lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

controls on the geometry of transfer zones in the suez rift and northwest red sea implications for the structural geometry of rift systems two main types of transfer zones are mapped in the suez rift and the northwestern red sea these are transfer zones between indi vidual faults fault to fault transfer zones f f tz and transfer zones between half grabens with different fault domains half graben to half graben transfer zones h h tz relay ramps and linking transfer faults are two main recognized types of f f tz different varieties of these f f tz have the form of en echelon fault belts with single or mixed polarity tilted fault blocks at rift offsets and zigzag fault arrays at major fault or rift jumps relay ramps transfer throw between overlapping normal faults of the same polarity linking transfer faults join normal faults that have the same polarity with no overlapping segments leading to a characteristic zigzag pattern if the linked faults are listric local anticlines and synclines develop at the corners of the zigzag fault array three h h tz in the suez rift and northwest red sea show the effect of prerift structures both faults and major folds on the location orientation and style of deformation of the transfer zones of continental rifts the widest one of these three h h tz 40 60 km has a broad antiformal structure lying between the overlapping ends of the listric faults bounding the two linked half grabens and is deformed by dip slip normal faults of opposite polarity the other two h h tz reactivate prerift faults by strike slip faulting due to the angular relationship between these faults and the direction of extension in the rift

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

miocene extension and extensional folding in an anticlinal segment of the black mountains accommodation zone colorado river extensional corridor southwestern united states 1 recent studies demonstrate that rifts are characterized by linked tilt domains each containing a consistent polarity of normal faults and stratal tilt directions and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults the mid miocene black mountains accommodation zone of southern nevada and western arizona is a well exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains in the southeastern part of this zone near kingman arizona east dipping normal faults of the whipple tilt domain and west dipping normal faults of the lake mead domain coalesce across a relatively narrow region characterized by a series of linked extensional folds the geometry of these folds in this strike parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions whereas large fault overlaps produce elongate folds parallel to faults analysis of faults shows that the folds are purely extensional and result from east northeast stretching and fault related tilting the structural geometry of this portion of the accommodation zone mirrors that of the black mountains accommodation zone more regionally with both transverse and strike parallel antithetic segments normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of kingman arizona however isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the lake mead domain began to form between 1 m y to 0 2 m y prior to east dipping faults of the whipple domain the accommodation zone formed above an active and evolving magmatic center that prior to rifting produced intermediate composition volcanic rocks and that during rifting produced voluminous rhyolite and basalt magmas

tectonic history of the metamorphic basement rocks of the sierra del carmen coahuila mexico the ouachita interior zone arching through central texas into the big bend region of the rio grande has been defined chiefly on the basis of well samples and its associated gravity high the only known exposure of rocks of the interior zone lies at the base of the sierra del carmen normal fault escarpment under approximately 1100 m of lower cretaceous carbonates in northern coahuila mexico these lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist graphitic marble and minor quartzite with abundant quartz veins the dominant structures shown by the metamorphic rocks are isoclinal folds f2 that have a well defined axial planar foliation s2 earlier f1 folds refolded by f2 are locally preserved at the microscopic scale s2 is a crenulation cleavage of the s1 foliation that has been enhanced by pressure solution feldspar porphyroblasts overgrow a folded graphitic s1 foliation throughout the area a second crenulation cleavage s3 is present on the s2 foliation surface as an intersection lineation l3 subsequent laramide regionalto outcrop scale folds and associated thrusts overprint these ductile structures lower cretaceous carbonates form a large east verging overturned to recumbent anticline syncline fold pair with a northwest trending fold axis the s2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates indicating that the metamorphic rocks were also folded by this regional structure a basin and range normal fault is interpreted to have reactivated the laramide thrust fault responsible for the contractional deformation one to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline syncline fold pair the maximum offset along the normal fault is 1600 m rb sr data from the muscovite schist gives a six point mineral whole rock isochron age of 277 10 ma indicating that polyphase deformation and metamorphism occurred during the late paleozoic ouachita orogeny an initial sr ratio of 0 721 suggests an ancient source for these metasediments common pb ratios of the metamorphic whole rock and galena from the puerto rico mine are isotopically distinct from those of the north american craton suggesting that they were derived from a non north american source and thus may be equivalent to unmetamorphosed ouachita flysch sediments this indicates that the metamorphic rocks of the ouachita interior zone were derived from an ancient allochthonous source that was deformed and metamorphosed during the late paleozoic ouachita orogeny

retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

evolution of amphibolite facies structural features and boundary conditions for deformation during exhumation of high and ultrahigh pressure rocks nordoyane western gneiss region norway 1 geologic mapping and structural analysis of baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high pressure rocks where strain partitioning has preserved evidence for interpreting the evolution of late structural features the earliest of these were extensional detachments juxtaposing eclogite facies rocks against overlying amphibolite facies rocks that show no evidence for eclogite facies metamorphism these early detachments are strongly overprinted and complexly folded and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels younger more localized mylonite zones formed synchronously with tubular sheath isoclinal tight and open folding that shows a progression from wnw to ene trends the earliest mylonite zones interpreted as originally subhorizontal range in strike through a 20 angle from 110 to 90 later steeply dipping mylonite zones formed under lower amphibolite facies conditions strike 75 and locally truncate earlier structures the youngest mylonite zones formed under lowest amphibolite conditions strike 50 and truncate all earlier structures folds developed during this progression show the range in orientation from wnw to ene reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing these changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation l s fabrics absence of axial planar foliation and chaotic orientations of axial surfaces of granulite to amphibolite facies folds indicate formation in a constrictional strain field this is also supported by estimates of the finite strain accumulated at 780 c and 45 km and similar observations by previous workers assuming a simple monoclinic deformation for transtension strain estimates and structural measurements indicate apparent transtensional angles of 9 11 that increased to greater than 20 and then decreased to less than 20 these changes in transtensional angle agree with changes in the orientation of the x y plane of strain during exhumation from 45 km to less than 20 km depth and appear to reflect changes in the boundary conditions of deformation this upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of hp and uhp metamorphic rocks

pre caledonian granulite and gabbro enclaves in the western gneiss region norway indications of incomplete transition at high pressure the western gneiss region of norway is a continental terrane that experienced caledonian high pressure and ultrahigh pressure metamorphism most rocks in this terrane show either peak caledonian eclogite facies assemblages or are highly strained and equilibrated under late caledonian amphibolite facies conditions however three kilometre size rock bodies flatraket ulvesund and krakenes in outer nordfjord preserve pre caledonian igneous and granulite facies assemblages and structures where these assemblages are preserved the rocks are consistently unaffected by caledonian deformation the three bodies experienced high pressure conditions 20 23 kbar but show only very localized about 5 eclogitization in felsic and mafic rocks commonly related to shear zones the preservation of pre caledonian felsic and mafic igneous and granulite facies assemblages in these bodies therefore indicates widespread 95 metastability at pressures higher than other metastable domains in norway late caledonian amphibolite facies retrogression was limited the degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle whereby metastability is associated with a lack of deformation and lack of fluids either as a catalyst or as a component in hydration reactions the three bodies appear to have been far less reactive than the external gneisses in this region even though they followed a similar pressure temperature evolution the extent of metastable behaviour has implications for the protolith of the western gneiss region for the density evolution of high pressure terranes and hence for the geodynamic evolution of mountain belts

u pb zircon geochronology of eclogites from the scandian orogen northern western gneiss region norway 14 20 million years between eclogite crystallization and return to amphibolite facies conditionsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh reconstructing tectonic histories involving continental collision subduction and exhumation at plate tectonic rates of 1 cm year requires precise u pb zircon geochronology the western gneiss region has exceptional exposures of high pressure hp and ultra high pressure uhp rocks the strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation respectively the oldest eclogite sampled is 415 1 ma from layered probably uhp eclogite at tevik averoya also with a garnet hornblende assemblage at 410 1 ma the flem gabbro eclogite margin with implied uhp conditions is 410 2 ma hornblende eclogite at seth lepsoya never at uhp is 412 2 ma these compare to devonian ages of 401 1 ma for overgrowths on proterozoic baddeleyite in selnes gabbro 402 2 ma for coesite eclogite at hareidlandet 405 400 ma for coesite eclogite at flatraket and 405 2 ma for near uhp eclogite at hjelmeland

paleostress field reconstruction in the oslo region abstract the geodynamic history of a region is archived in its geologic record which in turn may reflect deformation patterns that causally can be related to certain configurations of paleostresses in the oslo region the exposed geological record ranges from precambrian high grade metamorphic rocks through cambro silurian sedimentary rocks to permo carboniferous sedimentary and magmatic rocks the latter being related to the development of the oslo rift system we investigate the kinematics of outcrop scale faults to derive the diversity of paleostress states responsible for the observed strain for this purpose we combine different graphical and numerical approaches to separate heterogeneous fault slip data sets and estimate the associated reduced stress tensors a reduced stress tensor consists of the directions of the three principal stress axes with 1 2 3 and the ratio of principal stress differences r 2 3 1 3 the present study gives evidence for three major regional paleostress fields that affected the oslo region a compressional stress field with a nw se directed maximum compression 1 has been identified as a caledonian imprint the most prominent regional stress field however is tensional characterised by a wnw ese directed minimum compression 3 and related to the permo carboniferous rifting later the area was affected by a wrench regime with a roughly n s directed 1 and a maximum age of permian whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than permian for a large number of estimated stress states none of the principal axes are sub vertical these oblique paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the permo carboniferous phase of rifting the present study tends to show that the oslo region remained unaffected by major tectonic activity for much of the mesozoic and cenozoic which is when the central european basin system to the south experienced several phases of intense deformation

zircon geochronology and ca 400 ma exhumation of norwegian ultrahigh pressure rocks an ion microprobe and chemical abrasion study understanding the formation and exhumation of the remarkable ultrahigh pressure uhp rocks of the western gneiss region norway hinges on precise determination of the time of eclogite recrystallization we conducted detailed thermal ionization mass spectrometry chemical abrasion analysis and sensitive high resolution ion microprobe analysis of zircons from four ultrahigh and high pressure hp rocks ion microprobe analyses from the flatraket eclogite yielded a broad range of apparently concordant caledonian ages suggesting long term growth in contrast higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi step chemical abrasion yielded moderate pb loss from the first lowest temperature abrasion step possible minor pb loss or minor growth at 400 ma from the second step and a 407 404 ma cluster of slightly discordant 206pb 238u ages most likely free from pb loss from the remaining abrasion steps we interpret the latter to reflect zircon crystallization at 405 400 ma with minor discordance from inherited cores zircon crystallization occurred at eclogite facies possibly post peak conditions based on compositions of garnet inclusions in zircon as well as nearly flat hree profiles and lack of eu anomalies in zircon fractions subjected to chemical abrasion these ages are significantly younger than the 425 ma age often cited for western norway eclogite recrystallization implying faster rates of exhumation 2 5 8 5 km myr and coeval formation of eclogites across the uhp portion of the western gneiss region

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

determining the three dimensional structure of a volcanic plume using unoccupied aerial system uas imagery abstract this study presents a photogrammetric method for 3d reconstruction of a volcanic plume outline to retrieve its spatial properties a dataset of sequential multi view images was collected using a drone mounted camera for a small scale volcanic plume emitted from volcan pacaya guatemala a space carving algorithm has been applied to estimate the plume shape top height volume and drift direction the complete method workflow is presented herein including data capture camera projection image segmentation and model reconstruction the process applied is considered the simplest approach to reconstruct a 3d plume model from sequential imagery whilst accounting for scene evolution within a probabilistic framework the algorithm is sensitive to the method of image segmentation scene resolution and number of images used with unquantifiable uncertainty in the estimated plume volume due to the lack of ground truth data this proof of concept investigation confirms that 3d quantification of volcanic plume geometry can be achieved using uas based photogrammetry and shows promising results for a new method of measuring volcanic source parameters to validate and adjust dispersion models of volcanic plumes

the unusual 3d interplay of basement fault reactivation and fault propagation fold development a case study of the laramide age stillwell anticline west texas usa abstract subsurface fault geometries have a systematic influence on folds formed above those faults we use the extraordinarily well exposed fold geometries of the laramide age stillwell anticline in west texas usa to develop a strain predictive model of fault propagation fold formation the anticline is a 10 km long nw trending ne vergent asymmetric fold system with an axis that displays a map view left stepping en echelon pattern we integrated field observations geologic and structural data cross sections and 2d kinematic modeling to establish an unusual 3d two stage model of contractional fold formation including 1 reverse reactivation of a pre existing nw striking sw dipping left stepping en echelon normal fault system in paleozoic basement rocks to generate monoclinal flexures in overlying layered cretaceous carbonate rocks and 2 the formation of a subsequent flat ramp fault system that propagated horizontally along a mechanically weak clay rich cretaceous unit before ramping up at the hinge of the pre existing monocline system strain is focused within the forelimb of the system in front of the propagating fault tip and is accommodated by a combination of interlayer slip flat ramp faulting and fracturing proximal to planes of slip this strain predictive model can be applied to similar less well exposed contractional systems worldwide and provides a new unusual example of laramide age contractional deformation

insights into u hp metamorphism of the western gneiss region norway a high spatial resolution and high precision zircon study abstract combining high spatial resolution and high precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh pressure uhp metamorphism resulting from the collision of baltica avalonia and laurentia during the scandian orogeny in the western gneiss region of norway zircons were extracted from a layered eclogite in the saltaneset region southern uhp domain and from an eclogite in the ulsteinvik region central uhp domain zircons were first analyzed for u pb and trace element compositions by laser ablation split stream lass inductively coupled plasma mass spectrometry icp ms followed by analysis of those same zircons that yielded scandian dates by integrated u pb isotope dilution thermal ionization mass spectrometry and trace element analysis tims tea lass results from a garnet quartz layer within the saltaneset eclogite give scandian dates of ca 413 397 ma with subsequent id tims analyses ranging from 408 9 0 4 ma to 401 4 0 2 ma 2 an omphacite rich layer from the same eclogite yields lass dates of ca 414 398 ma and a single id tims date of 396 7 1 4 ma in comparison the ulsteinvik eclogite lass results give dates spanning ca 413 397 ma whereas id tims analyses range from 409 6 0 6 ma to 401 3 0 4 ma id tims zircon data from the eclogites reveals two age populations 1 ca 409 407 ma and 2 ca 402 ma both in situ and solution trace element data show a distinct pattern for scandian zircons with strongly depleted hree and weakly negative eu anomalies eu eu whereas inherited zircon ree patterns are distinguished by steep hree slopes and marked negative eu eu when coupled with partition coefficients calculated for zircon and garnet these ree patterns indicate that zircon re crystallized during eclogite facies metamorphism at ca 409 407 ma and ca 402 ma at two widely separated uhp localities

microdiamond daughter crystals precipitated from supercritical coh silicate fluids included in garnet erzgebirge germany microdiamonds associated with phlogopite quartz paragonite phengite apatite and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh pressure p metamorphic gneiss lenses within migmatites of the gneiss eclogite unit erzgebirge germany these aggregates are interpreted to represent original inclusions of a supercritical dense coh fluid rich in k na and sio 2 from this fluid diamond was precipitated as a daughter crystal due to cooling at ultrahigh p conditions brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks during further cooling the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond as indicated by the presence of quartz paragonite and plagioclase it is proposed that the mica dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host the previously formed diamond daughter crystals being preserved metastably these diamond bearing inclusions provide an unequivocal record of dense supercritical coh fluids rich in alkalies and silica within subducted continental crust during ultrahigh p metamorphism

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

3d digital outcrop modelling of the lower cretaceous los santos formation sandstones mesa de los santos region colombia implications for structural analysis abstract the increasing development of the geological data acquisition and processing techniques for outcrops from photographs acquired with the unmanned aerial vehicles has had a great impact on various branches of geology such as structural geology when approaching structural analysis one of the main inputs is given by the fracture orientations therefore the use of unmanned aerial vehicles is a great benefit for acquiring in a very short time a large georeferenced dataset for fractures distribution and orientation even in normally non accessible outcrops access during the fieldwork as well as the ease of data processing and visualization constitute great advantages provided by the use of unmanned aerial vehicles in structural analysis and modelling in this study data were acquired from two outcrops from the lower cretaceous los santos formation sandstones mesa de los santos region colombia el duende and la mojarra outcrops the data was processed in photogrammetry software based on the structure from motion technique with which a dense point cloud was obtained for each outcrop the dense point cloud was processed with the discontinuity set extractor software obtaining the main families of discontinuities and their associated clusters later as a validation these orientations which obtained in a digital way were contrasted with field data previously acquired in the area obtaining a good consistency between both datasets finally the data was associated with the regional structural trends evidenced in the field data generating more data to improve the structural models of the area

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

a tem study of the oriented orthopyroxene and forsterite inclusions in garnet from otroy garnet peridotite wgr norway new insights on crystallographic characteristics and growth energetics of exsolved pyroxene in relict majoritic garnet regularly oriented orthopyroxene opx and forsterite fo inclusions occur as opx rutile rt or fo rt inclusion domains in garnet grt from otroy peridotite electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships cor with the garnet host in contrast orthopyroxene inclusions have two sets of cor that is cor i grt opx and 110 grt 100 opx 13 off and cor ii grt opx and 110 grt 100 opx 14 off in four garnet grains analysed both variants of orthopyroxene have a blade like habit with one pair of broad crystal faces parallel sub parallel to 110 grt plane and the long axis of the crystal opx for cor i and opx for cor ii along grt direction whereas the lack of specific cor between forsterite and garnet along with the presence of abundant infiltrating trails veinlets decorated by fo rt at garnet edges provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving infiltrating precipitating healing process at low temperatures the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious in this connection the reported cor the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed clarified the exsolved ortho pyroxene in a relict majoritic garnet follows cor iii 112 grt 100 opx and grt opx based on the detailed trace analysis on published sem images these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to 112 grt 100 opx and the long crystal axis along grt opx such a crystal habit can be rationalized by the differences in oxygen sub lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid state exsolution mechanism considering the very different cor crystal habit as well as crystal growth direction the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism s other than solid state exsolution regardless of their regularly oriented appearance in garnet and the cor specification between orthopyroxene and garnet in fact the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx rt inclusion domains fo rt inclusion domains trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving infiltration process as forsterite inclusions though probably at an earlier stage of metamorphism this work demonstrates that the oriented inclusions in host minerals with or without specific cor can arise from mechanism s other than solid state exsolution caution is thus needed in the interpretation of such cor so that an erroneous identification of exhumation from uhp depths would not be made

geometry of eclogite facies structural features implications for production and exhumation of ultrahigh pressure and high pressure rocks western gneiss region norway 1 well preserved eclogite facies structural features in metamorphosed gabbro and diorite gneiss in baltica crust formed at a minimum depth of 60 70 km during the scandian orogeny allow direct inferences to be made regarding the geometry of folds kinematics and original structural orientations related to production and exhumation of high pressure rocks folds associated with eclogite facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation strain estimates and l s or l s fabrics indicate that these structures were formed in a constrictional strain field locally they are well preserved in eclogite facies mylonite zones at least 40 m thick that cut proterozoic gabbro and adjacent gneiss where steeply dipping they show a north side up shear sense across a younger northeast trending shallowly plunging amphibolite facies anticlinorium formed in a constrictional noncoaxial strain field where original structural facing direction can be inferred small circle rotation of the eclogite facies lineation about the anticline axis indicates that the relative motion vector of baltica with respect to overlying imbricated crust and laurentia was oriented 320 in a present day reference frame this result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with late silurian to early devonian oblique sinistral convergence between baltica and laurentia results of this study indicate that coherent baltica crust at 60 70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops southeast thrusting parallel to plate motion high pressure rocks were progressively imbricated and stacked along eclogite facies mylonite zones with the deforming zone separating coherent baltica crust from imbricated basement above the constrictional strains under eclogite facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

coesite micro inclusions and the u pb age of zircons from the hareidland eclogite in the western gneiss region of norway abstract we have identified by laser micro raman spectroscopy that inclusions of coesite occur together with other eclogite facies mineral phases within metamorphic zircons separated from the large eclogite body at ulsteinvik dimnoy on hareidland this is the first identification of coesite from this portion of the northwestern western gneiss region wgr and supports continuity of ultrahigh pressure uhp metamorphism between the documented coesite occurrences on stadlandet to the south and the microdiamond and coesite pseudomorph localities on fjortoft in the nordoyene to the north the zircons first analysed by u pb tims in 1973 have been re analysed and have yielded a much more precise age of 401 6 1 6 ma that overlaps with the previously determined age our discovery of coesite and the indication of a close to 402 ma formation age add to a growing number of mid late early devonian ages that signal that the uhp metamorphism in this part of west norway occurred relatively late in the caledonian orogenic cycle these observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite facies mineralogies

drone high resolution infrared imaging of the lusi mud eruption abstract the use of low cost hand held infrared ir thermal cameras based on uncooled micro bolometer detector arrays became more widespread during the recent years thermal cameras have the ability to estimate temperature values without contact and therefore can be used in conditions where targets are difficult or dangerous to reach such as volcanic eruptions since may 2006 the indonesian lusi mud eruption continues to spew boiling mud water aqueous vapour co2 ch4 and covers a surface of nearly 7 km2 here we performed surveys above and around the erupting crater using a specifically equipped remote controlled aerial vehicle drone despite the harsh logistics and the continuously varying gas concentrations we managed to collect ir images composing mosaics to estimate the crater zone spatial and temporal thermal variations as well as that in the surrounding regions in this manuscript we provide a a description of the main processes that affect and control the acquisition of ir images b an overview of still non disclosed physical model used by the thermal camera employed during our survey i e flir i7 ir c a method for capturing high resolution infrared images over an erupting clastic system d analysis and interpretation of the acquired data and scientific results the results show that it is possible to obtain good quality mosaics also in inaccessible areas such as erupting craters where fixed reference points are not constant and where the presence of ir attenuation factors introduce errors in terms of temperature estimates however the ir camera radiative transfer model based on lowtran model allows the control of only some of the parameters that affect the attenuation of the ir spectrum this results in obtained crater temperature estimates up to 20 lower than those measured with hand held thermometers providing however very important information about the thermal gradient and the potential radioactive absorption factors the imaged lusi vent thermal pattern suggests the presence of shallow convective chambers inside the caldera zone these are activated by the rise of boiling mud breccia that suddenly cools and sinks the thermal survey conducted on the dry mud region to the ne of the crater reveals temperatures matching with those measured directly on the field with a hand held thermometer here the presence of hot spot anomalies and colder circular features is consistent with the migration of deeper warm fluids along a faulted and fractured area and with widespread pools distribution

raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh pressure metamorphic rocks from diamonds in sample preparation materials natural diamond from three ultrahigh pressure metamorphic uhpm terranes erzgebirge massif germany kokchetav massif northern kazakhstan rhodope metamorphic province greece and synthetic diamond from cutting and polishing materials paste spray saw blade were studied by means of optical microscopy and raman microspectroscopy to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones i e diamond as residual particles contaminants from the cutting and polishing material such as the diamond size the presence of inclusions coatings or coexistent phases and two diamond raman band parameters i e the raman shift and the full width at half maximum height fwhm

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

collecting field data in volcanic landscapes using small uas suas drones abstract the last several years have seen a very rapid increase in the development and availability of unmanned aerial vehicles systems uav or uas more commonly called drones uas are remotely operated aerial vehicles that can be fixed wing planes helicopters or aerial systems with multiple propellers small versions of uas here called suas in particular have become so prevalent that they can be easily purchased at general retail stores this paper seeks to review and summarize the use of suas in volcanic research given their size low cost and relative durability suas provide a light weight tool platform that is easy to transport to field locations quick to set up and easy to launch and control they require very little launch and recovery space because of these characteristics suas are useful in collecting immediate and real time aerial data especially in remote inaccessible dynamic and or hazardous volcanic environments in volcanic areas suas have been used for mapping sample collection thermal imaging magnetic surveys slope stability studies and as platforms for sensors to measure outgassing of co 2 and so 2 they are also becoming invaluable for real time hazard assessment during and after an eruption they are however limited by their flight time which is greatly affected by wind speeds since they are predominately made of plastic they are also impacted by the high thermal temperatures found in active volcanic areas which can degrade their structure and performance with continued technological improvement however suas have the potential to dramatically improve our ability to collect field data because of additional natural technological and legal challenges related to their use it is critical that users be aware of and adhere to all national and local laws associated with suas

mechanical stratigraphic controls on natural fracture spacing and penetration abstract fine grained low permeability sedimentary rocks such as shale and mudrock have drawn attention as unconventional hydrocarbon reservoirs fracturing both natural and induced is extremely important for increasing permeability in otherwise low permeability rock we analyze natural extension fracture networks within a complete measured outcrop section of the ernst member of the boquillas formation in big bend national park west texas results of bed center dip parallel scanline surveys demonstrate nearly identical fracture strikes and slight variation in dip between mudrock chalk and limestone beds fracture spacing tends to increase proportional to bed thickness in limestone and chalk beds however dramatic differences in fracture spacing are observed in mudrock a direct relationship is observed between fracture spacing thickness ratio and rock competence vertical fracture penetrations measured from the middle of chalk and limestone beds generally extend to and often beyond bed boundaries into the vertically adjacent mudrock beds in contrast fractures in the mudrock beds rarely penetrate beyond the bed boundaries into the adjacent carbonate beds consequently natural bed perpendicular fracture connectivity through the mechanically layered sequence generally is poor fracture connectivity strongly influences permeability architecture and fracture prediction should consider thin bed scale control on fracture heights and the strong lithologic control on fracture spacing

structural petrological and chemical analysis of syn kinematic migmatites insights from the western gneiss region norway evidence of melting is presented from the western gneiss region wgr in the core of the caledonian orogen western norway and the dynamic significance of melting for the evolution of orogens is evaluated multiphase inclusions in garnets that comprise plagioclase potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies the multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting such as segregations in mafic rocks leucosomes and pegmatites hosted in mafic rocks and in gneisses based on field studies these lithologies are found in three structural positions 1 as zoned segregations found in high pressure hp ultra mafic bodies 2 as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss and 3 as undeformed pegmatites cutting the main caledonian structures segregations post date the eclogite facies foliation and predate the amphibolite facies deformation whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post kinematic and were formed at the end of the deformation history geochemistry of the segregations leucosomes and pegmatites in the wgr defines two trends which correlate with the mafic or felsic nature of the host rocks the first trend with ca poor compositions represents leucosome and pegmatite hosted in felsic gneiss whereas the second group with k poor compositions corresponds to segregation hosted in ultra mafic rocks these trends suggest partial melting of two separate sources the felsic gneisses and also the included mafic eclogites the ree patterns of the samples allow distinction between melt compositions fractionated liquids and cumulates melting began at high pressure and affected most lithologies in the wgr before or during their retrogression in the amphibolite facies during this stage the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward buoyant crust partial melting of both felsic and mafic sources at temperatures below 800 c implies the presence of an h2o rich fluid phase at great depth to facilitate h2o present partial melting

variation in fold geometry in the yuso basin northern spain implications for the deformation regime first generation structures in greywackes of the yuso group from the cantabrian mountains of northern spain show a distinct variation in geometry with depth in a regional synclinal structure curavacas and lechada synclines they are easily distinguished from other deformation events in the structurally uppermost level we find flap folds flap folds are recumbent structures with the inverted limb preserved below this level cascade folds are found these structures have a vergence opposite to that of parasitic folds the nomenclature adopted is from harrison and falcon characteristically these structures have shallowly dipping axial surfaces in agreement with the shallow dip of the axial plane regional cleavage in the lowermost structural level upright parasitic folds with a steep cleavage are present the variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth in the absence of overprinting relationships the f1 fold geometries are included in a single deformation event the steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub vertical in the upper part of the syncline to sub horizontal in the lower part with increasing depth the deformation regime during f changed from bending to buckling the deformation regime on the regional scale however is associated with basement subsidence and passive formation of the regional synclinal structure furthermore the absence of a distinct microfabric for the different f1 folds indicates that on a small scale a similar deformation regime was present we conclude therefore that the scale at which we study a structure only reflects the deformation regime at that particular scale consequently the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone

surface expression of an accommodation zone within the gulf of suez rift egypt the gebel gharamul region in the western gulf of suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination the surface expression of the accommodation zone is dominated by a basement promontory which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half grabens both of which face the gulf of suez the southern half graben represents the breakaway fault of the southwest dipping tilt block domain and the northern half graben represents the collapse of the upper plate above the northeast dipping tilt block domain basement and nubian rocks in the hanging walls above the opposing low angle detachments on either side of the accommodation zone are broken by gulf parallel synthetic normal faults and orthogonal transfer faults which act as block terminations and incrementally accommodate the large scale rotation and displacement between adjacent blocks the prerift and synrift stratigraphic succession is draped over this fault template resulting in a complex and variable distribution of structures the product of an inhomogeneous stratigraphic succession within an accommodation zone

fluid and deformation induced metamorphic processes around moho beneath continent collision zones examples from the exposed root zone of the caledonian mountain belt w norway abstract exposed high pressure hp and ultra high pressure uhp metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near moho depth in continental root zones the investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle this applies to a formation of granulite facies areas b transformation of granulites to eclogites c retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d the spinel lherzolite to garnet lherzolite transition dry rocks both feldspar bearing and ultramafic remain with their pre hp and uhp structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones a mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle pseudotachylytes frictional melts or ultracomminuted material are observed in both ultramafic and feldspar bearing lithologies spatially associated with hp and uhp rocks suggesting that rock properties at moho depth allow earthquakes seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx ductile eclogite facies shear zones nucleate along the brittle structures these observations point to moho as a rock processing zone with the following facets 1 a metastable dry and strong lower crust and upper mantle 2 earthquakes and tremors result in fluid flow and hp metamorphism 3 a pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics lpo in the transformed rocks deep tremor and earthquakes at moho depth may record ongoing metamorphic transitions

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

fluid generation and evolution during exhumation of deeply subducted uhp continental crust petrogenesis of composite granite quartz veins in the sulu belt china

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

high resolution imaging of hydrothermal heat flux using optical and thermal structure from motion photogrammetry abstract quantifying hydrothermal heat flux at meter scale resolution over 0 25 km2 is required to bridge in situ heat flux and satellite based measurements we advance a methodology that blends ground based daytime optical and nighttime thermal infrared tir imagery using structure from motion photogrammetry to map radiant hydrothermal heat flux over these scales at sites with low signal to noise ratios that would otherwise be difficult to characterize using for example unmanned aerial systems the improved method uses a computerized telescopic mount to relocate and align daytime optical acquisitions with nighttime tir imagery thereby enabling tir acquisition from multiple camera orientations positioned throughout a study area this facilitates mapping of thermal features at sites of varying size and complexity and helps to ameliorate topographic occlusion effects and geometric distortions that can bias radiant hydrothermal heat flux estimates derived from the resulting orthorectified thermal maps we assessed detection thresholds of this method at three sites across central california which range in size topography and heat flux conditions we found that blending of optical and thermal acquisitions successfully detected anomalous heat flux even in cases where temperatures were only slightly greater than background this approach might be applied to a variety of volcanic and hydrothermal systems to quantify the spatial distribution of heat flux and how this may relate to factors such as the distribution of ground fractures and lava flow rheology

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

diamond former coesite and supersilicic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established abstract we report here the first discovery of ultrahigh pressure uhp indicator minerals and textures from the rhodope metamorphic province rmp northern greece in particular we document the exsolution of quartz rutile and apatite in sodic garnet from metapelites garnet biotite kyanite gneisses which attests to the presence of a si ti na p rich precursor garnet phase similar textures in garnet have been reported in the literature only once before for eclogites from the su lu uhp metamorphic province china we also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite included in garnet from both eclogites and metapelites we then argue that these rocks had once been transported to depths exceeding 220 km well into the upper mantle thus establishing the rmp as another important uhp metamorphic belt in the world

early basin and range development in trans pecos texas and adjacent chihuahua magmatism and orientation timing and style of extension magmatism during early basin and range extension in trans pecos texas consisted of widespread but volumetrically minor alkali basalts 24 17 ma an alkalic mafic to intermediate composition stratovolcano 28 27 ma and a dominantly rhyolitic possibly bimodal caldera complex in adjacent chihuahua 31 28 ma this magmatism contrasted sharply with the much more voluminous and compositionally more diverse eocene oligocene magmatism the age and orientation of dikes emplaced during this magmatic episode have been used to determine the orientation of least principal stress 3 an ene orientation of 3 similar to early extension throughout the basin and range province can be well documented by 28 ma and possibly as early as 31 ma faulting consisting almost exclusively of movement along high angle normal faults was underway by 23 ma some initial extension was taken up along preexisting wnw trending structures oblique to ene oriented 3 the least principal stress probably changed to wnw as it did elsewhere in the basin and range province but the timing of this change is not well constrained in texas ene and ne striking silver copper lead veins in precambrian and cretaceous red beds may have formed during the later basin and range extension the present basin and range topography of this region is the result of movement along high angle and dominantly nnw trending faults established early under 3 oriented ene and reactivated after 3 changed to wnw

internal structure and deformation of an accommodation zone in the northern part of the suez rift abstract detailed structural study of the eastern onshore part of the gharandal accommodation zone that separates the northern sw dipping and central ne dipping half grabens of the suez rift helped decipher the internal structure and deformation of accommodation zones of continental rifts this 60 km wide zone is affected by pure normal faulting the ne dipping faults of the northern half graben extend southward into the accommodation zone where they interfinger with sw dipping faults extending from the central half graben these two sets of rift parallel faults form several horsts and grabens in the accommodation zone areas dipping parallel to the northern or southern half grabens form several intermixing dip domains in the accommodation zone smaller scale accommodation of dip between these dip domains proceeds by the development of rift parallel folds twist zones in contrast to the southern accommodation zone of the suez rift the internal structure of the gharandal accommodation zone is believed to be representative of accommodation zones in regions a unaffected by prerift structures lying at high angles to the rift and b experiencing relatively small extension accommodation zones in areas having pre rift structures lying at high angle to the rift have relatively narrow width and are characterized by transverse strike slip faults strike slip movement on these faults is related to the torsional strain resulting from the opposite tilt directions and transport of fault blocks of adjacent half grabens

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

unmanned aerial vehicle measurements of volcanic carbon dioxide fluxes i we report the first measurements of volcanic gases with an unmanned aerial vehicle uav the data were collected at la fossa crater vulcano italy during april 2007 with a helicopter uav of 3 kg payload carrying an ultraviolet spectrometer for remotely sensing the so 2 flux 8 5 mg d 1 and an infrared spectrometer and electrochemical sensor assembly for measuring the plume co 2 so 2 ratio by multiplying these data we compute a co 2 flux of 170 mg d 1 given the deeper exsolution of carbon dioxide from magma and its lower solubility in hydro thermal systems relative to so 2 the ability to remotely measure co 2 fluxes is significant with promise to provide more profound geochemical insights and earlier eruption forecasts than possible with so 2 fluxes alone the most ubiquitous current source of remotely sensed volcanic gas data

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

composition of barian mica in multiphase solid inclusions from orogenic garnet peridotites as evidence of mantle metasomatism in a subduction zone setting multiphase solid inclusions in minerals formed at ultra high pressure uhp provide evidence for the presence of fluids during deep subduction this study focuses on barian mica which is a common phase in multiphase solid inclusions enclosed in garnet from mantle derived uhp garnet peridotites in the saxothuringian basement of the northern bohemian massif the documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite kmg3 si3al o10 oh 2 kinoshitalite bamg3 al2si2 o10 oh 2 and ferrokinoshitalite bafe3 al2si2 o10 oh 2 in addition to ba 0 24 0 67 apfu mica is significantly enriched in mg xmg 0 85 to 0 95 cr 0 03 0 43 apfu and cl 0 04 0 34 apfu the substitution vector involving ba in the i site which describes the observed chemical variability can be expressed as bafeivalclk 1mg 1si 1 oh 1 a minor amount of cr and vial enters octahedral sites following a substitution vector vi cr al 2 vi mg fe 3 towards chromphyllite and muscovite as demonstrated by variable ba and cl contents positively correlating with fe barian mica composition is partly controlled by its crystal structure textural evidence shows that barian mica together with other minerals in multiphase solid inclusions crystallized from fluids trapped during garnet growth the unusual chemical composition of mica reflects the mixing of two distinct sources 1 an internal source i e the host peridotite and its garnet providing mg fe al cr and 2 an external source represented by crustal derived subduction zone fluids supplying ba k and cl at uhp uht conditions recorded by the associated diamond bearing metasediments c 1100 c and 4 5 gpa located above the second critical point in the pelitic system the produced subduction zone fluids transporting the elements into the overlying mantle wedge had a solute rich composition with properties of a hydrous melt the occurrence of barian mica with a specific chemistry in barium poor mantle rocks demonstrates the importance of its thorough chemical characterization

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

dynamics of mineral crystallization from precipitated slab derived fluid phase first in situ synchrotron x ray measurements precipitation of daughter minerals we performed for the first time a single crystal x ray diffraction experiment by synchrotron radiation at diamond light source in combination with electron probe microanalyses this measurement allowed the unique identification of each mineral phase and reciprocal orientations we demonstrated the epitaxial relationship between spinel and garnet and between some hydrous minerals such information is discussed in relation to the physico chemical aspects of nucleation and growth shedding light on the mode of mineral crystallization from a fluid phase trapped at supercritical conditions

graben hydrocarbon occurrences and structural style major hydrocarbon occurrences types of traps and structural styles have been synthesized from the sirte basin the suez and viking grabens and from other normal faulted regions hydrocarbons occur in a stacked succession of one or more basins pregraben graben fill and interior sag preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders furthermore a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault block closures hydrocarbon traps in the pregraben and graben fill sediments are primarily dependent on the multidirectional orientation of normal faults tilting of fault blocks and in many cases flexing or erosion parallel with block edges the fault pattern is dominated by longitudinal faults subparallel with the graben axis and secondarily by oblique faults the direction and degree of block rotation are influenced by fault profile planar or listric degree of extension fault pattern doglegs junctions terminations downwarping of the subsequent sag basin and isostatic adjustments between large blocks fold closures result from the upward termination of faults into forced folds which may be accentuated by drag during renewed faulting passive drape and differential compaction extend he folds to shallower depths because they form along block edges the flexures have trends that mimic the multidirectional fault patterns truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns where the sedimentary succession is complete an interior sag basin is superimposed across the graben its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault block topography the upper part is usually an unsegmented syncline and hydrocarbon traps at this level include depositional buildups reefs turbidite mounds and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics

raman confirmation of microdiamond in the svartberget fe ti type garnet peridotite western gneiss region western norway introductionvestiges of ultra high pressure meta morphism uhpm havebeenfoundincontinental and oceanic rocks of dif ferentorogensworldwide smith 1988 coleman and wang 1995 carswelland compagnoni 2003 indirectuhpmevidencecomesfromtheappli cation of geothermobarometric tech niques lappin and smith 1978 ravna krogh and paquin 2003 ap plied to uhpm mineral assemblagesand directly from mineralogical indi cators such as coesite diamond andmajoritic garnet chopin 1984 smith 1984 sobolev and shatsky 1990 do brzhinetskaya et al 1995 van roer mund and drury 1998 nasdala andmassonne 2000 mposkos andkosto poulos 2001 van roermund et al 2002 godard et al 2003 neverthe less the mechanisms responsible forthe formation preservation and exhu mation of uhpm rocksin orogens arestill not fully understood the volume and spatial distributionof uhpm rocks within uhpm terr anes including the observed contrastin metamorphic grade between rocksthat contain uhpm minerals andadjacent country rocks that do notcontain them have led to di erentexplanations concerning the origin emplacement and preservation ofthese rocks to explain the occur rence volume and areal extent ofuhpm rocks in the western gneissregion wgr the following mecha nisms that assume a continental sub duction scenario have been proposed a the existence of major and orminor post uhpm tectonic contactssuch as nappes and or tectonic mel anges e g lappin and smith 1978 smith 1984 1988 wain 1997 cuth bert et al 2000 terry and robinson 2003 2004 or b the presence of uids and or deformation leading todi erential prograde or retrogradereactions and di erences in reaction kinetics between rock types austr heim et al 1997 cuthbert et al 2000 wain et al 2001 carswell andcuthbert 2003 cuthbert and car swell 2003 hypotheses that do notnecessarily involve continental sub duction may involve local regional tectonic overpressure in rocks smith 1984 mancktelow 1993 petrini and podladchikov 2000 it isconsidered here that the volume andareal extent of uhpm domains withina single metamorphic terrain are thusimportant parameters to determine asthey can modify or identify the loca tions of inferred or observed tectoniccontacts the relations between uidsand uhpm and or the scales atwhich continental crust subductionor tectonic overpressure might act here we report raman data thatcon rm the previously reported butnot proven presence of microdia monds at the svartberget uhpmlocality in the wgr of norway vrijmoed et al 2006 micro diamond con rmation is consistentwith the results of vrijmoed et al 2006 that the volume and arealextent of the northern uhpm domainof the wgr fig 1 are considerablyexpanded eastwards and followingthe reasoning in the paragraph above the new microdiamond nding willput important constraints on themodels described above ultra high pressure metamorphismin the western gneiss regionthe wgr represents a window inthe scandinavian caledonides wherescandian reworked high grade gneis sic rocks of proterozoic age presum ably belonging to the baltic plate are exposed in addition variableamounts of metadolerites amphi abstract

sinking intrusion model for the emplacement of garnet bearing peridotites into continent collision orogens garnet and probably spinel bearing ultramafic rocks that form tectonic lenses within high metamorphic grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge this model is most credible for garnet peridotites associated with ultrahigh pressure metamorphic terranes in mountains formed by continent continent collision the extremely high pressures implied by ultrahigh pressure metamorphism indicate that continental crust was subducted to mantle depths 100 120 km where the mantle is in the garnet peridotite facies exchange between mantle and crust seems likely particularly the transfer of mantle from the hanging wall into the crustal footwall the mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable if the interface becomes hot or wet enough so that peridotite can deform by ductile flow it would sink as mantle blobs into the underlying crustal rocks if not the exchange presumably occurs through a brittle mechanism the intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface this mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh pressure western gneiss region of the norwegian caledonides during the caledonian orogeny

structure from motion photogrammetry a low cost effective tool for geoscience applications abstract high resolution topographic surveying is traditionally associated with high capital and logistical costs so that data acquisition is often passed on to specialist third party organisations the high costs of data collection are for many applications in the earth sciences exacerbated by the remoteness and inaccessibility of many field sites rendering cheaper more portable surveying platforms i e terrestrial laser scanning or gps impractical this paper outlines a revolutionary low cost user friendly photogrammetric technique for obtaining high resolution datasets at a range of scales termed structure from motion sfm traditional softcopy photogrammetric methods require the 3 d location and pose of the camera s or the 3 d location of ground control points to be known to facilitate scene triangulation and reconstruction in contrast the sfm method solves the camera pose and scene geometry simultaneously and automatically using a highly redundant bundle adjustment based on matching features in multiple overlapping offset images a comprehensive introduction to the technique is presented followed by an outline of the methods used to create high resolution digital elevation models dems from extensive photosets obtained using a consumer grade digital camera as an initial appraisal of the technique an sfm derived dem is compared directly with a similar model obtained using terrestrial laser scanning this intercomparison reveals that decimetre scale vertical accuracy can be achieved using sfm even for sites with complex topography and a range of land covers example applications of sfm are presented for three contrasting landforms across a range of scales including an exposed rocky coastal cliff a breached moraine dam complex and a glacially sculpted bedrock ridge the sfm technique represents a major advancement in the field of photogrammetry for geoscience applications our results and experiences indicate sfm is an inexpensive effective and flexible approach to capturing complex topography

tectonic overpressure in competent mafic layers and the development of isolated eclogites variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions whether this is mean stress or the normal stress acting across the reacting interface the magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks the magnitude of the mean stress difference across a non slipping interface between two materials with viscosity ratio c 20 1 is effectively equal to the maximum shear stress for flow in the more viscous material progressive shortening of the interface results in a higher mean stress in the more competent material whereas extension results in a lower mean stress for high p low t eclogite facies conditions current experimental data indicate that clinopyroxene and garnet rich layers of eclogite should be very strong and that pressure differences of up to 800 mpa 8 kbar between competent layer and weaker matrix may be possible such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower by a factor approaching the viscosity ratio extrusion of material between more rigid plates which has been proposed as a regional mechanism of lateral continental escape for both the alps and the himalayas should also be accompanied by a lateral gradient in effective pressure otherwise extrusion could not occur maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width e g around 20 times for a width to thickness ratio of 10 tectonic overpressure in progressively shortened competent layers particularly in regions of extrusion between more rigid plates might help explain the occurrence of isolated layers and pods of low t eclogite c 550 c the dramatic weakening of clinopyroxene in the power law creep field precludes the development of significant overpressures in eclogite layers

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

magnetism at depth a view from an ancient continental subduction and collision zone recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle two approaches to constraining magnetizations are developed providing minimum 0 01 si and maximum 0 04 si susceptibility estimates given some assumed thickness 15 km here these values are higher than are found in many continental rocks are there rocks deeper in the crust or upper mantle that are more magnetic than expected or are the model assumptions incomplete what is the magnetic behavior of deep crustal and upper mantle rocks when slightly cooler than the curie or neel temperatures of their magnetic minerals after being exhumed from locations of high grade metamorphism at greater depth different sets of equilibrium metamorphic minerals can be considered that would form under different conditions results on 1501 samples from the western gneiss region wgr norway mainly from mafic and ultramafic bodies subducted to depths of 60 200 km and temperatures of 750 up to 950 c at the very highest pressures show that rocks did not fully equilibrate to the dominant metamorphic facies conditions there is a large variation in petrophysical properties oxide minerals and mineral assemblages in wgr samples though they cannot explain the broad high amplitude deep seated anomalies measured in this region the presence of magnetite and exsolved titanohematite and hemo ilmenite in samples shows those magnetic phases are preserved even at eclogite facies conditions in part because complete eclogite facies equilibrium was rarely achieved

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

raman study of diamond based abrasives and possible artefacts in detecting uhp microdiamond abstract raman spectral characteristics of a range of diamond based abrasives powders and sprays and drilling and cutting tools originating from preparation laboratories worldwide are presented some abrasives show strong broadening of the main diamond band fwhm full width at half band maximum 5 cm 1 accompanied by strong band downshift 1316 1330 cm 1 others are characterised by moderate band broadening fwhm 1 8 5 cm 1 at rather regular band position 1331 1333 cm 1 in addition we found that a fresh abrasive and its used analogue may in some cases show vast differences in their raman spectra the raman parameters of diamond based abrasives overlap widely with raman parameters of uhp ultra high pressure microdiamond it is hence impossible to assign diamond detected in a geological specimen to either an introduced artefact or a genuine uhp relict from the raman spectrum alone raman is an excellent technique for the detection of minute amounts of diamond however it does not provide conclusive evidence for the identification of uhp microdiamond the latter requires thorough verification for instance by optical microscopy or if doubts cannot be dispelled transmission electron microscopy

a major synmetamorphic early devonian thrust and extensional fault system in the mid norway caledonides relevance to exhumation of hp and uhp rocks

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

coesite in the straumen kyanite eclogite pod norway the occurrence of coesite in the straumen kyaniteeclogite pod in the western gneiss region wgr norway is described although the microtextures of coesite and of its partial or complete transformation into quartz are similar to those described at the first coesite occurrence in norway the grytting dolomite eclogite pod there are some significant differences at this second locality these include the host mineral of the sio2 inclusions being garnet instead of clinopyroxene and also occasionally kyanite an initial coesite eclogite sub facies paragenesis was followed by a quartz eclogite sub facies paragenesis which in addition to rutile and dolomite or calcite contains the highest number 7 of silicate phases known in any norwegian eclogite garnet clinopyroxene kyanite quartz phengite zoisite and clinoamphibole this second occurrence of definite coesite i consolidates the very high pressury estimates about 30 kbar minimum previously deduced for certain eclogite pods occurring in the wgr and ii justifies the recognition of the norwegian coesite eclogite province constituted of a nappe terrane of about 30 kbar parageneses now residing in the wgr

long term tir imagery processing for spatiotemporal monitoring of surface thermal features in volcanic environment a case study in the campi flegrei southern italy the tir monitoring system was partially funded by the 2000 2006 national operating programme and by the italian civil protection department in the framework of the 2004 2006 agreement with the istituto nazionale di geofisica e vulcanologia

review of the tectonics of the levant rift system the structural significance of oblique continental breakup abstract the levant rift system is an elongated series of structural basins that extends for more than 1000 km from the northern red sea to southern anatolia the system consists of three major segments the jordan rift in the south el gharb kara su rift in the north and the lebanese fault splay in between the rifted parts of this structural system are accompanied by intensively uplifted margins that mirror image the basinal pattern namely the deeper the basin the higher its margins and vice versa uplifts also occur along the fault splay section the jordan rift comprises axial basins that diminish in size from the south northwards and are separated from each other by shallow threshold zones along the axis of the rift where the margins are also subdued the lebanese fault splay consists of five faults that emerge from the northern edge of the jordan rift and trend like a fan between the north and the northeast one of these faults connects the jordan and el gharb kara su rifts the levant rift and its uplifted margins started to develop in the middle late miocene and most of the structural development occurred in the plio pleistocene the levant rift system is characterized by its oblique displacement and evidence for both dip slip and strike slip displacement was measured on its faults earthquakes also indicate that same mixed pattern some of them show strike slip offset and others normal it is generally conceded that the amount of normal offset along the boundary faults of the rift system reaches 8 10 km but the lateral displacement is disputed and offsets ranging from 11 to 107 km were suggested assessment of the available data led us to suggest that the sinistral offset along the levant rift system is approximately 10 20 km the similarity between the vertical and the lateral displacements the basin and threshold structural pattern of the rift model experiments in oblique rifting as well as the significant tectonic resemblance to the red sea and the east african rifts indicate that the levant rift is the product of continental breakup and it is probably an emerging oceanic spreading center

drone with thermal infrared camera provides high resolution georeferenced imagery of the waikite geothermal area new zealand abstract drones are now routinely used for collecting aerial imagery and creating digital elevation models dem lightweight thermal sensors provide another payload option for generation of very high resolution aerial thermal orthophotos this technology allows for the rapid and safe survey of thermal areas often present in inaccessible or dangerous terrain here we present a 2 2 km 2 georeferenced temperature calibrated thermal orthophoto of the waikite geothermal area new zealand the image represents a mosaic of nearly 6000 thermal images captured by drone over a period of about 2 weeks this is thought by the authors to be the first such image published of a significant geothermal area produced by a drone equipped with a thermal camera temperature calibration of the image allowed calculation of heat loss 43 12 mw from thermal lakes and streams in the survey area loss from evaporation conduction and radiation an rgb visible spectrum orthomosaic photo and digital elevation model was also produced for this area with ground resolution and horizontal position error comparable to commercially produced lidar and aerial imagery obtained from crewed aircraft our results show that thermal imagery collected by drones has the potential to become a key tool in geothermal science including geological geochemical and geophysical surveys environmental baseline and monitoring studies geotechnical studies and civil works

dead horse graben a west texas accommodation zone the cretaceous rocks of the dead horse graben area underwent two superposed deformations laramide latest cretaceous to early tertiary left transpression structures are overprinted by a basin and range miocene to recent accommodation zone during laramide shortening two northwest trending sets of monoclines developed an en echelon set over a left slip basement fault and a single monocline draped over the uplifted edge of a rotated basement block basin and range extension caused northerly trending half grabens that have opposing asymmetry downthrown to the east north of the study area and downthrown to the west south of the area these half grabens are separated by an accommodation zone expressed as a northwest trending right lateral pull apart graben the normal faults of the pull apart graben flatten at a shallower depth than those of the half grabens this suggests not only that accommodation zones can be mechanically layered but that this layering can occur on a scale different from the adjacent half grabens the northwest trend and superposition of both sets of structures implies a tectonic control by the preexisting structural fabric of the texas lineament

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin

the timing of stabilisation and the exhumation rate for ultra high pressure rocks in the western gneiss region of norway new petrographic evidence and a review of the latest radiometric age data are taken to indicate that formation of the ultra high pressure uhp eclogites within the western gneiss region of norway probably occurred within the 400 410 ma time frame thus this event took place significantly later than the previous widely accepted age of c 425 ma for the timing of the high pressure metamorphism in this part of the scandinavian caledonides garnet growth under uhp coesite stable conditions is recognised as a discrete younger event following on from earlier garnet formed under firstly amphibolite facies then quartz stable eclogite facies conditions currently the best constrained and most precise age specifically for uhp mineral growth is the 402 2 ma u pb age for metamorphic zircon some of which retain coesite inclusions from the hareidland eclogite exhumation must have followed shortly thereafter and based on synoptic pressure temperature and depth time curves must have been very fast our data and those of others indicate an initial fast exhumation to about 35 km depth by about 395 ma at a mean rate of about 10 mm a 1 this rapid exhumation rate may have been driven by the appreciable residual buoyancy of the deeply subducted continental crustal slab due to incomplete eclogitization of the dominant proterozoic orthogneisses during the short lived uhp event subsequent exhumation to 8 10 km depth by about 375 ma occurred at a much slower mean rate of about 1 3 mm a 1 with the late stage extensional collapse of the caledonian orogen playing an increasingly important role especially in the final unroofing of the western gneiss region with some remarkably preserved uhp rocks