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

metastable olivine wedge and deep dry cold slab beneath southwest japan abstract oceanic plates subducted at trenches penetrate into the deep mantle and encounter a structural boundary at a depth of 410 km where olivine the dominant element of mantle rocks transforms into a higher density form wadsleyite this transformation may be delayed within the coldest core of subducting plates slabs due to kinetic effects and it has been suggested that metastable olivine may persist deeper than 410 km using high density seismic array data in japan we show the direct image of the structure corresponding to this metastable olivine wedge mow beneath southwest japan numerical simulation of a subducting slab including the kinetic effect of water h 2 o on the olivine wadsleyite transformation indicates that the presence of the imaged mow requires an insignificant amount of water less than 100 wt ppm be present in the slab mantle thus a deep dry cold slab we infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab but no significant amount within the slab itself we also demonstrate that a numerical simulation including the kinetics of 660 km phase transformation can reconcile the observed deep depression of the 660 km discontinuity with a gentle clapeyron slope

a fractal model for crustal deformation abstract we hypothesize that crustal deformation occurs on a scale invariant matrix of faults for simplicity we consider a two dimensional pattern of hexagons on which strike slip faulting occurs the behavior of the system is controlled by a single parameter the fractal dimension deformation occurs on all scales of faults the fractal dimension determines the fraction of the total displacement that occurs on the first order or primary faults the value of the fractal dimension can be obtained from the frequency magnitude relation for earthquakes our results are applied to the san andreas fault system in central california earthquake studies give d 1 90 we associate the main strand of the san andreas fault with the primary faults of our fractal system we predict that the relative velocity across the main strand is 2 93 cm yr the remainder of the relative velocity of 5 5 cm yr between the pacific and north american plates occurs on higher order faults the predicted value is in reasonably good agreement with the value 3 39 0 29 cm yr obtained from geological studies

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

geologic and geophysical investigation of two fine grained granites sierra nevada batholith california evidence for structural controls on emplacement and volcanism the johnson granite porphyry and the silver pass porphyry are two late stage porphyritic intrusions in the cretaceous sierra nevada batholith california both intrusions are characterized by fine grained textures miarolitic cavities pegmatite veins and abundant xenoliths microstructures in the johnson granite porphyry record magmatic flow magnetic fabrics in the johnson granite porphyry determined by anisotropy of magnetic susceptibility show that the poles to magnetic foliations define a nne ssw girdle and magnetic lineations plunge shallowly to moderately to the wnw and ese a gravity survey and three dimensional inversion indicate that the johnson granite porphyry is shallow and tabular extending to 1 5 km depth beneath the southwestern margin a second gravity anomaly was delineated southeast of the johnson granite porphyry suggesting the intrusion continues to depth the silver pass porphyry is deformed by the dextral rosy finch shear zone and records a gradient of high to low temperature solid state fabrics field and magnetic foliations in the silver pass porphyry are subvertical and lineations are shallowly plunging we propose that regional shear zones controlled the emplacement of these intrusions the johnson granite porphyry was emplaced in a three dimensional tension gash associated with the termination of the bench canyon shear zone the rosy finch shear zone facilitated the emplacement of the silver pass porphyry probably as a tension fracture associated with the shear zone the field observations and magnetic fabrics suggest that the johnson granite porphyry may record an extrusive event suggesting that volcanism may be related to deformation along active shear zones

the multiple inverse method a new technique to separate stresses from heterogeneous fault slip data abstract a new inverse technique is presented here to separate stresses from heterogeneous fault slip data without a priori information on the stresses or on the classification of faults according to the stresses four parameters are determined by the inversion one for the shape of the stress ellipsoid and three for the direction of the stress axes accordingly the inversion is equivalent to the projection of the fault slip data to a point in four dimensional parameter space the data are divided into k element subsets to which the inverse technique is applied where k 4 or 5 is the optimal choice for the stability of solutions and for the reduction of computation significant solutions are identified as clusters in parameter space the technique is demonstrated first by simulated fault slip data as an example field data obtained from miocene fore arc sediments in western japan were processed by the method

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

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

gutenberg richter law for deep earthquakes revisited a dual mechanism hypothesis deep earthquake b values appear to vary with slab thermal state and earthquake magnitude the physical reason for the variations and the relation with deep rupture mechanisms are still unclear here i confirm the spatial variations of b value and the dependence on slab temperature using about 40 yr of data from the global centroid moment tensor catalog a new bimodal pattern is observed for the 500 700 km depth range b is close to 1 in the cold tonga slab while in warmer slabs e g south america japan kuril izu bonin mariana b is close to 0 5 for intermediate magnitudes m w 5 3 6 5 and increases to 1 for large magnitudes m w 6 5 to explain these observations i propose a dual mechanism hypothesis in which deep earthquakes nucleate only within the metastable olivine wedge mow but can rupture outside mow by a different mechanism the fractal dimension of earthquake size distribution changes from 2 to 1 as the thermally controlled mow thickness decreases and back to 2 as the mechanism outside mow dominates

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

multistage emplacement of the mount givens pluton central sierra nevada batholith california the ca 90 ma mount givens pluton is one of the largest granodioritic to granitic intrusions in the sierra nevada batholith of california emplacement of the pluton occurred during a critical time in the tectonic evolution of the central sierra nevada magmatic arc marked by a transition from regional contraction to dextral transcurrent shear a model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility ams at 351 stations one of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain the 80 km long 15 30 km wide pluton crystallized from a multiphase three segment magma chamber marked by a bulbous northern lobe and linear central and southern segments the pluton is interpreted to be tabular in shape with a thickness of 5 km most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets the most important of which were steeply dipping northwest trending fractures formed parallel to the structural grain of the arc and vertical north trending extension fractures formed in response to a component of arc parallel dextral shear some of these fractures acted as magma conduits episodically filling the pluton as source rocks became depleted in melt an initial voluminous intrusive event stage 1 quickly filled the southern chamber with granodiorite magma but only partially filled the northern and central chambers stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber the latter delineated by a 20 km long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments floor downdrop eventually ceased to be an effective space making process in the northern lobe and renewed magmatism stage 3 led to expansion and doming of the chamber as the northern lobe cooled a ring fault ruptured within the viscoelastic stage 1 2 carapace allowing ring dike intrusion stage 4 and sinking of a central flap of consolidated material the temporal and spatial variations in emplacement mechanisms demonstrated for the mount givens pluton i e fracture generation floor downdrop underplating inflation ring diking suggest that end member models e g fracture vs diapir are oversimplifications of the pluton assembly process

kilometer scale kinking of crystalline rocks in a transpressive convergent setting central sierra nevada california structural analyses in the lake edison area of the central sierra nevada california show that prominent north northeast trending lineaments represent traces of joints and faults that have accommodated slip within a 4 8 km wide right lateral north northwest trending monoclinal kink band the kilometer scale kink band formed in cretaceous granitic rocks and the closely spaced mineralized joints and faults formed at 79 ma joints and faults strike 070 within the kink band and 020 030 outside the kink band a kink band kinematic model indicates that 3 7 2 1 1 9 km of right lateral displacement was accommodated across the kink band and that as much as 148 m of left lateral slip on faults within the kink band occurred a sensitivity study on four model parameters varied the angle made by the inner and outer limbs of the kink band the number of faults within the kink band and the kink band width and length this analysis yields estimates for right lateral displacement across the kink band and left lateral slip on faults within the kink band that fall into the range constrained by geologic relationships kinking may have been the last stage of right lateral deformation which started with development of the rosy finch shear zone the presence of curved lineaments in the southern sierra nevada indicates that kilometer scale kink bands may exist elsewhere in the sierra nevada batholith such arc parallel kinking has been described in several other transpressive settings and kilometer scale kink bands may be a process by which late stage arc parallel deformation is accommodated in convergent margins

the 2013 okhotsk deep focus earthquake rupture beyond the metastable olivine wedge and thermally controlled rise time near the edge of a slab the 2013 m8 3 okhotsk earthquake involves two primary mechanisms of deep focus earthquake rupture mineral phase transformation of olivine to spinel and thermal shear instability backprojection imaging of broadband seismograms recorded by the north american and european networks indicates bilateral rupture toward ne and sse the rupture paths of the ne segment and other regional m7 earthquakes are confined in narrow regions along the slab contours consistent with the phase transformation mechanism however the sse rupture propagates a long distance across the slab and aftershocks are distributed across a 60 km wide zone beyond the plausible thickness of the metastable olivine wedge favoring thermal shear weakening while the ne rupture is only visible at high frequencies the sse rupture is consistently observed across a broad frequency range this frequency dependent rupture mode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end

late jurassic early cretaceous continental convergence and intracontinental orogenesis in east asia a synthesis of the yanshan revolution abstract the basic tectonic framework of continental east asia was produced by a series of nearly contemporaneous orogenic events in the late middle jurassic to early cretaceous commonly the late mesozoic orogenic processes were characterized by continent continent collision large scale thrusting strike slip faulting and intense crustal shortening crustal thickening regional anatexis and metamorphism followed by large scale lithospheric extension rifting and magmatism to better understand the geological processes this paper reviews and synthesizes existing multi disciplinary geologic data related to sedimentation tectonics magmatism metamorphism and geochemistry and proposes a two stage tectono thermal evolutionary history of east asia during the late middle jurassic to early cretaceous ca 170 120 ma in the first stage three orogenic belts along the continental margins were formed coevally at ca 170 135 ma i e the north mongol okhotsk orogen the east paleo pacific coastal orogen and the west bangong nujiang orogen tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the late jurassic as well as crustal anatexis that generated syn kinematic granites adakites and migmatites the lithosphere of the east asian continent was thickened reaching a maximum during the latest jurassic or the earliest cretaceous in the second stage ca 135 120 ma delamination of the thickened lithosphere resulted in a remarkable 120 km lithospheric thinning and the development of mantle derived magmatism mineralization metamorphic core complexes and rift basins the middle jurassic early cretaceous subduction of oceanic plates paleo pacific meso tethys and mongol okhotsk and continent continent collision e g lhasa and qiangtang along the east asian continental margins produced broad coastal and intracontinental orogens these significant tectonic activities marked by widespread intracontinental orogeny and continental reconstruction are commonly termed the yanshan revolution movement in the chinese literature

structural control on the shape of intrusions in the koktokay ore district chinese altai north western china abstract the koktokay pegmatite type rare metal bearing ore district in the altai orogen is famous for both its large scale and its diversity of rare metals however the emplacement mechanisms of the ore bearing pegmatite intrusions in the koktokay ore district are still unclear based on field observations the emplacement of the ore bearing pegmatite intrusions falls into two types the first type is typical of the formation of dykes and sills whereby they intruded into fan shaped moderate dipping joints within plutonic rocks the second type involves the formation of a punched laccolith that was fed by a pegmatite sill magmatic stoping is the main mechanism of the laccolith emplacement the peripheral faults played an important role in helping the emplacement of the laccolith the trend of dykes and sills indicate two potential prospecting areas which are located in the western and northern regions of the koktokay ore district

folding and granite emplacement inferred from structural strain tem and gravimetric analyses the case study of the tulle antiform sw french massif central abstract in the south limousin french massif central the tulle anticline is a kilometre scale upright open fold which re folds an already foliated variscan stack of nappes three main granodioritic plutons associated with numerous folded dykes and boudinaged sills crop out in the core of the tulle anticline microstructural study including quartz c axis petrofabrics and transmission electron microscope tem analysis allows us to recognize the timing of deformation during pluton emplacement and cooling the earliest fabric is a pre full crystallization deformation the second stage corresponds to a high temperature ca 650 c solid state deformation the last deformation is responsible for the c axis preferred orientation at low temperature several methods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain this strain is characterized by a nw se maximum stretching axis x parallel to the tulle anticline axis and an intermediate ne sw trending shortening axis y also responsible for large open folding of the host rocks gravimetric modelling allows us to determine a laccolithic shape for the plutons hot stokes diapirism is ruled out and the hypothesis of granitic magma channelling through dykes is preferred a laccolith emplacement model based on a syn folding decollement level is proposed

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

trace elemental and sr nd hf isotopic compositions and u pb ages for the kitakami adakitic plutons insights into interactions with the early cretaceous trt triple junction offshore japan abstract we conducted major trace element sr nd hf isotopic and u pb geochronologic analyses of early cretaceous kitakami granitic plutons northeast japan we suggest that these plutons include rocks of adakitic affinity which indicate partial melting of an eclogitic slab the kitakami adakites were mostly derived from juvenile oceanic crustal sources but include inherited zircons with archean to neoproterozoic hf model ages rather than being directly derived from crustal rocks of these ages zircons may have been detrital in the voluminous trench fill sandstone within the jurassic northern kitakami accretionary prism the exhumation of older basement that served as the detrital zircon source may have been triggered by ridge subduction although the plutons show a general southward younging from northern kitakami 120 130 ma southern kitakami 115 125 ma and abukuma 100 115 ma restoration of post crystallization left slip faults inverts this spatial pattern and results in a northward younging pattern this suggests northward migration of a trench ridge transform triple junction that included the izanagi farallon ridge the sambagawa eclogite may be genetically related to the kitakami adakite in time and space if the transform duplexing is restored

reactivation of a collisional suture by miocene transpressional domes associated with the red river and song chay detachment faults northern vietnam abstract elongate miocene gneissose and granitic domes in northern vietnam formed in a dextral transpressional ductile shear regime possibly associated with large scale restraining step overs along dextral faults initial anticlinal d1 doming involved folding of both basement and hanging wall rocks with d1 secondary folds that verge toward the anticlinal axes such folds reflect dome scale flexural slip folding with continued shortening d2 detachment faults developed on the flanks of the anticlines along the hanging wall basement interface so that the basement was extruded vertically into the overlying hanging wall rocks the detachment faults were associated with d2 drag folds that verge away from the anticlinal axes the hanging wall assemblage lacks a well ordered stratigraphy displaying primarily block in matrix fabric we identified bedded cherts associated with umbers and alkalic basaltic intrusions within these hanging wall rocks a first report of such rocks from vietnam the association of cherts umbers and basaltic intrusions and extrusions with block in matrix units with clastic rocks strongly suggest that the hanging wall rocks comprise part of a subduction complex because the base of a subduction complex is a former subduction megathrust horizon the hanging wall basement interface represents a reactivated collisional suture such a suture was probably associated with the indosinian orogeny and the basement should be the indochina continental block this structure may have influenced the position of miocene dextral faulting in addition to controlling the position of the dome detachments the well known red river fault marks the boundary of one of the domes but in this region it appears to be a detachment normal fault rather than a dextral strike slip fault however the association with the dome evolution with large scale restraining step overs suggests that dextral faulting associated with dome development may lie further away from the dome axes than the detachment

frequency moment distribution of deep earthquakes implications for the seismogenic zone at the bottom of slabs abstract we present a systematic investigation of the variation with depth of the frequency of earthquake occurrence vs seismic moment based on 16 years of harvard centroid moment tensor cmt solutions we analyze depth variations of earthquake size distribution in terms of variations in the absolute value of the slope of the regression of the logarithm of the population vs seismic moment a quantity known as the parameter the shallowest earthquakes 0 50 km depth exhibit a well defined and robust size distribution regime characterized by a discontinuous increase in with increasing moment others have shown that this increase probably represents the effects of a physical limit in the dimensions of the area of seismogenic slip of shallow earthquake sources the population of deep earthquakes in the depth interval 500 600 km shows two markedly different distributions the deep earthquakes in the tonga region feature an initially high value 0 92 at small moments and a lower value 0 41 at high moments in contrast the size distribution of non tonga deep events shows the reverse of those changes 0 41 at low moment and 1 17 at higher moment to help explain these observations we propose a model of deep seismogenesis that assumes three dimensional earthquake source regions that vary principally in their transverse dimensions the two segment behavior in the tonga region and other subduction zones is thought to represent in part constraints owing to the threshold of completeness of the cmt catalog and to its short time interval of sampling we interpret the differences between tonga and other deep wadati benioff zones as being a consequence of tonga s markedly higher subduction rate and hence its colder thermal structure and presumably thicker region of seismogenesis we interpret the critical moments at which values change in terms of variations in the transverse thickness of deep seismogenic zones and estimate that it is about 11 km for the tonga region and about 3 km for other zones at depths of 500 600 km these results are generally consistent with deep earthquakes being restricted to wedge shaped regions of peridotite persisting metastably to as deep as 700 km in old rapidly descending and hence cold slabs failure is thought to occur in metastable peridotite by transformational faulting great deep earthquakes present special challenges to any theory of deep earthquakes based on slab thermal structure for example a continuing question is how such large events can fit in a thermally controlled seismogenic zone that is diminishing in its transverse dimensions with increasing depth the very concept of a scale invariant earthquake size distribution may be inappropriate for these rare events and the unusual settings in which they are found

block in matrix fabrics that lack shearing but possess composite cleavage planes a sedimentary melange origin for the yuwan accretionary complex in the ryukyu island arc japan while melanges have been taken to be a significant component of accretionary complexes the use of this term has been mostly descriptive given the lack of consensus concerning their genesis however many workers have interpreted asymmetric shear fabrics in melanges as evidence of a tectonic origin i e giving rise to tectonic melanges in contrast to olistostromes which are considered to be purely sedimentary in origin due at least in part to the lack of such shear fabrics it is still uncertain whether accretionary processes include those that involve the intense shearing of oceanic lithologies along with for example the incorporation and entrapment of exotic blocks by muddy matrices during sediment deposition and deformation in axial deep sea trench settings here we argue that the so called block in matrix fabrics that characterize accretionary complexes are primarily sedimentary in origin and are distinct from shear fabrics represented by tectonic melanges we also propose that following their accretion subsequent recycling of such oceanic materials is a common feature within evolving accretionary prisms lastly we draw attention to the observation that d1 pressure solution cleavage planes notably widespread in the lower cretaceous yuwan complex amami oshima island in the ryukyu arc sw japan are absent from the minor asymmetric shear structures that often characterize tectonic melanges in contrast we suggest that the yuwan complex represents the effects of broadly symmetric flattening this scenario is supported by our observation that the number of sedimentary and deformation events differs significantly compared to the single phase of shearing expected in tectonic melanges d minus 1 d 1 pressure solution and axial planar cleavages are only observed in exotic blocks and exhibit older accretionary deformation events within a preexisting accretionary prism of the existing structural associations sd1 is the oldest predating the d 1 sedimentary episode and it represents an oceanic sedimentary phase associated with the formation of a distinct type of block in matrix fabric produced by the accumulation of debris flows at seamount margins sd2 represents a succeeding sedimentary episode characterized by trench axis debris flow including oceanic inclusions derived from collapsed material from an older accretionary prism the resulting sd2 block in matrix fabric is one of the main lithologic components of the amami oshima yuwan complex here the sd2 episode typically includes an igneous fabric consisting of nonexotic intrusive and extrusive basalts within a terrigenous matrix suggesting that the yuwan prism may have also been associated with a trench trench ridge triple junction d1 coaxial deformation is observed to overprint all of the episodes represented although it has not destroyed or significantly sheared their respective fabrics especially in regard to the main sd2 block in matrix structures which are therefore considered to represent original sedimentary structures

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

a great earthquake doublet and seismic stress transfer cycle in the central kuril islands temporal variations of the frictional resistance on subduction zone plate boundary faults associated with the stick slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate here we report on two great earthquakes that occurred near the kuril islands which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting on 15 november 2006 an event of moment magnitude 8 3 ruptured the shallow dipping plate boundary along which the pacific plate descends beneath the central kuril arc the thrust ruptured a seismic gap that previously had uncertain seismogenic potential although the earlier occurrence of outer rise compressional events had suggested the presence of frictional resistance within minutes of this large underthrusting event intraplate extensional earthquakes commenced in the outer rise region seaward of the kuril trench and on 13 january 2007 an event of moment magnitude 8 1 ruptured a normal fault extending through the upper portion of the pacific plate producing one of the largest recorded shallow extensional earthquakes this energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults

evaluation of multiple emplacement mechanisms the huichizi granite pluton qinling orogenic belt central china three end member granite emplacement mechanisms host rock ductile shortening hrds host rock rigid displacement hrrd and magmatic absorption ma are identified in this study of the huichizi pluton the internal fabrics are mainly flattening typesona 0 4 0 9u and strain is highoesa 1 0 0 6u near the boundaries and lowoesa 0 7 0 30u toward the central parts of the pluton in the 1 5 km wide contact aureole extension e is measured in syn emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1 14 km and 0 74 km respectively from this we conclude that the hrds produced only about 20 space for the present volume of the pluton the regional hrrd derived from the uplift by about 8 km of the qinling group in a flower structure contributed 19 032 space ms including stoping and material transfer processes yielded another 37 space of this stoping contributed a minimum of 1 space estimated from the surface area of 2 8 km 2 covered by xenoliths transfer processes provided about 36 of the needed space deduced from the average 64 36 ratio of hybridization of mantle derived to crustal qinling group derived magma the three mechanisms may act nearly simultaneously as a multiple emplacement mechanism this situation may apply to many other plutons as well 2000 elsevier science ltd all rights reserved

coupling of early tertiary extension in the great valley forearc basin with blueschist exhumation in the underlying franciscan accretionary wedge at mount diablo california in the california coast ranges east of san francisco post 3 5 ma folding and erosion of the mount diablo anticline have created unusual three dimensional exposures of late mesozoic cenozoic stratigraphic and structural relations among rocks of the franciscan subduction complex coast range ophiolite and great valley forearc basin these relations offer new insights into the kinematics of late cretaceous early tertiary synsubduction extension attenuation and blueschist exhumation within the ancestral california forearc region map relations and subsurface data reveal that marine strata of the great valley forearc basin northeast of mount diablo accumulated in an 100 km long north south trending extensional graben system normal faults in the graben system cut steeply downsection through late cretaceous and early tertiary growth strata and terminate against the clayton marsh creek fault which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system the clayton marsh creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried including the coast range ophiolite and basal great valley strata these faults are the local expression of the coast range fault in this area franciscan rocks exposed beneath the faults were metamorphosed at 20 km depths after 108 ma based on jadeitic pyroxene and 108 ma detrital zircons u pb in metagraywackes apatite fission track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from 9 km to 3 km depths in latest cretaceous early tertiary time coeval with the subsidence and extension in the structurally overlying forearc graben system much of their earlier rise from 20 km to 9 km depth presumably also occurred coeval with graben development although our data cannot determine this directly these relations demonstrate that the california forearc around mount diablo was in an extensional regime undergoing active sedimentation during major franciscan exhumation and thus support models for exhumation of franciscan blueschist facies rocks via synsubduction extensional processes rather than via erosional processes

late devonian early permian subduction accretion of the zharma saur oceanic arc west junggar nw china insights from field geology geochemistry and geochronology abstract migration of arc magmatism is of significant importance for accretionary orogens but few examples have been documented in ancient orogens in this paper we report late paleozoic early permian migrating magmatism of the intra oceanic zharma saur arc in the tarbagatay mountains west junggar nw china in order to address the general role of a growing arc in accretionary orogenesis and the debates on the formation and timing of the kazakhstan orocline in the central asian orogenic belt caob our detailed field work suggests that in the zharma saur arc plutons and dikes intruded and lavas erupted in association with accretionary complexes that were dominated by ocean plate stratigraphy ops mafic to intermediate magmas have arc type geochemical signatures such as depleted hfses and enrichments in liles and lrees coupled with high end t 5 7 to 6 8 to low 87sr 86sr initial values 0 7035 to 0 7037 the tarbagatay accreted rocks contain early paleozoic pillow lavas which have depleted liles high end t 7 4 to 8 2 and high 87sr 86sr initial values 0 7041 to 0 7063 these data suggest that the zharma saur arc was an intra oceanic island arc that developed on accreted ops material u pb zircon isotopic data the zwtb i diorite pluton 322 3 ma the jldk diorite pluton 318 4 ma andesitic lavas 312 2 ma 310 4 ma 301 3 ma demonstrate that the zharma saur arc magmatism occurred in the late paleozoic integration with published geochronological data on the zharma saur arc leads to a complex model of arc growth with a general southward younging of the arc magmatism from 383 ma to 262 ma these relations imply a southward growth and migration of the magmatic axis of the zharma saur arc that was associated with a short period of ridge subduction the magmatic activity of the zharma saur arc that probably continued until the early permian resulted from migrations towards the forearc and backarc as well as oblique or parallel motion to the trench these new results not only provide robust evidence for resolving controversies about the phanerozoic accretionary and continental growth of the central asian orogenic belt but also shed light on the migration of magmatism in accretionary orogens in general

magmatic structures and kinematics emplacement of the variscan granites from central portugal serra da estrela and castro daire areas abstract the late d3 variscan granites which represent the most important volume of granites of central portugal are studied here for their magmatic and solid state fabrics by means of the anisotropy of magnetic susceptibility ams technique in a 5000 km 2 area located between guarda and castro daire including the serra da estrela region two main directions n150e and n20e of stretching lineations are found indicating that the emplacement of these late kinematic granitoids was tectonically controlled a three stage model is proposed in which a openings along n150 striking dextral strike slip faults allow the first granitic magmas to be emplaced in the upper crust of palaeozoic metasediments and syn d3 granitoids b openings along n20 striking possibly sinistral strike slip faults conjugates to the prior strike slip faults and allow the enlargement of the granitic plutons and c the final emplacement of this huge volume of granitic rocks was mainly controlled by the magma pressure which explains the rarity of structures related to this stage of granites emplacement in the country rocks this structural study of granitic rocks yields new constraints on the tectonic evolution of this part of the variscan orogenic belt at c 300 ma

development of cretaceous transpressional cleavage synchronous with batholith emplacement central sierra nevada california metamorphosed strata of the ritter range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels 8 km in the late cretaceous sierra nevada magmatic arc paleozoic to middle jurassic strata of the pendant have been penetratively shortened 30 50 forming slates phyllites and schists with bedding inclined cleavage and associated linear structures the cleavage secondary cleavages and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the farallon and north american plates microstructural relations show that cleavage development accompanied greenschist to amphibolite facies metamorphism metamorphic hornblende muscovite and biotite in the wall rocks show weak to strong preferred orientation and have 40ar 39ar plateau ages ranging from about 85 to 80 ma hornblende and possibly muscovite are expected to retain 40ar at or near estimated peak metamorphic temperatures of 500 c thus their 40ar 39ar ages closely correspond to the time of cleavage formation granitic plutons adjacent to the ritter range pendant have yielded u pb and other ages that cluster from 91 to 82 ma we conclude that bedding inclined cleavage and metamorphism in the ritter range wall rocks developed in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

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

folding of granite and cretaceous exhumation associated with regional scale flexural slip folding and ridge subduction kitakami zone northeast japan abstract the early cretaceous granitic plutons intrude the kitakami zone northeast japan whose southern and northern regions consist of forearc basin and accretionary complex rocks respectively all country rock of the kitakami zone exhibit prominent pressure solution cleavage and associated folds formed during shortening with a small component of sinistral shear whereas most plutons show only igneous textures the kesengawa granite and some other plutons however have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks we document a tectonic fold with axial planar foliation in part of the kesengawa granite the metamorphic minerals associated with the contact aureole and country rocks of the kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled the kesengawa pluton and country rocks of the southern kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern kitakami zone are overturned and verge to the east the kitakami basement is not bounded by normal or reverse faults so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions nor is the exhumation associated with extensional doming instead the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines the regional shortening of accretionary prism forearc basin and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation the adakitic magmatism and forearc shortening resulted from subduction of the buoyant izanagi kula ridge a regional event known as the oshima orogeny

frequency distribution and quantification of deep earthquakes the frequency distribution of deep earthquakes d 350 km is analyzed on a regional scale using magnitude data from 4660 events reported in the international seismological center bulletin 1964 1982 and 391 centroid moment tensor solutions 1977 1987 we use the moment magnitude mw to define b values comparable with studies of shallow seismicity scaling laws relating mw mb and mo are derived mw 1 6mb 3 2 mb log mo 4 3 2 4 which compare very favorably with published laws uncertainties and biases of the mb and mo data sets are considered the slope of the frequency distribution is evaluated over a restricted magnitude range the results are very robust and independent from the magnitude range used from the mb magnitude data we obtain regional b values in the range 0 4 b 1 22 with a worldwide average value b 0 87 these results are confirmed by the analysis of the seismic moment data the steepest frequency distribution is observed in the tonga islands region b 1 22 although 60 of the worldwide deep seismicity occurs here only few events since 1977 had seismic moment exceeding 1019 n m the high fractal dimension d 2 4 implies a volume partially filled with fault planes in agreement with the observations on the seismic morphology of the region at the other extreme south america is characterized by the largest recorded deep events and an almost complete absence of small ones b 0 4

exhumation of triassic hp lt rocks by upright extrusional domes and overlying detachment faults ishigaki jima ryukyu islands abstract the tomuru formation of ishigaki jima in the southernmost part of the ryukyu arc comprises blueschist facies subduction complex rocks metamorphosed in triassic time d1 structures related to subduction blueschist facies mineral growth and possibly early stages of exhumation are deformed by d2 structures that appear to reflect the last stage of exhumation d2 structures define several anticlines with parasitic overturned folds verging away from anticlinal axes the shortening recorded by this deformation appears to reflect upward extrusion relative to flanking material the anticlines are flanked by detachment faults with normal sense of shear parallel to d2 vergence hanging wall rocks that include the fusaki formation an accretionary prism with early cretaceous metamorphic ages and late eocene limestone conglomerate and andesitic volcanics the eocene strata contain metamorphic detritus derived from the tomuru and fusaki formations indicating pre late eocene surface exposure of these units ultramafic rocks and gabbro blocks of the tomuru formation were incorporated by sedimentary sliding into the trench prior to subduction and high pressure metamorphism rather than being emplaced as diapirs along a post metamorphic fault as previously proposed geochronologic metamorphic and thermal considerations suggest exhumation of the tomuru formation to relatively shallow crustal depths prior to or concurrent with early cretaceous metamorphism of the fusaki formation arcward vergent thrusting may have placed the younger and formerly structurally lower subduction complex fusaki formation over the older one tomuru formation d2 extrusional doming began after the emplacement of the fusaki formation at high structural levels the d2 transport directions are subparallel to the strike of the orogen suggesting that the upright extrusion may have occurred along a forearc strike slip fault system this final stage of exhumation concluded in the late eocene with extensional collapse and the development of detachment faults the progression from initial exposure of the tomuru and fusaki formations deposition of late eocene strata extrusional doming and late detachment faulting may have been associated with migrating step overs rather than changes in regional tectonics

temporal variation in the magnitude frequency distribution during the guy greenbrier earthquake sequence the recent increase in earthquake activity in the central u s has led to concerns about the hazard posed by induced earthquakes understanding earthquake phenomena and monitoring in all settings can be improved by the detection of small events however the catalog of induced earthquakes is incomplete for small events due to the sparse instrumentation this is particularly true in settings of low background seismicity like the central u s we apply single station template matching to detect small earthquakes during the guy greenbrier sequence in central arkansas and find over 100 times more earthquakes than are registered in the advanced national seismic system catalog between july 2010 and october 2011 a complete catalog over nearly 4 units of magnitude enables us to analyze the magnitude frequency distribution of induced earthquakes we find that earthquakes deviated from the gutenberg richter statistics during the operation of nearby injection wells but returned to gutenberg richter statistics after the wells were shut in

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

rupture complexity of the 1994 bolivia and 2013 sea of okhotsk deep earthquakes the physical mechanism of deep earthquakes depth 300 km remains enigmatic partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations the two largest deep earthquakes the recent great 2013 sea of okhotsk earthquake m 8 3 depth 607 km and the great 1994 bolivia earthquake m 8 3 depth 637 km together provide a unique opportunity to compare their rupture patterns in detail here we extend a travel time sub event location method to perform full teleseismic p waveform inversion this new method allows us to explain the observed broadband records with a set of sub events whose model parameters are robustly constrained without smoothing we find that while the okhotsk event is mostly unilateral rupturing 90 km along strike with a velocity over 4 km s the bolivia earthquake ruptured about half this distance at a slow velocity about 1 5 km s and displayed a major change in rupture direction we explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered

structural evolution of underthrusted sediments kodiak islands alaska the late cretaceous to early tertiary sediments of the kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin because of the thick trench fill this early deformation resulted in the formation of two distinct structural terranes 1 coherent terranes of layered turbidites and 2 melange terranes the earliest deformation d1 in the coherent terranes is marked by normal faults mineralized layer perpendicular veins in sandstones and microfaults in shales these features suggest that 1 was vertical and that 3 was horizontal and nw trending during d1 we interpret d1 structures as hydrofractures that formed in flat lying sediments with high fluid pressures caused by rapid tectonic loading in contrast the melanges appear to record a more protracted and complex history that culminated in structures similar to the d1 structures above we have recognized three successive transitional stages in the melanges i breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix resulting in the blocks in matrix character of the melange ii development of ductile shear bands that crosscut the melange fabric at a low angle and iii hydrofracturing with the formation of calcite and quartz filled veins in sandstone inclusions and microfaults in the shales the sequence of microstructures associated with stages i iii records increased cohesion of the sediments during deformation dominated by layer parallel shear intense mineralization and microfaulting during stage iii suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability stage iii structures are texturally indistinguishable and contemporaneous with the less pervasive d1 structures in the coherent terranes based on these observations we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement the coherent terranes represent the structurally and stratigraphically lowest sediments and they bypassed the stage i and ii deformations of the melanges because they were the greatest distance from the decollement when they were deformed moreover both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism