palaeomagnetic confirmation of palaeozoic clockwise rotation of the famatina ranges nw argentina implications for the evolution of the sw margin of gondwana summary palaeomagnetic results from palaeozoic volcanic and sedimentary units of the famatina ranges in nw argentina 28 7 s 67 8 w are reported a late early to late middle ordovician palaeomagnetic pole was obtained from a pre tectonic remanence carried by magnetite and isolated in volcanics of the molles formation and the cerro morado group mcm1 16 7 s 357 2 e a95 6 5 k 38 5 n 14 sites this pole position is rotated 39 clockwise respect to the coeval reference pole for gondwana but it is consistent with previous early ordovician poles from the famatina belt and the faja eruptiva oriental in the puna region of nw argentina the sedimentary layers of the molles formation however present a secondary magnetization carried by hematite which is interpreted of permian age and yields a pole position mcm2 at 78 7 s 330 8 e a95 7 2 k 16 1 n 27 samples two additional independent palaeomagnetic poles were obtained from the permian de la cuesta formation exposed at two different localities in the same area while one consisted in a exclusively reverse polarity magnetization and a pole position lc1 76 9 s 345 2 e a95 6 0 k 21 1 n 29 samples compatible with the late early to early late permian palaeomagnetic poles from south america the other presented only normal polarities and a pole position lc2 74 5 n 275 4 e a95 2 0 k 258 3 n 21 samples suggestive of a cretaceous remagnetization these new palaeomagnetic results confirm on a much more robust database previous proposals that the ordovician rocks of the famatina belt have undergone a large clockwise rotation they also constrain the rotation to pre permian times different tectonic models involving the late ordovician docking of a large para authochthonous terrane or a pattern of systematic large scale rotations in the early palaeozoic continental margin of western gondwana are discussed

magnetostratigraphy of miocene pliocene zagros foreland deposits in the front of the push e kush arc lurestan province iran abstract the timing of the deformation in the zagros simply folded belt is poorly constrained because of the lack of an accurate absolute chronology of the syntectonic sedimentary sequences the foreland basin infill at the front of the push e kush arc is composed of fine grained fluvial plain deposits agha jari fm and coarse conglomerates at the top of the section bakhtyari fm a magnetostratigraphic study was carried out in a composite section spanning about 2800 m in order to date growth strata and to constrain the timing of the deformation in the mountain front flexure mff magnetostratigraphic correlation of the base of the agha jari fm with chron c5a yields an age of 12 8 to 12 3 ma for this base the transition to the conglomerates of the bakhtyari fm correlates with the upper gauss chron c2an at approximately 3 ma the deposition age of the top of the preserved bakhtyari fm is extrapolated around the pliocene pleistocene boundary the base of the agha jari fm growth strata and thus the beginning of the deformation in the front of the push e kush arc is dated at 8 1 7 2 ma the topmost preserved bakhtyari is folded in the ne flank of the changuleh anticline and is unconformably overlying the sw flank of the anaran anticline this indicates that the tectonic deformation in the front of the push e kush arc was active at least during 5 my the mff is a relatively long lived structure active from 8 1 to 7 2 ma to about the pliocene pleistocene boundary partly synchronous with the changuleh anticline to the foreland after mff tectonic cessation only the changuleh anticline remained active

compaction induced inclination shallowing in synthetic and natural clay rich sediments experiments using single clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite illite montmorillonite or chlorite were conducted to determine the effect of compaction on inclination a natural marine sediment and a reconstructed marine sediment made up of the clays used in our single clay experiments were also studied a laboratory postdepositional remanence imparted to the slurries by stirring had its inclination shallowed by compaction for both the single clay samples as seen previously anson and kodama 1987 and the natural and reconstructed marine sediments suggesting that our single clay sediments are valid analogues of natural sediments magnetic intensity also decreased by 30 50 during compaction the greatest decrease occurring for samples with the steepest initial inclination altering the ph of a slurry so that the magnetite would have a negative surface charge and be repelled by clay particles did not affect a slurry s inclination shallowing behavior suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing however most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high comparison with a clay microfabric compaction study mcconnachie 1974 suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay particle aggregates clay domains since pore spaces are large at these low pressures inclination shallowing is probably caused by magnetite particles either being attached to clay particles perhaps by van der waals forces or having been incorporated into clay domains alternating field demagnetization of compacted samples which contain magnetite with a range in grain sizes supports this model indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains our results suggest that a sediment s clay content may be an important contributor to inclination shallowing

cenozoic provenance history of synorogenic conglomerates in western argentina famatina belt implications for central andean foreland development in famatina at the modern northern flat slab segment of the central andean belt synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting stratigraphically constrained provenance analyses record a multi event history of the andean foreland and a complex pattern of exhumed uplands which in turn controlled the depocenter history western and eastern sources can be differentiated the western domain was composed of equigranular granitoids and lower and upper paleozoic volcano sedimentary and sedimentary units whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower paleozoic clastic rocks the surrounding regions expose medium to high grade metamorphic rocks which are not recorded in any cenozoic conglomerate of famatina unroofing sequences can be deduced from the compositional trends conglomerate composition and paleocurrent data show input from intraba sinal sources and important participation of basement since the beginning of the neogene we infer an early miocene broken foreland during deposition of the del creston formation a significant source inversion with clasts supplied from the eastern basement domain is recorded 2000 m from the base within the middle miocene del abra formation reactivation of western granitic thrust sheets occurred by the late middle miocene and late miocene as indicated by provenance studies of the santo domingo and el durazno formations in contrast to simple asymmetric foreland models provenance analysis documents early participation of crystalline rocks suggesting early development of intrafore land basement highs within the southern central andes at a regional scale basement thrusting and associated intermontane basin development were diachronous this interpretation is supported by broad propagation of basement thrusting from the northern sierras pampeanas and famatina toward the central and eastern sierras pampeanas between early middle miocene and late miocene pliocene time early miocene reconstructions of the juan fernandez ridge position on the central andean margin suggest that during this interval shallow subduction was likely unrelated to oceanic plateau subduction under the sierras pampeanas

magnetostratigraphy and paleomagnetism of early and middle miocene synorogenic strata basement partitioning and minor block rotation in argentine broken foreland magnetostratigraphic and paleomagnetic studies on early andean synorogenic strata del creston fm in the famatina belt 28 7 s 67 5 w clarify details of chronology that permit calculation of sedimentation rates within the broken foreland of west argentina the del creston fm represents the first record of broken foreland sedimentation within the southern central andean belt and the earliest retroarc volcanic rocks exposed several hundred kilometers from the trench twenty five out of 49 sites collected along the succession presented a primary remanence as determined through positive fold and reversal tests correlation of the local magnetic polarity section with the global polarity time scale indicates that the sedimentation of del creston fm started at 16 7 ma and continued until 14 5 ma the youngest strata are represented by conglomerates bearing abundant lower paleozoic granite boulders indicating unroofing of the crystalline basement within the nw sierras pampeanas this result supports the hypothesis of an early broken foreland stage at these latitudes of the andes with involvement of the basement in deformation and coeval retroarc volcanism common attributes of flat subduction regimes a mean site paleomagnetic direction of dec 6 3 inc 43 6 95 8 0 n 24 confirm our earlier intrepretation that the central part of the famatina belt within the sierras pampeanas did not undergo large vertical axes rotations since the middle miocene

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina comment and discussion in a recent contribution to the geological society of america bulletin carrapa et al 2008 1 provided an interesting geochronological and thermochronological database for the fiambala basin northern end of the sierras pampeanas which together with stratigraphic sedimentological and

processes of late quaternary turbidity current flow and deposition on the var deep sea fan northwest mediterranean sea late quaternary sedimentation patterns on the var deep sea fan are known from high resolution seismic boomer profiles vertical resolution 120 m where it spilled over the eastern var sedimentary ridge at a velocity of 2 5 ms sup minus 1 other holocene turbidity currents with a 103 yr recurrence interval were muddier and thicker but also deposited sand on middle fan valley levees and are inferred to have had a similar slide related origin late pleistocene turbidity currents deposited on the high var sedimentary ridge the presence of sediment waves and the cross flow slope inferred from levee asymmetry indicate that some flow were hundreds of meters thick with velocities of 0 35 ms sup minus 1 more estimated times for deposition of thick levee mud beds are many days or weeks late pleistocene flows therefore are interpreted to result from hyperpycnal flow of glacial outwash in the var river variation in late pleistocene holocene turbidite sedimentation thus is controlled more by changes in sediment supply than by sea level less

high resolution magnetostratigraphy of the neogene huaitoutala section in the eastern qaidam basin on the ne tibetan plateau qinghai province china and its implication on tectonic uplift of the ne tibetan plateau abstract the closed inland qaidam basin in the ne tibetan plateau contains possibly the world s thickest 12 000 m continuous sequence of cenozoic fluviolacustrine sedimentary rocks this sequence contains considerable information on the history of tibetan uplift and associated climatic change however work within qaidam basin has been held back by a paucity of precise time constraints on this sequence here we report on a detailed paleomagnetic study of the well exposed 4570 m huaitoutala section along the keluke anticline in the northeastern qaidam basin where three distinct faunas were recovered and identified from the middle miocene through pliocene constrained by these faunal ages the observed thirty three pairs of normal and reversed polarity zones can be readily correlated with chrons 2n 5br of the gpts this study assigns the age of the section to the interval between ca 15 7 ma to 1 8 ma in addition the widely used stratigraphic units the xia youshashan shang youshashan the shizigou and qigequan formations were formed at 15 3 ma 15 3 8 1 ma 8 1 2 5 ma and

cenozoic dextral transpression and basin development at the southern edge of the puna plateau northwestern argentina abstract in northwestern argentina the southern edge of the altiplano puna is the most remarkable positive topographic anomaly average altitude 3700 m of the andean chain to the south are the sierras pampeanas a less elevated domain made of alternating compressive basins and ranges developed during the cenozoic at about 27 s the transition between the puna and the sierras pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the pacific subduction zone nazca plate from 30 e below the plateau to subhorizontal below the sierras pampeanas we argue that this structural transition which can be identified at the scale of the andean topography is a major dextral transpressional transfer zone the tucuman transfer zone in this region ene wsw to e w directed subhorizontal shortening related to bulk convergence between the nazca and south american plates interferes with nw se directed subhorizontal shortening strains due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics structural observations digital mapping spot satellite images and a microtectonic analysis of fault populations provide general insights into the neogene kinematic evolution of transpressional basins and ranges at the edge of the puna

chronology of motion in a complete thrust belt the precordillera 30 31 s andes mountains in the precordillera thrust belt of west central argentina faulted rotated and onlapping relations of piggyback basin units and foreland basin units yield an accurate but imprecise chronology of thrust motion addition of data from provenance history strata accumulation rates and presence of thick conglomerate units in the bermejo foreland basin results in a thrust chronology that is more precise but not entirely independent of assumptions about foreland basin dynamics seven new radiometric dates allow substantial revision of the interpreted age of first deformation in the precordillera at about 30 south units previously assigned to the paleozoic and mesozoic are of oligocene and miocene ages a redbed unit that began before 21 6 pm 0 8 ma appears to have pre dated thrusting deposits in the early bermejo foreland basin were dominated by eolian sandstones unlike the younger fluvial and alluvial units an andesitic volcanic center within the miocene foreland basin had long term effects on sub

concentrated perchlorate at the mars phoenix landing site evidence for thin film liquid water on mars 1 nasa s phoenix mission which landed on the northern plains of mars in 2008 returned evidence of the perchlorate anion distributed evenly throughout the soil column at the landing site here we use spectral data from phoenix s surface stereo imager to map the distribution of perchlorate salts at the phoenix landing site and find that perchlorate salt has been locally concentrated into subsurface patches similar to salt patches that result from aqueous dissolution and redistribution on earth we propose that thin films of liquid water are responsible for translocating perchlorate from the surface to the subsurface and for concentrating it in patches the thin films are interpreted to result from melting of minor ice covers related to seasonal and long term obliquity cycles

hirise observations of new impact craters exposing martian ground ice twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid latitudes and high latitudes on mars in the northern hemisphere the craters are widely distributed geographically and occur at latitudes as low as 39 n stability modeling suggests that this ice distribution requires a long term average atmospheric water vapor content around 25 precipitable micrometers more than double the present value which is consistent with the expected effect of recent orbital variations alternatively near surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric co2 or the vapor pressure at the ice table could be lower due to salts ice in and around the craters remains visibly bright for months to years indicating that it is clean ice rather than ice cemented regolith although some clean ice may be produced by the impact process it is likely that the original ground ice was excess ice exceeding dry soil pore space in many cases observations of the craters suggest small scale heterogeneities in this excess ice the origin of such ice is uncertain ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders but in some cases nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

magnetic polarity stratigraphy of the miocene rio azul section precordillera thrust belt san juan province argentina the chronostratigraphy of synorogenic strata of rio azul in the bermejo foreland basin argentine andes reveals details of miocene tectonic activity and deposition chronostratigraphic interpretations are based on magnetic polarity stratigraphy and zircon fission track dates of interbedded volcanic ashes the rio azul section is located within the precordillera thrust belt uplifted and rotated by motion on subjacent faults the cenozoic strata are themselves broken by two principal zones of faulting whose displacements are great enough to affect correlation of the local magnetic stratigraphy to the magnetic polarity time scale the 3200 m thick section coarsens upward from sandstone and mudstone cuculi formation to conglomerate el corral formation the magnetostratigraphic approach could be used only in the relatively fine grained strata of the lower part of the section the radiometric ages indicate the entire section is of middle to late miocene age and we correlate the magnetic polarity patter

reinterpretation of the ordovician rotations in nw argentina and northern chile a consequence of the precordillera collision early paleozoic paleomagnetic data from nw argentina and northern chile have shown large systematic rotations within two domains one composed of the western puna that yields very large up to 80 counter clockwise rotations and the other formed by the famatina ranges and the eastern puna that shows 40 clockwise rotations around vertical axes in several locations lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the paleozoic previous tectonic models have explained these rotations as indicative of rigid body rotations of large para autochthonous crustal blocks or terranes a different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of precordillera in the late ordovician this collision should have generated dextral shear zones in the back arc region of the convergent sw gondwana margin where systematic domino like clockwise rotations of small crustal blocks accommodate crustal shortening the western puna block bordering the precordillera terrane to the north might have rotated counterclockwise as an independent microplate due to tectonic escape processes in a fashion similar to the present day relationship between the anatolia block and the arabian microplate

comment on the combined analysis of remagnetization circles and direct observations in paleomagnetism by p l mcfadden and m w mcelhinny

chronology of neogene sedimentation stratigraphy and tectonism in the campo de talampaya region la rioja province argentina abstract chronostratigraphic paleoenvironmental and tectonic analysis of a 2320 m thick neogene continental section was conducted along the southern limit of the campo de talampaya magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology the strata record a period of mixed evaporitic and terrigenous elastic sedimentation 20 6 1 ma that persisted in the distal bermejo basin during eastward migration of the precordillera fold thrust belt a critical transition of depositional environments and a change from western to eastern source areas occurred between 12 9 and 12 3 ma the saline mud flat environment of the rio manero formation derived from a western source and characterized by fine grained sandstones mudstones and evaporites yielded to a short period of playa lake deposition this was soon overwhelmed by deposition of the desencuentro formation composed of sandstones and mudstones with an eastern source area and deposited in a sand flat mud flat environment we propose that this stratigraphic transition resulted from initiation of uplift of the sierra de velasco a range of the sierras pampeanas located 65 km to the east northeast of the section basement clasts probably derived from the sierra de velasco are found in conglomeratic lenses beginning in the lower part of the desencuentro formation the modern campo de talampaya basin formed after deposition of the top of the alto de san nicolas paleomagnetic section 6 1 ma when west verging reverse faults uplifted the sierra morada and sierra de los tarjados all strata in the section were tilted eastward during these culminating uplift events

neogene deformation of the flat subduction segment of the argentine chilean andes magnetostratigraphic constraints from las juntas la rioja province argentina substantial neogene deformation occurred in the part of the andes mountains and the sierras pampeanas that overlie a subhorizontal segment of the subducting nazca plate in northern argentina the history of the volcanic arc suggests that the nazca plate attained the present low angle inclination between about 17 and 10 ma with the region underlain by the flat nazca plate widening eastward thereafter we compare the history of deformation provided by magnetostratigraphy of miocene strata in foreland and intermontane basins to the history of the subducting plate at las juntas in the precordillera thin skinned thrust belt 4 780 m of neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin magnetostratigraphic data complemented by fission track dating of an interbedded tuff indicate that the strata span from 18 to 9 ma the earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the frontal cordillera subsequently the deformation front migrated eastward generating the precordillera thrust belt and finally the sierras pampeanas basement uplifts the rate of strata accumulation varied systematically as new uplifts arose with a maximum rate exceeding 1 0 mm a between 15 and 12 ma a correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed although large scale factors affecting the plate margin beyond the flat slab segment may have partly induced the compressional deformation this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

magnetostratigraphy of neogene andean foreland basin strata lat 33 s mendoza province argentina neogene nonmarine strata in the southernmost part of the precordillera thrust belt of the andes of argentina accumulated during the miocene and pliocene in response to eastward advance of thrust sheets located to the west to link the sequence of deformational events with the sedimentary record five stratigraphic sections in the mendoza province were dated using magnetic polarity stratigraphy calibrated with 40 ar 39 ar dates from interbedded tephras these data establish age control on tectonic activity volcanism and deposition the existence of a backarc foreland basin that developed prior to or ca 16 ma is recorded by the oldest strata the ca 15 7 12 2 ma baksi time scale marino formation deposition of this unit is linked to a major phase of thrust activity in the central part of the aconcagua fold and thrust belt in the principal cordillera initiation of deposition coincides with an eastward shift of the subduction related magmatic foci to the argentine principal cordillera the overlying ca 11 7 9 0 ma la pilona formation is thought to record the initiation of exhumation of the frontal cordillera to the northwest of the study region an extensive ash rich unit the 8 9 locally 9 5 to 8 7 ma tobas angostura formation is correlated with the latest and probably the largest extrusive episode in the principal cordillera chronologic data constrain the timing of oil generation in the cuyo basin and development of prospective closures the new data also require that land mammalian fossils found in the rio de los pozos formation are late miocene rather than pliocene as previously believed

temporal constraints on the tectonic evolution of sierra de famatina northwestern argentina using the fission track method to date tuffs interbedded in synorogenic clastic sedimentary strata fission track dates from seven volcaniclastic units allow time constraints to be placed on synorogenic clastic strata exposed along the flanks of sierra de famatina northwest argentina four formations are exposed along the western margin of sierra de famatina the el abra conglomerate is composed of clasts eroded from the basement exposed just to the west of cadena de paiman the el buey formation contains sediments probably derived from the precordillera fold and thrust belt to the west fission track dates give a maximum age of 6 ma for the deposition of both of these formations the el durazno formation deposited between 6 and 4 ma contains proximal volcaniclastic and other clastic rocks derived from the core of sierra de famatina the santa florentina conglomerate was derived from the rising sierra de famatina and deposited in the past 4 m y sierra de famatina probably become a positive topographic feature approximately 6 ma prior to that time this region was receiving fine grained sediments fro

compositions of subsurface ices at the mars phoenix landing site 1 nasa s phoenix lander uncovered two types of ice at its 2008 landing site on the northern plains of mars a brighter slab like ice that broke during robotic arm operations and a darker icy deposit spectra from the phoenix surface stereo imager ssi are used to demonstrate that the brighter material consists of nearly pure water ice which probably formed by migration and freezing of liquid water the darker icy material consists of 30 20 wt ice with the remainder composed of fine grained soil indicating that it probably formed as pore ice these two types of ice represent two different emplacement mechanisms and periods of deposition

mio pliocene volcaniclastic deposits in the famatina ranges southern central andes a case of volcanic controls on sedimentation in broken foreland basins abstract a well constrained record of miocene pliocene explosive volcanism is preserved within the broken foreland of western argentina along the famatina ranges this paper focuses on the volcaniclastic record known as the rio blanco member of the el durazno formation three facies can be recognized in the study area 1 massive tuffs 2 volcaniclastic conglomerates and 3 pumiceous sandstones these facies are interpreted as primary pyroclastic flow deposits ignimbrites and reworked volcanogenic deposits within interacting volcanic fluvial depositional systems alternation between ignimbrites and volcanogenic sandstones and conglomerates suggest a recurrent pattern of sedimentation related to recurrent volcanic activity considering the facies mosaic and relative thicknesses of facies short periods of syn eruption sedimentation volcaniclastic deposits seem to have been separated by longer inter eruption periods where normal stream flow processes were dominant the volcaniclastic component decreases up section suggesting a gradual reduction in volcanic activity the mean sedimentation rate of the rio blanco member is higher 0 44 mm year than those obtained for the underlying and overlying units this increase cannot be fully explained by foreland basement deformation and tectonic loading hence we propose subsidence associated with volcanic activity as the causal mechanism volcanism would have triggered additional accommodation space through coeval pyroclastic deposition modification of the stream equilibrium profile flexural loading of volcanoes and thermal processes these mechanisms may have favored the preservation of volcaniclastic beds in the high gradient foreland system of famatina during the mio pliocene thus the rio blanco member records the response of fluvial systems to large volcanism induced sediment loads

magnetostratigraphy of paleogene sediments from northern qaidam basin china implications for tectonic uplift and block rotation in northern tibetan plateau abstract we have carried out a detailed magnetostratigraphic study of tertiary sediments in the northern margin of the qaidam basin northern tibetan plateau a total of 400 paleomagnetic samples with an interval of about 2 m were collected from the shanggancaigou and xiagancaigou formations in the hongsanhan section stepwise thermal demagnetization was used to isolate the magnetic components in these samples a higher temperature component htc was separated between 300 and 680 c from most of samples positive fold and reversal tests suggest a primary origin for the htc a magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale ostracoda assemblages and also a fission track age from an adjacent section suggest a tertiary age of the shanggancaigou and xiagancaigou formations in the hongsanhan area the magnetozones in the xiagancaigou formation may correspond to subchrons c16n c18 ranging from 40 to 35 5 ma mid eocene and the shanggancaikou formation to subchrons c8 2n c15 age from 35 5 to 26 5 ma late eocene to late mid oligocene the accumulation of sandy conglomerate and conglomerate layers on top of the shanggancaikou formation and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north qaidam at about 30 ma when compared with coeval directions expected from eurasia the mean declinations indicate a slight vertical axis rotation of the northern qaidam basin since the mid eocene sinistral shear strain may be concentrated on the altyn tagh fault atf

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference