an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

miocene recent sediment flux in the south central alaskan fore arc basin governed by flat slab subduction the cook inlet in south central alaska contains the early oligocene to recent stratigraphic record of a fore arc basin adjacent to a shallowly subducting oceanic plateau our new measured stratigraphic sections and detrital zircon u pb geochronology and hf isotopes from neogene strata and modern rivers illustrate the effects of flat slab subduction on the depositional environments provenance and subsidence in fore arc sedimentary systems during the middle miocene fluvial systems emerged from the eastern western and northern margins of the basin the axis of maximum subsidence was near the center of the basin suggesting equal contributions from subsidence drivers on both margins by the late miocene the axis of maximum subsidence had shifted westward and fluvial systems originating on the eastern margin of the basin above the flat slab traversed the entire width of the basin these mud dominated systems reflect increased sediment flux from recycling of accretionary prism strata fluvial systems with headwaters above the flat slab region continued to cross the basin during pliocene time but a change to sandstone dominated strata with abundant volcanogenic grains signals a reactivation of the volcanic arc the axis of maximum basin subsidence during late miocene to pliocene time is parallel to the strike of the subducting slab our data suggest that the character and strike orientation of the down going slab may provide a fundamental control on the nature of depositional systems location of dominant provenance regions and areas of maximum subsidence in fore arc basins

dependence of joint spacing on rock properties in carbonate strata joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories the kimmeridgian limestones of the chay peninsula western france with a mean joint density of 6 37 fractures per meter fr m 1 94 fr ft and the bathonian limestones of the bouye outcrop western france with a mean joint density of 1 9 fr m 0 58 fr ft the chay carbonates are characterized by a lower caco3 content a higher average porosity and a lower sound velocity than values recorded in the bouye limestones the compressive strength and young s modulus of the bouye carbonates are respectively 10 and 3 times higher than in the chay carbonates a statistical analysis was used to identify relationships between joint density and carbonate rock properties when facies variations are marked the joint density at outcrop scale is related to the mean bed thickness the facies descriptors and the young s modulus when textural variations are more limited the joint density is controlled by the porosity at the scale of a sedimentary basin 63 8 of the variation in joint density may be accounted for by young s modulus and the sparite micrite ratio the decrease in the sparite micrite ratio reflects an increased number of grain boundaries in the carbonate rock which limits grain deformation and enhances joint density the variations in young s modulus depend essentially on the porosity and mineralogy of the studied rocks any increase in caco3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density

intense localized rock uplift and erosion in the st elias orogen of alaska the st elias orogen in alaska is one of the world s highest coastal mountain ranges an age analysis of detrital material in active sediment systems in the inaccessible ice covered valley bottoms reveals localized exhumation that is driven by coupling between erosion and active tectonic rock uplift

strain accommodated by brittle failure in adjacent units of the monterey formation u s a scale effects and evidence for uniform displacement boundary conditions abstract extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at arroyo burro beach california the dolostone failed in effective tension whereas the mudstone failed in shear both in response to the same extensional tectonic event in the mudstone unit initial bed length and fault displacement methods document extensional strains of 6 6 0 3 and 6 0 0 3 respectively upon adjusting the displacement estimates according to theoretical fault displacement population analysis the corrected strain becomes 9 7 0 3 for the mudstone measurements of vein apertures in the dolostone document extensional strain that varies according to scale with outcrop vein scanlines indicating a strain of 3 4 0 1 and thin section scanlines yielding a strain of 5 8 0 2 applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture related strain within the dolostone this difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness whereas microscopic veins measured in thin section terminate without regard to a bounding layer the corrected dolostone strain becomes 10 2 1 0 matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies the 10 extensional strain at arroyo burro indicates significant strike parallel nw se extension accommodated by brittle failure during development of the western transverse ranges fold and thrust belt

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

denudation and deformation in a glaciated orogenic wedge the st elias orogen alaska apatite u th he dating from the st elias orogen in southern alaska illustrates a potential association between long term denudation and glacier sliding cooling ages as young as 0 4 ma exhumation 4 5 mm yr are concentrated in a narrow band near the glacier equilibrium line altitude ela front where mean quaternary ela intersects the windward flank of the orogen this band of denudation is not correlated with individual faults structural trends or known concentrations of precipitation and we propose that it is produced by focused glacier sliding at or near the ela front this implies that long term glacial erosion is a maximum at ela which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens denudation rates do not covary with fluctuations in glacier size along the ela front suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift ice discharge may thus play a critical but complex role in excavating glaciated orogens

geophysical insights into the transition fault debate propagating strike slip in response to stalling yakutat block subduction in the gulf of alaska on the basis of faulting mapped on seismic reflection and bathymetric data seismicity current plate motions and evidence that the yakutat block may be anomalously thick we propose a tectonic model for yakutat pacific interactions including the often debated transition fault to the east deformation associated with the queen charlotte fairweather fault system is extending offshore facilitating westward propagation of strike slip motion along the eastern segment of the transition fault to the west the oblique slip pamplona zone and transition faults merge at an embayment in the continental margin where a north south dextral strike slip fault within the pacific plate illuminated by the 1987 1992 earthquake swarm intersects the pacific yakutat tectonic boundary these fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the yakutat block a possible moderate sized oceanic plateau

evolution of the stress field near the arava basin located along the dead sea fault system as revealed by joint sets abstract to reveal the development of the stress field near the arava basin located along the dead sea fault system we characterized joint sets that were exposed mainly in the santonian and eocene chalk rock outcropping in the study area in total 2600 strike orientations of single layer joints slj and multilayer joints mlj mirrored four main stress fields 1 hnw j1 326 2 hnnw j2 343 3 hnne j3 033 and 4 hn j4 360 the results showed that the relatively lower values of the fracture spacing ratio fsr and their standard deviations in j1 implied that j1 was formed during the burial stage of the eocene chalk formations the inconsistency between the statistical parameters of the log normal distributions of joint spacing and fsr raised doubt about the applicability of the saturation joint density model in our studied area the joint sets characteristics suggest that the regional hnw operated during the eocene during the miocene the regional stress field changed clockwise to hnnw probably in association with the early stage of transform motion along the dead sea fault in the pliocene pleistocene the hnne and hn local stress fields both operated and were accompanied by a tensional stress field that can be identified in the intensive normal faulting evolved along the western arava basin margins

the relationship between joint aperture spacing distribution vertical dimension and carbonate stratification an example from the kimmeridgian limestones of pointe du chay france joint aperture and joint development have been studied in the kimmeridgian limestones of the pointe du chay at the northern boundary of the jurassic biscay basin france at belette outcrop in some layers the mean joint spacing of the n120 joint set is close to mean layer thickness there the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix at pillar outcrop the n10 joint set is characterised by a high level of joint density and a non saturated spacing distribution as indicated by the mode mean ratio values and the cv values cv is the ratio of standard deviation to mean fracture spacing the classical relationship between layer thickness and fracture spacing has not been observed at the pillar outcrop joint aperture reaches larger values at the pillar outcrop than at the belette outcrop where aperture is more homogeneously distributed almost all the joints are opened with moderate aperture values at belette outcrop whereas most joints with large vertical dimension have large apertures at pillar outcrop and smaller fractures are closed or poorly opened from two outcrops that have been subjected to the same geological conditions apertures of non stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened

the transition from strike slip to oblique subduction in southeastern alaska from seismological studies abstract body waveform modeling of 11 moderate to large earthquakes within southeastern alaska has been incorporated with earthquake relocations and the results of previous seismicity studies to examine the transition from strike slip to oblique subduction in southeastern alaska in the sitka region of extreme southeastern alaska earthquakes indicate seismic slip is parallel to the direction of motion between north america and the pacific plate as the plate margin begins to bend to the west near cross sound and encounters the southeastern edge of the yakutat block partitioning of seismic slip is evident both onshore and offshore although the largest earthquakes magnitude 6 0 in this region have slip parallel to plate motion most moderate sized events have slip vectors rotated clockwise from plate motion offshore the 1973 cross sound sequence indicates that the southern end of the transition zone is seismogenic with the pacific plate being thrust beneath the yakutat block while onshore strike slip faulting has occurred along the fairweather border ranges and denali faults in the st elias region thrust faulting is occurring along and above the plate interface moderate sized events in the st elias show a mix of slip vector orientations in the pamplona zone slip vectors of offshore earthquakes and deformation directions determined onshore from gps studies show a counter clockwise rotation relative to plate motion suggesting that a change in strain field occurs just west of the st elias region

age and progression of volcanism wrangell volcanic field alaska the wrangell volcanic field covers more than 10 000 km2 in southern alaska and extends uninterrupted into northwest yukon territory lavas in the field exhibit medium k calc alkaline affinities typical of continental volcanic arcs along convergent plate margins eleven major eruptive centers are recognized in the alaskan part of the field more than 90 k ar age determinations in the field show a northwesterly progression of eruptive activity from 26 ma near the alaska yukon border to about 0 2 ma at the northwest end of the field a few age determinations in the southeast extension of the field in yukon territory canada range from 11 to 25 ma the ages indicate that the progression of volcanism in the alaska part of the field increased from about 0 8 km ma at 25 ma to more than 20 km ma during the past 2 ma the progression of volcanic activity and its increased rate of migration with time is attributed to changes in the rate and angle of pacific plate convergence and the progressive decoupling of the yakutat terrane from north america subduction of yakutat terrane pacific plate and wrangell volcanic activity ceased about 200 000 years age when pacific plate motion was taken up by strike slip faulting and thrusting

ice surface morphology and flow on malaspina glacier alaska implications for regional tectonics in the saint elias orogen the saint elias mountains in southern alaska are located at a structural syntaxis where the coastal thrust and fold belt of the fairweather plate boundary intersects thrust faults and folds generated by collision of the yakutat terrane the axial trace of this syntaxis extends southeastward out of the saint elias mountains and beneath malaspina glacier where it is hidden from view and cannot be mapped using conventional methods here we examine the surface morphology and flow patterns of malaspina glacier to infer characteristics of the bedrock topography and organization of the syntaxis faults and folds beneath the eastern part of the glacier trend northwest and reflect dextral transpression near the terminus of the fairweather fault system those beneath the western part of the glacier trend northeast and accommodate folding and thrust faulting during collision and accretion of the yakutat terrane mapping the location and geometry of the structural syntaxis provides important constraints on spatial variations in seismicity fault kinematics and crustal shortening beneath malaspina glacier as well as the position of the collisional deformation front within the yakutat terrane we also speculate that the geometrical complexity of intersecting faults within the syntaxis formed a barrier to rupture propagation during two regional mw 8 1 earthquakes in september 1899

an 18 million year record of vegetation and climate change in northwestern canada and alaska tectonic and global climatic correlates abstract we reconstruct long term vegetation paleoclimatic trends spanning the last 18 million years in alaska yukon and far western northwest territories twenty one average percentage spectra for pollen and spores are assembled from eight surface subsurface sections the sections are dated independently or by correlation pollen and spore ratios indicate the direction of change in vegetation and climatic parameters growing season temperature test tree canopy density cest and paludification at study sites pest a global warm peak ca 15 ma is shown by the abundance of thermophilous taxa including fagus and quercus a temperature decline immediately following 15 ma parallels climatic reconstructions based on marine oxygen isotopes subsequent declines correlate to the messinian event and the onset of late pliocene pleistocene glaciation after 7 ma herbs and shrubs become more important elements of the palynological assemblages suggesting a more continental colder drier climate however a late pliocene warm interval is evident vegetation climatic changes during the early to late miocene show synchrony with and are most economically attributable to global events after 7 ma vegetation climate change is attributed primarily to latest miocene to pleistocene uplift of the alaska range and st elias mts the continuing influence of global climatic patterns is shown in the late pliocene warm interval despite uplift to the south the opening of the bering strait ca 3 ma may have moderated the climate in the study area

new constraints on relative motion between the pacific plate and baja california microplate mexico from gps measurements summary we present a new surface velocity field for baja california using gps data to test the rigidity of this microplate calculate its motion in a global reference frame determine its relative motion with respect to the north american and the pacific plates and compare those results to our estimate for pacific north america motion determination of pacific plate motion is improved by the inclusion of four sites from the south pacific sea level and climate monitoring project these analyses reveal that baja california moves as a quasi rigid block but at a slower rate in the same direction as the pacific plate relative to north america this is consistent with seismic activity along the western edge of baja california the baja california shear zone and may reflect resistance to motion of the eastern edge of the pacific plate caused by the big bend of the san andreas fault and the transverse ranges in southern california

stability analysis and optimization of the support system of an underground powerhouse cavern considering rock mass variability input parameters such as rock mass strength parameters and deformation modulus considered in the design of underground openings involve some uncertainty the current uncertainty in these parameters is due to the inherent variability of these parameters to quantify these parameters and design underground openings the statistical methods must be utilized in this research a statistical method was used to define the gsi of rock mass geological strength index block volume vb and joint conditions jc using the gsi distribution function obtained from field data and intact rock strength characteristics the statistical distribution functions of rock mass parameters were defined using the monte carlo method the statistical analysis of the stability in azad pumped storage powerhouse cavern was carried out through the point estimate method the appropriate support system was suggested according to the support pressure and the plastic zone around the cavern this study showed the application of the statistical method by combining the uncertainties of the intact rock strength and discontinuity parameters in the assessment of the strength and deformability of rock masses and the support selection process in comparison with the deterministic methods

eocene basalts from the yakutat terrane evidence for the origin of an accreting terrane in southern alaska basalts from the yakutat terrane a composite oceanic and continental tectonostratigraphic terrane are chemically diverse large ion lithophile element lile depleted and lile enriched tholeiites interpreted as normal mid ocean ridge and oceanic island basalt that originated on seamounts near the kula farallon spreading center during the early to middle eocene coeval and geochemically similar basalts occurring sporadically in a linear belt from southern vancouver island to the southern oregon coast range are correlative with the yakutat terrane basalts both basaltic sequences were accreted to the continental margin during subduction of the kula farallon ridge and kula plate about 48 ma the yakutat basalts were emplaced along the coast of washington or british columbia the yakutat terrane probably originated during the neogene as a composite terrane when it was sliced off the continental margin and started to move northward along the queen charlotte fairweather transform fault toward its present location in southern alaska

low pressure facies series metamorphism in an accretionary sedimentary prism southern alaska the low pressure high temperature metamorphism of the chugach metamorphic complex alaska occurred in an ocean continent convergence zone to achieve the high temperatures at a relatively shallow depth in an accretionary prism we propose the large scale transport of heat by fluids which preheated the metamorphic belt by tectonic focusing of fluids followed by injection of melts both of which were generated downdip in a shallow subduction zone

natural long term annealing of the zircon fission track system in vienna basin deep borehole samples constraints upon the partial annealing zone and closure temperature abstract fission track ft analysis of zircon from cretaceous and tertiary sandstone samples in vienna basin boreholes has provided constraints for the thermal stability of the zircon ft system over a geological timescale confined track lengths and ages were measured for samples from depths of 1 5 7 5 km the deepest with a present environmental temperature of 200 c mean track lengths range from 10 3 to 10 8 m indistinguishable within error from the unannealed reference length of 10 7 0 1 m 1 standard error thus showing no sign of systematic downhole reduction central zircon ages are consistently older than stratigraphic ages of parent rocks with no single grain ages significantly younger than their respective stratigraphic ages such ft age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition because the present geothermal regime as well as sample burial depths have been near constant for at least the past 5 m y the lower temperature limit of the zircon ft partial annealing zone is 200 c for a heating duration of the order of 5 10 m y such evidence from long term natural annealing is compatible with a zircon ft partial annealing zone cf 200 350 c derived by the extrapolation of laboratory annealing results based on onfined track length measurements but cannot alone discriminate between different annealing models for the zircon ft closure temperature a perhaps over simplified concept these results are consistent with the oft quoted temperature of 250 c for cooling rates of 10 100 c m y

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

exhumation along the fairweather fault southeastern alaska based on low temperature thermochronometry 1 the southern alaskan syntaxis marks the spectacular junction between the 1000 km long pacific north america transform margin and the chugach st elias belt where subduction and terrane accretion drive rapid convergent deformation and rock uplift new low temperature thermochronometry reveals that intense orogenic deformation is not restricted to one side of the syntaxis but extends nearly 300 km south along the dextral fairweather fault apatite and zircon u th he ages as young as 0 9 and 2 0 ma suggest maximum exhumation rates of nearly 2 mm a in close proximity 0 5 mm a along the entire plate margin we estimate that long term rock uplift accommodates 3 mm a of fault normal convergence in this area this suggests that the fairweather fault is slightly transpressive and highly partitioned analogous to the central san andreas fault this convergence only accounts for 1 5 of the obliquity between pacific plate motion and the continental margin however implying the deficit is taken up by 1 2 cm a thrust sinistral motion along the offshore transition fault additionally thermochronometry shows a marked increase in bedrock cooling coincident with onset of heavy glaciation similar to what has been observed in other parts of the pacific northwest the tectonically active fairweather corridor is distinguished however by the magnitude of the acceleration and the depth of exhumation since pliocene climate change

tectonic block motion and glacial isostatic adjustment in southeast alaska and adjacent canada constrained by gps measurements 1 we use data from campaign and continuous gps sites in southeast alaska and the neighboring region of canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self consistent set of fault slip rates from the block motions present day tectonics in southeast alaska is strongly influenced by the collision of the yakutat block our model predicts a velocity of 50 3 0 8 mm a toward n22 9 0 6 w for that block our results suggest that the eastern edge of the yakutat block is deforming along this edge the fairweather fault accommodates a large portion of the pacific north america relative plate motion through 42 9 0 9 mm a of dextral slip further south along the queen charlotte fault our model predicts an average of 43 9 0 6 mm a of dextral slip and a southward increasing amount of transpression strain from the yakutat collision is transferred far to the east of the strike slip system this strain transfer causes the region north of glacier bay to undergo a clockwise rotation south of glacier bay and inboard of the queen charlotte fault a smaller but clearly defined clockwise rotation is observed the heterogeneous block motion north and south of glacier bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity the northern cordillera of canada displays a small northeasterly motion our block model suggests that the entire southeastern alaska northwestern canada margin is mobile

spatial extent of rapid denudation in the glaciated st elias syntaxis region se alaska 1 the ongoing collision of the yakutat terrane with the north american plate formed the st elias range in southeastern alaska previously published detrital zircon fission track dating of sand size material detected very rapid rock denudation within the syntaxis region of the st elias range but the size and spatial extent of the rapid exhumation are elusive due to the glacial cover that inhibits direct bedrock studies two possible mechanisms were suggested to explain the rapid denudation occurring underneath the seward malaspina glacier which differ in their spatial extent of rapidly exhumed rocks this study evaluates the spatial extent of rapid denudation at the st elias syntaxis region we investigate the cobble sized material of the malaspina glacier petrological classifications are used as a provenance tool and combined with zircon and titanite u th he analysis to reveal cooling rates of the glacial detritus in total 1998 cobbles are classified in the field petrographic characterization is conducted on 91 thin sections and the u th he ages of 59 cobbles are measured fourteen cobbles reveal young cooling ages of 3 2 ma originating from various lithologies of undeformed and deformed magmatic and metamorphic rocks older age populations peak at 12 27 and 36 ma the cooling ages and the mapped known extent of the different lithologies suggest that a larger then previously assumed area of the st elias syntaxis region experienced rapid exhumation rapid rock exhumation extends beyond the contact fault and comprises most of the seward malaspina glacier catchment

exhumation at orogenic indentor corners under long term glacial conditions example of the st elias orogen southern alaska abstract syntaxial bends in convergent plate boundaries or indentor corners display some of the most intriguing deformation patterns on earth and are type localities for aneurysms of coupled erosion thermal weakening and strain the st elias orogen in alaska is a small young convergent system that has been dominated by a glacial climate for much of its history and exhibits two prominent indentor corners that are not well understood we have added 40 new apatite u th he ages to the already extensive dataset for the low temperature cooling history of this orogen to constrain the pattern of exhumation in these indentor corners ages from the western syntaxis show minor variation across the structural hinge suggesting that the bend has little effect on the pattern of exhumation and that structures including the bagley fault connect smoothly from the orogen core to the subduction zone to the southwest rock uplift on the north flank of the range appears to increase steadily towards the eastern syntaxis which represents the apex in the right angle bend between a transform fault in the south and the collision zone in the west based on age elevation relationships zones of relative rock uplift can be defined in which the mt logan massif or the area just north of the eastern syntaxis experienced 4 8 km greater rock uplift than background levels northwest of the western syntaxis a bulge in relative rock uplift is symmetric about the hinge in the eastern indentor corner however rates of denudation in this bulge are not as rapid as the core of the fold and thrust belt and are lower than those implied by detrital cooling ages from beneath the seward glacier this implies that a large bull s eye of ultra rapid 5 mm yr exhumation does not occur and that the subpopulation of young detrital ages may be sourced from a narrow transpressional zone along the fairweather fault unlike the himalayan syntaxes it thus appears that an aneurysm of coupled erosion strain has not developed in either indentor corner of the st elias orogen this may indicate a limit to the degree to which glacial erosion can partition strain in convergent orogens similarly the likely existence of a through going dextral fault the totschunda fault through the eastern syntaxis implies that tectonics rather than surface processes exerts the main control on strain partitioning in these corners

transport of the yakutat terrane southern alaska evidence from sediment petrology and detrital zircon fission track and u pb double dating abstract two hypotheses have been offered to account for the transport and accretion history of the yakutat terrane in southern alaska to investigate these two options we deconvolved fission track ft and u pb ages of detrital zircons from stratigraphically coordinated samples collected in the northern robinson mountains into component populations the strata of the yakutat terrane include the middle eocene kulthieth formation the lower oligocene to lower miocene poul creek formation and the miocene pleistocene yakataga formation the kulthieth and poul creek formations record erosion of a simple uniform long lived nonvolcanic source terrain that crystallized from 50 to 220 ma and cooled from 40 to 110 ma miocene cooling episodes recorded in the source to the kulthieth and poul creek formations are likely associated with plutons in the northern coast plutonic complex and the kuiu etoilin belt the upper miocene to pleistocene yakataga formation records erosion of rocks that crystallized from 5

the structural setting of the red sea axial valley and deeps implications for crustal thinning processes abstract a lineamental analysis of the bathymetry in the northern red sea axial valley in conjunction with reflection seismic models shows a poor geometric correlation between the axial valley and the red sea margins however there is good accordance with the lineamental patterns of the bordering continental plates we conclude that the present axial tectonics of the northern red sea is a rejuvenation of an inherited continental structure in a regime of weak distension the distension appears distributed rather than focussed except at some points where the crust has been torn or collapsed to form deeps these areas have become starting points for limited crustal oceanization based on these conclusions we discuss some models of crustal thinning with continental volume conservation this process assumes a role for ascendant decontinentalisation of the basement

timing of late cenozoic tidewater glaciation in the far north pacific the onset of late cenozoic glacial events in the far north pacific ocean is recorded by ice rafted debris in the yakataga formation in the gulf of alaska the dating of these events is controversial ages based on molluscs suggest that initial late cenozoic tidewater glaciation occurred in the early middle miocene 15 16 ma previous work on planktic foraminifera indicates that this event is no older than late miocene probably latest miocene 5 6 ma resolution of this problem is important because the yakataga formation is the thickest most complete and best exposed repository of late cenozoic glaciomarine rocks in the northern hemisphere investigation of this record has broader implications for global temperature gradients paleoceanographic development of the far north pacific and northern hemisphere glacial history new planktic foraminiferal data from onshore and offshore yakataga formation sections in the gulf of alaska are compared to a regional depositional and chronostratigraphic framework in order to test whether the yakataga record is anomalous or consistent with the paleoclimatic record of the north pacific this comparison reconfirms that initial glaciomarine rocks of the yakataga formation are no older than late miocene new paleomagnetic data suggest that the base of the yakataga formation is within the lower gilbert polarity chron consistent with the planktic foraminiferal dating and k ar dates on glauconite correlation of the yakataga record to key offshore sections in the far north pacific deep sea drilling project dsdp sites 178 183 192 provides additional insights into the paleoclimatic significance of these rocks uplift of the alaska coastal ranges a necessary prelude to alpine glaciation occurred during late miocene time diatom biofacies dsdp sites 183 192 indicate warm middle miocene conditions climatic deterioration during the late middle and early late miocene time and initial tidewater glaciation between 5 0 and 6 7 ma a relatively warm mid pliocene interval succeeded this initial glaciation which was in turn followed by hemisphere scale glaciation beginning at 2 5 ma this second phase of yakataga glaciation may have begun as early as 3 0 3 5 ma glaciomarine rocks in the basal yakataga formation mark an important climatic event in the north pacific ocean they are latest miocene in age consistent with regional climatic information and not a middle miocene climatic anomaly

deformation during terrane accretion in the saint elias orogen alaska the saint elias orogen of southern alaska and adjacent canada is a complex belt of mountains formed by collision and accretion of the yakutat terrane into the transition zone from transform faulting to subduction in the northeast pacific the orogen is an active analog for tectonic processes that formed much of the north american cordillera and is also an important site to study 1 the relationships between climate and tectonics and 2 structures that generate large to great magnitude earthquakes the yakutat terrane is a fragment of the north american plate margin that is partly subducted beneath and partly accreted to the continental margin of southern alaska interaction between the yakutat terrane and the north american and pacific plates causes significant differences in the style of deformation within the terrane deformation in the eastern part of the terrane is caused by strike slip faulting along the fairweather transform fault and by reverse faulting beneath the coastal mountains but there is little deformation immediately offshore the central part of the orogen is marked by thrusting of the yakutat terrane beneath the north american plate along the chugach saint elias fault and development of a wide thin skinned fold and thrust belt strike slip faulting in this segment may be localized in the hanging wall of the chugach saint elias fault or dissipated by thrust faulting beneath a north northeast trending belt of active deformation that cuts obliquely across the eastern end of the fold and thrust belt superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen where the sedimentary cover of the yakutat terrane is accreted into the upper plate of the aleutian subduction zone these three structural segments are separated by transverse tectonic boundaries that cut across the yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the saint elias mountains onto the coastal plain the malaspina fault pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding onshore most of this boundary is buried beneath the western or agassiz lobe of the malaspina piedmont glacier the boundary between the central fold and thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the bering piedmont glacier

a zero damage model for fission track annealing in zircon a zircon fission track annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in decay event damage free zircon samples empirically derived parallel and fanning equations for this zero damage model yield an excellent fit to the data with the fanning model providing slightly better statistical parameters a comparison between annealing models with fanning iso annealing lines but different decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero damage model compilations of existing geologic constraints on the zircon partial annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial annealing zone and closure temperature this finding is consistent with the fact that the annealing behavior of zircon from long duration temperature evolutions is increasingly influenced by the accumulated decay event damage zircon samples of young age or low u content show a behavior closest to the predictions of the zero damage model and are in the predicted range of published models with low decay event damage density for thermal events of more than 10 myr duration however constraints from field studies show marked differences from proposed partial annealing zone boundaries of the zero or low damage models the applicability of the zero damage model is threefold 1 it predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no decay event damage is accumulated 2 it predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age as found in sedimentary samples and in this case may be used as a thermometer 3 it represents an important reference for the establishment of a more comprehensive model of zircon fission track annealing that also includes the influence of decay event damage for such a model two different equations are discussed however additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of decay event damage annealing

investigating the effect of mechanical discontinuities on joint spacing in rocks without systematic mechanical discontinuities e g granite joint spacing follows an approximately log normal frequency distribution i e the distribution has a kurtosis near zero joint spacing in rocks with systematic mechanical boundaries differs from the spacing in isotropic rocks exhibiting consistently positive values of kurtosis i e the distribution is more clustered around the mode than a perfectly log normal distribution we attribute this difference in joint spacing distribution to mechanical boundaries such as bed partings in sedimentary rocks that constrain joint height and control joint spacing existing systematic joints can also act as mechanical boundaries during the development of later cross joints two parallel mechanical boundaries determine the mechanical layer thickness that influences joint spacing in this paper we investigate the effect of sampling geometry and mechanical discontinuities on joint spacing statistics in many situations neither pavement surfaces nor properly oriented boreholes are available for measuring the spacing of cross joints that develop between existing systematic joints when joint spacing data come from scanlines that are oblique to a systematic joint set e g crossing many systematic joints on a sub vertical outcrop face we consider whether the median spacing of the systematic joint set is statistically equivalent to the mechanical layer thickness thought to control cross joint spacing between individual pairs of systematic joints the basis of our analysis is the fracture spacing index fsi which is the slope of a line fitted to a plot of mechanical layer thicknesses vs median joint spacing we collected joint spacing data along oblique scanlines from a large outcrop of the devonian brallier formation a distal turbidite sequence near huntingdon pa our analysis indicates that the spacing of cross joints correlates better with a mechanical layer thickness defined by the median systematic strike joint spacing r 2 d 0 78 fsid 1 02 than with a mechanical layer thickness defined by the stratigraphic bed thickness r 2 d 0 69 fsid 0 97 this is consistent with the conclusions of previous workers we also note that the spacing data from the cross joints exhibit a higher i e more positive value of kurtosis than the data from the earlier strike joints 1 66 vs 0 98 this is consistent with the idea that mechanical discontinuities alter joint spacing in a systematic manner in this case the cross joints may have been influenced by two sets of mechanical boundaries bedding and existing joints whereas the earlier strike joints were constrained by only one set bedding 1998 elsevier science b v all rights reserved

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

late miocene to pleistocene source to sink record of exhumation and sediment routing in the gulf of alaska from detrital zircon fission track and u pb double dating

constraining the area of rapid and deep seated exhumation at the st elias syntaxis southeast alaska with detrital zircon fission track analysis the chugach st elias orogen southeast alaska southwest yukon formed in response to the ongoing yakutat microplate subduction collision with the north american plate due to heavy glaciation the region is a prime location to study active convergent orogenesis and climate tectonic interactions this study focuses on the long term distribution of deformation in the st elias syntaxis area where dextral motion along the plate bounding fairweather fault transitions into convergence we present 2718 new zircon fission track single grain ages from 26 glaciofluvial outwash samples the grain ages range from 293 ma to 0 2 ma and each sample contains two to five age populations with peaks between 267 64 ma and 1 2 0 7 ma 1 the rocks of the yakutat microplate are dominated by latest cretaceous and eocene fission track ages while the rocks of the north american plate in the syntaxis region reveal two exhumation phases at 5 1 ma and 2 7 ma the spatial pattern of 5 ma cooling ages shows that the area of rapid and deep exhumation at the st elias syntaxis is more extensive than previously known and confined to the south by the fairweather fault to the west by the seward glacier catchment and to the east possibly by the inferred connector fault the area seems to be 4800 km2 large and may extend farther to the north the new zircon fission track data further suggest a transpressional plate boundary since 30 ma and the onset of plate collision 15 12 ma

formation of a metamorphic complex along an obliquely convergent margin structural and thermochronological evolution of the chugach metamorphic complex southern alaska this study documents the structural and metamorphic evolution of middle to lower crust along an oblique convergent curved continental margin during a time span of 60 myr our study documents the importance of variable obliquity during convergence which led to the development of overprinting fabrics and distinct exhumation histories along strike of the margin we present structural analyses 40ar 39ar rb sr and zircon fission track ages from middle to lower crust exposed along the southern alaskan margin in the chugach metamorphic complex together with the metamorphic history and additional geochronology from the literature we derive the following tectonic evolution for this area accretion of sediments during dextrally oblique convergence led to strain partitioned d 1 structures and greenschist facies metamorphism prior to circa 55 ma at 55 51 ma a margin parallel stretching phase with vertical thinning d 2 affected the margin and led to andalusite sillimanite grade metamorphism and the onset of partial melting a switch back to dextral transpression d 3 shortly after d 2 led to rapid cooling of the western and central parts of the complex associated with exhumation of parts of the core of the complex until circa 46 ma the southeastern part of the complex cooled and exhumed regularly and slowly until circa 5 ma due to its highly oblique orientation relative to the convergence direction an increase in cooling and exhumation occurred after circa 5 ma in the entire southeastern part of the complex associated with the neogene collision of the yakutat terrane

thermotectonic history of mt logan yukon territory canada implications of multiple episodes of middle to late cenozoic denudation abstract the extreme relief of the st elias mountains including mt logan is suggestive of a young mountain belt new apatite fission track data indicate that the mt logan massif experience rapid low temperature cooling during three distinct periods during the middle eocene middle miocene and pliocene each cooling episode the magnitude of which varies due to temporal variations in recorded paleogeothermal gradients can be tentatively linked to a tectonic cause eocene cooling may be due to re equilibration of isotherms following normal faulting caused by a combination of thermal weakening of the crust and a decrease in regional compressional stress due to the 43 ma change in relative motions between the pacific and north american plates the cause of miocene cooling is problematic and could reflect denudation in response to initial underplating of the yakutat terrane or a recorded change in heat flow unrelated to denudation pliocene cooling reflects erosion due to surface uplift that produced the spectacular present day topography of the st elias mountians this surface uplift is probably related to the coeval significant change in relative motion between the north american and pacific plates and or resistance to subduction by the yakutat terrane

cooling history of the st elias syntaxis southeast alaska revealed by geochronology and thermochronology of cobble sized glacial detritus

thermochronology in southeast alaska and southwest yukon implications for north american plate response to terrane accretion abstract this study presents the first comprehensive dataset of low temperature thermochronology from 43 bedrock samples collected north of the active yakutat north american plate boundary our apatite and zircon u th he and fission track data reveal the cooling history of the inboard wrangellia composite terrane that is dominated by rapid cooling after late jurassic to early cretaceous arc magmatism followed by very little cooling and exhumation until today deformation resulting in rock exhumation due to the collision of the yakutat microplate is spatially very limited 20 30 km and is concentrated mainly in the chugach prince william terrane and rocks near the border ranges fault focused exhumation from greater depths of ca 10 km with very high rates 5 km myr is localized at the syntaxis region starting ca 10 ma and shifted south through time the rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion which started already before glaciation of the area the shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate which is changing on local and global scales since the late miocene

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

curving cross joints and the lithospheric stress field in eastern north america cross joints are late formed nonsystematic fractures that extend across intervals between systematic joints traces of such cross joints are seen on bedding plane surfaces of devonian catskill clastic sedimentary rocks of the appalachian plateau of western new york state where the maximum horizontal principal stress s h is oriented n65 e as indicated by in situ stress measurements between pairs of closely spaced systematic joints traces of cross joints are commonly planar and orthogonal to the preexisting joints however in the mid region between some widely spaced systematic joints in western new york cross joints strike parallel to the s h of the present lithospheric stress field but then curve to abut the preexisting joints at right angles a curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints and the perpendicular termination indicates that the preexisting joints were open depending on their age the strike of the mid region of curving cross joints denotes the orientation of either the neotectonic stress field or its tertiary predecessor in the north american lithosphere

sandstone petrographic evidence and the chugach prince william terrane boundary in southern alaska the contact between the upper cretaceous valdez group and the paleocene and eocene orca group has been inferred to be the boundary between the chugach and the prince william tectonostratigraphic terranes sandstone petrographic data from the prince william sound area show no compositional discontinuity across this contact these data are best explained by considering the valdez and orca groups to be part of a single terrane a thick flysch sequence derived primarily from a progressively unroofing magmatic arc with increasing input from subduction complex sources through time

long term glacial erosion of active mountain belts example of the chugach st elias range alaska an emerging paradigm that equates glaciers to buzz saws of exceptional erosional efficiency has been strengthened by short term 10 2 yr sediment yields from southern alaska new low temperature cooling ages from this area the glaciated chugach st elias mountains constrain long term 10 6 yr exhumation rates vertically averaged exhumation rates reach 3 mm yr but are an order of magnitude lower than rates based on short term sediment yields whereas these exhumation rates are not exceptional for oro genic belts denudation patterns are strongly correlated with the distribution of glaciers and erosion appears to keep pace with con vergence and uplift these findings imply a coupling between gla cially dominated erosion and tectonics

indentor corner tectonics in the yakutat st elias collision constrained by gps the yakutat st elias collision in se alaska and adjacent canada represents a prime example of present day tectonics associated with an indentor corner its eastern syntaxis is marked by high exhumation a sharp structural bend and strain concentration at the transition from shortening to oblique transpression here we present gps velocity and strain rate fields that cover the syntaxis including 11 new stations in the core of the st elias mountains these data are corrected for transient deformation glacial isostatic adjustment and postseismic and interseismic loading to produce residual velocities and strain rates representative of long term tectonics the main features of these velocity and strain rate fields are a peak in strain rates strain knot in the syntaxis at the junction between the main fault systems and a rapid rotation from convergence parallel to convergence normal orientations of the velocities and shortening axes around the syntaxis leading to shortening across the southern denali fault these features are consistent with the strain and tectonic patterns expected near an indentor corner at the transition from shortening to transpression with a combination of diffuse and localized deformation the gps velocities and strain rates show diffuse deformation around the syntaxis from pure convergence parallel shortening in the orogenic wedge to oblique extension that accommodates the strain rotation at the front of the syntaxis this indentor corner model also results in a near zero strike slip rate on the southern denali fault and shows no clear evidence for a throughgoing fault hypothesized to link the fairweather and totschunda faults

the yakutat terrane dramatic change in crustal thickness across the transition fault alaska we present new constraints on the crustal structure of the yakutat terrane and evidence of the role of the transition fault in southern alaska the yakutat terrane south of yakutat bay includes crystalline crust that is 24 27 km thick overlain by sedimentary units that are 4 5 7 5 km thick the yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin the southern edge of the yakutat terrane is bounded by the transition fault which is imaged as a near vertical fault zone 1 km wide the transition fault is coincident with a dramatic change in moho depth from 32 km for yakutat oceanic plateau crust to 11 5 km for pacific ocean crust occurring over a horizontal distance of 0 5 km there is no evidence for underthrusting of the pacific ocean crust beneath the yakutat terrane at the transition fault we argue that the yakutat terrane formed on the kula or farallon plate and was later juxtaposed next to younger pacific ocean crust by the transition fault

controls on fracture distribution in cretaceous sedimentary rocks from the isfahan region iran in this study relationships between fracture patterns lithology thickness diagenetic processes and grain size are evaluated within cretaceous sediments in two sections of dizlu and kolah ghazi of isfahan this study area was selected based on its outcrops of different rock units and its well developed tectonic fractures the fracture patterns within stratigraphic units of these sections are studied using geometrical and statistical analyses this study finds that variable fracture spacing and fracture spacing ratios can be affected by lithology thickness grain size of sediments and diagenetic processes a study of fracture stratigraphy based on fracture pattern evaluation within different cropped out sedimentary rocks can be used to improve understanding of the same types of sedimentary rock units below the surface or throughout other sedimentary basins consequently this could improve information regarding storage and fluid flow pattern throughout sedimentary rocks in different regions even for subsurface purposes

crustal scale structural architecture shortening and exhumation of an active eroding orogenic wedge chugach st elias range southern alaska 1 active mountain building associated with the accretion of the yakutat microplate yt in southern alaska is characterized through the combination of a new balanced cross section and new low temperature cooling ages this analysis constrains the amount and timing of shortening the spatial and temporal trends of exhumation and the interplay between structural development and exhumation a fold and thrust belt comprising three principal thrust sheets the hope creek sullivan and an offshore thrust sheet from north to south characterizes the yt internal structure which has absorbed a minimum of 82 km shortening assuming shortening and foreland basin development occurred contemporaneously the shortening rate across the yt is 13 14 mm a after 5 6 ma detrital apatite fission track ages from south to north are unreset along the southern edge of the sullivan thrust sheet at the coast are reset and have ages younger than 6 3 ma within the thrust belt and have reset cooling ages of 13 ma in the north american upper plate only the zircon samples from the northern internal yt are potentially reset exhumation rates within the thrust belt vary from 0 3 0 1 mm a to 4 1 8 mm a combining the thermochronometric and structural data indicate that wedge exhumation since 6 ma is 500 km2 and that particle trajectories have larger horizontal than vertical components whereas exhumation and shortening have been focused on the windward side of the chugach st elias range since 6 ma the cooling ages do not uniquely distinguish between orographically versus tectonically controlled erosion

low frequency radar sounding of temperate ice masses in southern alaska we present the warm ice sounding explorer wise a low frequency 2 5 mhz radar for the sounding of temperate ice wise deployment in southern alaska in 2008 and 2012 provides comprehensive measurements of glacier thickness reveals deep valleys beneath glaciers and the full extent of zones grounded below sea level the east branch of columbia glacier is deeper that its main branch and remains below sea level 20 km farther inland ice is 1000 m deep on tazlina glacier on bering glacier two sills separate three deep bed depressions 1200 m that coincide with the dynamic balance lines during surges the piedmont lobe of malaspina glacier and the lower reaches of hubbard glacier are entirely grounded below sea level 40 and 10 km respectively from their termini knowledge of ice thickness in these regions helps better understand their glacier dynamics mass balance and impact on sea level key points low frequency radar sounding works extremely well in alaska ice is much thicker than suggested by limited surveys data reveals full extent of areas grounded below sea level 2013 american geophysical union all rights reserved

architecture kinematics and exhumation of a convergent orogenic wedge a thermochronological investigation of tectonic climatic interactions within the central st elias orogen alaska abstract the kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate but little research has been devoted to the long term effects of glacial erosion on orogenesis here we use low temperature apatite and zircon u th he and fission track thermochronometry along with subsidiary structural relationships and seismicity to develop a new architectural model of the st elias orogen in southern alaska which is one of the best examples of a glaciated orogenic wedge worldwide these data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding thin skinned fold and thrust belt on the windward flank a structure beneath the bagley ice field separates these distinct deformational domains which we propose is a backthrust that makes the orogen doubly vergent thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion long term exhumation at rates of up to 4 mm yr is concentrated within a narrow zone along the windward flank where glacier equilibrium lines intersect the orogenic wedge the onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge accelerated backthrust motion and a major shift in deformation away from the north american yakutat terrane suture chugach st elias fault we propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation which in turn are systematically truncated by the backthrust this focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially dominated orogen

mechanisms controlling rupture shape during subcritical growth of joints in layered rocks during joint propagation the orientation of plume lines on joint faces allows for mapping the path taken by the crack tip line during rupture growth rupture shapes in layered clastic sediments of the devonian catskill delta finger lakes district new york indicate joint growth through three stages where velocity of the tip line v tl varies as a function of the crack tip stress intensity k i the initial stage of growth is characterized by a rupture of approximately circular or elliptical shape that expands from an initiation fl aw with a velocity v tl in some cases primary growth involves self correction where an elliptical rupture redistributes the crack tip stress so that the rupture returns to a more stable circular shape the crack tip line eventually propagates to a bedding interface where properties of either a plastic or a noncohesive bedding interface cause v tl 0 along a portion of the tip line the onset of secondary growth occurs when the rupture tip line intersects both bedding interfaces and splits into two discontinuous segments that propagate synchronously as a single coherent rupture nonuniform nonsystematic v tl eventually interferes with the rupture s ability to grow coherently and thus leads to the transition to tertiary growth characterized by the detachment of the coherent rupture into one or more independently propagating noncoherent tip lines throughout all three stages of rupture growth the k i dependent v tl points to subcritical propagation variation from a smooth to rough plume morphology is consistent with propagation through region i of the subcritical regime with a transition at v tl 10 5 m sec rupture velocity may enter region ii of the subcritical regime but such a broad range of k i must be crossed to reach region iii and quasistatic behavior at the critical stress intensity k ic that region ii acts as a barrier through which few joints pass thus greatly limiting the number of postcritical joints in the catskill delta if not in the crust of the earth

depositional and microfaunal response to pliocene climate change and tectonics in the eastern gulf of alaska abstract the yakataga formation provides an excellent record of pliocene paleoclimate in the gulf of alaska and northeastern pacific ocean evidence for glaciomarine influence and paleoclimatic change includes the distribution of ice rafted debris and diamictite planktic and benthic foraminiferal biofacies and variable sediment accumulation rates three paleoclimatically defined intervals within the pliocene yakataga formation are recognized in onshore and offshore sections interval p1 5 35 to 4 2 ma contains the first evidence of neogene glaciomarine deposition in the form of ice rafted debris and rare diamictite interval p2 4 2 to 3 0 3 5 ma exhibits a reduction in glaciomarine influence and warming to cool temperate conditions interval p3 3 0 3 5 to 1 8 ma contains massive amounts of glaciomarine diamictite and correlates with other evidence indicating the onset of major northern hemisphere glaciation sediment accumulation rate and foraminiferal based paleobathymetric curves show some correlation to the above paleoclimatic subdivision but also significant independent relationships indicating a strong tectonic influence on those parameters comparison of the yakataga record to odp sites in the north pacific shows that there is good agreement in the ice proximal yakataga and deep sea records of pliocene paleoclimate change

cooperation among tectonic and surface processes in the st elias range earth s highest coastal mountains investigations of tectonic and surface processes have shown a clear relationship between climate influenced erosion and long term exhumation of rocks numerical models suggest that most orogens are in a transient state but observational evidence of a spatial shift in mountain building processes due to tectonic climate interaction is missing new thermochronology data synthesized with geophysical and surface process data elucidate the evolving interplay of erosion and tectonics of the colliding yakutat microplate with north america focused deformation and rock exhumation occurred in the apex of the colliding plate corner from 4 to 2 ma and shifted southward after the 2 6 ma climate change the present exhumation maximum coincides with the largest modern shortening rates highest concentration of seismicity and the greatest erosive potential we infer that the high sedimentation caused rheological modification and the emergence of the southern st elias intercepting orographic precipitation and shifting focused erosion and exhumation to the south

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

the border ranges fault system in glacier bay national park alaska evidence for major early cenozoic dextral strike slip motion the border ranges fault system of southern alaska the fundamental break between the arc basement and the forearc accretionary complex is the boundary between the peninsular alexander wrangellia terrane and the chugach terrane the fault system separates crystalline rocks of the alexander terrane from metamorphic rocks of the chugach terrane in glacier bay national park mylonitic rocks in the zone record abundant evidence for dextral strike slip motion along north northwest striking subvertical surfaces geochronologic data together with regional correlations of chugach terrane rocks involved in the deformation constrain this movement between latest cretaceous and early eocene 50 ma these findings are in agreement with studies to the northwest and southeast along the border ranges fault system which show dextral strike slip motion occurring between 58 and 50 ma correlations between glacier bay plutons and rocks of similar ages elsewhere along the border ranges fault system suggest that as much as 7

coupled stratigraphic and structural evolution of a glaciated orogenic wedge offshore st elias orogen alaska 1 the st elias orogen is the result of 10 myr of oblique convergence and flat slab subduction in the gulf of alaska between north america and the yakutat microplate extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge reflection seismic profiles across the offshore pamplona zone fold thrust belt the frontal st elias orogenic wedge provide constraints for quantifying pleistocene deformation recorded in the glaciomarine yakataga formation the total amount of pleistocene shortening observed varies from 3 to 5 mm yr compared to the current gps derived yakutat north america convergence rate across the st elias orogen of 45 mm yr growth strata and kinematic fold analysis allow comparison of relative timing of fault activity which reveals temporal and spatial shifting of active deformation during the glacial period faulting localized adjacent to the coastline and at the current submarine deformation front the abandoned currently inactive region is colocated with the major glacial depocenter in the region the bering trough these observations imply that glacial processes such as sediment loading and focused erosion during advance retreat cycles has a direct effect on the evolution of individual faults within the pamplona zone and the overall deformation pattern in the offshore st elias margin this information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during pleistocene glacial interglacial periods

extreme localized exhumation at syntaxes initiated by subduction geometry some of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments the initiating perturbation of this deformation has long been attributed to vigorous erosional processes as observed at nanga parbat and namche barwa in the himalaya and at mount st elias in alaska however an erosion dominated mechanism ignores the 3 d geometry of curved subducting plates here we present an alternative explanation for rapid exhumation at these locations based on the 3 d thermomechanical evolution of collisions between plates with nonplanar geometries comparison of model predictions with existing data reproduces the defining characteristics of these mountains and offers an explanation for their spatial correlation with arc termini these results demonstrate a bottom up tectonic rather than top down erosional initiation of feedbacks between erosion and tectonic deformation hence the importance of 3 d subduction geometry

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

relationships between thrusting and joint systems in the jaca thrust top basin spanish pyrenees abstract the oligo miocene rocks of the west jaca thrust top basin and adjacent parts of the ebro basin are cut by up to eight sets of joints and allied mesofractures the fractures belong to three groups that can be distinguished on the basis of their relative ages and geometry an older group of joints strikes normal or subnormal to the pyrenean mountain front and is restricted to subareas here called front normal joint domains coincident with the immediate footwalls of thrusts joints striking parallel to a buried lateral ramp characterize a lateral ramp joint domain younger joints striking parallel or subparallel to the mountain front occur throughout most of the west jaca and ebro basins and define front parallel joint domains the joint domains appear to reflect the geometry and evolution of thrust sheets joints in front normal domains were formed during stretching of footwalls as a result of their loading by overriding thrust sheets stretching above a lateral ramp is thought to be responsible for the development of joints in the lateral ramp domain joints in the front parallel domains of the west jaca basin are related to stretching in growth folds that were amplifying during salt doming front parallel joints in the ebro basin are attributed to stretching of a foreland basin sequence above a basement flexure related to thrust loading

low temperature thermochronology of the yakutat plate corner st elias range alaska bridging short term and long term deformation abstract this study investigates the long term deformation and rock exhumation of the subducting yakutat microplate corner at the st elias syntaxis region with a special focus on the area east and west of yakutat bay where major coseismic surface uplift occurred during a mw 8 2 earthquake in 1899 we use low temperature thermochronology to quantify the long term activity of thrust and reverse faults we present new apatite and zircon u th sm he ages from 35 bedrock samples collected along profiles crossing major faults that compose a one sided flower structure at the northern end of the fairweather fault the cooling ages reveal that rapid rock exhumation is accommodated by the eastern esker creek fault and the yakutat fault with rates of 2 3 mm yr and slower rates of

exhumation rates in the st elias mountains alaska abstract reprocessing and interpretation of seismic profiles from the gulf of alaska in the vicinity of the st elias mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch approximately 10 000 years ago this estimate of the sediment volume does not include material moved out of the region by long shore currents transportation from the shelf to deeper waters or material trapped in fiords and basins on and near land thus an estimated sedimentation rate offshore of 7 9 mm year and the average erosion rate of 5 1 mm year from the st elias mountains derived from the estimated sedimentation rate represent minimum estimates we infer that these estimated rates are not typical of the approximately 5 m y history of mountain building in the area but rather they represent the result of efficient erosion by glaciers during this interglacial period

mid pleistocene climate transition drives net mass loss from rapidly uplifting st elias mountains alaska erosion sediment production and routing on a tectonically active continental margin reflect both tectonic and climatic processes partitioning the relative importance of these processes remains controversial gulf of alaska contains a preserved sedimentary record of the yakutat terrane collision with north america because tectonic convergence in the coastal st elias orogen has been roughly constant for 6 my variations in its eroded sediments preserved in the offshore surveyor fan constrain a budget of tectonic material influx erosion and sediment output seismically imaged sediment volumes calibrated with chronologies derived from integrated ocean drilling program boreholes show that erosion accelerated in response to northern hemisphere glacial intensification 2 7 ma and that the 900 km long surveyor channel inception appears to correlate with this event however tectonic influx exceeded integrated sediment efflux over the interval 2 8 1 2 ma volumetric erosion accelerated following the onset of quasi periodic 100 ky glacial cycles in the mid pleistocene climate transition 1 2 0 7 ma since then erosion and transport of material out of the orogen has outpaced tectonic influx by 50 80 such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out of sequence fault patterns the 1 2 my mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response millions of years the st elias range provides a key example of how active orogenic systems respond to transient mass fluxes and of the possible influence of climate driven erosive processes that diverge from equilibrium on the million year scale

single grain heavy mineral provenance of garnet and amphibole in the surveyor fan and precursor sediments on the gulf of alaska abyssal plain implications for climate tectonic interactions in the st elias orogen abstract the st elias orogen formed as a result of the northwestward drift of the yakutat terrane along and final collision with the alaska margin in the miocene its exhumation coincided with changing glacial conditions that are considered to have strongly interacted with mountain building processes a significant part of the record of these tectonic climatic interactions is stored in sediments on the gulf of alaska abyssal plain including the surveyor fan our study examines temporal provenance changes of miocene through pleistocene sediments of the surveyor fan gulf of alaska to constrain the dynamics of exhumation and mass transfer from the evolving st elias orogen to the adjacent surveyor deep sea fan we present single grain geochemical data of amphibole and garnet and 40 ar 39 ar cooling ages of biotite and amphibole together with point counting data of heavy minerals from sands and silts from two sites in the distal and proximal fan drilled by iodp expedition 341 in 2013 sites u1417 and u1418 respectively a shift in heavy mineral composition during the miocene predating the onset of glaciation points to a tectonically induced change in erosion centers and sediment transport probably caused by the rise of the st elias mountains garnet and amphibole data suggest the chugach metamorphic complex is the main sediment source implying input to the surveyor fan from sources relatively far in the north during the miocene changing provenance signals from the miocene to pliocene suggest rising input from the lower grade metamorphic areas at the southern flanks of the orogen indicating the advance of glaciers to the tidewater line providing material from this flanking region higher input from the chugach metamorphic complex in all pleistocene sediments suggests the northern hemisphere glaciation at the plio pleistocene boundary caused erosion and sediment yield from the interior of the orogen climatic changes at the mid pleistocene transition did not cause significant changes in the provenance signal

orthogonal fracture systems at the limits of thrusting an example from southwestern wales abstract a well developed orthogonal vein system is located in the autochthonous foreland just beyond the limits of variscan thrusting in southwestern wales the two vein sets trend 200 and 290 perpendicular and parallel respectively to variscan structures the veins contain stretched quartz fibers indicating horizontal extensions of between 0 3 and 2 5 the 200 set has greater extension with a distinctive morphology including shorter more curvilinear fractures that have components of normal offset vein intersections are simple indicating episodic growth by alternating regional propagation of each set during propagation maximum compression was subvertical and the horizontal principal stresses switched direction with the alternation of set growth the system formed during waning variscan compression when the directions of horizontal principal stresses were still controlled by the tectonic compression the differential stress may have been greater during propagation of the 200 set this example indicates that orthogonal fracture systems in forelands beyond thrusting 1 are initiated by fracturing parallel to tectonic compression 2 form in a few stages that are controlled by changes in regional stress and 3 form during waning tectonic compression that controls fracture orientations

the thermochronological record of tectonic and surface process interaction at the yakutat north american collision zone in southeast alaska we investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing yakutat north american collision in southeast alaska many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility this paper reviews new and published low temperature thermochronological data from southeast alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change we find that the southeastern margin of alaska was exhumed and eroded long before the late miocene pliocene yakutat collision but since the beginning of the subduction of the yakutat lithosphere in the oligocene early miocene today there is a distinct pattern of exhumation in southeast alaska with a localized very rapid and deep seated exhumation at the yakutat plate corner st elias syntaxis where strike slip motion changes to convergence exhumation is also rapid but less deep along the dextral fairweather fault and in the evolving fold and thrust belt we present a re interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike the locus of most rapid exhumation migrated from northwest to southeast with recent exhumation occurring near the st elias syntaxis exhumation of the chugach terrane rocks is still active however to a lesser degree than on the south side of the orogen where precipitation rates are much higher the wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast alaska since the early cretaceous apatite u th he ages give the first evidence that rocks of the wrangell range have only been recently uplifted and eroded as a consequence of the continuing yakutat collision in general the thermochronology in southeast alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes

source to sink evaluation of sediment routing in the gulf of alaska and southeast alaska a thermochronometric perspective in this study we present a source to sink evaluation of sediment routing at the glaciated convergent margin in southeast alaska we investigate the efficacy of thermochronology to record spatial and temporal exhumation patterns in synorogenic sediment using neogene strata drilled by integrated ocean drilling program expedition 341 in the gulf of alaska we present 1641 and 529 new detrital zircon and apatite fission track ages respectively from strata deposited on the continental shelf slope and deep sea fans these data are compared to results from the proposed source terrains including the st elias mountains and new data from the alsek river we find that the offshore bagley bering sediment contains grains recording cooling ages much older 80 35 ma than those reported from the st elias syntaxis 3 2 ma indicating that extreme rapid exhumation does not extend west of the seward bagley divide data from the sediment on the continental shelf slope and proximal deep sea all yield similar results suggesting the same general source region since 1 2 ma and limited sediment mixing along this glaciated margin data from sediment in the distal deep sea show that extreme rapid and deep seated exhumation was ongoing at 11 8 ma overall this study demonstrates the strengths and limitations of using detrital fission track thermochronology to understand sediment routing on a glaciated convergent margin and to record changes in exhumation rates over geologic time scales