early diagenetic silica deposition in algal cysts and spores a source of sand in black shales abstract devonian black shales deposited on the north american craton contain abundant tasmanites cysts although these are typically flattened because of compaction a small proportion are filled with diagenetic silica the latter are spherical to ellipsoidal 0 1 0 5 mm and filled with chalcedony microquartz and megaquartz and single quartz crystals chalcedonic cyst fillings are preserved best in cheat and phosphate nodules whereas megaquartz and single quartz crystals are most common in shale matrix together with colloform textures this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits thin sandstone beds in the chattanooga shale e g bransford sandstone contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales because of this their quartz component is thought to have been transported over considerable distances from the basin margin however because certain shale horizons contain as much as 10 silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size the quartz component of these sandstone beds may actually have formed in situ indeed petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of cyst quartz e g rounding sphericity chalcedony pyrite inclusions lobate grain margins thus silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences

combining in situ isotopic trace element and textural analyses of quartz from four magmatic hydrothermal ore deposits this study couples in situ 16o 17o and 18o isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic hydrothermal quartz crystals euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic hydrothermal ore deposits el indio au ag cu deposit summitville au ag cu deposit north parkes cu au deposit and kingsgate quartz mo bi w deposit the internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry for 40 trace elements in quartz and sensitive high resolution ion microprobe for 3 isotopes in quartz imaging revealed fine oscillatory zoning sector zoning complex macromosaic textures and hidden xenocrystic cores in situ oxygen isotope analyses revealed a 18o range of up to 12 4 0 3 in a single crystal the largest isotopic range ever ascribed to oscillatory zonation in quartz some of these crystals contain a heavier 18o signature than expected by existing models while sector zoned crystals exhibited strong trace element variations between faces no evidence for anisotropic isotope fractionation was found we found 1 isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis e g d 18o using bulk analytical techniques 2 the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and 3 in three of the four samples both textural and isotopic data indicate non equilibrium deposition of quartz

slow diffusive fault slip propagation following the 6 april 2009 l aquila earthquake italy 1 two laser strainmeters that operate at 1400 m depth about 20 km ne of the epicenter of the 6 april 2009 magnitude 6 3 l aquila italy earthquake have produced a clear record of postseismic strain here we show the results from the analysis of the data related to the first few days after the event strain after about 1 5 days is fully consistent with afterslip on a stationary region of the earthquake causative fault the preceding few hour long transient whose seismic moment history is quasi exponential is fully consistent with unilateral diffusive slip propagation toward the shallower part of the same fault the propagation path ends where later afterslip probably occurred slip propagation similar to heat diffusion has been suggested to explain the observed scaling law between amplitude and duration of slow earthquakes here we give the first observational evidence of the role and details of slow rupture propagation

itrf2014 a new release of the international terrestrial reference frame modeling nonlinear station motions for the first time in the international terrestrial reference frame itrf history the itrf2014 is generated with an enhanced modeling of nonlinear station motions including seasonal annual and semiannual signals of station positions and postseismic deformation for sites that were subject to major earthquakes using the full observation history of the four space geodetic techniques very long baseline interferometry vlbi satellite laser ranging slr global navigation satellite systems gnss and doppler orbitography and radiopositioning integrated by satellite doris the corresponding international services provided reprocessed time series weekly from slr and doris daily from gnss and 24 h session wise from vlbi of station positions and daily earth orientation parameters itrf2014 is demonstrated to be superior to past itrf releases as it precisely models the actual station trajectories leading to a more robust secular frame and site velocities the itrf2014 long term origin coincides with the earth system center of mass as sensed by slr observations collected on the two lageos satellites over the time span between 1993 0 and 2015 0 the estimated accuracy of the itrf2014 origin as reflected by the level of agreement with the itrf2008 both origins are defined by slr is at the level of less than 3 mm at epoch 2010 0 and less than 0 2 mm yr in time evolution the itrf2014 scale is defined by the arithmetic average of the implicit scales of slr and vlbi solutions as obtained by the stacking of their respective time series the resulting scale and scale rate differences between the two solutions are 1 37 0 10 ppb at epoch 2010 0 and 0 02 0 02 ppb yr while the postseismic deformation models were estimated using gnss gps data the resulting parametric models at earthquake colocation sites were applied to the station position time series of the three other techniques showing a very high level of consistency which enforces more the link between techniques within the itrf2014 frame the users should be aware that the postseismic deformation models are part of the itrf2014 products unlike the annual and semiannual signals which were estimated internally with the only purpose of enhancing the velocity field estimation of the secular frame

desert loess versus glacial loess quartz silt formation source areas and sediment pathways in the formation of loess deposits abstract pathways are presented which show the proposed sequence of events involved in the formation of loess deposits both hypothetical deposits and those of central china hungary nigeria and tunisia these pathways illustrate the potential role of and inter relationships between a variety of silt generating mechanisms in the formation of loess using these pathways the existence of desert loess is discussed and it is concluded that this depends on how the terms glacial loess and desert loess are applied if desert loess is taken either to signify nonglacial mechanisms of silt production or to characterise the environment from which aeolian entrainment took place then desert loess is a very real and widely distributed deposit however if the term glacial loess is used to refer to the global climatic regime under which the loess deposit has formed rather than the geomorphic mechanisms responsible for silt generation then desert loess would not be an appropriate term for the classification of loess deposits it is suggested that classifying loess deposits with reference to the global climatic regime under which they accumulated would facilitate a greater understanding of the role of environmental conditions at all stages in the formation of loess deposits

sources of non glacial loess size quartz silt and the origins of desert loess abstract descriptions of peridesertic loess deposits and possible non glacial origins for silt size quartz are reviewed including direct release from weathered bedrock results from salt and frost weathering experiments are presented and methods by which sufficient energy can be imparted to sand size grains to release silt size fragments are identified these fragments may then be entrained by aeolian or fluvial action results from fluvial comminution experiments indicate that high energy mixed sediment fluvial systems may be very effective in generating quartz silt purely aeolian attrition is also seen as a major source of silt size and finer material results from aeolian abrasion experiments performed using crushed quartz sand are complemented by trials using sands from deeply weathered granite and pannonian sands from hungary field and laboratory observations are therefore seen to argue for the existence of deposits that can be classified as desert loess

finite fault inversion of dinsar coseismic displacement of the 2009 l aquila earthquake central italy 1 we define the geometric and kinematic characteristics of the fault activated during the mw 6 3 6 april 2009 l aquila earthquake from the modeling of envisat and cosmo skymed the first ever x band interferogram inverted for a coseismic dislocation study dinsar interferograms our best fit solution for the main shock is represented by a normal fault 16 km long and 12 km wide with a small right lateral component dipping 47 sw with a maximum slip of 90 cm although the seismic dislocation probably ended at 1 km depth the updip projection of the fault plane corresponds to the northern segment of the mapped paganica s demetrio fault where alignment of surface breaks was observed in the field the absence of this fault in existing seismic source catalogues suggests that an improved approach involving detailed surface and subsurface geological and geophysical investigations is needed for a better assessment of the seismic hazard at the local scale

making loess sized quartz silt data from laboratory simulations and implications for sediment transport pathways and the formation of desert loess deposits associated with the sahara abstract results from a series of laboratory simulations designed to investigate quartz silt generation by a number of geomorphic mechanisms are reviewed these findings are then used to discuss potential sediment pathways involved in the formation of desert loess deposits and a pathway is presented to show a proposed sequence of events involved in the formation of the loess deposits associated with the sahara the results indicate that both fluvial and aeolian systems can be very effective at producing quartz silt it is suggested that fluvial systems within hot desert environments may play a central role in both the production and distribution of loessic silt first the highly energetic but periodic fluvial regimes associated with arid and semi arid environments may be responsible for considerable quantities of particle comminution second the intermittent nature of these systems will help maintain geomorphologically active surfaces from which silt sized material can be easily deflated as well as fluvial comminution a range of weathering mechanisms and aeolian abrasion may also be responsible for the generation of silts in desert loess consequently silt sources and mechanisms of silt generation are likely to be much more diverse for desert loess than those associated with glacial loess

oxygen isotope analyses of fine silica grains using laser extraction technique comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation the laser fluorination technique reported here for analyzing the oxygen isotope composition d 18 o of fine quartz size fractions 50 20 20 10 10 5 5 2 2 1 and 1 lm has been validated by comparison with the ion microprobe technique it yields accurate d 18 o data with an external precision better than 0 15 this is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis both techniques were used to investigate d 18 o composition of a cretaceous quartzite and silcrete sequence from the southeast of france quartzite cements average 31 04 1 93 they formed from mid cretaceous seawater higher in the series silcretes cements average 26 66 1 36 they formed from upper or post upper cretaceous soil water and groundwater oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another including from an upper pedogenic silcrete to a lower groundwater silcrete occurred in a closed or weakly evaporating hydrological system

earthquake supercycles inferred from sea level changes recorded in the corals of west sumatra records of relative sea level change extracted from corals of the mentawai islands sumatra imply that this 700 kilometer long section of the sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years the moment magnitude 8 4 earthquake of september 2007 represents the first in a series of large partial failures of the mentawai section that will probably be completed within the next several decades

the active main marmara fault abstract this paper presents selections from and a synthesis of a high resolution bathymetric sparker and deep towed seismic reflection data set recently acquired by the french ifremer r v le suroit in an e w deep trough that forms the northern half of the sea of marmara in nw turkey it includes the first high resolution complete bathymetric map of this area a single throughgoing dextral strike slip fault system which is the western continuation of the northern branch of the north anatolian fault cuts this trough lengthwise and joins the 1999 8 17 kocaeli earthquake fault with the 1912 8 09 arkoy murefte earthquake fault both of which display strike slip offset in its eastern one fourth the structure follows closely the northern margin of the deep trough whereas in the west it hugs its southern margin the eastern one fourth of the structure has a minor component of its displacement distributed across the deep trough owing to a possible original bend in the course of the dextral structure the present course of the north anatolian fault in the sea of marmara originated some 2 105 a ago by cutting across the older basin fabric generated by a dominant nne ssw extension before it began taking up major motion in the pliocene

cenozoic extension in coastal dhofar southern oman implications on the oblique rifting of the gulf of aden a detailed analysis of fault populations that affect the paleocene to early miocene sedimentary succession of southern dhofar allows the definition of a time sequence of paleostresses and the reconstruction of the rifting history of the eastern gulf of aden two distinct phases of n20j and n160j trending extensions took place during the deposition of the synrift dhofar group from late priabonian 35 ma to middle burdigalian 18 ma a period of tectonic instability preceded these events marked by normal and strike slip faulting as early as the mid eocene the two main directions of extension also recorded in southern yemen developed respectively oblique and normal to the n75e trend of the gulf of aden an anticlockwise succession of the two episodes from n20 to n160 only established in yemen accounts for the observed synrift fault pattern which is consistent with the type and geometry of faults predicted in analogue models of oblique rifting a latest phase of ns compression marked by strike slip faulting may represent a far echo of the arabia eurasia collision d 2002 elsevier science b v all rights reserved

silica hydrate preserved with δ18o rich quartz in high temperature hydrothermal quartz in the high sulfidation copper gold deposit at el indio chile quartz microcrystals from the el indio au ag cu deposit chile preserve a rare glimpse into the high temperature evolution of silica here we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates opal and moganite we propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase evidence for sequential dehydration to from silica hydrate to quartz silica hydrate opal moganite quartz is provided by shrimp 18o microanalytical data that show oscillatory isotopic zoning from 3 6 to 16 2 18o 0 5 coupled with k and al variations we estimate that the precursor silica hydrate deposited between 480 680 c and contained 32 63 wt h2o silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the el indio copper gold deposit modern understanding of ore forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions the discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids quartz matured from silica hydrate would record pseudo primary fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while 18o data may bias fluid provenance interpretation within sub volcanic systems

rupture across arc segment and plate boundaries in the 1 april 2007 solomons earthquake the largest earthquakes are generated in subduction zones and the earthquake rupture typically extends for hundreds of kilometres along a single subducting plate these ruptures often begin or end at structural boundaries on the overriding plate that are associated with the subduction of prominent bathymetric features of the downgoing plate here we determine uplift and subsidence along shorelines for the 1 april 2007 moment magnitude m w 8 1 earthquake in the western solomon islands using coral microatolls which provide precise measurements of vertical motions in locations where instrumental data are unavailable we demonstrate that the 2007 earthquake ruptured across the subducting simbo ridge transform and thus broke through a triple junction where the australian and woodlark plates subduct beneath the overriding pacific plate previously no known major megathrust rupture has involved two subducting plates we conclude that this event illustrates the uncertainties of predicting the segmentation of subduction zone rupture on the basis of structural discontinuities

on the mechanics of earthquake afterslip we propose a model for earthquake afterslip based on rate and state variable friction laws in the model afterslip is attributed to the interaction of a velocity weakening region at depth within which earthquakes nucleate with an upper region of velocity strengthening frictional behavior the existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments in our model afterslip is the result of relaxation of a stress perturbation within the velocity strengthening region which arises when an earthquake propagates into that region from below we derive the stress perturbation and its decay from the friction constitutive law using a simple 1 degree of freedom approximation for the elastic interaction between the fault and its surroundings this approximation is based on thickness averaged displacements and slip velocities within the velocity strengthening region which is assumed to slip as a rigid block coseismic and postseismic slip are coupled through the thickness averaged stiffness k of the velocity strengthening region we assume k to be inversely proportional to the thickness of this region which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip we model the afterslip time histories of the 1966 parkfield and 1987 superstition hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults in accord with field observations our model predicts 1 that afterslip on some faults scales with the thickness of the unconsolidated sedimentary cover and 2 that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth velocity strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones about 500 m within their shallow regions 3 5 km combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip

a combined visual geochemical approach to establishing provenance for pegmatite quartz artifacts a technique for establishing source provenance for pegmatite quartz artifacts was developed using a quarry sample from the churchill river basin of northern manitoba and saskatchewan secondary ion mass spectrometry sims was used to quantify ti ge th and u trace element concentrations and pb isotope ratios coupled with qualitative visual characteristics to distinguish between pegmatite quartz from different archaeologically exploited sources in the churchill river basin region the technique was also applied to a small sample of quartz artifacts recovered from sites in and around the study area in a preliminary attempt to assign or rule out raw material provenance to characterized sources

pore water evolution in oilfield sandstones constraints from oxygen isotope microanalyses of quartz cement abstract oxygen isotope microanalyses of authigenic quartz in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the brae formation in the miller and kingfisher fields north sea oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a cameca ims 4f ion microprobe in conjunction with quartz cement paragenesis in the reservoirs constrained from textural and cathodoluminescence cl microscopy studies pore water evolution was reconstructed from the time of deposition of the sandstones in the upper jurassic until the present cl photomicrographs of quartz overgrowths in the brae formation sandstones show three cement zones a b and c which can be related to different oxygen isotope compositions 1 the earliest and thinnest zone a homogeneous cl pattern with probable 18 o values between 23 and 26 direct measurements were not possible precipitated in the sandstones at temperatures 18 o values between 15 and 18 precipitated in the sandstones most likely between 70 and 90 c 3 the third zone c homogeneous cl pattern and directly measured 18 o values between 16 and 22 precipitated in the sandstones most likely at temperatures 90 c calculated oxygen isotope compositions of pore waters show that zone a quartz cements and enclosing concretionary calcite precipitated from a meteoric type fluid 7 during shallow burial 18 o 7 to a more 18 o enriched fluid 18 o 4 as burial continued to 3 0 km data from zone c quartz cements are consistent with further fluid evolution from 18 o 4 to basinal type fluids with 18 o similar to the present day formation water oxygen isotope composition 0 6 at 4 0 km burial a similar pore water evolution can be derived for all three reservoirs studied indicating that hydrogeologic evolution was similar across sandstones of the whole brae formation the quartz cement zones observed in the brae formation sandstones and the pore water history derived for the area studied is analogous to published petrographic and pore water evolution data from the nearby brent group reservoirs and from reservoirs located in the haltenbanken area on the atlantic margin offshore norway considering quartz cement is a major porosity occluding phase in many reservoir sandstones and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally

cherty stringers in the barnett shale are agglutinated foraminifera abstract masses of microcrystalline quartz are abundant within several lithologies in the barnett shale lower mississippian of central texas a typical quartz mass is elongated parallel to bedding and contains a central planar microporous structure that suggests collapse of a formerly hollow spheroidal or tubular body an integrated imaging approach utilizing transmitted polarized light microscopy secondary and back scattered electron imaging cathodoluminescence imaging and x ray mapping reveals these quartz masses to be composed primarily of quartz cemented silt size detrital quartz with a minor admixture of detrital ca plagioclase and dolomite microcrystalline quartz rich masses in the barnett shale are interpreted as agglutinated foraminifera that have been dramatically collapsed during compaction locally a significant portion of the total detrital quartz resides within these biogenic accumulations this study highlights the potential for using these imaging techniques to investigate agglutinated foraminifera in lithified materials that are not amenable to disaggregation and extraction of three dimensional specimens the combined imaging techniques provide an unambiguous view into aspects of skeletal ultrastructure such as particle size and sorting that cannot be readily obtained from either conventional light microscopy or sem these techniques reveal that agglutinated foraminifers are abundant in several lithologies of the barnett shale pointing to the possibility that application in other organic rich shales may reveal these organisms to be more widespread than previously recognized comparative observations in devonian shales of the central eastern united states support this prediction

regional grain size variations in aeolian sediments along the transition between tibetan highlands and north western chinese deserts the influence of geomorphological settings on aeolian transport pathways sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains qilian shan and their arid forelands hexi corridor in north western china six main sediment types were classified medium scale 103 m geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability a tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident aeolian sediments e g loess and dune sands are widespread they are used to identify aeolian transport pathways the mu fs ratio 5 11 m 48 70 m among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions the first fraction is attributed to long distance transport in high suspension clouds whereas the latter represents local transport in saltation mode the ratio shows strong correlation with elevation r2 0 77 thus it indicates a relatively higher far traveled dust supply in mountainous areas 3000 m above sea level a s l compared to the foreland the contribution of westerlies to high mountain loess deposits is considered likely hereby the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent the contribution from distant dust sources is ubiquitous in the study area however the far distance contribution may be reduced by the availability of fine sand provided in low topography settings plain foreland areas support fine sand deflation from supplying river beds allowing the formation of sandy loess in foreland areas and intramontane basins in contrast high mountain topography inhibits strong sand deflation into loess deposits eastern parts of the hexi corridor show higher aeolian sand occurrence in contrast the western parts are dominated by gravel gobi surfaces this is attributed to higher sand supply in eastern parts provided by the badain jaran desert and fluvial storages as sand sources copyright 2014 john wiley sons ltd

gradual carbonate to clastic transition and provenance of quartz along the nw trento plateau southern alps italy abstract over the drowned site of the nw trento carbonate platform a gradual transition from early cretaceous pelagic carbonate to more siliciclastic sedimentation during the early tertiary is denoted by mineralogical and geochemical trends decreasing oxygen isotope ratios of quartz in the matrix from 34 33 to 18 47 smow suggest a mainly biogenic origin for the silica with insignificant terrigenous quartz input during the early cretaceous and only minor introduction of terrestrial quartz during the late cretaceous and early tertiary the insignificance of terrigenous input during the late cretaceous and early tertiary is consistent with existing depositional models that involve bypassing of the nw trento plateau margin the bulk of the terrigenous deposition being funnelled into the adjacent lombardian flysch basin bernoulli and winkler 1990 during the eoalpine tectonic phase doglioni and bosellini 1987

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

tectonic and climatic controls on provenance changes of fine grained dust on the chinese loess plateau since the late oligocene abstract provenance variations of late cenozoic aeolian deposits on the chinese loess plateau clp were closely associated with regional tectonic activity and climatic change previous studies however have not reached a consensus regarding the nature and origin of past variations in source this study presents the results of oxygen isotope 18 o analyses of fine grained quartz 18 o variations exhibit distinct trends and patterns of variation over six time intervals i e 25 20 20 12 12 7 7 2 6 2 6 1 2 and 1 2 0 ma in comparison with quartz 18 o results from east asian dust sources and previous provenance studies of the same aeolian sequences we identify three significant composition changes of the dust source system at around 20 12 and 2 6 ma the dust source system was also rather unstable at 25 20 12 7 and 1 2 0 ma while three stable stages occurred at 20 12 7 2 6 and 2 6 1 2 ma the correlation between the provenance changes and paleoclimatic and tectonic evidence suggests that both tectonic and climatic factors were important in driving the observed stepwise provenance changes however the changes were mainly constrained by tibetan plateau uplift prior to the quaternary and by global climate change thereafter

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

possible formation mechanisms for the modal coarse silt quartz particles in loess deposits abstract the first major event in the formation of a loess deposit is the production of silt sized quartz particles many particle production methods have been proposed the bulk of loess particles appear to be produced by a glacial grinding to give ice sheet loess or b cold weathering in high regions to give mountain loess there are close relationships between mountain loess and desert loess loess material can be seen as a product of the cold phase of the glacial cycle

comment on john s stone a possible fragment of the 1908 tunguska meteorite anfinogenov et al 2014 icarus 243 139 147 anfinogenov et al anfinogenov et al 2014 icarus 243 139 147 argue that a highly silicified gravelite sandstone referred to as john s stone could be a fragment of the long sought after impactor from the 1908 tunguska event we have analyzed the oxygen isotopic composition of two distinct samples of john s stone both kindly provided by dr yana anfingenova both samples had very similar isotopic compositions such that the average of the two was as follows 17o 6 67 0 15 18o 12 78 0 28 17o 0 01 0 01 errors 1sd this value is well within the range for samples of terrestrial quartz bearing sedimentary and metamorphic rocks john s stone has an oxygen isotope composition and mineralogy that is unlike any known group of meteorites if john s stone is extraterrestrial this would imply that it represents a previously unknown type of meteorite as suggested by anfinogenov et al however since sandstones can only form on a parent body with liquid water and by inference also an atmosphere we argue that there are only two possible parent bodies in the solar system the earth and mars mars can be ruled out for several reasons including the oxygen isotopic and noble gas composition of john s stone which is clearly distinct from martian meteorites consistent with the o study the noble gases do not provide any evidence for an extraterrestrial origin of the samples the lack of any cosmogenic noble gases particularly striking in 3he 21ne 38ar is consistent with a terrestrial origin or an extraterrestrial origin under large shielding based on the combined evidence obtained in this study we infer that john s stone is a terrestrial rock unrelated to the tunguska impactor

geochemistry and mineralogy of late quaternary loess in the upper mississippi river valley usa provenance and correlation with laurentide ice sheet history abstract the midcontinent of north america contains some of the thickest and most extensive last glacial loess deposits in the world known as peoria loess peoria loess of the upper mississippi river valley region is thought to have had temporally varying glaciogenic sources resulting from inputs of sediment to the mississippi river from different lobes of the laurentide ice sheet here we explore a new method of determining loess provenance using k rb and k ba values in k feldspars and micas in loess from a number of different regions in north america results indicate that k rb and k ba values can distinguish loess originating from diverse geologic terrains in north america further different loess bodies that are known to have had the same source sediments using other criteria have similar k rb and k ba values we also studied three thick loess sections in the upper mississippi river valley region at each site the primary composition of the loess changed over the course of the last glacial period and k rb and k ba values parallel changes in carbonate mineral content and clay mineralogy we thus confirm conclusions of earlier investigators that loess composition changed as a result of the shifting dominance of different lobes of the laurentide ice sheet and the changing course of the mississippi river we conclude that k rb and k ba values are effective robust and rapid indicators of loess provenance that can be applied to many regions of the world

distribution and magnitude of post seismic deformation of the 2009 l aquila earthquake m6 3 surface rupture measured using repeat terrestrial laser scanning we captured post seismic deformation close to the surface rupture of the 2009 l aquila earthquake m6 3 central italy using repeat terrestrial laser scan tls methods from 8 to 126 d after the earthquake we repeatedly laser scanned four road surfaces that intersected the earthquake surface rupture we modelled vertical near field deformation at millimetre level precision by comparing subsequent laser scan data sets to the first acquired at each site the horizontal post seismic deformation at each site was measured between reflectors paired across the rupture the tls data were supplemented by total station data from a fifth site which measured the vertical and horizontal components of post seismic deformation between two points spanning the rupture we find post seismic deformation increased between 44 and 126 d at the southeastern end of the rupture beneath which a significant gradient in coseismic slip exists within the fault zone the location rate of decay and spatially localized nature of the post seismic deformation within tens of metres of the surface rupture suggests it is due to afterslip in the fault zone driven by increased shear stresses at the edges of regions which slipped coseismically we note that the magnitude of post seismic deformation in the far field obtained from insar and gps is not significantly greater than the deformations we have measured close to the rupture we suggest that shallow localized afterslip within the fault zone is responsible for the majority of the regional post seismic deformation field

coseismic ground deformation of the 6 april 2009 l aquila earthquake central italy mw6 3 1 we provide field data of coseismic ground deformation related to the 6 april mw 6 3 l aquila normal faulting earthquake three narrow fracture zones were mapped paganica colle enzano p e mt castellano mt stabiata c s and san gregorio sg these zones define 13 km of surface ruptures that strike at 130 140 we mapped four main types of ground deformation free faces on bedrock fault scarps faulting along synthetic splays and fissures with or without slip that are probably due to the near surface lithology of the fault walls and the amount of slip that approached the surface coseismically the p e and c s zones are characterized by downthrow to the sw up to 10 cm and opening up to 12 cm while the sg zone is characterized only by opening afterslip throw rates of 0 5 0 6 mm day were measured along the paganica fault where paleoseismic evidence reveals recurring paleo earthquakes and post 24 8 kyr slip rate 0 24 mm yr

partitioned postseismic deformation associated with the 2009 mw 6 3 l aquila earthquake surface rupture measured using a terrestrial laser scanner using 3d terrestrial laser scan tls technology we have recorded postseismic deformation on and adjacent to the surface rupture formed during the 6th april 2009 l aquila normal faulting earthquake mw 6 3 using surface modeling techniques and repeated surveys 8 124 days after the earthquake we have produced a 4d dataset of postseismic deformation across a 3 65 m area at high horizontal spatial resolution we detected millimetre scale movements partitioned between discrete surface rupture slip and development of a hangingwall syncline over 10 s of meters we interpret the results as the signal of shallow afterslip in the fault zone we find 52 of the total postseismic hangingwall vertical motion occurs as deformation within 30 m of the surface rupture the total postseismic vertical motions are approximately 50 that of the coseismic we highlight the importance of quantifying partitioned postseismic contributions when applying empirical slip magnitude datasets to infer palaeoearthquake magnitudes

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

in situ evidence for dextral active motion at the arabia india plate boundary the arabia india plate boundary also called theowen fracture zone is perhaps the least known boundary among large tectonic plates1 6 although it was identified early on as an example of a transform fault converting the divergent motion along the carlsberg ridge to convergent motion in the himalayas7 its structure and rate of motion remains poorly constrained here we present the first direct evidence for active dextral strike slip motion along this fault based on seafloor multibeam mapping of the arabia india somalia triple junction in the northwest indian ocean there is evidence for 12km of apparent strike slip motion along the mapped segment of the owen fracture zone which is terminated to the south by a 50 km wide pull apart basin bounded by active faults by evaluating these new constraints within the context of geodetic models of global plate motions we determine a robust angular velocity for the arabian plate relative to the indian plate that predicts 2 4mmyr 1 dextral motion along the owen fracture zone this transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8 11 which induced a change in configuration of the triple junction infrequent earthquakes of magnitude 7 and greater may occur along the arabia india plate boundary unless deformation is in the formof aseismic creep

holocene sea levels and palaeoenvironments malay thai peninsula southeast asia sedimentological and palynological investigations of great songkhla lakes east coast of the malay thai peninsula southeast asia reveal sedimentary sequences rich in palynomorph assemblages dominated by pollen of mangroves and freshwater swamps compared with other regions in southeast asia the assemblages are of relatively low diversity geochronological data indicate that the great songkhla lakes record one of the earliest mangrove environments in southeast asia 8420 8190 cal yr bp which are subsequently replaced by a freshwater swamp at 7880 7680 cal yr bp owing to the decline of marine influence sea level observations from great songkhla lakes and other areas of the malay thai peninsula reveal an upward trend of holocene relative sea level from a minimum of 22 m at 9700 9250 cal yr bp to a mid holocene high stand of 4850 4450 cal yr bp which equates to a rise of c 5 5 mm yr the sea level fall from the high stand is steady at c 1 1 mm yr geophysical modelling shows that hydroisostasy

postseismic grace and gps observations indicate a rheology contrast above and below the sumatra slab more than 7 years of observations of postseismic relaxation after the 2004 sumatra andaman earthquake provide an improving view on the deformation in the wide vicinity of the 2004 rupture we include both gravity recovery and climate experiment grace gravity field data that show a large postseismic signal over the rupture area and gps observations in the back arc region with increasing time gps and grace show contrasting relaxation styles that were not easily discernible on shorter time series we investigate whether mantle creep can simultaneously explain the far field surface displacements and the long wavelength gravity changes we interpret contrasts in the temporal behavior of the gps grace observations in terms of lateral variations in rheological properties of the asthenosphere below and above the slab based on 1 d viscoelastic models our results support an almost order of magnitude contrast between oceanic lithosphere viscosity and continental viscosity which likely means that the low viscosities frequently found from postseismic deformation after subduction earthquakes are valid only for the mantle wedge next to mantle creep we also consider afterslip as an alternative mechanism for postseismic deformation we investigate how the combination of grace and gps data can better discriminate between different mechanisms of postseismic relaxation distributed deformation mantle creep versus localized deformation afterslip we conclude that the grace observed gravity changes rule out afterslip as the dominant mechanism explaining long wavelength deformation even over the first year after the event

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

new evidence for the origin of quartz cements in hydrocarbon reservoirs revealed by oxygen isotope microanalyses quartz cements degrade hydrocarbon reservoir quality by choking off fluid flow paths therefore it is important to understand the sources and timing of quartz cementation we present in situ microanalyses of oxygen isotope ratios in quartz cements from a jurassic hydrocarbon reservoir located offshore norway showing systematic variations that reveal the isotopic history of the pore fluids that precipitated the quartz earliest quartz cements have 18o values of 20 26 subsequent cements drop in 18o value as low as 13 but then increase back to 23 with increasing cement thickness this isotopic trend was observed in multiple overgrowths with temperatures constrained by fluid inclusions and the burial history of the reservoir the isotopic composition of porewaters can be estimated from the measured isotopic ratios in quartz cement results indicate that 97 of the quartz cement precipitated from negative 18o waters originally meteoric that were gradually modified by burial diagenetic reactions as temperatures increased precipitation began in the middle to late cretaceous however the rate of quartz precipitation increased dramatically in the last 3 5 ma as the reservoir was rapidly buried the data show that quartz precipitation is a continuous process during burial and that silica is derived locally from silicate reactions occurring within the sandstone reservoir the isotopic data do not record evidence of episodic cementation from convection driven hot basinal fluids

vertical motion of phuket island 1994 2018 due to the sumatra andaman mega thrust earthquake cycle impact on sea level and consequences for coral reefs abstract macro tidal coral reefs are particularly sensitive to medium to long term changes in sea level vertical motions of the seabed contribute to both lower or higher relative sea level changes particularly in tectonic plate boundary deformation zones along active subduction trenches phuket island in southern thailand is subject to both horizontal and vertical land deformations during the seismic cycle of mega thrust earthquakes along the sumatra and andaman trenches the relative sea level changes in this region were historically monitored using the space geodetic techniques gps and satellite altimetry alongside the traditional tide gauge measurements over a period of almost 25 years the gps results show that the south of thailand is still undergoing post seismic deformations from the 2004 mw 9 2 sumatra andaman earthquake after a significant change in the vertical motion of phuket from stable quasi linear uplift at 2 5 0 2 mm yr to temporary non linear subsidence rates of 5 10 mm yr in the past 14 quake aftermath years the satellite altimetry data estimates the absolute sea level rise in the andaman sea around the island at 3 9 0 5 mm yr therefore relative sea level changes in phuket appear to have been slightly positive until the end of 2004 followed by a significant increase averaging 9 mm yr that accumulated in 12 1 cm by the end of 2018 tide gauge data gives a very similar result the increased sea level rise from 2005 onwards due to tectonic land subsidence correlates with the enhanced shallow coral reef growth and swift recoveries from bleaching events reported by long term coral studies in this area

evolution of quartz cementation and burial history of the eau claire formation based on in situ oxygen isotope analysis of quartz overgrowths abstract individual quartz overgrowths in siltstone of the late cambrian eau claire formation fm are systematically zoned in oxygen isotope ratio 18 o in situ analysis of 18 o was performed with 3 and 15 m beam spots by secondary ion mass spectrometer sims on detrital quartz grains and quartz overgrowths these results from thin lenses within impermeable mudstones reflect samples that were sealed from basin wide fluid flow and compliment previous studies of more permeable sandstones individual grains of detrital quartz dq are homogeneous in 18 o the average 18 o values in fine grained detrital quartz in mudstones and siltstones and in coarser grained quartz in the eau claire fm mt simon and st peter sandstones ss are essentially identical at 18 o 10 vsmow suggesting that detrital quartz is dominantly igneous in origin the 18 o values of overgrowth quartz oq of buried samples from the illinois basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores these gradients are similar to those from the underlying mt simon ss and are best explained by increasing temperatures during burial pressure solution is evident in thin section and may have supplied significant silica for overgrowths in contrast to the deeply buried samples from the illinois basin quartz overgrowths in samples from the wisconsin arch are homogeneous and higher in 18 o those overgrowths are interpreted as quartz cements formed in a near surface environment based on these 18 o oq results and the modeled thermal history during burial of the basin the earliest formed quartz overgrowths were produced at low temperature from low 18 o water around 450 ma the 18 o values in traverses of single overgrowths decrease by up to 9 1 showing continued cementation with increased burial pressure solution and heating until 250 ma in traverses of the outermost zone of some overgrowths oxygen isotope values become constant or increase slightly possibly due to clay mineral dehydration reactions or later fluid infiltration we present a new cementation and basin evolution model in which the 18 o of cement correlates to the age of formation and the late overgrowths formed between 270 and 250 ma during and or after the migration of brines that formed the pb zn deposits of the upper mississippi valley district 270 ma cementation around 270 ma would have reduced permeability possibly ending the flow of ore forming brines

the nature origin and accumulation of loess abstract loess can be defined simply as a terrestrial clastic sediment composed predominantly of silt size particles which is formed essentially by the accumulation of wind blown dust there are three fundamental requirements for its formation 1 a sustained source of dust 2 adequate wind energy to transport the dust and 3 a suitable accumulation site most loess has been modified to some degree by local reworking bioturbation and syn depositional weathering but a process of loessification is not necessary for a dust deposit to qualify as loess the terms primary loess and secondary loess have previously been used to described wind deposited loess and redeposited loess respectively but it is more accurate to describe material reworked and redeposited by non aeolian processes as loess derived colluvium or loess derived alluvium during the quaternary loess formed in three main situations 1 mid continental shield areas beyond the limits of ice sheets periglacial loess 2 on the margins of high mountain ranges perimontane loess and 3 on the semi arid margins of some lowland deserts peridesert loess peridesert loess is relatively poorly developed partly due to the limited efficiency of silt generating mechanisms in low land deserts compared with glaciation but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition particularly in areas which experience frequent climatic changes the remarkably thick loess of china and central asia results from an unusual combination of conditions which has persisted for much of the last 2 3 million years namely rapid uplift of the tibetan plateau and surrounding mountain ranges high rates of sediment production and supply to adjacent basins a strong northwesterly and westerly wind regime and the existence of effective dust traps downwind of the source regions

source analysis of siliceous minerals and uranium in early cambrian shales south china significance for shale gas exploration abstract efficient shale gas exploration in the early cambrian shales of southern china requires the identification of favorable target areas this study used logs for u si al and total organic carbon toc from three typical wells to assess a new framework for gas exploration in this area the excess siliceous mineral content from a non terrigenous clastic source ranged from 20 to 30 in most layers reaching 50 analysis of these excess siliceous minerals using an al fe mn ternary plot showed that most were probably derived from hydrothermal fluids within the boundary of the yangtze and cathaysian plates excess siliceous mineral content and uranium content had a good positive correlation both tending to decrease away from the plate boundary indicating that both were derived from hydrothermal fluids radioactive uranium in hydrothermal fluids can enhance biological productivity which is conducive to the increased accumulation of sedimentary organic matter therefore shale gas exploration in the early cambrian layers of south china should focus on target areas near the plate boundary that have increased uranium levels moderate maturity and good preservation conditions

surface displacements associated with the l aquila 2009 mw 6 3 earthquake central italy new evidence from sbas dinsar time series analysis 1 we investigate the surface displacements in the area affected by the april 2009 l aquila earthquake central italy through an advanced dinsar analysis in particular we apply the sbas approach to retrieve deformation maps and displacement time series from envisat data acquired between february 2003 and october 2009 and from cosmo skymed data relevant to the six month interval following the earthquake our analysis shows no evidence of pre seismic surface deformation at the 35 day temporal sampling of the envisat sensor on the other hand by benefiting of the high spatial resolution and temporal sampling of the cosmo skymed satellites we measure post seismic displacements at an unprecedented level of detail for a dinsar analysis this allows us to identify three main areas continuing to deform after the earthquake in addition our modeling shows that post seismic displacements are very likely related to the fault afterslip since their decay times are on the order of 101 102 days

strain localization in the sea of marmara propagation of the north anatolian fault in a now inactive pull apart 1 the sea of marmara is presently the site of pure dextral strike slip faulting that connects the izmit fault in the east to the ganos fault in the west along a linear strike slip fault that is part of the northwestern branch of the north anatolian fault this active main marmara fault mmf cuts across a succession of depocenters and highs analysis of a large data set that includes multichannel seismic lines sparker and deep towed high definition seismics shows that most of the n120 e trending normal faults framing these depocenters are now inactive as they are sealed by an average thickness of 300 m of sediments the highs correspond to a series of ne sw trending anticlines and reverse faults blanketed by a thin cover of undeformed sediments we interpret these structures as inverted former transfer faults of the now inactive sea of marmara pull apart system depocenters and highs are the relicts of a former pull apart basins and transfer faults system that accommodated the north anatolian fault motion within the sea of marmara prior to the formation of the mmf we propose that a relatively short a few hundreds of thousands of years period of basin inversion accompanied the localization of strain along the present main marmara fault until it was fully formed 200 100 kyr ago the preexisting pull apart basin system accounts for about 30 km of motion along this fault system and thus probably appeared sometime in late pliocene or early pleistocene as a reactivated portion of the old thrace basin during the plio quaternary the sea of marmara has been the site of progressive localization of strain

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

diagenetic origin of quartz silt in mudstones and implications for silica cycling mudstone the most abundant sedimentary rock type1 composed primarily of clay or silt sized particles contains most of the quartz found in sedimentary rocks2 these quartz grains which are chemically and mechanically resistant and therefore preserve their characteristics well have long been considered to be derived from the continental crust1 here we analyse quartz silt from black shales in the eastern usa dating back to the late devonian period about 370 million years ago using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe our results indicate that up to 100 of the quartz silt in our samples does not originate from the continental crust instead it appears to have precipitated early in diagenesis in algal cysts and other pore spaces3 with silica derived from the dissolution of opaline skeletons of planktonic organisms such as radiolaria and diatoms transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 myr ago4 and molecular evidence for a much earlier appearance between 266 or even 500 myr ago5 6 moreover if many other mudstone successions show similarly high proportions of in situ precipitated rather than detrital quartz silt the sedimentary record in mudstones may have been misinterpreted in the past with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica

oblique rifting and segmentation of the ne gulf of aden passive margin 1 the gulf of aden is a young obliquely opening oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid oceanic ridge it is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading the northeastern margin of the gulf of aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin on either side of the socotra fracture zone exhibiting distinct morphologic stratigraphic and structural features the western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits though the morphostructural features of the margin segments are different the stress field analysis provides coherent results all along the margin four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses 2 and 3 the two dominant directions of extension n150 e and n20 e are perpendicular to the mean trend of the gulf of aden n75 e and parallel to its opening direction n20 e n30 e respectively unlike another study in the western part of the gulf our data suggest that the n150 e extension stage is older than the n20 e extension stage these conflicting chronologies which are nowhere unambiguously established suggest that the two extensions coexisted during the rifting on land data are compared with offshore data and are interpreted with reference to oblique rifting the passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field which reveals the asymmetry of the rifting process the first order segmentation of the sheba ridge is inherited from the prior segmentation of the passive margin

oxygen isotope signatures of quartz from major asian dust sources implications for changes in the provenance of chinese loess abstract we present a systematic investigation of the oxygen isotopic composition of quartz in both fine and coarse fractions 18 o values of both the fine and coarse fractions from the various sources are overlapped to varying degrees thus making it difficult to differentiate them nevertheless the quartz 18 o values of both fractions exhibit an increasing trend from the mongolian gobi to the northern chinese deserts and then to the taklimakan desert this implies that the geological settings of the source areas are different which in turn results in differing contributions of high temperature igneous rocks the combination of quartz 18 o results with other quartz based provenance tracers can clearly differentiate the three major source areas i e the taklimakan desert the mongolian gobi and the northern chinese deserts in addition comparison of our results with previous 18 o measurements of fine grained quartz from the luochuan loess sequence suggests the likely glacial interglacial fluctuations in dust provenance finally we suggest that the combination of quartz 18 o signatures and other dust provenance tracers can potentially improve the recognition of long term fluctuations in the provenance of chinese loess red clay deposits

holocene sea levels along the andaman sea coast of thailand for the malay thai peninsula several sea level curves for the younger holocene based on field evidence as well as on hydro isostatic modelling of a far field site have been published the general assumption is a rapid rise to a mid holocene maximum up to 5 m above present sea level followed by a constant or oscillating regression however from the andaman sea coast of thailand which was affected by the 2004 tsunami only isolated observations are available regarding holocene sea levels thus the timing and magnitude of the holocene highstand as well as the course of the regression remain to be defined as several palaeotsunamis could be detected in the meantime it is important to know the related sea levels as exactly as possible to judge the energy inundation width and potential wave height of these events therefore fixed biological indicators from the rocky coasts of the phang nga bay and phuket as well as morphological indicators from beach ridge and swale sequences along the exposed west coast ko phra thong were studied to gain information about the holocene sea level development in this region while oyster and coral data from the phang nga bay and phuket document a holocene maximum of 2 6 m at 5700 cal bp the ridge crests and swale bases in the northwest of the study area point to maximum heights of 1 5 2 0 m above the present level around 5300 years ago during the last 3000 years to when the largest part of the holocene palaeotsunami deposits from thailand was dated relative sea levels rsl in both areas did not exceed 1 5 m

an overview of the role of weathering in the production of quartz silt abstract quartz silt is a widespread detrital sediment with large aeolian loess and alluvial silt deposits forming important components of many contemporary landscapes despite research findings which indicate that a range of opportunities exist for the comminution of quartz into silt particles within a wide variety of geomorphic environments e g glacial grinding fluvial comminution aeolian abrasion frost weathering salt weathering insolation weathering and deep weathering the glacial aeolian hypothesis has traditionally been favoured as the most likely explanation for loess formation and loess is seen as primarily a quaternary phenomenon as a consequence there has been a tendency to underestimate sediment inputs into loess systems by geomorphological processes operating within pre quaternary environments in particular earth scientists may have considerably underestimated the role of weathering in global silt generation as there are now many references to the existence of a patchy but widespread distribution of pre quaternary weathering profiles across the glacial and periglacial landscapes of the northern hemisphere e g north america british isles north and central europe landscapes within which many of the classical loess deposits are located these observations suggest that weathering profiles may have covered large tracts of the northern hemisphere land surface prior to the quaternary glaciations this in turn may have important implications for quartz silt generation as 1 experimental studies indicate that many weathering processes are capable of generating significant quantities of silt sized debris and 2 particle characteristics displayed by saprolitic material that has developed on quartz rich crystalline source rocks often include significant quantities of either silt sized material or quartz grains that are weakened by weathering derived microfractures thus weathering profiles may represent mines of actual and potential quartz silt available for release into sedimentary systems when these profiles are subsequently reworked by geomorphological processes such as glacial fluvial and aeolian erosion

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

neotectonics of the owen fracture zone nw indian ocean structural evolution of an oceanic strike slip plate boundary the owen fracture zone is a 800 km long fault system that accommodates the dextral strike slip motion between india and arabia plates because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the early pliocene the fault is clearly observed on bathymetric data it is made up of a series of fault segments separated by releasing and restraining bends including a major pull apart basin at latitude 20 n some distal turbiditic channels from the indus deep sea fan overlap the fault system and are disturbed by its activity thus providing landmarks to date successive stages of fault activity and structural evolution of the owen fracture zone from pliocene to present we determine the durability of relay structures and the timing of their evolution along the principal displacement zone from their inception to their extinction we observe subsidence migration in the 20 n basin and alternate activation of fault splays in the vicinity of the qalhat seamount the present day owen fracture zone is the latest stage of structural evolution of the 20 myr old strike slip fault system buried under indus turbiditic deposits whose activity started at the eastern foot of the owen ridge when the gulf of aden opened the evolution of the owen fracture zone since 3 6 myr reflects a steady state plate motion between arabia and india such as inferred by kinematics for the last 20 myr period the structural evolution of the owen fracture zone since 20 myr including fault segments propagation and migration pull apart basin opening and extinction seems to be characterized by a progressive reorganisation of the fault system and does not require any major kinematics change

anatomy of the dead sea transform does it reflect continuous changes in plate motion a new gravity map of the southern half of the dead sea transform offers the first regional view of the anatomy of this plate boundary interpreted together with auxiliary seismic and well data the map reveals a string of subsurface basins of widely varying size shape and depth along the plate boundary and relatively short 25 55 km and discontinuous fault segments we argue that this structure is a result of continuous small changes in relative plate motion however several segments must have ruptured simultaneously to produce the inferred maximum magnitude of historical earthquakes

absorption band q model for the earth body wave surface wave and normal mode data are used to place constraints on the frequency dependence of q in the mantle with a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range the quality factor qs is proportional to in the band and to and 1 at higher and lower frequencies respectively as appropriate for a relaxation mechanism with a spectrum of relaxation times the parameters of the band are q min 80 0 15 and width 5 decades the center of the band varies from 101 seconds in the upper mantle to 1 6 103 seconds in the lower mantle the shift of the band with depth is consistent with the expected effects of temperature pressure and stress high qs regions of the mantle are attributed to a shift of the absorption band to longer periods to satisfy the gravest fundamental spheroidal modes and the scs data the absorption band must shift back into the short period seismic band at the base of the mantle this may be due to a high temperature gradient or high shear stresses a preliminary attempt is also made to specify bulk dissipation in the mantle and core specific features of the absorption band model are low q in the body wave band at both the top and the base of the mantle low q for long period body waves in the outer core an inner core qs that increases with period and low qp qs at short periods in the middle mantle the short period qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km the deformational q of the earth at a period of 14 months is predicted to be 463

insight into the 2004 sumatra andaman earthquake from gps measurements in southeast asia data collected at 60 global positioning system gps sites in southeast asia show the crustal deformation caused by the 26 december 2004 sumatra andaman earthquake at an unprecedented large scale small but significant co seismic jumps are clearly detected more than 3 000 km from the earthquake epicentre the nearest sites still more than 400 km away show displacements of 10 cm or more here we show that the rupture plane for this earthquake must have been at least 1 000 km long and that non homogeneous slip is required to fit the large displacement gradients revealed by the gps measurements our kinematic analysis of the gps recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre it also provides evidence that the rupture propagated northward sufficiently fast for stations in northern thailand to have reached their final positions less than 10 min after the earthquake hence ruling out the hypothesis of a silent slow aseismic rupture data culled from gps stations in indonesia malaysia singapore thailand india and china have answered more questions relating to the 26 december sumatra andaman megathrust earthquake permanent displacements were seen 3 000 km away final displacements occurred within 10 minutes of the earthquake ruling out slow aseismic slip on the fault

effect of annual signals on geodetic velocity 1 our analysis of global positioning system gps site coordinates in a global reference frame shows annual variation with typical amplitudes of 2 mm for horizontal and 4 mm for vertical with some sites at twice these amplitudes power spectrum analysis confirms that gps time series also contain significant power at annual harmonic frequencies with spectral indices 1 2 which indicates the presence of repeating signals van dam et al 2001 showed that a major annual component is induced by hydrological and atmospheric loading unless accounted for we show that annual signals can significantly bias estimation of site velocities intended for high accuracy purposes such as plate tectonics and reference frames for such applications annual and semiannual sinusoidal signals should be estimated simultaneously with site velocity and initial position we have developed a model to calculate the level of bias in published velocities that do not account for annual signals simultaneous estimation might not be necessary beyond 4 5 years as the velocity bias rapidly becomes negligible minimum velocity bias is theoretically predicted at integer plus half years as confirmed by tests with real data below 2 5 years the velocity bias can become unacceptably large and simultaneous estimation does not necessarily improve velocity estimates which rapidly become unstable due to correlated parameters we recommend that 2 5 years be adopted as a standard minimum data span for velocity solutions intended for tectonic interpretation or reference frame production and that we be skeptical of geophysical interpretations of velocities derived using shorter data spans

the formation of silt material for terrestrial sediments particularly loess and dust abstract there are certain controls which operate to demarcate the nature and size range of quartz particles in terrestrial detrital sediments the particles fall into discrete and definable populations which are separated by tanner gaps quartz sand nature is determined largely by a eutectic like reaction which takes place in the original igneous rock this reaction delivers a fine mix of quartz and feldspar units which on weathering deliver quartz sand the formation of quartz silt is more contentious it appears that silt production is essentially controlled by the presence in larger quartz particles of moss defects crystalline defects introduced into the quartz structure by earlier events e g the high low displacive transformation in cooling quartz there appear to be two modes in airborne quartz particles which have been called large dust and small dust the larger mode is the loess mode around 20 60 m and the smaller mode falls into the classification with high level long travel dust mode around 2 6 m the quartz in small dust is around the same size as the clay mineral agglomerate particles cma which form a considerable proportion of high level dust clouds the cma particles form from lake deposits the size is controlled by the openness of the packing in the lake sediments this can be demonstrated by a simple monte carlo model

coseismic and initial post seismic slip of the 2009 mw 6 3 l aquila earthquake italy from gps measurements summary here we report the preliminary results of gps data inversions for coseismic and initial afterslip distributions of the mw 6 3 2009 april 6 l aquila earthquake coseismic displacements of continuous and survey style gps sites show that the earthquake ruptured a planar sw dipping normal fault with 0 6 m average slip and an estimated moment of 3 9 1018 nm geodetic data agree with the seismological and geological information pointing out the paganica fault as the causative structure of the main shock the position of the hypocentre relative to the coseismic slip distribution supports the seismological evidence of southeastward rupture directivity these results also point out that the main coseismic asperity probably ended downdip of the paganica village at a depth of few kilometres in agreement with the small 1 10 cm observed surface breaks time dependent post seismic displacements have been modelled with an exponential function the average value of the estimated characteristic times for near field sites in the hanging wall of the fault is 23 9 5 4 d the comparison between coseismic slip and post seismic displacements for the first 60 d after the main shock shows that afterslip occurred at the edges of the main coseismic asperity with a maximum estimated slip of 25 cm and an equivalent seismic moment of 6 5 1017 nm the activation of the paganica fault spatially intermediate between the previously recognized main active fault systems suggests that strain accumulation in the central apennines may be simultaneously active on distinct parallel fault systems

the tectonic geomorphology of bedrock scarps on active normal faults in the italian apennines mapped using combined ground penetrating radar and terrestrial laser scanning using combined datasets from ground penetrating radar gpr and terrestrial laser scanning tls we document the variety of tectono geomorphic features that contribute to the morphology of bedrock scarps associated with active extensional faulting in central italy measurement of faulted offsets across such scarps can provide important fault slip rate data relevant to seismic hazard analysis if ages can be established for offset features however interpretation of these offsets is challenging when geomorphic processes as well as fault slip contribute to exhumation of the bedrock during scarp development through the integration of surface tls and subsurface gpr datasets we show that the surface expression of three bedrock scarps results from the interaction between footwall incision hangingwall sedimentation channel incision and landsliding as well as fault slip and fault linkage we further illustrate how these processes can be differentiated to identify locations suitable for determining fault slip rates the identification of such features has important implications for our understanding of the relationship between bedrock faulting and geomorphic processes in turn contributing to improved assessment of fault slip rates in the central apennines the use of combined gpr and tls datasets here has shown that the tectono geomorphic features of the studied scarps vary greatly over short distances and we emphasise the need for detailed scarp analysis to elucidate the processes responsible for bedrock exhumation before estimating the rates of fault slip over the timescale of scarp formation

the geochemical record of hydrothermal mineralization and tectonism inboard of the appalachian orogen the ottawa embayment abstract the history of mineralization of the ottawa embayment includes migration of basin brines within the confines of an early paleozoic basin and cross stratal fracture flow linked to structural reactivation of an underlying neoproterozoic failed rift system paleoporosity consists of paleokarst pal in lower ordovician dolostone beekmantown group and fractures now veins that cross cut both lower paleozoic pl and precambrian pc strata a series of contrasting mineral assemblages which define changing fluid compositions fill paleokarst cavities and rare interconnected paleofractures now veins first there is an early pal succession of quartz saddle dolomite then calcite with accessory hydrocarbon as well as sulphide and sulphate minerals the quartz defines migration of saline 20 wt silica brines and hydrocarbon that spans a temperature increase defined by fluid inclusions from 13 c of the dolomite identifies strong sedimentary rock water interaction but a depleted 13 c composition 14 for the calcite illustrates that with cooling paleokarst cavities were sites of bacterially mediated sulphate reduction remaining paleokarst porosity is filled with a late pal mineral assemblage that contains low temperature mostly

surface faulting of the 6 april 2009 mw 6 3 l aquila earthquake in central italy this paper documents evidence of surface faulting associated with the 6 april 2009 moderate sized earthquake ml 5 8 mw 6 3 in the central apennines of italy which caused major damage to the town of l aquila and its surroundings coseismic surface ruptures were mapped for a minimum distance of 2 6 km along the paganica fault a fault still poorly investigated relative to the other active faults nearby which bound much wider range fronts surface rupture length srl and maximum displacement parameters 2 6 km minimum and 10 15 cm respectively are in agreement with what is expected for an mw 6 3 event in the italian apennines tectonic environment different viewpoints exist on the amount of srl and the number of activated faults we propose a pattern of sympathetic and secondary slip on an array of faults around the master seismogenic structure past seismicity and evidence for larger holocene offsets on this and other capable faults nearby prove that the 2009 event is not a good reference event for assessing the seismic hazard of the region nevertheless the 2009 l aquila earthquake once more confirmed the importance of detailed geological studies for a proper seismic hazard assessment and it clearly illustrates the need to pay attention to moderate events and supposedly minor active faults indeed this type of earthquake is rather frequent in the whole mediterranean region and is potentially much more destructive than in the past due to the expanding urban centers and infrastructures inside their epicentral regions and even right above the traces of capable faults

three dimensional moho topography in italy new constraints from receiver functions and controlled source seismology in complex tectonics regions seismological geophysical and geodynamic modeling require accurate definition of the moho geometry various active and passive seismic experiments performed in the central mediterranean region revealed local information on the moho depth in some cases used to produce interpolated maps in this paper we present a new and original map of the 3 d moho geometry obtained by integrating selected high quality controlled source seismic and teleseismic receiver function data the very small cell size makes the retrieved model suitable for detailed regional studies crustal corrections in teleseismic tomography advanced 3 d ray tracing in regional earthquake location and local earthquake tomography our results show the geometry of three different moho interfaces the european adriatic ionian and tyrrhenian the three distinct moho are fashioned following the alpine and apennines subduction collision and back arc spreading and show medium to high frequency topographic undulations reflecting the complexity of the geodynamic evolution

oxygen isotope microanalyses of diagenetic quartz possible low temperature occlusion of pores abstract secondary ion mass spectrometry sims was used to obtain 18 o measurements on 20 m spots in authigenic and detrital quartz from mississippian sandstones of the western canadian sedimentary basin detrital quartz shows a large range in 18 o from 5 to 25 which overlaps the analyses of authigenic cements 20 34 combining these data with the burial history and precipitation sequence of minerals indicates that meteoric water may have precipitated quartz at temperatures between 15 and 35 c over a period of 180 ma 300 120ma this nearly isothermal precipitation may have occluded pores in samples from the central portion of the basin but quartz precipitation continued up 75 c at the basin margin conventional bulk analyses of these samples gave 18 o values of 13 4 for detrital quartz and 19 8 for authigenic quartz models based on bulk isotopic analyses can bias the interpretation of the timing of diagenetic quartz precipitation toward much higher temperatures the sims measurements in diagenetic silicates can identify discrete episodes of precipitation allowing a more accurate interpretation for the timing of diagenetic events

a mid pleistocene deformation transition in the hula basin northern israel implications for the tectonic evolution of the dead sea fault the dead sea fault dsf plate boundary has accommodated relative sinistral motion between the sinai and arabian plates since the neogene geologically based models point to a long term 5 0 ma n trending relative motion roughly parallel to the central sector of the dsf gps based calculations of present day relative motion between the sinai and arabian plates indicate a northward increasing convergent component across the dsf suggesting a kinematic change over the plio pleistocene period we study the evolving deformation of the hula rhomb shaped graben situated along the central sector of the dsf in order to examine the possible kinematic change the hula graben is widely accepted as a pull apart basin part of a series of basins extending southward we use a 3 d approach combining mapped surface structures with subsurface seismic reflection profiles 173 km and borehole data 38 boreholes gathered here into a single geographic information systems database results indicate that during the mid pleistocene a major tectonic transition modified the structure of the basin fill a subvertical nnw trending left lateral throughgoing strike slip fault developed diagonally across the basin consequently the basin entered a new geodynamic phase where subsidence was controlled by both the basin bordering faults and the diagonal fault while the vertical displacement across the transverse faults is minor synchronous structural changes recorded in additional localities along the dsf attests to a mid pleistocene 1 ma regional tectonic transition that was associated with a northward increasing convergent component across the central and northern sectors of the dsf

structural geometry and evolution of the dead sea jordan rift system as deduced from new subsurface data abstract analysis of deep drilling data and seismic reflection surveys obtained in recent years permit establishment of a tectonic model of the dead sea jordan rift system that is in part conformable with some of the many often controversial theories on this subject but which also introduces new information negating some concepts that are widely accepted in the literature the dead sea jordan rift system is not a tensional graben between two parallel sets of faults but instead can be best defined as a sinistral transform connecting an incipient oceanic ridge the red sea with an upthrusted collision zone the taurus range the shear is caused by the opening of the red sea and the motion of the arabian sub plate away from the african plate the shearing occurs along a largely south north trending slightly arcuate une consisting of a series of en echelon left stepping left lateral strike slip master faults of varying lengths these faults characteristically die out to the north by bending outward northeast and the movement is taken up by the strike slip faults to the left i e to the west where overlap of the two faults occurs very narrow and very deep grabens have evolved in places the width being equal to the depth the narrowness of these grabens is dictated by the closeness of the overlapping segments of the successive strike slip faults this feature points to the probability that the en echelon strike slip faults have a common root zone at great depth these grabens are not typical leaky pull aparts in that they have no gravimetric magnetic or heat flow anomalies nor are they closed on all four sides by large faults their northern ends are characterized by gradually rising graben floors without major diagonal cross faults the dead sea jordan rift system sensu stricto extends over a distance of 420 km from the northern shore of the gulf of elat to the northern margin of the hula valley southward it is connected with the red sea along the gulf of elat transition zone which has elements of strike slip as well as tensional opening a northern transition zone exists between the hula valley and the taurus arc where diminishing strike slip movement is compensated by several underthrust belts the time of the beginning of the movement along the rift can be established as pre middle miocene because mid miocene sediments overlie an upper cretaceous graben floor rifting has continued intermittently to the present volcanism in the northern fifth of the rift is genetically associated with the seismically active tensional golan hauran jebel drouz wadi sirhan tectonic trend which has a north west southeast orientation and not with the north south trending dead sea jordan rift

the 2009 l aquila earthquake central italy a source mechanism and implications for seismic hazard an edited version of this paper was published by agu copyright 2009 american geophysical union

crustal deformation in thailand and tectonics of indochina peninsula as seen from gps observations 1 the indochina peninsula has been hypothesized to be in the sunda block a rigid block in se asia however its northern boundary is not obvious to elucidate the boundary and the deformation on the peninsula six permanent gps sites were established in thailand and observations have been conducted since march 1998 estimated velocities suggest that most of the indochina peninsula is rigid and can be considered in the sunda block however close examination suggest small but significant strain in the order of 10 8 exists in thailand statistical test suggests that there is no significant differential motion between the sunda block and the south china block on the other hand acute clockwise rotation of east himalayan syntax indicates left lateral shear in the north of thailand which may be partially accommodated by left lateral motion along mae chan fault

relative and absolute sea level rise in western canada and northwestern united states from a combined tide gauge gps analysis 1 empirical studies and climate models suggest large variations of absolute sea level asl changes between oceanic basins such potential variations raise concern on the applicability of global mean asl predictions to specific regions and on estimates of relative sea level rsl hazards we address this issue for the western canada and northwestern united states coastline by estimating the 20th century asl rate using a combination of 34 colocated tide gauge and global positioning system gps stations the tide gauge data are quality controlled and corrected for spatially and temporally correlated sea level transients in order to derive robust rsl trends and standard errors reference frame and other gps specific issues are considered as part of the error budget in absolute gps vertical velocities our combined tide gauge gps analysis aligned to the international terrestrial reference frame 2000 indicates a northeast pacific asl rise of 1 8 0 2 mm a through the 20th century which is similar to accepted rates for the global eustatic mean for the period 1993 2003 we find a regional asl rate of 4 4 0 5 mm a consistent with satellite altimetry on the basis of the intergovernment panel on climate change assessment report 4 mean scenario and our assessment of coastal motions from gps and tide gauge data we derive a map of predicted 21st century rsl rise in western canada and the northwestern united states variations in coastal uplift strongly affect spatial rsl patterns subsidence of southern puget sound may significantly increase rsl rise in the seattle tacoma metropolitan area conversely tectonic uplift along parts of the outer west coast may reduce future rsl rise by up to 50 100

glacial comminution of quartz sand grains and the production of loessic silt a simulation study abstract traditional views have attributed the formation of loess sized quartz silt particles to glacial processes glacial grinding was considered to be the only mechanism capable of producing loessic silt in the quantities necessary for the formation of loess deposits this idea has been strengthened by the apparent close association between present or former glacierised regions and major global loess deposits however some doubts exist as to whether or not glacial grinding is capable of producing sufficient quantities of quartz silt because of the difficulties entailed in the observations and direct measurement of subglacial abrasion and crushing this paper describes and discusses the results of a simulation study designed to examine abrasion and size reduction of quartz grains under grinding conditions analogous to those experienced in a subglacial environment the findings from these experimental runs suggest that although particle breakage and comminution does occur little silt sized material is actually produced the amount of silt produced ranges from approximately 9 of the original sample in one experimental run to less than 1 for the other seven runs it is suggested that the results obtained may show considerably less abrasion and loessic silt production than may be expected to occur within a natural subglacial environment because 1 subglacial grains experience much greater overburden pressures for much longer periods of time 2 the use of sand sized grains may represent a stable component resistant to further size reduction through crushing or abrasion and 3 the use of a monomineralic sample of relatively sound quartz grains although care must be taken when relating results obtained from these experimental runs to silt production within a glacial environment findings from this work do question whether large amounts of quartz silts are produced within subglacial environments the importance of glacial processes in the formation of loess deposits may lie in the reworking of pre existing sediments by ice rather than the actual production of vast quantities of silt this in turn suggests that care should be taken in the interpretation of the provenance of particles comprising loess deposits

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

perspectives oxygen isotopes and the origin of quartz published data indicate that in igneous rocks the delta 18 o of quartz averages about 9 per thousand in metamorphic rocks 13 to 14 per thousand in sandstones 11 per thousand in shales 19 per thousand quartz overgrowths in sandstones 20 per thousand and cherts 28 per thousand either a very large proportion of the silt and clay size quartz in shales about 90 of the total quartz in shales is secondary or the isotopic data are incomplete it is concluded that the latter interpretation is correct because published isotopic analyses of metamorphic rocks consider only the coarser grained rocks phyllites and slates whose quartz is nearly all of silt and clay size and has more positive delta 18 o values than quartz from schists and gneisses probably supplies the bulk of quartz to mudrocks