causes of unusual distribution of coseismic landslides triggered by the mw 6 1 2014 ludian yunnan china earthquake abstract the mw 6 1 2014 ludian yunnan china earthquake triggered numerous coseismic landslides that do not appear to be associated with any previously known seismogenic fault traditional models of triggering for seismically generated landslides do not provide a reasonable explanation for the landslide pattern observed here here the newmark method is applied to a grid to calculate the minimum accelerations required for slope failures throughout the affected region the results demonstrate that for much of the study area the distribution of failure prone slopes is similar to the actual pattern of coseismic landslides however there are some areas where the model predicts considerably fewer failures than occurred we suggest that this is a result of the complex source faults that generated the ludian earthquake which produced a half conjugate rupture on nearly ew and nnw trending faults at depth the rupture directed much of its seismic moment southeast of the epicenter increasing ground shaking and the number of resulting landslides

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

a different pattern of ridge segmentation and mantle bouguer gravity anomalies along the ultra slow spreading southwest indian ridge 15 30 e to 25 e abstract the results of a recent bathymetric and geophysical investigation of a 650 km long portion of the very slowly opening 16 mm yr full rate southwest indian ridge swir between 15 30 e and 25 e are presented bathymetry and mantle bouguer gravity anomalies mba caused by variations in crustal thickness and or crustal and upper mantle densities show different characteristics from those observed at faster spreading centers like the mid atlantic ridge mar 20 30 mm yr full rate with the exception of the du toit transform none of the ridge axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off axis traces the mba patterns associated with individual segments are much more complex than the simple circular bull s eyes lows reported along the mar while the short wavelength ridge segment length is comparable to that of the mar there is little correlation with mba amplitude and segment length and axial relief furthermore an eastward propagating magma source and an 84 km long zone of oblique spreading appears to define a fundamental boundary along the swir between two 250 300 km long sections characterized by distinctly different axial morphology and gravity signatures we interpret these results to indicate a long wavelength segmentation pattern of the underlying upwelling mantle melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes fissure eruptions within the rift valley build linear ridges defining a short wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle our results and interpretation are quite different than that predicted by extending current models of the faster spreading mar to these ultra slow spreading rates

the 2002 denali fault earthquake alaska a large magnitude slip partitioned event the mw moment magnitude 7 9 denali fault earthquake on 3 november 2002 was associated with 340 kilometers of surface rupture and was the largest strike slip earthquake in north america in almost 150 years it illuminates earthquake mechanics and hazards of large strike slip faults it began with thrusting on the previously unrecognized susitna glacier fault continued with right slip on the denali fault then took a right step and continued with right slip on the totschunda fault there is good correlation between geologically observed and geophysically inferred moment release the earthquake produced unusually strong distal effects in the rupture propagation direction including triggered seismicity

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

transtensional model for the sierra nevada frontal fault system eastern california active strike slip and normal faults along the eastern margin of the sierra nevada primarily accommodate northwestward translation of the sierra nevada central valley i e sierran microplate with respect to stable north america strike slip faults bordering the eastern sierran microplate are subparallel to small circles about the sierra nevada north american euler pole normal faults of the sierra nevada frontal fault system strike 45 clockwise of the small circle trajectories and exhibit well defined left stepping en echelon patterns consistent with formation in dextral transcurrent regime major graben bordering the northeastern sierran microplate are located in regions where the locus of range front deformation steps abruptly eastward in a releasing geometry relative to sierra nevada north american motion crustal shortening occurs at the northern end of the sierran microplate where a component of northwest dextral shear steps westward in a left restraining geometry across the sierran crest to the sacramento valley kinematic inversions of earthquake focal mechanisms from the walker lane belt bordering the eastern sierra nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the sierra nevada north american euler pole normal faulting along most of the eastern sierran range front thus appears to primarily accommodate microplate translation rather than sierran uplift or regional basin and range extension

coseismic conjugate faulting structures produced by the 2016 m w 7 1 kumamoto earthquake japan abstract field investigations and analyses of airborne lidar data reveal that the 2016 mw7 1 kumamoto earthquake produced a 40 km long surface rupture zone with a typical conjugate riedel shearing fault structure along the pre existing right lateral strike slip hinagu futagawa fault zone hffz the conjugate riedel shearing structure comprises two sets of coseismic shear fault zones that are oriented to ne sw to ene wsw and wnw ese to e w the ne sw to ene wsw trending shear fault zone is characterized by r riedel shear structures with right lateral strike slip displacement of up to 2 5 m including left stepping en echelon cracks t shear and mole tracks p shear in contrast the wnw ese to e w trending shear fault zone is dominated by r riedel shear structures with left lateral displacement of up to 1 3 m including right stepping en echelon tension cracks t and mole tracks p which are concentrated in a zone of

distribution pattern of earthquake induced landslides triggered by the 12 may 2008 wenchuan earthquake abstract this paper presents the preliminary results of an extensive study of the mapping the distribution of landslides triggered by the wenchuan earthquake in sichuan province china on 12 may 2008 an extensive landslide interpretation was carried out using a large set of optical high resolution satellite images e g aster alos cartosat 1 spot 5 and ikonos as well as air photos for both the pre and post earthquake situation landslide scarps were mapped as points using multi temporal visual image interpretation taking into account shape tone texture pattern elevation and ridge and valley orientation nearly 60 000 individual landslide scarps were mapped the landslide distribution map was compared with the inventory map that was prepared directly after the earthquake which contains about 11 000 individual landslide points through the calculation of normalized landslide isopleths maps remarkable differences were observed as the earlier inventory mapping did not consider the pre earthquake situation and did not consider all individual landslides as part of the landslide inventory landslides were identified that had blocked the drainage and had formed landslide dams the landslide distribution was compared with a number of aspects such as the seismic parameters distance to epicenter distance to fault rupture co seismic fault geometry and co seismic slip distribution and geology the most remarkable correlation found was with the co seismic slip distribution and the fault geometry landslide distribution in the section of the fault that had mainly a thrust component with low angle fault plane was found to be much higher than the sections that had steeper fault angles and a major strike slip component

impact of chi chi earthquake on the occurrence of landslides and debris flows example from the chenyulan river watershed nantou taiwan abstract the chenyulan river watershed in central taiwan was chosen for evaluating the impact of the chi chi earthquake on the occurrence of landslides and for exploring the initial conditions triggering debris flows interpretations of aerial photographs and spot images as well as field investigations were used to identify landslide and debris flow occurrences precipitation data were then utilized to investigate critical conditions leading to the debris flow comparison of five spot images taken between june 1996 before the july 1996 typhoon herb and september 1999 before the chi chi earthquake shows that the landslide area increased from 7 1 106 to 7 5 106 m2 during that time however by january 2000 after the chi chi earthquake the landslide area almost tripled to 20 8 106 m2 measurement of images taken in march and august 2001 reveals that the landslide area had further expanded to 24 2 106 and 27 5 106 m2 respectively significant differences before and after the earthquake are also noticed in 1 the intensity and amount of precipitation required for triggering debris flow 2 the size of contributing drainage basin in which debris flows occurred and 3 the frequency of debris flows after the chi chi earthquake maximum hourly rainfall intensity and critical accumulated precipitation necessary to initiate debris flow reduced to as low as 1 3 of the pre earthquake figures prior to the chi chi earthquake most debris flows occurred in gullies with slopes greater than 10 and in drainage basins larger than 0 1 km2 conversely after the earthquake debris flows were observed even in gullies with effective drainage area smaller than 0 03 km2 furthermore before the earthquake the debris flow recurrence time in the study area was greater than 5 years whereas six debris flow events have been observed in the 2 years since the earthquake

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

is the north altyn fault part of a strike slip duplex along the altyn tagh fault system although the altyn tagh fault system has played an important role in the indo asian collision its geometry and tectonic evolution remain poorly known between 86 and 92 e this system is at least 100 km wide and is bounded to the north and south by the north altyn and altyn tagh faults respectively mapping along the jianglisai reach of the north altyn fault indicates that miocene to pliocene motion was predominantly left to left reverse slip with transport vectors trending n45 60 e map relationships suggest that total offset on the fault is 120 km these results are inconsistent with previous models of the altyn tagh fault system in which oblique convergence along the northern margin of the tibetan plateau is partitioned into thrusting on the north altyn fault and left slip on the altyn tagh fault an alternative hypothesis is that the north altyn fault is the northern boundary of a transpressional strike slip duplex within which the structurally elevated altyn mountains were created our model suggests that transpressional deformation may be restricted to this strike slip duplex and need not characterize the entire margin

displacement features associated with fault zones a comparison between observed examples and experimental models abstract fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures tension cracks r or p type fractures which give the brittle shear zone a width translation along these faults and related features can occur by sliding on the elementary planes and or by solution deposition processes with opening of transtension zones and possible dilatation of the shear zone in order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations displacement along an analogue fault model was examined under conditions of direct shear the experimental fracture patterns were then compared with the natural features the structural elements of certain faults created by testing and showing transpression and transtension zones were found to be similar to natural domino structures bounded by elementary shears and could be compared with computed situations it can be inferred that stresses are reoriented inside the shear zone the angle between elementary fractures depends on their order of development transpression and transtension zones occur systematically and the shear zone undergoes dilatancy under low normal stresses

topographical and geological amplifications determined from strong motion and aftershock records of the 3 march 1985 chile earthquake site response experiments were performed 5 months after the ms 7 8 central chile earthquake of 3 march 1985 to identify amplification due to topography and geology topographical amplification at canal beagle a subdivision of vina del mar was hypothesized immediately after the main event when extensive damage was observed on the ridges of canal beagle using frequency dependent spectral ratios of aftershock data obtained from a temporarily established dense array it is shown that there is substantial amplification of motions at the ridges of canal beagle the data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event dense arrays established in vina del mar also yielded extensive data which are quantified to show that in the range of frequencies of engineering interest there was substantial amplification at different sites of different geological formations to substantiate this spectral ratios developed from the strong motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site similarly spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong motion records in case of weak motions the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs to quantify the degree of frequency dependent amplification and used in microzonation of closely spaced localities

mechanics of the earthquake induced hongshiyan landslide in the 2014 mw 6 2 ludian earthquake yunnan china abstract the hongshiyan landslide the largest landslide volume 12 24 mm3 triggered by 2014 mw 6 2 ludian earthquake blocked the niulan river forming a rockslide dam the failure potential of the dam created an elevated risk to population and infrastructure downstream we provide insight into the failure mechanics of the hongshiyan landslide using data obtained by means of remote sensing techniques and traditional in situ surveys geological data obtained by these methods was then used for kinematic and numerical analyses our study shows that failure of the hongshiyan landslide involved a high rock slope with an average slope of 55 and aspect of 185 ssw in which the geological setting consists of an upper strong limestone slab and an underlying weak silty mudstone the geological setting allowed the landslide to develop as follows i the silty mudstone due to its poor mechanical properties was subject to ductile deformation under the compressive loading from the overlying strong limestone slabs ii a large number of release fractures in the limestone developed as a consequence of pre failure progressive deformation in the underlying weak mudstone iii the oxidation and solution along steep joint walls in the upper limestone indicates that the joints are pathways that allow water ingress into the mudstone further promoting the degradation of shear strength in this layer distinct element modelling udec was used to back analyse the failure mechanism and develop the geological model that best reproduced the hongshiyan failure geometry results show that the failure surface consisted of two elements the upper steeply dipping release surface formed along the slope parallel steep cross joints defining a jointed strong limestone slab and the toe of the failure formed a curved failure surface through the ductile weak silty mudstone underlying the limestone slab independent of any discontinuities within it the pre failure observations and numerical analysis suggest that the hongshiyan slope was in a state of marginal static stability prior to the earthquake as a result of pre failure progressive deformation controlled by the shear strength of the underlying mudstone overall the marginal pre earthquake stability of the hongshiyan slope coupled with the strongest seismic loading which the hongshiyan slope has been historically subjected to are the reasons why this moderate earthquake induced a large scale catastrophic rock slope failure

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

spatial distribution analysis of landslides triggered by 2008 5 12 wenchuan earthquake china abstract the 2008 5 12 wenchuan earthquake seismic magnitude mw 7 9 according to the usgs triggered a great number of landslides a spatial database of landslides is built by interpreting the remote sensing rs data which covers 11 counties severely damaged by the earthquake with area of about 31 686 12 km2 the geological settings of the study area are detailed mapped including geologic structure and lithology the digital elevation model dem of the study area with resolution of 30 m 30 m is presented to address the topographic characteristics correlations between the occurrence of landslides with geological settings i e active faults the coseismic surface ruptures geological unit slope gradient slope elevation as well as slope aspect are analyzed using spatial modeling in gis with the aids of field investigations it can be concluded that the distance to the causative faults and slope gradient are much more important than other factors landslides are clustered distributed along the causative faults of f2 and f3 and landslides incidence are dramatically decreasing with the increasing of the distance to the causative faults the landslides incidence has an obvious increase near the active faults i e f1 landslide concentration lc landslides km2 has an obvious exponent increase with slope gradient however lc has no obvious relationship with geological unit and slope elevation the difference of lc value in each geological unit each slope elevation category maybe have been the result affected by slope gradient for each geological unit each slope elevation category and distance of each geological unit each slope elevation category to causative fault zone isoseismal map reflects the incidence of landslides in some degree and lc increases dramatically with intensity increasing almost all landslides occurred in the region above vi degree statistics result also indicates that in the near field of causative faults landslides tend to have the initial sliding direction similar to the movement of the causative faults and preliminary study shows that landslides incidence varies in different slopes with different structure and consequent slopes and obsequent slopes have a higher landslides incidence than other layered slopes

anomalous co seismic surface effects produced by the 2014 mw 6 2 ludian earthquake yunnan china an example of complex faulting related to riedel shear structures abstract riedel shear structures are often developed within strike slip fault zones this study focuses on the riedel shear structures within the co seismic surface rupture zone and the co seismic landslides produced by the 2014 mw 6 2 ludian earthquake all the geologic seismic and geodesic data indicate a wnw ese principal compressive stress acting on the baogunao xiaohe fault zone bxfz based on the idealized riedel shear model together with evidences form the particular distribution of aftershocks and co seismic surface effects the 2014 ludian earthquake could be with a complex faulting process related to the riedel shear structures under this present tectonic stress i e nnw striking sinistral strike slip fault plane y shear which trends approximately n40 w and ene striking dextral strike slip fault plane r shear which is nearly perpendicular to former field surveys and structural analysis of the co seismic surface rupture zone are performed based on unmanned aerial vehicle uav borehole drilling and adit excavation to the north of wangjiapo village southeast of the epicenter the results indicate that these co seismic surface ruptures were produced following a pre existing sinistral transtensional duplex structure in plan view which mainly comprises i typical riedel shear structures characterized by en echelon cracks running northwest and ii tensile cracks subparallel to the slope contours running northeast and the inward dipping geometry of faults under the sinistral transtentional duplexes converges downwards into a single shear zone which is defined as a negative normal faulted flower structure in vertical cross section in addition on the basis of the analyses of three large scale landslides this study demonstrates that the slopes in a state of marginal static stability just situated inside the region of main energy release which was predominantly controlled by the y shear fault are the reasons why this moderate earthquake induced anomalous co seismic landslides

research on the geometry of shear fracture zones the geometry of brittle shear fracture zones has been studied by the examination of some strike slip faults and shear joints in china pinnate and en echelon deformational patterns are two fundamental arrangements of fractures within brittle shear fracture zones the pinnate angle is usually less than 25 and the en echelon is more than 25 the deformation in the rock bridge the area between two secondary fractures depends on the arrangements of secondary fractures and the sense of shear some tensile ruptures are formed at the terminations of both primary shear fracture zones and secondary fractures the direction of a tensile rupture is commonly parallel to the orientation of the principal compression along the direction of propagation of the shear fracture the horizontal displacement is large and the attenuation of displacements is slow the maximum horizontal displacement of the shear zone may be located in the region of the initial fracture under certain conditions the shear fracture zones may possess pivotal movement this type of movement may be expressed by the behavior of the fault or by the geomorphology of the fault zone

transpressive duplex and flower structure dent fault system nw england abstract revised mapping along the dent fault northwest england has improved the resolution of folds and faults formed during variscan late carboniferous sinistral transpression a nne trending east down monocline comprising the fell end syncline and taythes anticline was forced in carboniferous cover above a reactivated precursor to the dent fault within the lower palaeozoic basement the taythes anticline is periclinal due to interference with earlier acadian folds the steep limb of the monocline was eventually cut by the west dipping dent fault the hangingwall of the dent fault was dissected by sub vertical or east dipping faults together forming a positive flower structure in cross section and a contractional duplex in plan view the footwall to the dent fault preserves evidence of mostly dip slip displacements whereas strike slip was preferentially partitioned into the hangingwall faults this pattern of displacement partitioning may be typical of transpressive structures in general the faults of the taythes duplex formed in a restraining overlap zone between the dent fault and the rawthey fault to the west the orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly kinematic constraints provided by the dent and neighbouring variscan faults yield a nnw sse regional shortening direction in this part of the variscan foreland

mechanical stratigraphy and fault fold interaction absaroka thrust sheet salt river range wyoming abstract a distinct contrast in deformation style from tightly folded and faulted cambrian through ordovician strata to broadly folded and faulted mississippian and younger strata is displayed in the hanging wall of the absaroka thrust salt river range wyoming based on style the paleozoic section can be divided into two main structural lithic units each unit is characterized by a relatively weak ductile anisotropic lower section and a relatively strong brittle more isotropic upper section where the two units were stacked within the thrust sheet inverted fault propagation folds formed in the center of each unit and the overall transition upward from close to wide spaced folds and imbricate faults developed in the multilayer where the upper unit was isolated deformation was dominated by imbricate faulting with little associated folding and inverted fault propagation folds did not form observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response it is note worthy that the boundary between the two structural lithic unts in part defined by the roof thrust of the stewart passive roof duplex occurs within a dolostone rather than in overlying shale of the darby formation i propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions cross cutting relations suggest that the absaroka sheet was cut by large imbricate faults during late stage movement over a major dip ramp a high deviatoric and low mean compressive stress state just beyond the upper ramp flat corner promoted break back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation

the southwest indian ridge between 49 15 e and 57 e focused accretion and magma redistribution abstract bathymetric gravity magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow spreading southwest indian ridge swir full rate 14 mm yr between 49 15 e and 57 e this analysis reveals that the segmentation defined by along axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along axis variations of backscattering strength mantle bouguer anomaly mba and amplitude of the central magnetic anomalies cma at axial discontinuities with offsets larger than 15 km thin crust and reduced volcanic production are suggested by the occurrence of mba highs almost non existent cma and 50 lower backscattering strength relative to the segment centres by contrast smaller non transform discontinuities with offsets smaller than 15 km correspond to very weak variations or to no variation of the mba the cma or the reflectivity of the seafloor suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities these small axial discontinuities bound low relief bathymetric segments 500 700 m corresponding to weak or no mba lows amplitude 1000 m corresponding to large mba lows amplitude 30 mgal we suggest that the magma supply to these low relief segments is controlled by near surface processes such as melt migration and or crustal plumbing from adjacent high relief segments pronounced mba lows at high relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle these spreading cells are spaced by about 100 km along the swir axis we suggest that the spacing of spreading cells on slow spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow spreading ridges than on slow spreading ridges

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

riedel shear structures in the co seismic surface rupture zone produced by the 2001 mw 7 8 kunlun earthquake northern tibetan plateau abstract we present a case study of riedel shear structures related to the co seismic surface ruptures produced during the 2001 mw 7 8 kunlun earthquake in northern tibet which are related to strike slip movement along the kunlun fault field investigations and interpretations of high resolution remote sensing images show that the 2001 co seismic surface ruptures striking wnw ese are mainly characterized by riedel shear structures including t fractures and r y and p shears a left lateral shear sense is indicated to assess the riedel shear fabrics quantitatively we measured the azimuth of 19 455 co seismic surface rupture strands the width of rupture zones at 474 profiles and 336 fold axes of mole track structures using 1 m resolution ikonos and 0 61 m resolution quickbird images acquired soon after the earthquake the analytical results show that i the co seismic surface ruptures are generally concentrated in up to five subparallel sub rupture zones with individual sub rupture zones varying in width from 3 to 350 m generally 1 2 km iii t fractures are mainly developed within the alluvial deposits at counterclockwise angles of 15 40 relative to the general trend of the rupture zone iv p shears are also mainly found in the alluvial deposits but at counterclockwise to clockwise angles of 5 10 relative to the general trend and v y and r shears are developed within both the alluvial deposits and basement rocks mainly representing the reactivation of pre existing fault traces the results demonstrate that the co seismic riedel shear structures are primarily controlled by the local geology during surface rupture formation consistent with the idea that riedel shear structures are common fault patterns within strike slip shear zones and that their development is related to the early stages of fault evolution

lithospheric stress and uppermantle dynamics in mainland china due to mantle flow based on combination of global and regional scale seismic tomography abstract in order to explore the importance of mantle flow to lithospheric stress field in mainland china seismic tomography based mantle flow models are used to predict the most compressive principal horizontal stress directions mcphsds considered that regional scale seismic tomography models have higher horizontal resolution to map the mantle structure while global scale models can present the information out of the imaging domains of regional scale models although this information has relatively poor horizontal resolution the combined global and regional scale seismic tomography based mantle flow models hereafter called combined models are mainly used in this paper after the comparison of the observed and our predicted mcphsds it is suggested that 1 a combined model compared with a only global scale seismic tomography based model could improve greatly the predictions in some regions of mainland china such as sichuan yunnan south china and north china blocks 2 the mantle flow model driven by both plate motions and mantle density heterogeneity hereafter called plate density driven model compared with the flow model driven only by mantle density heterogeneity hereafter called density driven model has much better predictions in the eastern china 3 the presence of density variations above 250 km could better dramatically the predictions in the eastern china and 4 sublithospheric mantle flow causes the lithosphere under compression in mainland china and plays an important role in forming the lithospheric stress in alashan qaidam western tibetan and eastern tarim blocks as well as the east of the eastern china

coseismic surface ruptures associated with the 2014 mw 6 9 yutian earthquake on the altyn tagh fault tibetan plateau field investigations reveal that the 2014 m w 6 9 yutian earthquake on the left lateral strike slip altyn tagh fault atf system tibetan plateau produced an 25 km long surface rupture zone that contains conjugate riedel shear faults the coseismic surface ruptures occurred mainly along two parallel east northeast trending active left lateral strike slip faults rupture also occurred in a conjugate west northwest trending zone along an active right lateral strike slip fault the east northeast trending ruptures are concentrated in a zone of 500 m wide and 25 km long and are characterized by riedel shear structures including distinct shear faults y with a maximum sinistral displacement of 1 m right stepping en echelon cracks and mole tracks in contrast the west northwest trending ruptures occur within a zone of up to 1 5 km wide and 4 km long in the jog area between the two parallel east northeast trending faults and this zone is characterized by discontinuous shear faults with dextral displacements of 0 5 m left stepping en echelon cracks and mole tracks all oriented oblique to the east northeast trending rupture zones at an angle of 30 40 the lengths and displacements of the coseismic surface ruptures measured in the field are comparable with those obtained from the empirical relationships between magnitude and coseismic surface rupture length and displacement our findings demonstrate that the coseismic conjugate riedel faulting was controlled mainly by preexisting active faults of the atf system reflecting the present day tectonic stress field associated with the ongoing penetration of the indian plate into the eurasian plate

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

co seismic riedel shear structures produced by the 2010 mw 6 9 yushu earthquake central tibetan plateau china abstract riedel shear structures are often developed within strike slip fault zones this study focuses on the riedel shear structures within the co seismic strike slip surface rupture zone produced by the 2010 mw 6 9 yushu earthquake in the central tibetan plateau china field surveys and structural analysis of the surface ruptures reveal that the co seismic riedel shear structures are characterized by i tension cracks t fractures ii compression structures mainly mole tracks p shears and iii discrete shear faults r and y shears developed in unconsolidated alluvial deposits along the pre existing left lateral strike slip ganzi yushu fault zone the t fractures generally show a right stepping en echelon pattern in contrast the p and r shears indicate a left stepping en echelon pattern the riedel shears are inclined to the general trend of the co seismic surface rupture zone at mean counterclockwise angles of 41 t fractures and 21 r shears and a mean clockwise angle of 35 for the p shears the present results show that i the co seismic riedel shear structures indicate a left lateral strike slip sense for the seismogenic fault and ii the t fractures and p and r shears are the primary riedel shear structures formed during the early stages of the evolution of a strike slip fault constrained by pre existing geologic structures

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

evidence from gravity anomalies for interactions of the marion and bouvet hotspots with the southwest indian ridge effects of transform offsets abstract the ultra slow spreading southwest indian ridge swir presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry using recently available satellite free air gravity and shipboard bathymetry data we obtain mantle bouguer mba and residual mantle bouguer anomalies rmba by removing from free air gravity the attractions of seafloor topography sediment thickness variations a reference crust and theoretically predicted effects of lithospheric cooling the bouvet hotspot previously observed to be associated with anomalous bathymetry and geochemistry near the bouvet triple junction has an mba axial gravity low of 100 mgal implying pronounced localized crustal thickening off axis the rmba lows along previously calculated bouvet hotspot tracks are variable in amplitude suggesting the possibilities that bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center along axis geophysical anomalies suggest that the marion hotspot has a significant effect on accretionary processes in the central portion of the swir importantly the marion axial anomaly appears to be compartmentalized between the andrew bain and discovery ii fracture zones implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented ultra slow spreading system

tectonic stress field of china inferred from a large number of small earthquakes mean principal tress axes were inferred for china using 9621 p wave first motion polarity readings from 5054 small earthquakes 1 ml 5 the area studied was divided into 76 subregions the mean p compressive b intermediate and t relatively tensional axes corresponding to composite focal mechanism solutions of multiple earthquakes for each subregion were determined by a grid test to all possible orientations of the p b and t axes with a step of 5 or 10 in order to get a relatively homogeneous sampling in space we have avoided using the readings from spatially and temporally clustered earthquakes we have rechecked most of the polarity readings by inspection of original seismograms the focal mechanism solutions of large m 6 individual earthquakes are also presented for comparison the results indicate the existence of an overall radial pattern of the present day maximum horizontal stress orientations throughout the continental area of china this pattern is thought to be closely related to indentor models of continental collision between the indian ocean and the eurasian plate

gravity evidence of very thin crust at the gakkel ridge arctic ocean abstract gakkel ridge the active spreading center in the arctic ocean is the slowest spreading portion of the global mid ocean ridge system total spreading rates range from 0 6 cm yr in the east where the ridge disappears beneath the laptev shelf to 1 3 cm yr in the west near greenland bathymetry and gravity surveys of four sections of the gakkel ridge were carried out in 1996 by the u s navy nuclear submarine uss pogy as part of scicex 96 in order to sample variations in seafloor morphology and gravity anomalies as a function of spreading rate the ridge axis throughout the survey area is characterized by a continuous axial rift valley similar to that observed at other slow spreading ridges the continuous rift axis suggests that well organized seafloor spreading is occurring at total spreading rates of less than 1 cm yr in three faster spreading 1 13 1 24 cm yr western survey areas located between 7 e and 54 e the gakkel ridge is deep compared with other ridge axes axial depths range between 4600 and 5100 m and ridge flanks at about 3200 m the ridge flank morphology is very blocky and is characterized by large scarps and deep fault bounded troughs very large amplitude free water anomalies with peak to trough amplitudes of 85 150 mgal are observed centered on the axis of the gakkel ridge modeling of the free water anomalies by varying the crustal thickness and average crustal density including the gravity effect of the cooling of the mantle away from the axis implies that if the average crustal density is less than 2900 kg m3 the crustal thickness must be less than 4 km the axial rift valley at the fourth survey area near 98 e where the total spreading rate is 0 99 cm yr is buried by sediments the axis in this region is associated with a continuous 70 mgal gravity minimum implying the presence of a large buried rift valley the rift flanks at 95 e are at a depth of greater than 3800 m 600 m deeper than the average depth at the gakkel ridge axis west of 60 e simple isostatic calculations suggest that the crust in this region may be vanishingly thin beneath the sediment cover these observations indicate a relationship between melt production and seafloor spreading rate at very slow spreading rates suggesting that ultra slow spreading may suppress melt production or delivery at the gakkel ridge

rifting in the northern norwegian greenland sea thermal tests of asymmetric spreading the analysis of heat flow seismic and topographic data collected in the northern norwegian greenland sea reveals an asymmetric evolution of the eurasian and north american plates these data are compared to predictions from three kinematic models of extension which produce asymmetry about the knipovich ridge 1 uniform asymmetric pure shear 2 lithospheric simple shear and 3 rift ridge jumping the data are consistent with a range of deformation scenarios from one ridge jump occurring at about 25 m y after the initiation of spreading to continuous asymmetric extension the simple shear model can match the data only when a detachment fault dips more steeply than 45 under svalbard tectonic and heat flow evidence suggests that asymmetry may have evolved from a combination of all three models when the mohns ridge propagating to the east encountered the preexisting northward trending spitsbergen shear zone the direction of ridge propagation shifted to the north being influenced by the abrupt change in the regional stress field at the shear zone ridge axis intersection as a result the nascent knipovich ridge entered into and propagated along the shear zone consequently formerly active shear faults became the new detachment surfaces along which new crust is minted and asymmetrically extended the high level of deviatoric stress about the mohns and knipovich ridge intersection may cause a gradual eastward migration of the knipovich ridge resulting in multiple zones of magma intrusion off axial bands of high heat flow and volcanism located along the barents sea and svalbard margin along the yermak plateau and on continental svalbard just southeast of the yermak plateau may be evidence for this migration propagation from the nansen ridge may have entered the same shear zone from the north explaining the creation of the small molloy ridge and off axis volcanism on the yermak plateau

new gravity data in the arctic ocean comparison of airborne and ers gravity new gravity fields from airborne gravimetry and from ers 1 and 2 satellite altimetry cover extensive portions of the arctic ocean these two data sets may constitute as much as 60 of the data contributions to the arctic gravity project compilation here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation both gravity determinations compare favorably with geological survey of canada surface measurements in the beaufort sea airborne 1 86 2 09 mgal rms ers 2 64 3 11 mgal rms comparisons between the airborne and ers data over the chukchi borderlands reveal a 4 38 mgal rms difference over the smoother region of the field and 7 36 mgal rms over the rugose field generated by the shallow ridges and deep troughs coherency between the two data sets in the chukchi region implies a resolution of 19 km comparison with science ice expedition submarine measurements over chukchi plateau suggests that the ers field resolves even shorter wavelength signal than the airborne data whereas in the beaufort sea the airborne data showed better coherence to ground truth data long wavelength differences exist between the two data sets expressed as a 2 3 mgal offset over the chukchi region this study highlights the respective strengths of the two data sets the ers gravity field has the advantage of ubiquitous coverage of the ocean south of 81 5 degreesn a denser sampling of the gravity field and a recovery of signal down to similar to 15 km the airborne data cover a significant portion of the polar hole in the satellite coverage have lower measurement noise and recover somewhat higher anomaly amplitudes in the 25 100 km wavelength range

landslides triggered by the avaj iran earthquake of june 22 2002 abstract the 2002 avaj iran earthquake mw 6 5 triggered many landslides over an area of about 3600 km 2 in this paper we describe a ground based field study conducted during several days immediately after the earthquake and pre and post earthquake landslide inventory maps that were prepared the landslides due to earthquake include 47 fall and topple zones 9 slides and 3 lateral spreads the largest slide is a rockslide 150 100 m which occurred southeast of the village of changureh the main results of this study are the density of earthquake triggered landslides decreases away from the fault zone in a manner that is asymmetric with respect to direction although several slides and lateral spreads were seen the most common types of triggered landslides are falls and topples the area throughout which landslides occurred and maximum epicentral and fault zone distances of landslides are comparable to similar data from other historical earthquakes landslides mostly occurred in the most susceptible geologic units in which there were many landslides before the earthquake new disrupted slides and falls generally occurred in ancient fall zones but no information about reactivation of coherent slides is available

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

co seismic strike slip fault displacement determined from push up structures the selsund fault case south iceland abstract we analysed push up structures along the selsund fault a n s right lateral strike slip fault activated during the 1912 earthquake in the south iceland seismic zone volume changes and syn tectonic collapse affected push ups during the earthquake followed by post seismic gravitational sagging to determine the push up shortening and hence the strike slip fault motion we define a virtual push up structure without volume change and collapse and we compare it with the present day configuration whereas length comparisons are subject to errors volumetric analysis allows determination of shortening through evaluation of the thickness of the deformed layer affected by the push ups we determine a co seismic peak displacement of 2 4 m along the rupture trace this value is consistent with the magnitude 7 of the earthquake based on empirical relationships neglecting volume changes and collapse effects gives underestimated displacement the new method for analysing push up structures thus allows better determination of magnitudes of ancient earthquakes along strike slip faults

deformation partitioning in a slow spreading ridge undergoing oblique extension mohns ridge norwegian sea although oceanic spreading is often perpendicular to the ridge trends in some cases the angle between these two directions can be significantly less than 90 40 50 this occurs because of either a bend of the ridge trend or a change of the spreading direction we here describe oblique spreading in the mohns ridge resulting in deformation partitioning between the valley walls which are dominantly affected by strike slip displacements and the axial valley which is subject to nearly pure extension the axial valley walls are characterized by en echelon normal faults affecting the walls while the axial valley is affected by parallel faults grouped into oblique sets these fault sets define different structures horst or tilted blocks that are regularly spaced inside the axial valley moreover some ridge segments mainly undergo pure extension whereas others are affected by oblique extension we explain this faulting pattern including the along strike and transverse variations as a consequence of depth variations of the brittle ductile transition

preliminary analysis of the knipovich ridge segmentation influence of focused magmatism and ridge obliquity on an ultraslow spreading system abstract bathymetry gravity and deep tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow spreading knipovich ridge in the norwegian greenland sea northeast atlantic ocean discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle bouguer anomaly mba lows generally resemble those of the gakkel ridge and the easternmost southwest indian ridge these magmatically robust segment centers are regularly spaced about 85 100 km apart along the ridge and are characterized by accumulated hummocky terrain high relief off axis seamount chains and significant mba lows we suggest that these eruptive centers correspond to areas of enhanced magma flux and that their spacing reflects the geometry of underlying mantle upwelling cells the large scale thermal structure of the mantle primarily controls discrete and focused magmatism and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge segment centers along the southern knipovich ridge are characterized by lower relief and smaller mba anomalies than along the northern section of the ridge this suggests that ridge obliquity is a secondary control on ridge construction on the knipovich ridge as the obliquity changes from 35 to 49 from north to south respectively while spreading rate and axial depth remain approximately constant the increased obliquity may contribute to decreased effective spreading rates lower upwelling magma velocity and melt formation and limited horizontal dike propagation near the surface we also identify small magmatically weaker segments with low relief little or no mba anomaly and no off axis expression we suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller discrete mantle upwelling centers with short lived melt supply

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

rupture mechanism and seismotectonics of the ms6 5 ludian earthquake inferred from three dimensional magnetotelluric imaging a three dimensional 3 d resistivity model around the 2014 ms6 5 ludian earthquake was obtained the model shows that the aftershocks were mainly distributed in a shallow inverse l shaped conductive angular region surrounded by resistive structures the presences of this shallow conductive zone may be the key factor leading to the severe damage and surface rupture of the ludian earthquake a northwest trending local resistive belt along the baogunao xiaohe fault interrupts the northeast trending conductive zone at the zhaotong lianfeng fault zone in the middle crust which may be the seismogenic structure of the main shock based on the 3 d electrical model combining with gps thermal structure and seismic survey results a geodynamic model is proposed to interpret the seismotectonics deep seismogenic background and deformation characterized by a sinistral strike slip with a tensile component of the ludian earthquake

magnitude distance relations for earthquake induced landslides in greece a data set of 47 earthquake induced landslides occurring in greece from ad 1650 to 1995 has been compiled and the landslide distribution has been examined the spatial distribution indicates landslides occurrence almost everywhere in greece with the exception of the north greek mainland which is likely due to the low occurrence frequency of large earthquakes the earthquake magnitude m s ranges between 5 3 and 7 9 while the main bulk of events falls within the range 7 5 m s 5 3 with peaks at m s 6 4 and 6 7 the upper bound of the maximum epicentral distance r e from a reported landslide has been approximated by the line log r e 2 98 0 75m s for m s 5 3 which is compatible with curves proposed world wide and constitutes a realistic solution at least for hazard assessment purposes we discuss the importance of the results for assessing hazards from earthquake induced landslides and for calculating focal parameters of historical earthquakes 2000 elsevier science b v all rights reserved

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

field study of a highly active fault zone the xianshuihe fault of southwestern china the xianshuihe fault of western sichuan province china is one of the world s most active faults having produced 4 earthquakes during this century of magnitude 7 along a 350 km length of the fault at least 8 such events have occurred since 1725 in the more limited 150 km long segment including luhuo and daofu major earthquakes in 1904 1923 1973 and 1981 m 7 7 7 6 6 9 were associated with overlapping surficial fault ruptures and with individual left lateral displacements as large as 3 6 m field studies indicate that this high degree of activity is typical of the fault s longer term history the holocene left lateral slip rate on the north western segment of the fault has been 15 5 mm yr decreasing to about 5 mm yr on its southeastern segment based on radiometrically dated offset stream channel and terrace deposits and on offset glacial moraines physiographic features of active faulting are fully as diagrammatic as those of california s san andreas fault mainly because of high altitude preservation and the absence of cultural modification on this eastern margin of the tibetan plateau detailed en echelon tensional and pushup features resulting from surface ruptures in 1973 1955 1923 and 1893 can still be recognized today and new data have been collected bearing on the offsets and fault rupture lengths during these and other events the locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the xianshuihe fault obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events whereas segmentation control for others remains obscure based on the historic record repeat times estimated from slip rates and current seismic gaps two segments are particularly likely sites for m 7 earthquakes in the near future the 65 km long segment between daofu and qianning and the 135 km long segment bracketing kangding continuing creep has been documented along some segments of the fault and this together with the high degree of activity and other unique attributes makes the xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard evaluation studies

transtensional faulting patterns ranging from pull apart basins to transform continental margins an experimental investigation abstract seventeen small scale experiments were performed to study the deformation induced in brittle ductile models by a releasing stepover between two transform faults and by intersection between transform and divergent plate boundaries in both cases faulting depends on the rheological layering presence and strength of a ductile layer at depth and the width of the basal stepover successive types of pull apart basins are observed in the first set of experiments which are compared with natural examples firstly a lazy z shaped basin appears which is bounded by y faults above transform boundaries and r faults propagating from each corner of the stepover then r faults replace r faults leading to a rhomb shaped graben at every stage the basin length to width ratio remains between 2 2 and 3 8 suggesting that scale independence of pull apart basins is related to the geometrical shape of bounding faults in a second set of experiments deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin within the horsetail splay block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin a feature that can be compared with marginal ridges at transform margins

interdecadal variability in dissolved oxygen in the intermediate water layer of the western subarctic gyre and kuril basin okhotsk sea using dissolved oxygen do concentration at isopycnal surfaces we have observed an oscillation in the intermediate water layer of the western subarctic gyre wsg and the kuril basin of the okhotsk sea with a period of about 20 years the interdecadal oscillation in do is strongly correlated with the intensity of the aleutian low pressure cell with a 2 year time lag the interdecadal variations in do in the intermediate water layer of the wsg and the okhotsk sea may be a result of oscillation in the atmospheric forcing leading to variations in the isopycnal fluxes of do at the boundaries of the okhotsk sea western subarctic gyre subtropical region the isopycnal surface replacement in the wsg and in changes in the intermediate water formation rate in the okhotsk sea

statistical analysis of an earthquake induced landslide distribution the 1989 loma prieta california event the 1989 loma prieta california earthquake moment magnitude m 6 9 generated landslides throughout an area of about 15 000 km2 in central california most of these landslides occurred in an area of about 2000 km2 in the mountainous terrain around the epicenter where they were mapped during field investigations immediately following the earthquake the distribution of these landslides is investigated statistically using regression and one way analysis of variance anova techniques to determine how the occurrence of landslides correlates with distance from the earthquake source slope steepness and rock type the landslide concentration defined as the number of landslide sources per unit area has a strong inverse correlation with distance from the earthquake source and a strong positive correlation with slope steepness the landslide concentration differs substantially among the various geologic units in the area the differences correlate to some degree with differences in lithology and degree of induration but this correlation is less clear suggesting a more complex relationship between landslide occurrence and rock properties

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

volcano tectonic events and sedimentation since late miocene times at the mohns ridge near 72 n in the norwegian greenland sea abstract the combined analysis of structural and magnetic data from the mohns ridge near 72 n in the norwegian greenland sea shows the existence of different phases of seafloor spreading from anomaly 5a 12 ma which represents the limit of the magnetic survey onwards for the period comprised between anomaly 5 9 ma and anomaly 3a 6 5 ma most particularly a dramatic drecrease of spreading rate is observed by as much as 50 between anomaly 3a and 2a 3 2 ma an acceleration of spreading is observed together with a global uplift of the acoustic basement this latter period may correspond to a local enhancement of tectono volcanic activity the existence of ridges located at 25 and 50 km from the ridge axis and parallel to it suggests that both the beginning 5 4 5 ma and the end 3 2 5 ma of this phase were marked by intense volcanism in the sedimentary basins of the survey area two at least intra sedimentary horizons appear at respectively 140 160 ms and 340 360 ms two way travel time beneath the sediments surface using an inferred value of sedimentation rate of 50 70 m myr the age of the first horizon between 3 and 2 ma corresponds to the beginning of mid pliocene glaciations in the northern hemisphere the age of the second one between 6 5 and 4 5 ma to the pliocene miocene boundary our data and other data from other areas indicate that the three major sources of sediments western spitzbergen margin bear island trough and storfjord trough which supply the mohns ridge area near 72 n have been dramatically more active after 5 5 ma in addition local unconformities recorded in the single channel seismic sections indicate that some tectonic event occurred off axis little after the date corresponding to the deposition of the second horizon s2 if one accepts that the knipovich ridge migrated eastward 6 5 ma ago it can be hypothesized that the tectonic release of the lena trough knipovich ridge mohns ridge system forced the sediment production to increase at each one of the three sediment sources after the miocene pliocene transition

late pleistocene holocene activity of the zemuhe fault on the southeastern margin of the tibetan plateau abstract a n s trending active fault zone which includes the anninghe zemuhe and xiaojiang faults forms the tectonic boundary of the tibetan plateau on its southeastern margin marking the limit of the eastward extrusion of the tibetan plateau in this paper we document the late pleistocene holocene activity of the zemuhe fault which is the central segment of the fault zone we have been able to determine slip rate and the average recurrence interval of large earthquakes upon the fault based on aerial photograph interpretations field investigations trench excavations and radiocarbon age data the zemuhe fault is mainly characterized by left lateral offsets with a horizontal slip rate of 10 12 mm yr and a vertical slip component of 2 0 2 4 mm yr trench investigations and 14 c ages reveal that at least four large earthquakes occurred during the last 1100 1500 years indicating an average recurrence interval of 300 400 years for m 7 0 7 5 earthquakes consistent with the documented history of the region our results confirm that the zehume fault plays an important role as a tectonic boundary in releasing the strain energy caused by the southeastwards motion of the tibetan plateau the energy is released in the form of repeated large earthquakes

tobi sidescan sonar imagery of the very slow spreading southwest indian ridge evidence for along axis magma distribution new deep tow sidescan sonar data from the southwest indian ridge reveal complex volcanic tectonic interrelationships in the axial zone of this ultra slow spreading ridge while some constructional volcanic features resemble examples documented at the slow spreading mid atlantic ridge such as axial volcanic ridges hummocky and smooth lava flows their distribution and dimensions differ markedly the largest axial volcanic ridges occur at segment centres but fresh looking volcanic constructions also occur at segment ends and in the deep basins marking the non transform discontinuities the orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes this observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes with associated fractures opening in response to magma pressure may control local emplacements of axial volcanic ridges at obliquely spreading ridges this non systematic pattern of ridge characteristics suggests an along axis variation between focused and distributed magmatic supply a model which is supported by our interpretation of low amplitude mantle bouguer anomalies calculated for the area we propose that a change of the axial segmentation pattern from two segments to the present day three segments may have introduced additional instability into the crustal accretion process

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses