basement of the south coast range of chile with those classical ophiolites complexes like cyprus oman the apennines magellean etc allows to find many similarities between them ultramafic rocks mainly wherlites harzburgites and dunites with strong serpentinization and rodingites tholeiitic basalts with pillow structures volcano clastic sediments green schists of tholeiitic composition and meta pelites grey schists

giant earthquakes in south central chile revealed by holocene mass wasting events in lake puyehue abstract very high resolution reflection seismic profiling 3 5 khz revealed nine holocene mass wasting events in lake puyehue south central chile these events are made up of numerous coeval mass wasting deposits and some homogenites which are radiocarbon dated the two youngest mass wasting events could be attributed to the giant ad 1960 and ad 1575 valdivia earthquakes the most extensive event took place around 1660 cal yr bp evaluation of all possible slope failure processes led us to infer that giant earthquakes roughly comparable to the ad 1960 valdivia earthquake mw 9 5 are the most likely trigger mechanisms of all mass wasting events in lake puyehue these occurred with a mean recurrence rate of 1000 yr although relatively aperiodically ranging between 500 and 2000 yr quantitative comparison of mass wasting events related to historically reported earthquakes ad 1960 and ad 1575 showed significant differences although these earthquakes are assumed to have had a comparable strength a lowered background sedimentation rate could be responsible for this variable earthquake recording which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis

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

new paleomagnetic data from thurston island implications for the tectonics of west antarctica and weddell sea opening palcomagnetic data from three west antarctic crustal blocks antarctic peninsula ap thurston island eights coast ti and the ellsworth whitmore mountains ewm indicate that there has been motion between the individual blocks and motion relative to east antarctica during the mesozoic a triassic palcomagnetic pole from the ti block 116oe 61os a95 19 4o n 3 vgps appears to indicate that the block has rotated 90 o relative to east antarctica between 230 ma and 110 ma our previously reported middle jurassic palcomagnetic pole from the ewm block indicates that a 90 o rotation relative to east antarctica occurred sometime between the cambrian and 175 ma we believe that the 90 o counterclockwise ewm rotation occurred between 220 ma and 175 ma related to the development of post gondwanide orogeny shear zones the motion of the ap ti and ewm blocks appears to be linked during the mid to late mesozoic to three major events in the evolution of the southem ocean basins opening in the mozambique somali weddell sea basins may have produced major counterclockwise rotation of the ti block with respect to east antarctica between the jurassic and early cretaceous based on new late jurassic 145oe 64 5os a95 7o n 5 vgps poles we believe that the ti rotation as well as deformation in the southem ap block was caused by collision and shearing of the ewm block against the other two as the ewm block moved southward with east antarctica an early cretaceous palcomagnetic pole 232oe 49os a95 7 9o n 5 vgps from the ti block requires that between the early and mid cretaceous there was clockwise rotation with respect to east antarctica of the ap ti ewm blocks an entity we call weddellia a change in the opening history of the weddell sea basin caused by initiation of spreading in the south atlantic ocean basin at 130 ma probably started weddellia s clockwise rotation two new 110 and 90 ma poles from the ti block 210oe 73 os a95 7 6o n 7 vgps and 161oe 81os a95 3 9o n 18 vgps respectively are similartoequivalent agepoles from the apblock and east antarctica and indicate that weddellia was at ornearits present day position with respect to east antarctica by 110 ma this corresponds to a time of major plate reorganizaion in the south atlantic and southeast indian oceans based on both the new ti palcomagnetic data and previously reported data from marie byrd land mbl dextral shearing would be expected to have occurred between mbl and weddellia since the mid cretaceous pine island bay the area between the ti and mbl blocks marks a fundamental and complex tectonic boundary in west antarctica that we propose has largely been a zone of transcurrent sheafing

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

the palaeogeographic position of central iberia in gondwana during the ordovician evidence from zircon chronology and nd isotopes terra nova 22 341 346 2010 abstract the abnormally elevated abundance of inherited zircon locally up to 80 90 of zircon grains contain pre magmatic cores in the cambro ordovician magmatic rocks of central iberia made possible an estimation of the distribution of ages of their source rocks the comparison of inherited u pb ages and whole rock nd model ages with those of three main north african neoproterozoic terranes reveals that the iberian magmas can only have been generated from a crust similar to that of the east african orogen west of the arabian nubian shield such crust is currently found in the three precambrian inliers of the western desert of egypt east of the archaean terranes of gebel kamel palaeontological evidence also indicates that the ordovician fauna of central iberia is similar to that of the region between eastern algeria and arabia therefore we conclude that during the ordovician iberia was not attached to north west africa near morocco as is usually assumed but instead to the east african orogen in northern egypt

mesozoic rifting evolution of sw gondwana a poly phased subduction related extensional history responsible for basin formation along the argentinean atlantic margin abstract the opening of the south atlantic in the early cretaceous was the final stage of the complex rifting history of sw gondwana in this contribution we reassess the chronology of mesozoic basin formation in southern south america and africa and integrate it in the long term breakup history of sw gondwana triassic rifting is characterized by intracontinental rifting in africa karoo i phase and retro arc extension on the sw margin of gondwana in the early jurassic the impingement of the karoo plume triggered rifting in eastern africa producing the karoo ii basins and the colorado and salado basins on the argentinean shelf east africa rifting ultimately lead the breakup of eastern from western gondwana in the middle jurassic in patagonia the austral malvinas and other related basins formed in association with the synextensional emplacement of the chon aike magmatic province in the patagonian retro arc in the late jurassic the rocas verdes back arc basin opened in southern patagonia while oblique rifting in the core of the late paleozoic gondwanides orogen produced the outeniqua and rawson valdes basins the south atlantic rift initiated in the early cretaceous associated with present day e w extension rifting occurred diachronically from south to north initiating in the previously thinned rawson valdes outeniqua segment a precursor oblique rift system and a larger degree of extension in this segment could explain the lack of seaward dipping reflectors sdr south of the colorado cape fracture zones rifting and sdr emplacement occurred progressively to the north along different rift segments producing strongly asymmetric conjugate margins

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