tectonic wedging blueschist metamorphism and exposure of blueschists are they compatible tectonic wedging is currently deforming rocks along the westernmarginofthecentralvalleyofcalifornia structuralrelations in the strata of the mesozoic great valley group fore arc basin indicatethatearlierphasesofwedgingwerecoevalwithsubduction and accretion of the franciscan complex to the west the presentday geometry of tectonic wedging is incompatible with blueschist metamorphism in the franciscan complex because the detachment for the wedge system would have prevented the transport of accretionary wedge materials to great depth we propose a sequence of tectonic events that reconciles these tectonic processes franciscan subduction and metamorphism began at 165 ma and great valley group deposition began in the tithonian shortening affected great valley group rocks in the hauterivian barremian and may havecoincidedwithagapinblueschistmetamorphisminthefranciscan major uplift of franciscan blueschist relative to the unmetamorphosed great valley group rocks began in the albian cenomanian this uplift was accommodated along an east dipping extensional fault system between the franciscan complex and the structurally overlying coast range ophiolite and great valley group movement on this fault system caused the western side of thefore arcbasintosubside resultinginuninterrupteddeposition ofgreatvalleygroupsedimentsfromthecenomaniantothecampanian maastrichtian the youngest exposed blueschists in the franciscan were metamorphosed at 80 90 ma uplift of blueschist relativetogreatvalleygrouprocksmusthavebeencompleteprior todevelopmentofthepresentstyleofeast vergenttectonicwedging this style of deformation probably began between the maastrichtian and paleocene resulting in exhumation and deformation of bothfranciscanandgreatvalleygrouprocks thedevelopmentof wedging was coincident in part with the shallowing of subduction dip and voluminous frontal accretion of the coastal belt of the franciscan

extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

seismic imaging of crust beneath the dharwar craton india from ambient noise and teleseismic receiver function modelling we use cross correlation of continuous 18 months 2009 february to 2010 august ambient noise data recorded over 35 broad band seismographs in the archean dharwar craton and the adjoining granulite terrain to generate rayleigh wave group velocity maps in the period 5 28 s this is supplemented with longer period data 40 70 s from earthquake source combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations includes 15 stations operated during 1998 2002 to produce shear velocity image of the crust the velocity image reveals thinner crust 34 38 km in the late archean 2 7 ga eastern dharwar craton edc while all other terrains mid archean and proterozoic have crustal thickness from 40 to over 50 km the mid archean 3 36 ga greenstone belt of the western dharwar craton wdc has the thickest crust 50 km the average crustal vs beneath the edc is 3 70 3 78 km s 1 as compared to 3 80 3 95 km s 1 beneath the wdc we observe significant lateral variation in the thickness of lower crust vs 3 8 4 2 km s 1 10 15 km in the edc compared to 20 30 km in the wdc the lowermost part of the crust vs 4 0 km s 1 is thin 5 km beneath the edc in contrast to more thickness 10 27 km beneath the wdc our analysis suggests intermediate composition for the crust beneath the edc similar to those for other cratons in contrast the mid archean exposed wdc crust has more mafic composition and exceptional thickness a scenario at variance with the global observations we interpret this thick mafic crust to represent undeformed geological segment of 3 36 ga the edc with a nearly flat moho felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late archean

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

clastic lawsonite glaucophane and jadeitic pyroxene in franciscan metagraywackes from the diablo range california textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in franciscan metagraywackes in the diablo range these clasts differ from the neometamorphic post depositional assemblage by having larger grain sizes and deformational features including bending cracking and optical strain polarization large strain polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane but none of these minerals has grown across clast matrix boundaries by comparison neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz albite chlorite white mica in recrystallized metagraywacke matrices similarly authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine grained mixtures of albite chlorite quartz or internally to pumpellyite albite backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable jd contents within a single clast compositional heterogeneity can be as much as jd 50 ac 39 aug 11 to jd 95 ac 4 aug 1 because their texture habit and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at new idria southern diablo range or to those of some jadeite quartz associations in extensively recrystallized metabasite knockers such a provenance is inferred for pyroxene clasts in the franciscan metasediments in metaconglomerates pebbles containing pyroxenes jd 59 98 formed original clasts that were overprinted later by the post depositional lawsonitic metamorphism metabasites at pacheco pass have compositionally variable na pyroxenes jd 30 92 which indicate tectonic emplacement of the igneous bodies within enclosing in some cases pyroxenefree metagraywackes the diablo range metagraywackes are weakly developed blueschist facies assemblages of lawsonite white mica albite chlorite titanite glaucophane aragonite pumpellyite which indicate minimum metamorphic conditions of p 3 5 5 0 0 5 kb at t 100 200 c

history and controls of subsidence in the late cretaceous tertiary great valley forearc basin california abstract analysis of the upper cretaceous tertiary fill of the great valley forearc basin of california reveals a complicated history of subsidence partitioned in time and space western oceanward parts of the basin record subsidence then uplift in apparent response to the angle and rate of descent of the underlying subducting plate specifically uplift in the forearc basin corresponds to the onset of the laramide orogeny and inferred low angles of subduction in contrast eastern arcward parts of the basin display subsidence histories suggestive of thermal contraction and compatible with continentward migration of sierran arc magmatism in the late cretaceous these results imply that subsidence histories of forearc basins are likely to be much more complicated and less predictable than those of rifted continental margins and foreland basins where single subsidence mechanisms predominate and time varying subduction is not an underlying controlling factor

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

deformation history of the yolla bolly terrane at leech lake mountain eastern belt franciscan subduction complex california coast ranges the roles of volume loss coaxial versus noncoaxial flow and blueschist exhumation in subduction related accretionary wedges are still poorly understood in our study at leech lake mountain in the eastern belt of the franciscan subduction complex we focus on these subjects in the specific example of the franciscan the tectonic significance of the boundary between the eastern and central belts remains controversial the leech lake mountain area in northern california is situated immediately above this boundary and therefore appears to be of crucial importance for understanding aspects of the tectonic evolution of the franciscan the structural development at leech lake mountain is characterized by three deformational events d2 produced the regional cleavage s2 during blueschist facies metamorphism d2 is the only entirely ductile deformation event and was associated with accretion related internal stacking within the yolla bolly terrane the s2 cleavage is folded by f3 folds which are overturned to the west spacing of a related s3 cleavage is generally at the decimeter scale but locally is spaced at the centimeter scale the s2 and s3 cleavages and f3 folds are overprinted by tight to open f4 folds at the decimeter to map scale map scale f4 folds are largely upright whereas small scale f4 folds are either upright or overturned to the east our finite strain analysis relates deformed lengths to original lengths in the rock and thus provides an absolute reference frame which allows us to detect deformation related volume changes absolute finite strain data from 20 samples support earlier studies indicating that the development of the moderately dipping s2 cleavage was accompanied by pronounced volume loss averaging 36 the directions of the principal finite strain axes are scattered therefore we calculated a tensor average of the data the principal stretches of the tensor average are s x s y s z 1 06 0 91 0 66 indicating that the accumulation of ductile strain was characterized by subvertical shortening which was largely compensated for by volume loss and not by orthogonal extension evidence for noncoaxial deformation in our samples is limited indicating that the accumulation of ductile strain was largely coaxial our preferred tectonic interpretation is that accretion and blueschist facies metamorphism of the yolla bolly terrane at leech lake mountain occurred during d2 under an almost coaxial deformation regime during and after d2 the rocks were considerably exhumed and were telescoped by d3 top to the west out of sequence thrusts at shallow crustal levels in the leech lake mountain area d3 thrusting juxtaposed the eastern and central belts along the red mountain fault and cut out the lower tectonic units of the eastern belt the difference in the degree of peak metamorphism between the eastern and central belts is modest indicating that displacement at the red mountain fault was not large we propose that d3 out of sequence thrusts attenuated the metamorphic and stratigraphic section across the entire franciscan and overlying great valley forearc the final d4 event produced large scale folds all three events resulted from horizontal shortening there is no evidence for a major phase of horizontal extension which might have aided exhumation of the eastern belt blueschists modest tectonic exhumation was due to vertical ductile shortening during d2

magmatic origin of low t mafic blueschist and greenstone blocks from the franciscan melange san simeon california abstract the franciscan melange exposure near san simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale matrix tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails major trace and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the melange were derived from sources with morb like composition along with some having trace element and ree patterns similar to oib compositions most blocks are low lree basalts that probably formed in an open ocean ridge setting but some blocks have high lree contents similar to off axis seamounts linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar variably fractionated tholeiitic magma blueschist blocks with sodic amphibole lawsonite epidote were pervasively recrystallized at 300 350 c and foliated during ductile deformation that included folding their protolith can only be identified as mafic a few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts but some may have been diabase or even gabbro where interlayered sediment was present the mafic protolith was enriched in k rb and na greenstones on the other hand contain abundant pseudomorphic evidence of magmatic textures alteration to albite chlorite and pumpellyite at temperatures of 100 200 c is intense especially in cataclastic margins and pinched tails some of the basaltic greenstones have attached radiolarian chert and a few have relict diabasic textures the mafic blueschists and greenstones in the melange near san simeon are probably fragments from the uppermost part of the farallon plate the blueschists may be mostly mafic slabs uprooted from the subducting crust underplated to the base of the north american plate and later detached and carried back towards the surface in upwelling melange the greenstones are most likely fragments detached from seamounts subducted during times of melange upwelling

imaging the upper crust of the korean peninsula by surface wave tomography cross correlation of seismic background motions campillo and paul 2003 shapiro et al 2005 is applied to observations from the korean meteorological administration seismic network to estimate the short period rayleigh and love wave dispersion characteristics of the region standard processing procedures are applied to the cross correlation except that signal whitening is used in place of one bit sampling to equalize power in signals from different times multiple filter analysis is used to extract the group velocities from the estimated green s functions which are then used to image the spatially varying dispersion at periods between 0 5 and 20 sec the tomographic inversion technique used inverts all periods simultaneously to provide a smooth dispersion curve as a function of period in addition to the usual smooth spatial image for a given period the gyeongsang basin in the southeastern part of the peninsula is clearly resolved with lower group velocities

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

kinematics of franciscan complex exhumation new insights from the geology of mount diablo california three major lithotectonic units of an ancient arc trench system are present in western california in ascending structural order they include the franciscan complex a remnant accretionary prism partly including high pressure blueschist facies metamorphic rocks in a subduction melange the relatively unmetamorphosed coast range ophiolite and the great valley group forearc sediments in the mount diablo study area east of san francisco the subduction complex and ophiolite are juxtaposed across the coast range fault and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late cenozoic mount diablo anticline the coast range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite shear sense indicators in the serpentinite such as s c fabrics rotated clasts and slick enfibers record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the franciscan rocks in the modern reference frame based on these kinematic relations and metamorphic facies contrasts between the franciscan and the coast range ophiolite as much as 6 18 km structural attenuation has occurred along the coast range fault after restoration of vertical axis rotation and late cenozoic fold deformation the coast range fault dips at a low angle toward the northeast and shear sense data restore to top to the northeast motion on the coast range fault implying normal displacement of the ophiolite with respect to the structurally underlying franciscan complex these relations are consistent with the hypothesis that the franciscan rocks were exhumed by crustal attenuation along a low angle fault system these data suggest that the coast range fault dominantly accommodated normal displacement and ductile thinning and that these processes were primarily responsible for the structural exhumation of high pressure assemblages in the franciscan complex relative to the structurally overlying forearc crust

kinematic evidence for extensional unroofing of the franciscan complex along the coast range fault northern diablo range california franciscan metagraywacke immediately below the del puerto ophiolite an outlier of the coast range ophiolite in the northern diablo range was sheared during top to the east displacement on the coast range fault this represents normal faulting and extensional offset it was accompanied by attenuation of the coast range ophiolite and great valley sequence in the hanging wall along layer parallel normal faults that sole into the coast range fault extension occurred as the franciscan complex moved relatively west out from under north american lithosphere and across the subducting ocean plate below this effected a lengthening and thinning in the wedge of material above the down going plate presumably in response to instability brought about by subduction shallowing krueger and jones 1989 and accretion of the franciscan central belt in the latest cretaceous to early paleocene as a result blueschist facies terranes of the uppermost part of the franciscan complex are now juxtaposed directly against hanging wall units that bear only low grade metamorphism

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

a comprehensive rocket and radar study of midlatitude spread f 1 an instrumented sounding rocket launched from wallops island virginia has flown through a midlatitude spread f msf event in conjunction with simultaneous ionosonde hf radar and 244 mhz scintillation observations from the ground the in situ measurements include the electric field horizontal neutral wind and plasma density within the spread f region the ground based hf radar measurements of wave signatures in the bottomside f region ledge reveal the presence of waves propagating to the north and northwest prior to and during the spreading event the periods of these bottomside waves range from 16 to 60 min and they are shown to be associated with a strong tropical storm located 2000 km southeast of the launch site enhancements in the auroral current system occur about an hour before the msf first appears but none of the observed waves can be attributed to this source the new phase sensitive ionosonde system operated at wallops island during the experiment confirms the long standing hypothesis that this particular spread f event arises from multipath echoes distributed over a wide field of view in the bottomside f region evidence of vertically displaced plasma that could produce such multipath echoes is observed in the rocket data at and above the f region peak over spatial scales smaller than the wavelengths observed on the bottomside ledge by the hf radar but similar to the range separation given by the high resolution ionosonde echoes when the scale lengths of the structures are interpreted in magnetic coordinates no significant plasma density structures smaller than a few kilometers are observed in the rocket data and no unusual scintillation is observed along a path coincident with the rocket trajectory

the latest cretaceous early paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern pacific margin episodic growth of accretionary complexes along the northwestern pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks the latest cretaceous to early paleogene rapid growth of the accretionary complexes observed in hokkaido and sakhalin islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone the mongolia okhotsk zone in the northern asian continent rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks now exposed as the kamuikotan complex in hokkaido and the susunai complex in sakhalin the forearc basin shallowed as a result of the uplift the uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region magmatic activity along the continental margin was also active at the same time subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time a large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge

tectonic significance of low temperature blueschist blocks in the franciscan melange at san simeon california abstract the blueschist bearing franciscan melange exposed near san simeon is very similar to the melange terrane forming the central belt north of san francisco epidote bearing low t blueschist blocks at san simeon n 3 are the same age as high t garnet bearing blocks in the northern and diablo range melange terranes 155 150 ma this age approximates the time of initiation of franciscan subduction during the initial stages of franciscan subduction part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high p conditions this underplated material was variably retrograded as subduction refrigerated the forearc blocks actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized a model is proposed in which exhumation was facilitated by gravity driven seaward oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80 ma that caused the laramide orogeny as this occurred blueschist and graphite schist blocks were plucked from the bottom of the hanging wall incorporated into the shale and water rich shear zone at the plate interface subduction channel shear zone and exhumed during the upward flow of melange driven by the movement of the downgoing plate while subduction was still active

normal faulting at convergent plate boundaries mylonitic extensional fabrics in the franciscan subduction complex in del puerto canyon california revisited 1 using a strain and rotation analysis we tested the hypotheses that top east mylonitic extensional structures in the uppermost franciscan subduction complex in del puerto canyon california accomplished exhumation of the franciscan blueschists we found no evidence of strongly noncoaxial deformation instead our data indicate overall coaxial deformation in the proposed zone of mylonitic extensional deformation there are no extensional strains moderate vertical shortening occurred without horizontal extension and was compensated by modest deformation related volume loss there is also no strain gradient toward and within the proposed shear zone therefore the results of our work indicate that mylonitic extensional structures in del puerto canyon do not exist overall we conclude that there is no currently identifiable structure adjacent to or within the franciscan in california that could have accommodated large scale horizontal extension exhumation of the franciscan blueschists was mainly achieved by erosion of an emergent forearc high our strain results and published data are also inconsistent with the development of a supercritically tapered franciscan wedge that ultimately should have triggered normal faulting strain magnitudes are low despite the relatively long residence time of the rocks in the accretionary wedge from this we infer that strain rates were low and that the wedge material flowed with a relatively high viscosity therefore ductile flow probably was not fast enough to form a supercritically tapered franciscan wedge

controls on the first appearance of jadeitic pyroxene northern diablo range california a reaction producing jadeitic pyroxene in metagreywackes of the northern diablo range has been identified on the basis of mineral distribution isograd patterns and composition of coexisting minerals the appearance of jadeitic pyroxene jd80 is closely followed by the disappearance of pumpellyite which indicates that pumpellyite plays a major role in the pyroxene producing reaction a new projection from hematite lawsonite chlorite quartz and h2o on to the naalo2 feo mgo ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form 1 00 pumpellyite 0 31 chlorite 8 71 albite 0 70 hematite 2 00 h2o 8 54 jadeite 0 57 glaucophane 3 09 lawsonite 5 26 quartz metagreywackes of the northern diablo range were metamorphosed under conditions of ph2o ptotal at 200 300 c and 7 5 10 0 kbar despite the low temperatures attained during metamorphism the assumption of equilibrium yields results consistent with field observations and phase relations

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

tectonic evolution of the jurassic cretaceous great valley forearc california implications for the franciscan thrust wedge hypothesis interpretation of seismic reflection data and restoration of depositional geometries of cretaceous forearc basin strata in the northwest great valley of california provide important controls on structural reconstructions of the western margin of the sacramento valley and northern coast ranges monoclinal eastward dips of great valley group strata and fault systems striking northwest southeast which are features proposed as evidence for a west dipping blind great valley franciscan sole thrust and related backthrusts instead are expressions of bedding geometry that resulted from folding of the paskenta and related synsedimentary normal faults depositional onlap and a major structural stratigraphic discontinuity the discontinuity separates east dipping aptian and younger great valley group strata from beds of lower great valley group and coast range ophiolite that were deformed and erosionally or structurally truncated by mid cretaceous time dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient great valley forearc but is also observed in modern forearc basins advocates of the franciscan thrust wedge model have also proposed that west dipping shingled patterns of seismic events imaged beneath the sacramento valley are imbricate thrust slices of the great valley group this hypothesis however is incompatible with borehole potential field and seismic refraction data that characterize the sacramento valley basement as ophiolitic seaward dipping reflections in the ophiolitic basement of the sacramento valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges thus late stage tectonic mechanisms are not required to interpret a forearc that owes much of its present day bedding architecture to processes coeval with deposition thickening of the great valley group stratigraphic section valanginian turonian in the hanging walls of the paskenta elder creek and cold fork fault zones combined with attenuation or complete omission of preextensional units including the coast range ophiolite and geometric evidence based on seismic reconstructions suggest that these faults are jurassic cretaceous normal faults that developed in a submarine setting down structure views of the great valley outcrop belt simplify otherwise complex map relations and portray the paskenta and related faults as half graben bounding faults that accommodated significant northwestward tectonic transport of hanging wall rocks it is significant that these faults sole into the coast range fault an enigmatic forearc structure that juxtaposes rocks of the franciscan complex blueschists with rocks of the coast range ophiolite and great valley group that have sustained only zeolite grade metamorphism discovery of jurassic cretaceous crustal scale extension in the great valley forearc suggests that a significant part of coast range fault related attenuation developed early in the history of the subduction complex

crustal structure of the ancestral northwestern california forearc region from seismic reflection imaging implications for convergent margin tectonics abstract analysis of a suite of 2 d seismic reflection profiles reveals that the northwestern sacramento valley and eastern coast range foothills northern california are underlain by a system of blind west dipping thrust faults homoclinally east dipping and folded mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast vergent fault propagation folds developed in the hanging walls of the thrusts exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern coast ranges are in the hanging wall of the blind thrust system and have been displaced from their roots in the footwall deep east dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared serpentinized and attenuated ophiolitic basement restoration of slip on the thrusts suggests that the coast range fault which is the exposed structural contact between the coherent blueschists and attenuated ophiolite originally dipped east and is associated with the east dipping magnetic reflectors in the footwall this interpretation of the reflection data is consistent with previous inferences about the deep structure in this region and supports a two stage model for blueschist exposure in the eastern coast ranges 1 blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east dipping coast range fault system in late cretaceous 2 blueschists attenuated ophiolite and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest cretaceous early tertiary the blind thrust system probably rooted in and was antithetic to the east dipping subduction zone beneath the forearc region active transpressional plate motion in western california is locally accommodated in part by reactivation of blind thrust faults that originally developed during the convergent regime

crustal structure from deep seismic soundings along the koyna ii kelsi loni profile in the deccan trap area india abstract the crustal depth section obtained from deep seismic soundings along the koyna ii kelsi loni profile which lies near latitude 18 n roughly in the east west direction in that part of the deccan trap maharashtra state india shows a number of reflection segments below the deccan traps down to the moho discontinuity a deep fault below the deccan traps 13 km east of mahad divides the entire cross section including the moho boundary into two crustal blocks the reflection segments show updip towards the west coast in the western block the moho discontinuity which is at a depth of 39 km near the deep fault starts rising towards the coast reaching a depth of 31 5 km at the west coast the eastern block is thrown up by 1 5 km with respect to the western block along the deep fault a structural contour map of the moho discontinuity for the koyna reservoir area has been prepared from the present results and the crustal information obtained along the koyna i profile kaila et al 1979a shows that the deep fault in the koyna area is aligned in the nnw sse direction refraction seismic data analysis by the wave front method reveals that the thickness of the deccan trap increases towards the west coast the deccan trap is 600 700 m thick in the eastern region between nira sp 130 and loni sp 200 and attains a thickness of 1500 m at 10 km east of the west coast the longitudinal wave velocity in the deccan traps along the profile varies from 4 8 to 5 0 km sec and in the crystalline basement from 6 0 to 6 15 km sec a tentative isopach contour map of the deccan traps and a tentative structural contour map of the pre deccan trap contact have been prepared for the koyna reservoir area from the results along the koyna ii and koyna i profiles a flexure aligned in a nnw sse direction in the pre deccan trap contact which is an expression of the deep fault into the basement has been clearly brought out the flexure coincides in general with the orientation of the deccan volcanic scarp in this area

extensional fault uplift of regional franciscan blueschists die to subduction shallowing during the laramide orogeny regional structural and stratigraphic relations in the northern california coast ranges require an episode of extensional tectonic unroofing of coherent blueschist terranes of the eastern belt of the franciscan complex along a crustal scale low angle normal fault system here named the cost range detachment this tectonic unroofing occurred during the latest cretaceous and early tertiary contemporaneous with the compressional laramide orogeny of the north american interior dynamic models of accretionary wedges predict that shallowing of subduction as has been inferred during the laramide orogeny will impart extensional stresses in the wedge above the subducting plate the authors therefore suggest a single casual event gradual shallowing of subduction in the latest cretaceous and early tertiary to link the extensional fault uplift of regional franciscan blueschists to laramide compression in the north american interior

investigations related to scientific deep drilling to study reservoir triggered earthquakes at koyna india artificial water reservoir triggered earthquakes have continued at koyna in the deccan traps province india since the impoundment of the shivaji sagar reservoir in 1962 existing models to comprehend the genesis of triggered earthquakes suffer from lack of observations in the near field to investigate further scientific deep drilling and setting up a fault zone observatory at depth of 5 7 km is planned in the koyna area prior to undertaking deep drilling an exploratory phase of investigations has been launched to constrain subsurface geology structure and heat flow regime in the area that provide critical inputs for the design of the deep borehole observatory two core boreholes drilled to depths of 1 522 and 1 196 m have penetrated the deccan traps and sampled the granitic basement in the region for the first time studies on cores provide new and direct information regarding the thickness of the deccan traps the absence of infra trappean sediments and the nature of the underlying basement rocks temperatures estimated at a depth of 6 km in the area made on the basis of heat flow and thermal properties data sets do not exceed 150 c low elevation airborne gravity gradient and magnetic data sets covering 5 012 line km together with high quality magnetotelluric data at 100 stations provide both regional information about the thickness of the deccan traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone acquisition of airborne lidar data to obtain a high resolution topographic model of the region has been completed over an area of 1 064 km2 centred on the koyna seismic zone seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

polyphase brittle and ductile deformation of late jurassic ophiolitic basalt argillite matrix melange and stratigraphic overlap sequence northwestern washington state abstract a belt of jurassic to cretaceous ophiolitic rocks borders the western margin of the u s cordillera and stretches from central california to northwestern washington state the northern end of this belt lies between the san juan islands and the northwest cascades within this region ophiolitic rocks consist of a succession of oceanic and arc affinity igneous and sedimentary rocks which form a sedimentary melange and sedimentary overlap sequence which is imbricated during the mid cretaceous the melange contains blocks and olistoliths of peridotite plagiogranite chert basalt and volcanoclastic conglomerate which range in size from a meter to over 1 km and are contained within a matrix of argillite and volcanoclastic breccia and conglomerate peridotites were exposed to the sub aqueous surface along serpentinized shear zones prior to their incorporation into the melange and the sedimentary matrix of the melange underwent brittle deformation during the earliest stages of its structural history melange rocks are overlain in angular unconformity by a jura cretaceous arc sourced sedimentary succession which is at least 500 meters thick and passes upward from a basal breccia containing clasts of plagiogranite gabbro tonalite chert and basalt into argillite containing late jurassic radiolarians the argillite is overlain by poorly sorted greywacke and conglomerate with clast populations similar to those of the basal breccia the conglomerate fines upward into a massive to bedded feldspathic lithic arenite and greywacke that yields mid cretaceous detrital zircons the overlap succession and the melange are deformed by two generations of highly penetrative structures d1a and d1b which produced north to east vergent tight to isoclinal folds and axial planar pressure solution cleavages all units are further deformed by three generations of penetrative structures the successively younger nne to nw ne and e w to wnw trending folds have foliations that cross cut the earlier structural fabrics and faults formation of the melange required differential elevations during the time of deposition and the presence of rocks which are sourced from both arc and oceanic crust extension within the forearc provides a mechanism to exhume peridotites and generate differential topography for arc and oceanic affinity rocks to erode and be incorporated into the melange as part of olistostromal deposits

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

finite strain patterns of nevadan deformation western klamath mountains california finite strain analyses of galice formation metagraywackes from the footwall block of the orleans fault western klamath mountains california show a dramatic increase in strain ratios structurally upward approaching the orleans fault r xz strain values range from less than 3 at a distance of 2 km from the fault to 6 5 adjacent to the fault strain ellipse orientations are relatively consistent x directions are commonly northwest southeast and subhorizontal and trend subparallel to the trace of the orleans fault finite strain ellipse orientations and the clockwise rotational strain recorded by microscopic kinematic indicators are both compatible with the interpretation of dextral oblique convergence on the orleans fault during the nevadan orogeny

constraints on the petrologic structure of the subduction zone slab mantle interface from franciscan complex exotic ultramafic blocks ultramafic blocks within mud matrix melange of the franciscan complex california preserve a series of metasomatic mineral zones generated by infiltration of si rich hydrous fluids during subduction we describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids fluid flow affected a cr spinel lherzolite protolith to form first serpentinite then a talc dominated rock and finally an amphibole rich assemblage a diverse suite of accessory minerals in the amphibole rich zone titanite clinozoisite zircon apatite suggests that the trace element signature of subduction zone fluids may be fractionated in this zone oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450 500 c whole rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism including elements such as ti which are currently considered to have low solubilities in such fluids taken as a whole the blocks appear to preserve the metasomatic structure of the slab mantle interface in subduction zones and imply that the chemistry of slab derived fluids is modified as they pass through these metasomatic zones in the mantle wedge our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas instead we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

deformation of the eastern franciscan belt northern california abstract the late jurassic and cretaceous eastern franciscan belt of the northern california coast range consists of two multiply deformed blueschist facies terranes the pickett peak and yolla bolly terranes four deformations have been recognized in the pickett peak terrane and three in the yolla bolly terrane the earliest recognized penetrative fabric d1 occurs only in the pickett peak terrane the later penetrative fabrics d2 and d3 occur in both the yolla bolly and pickett peak terranes d1 and d2 apparently represent fabrics that formed during subduction and accretion of the terranes fabrics from both d1 and d2 are consistent with sw ne movement directions with respect to their present geographic positions d3 postdates blueschist facies metamorphism of the terranes and may be related to emplacement of the terranes to higher structural levels a broad regional warping d4 is evident from the map pattern and folding of large metamorphosed thrust sheets d4 folds may be related to deformation associated with oblique convergence along the continental margin in late cretaceous and or early tertiary time

neogene transpressive evolution of the california coast ranges the san andreas fault has long been viewed as a vertical structure that extends from the ground surface to the base of the lithosphere and separates the pacific plate from the north american plate along a transform plate boundary analysis of neogene structures in the central coast ranges indicates however that crustal blocks defined by strands of the san andreas fault system have undergone pervasive compressive deformation that has shifted these blocks and their boundary faults east relative to north america this suggests that the blocks and their boundary structures are bounded below by an active decollement that appears to coincide with the midcrustal brittle to ductile transition at the base of the seismogenic zone these compressive structures may be related to a currently seismically active fold and thrust belt that characterizes the eastern front of the coast ranges seismic profiles from the central valley east of the range front show that tectonic wedges blind thrusts developed within stratified rocks are present throughout this zone of deformation and that crystalline basement that underlies these stratified rocks is not involved in the deformation crystalline basement may underlie most of the coast ranges as indicated by the presence of metamorphic rocks of possible sierran affinity preserved in neogene thrust sheets as far west as loma prieta and by seismic velocities in the lower crust appropriate for mafic to intermediate granitoid rocks on the basis of these relations we suggest that the san andreas fault system is confined to the brittle crust above the decollement and does not penetrate to the base of the lithosphere thus the san andreas fault should not be considered as a plate boundary instead the functional plate boundary within the coast ranges appears to be the inferred subhorizontal midcrustal decollement this decollement corresponds approximately to the 350 c isotherm the temperature at which quartz becomes ductile if our hypothesis is proven to be correct by geologic and geophysical investigations currently underway then extensive revisions in popular plate tectonic models applied to the coast ranges will be required in addition assessment of seismic hazards will be rendered more difficult but more realistic owing to realization of the greater likelihood for occurrence of thrust faulting on blind or buried faults or on faults currently deemed inactive

significance of klamath rocks between the franciscan complex and coast range ophiolite northern california small fault bounded slabs of low grade prehnite pumpellyite bearing slate metagraywacke and greenstone occur between the coast range ophiolite and south fork mountain schist for at least 60 km south of the klamath mountains northern california the metagraywacke slabs differ from typical franciscan complex metagraywacke to the west by the absence of blueschist facies minerals and the abundance of quartz and chert clasts and they differ from sandstone and mudstone of the great valley sequence to the east by the presence of a penetrative cleavage and low grade metamorphism the metasedimentary rock in the slabs is petrographically identical to the jurassic galice formation which occurs along the western and southern margins of the klamath mountains upward drag along a west dipping reverse fault best explains the present structural position of the slabs and is consistent with available geological and geophysical data these data imply that the coast range ophiolite structurally overlies sierran klamath basement at least locally

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

shear wave velocity structure of the koyna warna region in western india using ambient noise correlation and surface wave dispersion abstract the koyna warna region is a premier site of reservoir triggered seismicity in the deccan volcanic province of western india in the present study shear wave velocity structure of this region down to a depth of 10 km is estimated using the ambient noise correlation technique with data from a network of 11 seismic broadband stations asymmetric green s functions are obtained that are suggestive of an anisotropic distribution of noise sources in the indian subcontinent cross correlation of continuous noise data of 12 months duration recorded on the vertical components enables computation of group velocity dispersion curves from the green s functions this is supplemented by rayleigh waves from local earthquakes which in addition to those from noise data are inverted for the shear wave velocity structure using the multiple filter technique the study reveals on an average a 0 8 km thick basaltic layer of the deccan traps with a shear wave velocity of about 3 0 km s on the eastern side of the escarpment a low velocity possibly weathered granitic layer with a velocity of 3 3 km s is found below the traps and is underlain by the granitic basement with a velocity of 3 6 km s except for the surface topographic undulation the velocity structure in this region down to 10 km is similar on either side

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

oxygen isotope zoning in garnets from franciscan eclogite blocks evidence for rock buffered fluid interaction in the mantle wedge the oxygen isotope compositions of eclogite and amphibolite garnets from franciscan complex high grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories sims oxygen isotope analysis of single garnets from five eclogite blocks from three localities ring mountain mount hamilton and jenner beach shows an abrupt decrease in the 18o value by 1 3 from core to rim at a distance of 120 50 m from the rim in nine out of the 12 garnets analyzed in contrast amphibolite garnets from one block ring mountain analyzed show a gradual increase in 18o value from core to rim implying a different history from that of the eclogite blocks values of 18o in eclogite garnet cores range from 5 7 to 11 6 preserving the composition of the eclogite protolith the abrupt decrease in the 18o values of the garnet rims to values ranging from 3 2 to 11 2 suggests interaction with a lower 18o fluid during the final stages of growth during eclogite facies metamorphism 450 600 c we hypothesize that this fluid is sourced from the serpentinized mantle wedge high mg ni and cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185 240 c at ring mountain and mount hamilton rind formation temperatures together with the lower 18o garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low temperature retrogression we suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge as subduction continues metasomatic exchange between high grade blocks and surrounding ultramafic rock is recorded in low 18o garnet rims and later as temperatures decrease with rind formation