paleomagnetism from deception island south shetlands archipelago antarctica new insights into the interpretation of the volcanic evolution using a geomagnetic model deception island shows the most recent exposed active volcanism in the northern boundary of the bransfield trough the succession of the volcanic sequence in the island is broadly divided into pre and post caldera collapse units although a well constrained chronological identification of the well defined successive volcanic episodes is still needed a new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of deception island south shetlands archipelago antarctic peninsula region distributed in 1 volcanic breccia 3 sites and lavas 2 sites prior to the caldera collapse 2 lavas emplaced after the caldera collapse 10 sites and 3 dikes cutting pre and the lowermost post caldera collapse units 5 sites the information revealed by paleomagnetism provides new data about the evolution of the multi episodic volcanic edifice of this quaternary volcano suggesting that the present day position of the volcanic materials is close to their original emplacement position the new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model despite the uncertainties in the use of averaged paleomagnetic data per volcanic units the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre caldera units f1 and f2 erupted before 12 000 year bc the caldera collapse took place at about 8300 year bc and post caldera units s1 and s2 are younger than 2000 year bc

tectonic development of the bransfield basin and its prolongation to the south scotia ridge northern antarctic peninsula abstract the bransfield basin is located off the northern tip of the antarctic peninsula the analysis of multichannel seismic reflection profiles allows the shallow structure of the bransfield basin and its eastwards prolongation through the scotia sea to be imaged the bransfield basin is asymmetrical with most of the sediment input coming from the passive antarctic peninsula margin which is deformed by normal faults generally dipping less than 45 northwestwards much less sediment comes from the conjugate south shetland islands margin which is dominated by high angle normal faults generally dipping between 45 and 60 constituting the inner boundary of the south shetland block tectonic horst the detachment and moving apart of the south shetland block from the antarctic peninsula led to the opening of the bransfield basin where incipient oceanic spreading has been reported previously the south shetland block outer boundary to the northwest is made of the south shetland trench which to the east passes into the sinistral transpressive scotia antarctic plate boundary marked by the prominent south scotia ridge following cessation of spreading in the phoenix antarctic ridge northwest of the south shetland block during middle pliocene roll back played a major role in the development of the bransfield basin with maximum extension in the central subbasin in addition the eastern bransfield basin accommodates most of the sinistral transtensional deformation along the south scotia ridge normal and reverse faults were simultaneously active along the inner and outer boundaries respectively of the continental crust fragment constituting the south shetland block the development of the bransfield basin offers a good example of the tectonic processes involved in continental fragmentation associated with backarc basin development

recent achievements in archaeomagnetic dating in the iberian peninsula application to roman and mediaeval spanish structures archaeomagnetic studies in spain have undergone a significant progress during the last few years and a reference curve of the directional variation of the geomagnetic field over the past two millennia is now available for the iberian peninsula these recent developments have made archaeomagnetism a straightforward dating tool for spain and portugal the aim of this work is to illustrate how this secular variation curve can be used to date the last use of several burnt structures from spain four combustion structures from three archaeological sites with ages ranging from roman to mediaeval times have been studied and archaeomagnetically dated the directions of the characteristic remanent magnetization of each structure have been obtained from classical thermal and alternating field af demagnetization procedures and a mean direction for each combustion structure has been obtained these directional results have been compared with the new reference curve for iberia providing archaeomagnetic dates for the last use of the kilns the inferred ages obtained from this study are consistent with the archaeological evidence available

continental fragmentation along the south scotia ridge transcurrent plate boundary ne antarctic peninsula abstract the study of the south scotia ridge on the basis of swath bathymetry multichannel seismic and magnetometry profiles obtained during the hesant92 93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region the thinned continental crust fragments of the ridge are bounded by oceanic crust of the scotia sea to the north and powell basin to the south the northern boundary represents the contact between the scotia and antarctic plates this boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined both faults have associated steep scarps since they separate oceanic and continental crust types the most significant active deformation lies in the axial depression of the ridge within a band delineated by fault systems with wsw ene and sw ne strikes these faults develop pull apart basins which separate the northern and southern blocks of the ridge the northern block is being fragmented from the antarctic plate by a zone of transtensive faults and is probably a crustal element independent of the antarctic plate the axial depression which crosses the ridge slightly obliquely is characterized by deep basins locally more than 5000 m deep and associated high seismicity the fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system although it may locally have transpressive regimes depending on the fault plane and the stress field orientations the internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward deposits onlap the southern margins and are affected by normal faults in the northern margins the seismicity around the scotia plate shows that the present stresses are compressive along the northern boundary with the south american plate 1 sw ne and subhorizontal and along the western boundary with the antarctic plate 1 wnw ese and subhorizontal for the south scotia ridge however 1 is steeply inclined and 3 is subhorizontal with a nw se trend the stress distribution in bransfield strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central south scotia ridge the fragmentation of continental crustal blocks due to the tectonic activity along the transcurrent plate boundaries is a mechanism that contributes to the deformation of the northeastern end of the antarctic peninsula this area appears appropriate for the analysis of continental plate fragmentation processes

cooling rate of an active hawaiian lava flow from nighttime spectroradiometer measurements a narrow band spectroradiometer has been used to make nighttime measurements of the phase 50 eruption of pu u o o on the east rift zone of kilauea volcano hawaii on february 19 1992 a ger spectroradiometer was used to determine the cooling rate of an active lava flow this instrument collects 12 bit data between 0 35 to 3 0 m at a spectral resolution of 1 5 nm thirteen spectra of a single area on a pahoehoe flow field were collected over a 59 minute period 21 27 22 26 hst from which the cooling of the lava surface has been investigated a two component thermal mixing model flynn 1992 applied to data for the flow immediately on emplacement gave a best fit crustal temperature of 768 c a hot component at 1150 c and a hot radiating area of 3 6 of the total area over a 52 minute period within the time interval between flow resurfacings the lava flow crust cooled by 358 c to 410 c at a rate that was as high as 15 c min 1 our observations have significance both for satellite observations of active volcanoes and for numerical models of the cooling of lava flows during their emplacement

bransfield basin antarctic peninsula not a normal backarc basin the bransfield basin a marginal basin located northwest of the antarctic peninsula has been habitually considered as a backarc basin associated with the rollback process that took place along an inactive plate boundary the south shetland trench where the antarctic and the phoenix plates meet new geophysical and structural data discussed in this paper show that the basin opening is related to a sinistral simple shear couple between the scotia and the antarctic plates and not to the previously suggested rollback mechanism the widening of the bransfield basin and the lack of trench retreat are causing compression in the south shetland islands two different neotectonic stress directions with interchanged stress axes are found in the area of the south shetland block

paleomagnetism and 40ar 39ar ages from volcanics extruded during the matuyama and brunhes chrons near mcmurdo sound antarctica maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis these lobes persist in field models for the last few millenia the anomalous lobes are associated with observation sites at extreme southerly latitudes to determine whether these features persist for millions of years paleomagnetic vector data from the continent of antarctica are essential we present here new paleomagnetic vector data and ar 40 ar 39 ages from lava flows spanning the brunhes and matuyama chrons from the vicinity of mcmurdo sound antarctica oriented paleomagnetic samples were collected from 50 lava flows by e mankinen and a cox in the 1965 1966 austral summer season preliminary data based largely on the natural remanent magnetization nrm directions were published by mankinen and cox 1988 we have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection of these only one site fails to meet our acceptance criteria for directional data seven sites are reversely magnetized the mean normal and reverse directions are antipodal the combined mean direction has d over bar 12 i over bar 86 alpha 4 kappa 37 and is indistinguishable from that expected from a gad field we obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 zam 2 and a standard deviation of 28 zam 2 ar 40 ar 39 age determinations were successfully carried out on samples from 18 of the flows our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

magnetic anomalies of lava fields in the canary islands possible consequences for paleomagnetic records abstract measurements of the total magnetic field have been performed above the surface of 12 lava flows in the canary islands of la palma and tenerife directions can differ by up to 15 and intensity changes reach 20 the mean flow directions do not coincide and the mean flow virtual dipole moments vdms differ by more than 10 the deviations are linked to topographic features at the surface of the flows and should persist during emplacement of the next overlying flow such effects can thus limit the resolution of paleomagnetic records of detailed geomagnetic features and the field intensity variations above the flows and between the mean flow values could explain a significant part of the scatter inherent to many paleointensity studies historical field variations have been recorded from the same flows and are in agreement with the european curve for the same period however there is also some correlation with the present field which suggest that the underlying magnetized bodies could have affected the paleomagnetic records biases induced by the crustal field would increase considerably during periods of reduced field intensity like during reversals and induce erroneous records samplings performed over large distances within each lava flow would provide the best way to average out the contribution of the anomalies in any case such effects should not be neglected in paleomagnetic studies

volcanic and tectonic evolution of the northern antarctic peninsula late cenozoic to recent abstract field investigation together with a number of geochemical petrographical analyses as well as absolute k ar age determinations and geophysical data allow the recognition of an evolutionary sequence of geodynamic events which have affected the northern region of antarctic peninsula and the adjacent islands a significant volcanic calc alkaline belt which developed on the northwestern margin of the antarctic peninsula during the cretaceous to middle tertiary is indicative of active subduction of the antarctic plate in that area this activity decreases during the lower miocene giving way to an expansive phase represented by the bransfield rift these extensional processes are dominant during the pliocene creating a rift system in southeastern bransfield towards larsen both the bransfield and larsen systems comprise one fan like rift system associated with the prince gustav rift and the scotia arc micro plate ejection of abundant pyroclastic material generated a large plateau of palagonite hyaloclastites of basaltic alkaline composition during the pleistocene recent the extensional activity continued as evidenced by the active volcanic fractures represented in bransfield by the deception penguin and bridgeman volcanic centres in the prince gustav rift by paulet islands and others and in larsen by the coley seal nunatak and argo volcanic centres the latter is characterized by basaltic olivine alkaline effusions these rifts and the continental blocks are affected by a series of fractures with a n60 70 w strike which could be directly associated with the hero fracture zone extending northwest of the south shetland islands trench

holocene glacial history and sea level changes on james ross island antarctic peninsula a reconstruction of deglaciation and associated sea level changes on northern james ross island antarctic peninsula based on lithostratigraphical and geomorphological studies shows that the initial deglaciation of presently ice free areas occurred slightly before 7400 14 c yr bp sea level in connection with the deglaciation was around 30 m a s l a glacier readvance in brandy bay of at least 7 km with the initial 3 km over land reached a position off the present coast at ca 4600 yr bp the culmination of the advance was of short duration and by 4300 yr bp the coastal lowlands again were ice free a distinct marine level at 16 18 m a s l was contemporaneous with or slightly post dates the brandy bay advance thus indicating the relative sea level around 4600 4500 yr bp our results from james ross island confirm that over large areas in this part of antarctica the last deglaciation occurred late 1997 by john wiley sons ltd

effects of isostasy on large scale geoid signal i geoid anomaly over an earth like planet summary the geoid anomaly due to lithospheric changes such as cooling is a second order quantity which is highly sensitive to the definition of isostasy many different terms must be summed over a range of depths so a simplified test planet is required to make sure the algorithm is working correctly our model planet consists of an ocean underlain by lithosphere and asthenosphere there are two ocean ridges along 0 and 180 longitudes and two trenches along 90 and 270 longitudes the oceanic plates are moving away from the ridge with a velocity of 5 cm yr l at the equator pressure is assumed constant at the compensation depth which is itself an equipotential surface we use the method of rings in which a set of 18 rings about an arbitrary pole observation point cover the whole earth this method allows the anomaly source depth to be taken into account changes in the equipotential surface height both above and below any ring induce mass changes due to infilling with sea water and mantle material respectively these mass changes cause further changes in the geoidal height over and below all the other rings and the problem renders itself into a set of linear simultaneous equations the final geoid is a product of seven second order effects the direct dipole effect due to the density differences in the cooling lithosphere lister 1982 produces a geoidal elevation of sll m and 3 11 m over the ridge and the trench respectively the direct mass effect vening meinesz 1946 due to the fact that a column on a sphere is pie shaped and not straight sided as on a flat earth model produces a geoidal elevation of s12m and 3 11 m over the ridge and the trench respectively the final geoidal elevation due to the direct and indirect effects is s26 m and 3 26 m over the ridge and the trench respectively

isotopic and trace element constraints on mixing and melting models of marginal basin volcanism bransfield strait antarctica bransfield strait is a narrow marginal basin separating the south shetland islands from the northern end of the antarctic peninsula quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting and as two subaerial off axis volcanoes on the northern margin of the strait the bransfield strait lavas are similar to published analyses from other marginal basins ranging from basalts and basaltic andesites to trachytes they exhibit moderate enrichments in alkali and alkaline earth elements relative to high field strength elements that are typical of many back arc basin basalts the seamount basalts have trace element chemistry similar to enriched mid ocean ridge basalts e morb but with variously higher alkali and alkaline earth element concentrations and frequently lower nb low pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active melting of depleted mantle mixed with 0 5 2 crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes the off axis volcanoes are the products of less partial melting than the on axis volcanoes one of the off axis volcanoes also contains more of an enriched component in its source some of the lavas have rb sr too low to account for their 87sr 86sr perhaps due the loss of rb during mantle metasomatism or interaction between slab derived fluids and the mantle source of the basalts strontium neodymium and 207pb 204pb isotopic signatures remained constant during the transition from tertiary island arc volcanism to quaternary marginal basin volcanism but 206pb 204pb increased high cen smn of basalts from some volcanoes requires residual garnet in the source thus the young 15 25 ma at the trench subducted slab is either deeper than 60 km beneath the volcanoes or it has fractured and is no longer a coherent barrier to melts generated below it

late holocene tephrochronology of the northern antarctic peninsula abstract andesitic and basaltic andesitic tephra layers are abundant in holocene deposits from the antarctic peninsula visually discernible tephra horizons occur in three lakes on livingston island tephra in two other lakes and in a moss bank on elephant island with very low ash concentrations were detected magnetically deception island is the most likely volcanic source for the tephra with direct 14c dating age depth curves and cross correlations at least 14 tephra horizons dating to between ca 4700 and 250 yr b p were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the holocene climatic history of antarctica

a geomagnetic field model for the holocene based on archaeomagnetic and lava flow data abstract we propose a new geomagnetic field model for the holocene period based on archaeomagnetic and lava flow data avoiding the use of lake sediment data the source of data comes from the geomagia50v2 database which has been updated with the new archaeomagnetic and volcanic studies published during the last 3 yr the model called sha dif 14k allows us to analyse the behaviour of the geomagnetic field for the last 14 000 yr from 12 000 bc to 1900 ad for the model construction we use the spherical harmonic analysis in space and the penalized cubic b splines in time both spatial and temporal regularization norms are used to constrain the inversion problem and applied at the core mantle boundary cmb to assure the convergence of the model for the last 3 ka the model predictions agree with those given by the global model arch3k 1 and the european model scha dif 3k for older epochs the new model presents a clear improvement in field resolution with respect to other current models of the geomagnetic field for the holocene for the last 9 ka the time evolution of the dipolar moment obtained from the dipole field shows a clear minimum between 5500 bc and 3000 bc and the well known continuous decreasing trend of the geomagnetic field strength for the last millennium and a half a general view of the time average evolution of the geomagnetic field flux lobes at the cmb for the northern hemisphere suggests a marked lobe of positive magnetic flux when the dipole moment was maximum this lobe vanishes when the dipolar field is decreasing the north polar wander paths of both north magnetic dip and geomagnetic poles were obtained showing an average rate of motion of 5 1 km yr and 3 7 km yr respectively the model shows that the geomagnetic field can be averaged as axial dipolar in 2000 yr within an error of 5 the typical uncertainty of the palaeomagnetic studies finally and following the recent definition of archaeomagnetic jerks we found 8 critical events in the time evolution of the geomagnetic field for the last 8 ka characterized by a maximum in the hemispheric asymmetry of the proposed model the model is available in the earth ref digital archive at http earthref org erda 1897

impact of mesoscale eddy transfer on heat uptake in an eddy parameterizing ocean model abstractusing a set of experiments with an eddy parameterizing ocean model it is found that the strength of the atlantic meridional overturning circulation amoc intensifies with the decrease of

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

evidence of two holocene phreatomagmatic eruptions at stromboli volcano aeolian islands from paleomagnetic data 1 we present detailed stratigraphic sedimentological and paleomagnetic analyses of holocene phreatomagmatic deposits at stromboli volcano aeolian islands italy the investigated deposits belong to the secche di lazzaro succession sdl and to the advanced operations center of the department of civil defense coa succession both lying on the neostromboli lavas ca 13 5 ka the two stratigraphic successions have similar stratigraphic position and show a phreatomagmatic origin likely related to catastrophic magma water interaction processes during sector collapse events however the thermal remanent magnetization data of the lava lithics indicate that deposits of the sdl succession were emplaced at very low temperatures less than 140 c whereas the basal part of the coa deposit was emplaced at temperatures between 300 340 c paleomagnetic results indicate that the two investigated deposits may be related to two distinct eruptive events occurred during the holocene recent phreatomagmatic activity at stromboli apparently has occurred more frequently than previously believed consequently we must reconsider the recurrence interval of this dangerous eruptive scenario for stromboli

application of ams for reconstruction of the geological evolution of recent volcanic systems case of deception island south shetland islands antarctica abstract the analyses of recent volcanic systems can encounter limitations due to recent eruptions hiding part of the geological history where the geomorphological expression can present non univocal interpretations or where modern relief and abrupt cliffs can decrease the accessibility to places that are suitable for detailed analysis in the case of deception island bransfield trough the recent tectonic and volcanic history and the interactions with glacial periglacial and marine activities make traditional analysis difficult anisotropy of magnetic susceptibility ams a petrofabric approach has allowed for the identification of a clear correlation between the anisotropy of the magnetic ellipsoid the axial distribution characteristics at the site scale the type of defined magnetic fabrics and the imbrication angle between the magnetic foliation and the emplacement surface however these aspects are independent of the dip of the emplacement surface in some of the analysed sites the shear plane the plane that contains kmax and kmin axes and the flow direction shows non suitable kinematic patterns to respect the dip of the emplacement surface a tilting process is needed to recover the expected original distribution in some cases affecting the analysed units through a concentric fracture system with respect to the main caldera or following the regional tectonic structures a detailed analysis of ams data permits us to constrain the recent evolution of a volcanic system through three stages 1 a radial distribution of volcanic deposits 2 a coaxial tilting of the analysed zones respect the caldera collapse and 3 a tectonic related fracturing system responsible for the tilting of the volcanic system