northland ophiolite new zealand and implications for plate tectonic evolution of the southwest pacific the northland ophiolite and coeval ocean floor sedimentary rock form the northland allochthon obducted from the northeast onto new zealand in the late oligocene the ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon chemically the bulk of the igneous rocks are normal mid ocean ridge basalts but the ophiolite also includes a younger suite of hornblende modal within plate alkalic rocks believed to represent seamounts two alternative models one involving a subduction flip the other involving continuous westward subduction are proposed to account for the obduction of the upper part of the northland ophiolite the oceanic crust from which the ophiolite originated was formed simultaneously with tasman sea spreading on the western rim of a once much larger south fiji plate assemblage

a conceptual model for infiltration in two layered soils with a more permeable upper layer from local to field scale summary a conceptual model with a semi analytical formulation for estimating the expected field scale infiltration in two layered soils is proposed here at the local scale an upper layer with a vertically homogeneous saturated hydraulic conductivity k 1 s much greater than that of subsoil k 2 s is considered the basic element of the local model is the relatively simple mathematical representation of infiltration as a function of time this representation depends on whether the dynamic wetting front is entirely within the upper layer when surface saturation occurs or if the front has moved past the interface into the subsoil at the time to ponding this local scale infiltration model is found to be very accurate when compared to the richards equation moreover the local model clearly identifies if infiltration is controlled by rainfall the top soil layer or the bottom soil layer for upscaling to field scale k 1 s is represented by a spatial horizontal random field while k 2 s for the subsoil is assumed constant areally averaged infiltration is obtained by integrating the local infiltration equations monte carlo simulations over a vertical soil profile representative of a sandy loam top soil and a clay loam subsoil are performed for constructing the ensemble averages of field scale infiltration to be used for model testing the simulations reveal that the proposed model shows promise for field scale infiltration computations with errors referred to the aforementioned numerical benchmark typically less than 10 and 5 for infiltration rates and cumulative infiltration respectively conditions when spatial variability in infiltration properties of the top soil can be neglected are identified

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

formation mechanism and tectonic significance of millipede microstructures in the nw himalaya abstract this article focuses on the origin and tectonic significance of millipede inclusion trail microstructures preserved within garnet porphyroblasts of typical barrovian type schist from the swat region of the himalayas in pakistan two possible kinematic models are assessed envisaging bulk coaxial shortening and progressive simple shear 3d microstructural analysis of three samples hosting the millipedes reveals a poly phase deformation history that produced three suborthogonal foliations called sx sy sz successive inclusion of sx and sy in garnet porphyroblasts produced two sets of millipedes with distinctive subvertical and subhorizontal orientations respectively this kinematic reconstruction is supported further with a numerical simulation using finite difference code of millipede formation in a spherical porphyroblast subjected to a superposition of two orthogonal shortening directions measurements of inclusion trail curvature axes or fia foliation intersection inflexion axes in porphyroblasts in 61 additional samples from the study area which contain straight sigmoidal or spiral inclusion trails reflect two principle orogenic episodes during the first episode fias developed with wnw ese set 1 and e w set 2 trends associated with thrusting and collision tectonics a younger fia set 3 with nne ssw trend developed as a result of subsequent bulk e w crustal shortening and associated regional scale folding sigmoidal or straight inclusion trails defining this fia set can be correlated with the millipede microstructures based on their similar trends and timing relative to matrix fabrics these results are consistent with the meso and macro scale structures in the region and significant in terms of establishing relative timing of formation of different structures in the region well developed preferred vertical and horizontal orientations of both sigmoidal and millipede inclusion trails imply that their development involved limited or no relative rotations between porphyroblasts sigmoidal trails formed in zones where bulk shearing caused progressively reoriented matrix foliations around stable garnets while millipedes formed simultaneously in local coaxial strain pods

soil seal formation and its effect on infiltration uniform versus nonuniform seal approximation two different approaches to the solution of the problem of flow through the dynamic stage of seal formation as well as through soil with a fully developed seal are studied the first approach considers the disturbed seal layer and the undisturbed soil underneath as a continuous nonuniform soil profile the second replaces the nonuniform seal by a uniform equivalent layer thereby generating a homogeneous two layer flow system the depth dependent properties of the nonuniform seal are expressed in terms of the exponential model of mualem and assouline 1989 the dynamics of seal formation are modeled according to assouline and mualem 1997 during the first rainfall on an undisturbed soil profile when the seal layer is formed the application of the first or the second approach has only a minor effect on the calculated infiltration curves however there is a significant difference between the two solutions regarding the dynamic changes of the water content in the soil surface and consequently within the seal layer during subsequent rainfalls on a sealed soil profile when the seal layer is completely developed the differences between the two ways of accounting for the seal layer become evident and their effects on the infiltration curve are much more significant representing the seal as an equivalent uniform layer increases the ponding time and the infiltration rates at the early stage of the process the amplitude of these effects is increased when the rainfall rate is higher and the seal layer is thicker an important result is that the relationship between infiltration rate and cumulative infiltration is unique in the case of a completely developed seal and when the seal is considered as a nonuniform layer however this relationship is not unique during seal formation independent of the approach applied to represent the seal layer

rainfall intensity and slope gradient effects on sediment losses and splash from a saline sodic soil under coastal reclamation abstract rainfall intensity and slope gradient play important roles in soil erosion processes this study was conducted to investigate the effects of rainfall intensity and slope gradient as well as of their interactions on splash and overland flow transport on saline sodic soil slopes rainfall intensities of 85 95 110 and 125 mm h 1 were applied to slope gradients of 6 11 22 and 35 splash was partitioned into four directional components runoff sediment and splash samples were collected throughout the rainstorms and infiltration was calculated the final infiltrations under the lower rainfall intensities of 85 mm h 1 were the largest for every slope gradient but they decreased by 39 3 as the slope gradient increased from 6 to 35 however the final infiltration rates exhibited an increasing decreasing trend for rainfall intensities higher than 85 mm h 1 as the slope gradient increased the critical slope gradient was about 11 the influence of rainfall intensity on runoff was considerably reduced at the higher slope gradients due to increased infiltration the influence of slope gradient on sediment loss was reduced as rainfall intensity increased while the effect of rainfall intensity on sediment losses was greater on the gentler slopes than on the steeper slopes the upslope splash mean value initially increased from 4 8 to 7 2 g m 1 h 1 before it decreased to 3 g m 1 h 1 as the slope gradient increased the maximum measured value occurred when the slope gradient was 22 total splash increased to maximum levels as the slope gradient increased to 11 but decreased with further increases in gradient these results indicated that the effects of slope gradient and rainfall intensity on sediment losses runoff and splash were interconnected

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

modeling soil seal as a nonuniform layer a quantitative conceptual model of soil sealing is proposed based on the assumption that the compaction of the upper soil layer by raindrops and the washed in material result in a nonuniform change in bulk density which can be approximated by an exponential function the change in density causes in turn variations in soil properties such as porosity water retention and hydraulic conductivity empirical functions are suggested to simulate the pattern by which these properties vary as dependent upon the density change the model was calibrated using data of two experimental studies related to crust properties of three different soils sandy loam loam loess and clay loam the model reproduced quite well the observed features of the experimental sealing characteristics seals developed in two different flow systems subjected to simulated rain were analyzed 1 seal developed under saturated conditions over a shallow soil bed and 2 seal developed under unsaturated flow conditions the proposed model highlights some soil parameters that are quite important and should be observed in soil sealing investigations it also presents a basis for quantitative analysis of the soil sealing phenomenon in terms of measurable physical parameters

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

porphyroblast rotation and strain localization debate settled the topic of whether or not porphyroblasts rotate relative to a fixed external reference frame during deformation has engendered much debate mainly owing to the fact that pre deformational orientations of porphyroblasts and their inclusion trails can rarely be reconstructed in 20 years of

effect of rainfall induced soil seals on soil water regime wetting processes the effects of a rainfall induced soil seal on wetting processes are studied in the cases of two different soils hamra sandy loam and loess loam the soil seal is identified as the disturbed upper layer where the average hydraulic properties have been changed during rainfall from the initial properties that were identical to those of the undisturbed soil bed these properties are derived by applying the seal model of mualem and assouline 1989 calibrated for the two soils determination of the seal thickness bulk density retention curve and hydraulic conductivity function made it possible to solve systematically the flow equation for different boundary conditions allowing the simulation of the wetting processes within the seal layer as well as the undisturbed profile underneath the infiltration curves and the water content profiles during and after rainfall are derived for three rainfall intensities the water content and the capillary head profiles as well as the variation of these variables with time at different depths under the soil surface were also calculated smaller amounts of the rainfall infiltrate as the rainfall intensity increases the infiltration rate diminishes faster as the rainfall intensity increases the idea of a single infiltration curve as a function of the cumulative rainfall seems to be invalid each rainfall intensity yields a different infiltration curve a rough validation test of the model yielded predicted infiltration curves closely bounded by the experimental results the final infiltration rate after 50 mm of rainfall also seems to be a function of the rainfall intensity with the higher rate obtained for the higher rainfall intensity as experimentally observed in other studies unlike the explanations found in literature this study suggests that a shallower saturated zone within the seal layer is the reason for this phenomenon this study points out that the disturbed layer which forms the surface seal is not saturated from the very beginning of rainfall it has a significant water holding capacity and thus takes time to reach saturation furthermore under rainfall of high intensity the seal layer reaches saturation only at its upper zone close to the soil surface the lower part may remain unsaturated even after 50 mm rainfall and is larger for the soil of coarser texture

lateral shifts of apenninic foredeep depocentres reflecting detachment of subducted lithosphere abstract the hypothesis of lateral migration of slab detachment was formulated on the basis of seismic tomography results on the 3 d structure of subduction zones in the mediterranean region m j r wortel and w spakman structure and dynamics of subducted lithosphere in the mediterranenan region proc k ned akad wet 95 1992 325 347 the redistribution of slab pull forces associated with the tearing process is taken to affect foredeep development by adding a lateral component to internal external depocentre migrations in the case of the apenninic arc the direction of detachment and associated depocentre migration is expected to be southeast this prediction has been confronted with the tectonostratigraphy of oligocene to recent apenninic foredeeps results show there are two episodes marked by important lateral shifts of foredeep depocentres during the early late miocene tortonian the depocentre of the northern apennines foredeep shifted towards the southeast this is evidenced by the relative positions of the depocenters of the burdigalian serravallian marnoso arenacea and the tortonian umbrian marchean and lazian minor basins a further stepwise southeastward migration can be inferred from the plio pleistocene records of the central and southern apennines in a more general sense our results indicate that slab detachment may play a significant role in the evolution of foredeeps along convergent plate boundaries

soil mineralogy and slope effects on infiltration interrill erosion and slope factor 1 interactions between the effects of soil mineralogy and of slope gradient on seal formation infiltration rate ir runoff and soil loss were evaluated and the slope factor sf functions for various soils with differing mineralogy were examined three different soils on slopes of 9 15 20 and 25 were subjected to 80 mm of simulated rainfall the final ir was 20 5 mm h 1 in a clayey kaolinitic soil and 15 mm h 1 in smectitic soils the total runoff as a percentage of rainfall ranged from 24 to 18 65 to 43 and 50 to 35 from the kaolinitic clayey smectitic and sandy loam smectitic soils respectively the total soil loss ranged from 0 32 to 0 45 and from 1 14 to 3 93 kg m 2 for the kaolinitic and smectitic soils respectively in all three soils increasing the slope gradient increased the detachment of coarse particles 0 1 mm more sharply than that of small particles 0 1 mm especially in the smectitic soils combining these results with previous findings for six soils indicated that soils could be divided into two groups according to their sf values 1 soils that contained smectite and were therefore dispersive and 2 soils that did not contain smectite and were therefore stable for the former soils the regression sf 0 47exp 7 7 sin defined the sf values significantly whereas for the latter group a linear regression sf 0 81 1 77 sin was required it was suggested that the difference in the sf functions was mainly due to seal formation enhancement by the smectite

progressive deformation partitioning and deformation history evidence from millipede structures the progressive development and migration of patterns of deformation partitioning at all scales through the rock matrix commonly destroys any record of the ductile history associated with previous events making the problem of similar structures developing through multiple pathways generally intractable however records of the small scale geometries that form as deformation commences and begins to partition through a rock are routinely trapped and protected by porphyroblasts because these large crystals nucleate and or grow at this time this allows examination of the geometry of microstructures formed at the start of deformation partitioning that were destroyed by the same event in the matrix or which formed during an event prior to any preserved in the matrix porphyroblasts locally preserve oppositely concave microfolds millipedes which in all examples that we have found exclusively indicate a deformation history of bulk inhomogeneous shortening very similar structures have been formed experimentally during inhomogeneous simple shear but can readily be distinguished from those trapped in porphyroblasts that form during progressive bulk inhomogeneous shortening oppositely concave microfolds in some porphyroblasts reveal that deformation near orthogonal to a previously developed foliation occurred by axial plane shear driven rotation that led to rapid reactivational card deck like collapse of the pre existing foliation differentiated crenulation cleavages may result from the same process providing yet another reason for the cessation of porphyroblast growth at the start of differentiation

crenulation cleavage development by partitioning of deformation into zones of progressive shearing combined shearing shortening and volume loss and progressive shortening no volume loss quantification of solution shortening and intermicrolithon movement abstract an analytical method for determining amounts of cleavage normal dissolution and cleavage parallel shear movement that occurred between adjacent microlithons during crenulation cleavage seam formation within a deformed slate is developed for the progressive bulk inhomogeneous shortening pbis mechanism of crenulation cleavage formation the method utilises structural information obtained from samples where a diverging bed and vein are offset by a crenulation cleavage seam several samples analysed using this method produced ratios of relative cleavage parallel movement of microlithons to the material thickness removed by dissolution typically in the range of 1 1 3 4 1 the mean amount of solution shortening attributed to the formation of the cleavage seams examined is 24 the results indicate that a relationship may exist between the width of microlithons and the amount of cleavage parallel intermicrolithon movement the method presented here has the potential to help determine whether crenulation cleavage seams formed by the progressive bulk inhomogeneous shortening mechanism or by that involving cleavage normal pressure solution alone

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

seismic stratigraphy of a buried volcanic arc northland new zealand and implications for neogene subduction abstract for over 6 million years in the early miocene the northland peninsula of new zealand s north island was the site of a volcanic arc comprising north westerly trending twin belts of marine and subaerial andesitic volcanoes approximately 50 volcanic edifices from major massifs to small cones have been newly identified on marine seismic profiles of the western belt which is largely buried the volcanoes were important point sources of sediment and the volcaniclastic deposits of each of the major and intermediate sized volcanoes form distinct seismic sequences the three main elements of the volcanic sequence are i a main body ii a sloping apron and iii a wide almost flat ring plain the downlap of the oldest discernible internal clinoform reflector relative to a known regional marker reflector underlying it indicates the approximate time of onset of activity whereas downlap of the top bounding reflector relative to those of other volcanic sequences provides a relative order of extinction sequence stratigraphic principles and super position determine relative stratigraphic hierarchy the volcanoes studied here were active only in the early miocene with few exceptions most were active by 22 ma and extinct by 16 ma with many extinct by 19 ma there was no pattern in their order of inception or extinction although a few became dominant burying or surrounding their neighbours at the south eastern end of the chain there is a rapid change over a short distance in both age and orientation from the northwest trending early miocene volcanism in the northland basin to north east trending middle and late miocene volcanism in the contiguous taranaki basin the northland volcanoes are interpreted to have been an arc facing the south westwards subducting south fiji basin whereas the taranaki volcanoes are interpreted to have been part of a separate pacific facing volcanic arc the lau colville ridge

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

a parameterized model for local infiltration in two layered soils with a more permeable upper layer summary a simple conceptual model for local infiltration into a two layered soil profile with the upper layer much more permeable than the subsoil is proposed it is a reformulation of a conceptual model of corradini et al 2000 j hydrol 273 58 73 involving coupled ordinary differential equations that was developed for infiltration and soil water content distribution in a two layered soil under any rainfall pattern the simpler model framework proposed here is restricted to the estimate of infiltration rate alone in cases where the upper layer is more permeable than the lower one first the maximum value k 1 h of the saturated hydraulic conductivity of the upper soil k 1 s that yields the time to ponding while the dynamic wetting front is entirely within this homogeneous layer is defined by a conceptual approach then parameterized forms for the time when the wetting front reaches the interface for time to ponding associated with k 1 s k 1 h and for the suction head at the interface as a function of time are determined with these functional forms the current model utilizes algebraic equations for ponding time and soil infiltration capacity thus providing substantial savings in computational effort for describing infiltration in two layered soils when compared to the detailed conceptual model the proposed model represents the infiltration rate accurately with less than 10 average error on the dynamic cumulative infiltration depth this simplified model offers potential for upscaling to field scale infiltration over layered soils exhibiting spatial heterogeneity

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

linked debrites in sand rich turbidite systems origin and significance abstract the outer parts of a number of small late jurassic sandy deep water fans in the northern north sea are dominated by the stacked deposits of co genetic sandy and muddy gravity flows sharp based structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones pseudonodular siltstones and mudstones the contacts between the clast rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection the breccias contain fragments up to metre scale of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains often coarser than in the immediately underlying sand bed markov chain analysis establishes that the breccias consistently overlie sandstones and the character of the breccias and their external contacts rule out a post depositional origin via in situ liquefaction intrastratal flowage or bed amalgamation and disruption the breccias are interpreted as debrites that rode on a water rich sand bed just deposited by a co genetic concentrated gravity current as such they are referred to as linked debrites to distinguish them from debrites emplaced in the absence of a precursor sand bed the distinction is important because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath and they ride on a low friction carpet of liquefied sand this explains the paradox presented by fan fringes in which there are common debrites when conventional thinking might predict that deposits of low concentration gravity currents should be more important here in fact evidence for transport by low concentration turbidity currents is rare in these systems several possible mechanisms might explain the formation of linked flows but the ultimate source of both sandy and clast rich flow components must be in shallower water on the basin margin the debrites are not triggered from distal slopes flow partitioning may have occurred by upslope erosion and retardation of the mudclast charged portion of an erosional sandy density current partial flow transformation of a precursor debris flow and or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport linked debrites are not restricted to small sand rich fans and similar mechanisms may be responsible for the long runout of debris flows in other systems the recognition of a distinct class of linked debrites is of wider importance for facies prediction reservoir heterogeneity and even carbon fluxes and sequestration on continental margins

nonlinearity of ecogeomorphic processes along mediterranean arid transect the study was conducted along a climatic transect extending from the judaean mountains mean annual rainfall 700 mm annual mean temperature 17 c to the dead sea mean annual rainfall 100 mm annual mean temperature 23 c a high correlation was found between climatic conditions and factors and processes of the ecogeomorphological system the values of certain factors and processes such as soluble salt content and overland flow increase with increasing aridity whereas those of others such as soil organic matter content clay content aggregate size and stability and infiltration rate diminish with increasing aridity these trends express the long term effect of the prevailing mean climatic conditions in the research stations conditions that led to the development of various types of soil and of plant communities it was also found that for many of the factors and processes examined such as organic matter content salt concentration aggregate stability infiltration rate and overland flow yield the rates of change with respect to position along the transect are not linear and that a climatic threshold value exists in the zone with a mean annual rainfall of about 300 mm and in which the p t ratio is about 17 which creates a sharp boundary between arid and mediterranean ecogeomorphological systems while in arid systems erosion occurs mainly through physical mechanical processes under mediterranean systems it is governed mainly by chemical and biological processes the significance of the existence of a sharp boundary between the arid and the mediterranean zones is that even a relatively slight increase in the annual mean temperature and or a slight decrease in the mean annual rainfall is liable to shift the boundary towards the mediterranean zone and accelerate desertification processes

soil aggregation response to long and short term differences in rainfall amount under arid and mediterranean climate conditions soil structure variables aggregate stability as mean weight diameter mwd microparticles percentage mip and macroparticles percentage map organic matter content om electrical conductivity ec and sodium and potassium adsorption ratio spar were monitored for 3 years at seven research sites the sites are located along a climatic transect extending from the judean mountains mean annual rainfall 700 mm annual mean temperature 17 c to the dead sea mean annual rainfall 100 mm annual mean temperature 23 c and represent four climatic regions multiple regression relating soil structure variables to om spar and ec highlighted the importance of organic matter in determining soil structure a high correlation was found between mean annual rainfall representing its long term effect and annual rainfall variability representing its short term effect and some aggregation variables as increased while mip decreased with decreasing rainfall the rate of change along the climatic transect was non linear a step like threshold exists in the semi arid zone meaning that this area is threatened by land degradation even with a small decrease in mean annual rainfall mip was found to be very sensitive to rainfall amount even at the patch scale no patches of low mip values were found in the arid zone even in a very wet year and no patches of high mip values were found in the mediterranean zone even in a very wet year the main conclusion is that mip is the best indicator for assessing the impact of the expected climate change on soil structure and erosion

a conceptual model for infiltration and redistribution in crusted soils treating the crust as a single layer a simplified but accurate infiltration and redistribution model for a crust topped soil profile is developed the method is an extension of an earlier infiltration and redistribution model for homogeneous soil profiles the crust layer is conceptually subdivided into regions associated with surface water content and interface water content respectively the extension of the wetting front into the subsoil is described by the homogeneous soil method described earlier two ordinary differential equations are written which match pressure heads and fluxes at the soil interface and are solved by a runge kutta technique the model is tested against the results of the richards equation with variations in the key variables of capillary length scale and saturated hydraulic conductivity for both soil and crust and for cases where ponding occurs both in the crust region and after wetting has entered the subsoil region in all cases the profile water contents and infiltration rates are very reasonably simulated

modeling infiltration into a sealing soil rain infiltration is often controlled by a less permeable layer known as a seal infiltration through this layer was modeled numerically by continuously updating the seal properties as a function of rainfall characteristics the seal remains unchanged until the surface has reached incipient ponding subsequently a rapid change in the hydraulic properties of the seal is imposed to reflect the effect of raindrop impact finally an equilibrium state develops between seal formation due to raindrop impact and seal erosion due to the rainstorm intensity effects model predicted infiltration was similar to that observed during simulated rainstorms having various combinations of intensities and kinetic energy rates per millimeter of rain a sensitivity analysis was performed to determine the effects of changes in seal and bulk soil properties as well as simulation parameters on infiltration seal formation was dependent on cumulative rainfall energy and the rainstorm intensity

de bugging the millipede porphyroblast microstructure a serial thin section study and 3 d computer animation abstract seventy five spatially orientated serial thin sections cut from a single rock containing millipede porphy roblast microstructure from the robertson river metamorphics australia reveal thc three dimensional 3 dj geometry of oppositely concave microfolds ocms that define the microstructurc electronic animations showing progressive scrial sections of thc 3 d microstructurc arc made available via the world wide web the ocm amplitudes decrease regularly from a maximum in near central sections to a minimum in near marginal scctions whereas the ocm interlimb angles increase regularly from a minimum in near ccntral sections to a maximum in near marginal sections these observations illustrate that the ocms are noncylindrical surfaces with culminations located in near central scctions because the porphyroblast cores appear to have been present before significant development of the syn ocm foliation all of the ocms were formed by heterogcncous cxtension around thcse cores the overall gcometry of the ocms described hcre reflects the strain state and cannot be used to constrain defor mation paths

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

regional scale foliation reactivation and re use during formation of a macroscopic fold in the robertson river metamorphics north queensland australia abstract the rocks defining a macroscale antiform 25 km2 in the proterozoic robertson river metamorphics have been affected by four deformations of distinctive style with the fourth deformation d4 being responsible for formation of the macroscale geometry of the fold redistribution of progressive shearing strain due to strain partitioning during progressive d4 deformation resulted in the accommodation of d4 shearing strain along s2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations on the antiformal limbs this commonly resulted in reactivation of s2 because shearing during d4 was in a sense that was antithetic relative to that on the bulk scale of the fold continued d4 deformation caused unfolding of d2 crenulations resulting in straightening of sigmoidally folded s1 and its rotation toward the axial plane of the synchronously forming macroscopic d4 fold in zones where the sense of shear during d4 was the same as that operating on the bulk scale of the fold i e synthetic d4 shearing strain was accommodated dominantly by the favourably oriented approximately axial planar s1 fissility in zones where the progressive shearing component of d4 deformation was relatively more intense the s2 cleavage was also rotated into parallelism with s4 and was also re used as opposed to reactivated detailed microstructural analysis particularly of porphyroblast matrix relationships combined with field observations have resolved the processes operating during folding and reveal that despite the intensity of d4 deformation a separate cross cutting s4 cleavage has rarely been produced at either the meso or microscale similar processes have probably operated at all scales in other orogenic belts

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

infiltration during soil sealing the effect of areal heterogeneity of soil hydraulic properties 1 the combined effects of soil surface sealing and areal heterogeneity of the soil hydraulic properties on the mean infiltration curve are studied here for the first time seal formation during rainfall is simulated according to the dynamic model of assouline and mualem 1997 areal heterogeneity is represented by lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil and by related distributions of the other soil parameters the mean infiltration curve of the heterogeneous field is evaluated presuming that horizontal fluxes are negligible it is found that when soil surface sealing is taken into account in a heterogeneous field the ponding time is only slightly affected however significantly more runoff is produced compared to nonsealing field mulched with the relative area contributing to surface runoff being increased from 65 to 95 the relative effect of field heterogeneity on the infiltration curve increases for higher rainfall intensity the formation of soil surface seal apparently reduces the effect of the field areal variability on the steady infiltration rate the infiltration rate after 60 min of rainfall distributes lognormally under sealing conditions while it distributes normally in the case of the unsealed soil the final infiltration rate resulting from the assumption of a sealed uniform field is underestimated compared to sealed heterogeneous field with the relative effect being larger for a loam soil 51 than for a sandy loam soil 26 also the approximation that the infiltration curve is a unique function of cumulative rainfall independent of rainfall intensity is not valid in a sealing heterogeneous field and its application in such a case would be more erroneous than in uniform fields on the other hand one may disregard the variability of all soil parameters except the hydraulic conductivity when calculating infiltration

microstructural evidence for n s shortening in the mount isa inlier nw queensland australia the preservation of early w e trending foliations in porphyroblasts revealed by independent 3d measurement techniques 3d microstructural analyses of porphyroblast inclusion trails using both the asymmetry switch method for determining foliation intersection axes preserved in porphyroblasts fias and the recently developed fitpitch method reveal w e and n s trending fia sets in the white blow formation of the mount isa inlier each method reveals two subsets of fias centered on each of these major trends these were distinguished based on the relative timing trend and orientation of inclusion trail patterns thirty six samples were analyzed using both techniques and produced very similar results pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin sections and trends in horizontal sections yield distinct near orthogonal modal orientations from all the analyzed samples this indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes w e and n s trending fias have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages garnet and staurolite growth during bulk north south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain this period of bulk shortening and associated metamorphism formed during a period of orogenesis called o1 w e shortening formed n s striking foliations that preserve a period of orogenesis o2 and another succession of metamorphism involving more phases of porphyroblast growth preserving n s trending fias overprinting of successive fia trends wsw ene wnw ese nnw sse and ssw nne suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from n s to w e overall with a major tectonic break or decompression between o1 and o2 the porphyroblast inclusion trail patterns preserving w e trending fias provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations

distribution of pre folding linear indicators of movement direction around the spring hill synform vermont significance for mechanism of folding in this portion of the appalachians abstract three distinctly oriented sets of pre folding and one set of syn folding axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging spring hill synform in southeast vermont over one third of the samples contain consistent changes in the trend of these axes from the core to rim since the core grew before the rim this enabled the relative timing of each set of axes to be determined from the oldest to the youngest as ne sw e w nnw sse and nne ssw the youngest trend is parallel to the axial plane of the regional folds only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation the three pre folding sets of axes have the same orientation on both limbs this consistency in orientation has significant implications for the processes operating during folding and three mechanisms are presented that could potentially explain it these involve the classic card deck model of shear folding de sitter s model of clay bricks shortening as they shear past one another and the progressive bulk inhomogeneous shortening model the relative merits of each of these models are discussed

formation and emplacement of the northland ophiolite northern new zealand sw pacific tectonic implications petrological geochemical geochronological and palaeomagnetic data for rocks of the northland ophiolite terrane of northern new zealand suggest that it formed in a suprasubduction zone setting between c 29 and 26 ma at c 35 s close to its late oligocene obduction site cretaceous igneous rocks formerly considered to be part of the ophiolite probably represent the basement upon which the ophiolite was emplaced and are probably part of the mount camel arc related terrane the ophiolite is believed to have been generated in the southeastern south fiji basin close to a nw se oriented transform fault located to the sw of the vening meinesz fracture zone and was probably emplaced in response to the collision of the hikurangi plateau with eastern new zealand at the end of the oligocene this collision would have involved a major adjustment on the transform fault thereby allowing a portion of the upper crustal section of the southern south fiji basin to be emplaced southwestward onto northern new zealand as well as the coeval emplacement of the east cape allochthon to the south concomitant subduction of the lower crust mantle section led to the initiation of arc volcanism that resulted in the northland lower miocene volcanic plutonic suite

soil geomorphology relations in gypsiferous materials of the tabernas desert almeria se spain a detailed pedological study in an apparently homogeneous badlands area of gypsiferous mudstones in the tabernas desert almeria se spain with an annual precipitation of 200 mm has been shown to be composed of different soil units belonging to different stages of soil development twenty four soil profiles in four topographic transects within a small instrumented catchment have been described and analysed along with over 100 probings and observations a complementary approach to ascertain the relationships of soil units with topography made use of a 1 m resolution digital elevation model dem and derived terrain attributes moreover the relationships with soil cover surface hydrology and erosion have all contributed to understanding pedogenic and evolutionary processes the five soil units identified correspond to distinct topographic positions from steep s oriented slopes with incipient soil development under bare surfaces epileptic regosol to moderately sloping n oriented soils fairly well developed below a dense cover of annual and perennial plants haplic calcisol both the spatial distribution and the topographic position of soil units favour gypsum and salt washing processes and gypsum accumulation is restricted to higher positions with very small contributing areas and minimum overland flow and thus reduced leaching gypsic horizons and gypsisols while previously described in the area associated to gypseous rock outcrops are now described associated to gypsiferous mudstones

nature formation and effects of soil crusts formed by water drop impact abstract research was conducted to investigate 1 the structure formation and nature of soil crusts which develop as a result of raindrop impact under controlled conditions of simulated rain and 2 the crust effect on infiltration rates scanning electron microscope sem observations performed on sandy sandy loam and clayey soil samples show the crust to be composed of two layers 1 a skin 0 1 mm thick and 2 a layer 2 3 mm thick with a higher bulk density in which aggregates have been destroyed a washing in zone described by earlier investigators could not be detected the soil beneath the crust maintains its original structure and particle orientation the chemical composition of the rain water was found to have a significant but not very large effect on the infiltration rate vs cumulative rain curve of the clayey soil material only final infiltration rate was about 20 lower when distilled water was applied compared to tap water electrical conductivity 0 6 mmho cm 1 an increase of the bulk density from 1 35 1 48 g cm 3 in undisturbed soil to 1 74 1 88 g cm 3 in crusts was found the amount of eroded material increased gradually with increase of runoff and decrease of infiltration at steady state the erosion rates were 0 45 1 25 and 0 76 g m 2 min 1 in the sandy sandy loam and clayey soil specimens respectively percentages of clay and silt in the eroded material were higher than in the bulk samples three stages to describe the changes in nature and effects of crusts during their formation are suggested 1 the rate of infiltration decreases to a point at which runoff beings 2 the initiation of runoff until infiltration and runoff rates become stabilized and 3 infiltration and runoff maintain steady rates

the effect of raindrop impact on the dynamics of soil surface crusting and water movement in the profile abstract the infiltration of rainfall into bare soil was studied in field and laboratory experiments on two soil types in israel the main governing process is the formation of crust with an hydraulic conductivity several orders of magnitude lower than that of the soil profile the decrease in conductivity is attributed to the organization of a very thin layer on the soil surface the sealing efficiency of the crust is achieved by suction forces which arrange soil particles probably clay into a continuous dense layer the decrease in flux through the crust creates conditions of unsaturated flow in the soil profile below the crust the dynamics of the hydraulic relationship between the soil profile and the crust during a rain storm was analyzed by means of an analytical model

deformation partitioning in flat subduction setting case of the andean foreland of western argentina 28 s 33 s 1 the andean foreland of western argentina 28 s 33 s corresponds to back arc deformations associated with the ongoing flat subduction of the nazca plate beneath the south american lithosphere in this paper short and long term tectonic regimes for this andean region are investigated and compared to nuvel 1 and the new gps derived velocity field for the andes mountains from this comparison it appears that 1 the slight convergence obliquity at the plate boundary is accommodated by the subduction zone itself precluding any deformation partitioning at lithospheric scale 2 clockwise rotation of 1 and p axes within the andean foreland of western argentina suggests that plio quaternary deformation partitioning may occur in the upper plate at crustal scale and 3 comparison of seismic rates of shallow deformations determined from moderate to large earthquakes localized between 22 s and 36 s shows that the amount of shortening is about the same 2 4 cm yr 1 between the forearcs of the 30 dipping slab segments and the back arc of the flat segment all together those observations are in close agreement with the idea that interplate coupling in flat subduction settings may play a key role in the present day transfer of plate boundary forces consequently the andean back arc of western argentina might be regarded as an obliquely converging foreland where plio quaternary deformations are partitioned between strike slip and thrust motions that are localized in the east verging thin skinned argentine precordillera and west verging thick skinned pampean ranges respectively

deformation partitioning during folding and transposition of quartz layers abstract banded iron formation bif from the quadrilatero ferrifero southeastern brazil shows a compositional layering with alternating iron rich and quartz rich layers this layering was intensively folded and transposed at a centimeter millimeter scale through a component of bedding parallel shear related to flexural slip at middle to high greenschist facies conditions 400 450 c the microstructure and c axis fabrics of normal limbs inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy sem and digital image analysis in the normal limbs recrystallized quartz grains show undulose extinction relatively dry grain boundaries c axes at high angle to foliation and a pervasive grain shape fabric gsf indicating operation of crystal plastic processes in the inverted limb quartz grains show more serrated and porous wet grain boundaries the gsf is similar to that of the normal limb but c axes are oriented at 90 to those of the normal limb we interpreted these characteristics as reflecting operation of solution precipitation deformation in inverted limbs as a consequence of grains having been rotated to an orientation that was hard to basal a glide but easy to dissolution precipitation creep this deformation partitioning between crystal plasticity and solution transfer during folding transposition of quartz may explain the common occurrence of layered quartz rocks where individual layers show alternating c axis fabrics with opposite asymmetries but a consistent gsf orientation such characteristics may reflect an earlier event of pervasive folding transposition of a preexisting layering

microstructural analysis of the classical spiral garnet porphyroblasts of south east vermont evidence for non rotation new data strongly suggest that the classical spiral garnet porphyroblasts of south east vermont usa generally did not rotate relative to geographical coordinates throughout several stages of non coaxial ductile deformation the continuity of inclusion trails si in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities consisting of truncations and differentiation zones where quartz graphite si bend sharply into more graphitic si discontinuous tight microfold hinges with relatively straight axial planes are also present these microstructures form part of a complete morphological gradation between near orthogonally arranged discontinuous inclusion segments and smoothly curving continuous si spirals some 2700 pitch measurements of well developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike from specimens collected at 28 different locations around the chester and athens domes the results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations irrespective of variations in the external foliation attitude macrostructural geometry and apparent porphyroblast matrix rotation angles combined with evidence for textural zoning this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation dimensionally elongate idioblasts also have a preferred shape orientation with long axes orientated normal to the mica folia within which epitaxial nucleation occurred truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins in association with progressive and selective strain induced dissolution of matrix minerals and locally also the porphyroblast margin non rotation of porphyroblasts relative to geographical coordinates suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed relatively large and rigid heterogeneities which cause the progressive shearing rotational component of deformation to partition around them the spiral garnet porphyroblasts therefore preserve the most complete record of the complex polyphase tectonic and metamorphic history experienced in this area most of which was destroyed in the matrix by progressive foliation rotation and reactivation together with recrystallization

in situ determination of the soil surface crust hydraulic resistance summary this paper proposes a new method to determine the soil surface crust resistance in situ the method is based on the assumption that the crust is well established prior to the infiltration in this case and for steady infiltration regime the hydraulic resistance of the crust can be inferred from the infiltration rate and the relationship between soil conductivity and water pressure head the proposed method combines two types of in situ experiments i rain simulation experiments and ii single ring infiltration tests on the same soil after removal of the crust in order to assess the soil hydraulic properties the effectiveness of the proposed methodology was tested by reproducing numerically the experiments the comparison of the numerical results with observations was satisfactory further validation and tests on other soil types are required in order to confirm our results

the two parameters of the philip infiltration equation their properties and spatial and temporal heterogeneity in a red earth of tropical semi arid queensland abstract this paper assesses the magnitude and properties of philip s 1957b infiltration parameters sorptivity and transmission factor a on a red earth of central north queensland the soil supports an open eucalypt woodland with a discontinuous ground cover of grasses eight infiltration rings 0 3 m diameter 0 15 m deep were driven into the soil 0 10 m giving four replicates each of bare soil and part vegetation dominated by spinifex the rings were then left to consolidate followed by intermittent measurements taken over a 2 yr period which incorporated three wet seasons although the truncated equation of philip 1957b satisfactorily fitted the infiltration data the two derived parameters cannot be considered to be estimates of sorptivity and a of philip 1957b because the time over which his equation is appropriate is restricted to a few seconds in this soil therefore profile at infinity conditions of philip 1969 are rapidly attained such that our a parameter approaches field saturated hydraulic conductivity k with the consequence that cumulative infiltration i can be described by i k t this means that field saturated hydraulic conductivity under these ponded conditions is the dominant parameter this study highlighted that temporal heterogeneity in the a parameter was as significant as spatial heterogeneity because the soil fabric was apparently changing due to biological activity and raindrop compaction thus the work emphasises the limitation of point field measurements in time as representative estimate of infiltration properties for a comprehensive description of infiltration knowledge is required of the temporal as well as the spatial variability of the relevant parameters which in this soil was field saturated hydraulic conductivity

physical characterization spectral response and remotely sensed mapping of mediterranean soil surface crusts abstract soil surface crusting and sealing are frequent but unfavorable processes in mediterranean areas soil crust and seals form on bare soil subject to high intensity rainfall resulting in a hard impenetrable layer that impedes infiltration and hampers the germination and establishment of plants the adverse consequences of overland flow and reduced fertility can lead to erosion and ongoing degradation therefore information on the distribution of surface crusts and their physical properties is essential to combat the undesired effects of crust formation in e g soil erosion and soil compaction we studied the occurrence of crusts in a study area in mediterranean southern france our objectives were to compare the physical and hydrological properties of the crusts and underlying soil to identify the spectral characteristics 400 to 2500 nm of the crusted and non crusted soil surfaces using high resolution field spectra and to investigate the potential of mapping crust occurrence using airborne hyperspectral hymap images the differences in some physical properties between crusted and non crusted surfaces are significant while others are only marginal crusting markedly reduces the infiltration capacity and crust strength varies between the different soil types spectral differences are small mainly in albedo values overall reflectance and in absorption band depth and shape albedo differences range from 8 to 40 differences in absorption band features in the spectra of crusts and non crusted surfaces are small sixty percent of the crusted soil surfaces showed stronger absorption features in the clay mineral absorption bands at 2200 nm than non crusted soils increased absorption is due to a relative enrichment in fines spectral feature fitting and linear spectral unmixing algorithms were applied to airborne hymap images to evaluate the possibilities of mapping surface crusts crusts could be mapped in fallow agricultural fields but the spectral response of natural badlands was too fragmented for crust mapping

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

link between ssz ophiolite formation emplacement and arc inception northland new zealand u pb shrimp constraints cenozoic sw pacific tectonic implications abstract new u pb age data from zircons separated from a northland ophiolite gabbro yield a mean 206 pb 238 u age of 31 6 0 2 ma providing support for a recently determined 28 3 0 2 ma shrimp age of an associated plagiogranite and 29 26 ma 40 ar 39 ar ages n 9 of basalts of the ophiolite elsewhere miocene arc related calc alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206 pb 238 u ages of 20 1 0 2 and 19 8 0 2 ma the ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122 104 ma the early cretaceous zircons in the arc andesites are interpreted as xenocrysts from the mt camel basement terrane through which magmas of the northland miocene arc lavas erupted the inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation we postulate that the tholeiitic suite of the ophiolite represents the crustal segment of ssz lithosphere sszl generated in the southern south fiji basin sfb at a northeast dipping subduction zone that was initiated at about 35 ma the subduction zone nucleated along a pre existing transform boundary separating circa 45 20 ma oceanic lithosphere to the north and west of the northland peninsula from nascent back arc basin lithosphere of the sfb construction of the sszl propagated southward along the transform boundary as the sfb continued to unzip to the southeast after subduction of a large portion of oceanic lithosphere by about 26 ma and collision of the sszl with new zealand compression between the australian plate and the pacific plate was taken up along a new southwest dipping subduction zone behind the sszl renewed volcanism began in the oceanic forearc at 25 ma producing boninitic like ssz and within plate alkalic and calc alkaline rocks rocks of these types temporally overlap ophiolite emplacement and subsequent miocene continental arc construction

the geochemistry of tholeiitic and alkalic plutonic suites within the northland ophiolite northern new zealand magmatism in a back arc basin abstract the northland ophiolite of new zealand consists mainly of tholeiitic basalts with subordinate alkalic volcanic rocks the alkalic rocks occur together with but are generally younger than the tholeiitis both the tholeiitic and alkalic suites include minor plutonic intrusives which range from gabbro to plagiogranite variations within the tholeiitic plutonic suite can be modelled in terms of low pressure crystal fractionation the evolution of the alkalic suite is more problematic because each massif appears to be derived from a slightly different parental magma new trace element chemistry presented in this study shows that the tholeiitic rocks formed from a mantle source that was compositionally modified by supra subduction zone processes but that the alkalic suite has a different chemical signature nevertheless both suites of rocks are interpreted as having formed in a back arc spreading system in this environment the addition of fertile mantle to the supra subduction wedge facilitated by subduction roll back provided a source for the alkalic rocks with mixed geochemical characteristics

comment on geochemistry of the early miocene volcanic succession of northland new zealand and implications for the evolution of subduction in the southwest pacific by m a booden i e m smith p m black and j l mauk in a recent paper booden et al 2011 present new geochemical and petrological data of early miocene volcanics from the northland region northland volcanic belt in new zealand and interpret these data to support a particular regional tectonic model this tectonic model involves early miocene westward subduction of cretaceous pacific oceanic lithosphere below the northland volcanic belt and the authors interpret the volcanic belt as a continental magmatic arc although the new data are not in disagreement with such a tectonic model they provide more support for an alternative interpretation that involves a northeast dipping subduction zone furthermore geometric and plate kinematic data show that the west dipping subduction model is unviable geological and geophysical data contradict the model while geodynamic arguments indicate that the model is implausible here it will be shown that a subduction model involving a northeast dipping southwestward retreating slab made of the late cretaceous paleocene south loyalty backarc basin lithosphere that subsequently detaches is in agreement with the local geology geophysics and geochemistry is geometrically kinematically and geodynamically viable and fits within the regional southwest pacific tectonic framework

redefining the waitemata basin new zealand a new tectonic magmatic and basin evolution model at a subduction terminus in the sw pacific 1 the early miocene waitemata basin has long been described as an interarc intra arc basin formed between twin chains of arc volcanoes in northland new zealand however deep marine polymict volcaniclastic conglomerates within the basin reveal tectonic and magmatic signals that are not evident from neighboring volcanic edifices the conglomerates were deposited by high density turbidity currents and debris flows and include single sediment cycle megaclasts of lava these basaltic lavas have ocean island basalt oib like geochemical affinities and are precisely dated at 20 ma by 40ar 39ar methods their age and the occurrence of subordinate clasts derived from an ophiolitic nappe to the north indicate the basin postdates the initiation of collision in the wider region contemporaneous calc alkaline volcanism did occur some 250 km nw of the basin however the conglomerates lack clasts of calc alkaline arc affinities indicating an absence of arc like volcanism in the vicinity of the basin recent tectonic models for the sw pacific region and mantle tomography highlight the importance of wholesale slab detachment in driving early miocene calc alkaline volcanism and basin development although such models provide a slab window for the eruption of nonarc oib like magmas they would not explain their localized occurrence at the proposed leading edge of the tear waitemata basin rather than progressively along the entire length of the detachment northland as seen in other detachment settings in addition oib like volcanism predates the adjacent calc alkaline volcanism on the margin of the basin a transition that is the opposite of that found at other slab detachment settings the occurrence of oib like volcanism is better explained by a lateral slab termination in the vicinity of the waitemata basin that allowed asthenospheric derived magmas to erupt the basin is inferred to have developed in response to asthenospheric upwelling and associated lithospheric extension or deformation associated with a slab termination zone this study highlights the importance and potential of combined geochemical geochronological and sedimentological studies of conglomerates in reconstructing the geodynamic setting of a basin

foliation intersection axes preserved in garnet porphyroblasts from the swat area nw himalaya a record of successive crustal shortening directions between the indian plate and kohistan ladakh island arc abstract we have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the swat region of north pakistan nw himalaya the studied samples represent the indian plate cover sequence deformed and metamorphosed to medium and high grade during the himalayan orogeny two complementary techniques were applied to measure the orientation of foliation intersection inflection axes fia in porphyroblasts in the studied samples and reveal the existence of three principal fia sets in the study area measuring approximately 60 30 km each fia set is characterized by a distinctive regionally consistent trend and relative timing from old to young ese wnw fia set1 e w fia set 2 and broadly nne ssw fia set 3 this sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening perpendicular to fia through time either progressively or in a stepwise fashion consistent with previous kinematic and geochronological data regarding collisions between the indian plate kohistan ladakh island arc klia and eurasia between ca 55 and 40 ma fia set 1 and 2 correlate well with early formed ese wnw and e w trending thrust related structures in the study area respectively whereas the trend of fia 3 matches that of relatively young regional scale folds and shear zones with a conspicuous n s trend hence the fia succession has successfully resolved the relative timing of formation between the e w versus n s trending structures in the region which was a longstanding question in the tectonics of the nw himalaya in addition a comparison of orientation data for symmetrical millipede inclusion trails versus asymmetrical curved trails indicates that porphyroblasts experienced very limited differential rotations in the region despite the complex polyphase deformation history recorded perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between fia trends and successive vectors of relative indian plate motion since collision with the klia

crusting effects on erosion processes under simulated rainfall on a tropical alfisol sealing and crusting of soil surfaces have dramatic effects on water infiltration into and runoff from soils thereby greatly influencing erosion processes this study focused on the effect of the initial stage of crusting on inter rill erosion processes for a crust prone alfisol sampled from south central india soil aggregates ranging from 2 4 to 8 mm collected from ploughed pl and naturally vegetated nv treatments were subjected to rainfall simulation under laboratory conditions runoff from pl soil aggregates was 2 2 5 times higher while percolation was 20 100 lower than for nv aggregates soil wash and splash losses were 0 5 3 times greater for pl than for nv soil runoff and inter rill erosion were significantly higher during the wet simulation run compared with the dry run the results indicated that nv soil aggregates were more resistant to breakdown from raindrop impact and slaking and subject to less rapid sealing than pl soil total soil loss was influenced most by initial aggregate stability and the extent of seal development splash and wash losses of soil both increased as a result of surface sealing regardless of soil condition for short 30 60 min rainfall durations high drying rates resulted in the highest crust bulk densities increased crust strength for pl soil compared with nv soil reflected the greater susceptibility of cultivated soil to surface sealing and crusting 1998 john wiley sons ltd

modeling local infiltration for a two layered soil under complex rainfall patterns abstract a general model for local infiltration redistribution reinfiltration in a two layered soil profile is proposed as an extension and generalization of a formulation we developed earlier for infiltration and redistribution in crusted soils it is of analytical conceptual type and involves a runge kutta verner numerical solution of a system of two ordinary differential equations given the basic hydraulic properties of soils there are practically no parameters to estimate by calibration the wetting profile during infiltration is represented by a single similarity curve while in the reinfiltration stage which occurs for rewetting after a period of redistribution the profile of soil water content in each horizontal layer may be composed of two similarity profiles which evolve in time the model is tested by comparison with numerical solution of richards equation carried out for a variety of synthetic experiments upon the combination of three soil types of contrasting hydraulic characteristics it simulates particularly well the entire cycle infiltration redistribution reinfiltration in all cases where either layer may be less permeable the water content at the surface and the interface as well as the infiltration rates are very accurately estimated the usefulness of compound profiles of water content in the reinfiltration stage is also shown by comparison with the simple approach which relies on a single similarity curve

rainfall induced soil seal b application of a new model to saturated soils summary a conceptual model is proposed in which the saturated soil seal crust properties are derived on the basis of physical concepts estimated empirically the reduced hydraulic conductivity of the soil surface is attributed to its decreased porosity and increased ineffective water content the characteristics of the modeled soil seal were examined using data of sandy loam and loam loess soils an inverse approach was applied for calibration using infiltration rate data the results indicate that the importance of the 0 1 mm compact clay and silt seal layer the skin has been overestimated the sealing of bare soils formed under highenergy rainfall cannot be attributed to the skin the suggested theory allows the seal properties including its thickness and hydraulic conductivity to vary with respect to rain and soil properties as well as the flow boundary conditions seal thickness hydraulic conductivity and conductance were calculated as a function of the driving hydraulic head for the two soils the sandy loam had a thicker sealing layer with higher hydraulic conductivity and sensitivity to flow conditions than the loam while the hydraulic conductance of the loam seal decreased slowly as a function of the hydraulic gradient the conductance of the sandy loam soil dropped sharply with an increase in the hydraulic gradient and reached a lower value than that of the loam for large gradients

porphyroblast rotation and strain localization debate settled comment this comment addresses two inferences used by johnson 2009 to argue for porphyroblast rotation during bulk coaxial shortening firstly he interprets that porphyroblast inclusion trails that are inclined his figure 1 became inclined because the porphyroblast rotated during growth he used orientation data from millipede microstructures from one hand sample and concluded on the basis of the total spread of inclusion trail orientations and porphyroblast axial ratios that porphyroblasts had rotated relative to one another during ductile deformation secondly he presents a numerical model that indicates that asymmetrically shaped porphyroblasts rotated during coaxial deformation however i will show that the porphyroblasts did not rotate after they grew in spite of localized shearing along the developing s3 that rotated visible matrix s2 in johnson s figure 1

strain localization and porphyroclast rotation it has been debated for decades whether rigid inclusions such as porphyroclasts and porphyroblasts do or do not rotate in a softer matrix during deformation experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions whereas using mohr coulomb plasticity results in nonrotation because the rocks in which inclusions are found normally undergo deformation by dislocation creep we applied a full field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions we show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions inhibition of rotation can only occur after a significant amount of strain our results may help to explain why geologic rigid objects often show evidence for rotation but not necessarily in accordance with the viscous theory that is usually applied to these systems

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

holocene dune sourced alluvial fans in the nebraska sand hills abstract the large stabilized dunes of the nebraska sand hills are in a phase of degradation the deposition of small scale alluvial fans composed of well sorted fine to medium grained sand occurs when sand is transported via gullies on the lee side of large barchanoid ridge dunes during infrequent intense summer rain storms 5 cm h the hydraulic conductivity of the dune sand itself is too high to allow hortonian overland flow but organically bound mats and crusts on the surface of the dunes help to impede infiltration secondary eolian bed forms at the crests of the dunes focus shallow groundwater and overland flow that erodes gullies internally alluvial fans are dominated by laminated sands interpreted as sheetflood deposits depositional couplets of structureless bottom and laminated top sands may form from a combination of hyperconcentrated flow and sheetflood thickness of deposits varies with storm intensity and sediment supply in the gullies the alluvial fans are holocene in age and gully initiation may have begun at the onset of a wetter climate when vegetation was less dense than at present water laid sandstones associated with large scale eolian cross strata have been recognized in precambrian devonian and cretaceous sequences the alluvial fans from the sand hills may provide a new modern analogue for interpreting these ancient sandstones

the role of conjugate crenulation cleavage in the development of millipede microstructures abstract millipede microstructures are described from archean metaturbidites surrounding the back river volcanic complex eastern slave structural province they are locally developed within andalusite porphyroblasts that grew in response to the intrusion of late syn kinematic granitoids the porphyroblasts acted as a mechanical heterogeneity and resulted in the nucleation of conjugate crenulations which were then helicitically overgrown by the porphyroblasts this process produced the millipede morphology through a simple consistent mechanism the model presented here for the development of millipede microstructures represents an alternative to other models requiring complex deformation partitioning and restricted porphyroblast growth

the origin of garnets in andesitic rocks from the northland arc new zealand and their implication for sub arc processes an unusual andesitic suite from the miocene volcanic arc in northland new zealand comprises pyroxene andesite and garnetbearing hornblende pyroxene hornblende and biotite hornblende andesites garnet crystals occur as 1 10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses the andesitic rocks also contain enclaves of high mgo pyroxenite hornblendite and pyroxene hornblende gabbro as well as highal2o3 hornblende gabbro garnet hornblende gabbro and anorthosite garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges zoning patterns and inclusions which indicate that they share a common petrogenetic history they can be grouped into four distinct types on the basis of mode of occurrence chemical composition and zoning patterns which leads to their interpretation as antecrysts rather than orthocrysts the compositions of the garnets as well as their included mineral assemblages reflect a petrogenetic trend from high temperature pyroxene bearing high mg garnet to low temperature fe rich garnet at relatively constant pressure well preserved zoning patterns in particular those of the caand mg rich garnets reflect processes within a deep crustal arc environment later assimilation is suggested by some zoning patterns that show decreasing ca and increasing fe and mn contents the garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8 10 kbar the host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction related magmas ponded and interacted at or near the base of the crust together they represent a rare snapshot of the processes and components that produce arc type rocks

north eastward subduction followed by slab detachment to explain ophiolite obduction and early miocene volcanism in northland new zealand oligocene miocene models for northern new zealand involving south westward subduction to explain early miocene northland volcanism do not fit within the regional southwest pacific tectonic framework a new model is proposed which comprises a north east dipping south loyalty basin slab that retreated south westward in the eocene earliest miocene and was continuous with the north east dipping subduction zone of new caledonia in the latest oligocene the trench reached the northland passive margin which was pulled it into the mantle by the slab resulting in obduction of the northland allochthon during and after obduction the slab detached from the unsubductable continental lithosphere inducing widespread calc alkaline volcanism in northland the new model further explains contemporaneous arc volcanism along the northland plateau seamount chain and sinking of the northland basement followed by uplift and extension in northland

evidence for a contribution from two mantle plumes to island arc lavas from northern tonga lavas from the islands of tafahi and niuatoputapu at the northern end of the active tonga kermadec arc in the southwest pacific were erupted at a convergent plate margin yet they can be shown to contain a contribution from two different mantle plumes high concentrations of nb relative to other high field strength elements in these lavas compared to other tonga lavas reflect an ocean island basalt component in the mantle wedge derived from the nearby samoa mantle plume pb isotope compositions indicate that most of the pb in these lavas is derived from the oceanic crust of the plume generated louisville seamount chain which is being subducted beneath the tonga arc these two plume components were thus introduced into the arc lavas in very different ways and provide insight into upper mantle dynamics and magma generation processes occurring in an active arc back arc system

impact of slope aspect on hydrological rainfall and on the magnitude of rill erosion in belgium and northern france abstract the impact of slope aspect on hydrological rainfall and on the magnitude of rill erosion has rarely been studied the dominant wind direction in belgium and northern france is the southwest which brings large amounts of rain with it we investigated whether this leads to greater rates of rill erosion on the southwestern slopes due to the greater amount of hydrological precipitation on these slopes the study was executed on six barren conical spoil heaps which are excellent study objects to bring the slope aspect in relation with rill erosion three indicators for rill erosion volume of rills drainage density and mean distance of the head cut of the five uppermost rills till the top of the spoil heap were associated with four explanatory factors slope gradient vegetation density rock fragment cover and slope aspect the amount of hydrological rainfall is the greatest on the western and southwestern slopes as a result the largest volume of rills was measured on the slopes facing the southwest 96 which indicates that there is a connection between the slope aspect and the magnitude of rill erosion on the northeastern 32 and eastern 36 slopes the volume of rills is lower due to the effect of rain shadow the other explanatory factors showed little influence on the magnitude of rill erosion as the spoil heaps were selected to be similar in as many parameters as possible

transition from arc to oceanic magmatism at the kamchatka aleutian junction spatial geochemical variations of quaternary lavas erupted along the northern segment of the kamchatka arc are used to trace changes in magma generation across the subducting pacific slab edge the late pleistocene holocene lavas of the northern end of the central kamchatka depression north of the pacific slab edge show strong enrichment in high field strength elements and light rare earth elements relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios and oxygen isotope compositions similar to those of mid oceanic ridge basalts these geochemical characteristics are distinct from the southern central kamchatka depression volcanoes located above the subducting pacific slab extensive fluid triggered mantle melting dominates magma genesis beneath the largest kamchatka volcanoes in the south whereas low degree decompression melting of the pacific asthenospheric mantle is the major magma generation process north of the pacific slab edge quaternary detachment of the subducted pacific plate fragment resulted in the influx of fertile mantle beneath kamchatka we propose that upwelling and southward flow of this hotter more fertile mantle is the main reason for recent magmatism in northern kamchatka and for the exceptional productivity of the central kamchatka depression volcanoes klyuchevskoy and sheveluch the most active arc volcanoes on earth

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

arc continent collisional orogenesis in the sw pacific and the nature source and correlation of emplaced ophiolitic nappe components abstract southwest pacific ophiolitic nappes emplaced upon the former margin of eastern australia provide a record of cyclical episodes of arc continent collisional orogenesis that occurred throughout the paleogene cenozoic nappes in papua new guinea new caledonia and northland new zealand consist of two key components 1 a basal and apparently older late cretaceous eocene metamorphosed normal to weakly enriched morb like basaltic ocean floor terrane and 2 an uppermost supra subduction zone ssz ophiolite that represents basement of the associated magmatic forearc a detailed comparison of geochemical characteristics of unequivocal ssz basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal n and enriched e morb these slight ree enriched or transitional t morb have generally been considered as the result of seafloor spreading in a back arc basin bab or basins to the east of australia in the late cretaceous paleocene 85 55 ma the enrichment recorded by the bab basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern gondwana that left their signature on the mantle source consequently tapped by the bab basalts a subset of slightly more enriched basalts may owe their enrichment to source contamination minor assimilation of thinned gondwana derived continental crust associated lree depleted n mor basalts previously considered as part of the basaltic ocean floor terranes on the other hand are similar to ssz basalts of the northland and papuan ultramafic belt ophiolites refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying ssz ophiolites a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts nonetheless the geochemical characteristics displayed by lree depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite whether these and for that matter the majority of basalts of the basaltic ocean floor terranes indeed represent complementary forearc crust to the ssz ophiolite as opposed to a slice of late cretaceous eocene bab floor backthrust beneath the ssz ophiolite upon emplacement will have to await the acquisition of corresponding radiometric age data