precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

a detailed record of paleomagnetic field change from searles lake california 2 the gauss matuyama polarity reversal this new study of the gauss matuyama transition from searles lake california conjoined with other records from the western united states provides interesting insights into the structure of the reversing magnetic field the present study employs improved measurement and data reduction techniques multiple parallel strings of samples and a finer sampling interval than was used in the original study by liddicoat 1982 particularly crucial to this investigation was the use of overprint directions to reconstruct declinations required because the core was rotary drilled the results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric the fabric facilitated the alignment of core segments the new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study in addition it brings out several small scale variations that were absent in the old record some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes this alternating steady and rapid field change is similar to behavior observed in volcanic records which argues that such behavior is not merely an artifact of episodic volcanism the searles lake record is strongly nonzonal and is defined in the northern hemisphere by a swath of virtual geomagnetic poles vgps stretching from northern eurasia to west africa and to the northwest pacific glen et al 1994 showed that a collection spanning 15 myr of western north american transition and excursion records displays this same pattern indicating that the vgp swath is a persistent feature of the transitional field in addition the compilation reveals that the swath extends into the southern hemisphere outlining a region marked by an absence of poles that is centered on the indian ocean the fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component seven additional records are now available making the western north america data set perhaps the finest regional set of high resolution records consisting of both igneous and sedimentary records the new records which provide an important test of the existence of the vgp pattern strongly support the findings that reveal the presence of persistent long term 15 myr nondipolar transitional fields

a comparison of two sequential geomagnetic polarity transitions upper olduvai and lower jaramillo from the southern hemisphere abstract two normal to reverse n to r geomagnetic polarity transitions were obtained from a southern hemisphere deep sea sediment core 35 91 s 59 97 e the upper jaramillo reversal was continuously sampled by taking 0 5 cm thick samples across 55 cm of section and the upper olduvai reversal was sampled in a similar manner across 60 cm both records are characterized by full normal and reverse polarity directions which are in very good agreement with those predicted by axial dipole fields for the core site latitude each record also exhibits a zone yielding intermediate directions because of a large increase in both the within level and the between level scatter in the upper jaramillo transition zone it is not considered to represent an accurate record of the geomagnetic field the upper olduvai transition however appears to be a more coherent record it is characterized by directions that shallow early in the reversal but then rapidly steepen to nearly vertical upward directions the declinations do not change until the inclinations have passed through the vertical the total directional change occurs within a broad nrm intensity minimum the virtual geomagnetic pole vgp path calculated for this record is not longitudinally constrained but instead exhibits a westward progression through the reversal when considered together with the lower jaramillo transition previously reported from this core these records constitute a set of sequential southern hemisphere transitions a number of striking similarities exist in the two records such as steep upward directions and a shallowing which occurs between the near vertical and the full polarity directions the results of zonal harmonic modeling of these records are consistent with the idea of a standing field which persisted across both the upper olduvai and lower jaramillo reversals

geophysical and magneto structural study of the maâdna structure talemzane algeria insights on its age and origin the ma dna structure is located approximately 400 km south of algiers 33 19 n 4 19 e and emplaced in upper cretaceous to eocene limestones although accepted as an impact crater on the basis of alleged observations of shock diagnostic features such as planar deformation features pdfs in quartz grains previous works were limited and further studies are desirable to ascertain the structure formation process and its age for this purpose the crater was investigated using a multidisciplinary approach including field observations detailed cartography of the different geological and structural units geophysical surveys anisotropy of magnetic susceptibility paleomagnetism and petrography of the collected samples we found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies ones that might indicate the occurrence of a causative material which is at least in part identical geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface using soil magnetic susceptibility measurements the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite a primary magnetization is inferred from a stable remanence with both normal and reverse directions carried by these two minerals although this is supposed to be a chemical remagnetization its normal polarity nature is considered to be a pliocene component subsequent to the crater formation the pole falls onto the miocene pliocene part of the african apparent polar wander path apwp consequently we estimate then formation of the ma adna crater to have occurred during the time period extending from the late miocene to the early pliocene unfortunately our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones nor shocked minerals were found a dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis which can still be considered as plausible only a drilling may provide a definite answer

long lived transitional states of the geomagnetic field and the two dynamo families palaeomagnetic data from lavas produced at the hawaii and society islands hotspots suggest that one or more specific long lived field configurations recur during successive polarity reversals spanning perhaps several million years the apparent quasistationarity of these states and the relative placement of the two recording sites offer a rare opportunity to explore the fundamental geometrical properties of transitional fields

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

changes in glaciers in hidden valley mukut himal nepal himalayas from 1974 to 1994 glaciological investigations were carried out in 1994 on the glaciers in hidden valley mukut himal nepal himalayas in order to make a comparison with observations made in 1974 most of the glaciers were found to have retreated by 30 60 m in terminus elevation over the 20 years between the two studies rikha samba glacier the longest glacier in the valleys has retreated by about 200 m the areal average of the amount of surface lowering and the volume loss of the glacier was estimated to be 12 6 m ice equivalent and 13 of the total mass respectively the annual mass balance of 0 55 m a 1 water equivalent was obtained as an average for 20 years which is one of the largest negative values amongst small glaciers of the world

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

non axisymmetric behaviour of olduvai and jaramillo polarity transitions recorded in north central pacific deep sea sediments the causes of the reversals of the earth s magnetic field can be investigated by studying the palaeomagnetic record preserved in rocks a proper documentation of reversal phenomena should yield valuable information about the generation of the field deep sea sediments provide a globally and temporally distributed set of polarity transition records our studies2 4 of the olduvai 1 88 1 72 myr 1 and jaramillo 0 94 0 88 myr 1 subchrons using deep sea sediments showed that the time averaged behaviour of the geomagnetic field during these reversals proved to be reliably recorded independent of the latitude and longitude of the sample site here we test some phenomenological transition models and present directional data virtual geomagnetic poles vgps and related directions in rotated declination inclination d i space5 using two records of the olduvai reverse normal reverse r n r and two records of the jaramillo r n r polarity transitions these data are better explained by the flooding rather than the standing field model but they also indicate a degree of asymmetry of the transitional field

decadal change of the spring snow depth over the tibetan plateau the associated circulation and influence on the east asian summer monsoon abstract the decadal change in the spring snow depth over the tibetan plateau and impact on the east asian summer monsoon are investigated using station observations of snow depth data and the ncep ncar reanalysis for 1962 93 during spring march april both the domain averaged snow depth index sdi and the first principal component of the empirical orthogonal function eof analysis exhibit a sharp increase in snow depth after the late 1970s which is accompanied by excessive precipitation and land surface cooling the correlation between sdi and precipitation shows a coherent remote teleconnection from the tibetan plateau northern india to western asia it is found that the increased snow depth over the plateau after the mid 1970s is concurrent with a deeper india burma trough an intensified subtropical westerly jet as well as enhanced ascending motion over the tibetan plateau additional factors for the excessive snowfall include more moisture supply associated with the intensification of the south

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

a 24 000 year period climate signal in 1 7 2 0 million year old death valley strata continental records of plio pleistocene climatic fluctuations are often confined to fragmentary localized basins and not easily dated one exception is the confidence hills of southern death valley in this study we present a detailed description of a plio pleistocene section of interfingering alluvial fan and saltpan sediments from the confidence hills southern death valley this well exposed section accumulated with an average deposition rate of 25 cm 1000 yr with no bioturbation permitting more precise paleomagnetic dating than in oceanic records 1 we interpret the sequence of facies changes in the olduvai normal polarity zone of the confidence hills to represent alternations between wetter and drier regional climatic regimes the frequency of this variation 24 000 yr appears to correspond to the milankovitch precessional cycle 23 000 yr we believe this section records a milankovitch driven climatic variation in the death valley basin annual precipitation about 1 7 2 0 ma this is the first report of the precessional cycle in continental sediments of this age in the united states

the reunion subchronozone at odp site 981 feni drift north atlantic abstract at ocean drilling program site 981 the reunion subchron is recorded from the marine isotopic stage mis 81 82 boundary to mis 79 as a single normal polarity zone spanning the 2 115 2 153 ma interval according to the age model produced by matching the site 981 18 o record to an orbitally tuned reference record the duration of the reunion subchron 38 kyr is almost four times that assumed in current geomagnetic polarity timescales high interval sedimentation rates 18 cm kyr during mis 81 at site 981 have resulted in an unusually detailed record of the subchron virtual geomagnetic poles during the onset and demise of the reunion subchron trace a series of mainly anticlockwise loops over eastern asia

monitoring and prediction of shrinkage of a small glacier in the nepal himalaya in 1978 the first detailed glaciological studies were carried out on the small glacier ax010 in a glacier inventory of dudh kosi region in the nepal himalaya its length area and highest and lowest elevations in 1978 were 1 7 km 0 57 km2 5360 and 4950 m a s l respectively two resurveys of its terminus position and or surface elevations were carried out in 1989 and 1991 the glacier retreated by about 30 and 28 m during the periods 1978 89 and 1989 91 respectively in 1995 annual monitoring of this glacier by means of ground survey stake method and topographical mapping began in order to obtain its mass balance surface flow velocity and extent and link them with climatic conditions the results obtained in 1995 are summarized as follows 1 during the period 1991 95 the glacier retreated by 12 m 2 associated with the ice thickness loss in the lower part of the glacier the horizontal surface velocities along the center line in 1995 june october showed a remarkable decrease on the glacier tongue to about 50 of those in 1978 shrinkage of the glacier in the near future is predicted from a simple model calculation for the case that climatic conditions remain unchanged from 1995 the results show that the present shrinkage should continue and accelerate

the upper olduvai geomagnetic field reversal from death valley california a fold test of transitional directions high resolution records of the upper olduvai geomagnetic field reversal were obtained from two localities within steeply dipping lacustrine sediments of the confidence hills southern death valley california the difference in bedding attitude between the two localities allowed us to perform a fold test of both transitional and non transitional paleomagnetic directions this is the first positive fold test obtained for transitional directions from any geomagnetic field reversal in addition variations in lithology allowed a comparison of transitional records from different sedimentary environments alternating field and thermal demagnetization reveals that most samples have either single or simple two component magnetizations the low coercivity low blocking temperature component is most likely held by large multidomain grains of magnetite and aligns with the present day field when uncorrected for bedding orientation the high coercivity component is held primarily by single and pseudo single domain titanomagnetite as determined from standard magnetic studies and electron microscopy this component is interpreted as a primary magnetic signature based on the presence of stratigraphically bound reversals in the tilt corrected data and positive reversal tests short term field variations can be correlated between the two records and differences are readily explained by the recording process inherent to the two sedimentary environments the sampling site is located within one of the preferred bands of transitional virtual geomagnetic poles vgp s 1 3 yet the transitional vgp paths from both localities are clustered within two primary longitudinal bands which are approximately 90 away from the sampling site longitude one lies over the atlantic ocean 10 50 w and the other antipodal to this 130 170 e these bands are also inconsistent with eight previous studies of the upper olduvai transition 4 9 3 10 when all reported studies of this reversal are examined however no global consistency exists instead the transitional vgp s are better grouped in a sampling site reference frame

response of faults to climate driven changes in ice and water volumes on earth s surface numerical models including one or more faults in a rheologically stratified lithosphere show that climate induced variations in ice and water volumes on earth s surface considerably affect the slip evolution of both thrust and normal faults in general the slip rate and hence the seismicity of a fault decreases during loading and increases during unloading here we present several case studies to show that a postglacial slip rate increase occurred on faults worldwide in regions where ice caps and lakes decayed at the end of the last glaciation of note is that the postglacial amplification of seismicity was not restricted to the areas beneath the large laurentide and fennoscandian ice sheets but also occurred in regions affected by smaller ice caps or lakes e g the basin and range province our results do not only have important consequences for the interpretation of palaeoseismological records from faults in these regions but also for the evaluation of the future seismicity in regions currently affected by deglaciation like greenland and antarctica shrinkage of the modern ice sheets owing to global warming may ultimately lead to an increase in earthquake frequency in these regions

uplift and denudation of the central alaska range a case study in the use of apatite fission track thermochronology to determine absolute uplift parameters apatite fission track thermochronology aftt on granitic samples collected in the central alaska range in conjunction with geologic constraints from basins to the north nenana basin and south cook inlet of the range is used to constrain the timing amount rate and pattern of surface uplift rock uplift and denudation since the late miocene the conversion from a thermal frame of reference apatite fission track data to an absolute frame of reference with respect to mean sea level which requires constraining the paleoland surface elevation the paleomean annual temperature and the paleogeothermal gradient is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone paz apatite ages at denali mount mckinley range from 16 ma near the summit 6 km elevation to 4 ma at 2 km elevation a distinctive break in slope in the apatite age profile at an elevation of 4 5 km also marked by a change in confined track length distributions marks the base of an exhumed apatite paz rock uplift and denudation are greatest at denali decreasing southward away from the mckinley strand of the denali fault system as shown by progressively older apatite ages 7 35 ma from a suite of samples along the kahiltna glacier a correlative decrease in topography occurs southward from the fault the central alaska range lies within an arc defined by the denali fault with the highest peaks including denali concentrated at the arc apex patterns of rock uplift and denudation within the central alaska range mimic topography between early and late miocene and possibly earlier the central alaska range was most likely an area of relative tectonic and thermal stability rock uplift denudation and mean surface uplift of the denali region began by the late miocene 5 6 ma being 8 5 km 5 7 km and 2 8 km respectively at average rates of 1 5 km m y 1 km m y and 0 5 km m y the amount of rock uplift denudation and surface uplift decreases to 3 km 2 km and 1 km at little switzerland some 45 km south of the denali fault we conclude that the topographic and rock uplift patterns of the central alaska range the shape and proximity of the mckinley strand of the denali fault to these patterns the timing of the onset of rock uplift and denudation at 5 6 ma and a significant change in relative plate motion between north america and the pacific plates circa 5 6 ma are all inherently related

long term continental deformation associated with transpressive plate motion the san andreas fault a synthesis of transpressive mountain building as evidenced by rock uplift and topography along the entire san andreas fault reveals a complex crustal response to oblique plate motion convergent deformation increases toward the fault but does not correlate with the angle of plate motion obliquity the shortening estimated from rock uplift is also insufficient to account for the fault normal motion based on relative plate velocity this suggests that near field convergence is influenced by local structural complexity and is not purely driven by regional transpression and that the fault normal component of plate motion is partly accommodated elsewhere heterogeneity in deformation and degree of slip partitioning highlight the importance of other factors in shaping transpressive continental deformation including surface processes material anisotropy and strain weakening

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

height dependence of the tendency for reduction in seasonal snow cover in the himalaya and the tibetan plateau region 1966 2001 the dataset of northern hemisphere ease grid weekly snow cover and sea ice extent for the period october 1966 july 2001 is analyzed to examine the height dependence of declining tendencies of seasonal snow cover in the himalaya and the tibetan plateau region 25 458 n 70 1108 e it is found that the annual mean snow covered area is decreasing in the himalaya tibet region at a rate of 1 a 1 implying that the mean snow covered area has decreased by one third from 1966 to 2001 the rate of decrease is largest 1 6 at the lowest elevations 0 500 m on the other hand the length of the snow cover season is declining at all elevations with the greatest rate of decline in the 4000 6000 m height range on the tibetan plateau 4000 6000 m a s l the length of the snow cover season has decreased by 23 days and the end date for snow cover has advanced by 41 days over this 35 year period these rates might be somewhat overestimated by the binary definition of snow cover on satellite images it is likely that the reduction of the snow surface albedo by deposition of asian dust and anthropogenic aerosols may be at least partly responsible for earlier snowmelt

late cenozoic tectonics of the northern los angeles fault system california the northern los angeles fault system along the southern range front of the santa monica mountains includes potentially seismogenic faults directly beneath the los angeles metropolitan area for a better assessment of seismic hazards we mapped late cenozoic faults and folds in the northern los angeles basin using an extensive set of oil well and surface geologic data the northern los angeles fault system developed through early to late miocene transrotational and transtensional regimes and a pliocene and quaternary transpressional regime the santa monica san vicente and las cienegas faults are early to late miocene normal faults that were later reactivated as reverse faults suggesting that the orientation of reverse faults is largely controlled by miocene extensional tectonics rather than by the post miocene stress field tectonic inversion occurred at the beginning of pliocene time with the reactivation of miocene normal faults and initiation of reverse faults many pliocene contractile structures became inactive by the middle pleistocene and younger deformation is taken up by new active structures including the west beverly hills lineament and an active strand of the santa monica fault the west beverly hills lineament is the northernmost segment of the newport inglewood fault zone which may have propagated northward to the santa monica mountains in quaternary time the lineament acts as a segment boundary for the active left lateral santa monica hollywood fault system and bounds the hollywood basin to the west uplift of an oxygen isotope substage 5e marine terrace north of the city of santa monica and an assumed dip of 45 for the santa monica mountains thrust fault underlying and uplifting the santa monica mountains suggest that an average dip slip rate for the fault is 1 3 mm yr crustal shortening across the northern los angeles fault system accounts for less than a third of the current rate of shortening between the san gabriel mountains and palos verdes hills based on global positioning system observations

fault slip rates and interseismic deformation in the western transverse ranges california to better constrain fault slip rates and patterns of interseismic deformation in the western transverse ranges of southern california we present results from analysis of gps and interferometric synthetic aperture radar insar data and three dimensional mechanical and kinematic models of active faulting anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous gps site locations gps measures contraction rates across the ventura basin of 7 mm yr oriented west northwest with rates decreasing to the west and east the santa barbara channel is accommodating 6 5 mm yr in the east and 2 5 mm yr in the western portions of n s contraction inversion of horizontal gps velocities highlights a zone of localized fast contraction rates following the ventura basin using a mechanical model driven by geodetically calculated strain rates we show that there are no significant discrepancies between short term slip rates captured by geodesy and longer term slip rates measured by geology mechanical models reproduce the first order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the ventura basin due to the inadequate homogeneous elastic properties of the model existing two dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the gps data mechanical models predict zones of fast contraction in the santa barbara channel and offshore near malibu suggesting that offshore faults represent a significant seismic hazard to the region furthermore many active faults throughout the region may produce little to no interseismic deformation making accurate seismic hazard assessment challenging 2013 american geophysical union all rights reserved

restraining segments and reactivation of the santa monica dume malibu coast fault system offshore los angeles california 1 a regional west striking system of surface and blind faults transects northern metropolitan los angeles separating the santa monica mountains from two deep sedimentary basins the surface faults include the santa monica dume and malibu coast faults the three dimensional 3 d geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated how structural relief grows along a mountain front and how block translation is related to block rotation industry seismic reflection well and outcrop data were used to construct digital structure contour maps of several of these faults and three stratigraphic horizons that intersect them these maps swath bathymetry digital elevation models and seismicity were incorporated into a 3 d digital database modeling included new approaches to determining strike slip displacement by separate analyses of shortening and structural relief in a restraining double bend stratigraphic thicknesses indicate miocene extension across the santa monica and dume faults which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left lateral and left reverse displacement modeling indicates 5 km 8 1 km of left lateral displacement on part of the santa monica dume fault and 11 of clockwise rotation of the santa monica mountains during the last 4 1 m y modern displacement rates modeled from gps data are similar to our modeled post 4 ma rates of westward escape and clockwise rotation of the santa monica mountains

snow depth derived from passive microwave remote sensing data in china in this study we report on the spatial and temporal distribution of seasonal snow depth derived from passive microwave satellite remote sensing data e g smmr from 1978 to 1987 and smm i from 1987 to 2006 in china we first modified the chang algorithm and then validated it using meteorological observation data considering the influences from vegetation wet snow precipitation cold desert and frozen ground furthermore the modified algorithm is dynamically adjusted based on the seasonal variation of grain size and snow density snow depth distribution is indirectly validated by modis snow cover products by comparing the snow extent area from this work the final snow depth datasets from 1978 to 2006 show that the interannual snow depth variation is very significant the spatial and temporal distribution of snow depth is illustrated and discussed including the steady snow cover regions in china and snow mass trend in these regions though the areal extent of seasonal snow cover in the northern hemisphere indicates a weak decrease over a long period there is no clear trend in change of snow cover area extent in china however snow mass over the qinghai tibetan plateau and northwestern china has increased while it has weakly decreased in northeastern china overall snow depth in china during the past three decades shows significant interannual variation with a weak increasing trend

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

mass loss on himalayan glacier endangers water resources 1 ice cores drilled from glaciers around the world generally contain horizons with elevated levels of beta radioactivity including 36 cl and 3 h associated with atmospheric thermonuclear bomb testing in the 1950s and 1960s ice cores collected in 2006 from naimona nyi glacier in the himalaya tibet lack these distinctive marker horizons suggesting no net accumulation of mass ice since at least 1950 naimona nyi is the highest glacier 6050 masl documented to be losing mass annually suggesting the possibility of similar mass loss on other high elevation glaciers in low and mid latitudes under a warmer earth scenario if climatic conditions dominating the mass balance of naimona nyi extend to other glaciers in the region the implications for water resources could be serious as these glaciers feed the headwaters of the indus ganges and brahmaputra rivers that sustain one of the world s most populous regions

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

temporal clustering of major earthquakes along individual faults due to post seismic reloading palaeoseismic evidence suggests that earthquake recurrence intervals in some regions can be highly variable with clusters of multiple large events separated by much longer periods of quiescence because post seismic processes have a significant effect on the reloading rate of the coseismic fault we hypothesize that temporal variations in the amount of stress concentrated in the non seismogenic lithosphere can modulate large earthquake recurrence times we explore this hypothesis using simple analogue spring dashpot slider models we find that in the presence of small amounts of environmental noise post seismic stress transfer over timescales much longer than an earthquake cycle may be an important factor in generating clustering behaviour the propensity for the system to be clustered is a function of a non dimensional number that we call the wallace number w w is defined as the average earthquake stress drop divided by the product of the long term geologic strain rate across the fault and the effective viscosity of the system our results indicate that environments with relatively low strain rates and a relatively weak non seismogenic lithosphere are most susceptible to clustering driven by post seismic stress recycling mechanisms

confounding influence of magnetic fabric on sedimentary records of a field reversal recent compilations of geomagnetic reversal records have generated a controversy as to whether the geomagnetic field is geographically biased during polarity transitions at present there is general agreement that the virtual geomagnetic poles recorded from cenozoic sediments preferentially lie over the americas or the antipodal longitude yet such a preference is not statistically established however it is intriguing that the claimed preferred paths lie 90 away from the site longitude although this may be partly inherent in the very poor geographical distribution of the sites we prefer not to rely on fortuitous coincidences several authors have argued that sedimentary palaeomagnetic records may be modified by artefacts linked to the acquisition of magnetization here we report that in two sedimentary records of the upper olduvai reversal from confidence hills california the declinations of the remanent magnetization recorded during the reversal are similar to the directions of the maximum horizontal axes of the ellipsoids of magnetic anisotropy this supports the idea that at times of low geomagnetic intensity such as during a reversal factors other than the geomagnetic field influence the orientation of elongated grains

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

magnetostratigraphy of the orce region baza basin se spain new chronologies for early pleistocene faunas and hominid occupation sites abstract the orce fossil quarries in the baza basin of southeastern spain are a rich source of early pleistocene palaeolithic tools and vertebrate remains geologic fieldwork during the last decade has placed these fossiliferous strata within the context of a thick neogene continental sequence detailed lithostratigraphic and magnetostratigraphic results indicate that at least the upper 60 m of this sequence are of early pleistocene age the quarried strata venta micena barranco leon and fuentenueva 3 are from a narrow time span gazella borbonica equus stenonis and the beginning of more characteristic pleistocene fauna in the venta micena site with hippopotamus antiquus equus granatensis homo sp thus far no evidence for human occupation has been found within the earlier fuentenueva 1 quarry although many of the same terminal mn17 species have been found with hominids on the other side of europe at the dmanisi site earliest pleistocene in the republic of georgia