marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

folded continental and oceanic nappes on the southern side of monte rosa western alps italy anatomy of a double collision suture 1 previous work suggested a double collision suture including ophiolites from two oceanic basins valais and piemont liguria on the southern side of monte rosa penninic alps northern italy this area was studied using field mapping microstructural analysis and neutron texture goniometry after its formation and eclogite facies metamorphism of continental and oceanic units the suture was deformed by four successive folding and shearing events under greenschist facies conditions all of them taking place between 40 and 28 ma after fold retrodeformation the following tectonostratigraphy results from base to top monte rosa gneiss european margin balma serpentinite eclogite unit cretaceous crust of valais ocean stolemberg gneiss iberia brianconnais continent zermatt saas and tsate ophiolites jurassic crust of piemont ligurian ocean sesia nappe continental fragment off the adria margin the preservation of this lithological sequence suggests that deep seated deformation during multiple continent collision produces heterogeneous strain and extreme thinning of nappes but their original stacking order can still be reconstructed using kinematic analysis and overprinting criteria this is due to the ductile nature of the collisional deformation which retains the continuity of tectonic contacts

resonant slow fault slip in subduction zones forced by climatic load stress global positioning system gps measurements at subduction plate boundaries often record fault movements similar to earthquakes but much slower occurring over timescales of 1 week to 1 year these slow slip events have been observed in japan1 2 cascadia3 4 5 6 7 mexico8 9 alaska10 and new zealand11 the phenomenon is poorly understood but several observations hint at the processes underlying slow slip although slip itself is silent seismic instruments often record coincident low amplitude tremor in a narrow 1 5 cycles per second frequency range12 also modelling of gps data3 7 9 and estimates of tremor location13 indicate that slip focuses near the transition from unstable stick slip to stable friction at the deep limit of the earthquake producing seismogenic zone perhaps most intriguingly slow slip is periodic at several locations with recurrence varying from 6 to 18 months depending on which subduction zone or even segment is examined4 5 6 9 here i show that such periodic slow fault slip may be a resonant response to climate driven stress perturbations fault slip resonance helps to explain why slip events are periodic why periods differ from place to place and why slip focuses near the base of the seismogenic zone resonant slip should initiate within the rupture zone of future great earthquakes suggesting that slow slip may illuminate fault properties that control earthquake slip

tremor genic slow slip regions may be deeper and warmer and may slip slower than non tremor genic regions slow slip events sses are observed worldwide and often coincide with tectonic tremor notable examples of sses lacking observed tectonic tremor however occur beneath k lauea volcano hawaii the boso peninsula japan near san juan bautista on the san andreas fault california and recently in central ecuador these sses are similar to other worldwide sses in many ways e g size or duration but lack the concurrent tectonic tremor observed elsewhere instead they trigger swarms of regular earthquakes we investigate the physical conditions that may distinguish these non tremor genic sses from those associated with tectonic tremor including slip velocity pressure temperature fluids and fault asperities although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions slip velocities of sses at k lauea volcano 10 6 m s and boso peninsula 10 7 m s are among the fastest sses worldwide k lauea volcano the boso peninsula and central ecuador are also among the shallowest sses worldwide and thus have lower confining pressures and cooler temperatures in their respective slow slip zones fluids also likely contribute to tremor generation and no corresponding zone of high vp vs has been noted at k lauea or boso we suggest that the relatively faster slip velocities at k lauea volcano and the boso peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip while different conditions produce slower sse velocities elsewhere and trigger tectonic tremor

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

nappe geometry in the western swiss alps abstract detailed geological mapping during the last 20 years in the western swiss alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs moreover their cores are made of strongly deformed gneisses indicating that important ductile strain took place during the formation of the fold nappes it is therefore probably wrong to imagine deep basement nappes as rigid slices as often actually claimed especially when interpreting seismic profiles true brittle type thrust nappes involving basement rocks only occur in the internal and upper parts of the belt cover nappes on the contrary are in most parts of the alpine belt thrust sheets following more or less the rules of thin skinned tectonics many basement fold nappes lost part of their sedimentary cover during or just before their formation by decollement along ductile horizons the result is that many cover thrust nappes in the external part of the alps are directly related to their original basement fold nappes

the ecology of lower cambrian buildups from zuune arts mongolia implications for early metazoan reef evolution reefal buildups in western mongolia of mid early cambrian late atdabanian age flourished during a period when shelf seas were globally widespread the succession at zuune arts records the transition from shallow marine siliciclastic sediments bayan gol formation to shallow marine but still clastic influenced carbonates salaany gol formation the salaany gol formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow gently inclined shelf in a rapidly subsiding epicontinental sea cycles commenced with the growth of subtidal metazoan calcimicrobe aggregative communities on an open shelf the resultant buildups were commonly engulfed by extensive massive microbial stromatolites when they grew in agitated intertidal conditions latterly they were smothered by ooid shoals in response to rapid sea level rise four types of reefal buildup are distinguished 1 green coloured calcimicrobe tarthinia epiphyton gordonophyton and renalcis boundstones 2 red or green coloured cambrocyathellus tarthinia epiphyton bafflestones 3 red coloured okulitchicyathus bindstones and 4 red coloured radiocyath archaeocyath cribricyath bioherms each is interpreted as having grown at increasing depths and possibly sedimentation rates the buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones and are best developed towards the top of the formation when sea level was high thickets of large solitary archaeocyaths are also inferred in the deeper interbiohermal areas these buildups had synoptic relief and constructed porous structures with growth framework cavities housing diverse coelobiontic communities extensive synsedimentary cements are present including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids these early reefs thus have geological fabrics similar to later phanerozoic representatives it is proposed however that this ecosystem was largely composed of generalist and opportunistic filter and suspension feeders which were dependent upon a far higher input of nutrients than modern day reefal developments bacteria were probably the main primary producers from both planktic and benthic cyanobacterial communities the diversity of each buildup assemblage appears to be controlled by primary cavity size the richest fauna belonging to the highly tiered radiocyath dominated community inferred to have lived in the deepest waters the communities at zuune arts can be compared with other buildups from the early cambrian and with lower ordovician receptaculid calcimicrobe solitary sponge bioherms known from the usa and siberia

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

metamorphic evolution of the sesia lanzo zone western alps time constraints from multi system geochronology the sesia lanzo zone is a polymetamorphic unit containing hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the alpine orogeny different parts of the unit record different stages on the p t deformation evolution allowing multi system isotopic studies to unravel the precise timing of the metamorphic history new rb sr white mica and u pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60 70 ma this substantially alters the common view of early to mid cretaceous eclogite facies metamorphism in this unit the new results are more consistent with the established geotectonic framework for the alpine orogeny since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension it is also more concordant with recent studies of other units that demonstrate post cretaceous high pressure metamorphism step heated 40ar 39ar analysis of phengites yields good plateaux giving ages older than the corresponding rb sr age such anomalously high ages indicate the presence of radiogenic argon rich fluids in the grain boundary network under the fluid pressure conditions acting during this high pressure metamorphic event the u pb sphene ages are variable in polymetamorphic rocks and show inheritance of older pb or sphene crystals into the high pressure event two monometamorphic assemblages yield concordant ages at 66 1 ma reflecting crystallisation of the eclogite facies assemblage the gneiss minuti complex gmc lies structurally below the eclogitic micaschists and its pervasive greenschist facies fabric yields tightly clustered rb sr white mica ages at 38 39 ma this greenschist event did not affect the majority of the emc the 40ar 39ar ages of micas formed at this time were very disturbed whereas micas surviving from an earlier higher pressure assemblage had their 40ar 39ar system reset the greenschist event did not strongly affect u pb systematics in hercynian age sphenes suggesting that the gmc did not uniformly suffer an eclogite facies metamorphism during the alpine cycle but was juxtaposed against the emc later in the orogeny this model still requires that the locus of deformation and metamorphism and possibly fluid flux moved outboard with time leaving the sesia lanzo basement as a shear bounded unreactive block within the orogenic wedge

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

lu hf garnet systematics of a polymetamorphic basement unit new evidence for coherent exhumation of the adula nappe central alps from eclogite facies conditions the adula nappe in the central alps is a mixture of various pre mesozoic continental basement rocks metabasics ultrabasics and mesozoic cover rocks which were pervasively deformed during alpine orogeny metabasics ultrabasics and locally garnet mica schists preserve eclogite facies assemblages while the bulk of the nappe lacks such evidence we provide garnet major element data lu profiles and lu hf garnet geochronology from eclogites sampled along a north south traverse a southward increasing alpine overprint over pre alpine garnets is observed throughout the nappe garnets in a sample from the northern adula nappe display a single growth cycle and yield a variscan age of 323 8 6 9 ma in contrast a sample from alpe arami in the southernmost part contains unzoned garnets that fully equilibrated to alpine high pressure hp metamorphic conditions with temperatures exceeding 800 c we suggest that the respective eocene lu hf age of 34 1 2 8 ma is affected by partial re equilibration after the alpine pressure peak a third sample from the central part of the nappe contains separable alpine and variscan garnet populations the alpine population yields a maximum age of 38 8 4 3 ma in line with a previously published garnet maximum age from the central nappe of 37 1 0 9 ma the adula nappe represents a coherent basement unit which preserves a continuous alpine high pressure metamorphic gradient it was subducted as a whole in a single short lived event in the upper eocene controversial hp ages and conditions in the adula nappe may result from partly preserved variscan assemblages in alpine metamorphic rocks

evidence for episodic aseismic slip across the subduction seismogenic zone off costa rica cork borehole pressure observations at the subduction prism toe abstract slow slip events or silent earthquakes may relieve a significant amount of stress at many subduction plate boundaries both downdip of the limit of seismogenesis and within the seismogenic zone itself in cases where seismic energy release accounts for only a fraction of the plate tectonic displacement rate schwartz and rokosky 2007 slow slip has been identified in several instances downdip of the landward limit of the seismogenic zone and is often accompanied by seismic tremor or low frequency earthquake activity along and above the plate interface referred to as episodic tremor and slip or ets little is known however about the spatial distribution and history of slip between great earthquakes along the seismogenic thrust interface itself which lies mostly offshore in this article we present formation pressure transients observed in two deep sea boreholes near the toe of the subduction prism off nicoya peninsula costa rica which followed ets events observed on shore by 1 2 weeks the signatures of the transients are consistent with local slip on the shallow part of the thrust interface with the underthrusting plate experiencing relaxation and the outer prism experiencing contraction the delay between the tremor activity and the pressure transients observed c 100 km seaward at the prism toe suggests either slow propagation across the seismogenic zone or delayed deformation at the outer part of the prism triggered by the slip beneath nicoya such slip may serve generally to relieve stress at subduction zones but also to increase stress in parts of the plate boundary where interseismic slip does not occur

excess 40ar free phengite in ultrahigh pressure metamorphic rocks from the lago di cignana area western alps abstract ar ar analyses of phengites and paragonites from the ultrahigh pressure metamorphic rocks zoisite clinozoisite schist garnet phengite schist and piemontite schist in the lago di cignana area western alps were carried out with a laser probe step heating method using single crystals and a spot dating method on thin sections eight phengite and two paragonite crystals give the plateau ages of 37 42 ma with 96 100 of 39 ar released each rock type also contains mica crystals showing discordant age spectra with age fractions 20 35 ma significantly younger than the plateau ages phengite inclusions in garnet give ages of 43 2 1 1 ma and 44 4 1 5 ma which are significantly older than the spot age 36 4 1 4 ma from the matrix phengites and the plateau ages from the step heating analyses inclusion ages 43 and 44 ma are consistent with a zircon shrimp age 44 1 ma in this area these results suggest that the oceanic materials that underwent a simple subduction related uhpm form excess 40 ar free phengite and that the peak metamorphism is ca 44 ma or little older we suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40 ar from the crystals this has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age

yo yo subduction recorded by accessory minerals in the italian western alps high pressure metamorphic rocks form during subduction of earth s crust to mantle depths at convergent plate margins their exposure at the surface of earth today provides a record of the subduction zone process in general such metamorphic rocks record only a single cycle of subduction and exhumation yet tectonic models suggest that individual rock units should undergo multiple subduction exhumation cycles here we investigate the microstructure and chemical composition of metamorphic minerals in high pressure rocks exposed in the sesia zone in the italian western alps we find that the minerals white mica garnet allanite and zircon each exhibit multiple generations of mineral overgrowths in particular two generations of white mica with high silicon content indicative of formation at high pressure are separated by an overgrowth with low silicon content that formed during exhumation at low pressures furthermore the trace element signatures of distinct zones within zircon and allanite also reveal two episodes of high pressure metamorphism separated by a period of rapid exhumation we use uranium lead dating of zircon and allanite overgrowths to constrain the timing of this subduction exhumation subduction cycle to 79 65 myr ago we conclude that slices of the sesia zone crust experienced two cycles of burial to mantle depths in less than 20 million years the yo yo subduction occurred during oblique convergence between the african and european plates which involved a continental margin

upper cambrian archaeocyatha from antarctica geological mapping in the ellsworth mountains of west antarctica have established the existence of extensive middle and late cambrian strata forms tentatively referred to archaeocyatha have been examined by f d and a r who concluded that despite the commonly held opinion that the phylum became extinct at the boundary of the lower and middle cambrian the ellsworth mountains forms represented irregularian archaeocyatha which survived in protected niches into the late cambrian the age of the fauna containing the archaeocyaths is firmly dated by abundant trilobites including species of homagnostus pseudagnostus kormagnostella erixanium and onchopeltis modified journal abstract

origin of the monte rosa nappe in the pennine alps a new working hypothesis on the basis of lithological and structural evidence a new solution is proposed for the classical problem of the restoration of folded continental and oceanic nappes in the pennine alps of switzerland and italy according to this working hypothesis the monte rosa nappe a high pressure metamorphic gneiss unit represents basement of the northern european margin of alpine tethys its paleogeographic origin was formerly sought either in the brianconnais microcontinent or in the southern adriatic continental margin the monte rosa nappe is enveloped by the lithologically heterogeneous and extremely deformed furgg zone interpreted here as a melange zone formed during subduction and collisional closure of the valais ocean basin the northern subbasin of alpine tethys in a late stage of the alpine orogeny late eocene postdating closure of the valais ocean basin the european continental margin including the monte rosa basement was subducted southward under the pennine nappe stack and reached eclogite facies depth from there the monte rosa nappe quickly ascended back toward the surface in an internal southeastern position separated from the other europe derived units located farther northwest by the rootless brianconnais derived bernhard nappe system

metamorphic heterogeneities within a single hp unit overprint effect or metamorphic mix abstract eclogites and eclogites facies rocks in mountain belts provide evidence that material in subduction zones material can return from depths of more than 100 km to the surface in the early stages of orogenic processes their relationship with lower metamorphic grade rock of i e blueschist and greenschist facies can provide information on the late stages of orogenic processes but are often matter of debate petrography of metamorphic rocks in mountain belts has mainly focused on mafic rocks systems nevertheless metamorphic domains of recent mountain belts like the alps are not only constituted by mafic rocks but also by metasediments that continuously outcrop over very large areas such mountain belts are made of a large part of low temperature metasediments devoid of index minerals classically observed in mafic and quartzo feldspathic rocks systems allowing a direct comparison to be made these metasediments also have various chemical and mineralogical compositions that represent an important geothermobarometric potential thus we propose to study relationships between eclogites and less metamorphosed rocks for a significant case that has been long debate entrelor area western alps despite metasediments continuously outcropping over the whole area all previous metamorphic studies were carried out on dismembered sequences of mafic rocks they have evidenced two kind of metamorphic evolution three explanations are generally proposed to interpret this feature incertainties of the thermodynamic data the loss of hp mineralogy during the exhumation and a late stage tectonic juxtaposition by studying simultaneously both eclogites and metasedimentary contry rocks we show that rocks association in the entrelor area can be interpreted as a metamorphic mix this area consists of eclogites rocks embedded in a blueschist facies matrix mainly made of metapelites exhumation of hp metamorphic rocks reveals that different pressure peaks 1 2 gpa at 450 c vs 2 3 gpa at 550 c were contemporaneous the different types of rocks have been juxtaposed at a shallow crustal level within a subduction channel due to the fact the western alps do not reach the mature stage of a colliding belt as the central or as the eastern alps the rocks of the entrelor area can be viewed as an exhumed part of a frozen subduction channel attributed to a metamorphic mixing of rocks having different metamorphic evolution and accreted at great depths

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

rapid deformation of kilauea volcano global positioning system measurements between 1990 and 1996 campaign global positioning system gps measurements from 1990 to 1996 are used to calculate surface displacement rates on kilauea volcano hawaii the gps data show that the south flank of the volcano which has generated several large earthquakes in the past 3 decades is displacing at up to 8 cm yr to the south southeast the summit and rift zones are subsiding with maximum subsidence rates of 8 cm yr observed a few kilometers south of the summit caldera elastic dislocation modeling of the gps data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone fault slip along a subhorizontal fault near the base of the volcano and deflation near the summit caldera a nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry there is a broad range of model geometries that fit the data reasonably well however certain models can be ruled out including those that have shallow rift opening or shallow fault slip some offshore aseismic slip on a fault plane that dips between 258 north northwest and 88 south southeast is required best fitting slip and rift opening rates are 23 28 cm yr although rates as low as 10 cm yr are permitted by the data

tertiary age and paleostructural inferences of the eclogitic imprint in the austroalpine outliers and zermatt saas ophiolite western alps the austroalpine sesia lanzo inlier and upper austroalpine dent blanche mt mary and pillonet outliers occur on top of the western alpine orogenic wedge and as a whole override the structurally composite ophiolitic piemonte zone instead the mt emilius glacier rafray etirol levaz and other lower austroalpine eclogitic outliers are inserted within the piemonte zone between its upper combin and lower zermatt saas tectonic elements or within the latter rb sr dating on phengitic micas show that the eclogitic imprint in the lower austroalpine outliers conventionally regarded as late cretaceous by comparison with the sesia lanzo inlier is of eocene age 49 40 ma like the underlying zermatt saas ophiolite 45 42 ma between the aosta valley and gran paradiso massif 40ar 39ar plateau ages on the same mica concentrates of the ophiolitic zermatt saas nappe 46 43 ma are consistent with rb sr dating whereas that on the austroalpine glacier rafray klippe 92 ma is influenced by argon excess the lower austroalpine outliers underwent the subduction metamorphism concurrently with the zermatt saas nappe 20 25 ma later than the eclogitic sesia lanzo inlier and blueschist pillonet klippe the temporal gap and present intra ophiolitic position mean that the lower austroalpine outliers were probably derived from an intraoceanic extensional allochthon mt emilius domain stranded inside the piemonte ligurian ocean far from the dent blanche sesia domain and adriatic margin

eclogite from the ultrahigh pressure metamorphic unit at lago di cignana western alps a process oriented transmission electron microscope study abstract in the western alps the ocean derived lago di cignana unit of the piemonte zone has experienced ultrahigh pressure metamorphism we have studied the minerals of two eclogite samples from this unit using transmission electron microscopy tem in order to characterise their microstructures these microstructures are the result of deformation phase transformation and reactions and allow conclusions on the processes that had generated them the goal of this process oriented paper is to contribute to the knowledge of the formation and exhumation of ultrahigh pressure eclogites in our tem study we found omphacite amphibole barroisite glaucophane hornblende clinozoisite garnet albite and thin layers of chlorite and mica in amphibole the omphacite is well ordered and has the space group p2 n we observed antiphase domains apds dislocations that are organised in small angle grain boundaries and very rare crystallographic shear planes parallel to 010 and 110 deformation twin lamellae on 100 have not been observed most interesting is the first observation of faults parallel to 110 in a natural omphacite they are due to deformation chain multiplicity faults are common in the amphibole grains in one case the orientation of crystallographic shear planes was not only parallel to 010 but also parallel to 130 and 110 clinozoisite showed deformation twin lamellae on 100 with widths of a few nm up to about 50 nm dislocations organised into a small angle grain boundary have been found which have not been described before the garnet is almost free of dislocations while there are differences regarding the deformation microstructures from the uhp lago di cignana eclogite and other hp occurrences it is unlikely that these are due to higher pressures it is much more likely that the whole formation and exhumation history of the different geological units and the individual positions of the collected rocks within them are reflected in their microstructures a tentative correlation of the omphacite microstructures with the p t path of the lago di cignana unit is presented an important point is the early crystallisation of c2 c omphacite at a temperature below 500 c perhaps around 400 c the low temperature prevents rapid diffusion controlled ordering to p2 nomphacite and thus enables deformation of the metastable c2 c omphacite since c2 c omphacite is more easily to deform than p2 n omphacite this may influence the interplate coupling the onset of the transition to p2 n omphacite however will increase the shear strength of eclogite and the coupling with the overriding plate which may slow down the subduction and finally stop it

from passive margins to orogens the link between ocean continent transition zones and ultra high pressure metamorphism a lithostratigraphic association consisting of serpentinized mantle rocks continent derived allochthons mid oceanic ridge gabbros of jurassic age and post rift sediments typical of an ocean continent transition is found in the eclogitic piemonte units in the western alps in situ u pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement in contact with serpentinites primary magmatic zircons of permian age were overgrown by a second generation of zircon at ca 166 150 ma likely related to melt infiltration associated with the intrusion of the underlying gabbroic body this indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the jurassic and they were probably part of hyper extended crust related to the opening of the tethys therefore the complex lithological association described here which is also characteristic of several ultra high pressure melange zones worldwide was acquired prior to the orogenic event during which it was only partly reworked ocean continent transitions are in positions favorable to reach ultra high pressure conditions following negatively buoyant oceanic lithosphere into subduction and then being accreted to the orogen in response to the arrival of more buoyant continental lithosphere resisting subduction the ocean continent transition is now found in the immediate footwall of a 500 m thick shear zone which accommodated multiple episodes of deformation during eocene oligocene time suggesting an important link between alpine deformation and rift related structures

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

kinematic linkage between internal zone extension and shortening in more external units in the nw alps in the alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c 10 kbar implying of the order of 60 km of relative vertical movement in the period 45 36 ma the horizontal component of divergence was of the order of 100 km we use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites the brianconnais and or external zones this thrust related convergence during that time period was of the order of 44 56 km and therefore of the same order of magnitude as internal zone divergence extension may have been driven by internal density contrasts buoyant continental crust underneath the mafic eclogites or dense subducted slab offset from the orogen can induce extension any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen relative plate motions in the period 45 36 ma played a subsidiary role

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

implications of transient deformation in the northern basin and range western united states transient deformation events observed in global positioning system gps data from the basin and range extensional province may illuminate qualitatively similar transient events observed in subduction zones and other tectonic environments we model gps time series at 22 sites using a combination of hyperbolic tangent function analysis and elastic load deformation estimated from climatological data we identify two transient events 2000 4 and 2004 4 with roughly similar timing and displacement to those described previously by other researchers the first few years of gps observations adopted as a reference state in earlier studies are found to be anomalous our results differ from previous studies in two respects first a significant component of northward transient motion occurs during both events despite a reversal of sign in east component motion second sites move coherently in the eastern as well as the western basin and range surface mass loading the largest source of transient stress forcing in the region exhibits no evidence of a simple relationship to the deformation transients prior studies inferred slip on a single megadetachment at the moho but that hypothesis assumes negligible ductile deformation of the lower crust and a dry olivine rheology for the uppermost mantle recent measurements of crustal quartz abundance and effective elastic thickness suggest both assumptions are unlikely basin and range transients can be reconciled with the frictional slip mechanism widely accepted for subduction zone transients provided that slip is occurring on discontiguous detachment surfaces at midcrustal depths

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

tracing two orogenic cycles in one eclogite sample by lu hf garnet chronometry the adula nappe central alps was thought to be a melange of rock fragments some of which were subducted and then exhumed from the mantle lu hf dating of two populations of garnets in one rock sample instead indicates that the nappe twice experienced subduction and exhumation as one coherent unit

slow slip and tremor search at kilauea volcano hawaii 1 kilauea volcano hawaii has hosted a long series of slow slip events observed since the installation of the continuous gps network in 1996 kilauea s slow slip events are inferred to occur on the decollement fault at 8 km depth beneath its south flank with a location updip of the epicenters of large regular earthquakes fault slip typically lasts about two days and the events have magnitudes equivalent to mw 5 3 6 0 while slow slip events in subduction zones are commonly accompanied by tectonic tremor also called nonvolcanic tremor no tremor has yet been reported in association with kilauea s slow slip events instead there are swarms of small triggered earthquakes which is a characteristic only seen at select subduction zones e g boso and hikurangi a temporary array of seismometers was installed at kilauea in 2007 in anticipation of a slow slip event here we use several established methods to perform a systematic search for tectonic tremor during geodetically defined slow slip events as well as searching for tremor triggered by teleseismic surface waves we do not detect tectonic tremor using any of these methods although we are able to detect episodes of previously identified deep offshore volcanic tremor at 15 20 km depth and volcanic tremor from kilauea although kilauea s seismic network may not be adequate to observe tectonic tremor because hawaii is seismically noisy and its crust is highly attenuating it is also possible that the specific fault conditions on kilauea s decollement are not conducive to such tremor generation

the late early cambrian microbial reefs immediately after the demise of archaeocyathan reefs hunan province south china abstract the archaeocyath bearing reefs that first appeared in south china in the atdabanian were last seen in the lower toyonian e g in the tianheban and jindingshan formations the upper toyonian lower amgan qingxudong formation of hunan province examined in this study includes large sized reefs formed solely by calcimicrobes of epiphyton kordephyton girvanella hedstroemia and renalcis during the middle and upper cambrian small scale stromatolite reefs were formed the qingxudong reefs are therefore interpreted to be the most well developed microbial reefs of the post archaeocyath bearing reefs in the late early cambrian the transition from archaeocyath bearing to purely microbial reefs was caused by a decline in the diversity of metazoans and was accompanied by a reduction in the abundance of reefs possibly related in part to late early cambrian regression the late early cambrian reefs of south china contain a well preserved record of reef succession and provide important insights into the factors that led to the demise of archaeocyath bearing reefs and the subsequent long lasting hindrances to the development of skeletal dominated reefs during the middle and late cambrian

kilauea slow slip events identification source inversions and relation to seismicity 1 several slow slip events beneath the south flank ofkilauea volcano hawaii have been inferred from transient displacements in daily gps positions to search for smaller events that may be close to the noise level in the gps time series we compare displacement fields on kilauea s south flank with displacement patterns in previously identified slow slip events matching displacement patterns are found for several new candidate events although displacements are much smaller than previously identified events one of the candidates 29 may 2000 is coincident with a microearthquake swarm as are all of the previously identified slow slip events the microearthquakes follow the onset of slow slip implying that they are triggered by stress changes during slip the new slow slip event brings the total number of events on kilauea between 1997 and 2007 to eight the smallest having m w 5 3 and the largest having m w 6 0 while the recurrence time between the four largest events is 2 11 0 01 years the repeat time for all eight events is 0 9 0 6 years we invert for the fault geometry and distribution of slip during the slow slip events the optimal source depths of 5 km assuming uniform slip dislocations in an elastic half space are considerably shallower than the accompanying swarm earthquakes 6 5 8 5 km which would place the earthquakes in a zone of decreased coulomb stress inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths such that the earthquakes are located in a region of increased coulomb stress we also invert for time dependent fault slip directly from the 30 s gps phase observations constraining the source to the optimal uniform slip geometry on the basis of these inversions the larger events last between 1 5 2 2 days the data are unable to resolve migration of slip along the fault the temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate and state friction making the swarm earthquakes coshocks and aftershocks of the slow slip events

episodic slow slip events and rate and state friction 1 there are several ways of generating episodic slow slip events in models of rate and state friction here i explore the possibility that they arise on velocity weakening faults whose length is tuned in some sense unlike spring block sliders which have a unique critical stiffness for instability elastically deformable faults have multiple length scales stiffnesses relevant to nucleation slow slip events occur when the fault is large enough to initiate an event but too small for that event to reach dynamic instability the available window of lengths depends upon the effective fracture energy of the growing nucleation zone for the aging evolution law this fracture energy increases rapidly with slip speed and the range of permissible lengths increases nearly as b b a where a and b are the coefficients of the velocity and state dependence of the frictional strength this range becomes very large for faults that are near velocity neutral a b however existing lab data firmly favor the slip law as a predictor of nucleation style and for this law the effective fracture energy increases much less rapidly with slip speed the range of fault lengths capable of hosting slow slip events in this case seems too small to explain why they are common to so many subduction zones slow slip can be stabilized over an exceedingly large range of fault lengths if the fracture energy increases with slip speed much more rapidly than for the aging law an appealing mechanism is inelastic dilation of the fault zone coupled with pore pressure reduction and diffusive recovery

multiple metamorphic stages within an eclogite facies terrane sesia zone western alps revealed by th u pb petrochronology convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel studies of high pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels but the details of such dynamics are controversial to understand the dynamics of a subduction channel we report the results of a petrochronological study from the central sesia zone a key element of the internalwestern alps this comprises two polymetamorphic basement complexes eclogitic micaschist complex and gneiss minuti complex and a thin dismembered cover sequence scalaro unit associated with pre alpine metagabbros and metasediments bonze unit structurally controlled samples from three of these units eclogitic micaschist complex and scalaro bonze units yield unequivocal petrological and geochronological evidence of two distinct high pressure stages ages u th pb of growth zones in accessory allanite and zircon combined with inclusion and textural relationships can be tied to the multi stage evolution of single samples two independent tectono metamorphic slices showing a coherent metamorphic evolution during a given time interval have been recognized the fondo slice which includes scalaro and bonze rocks and the druer slice belonging to the eclogitic micaschist complex the new data indicate separate stages of deformation at eclogite facies conditions for each recognized independent kilometer sized tectono metamorphic slice between 85 and 60 ma with evidence of intermittent decompression p 0 5 gpa within only the fondo slice the evolution path of the druer slice indicates a different p t time evolution with prolonged eclogite facies metamorphism between 85 and 75ma our approach combining structural petrological and geochronological techniques yields field based constraints on the duration and rates of dynamics within a subduction channel

halogen geochemistry of the mcmurdo dry valleys lakes antarctica clues to the origin of solutes and lake evolution abstract we have determined the halogen and boron concentrations in the ice covered lakes of taylor valley antarctica to better establish the sources of salts and evolutionary histories of these unusual water bodies in addition we report on a series of 129 i measurements that were compared with previous 36 cl data that also help constrain the source of solutes and histories of the lakes the new data when put into context of previous work on these systems over the past forty years allow us to make the following conclusions the primary source of solutes to lake hoare the youngest of the lakes is the dissolution of marine aerosols and aeolian salts and the chemical weathering of dust on the glaciers the geochemistry of lake fryxell the brackish water lake is primarily dominated by the diffusion from a halite saturated brine at the sediment water interface and the recent infilling of the lake by glacier meltwater these waters have chemical weathering and marine aerosols components lake bonney has two distinct lobes whose hypersaline hypolimnia have different chemistries both of the lobes are remnants of ancient marine waters that have been modified by the input of weathering products this lake has also been modified by periods of cryogenic concentration when solutes have been lost via mineral precipitation thus the geochemistry of lake bonney owes its unusual geochemistry in part to variations in the climate in the taylor valley over at least the past 300kyr the 129 i data from the taylor valley are similar to those from fracture fluids in crystalline rocks from the northern hemisphere

dilatant strengthening as a mechanism for slow slip events 1 the mechanics of slow slip events sse in subduction zones remain unresolved we suggest that sse nucleate in areas of unstable friction under drained conditions but as slip accelerates dilatancy reduces pore pressure p quenching instability competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast we model sse with 2 d elasticity rate state friction and a dilatancy law where porosity evolves toward steady state ss over distance dc and ss 0 ln v v0 v is slip speed we consider two diffusion models membrane diffusion md is approximated by p p tf where p and p are shear zone and remote pore pressure and tf is a characteristic diffusion time homogeneous diffusion hd accurately models fault normal flow with diffusivity chyd for md linearized analysis defines a boundary 1 a b between slow and fast slip where f0e b p f0 a and b are friction parameters and is compressibility when 1 a b slip speeds remain quasi static for hd ep eh p defines dilatancy efficiency where h is shear zone thickness and v is plate velocity sse are favored by large eh and low effective stress the ratio ep to thermal pressurization efficiency scales with 1 p so high p favors sse consistent with seismic observations for ep 10 3 transient slip rates repeat times average slip and stress drops are comparable to field observations model updip propagation speeds are comparable to those observed along strike many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress model sse accommodate only a fraction of plate motion the remaining deficit must be accommodated during coseismic or postseismic slip

recognizing episodic lithospheric thinning along a convergent plate margin the example of the early oligocene alps large scale tectonic mode switches from lithospheric shortening to lithospheric extension may play a fundamental role in the tectonometamorphic evolution of orogens located along convergent plate margins short lived episodes of lithospheric extension can be inferred from the concomitant occurrence of bimodal magmatism rapid exhumation of rock units activity of extensional shear zones steep geothermal gradients and possibly topographic collapse of the orogenic belt here we provide a synthesis of existing data from the european alps supporting the hypothesis on tectonic mode switches data are interpreted to suggest that at ca 38 34 ma a period of n s orogen perpendicular shortening led to the formation of medium to ultra high pressure mineral assemblages by lithosphere scale thrust imbrication a tectonic mode switch took place at ca 34 ma marked by the onset of e w extension during this extensional event internal units of the western and central alps were rapidly exhumed to mid crustal level in the footwall of regional scale extensional shear zones tectonic thinning was also accomplished through pervasive e w stretching of the nappe pile lithospheric stretching resulted in the onset of an anomalous thermal regime which was responsible for the overprint of the older medium to ultra high pressure mineral assemblages by barrovian type mineral assemblages and by migmatization at the same time attenuation of the isotherms led to partial melting of mantle lithosphere and to bimodal magmatism contemporaneous topographic collapse of the orogen is suggested by the deposition in the foreland of flysch units rich in minerals that were probably derived from erosion of the early oligocene calc alkaline volcanics erupted in the hinterland this short lived episode of lithospheric stretching took place in no more than 4 myr from ca 34 ma to ca 32 30 ma renewed lithospheric thickening which is observed in seismic tomography studies resulted in large scale folding of the extensional structures and the in rapid generation of topographic relief feeding abundant clastic sedimentation in the peripheral basins

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

effect of pore pressure buildup on slowness of rupture propagation pore fluid pressure is known to play an important role in brittle fracture initiation and propagation yet the underlying mechanisms remain unclear we conducted triaxial experiments on saturated porous sandstones to investigate effects of pore pressure buildup on the slowness of shear rupture propagation at different confining pressures at low to intermediate confinements rocks fail by brittle faulting and pore pressure buildup causes a reduction in rock s shear strength but does not induce measurable differences in slip behavior when the confinement is high enough to prohibit dynamic faulting rocks fail in the brittle ductile transitional regime in the transitional regime pore pressure buildup promotes slip instability on an otherwise stably sliding fracture compared to those observed in the brittle regime the slip rate stress drop and energy dissipated during rupture propagation with concurrent pore pressure buildup in the transitional regime are distinctively different when decreasing confining pressure instead the slip behavior resembles the ones of the brittle regime emphasizing how the observed slowness is related to excess pore pressure beyond the effective pressure phenomenon analysis of the mechanical data using existing theoretical models confirms these observations quantitative microstructural analyses reveal that increasing pore pressure lessens the dilatancy hardening during failure thus enhances slip along the localized zone in the transitional regime our experimental results suggest that pore pressure buildup induces slow slip in the transitional regime and slip rates along a shear fracture may vary considerably depending on effective stress states

earthquakes triggered by silent slip events on kīlauea volcano hawaii silent earthquakes or slow slip events happen so slowly that they cannot be detected by conventional seismographs but now the availability of gps data means that they can be followed in detail segall et al report on a series of small earthquakes on k lauea volcano hawaii the earthquakes at depths of 7 to 8 km appear to be triggered by otherwise silent slow slips though small triggered earthquakes of this type can be used to help quantify the increased hazard due to slow slip events slow slip events or silent earthquakes have recently been discovered in a number of subduction zones including the nankai trough1 2 3 in japan cascadia4 5 and guerrero6 in mexico but the depths of these events have been difficult to determine from surface deformation measurements although it is assumed that these silent earthquakes are located along the plate megathrust this has not been proved slow slip in some subduction zones is associated with non volcanic tremor7 8 but tremor is difficult to locate and may be distributed over a broad depth range9 except for some events on the san andreas fault10 slow slip events have not yet been associated with high frequency earthquakes which are easily located here we report on swarms of high frequency earthquakes that accompany otherwise silent slips on k lauea volcano hawaii for the most energetic event in january 2005 the slow slip began before the increase in seismicity the temporal evolution of earthquakes is well explained by increased stressing caused by slow slip implying that the earthquakes are triggered the earthquakes located at depths of 7 8 km constrain the slow slip to be at comparable depths because they must fall in zones of positive coulomb stress change triggered earthquakes accompanying slow slip events elsewhere might go undetected if background seismicity rates are low detection of such events would help constrain the depth of slow slip and could lead to a method for quantifying the increased hazard during slow slip events because triggered events have the potential to grow into destructive earthquakes

episodic slow slip events and seaward flank motion at mt etna volcano italy abstract episodic aseismic slip events have recently been detected at a variety of tectonic and volcanic environments sparking the curiosity of seismic and geodetic communities here a sequence of 7 slow slip events occurring at mt etna since mid 2009 has been analyzed observed displacement fields evidence that the sequence involves two contiguous sectors of the unstable eastern flank delimited by the timpe faults the tectonic control played by these faults can also be recognized on the long term 2003 2015 velocity field elastic modelling of the long term velocity field infers a sub horizontal plane slightly dipping eastward and located within the sedimentary basement at shallow depth slip distribution models for each slow slip event highlight how the largest slip values were centred on the se edge of the sub horizontal plane during 4 events and on the ne edge during the remaining 3 ones the recognized events do not appear correlated with volcanic activity although there is a possible correlation between slow slip events and inflating episodes of the volcano

a new post early cambrian archaeocyath from antarctica a new species of archaeocyath sponge dictyocyathus neptunensis is described from the lower third of the nelson limestone of the neptune range antarctica based on the presence of the trilobites nelsonia schesis and amphoton oatesi these strata are interpreted to be mid to late middle cambrian in age d neptunensis shows a branching low integration modular form and a very simple skeletal type such an organization is predicted from known morphological trends within the early cambrian archaeocyatha specimens are not in life position and probably form part of a storm deposit the absence of dictyocyathus from latest early cambrian toyonian strata worldwide suggests a convergent nature for this highly simplified skeletal type in archaeocyaths

kinematics of syn and post exhumational shear zones at lago di cignana western alps italy constraints on the exhumation of zermatt saas ultra high pressure rocks and deformation along the combin fault and dent blanche basal thrust kinematic analyses of shear zones at lago di cignana in the italian western alps were used to constrain the structural evolution of units from the piemont ligurian oceanic realm zermatt saas and combin zones and the adriatic continental margin dent blanche nappe during palaeogene syn and post exhumational deformation exhumation of zermatt saas u hp rocks to approximately lower crustal levels at ca 39 ma occurred during normal sense top s e shearing under epidote amphibolite facies conditions juxtaposition with the overlying combin zone along the combin fault at mid crustal levels occurred during greenschist facies normal sense top se shearing at ca 38 ma the scarcity of top se kinematic indicators in the hanging wall of the combin fault probably resulted from strain localization along the uppermost zermatt saas zone and obliteration by subsequent deformation a phase of dominant pure shear deformation around 35 ma affected units in the direct footwall and hanging wall of the combin fault it is interpreted to reflect nw se crustal elongation during updoming of the nappe stack as a result of underthrusting of european continental margin units and the onset of continental collision this phase was partly accompanied and followed by ductile bulk top nw shearing especially at higher structural levels which transitioned into semi ductile to brittle normal sense top nw deformation due to vanzone phase folding from ca 32 ma onwards our structural observations suggest that syn exhumational deformation is partly preserved within units and shear zones exposed at lago di cignana but also that the combin fault and dent blanche basal thrust experienced significant post exhumational deformation reworking and overprinting earlier structures

the geometry and timing of orogenic extension an example from the western italian alps contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements low p upon high p contacts are commonly interpreted as extensional i e material points on either side of the contact moved apart relative to the palaeo horizontal but dating of deformation and metamorphism is essential in testing such models in the western alps the piemonte ophiolite consists of eclogites t 550 600 c and p 18 20 kbar structurally beneath greenschist facies rocks t 400 c and p 9 kbar mapping shows that the latter form a kilometre wide shear zone the gressoney shear zone gsz dominated by top se movement related to crustal extension rb sr data from micas within different gsz fabrics which dynamically recrystallized below their blocking temperature are interpreted as deformation ages ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping they show that the gsz had an extensional deformation history over a period of c 9 myr between c 45 36 ma this overlaps in time with the eclogite facies metamorphism the gsz operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation the discrete contact zone between eclogite and greenschist facies rocks is the last active part of the gsz and truncates greenschist facies folds in the footwall these final movements were therefore not a major component of eclogite exhumation pressure estimates associated with old and young fabrics within the gsz are comparable indicating that during extensional deformation there was no significant unroofing of the hangingwall since there are no known extensional structures younger than 36 ma at higher levels in this part of the alps exhumation since the final juxtaposition of the two units at 36 ma seems to have been dominated by erosion key words deformation age eclogite exhumation rb sr dating tectonic

recognizing remnants of magma poor rifted margins in high pressure orogenic belts the alpine case study magma poor rifted margins are being increasingly recognized in present day atlantic type systems however findings of fossil areas floored by exhumed mantle or hyper extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre existing rifted margins this discrepancy may be due to the common reactivation of lithological contacts during subduction orogeny potentially masking pre orogenic relationships and most importantly to the frequent lack of a preorogenic layer cake architecture hindering retro deformation of multiply deformed tectonic units this study outlines a methodology to detect sections of magma poor hyper extended rifted margins without a layer cake architecture in multiply deformed metamorphosed terrains this approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of alpine orogens in the latter domains continental basement and hydrated peridotites were exhumed at the basin floor during jurassic rifting along long offset detachment systems extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre rift sediments fromthe hanging wall blocks lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn to post rift meta sediments and or serpentinized subcontinental mantle are widespread within sections of alpine type orogenic belts that underwent high to ultra high pressure metamorphism however similar associations may arise from a variety of processes other than rift related lithospheric thinning in magma poor environments including subduction melange dynamics or deposition of sedimentary melanges along convergent divergent margins the partial preservation of rift related lithostratigraphic associations may still be assessed despite the lack of biostratigraphic evidence by 1 the consistency of the lithostratigraphic architecture over large areas despite pervasive alpine deformation which rules out chaotic mixing during subduction exhumation 2 the presence of clasts of basement rocks in the neighboringmeta sediments indicating the original proximity of the different lithologies 3 evidence of brittle deformation in continental basement and ultramafic rocks pre dating alpine metamorphism indicating that theywere juxtaposed by fault activity prior to the deposition of post rift sediments and 4 the similar alpine tectono metamorphic evolution of ophiolites continental basement and meta sediments a re assessment of basement cover relationships in the north western alps following this approach combined with published studies on exhumedmantle domains sampled in the rest of thewestern alps indicates that several tectono metamorphic units fromthemost deformed metamorphosed part of the belt between the canavese line and the penninic front sample hyper extended lithosphere related to the jurassic opening of the western tethys relative plate motion during cretaceous tertiary basin inversion was largely accommodated at the transition between areas floored by hyper extended crust or hydrated subcontinental mantle and domains consisting of thicker continental crust as a result distal margins were preferentially subducted whereas the proximal domains and the brianconnais paleo high underwent relatively minor deformation and metamorphism the high pressure alpine tectono metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present day distal margins the recognition of preserved pre alpine relationships between continental basement post rift sediments and or serpentinized ultramafic rocks calls for a re assessment of the relative role of subduction and rifting dynamics in establishing the present day orogen architecture

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

the timing of and nature of greenschist facies deformation and metamorphism in the upper pennine alps the upper pennine nappes in the western alps are cut by structures that locally place european basement rocks over those of the piemonte zone which represent the sutured remains of alpine ocean crust in the taschalp area of switzerland the mischabel backfold and associated se verging shearing carries gneisses of the grand saint bernard nappe over eclogitic material of the zermatt saas zone greenschist facies assemblages are developed locally in the foot wall and hanging wall of the mischabel structure as well as in the sheared rocks of the thrust zone rb sr 40ar 39ar and u pb studies of white micas amphiboles and sphenes show that greenschist assemblages developed at circa 44 ma in the zermatt saas zone of the footwall and at circa 40 ma in the thrust zone rb sr data from the literature document muscovite and biotite ages in strongly foliated grand saint bernard gneisses of the hanging wall of circa 36 and 32 ma respectively these are thought to indicate that deformation on the mischabel structure continued for at least 5 m y rapid cooling inferred for the zermatt saas ophiolites in the mid eocene may document either the tectonic emplacement of the ophiolite over the underlying monte rosa massif or their extensional unroofing it is not possible to determine with certainty whether the mischabel structure is extensional or contractional on a crustal scale the age of cooling and deformation reported here show that it is not possible that the mischabel structure was responsible for the exhumation of the high pressure assemblages in the zermatt saas and monte rosa rocks

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

overlapping regions of coseismic and transient slow slip on the hawaiian decollement abstract earthquakes and slow slip are different faulting processes that release accumulated stress on different time scales despite nearly two decades of study it is still unclear whether the same section of fault is capable of hosting both slow aseismic and fast seismic slip here we jointly invert gps strong motion records and empirically corrected tsunami waveforms for the 2018 m7 1 hawai i earthquake our inversion results suggest the earthquake had a slow rupture speed of 1 2 km s with a maximum slip of 3 m south east of the hypocenter extending to the south west through an area known to regularly host slow slip events after exploring how the choice of fault geometry and regularization parameters affect the slip distributions of the earthquake and slow slip events we find strong evidence of overlap between the two additionally we perform numerical modeling to simulate fast and slow slip behavior due to the homogeneity of the hawaiian decollement differences in effective stress are likely an important factor that causes the diversity of slip we find that an earthquake can completely penetrate into the slow slip zone provided the effective stress differences between fast and slow slip zones is large enough to facilitate this our results reinforce the idea that an individual section of fault is capable of hosting a variety of distinct slip behaviors this is important for estimating earthquake rupture extent and for better ground motion and tsunami hazard assessment

magmatically triggered slow slip at kilauea volcano hawaii we demonstrate that a recent dike intrusion probably triggered a slow fault slip event sse on kilauea volcano9s mobile south flank our analysis combined models of advanced land observing satellite interferometric dike intrusion displacement maps with continuous global positioning system gps displacement vectors to show that deformation nearly identical to four previous sses at kilauea occurred at far field sites shortly after the intrusion we model stress changes because of both secular deformation and the intrusion and find that both would increase the coulomb failure stress on possible sse slip surfaces by roughly the same amount these results in concert with the observation that none of the previous sses at kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation suggest that both extrinsic intrusion triggering and intrinsic secular fault creep fault processes can lead to sses

short term pre 2004 seismic subsidence near south andaman is this a precursor slow slip prior to a megathrust earthquake abstract we report here on the campaign gps data from the andaman islands just previous to the great 2004 sumatra andaman earthquake the campaign mode acquisitions at port blair showed that the site started to subside between 2003 and 2004 in addition during this period the horizontal displacement of port blair with respect to indian plate deduced from 1996 to 2000 gps data changed its orientation to that obtained during the 26th dec 2004 co seismic this short term subsidence can be modeled as slip in the up dip portion of the fault a slip that is equivalent to an earthquake with moment magnitude of 6 3 previously slow slip was thought to appear at intermediate depths roughly 35 55 km but simple models of the deformation at this single site suggest slow slip at much shallower depth than this this observation of subsidence obtained by gps methods is in rough agreement with subsidence observed from tide gauge data campaign mode gps data between 1996 and 2000 suggest uplift for port blair during the inter seismic period and so does the reported field observations of interseismic micro atoll emergence lack of exposed land with gps stations along the southern part of the thrust fault deprive of arriving at any indication of this preseismic subsidence in those areas although gps data is lacking the geological indices reported from some sites on the alaskan coast for example imply short term subsidence just previous to the great 1964 earthquake the pre earthquake subsidence recorded in port blair therefore may have global implications as a precursor signal of great earthquakes at least along some of the subduction zones

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

possible slip history scenarios for the hyuga nada region and bungo channel and their relationship with nankai earthquakes in southwest japan based on numerical simulations the nankai earthquakes are great interplate earthquakes with recurrence intervals of 100 200 years occurring along the nankai trough southwestern japan new geological evidences suggest that the rupture area of the 1707 earthquake one of the largest extended farther westward than previously thought and similar magnitude events may have recurrence intervals of several hundred years west of the nankai earthquake segments in the bungo channel region slow slip events sses have occurred with recurrence intervals of several years since 1980 farther westward no m 7 5 interplate earthquakes have occurred except for one in 1968 in the northern hyuga nada region near the bungo channel we numerically simulated the generation cycles of nankai earthquakes hyuga nada earthquakes and bungo channel sses to obtain possible slip histories in these regions particularly during 1707 1980 we showed that the nankai and hyuga nada earthquakes occur mostly independently with average recurrence intervals of 151 and 117 years respectively an sse patch slipped coseismically during both the nankai and hyuga nada earthquakes the rupture of the nankai earthquakes extended to the hyuga nada region every 500 600 years and was followed by quiescence of sses for more than 200 years subsequently sses occurred repeatedly every 8 years current observational findings are generally consistent with the characteristics obtained from our scenarios except regarding the recurrence intervals of hyuga nada earthquakes for which few observations are available further evidence of the slip history in these regions will be required to use information from such simulations to forecast future earthquakes

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

modeling of in situ crystallization processes in the permian mafic layered intrusion of mont collon dent blanche nappe western alps abstract the mont collon mafic complex is one of the best preserved examples of the early permian magmatism in the central alps related to the intra continental collapse of the variscan belt it mostly consists 95 vol of ol hy normative plagioclase wehrlites olivine and cpx gabbros with cumulitic structures crosscut by acid dikes pegmatitic gabbros troctolites and anorthosites outcrop locally a well preserved cumulative sequence is exposed in the dents de bertol area center of intrusion p t calculations indicate that this layered magma chamber emplaced at mid crustal levels at about 0 5 gpa and 1100 c the mont collon cumulitic rocks record little magmatic differentiation as illustrated by the restricted range of clinopyroxene mg number mg cpx 83 89 whole rock incompatible trace element contents e g nb zr ba vary largely and without correlation with major element composition these features are characteristic of an in situ crystallization process with variable amounts of interstitial liquid l trapped between the cumulus mineral phases la icpms measurements show that trace element distribution in the latter is homogeneous pointing to subsolidus re equilibration between crystals and interstitial melts a quantitative modeling based on langmuir s in situ crystallization equation successfully duplicated the ree concentrations in cumulitic minerals of all rock facies of the intrusion the calculated amounts of interstitial liquid l vary between 0 and 35 for degrees of differentiation f of 0 to 20 relative to the least evolved facies of the intrusion l values are well correlated with the modal proportions of interstitial amphibole and whole rock incompatible trace element concentrations e g zr nb of the tested samples however the in situ crystallization model reaches its limitations with rock containing high modal content of ree bearing minerals i e zircon such as pegmatitic gabbros dikes of anorthositic composition locally crosscutting the layered lithologies evidence that the mont collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions the proposed model is not able to resolve these complex open systems but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams absence of significant differentiation with recurrent lithologies in the cumulitic pile of dents de bertol points to an efficiently convective magma chamber with possible periodic replenishment

archaeocyathan limestone blocks of likely antarctic origin in gondwanan tillite from the falkland islands cambrian limestone clasts containing a rich well preserved archaeocyathan fauna have been recovered from the late carboniferous fitzroy tillite formation of the falkland islands since neither cambrian strata nor limestone are present anywhere in the indigenous rock succession the clasts are regarded as exotic erratics introduced during the permo carboniferous gondwana wide glaciation most recent reconstructions of gondwana rotate the falklands into proximity with the eastern cape south africa and the ellsworth mountains antarctica in both of these areas permo carboniferous diamictites correlated with the fitzroy tillite formation also contain rare exotic clasts of archaeocyathan limestone the transantarctic mountains seem the most likely source for all of these unusual erratics this interpretation sustains the requirement for substantial rotation of the falklands microplate into gondwana reconstructions and illustrates the extent of the late carboniferous ice sheet apparent differences in the tillite clast assemblages between east and west falkland suggest variable provenance within the regional ice flow regime

fault frictional parameters and material properties revealed by slow slip events at kilauea volcano hawai i 1 we categorize slow slip events at kilauea volcano into two distinct families based on gps measurements of the surface displacement patterns an event correlation filter confirms that eastern and western families are statistically distinguishable with the western family notably self similar the western family exhibits quasi periodicity with regular repeat times while eastern family events are aperiodic or have complicated periodicity if the decollement is the source fault for both families of events it must have varying frictional properties at the 10 km scale of separation the temporal slip and spatial scaling behavior are consistent with a simplistic rate and state dependent frictional formalism provided that the characteristic slip distance for state evolution dc is of the order of millimeters rather than the 10 100 m typically found in lab studies and the shear rigidity is around 2 gpa consistent with fault gouge material

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

evidence of major middle cambrian deformation in the ross orogen antarctica long lived polyphase tectonism in the cambrian history of the ross orogen is demonstrated by recent stratigraphic studies in both the central and weddell sea segments of the transantarctic mountains we postulate that strong early middle cambrian deformation preceded a more widely recognized late cambrian early ordovician period of contraction and magmatism these middle cambrian movements were locally the principal phase of orogenic deformation they profoundly altered sedimentation patterns along the antarctic margin so that in the weddell sea region when carbonate platform sedimentation resumed the platform margin was reestablished far oceanward of its early cambrian location intra cambrian tectonism along the paleo pacific margin of gondwana was not confined to its antarctic segment comparable activity recently has been described from australia and is known from south america these movements may be related to amalgamation of east and west gondwana

pressurized magma reservoir within the east rift zone of kīlauea volcano hawai i evidence for relaxed stress changes from the 1975 kalapana earthquake we use 2000 2012 insar data from multiple satellites to investigate magma storage in k lauea s east rift zone erz the study period includes a surge in magma supply rate and intrusion eruptions in 2007 and 2011 the kupaianaha area inflated by 5 cm prior to the 2007 intrusion and the n pau crater area by 10 cm following the 2011 intrusion for the n pau crater area elastic modeling suggests an inflation source at 5 2 km depth or more below sea level the reservoir is located in the deeper section of the rift zone for which secular magma intrusion was inferred for the period following the 1975 mw7 7 decollement earthquake reservoir pressurization suggests that in this section of the erz extensional stress changes due to the earthquake have largely been compensated for and that this section is approaching its pre 1975 state reservoir pressurization also puts the molten core model into question for this section of k lauea s rift zone

sm nd isotopic evidence on the age of eclogitization in the zermatt saas ophiolite rocks within the zermatt saas ophiolite of the western alps have undergone eclogite facies metamorphism during subduction prior to the alpine collision the metamorphic history of these rocks is well defined with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite subsequent greenschist alteration occurs adjacent to faults veins and metasediments away from such sources of water retrogression is very limited sm nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 18 ma the large uncertainty is due to the presence of very small amounts of nd rich epidote present as inclusions within garnet as the closure temperature of garnet to sm nd is thought to be 600dc resetting due to post high pressure diffusion is thought to be insignificant given the fine grained protolith to the sample analysed and its extensive deformation under eclogite facies conditions incomplete homogenization of pre metamorphic isotopic variations is also considered unlikely to be responsible for the young age a tertiary age of eclogitization means that models of early alpine evolution based on the cessation of high pressure metamorphism in the cretaceous need to be revised

geodetic evidence for en echelon dike emplacement and concurrent slow slip during the june 2007 intrusion and eruption at kīlauea volcano hawaii 1 a series of complex events at k lauea volcano hawaii 17 june to 19 june 2007 began with an intrusion in the upper east rift zone erz and culminated with a small eruption 1500 m3 surface deformation due to the intrusion was recorded in unprecedented detail by global positioning system gps and tilt networks as well as interferometric synthetic aperture radar insar data acquired by the envisat and alos satellites a joint nonlinear inversion of gps tilt and insar data yields a deflationary source beneath the summit caldera and an ene striking uniform opening dislocation with 2 m opening a dip of 80 to the south and extending from the surface to 2 km depth this simple model reasonably fits the overall pattern of deformation but significantly misfits data near the western end of an inferred dike like source three more complex dike models are tested that allow for distributed opening including 1 a dike that follows the surface trace of the active rift zone 2 a dike that follows the symmetry axis of insar deformation and 3 two en echelon dike segments beneath mapped surface cracks and newly formed steaming areas the en echelon dike model best fits near field gps and tilt data maximum opening of 2 4 m occurred on the eastern segment beneath the eruptive vent although this model represents the best fit to the erz data it still fails to explain data from a coastal tiltmeter and gps sites on k lauea s southwestern flank the southwest flank gps sites and the coastal tiltmeter exhibit deformation consistent with observations of previous slow slip events beneath k lauea s south flank but inconsistent with observations of previous intrusions slow slip events at k lauea and elsewhere are thought to occur in a transition zone between locked and stably sliding zones of a fault an inversion including slip on a basal decollement improves fit to these data and suggests a maximum of 15 cm of seaward fault motion comparable to previous slow slip events

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes